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Inductively coupled plasma mass spectrometry

Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It is known and used for its ability to detect metals and several non-metals in liquid samples at very low concentrations. It can detect different isotopes of the same element, which makes it a versatile tool in isotopic labeling.

Compared to atomic absorption spectroscopy, ICP-MS has greater speed, precision, and sensitivity. However, compared with other types of mass spectrometry, such as thermal ionization mass spectrometry (TIMS) and glow discharge mass spectrometry (GD-MS), ICP-MS introduces many interfering species: argon from the plasma, component gases of air that leak through the cone orifices, and contamination from glassware and the cones.

Inductively coupled plasma
An inductively coupled plasma is a plasma that is energized (ionized) by inductively heating the gas with an electromagnetic coil, and contains a sufficient concentration of ions and electrons to make the gas electrically conductive. Not all of the gas needs to be ionized for the gas to have the characteristics of a plasma; as little as 1% ionization creates a plasma. The plasmas used in spectrochemical analysis are essentially electrically neutral, with each positive charge on an ion balanced by a free electron. In these plasmas the positive ions are almost all singly charged and there are few negative ions, so there are nearly equal numbers of ions and electrons in each unit volume of plasma.

The ICPs have two operation modes, called capacitive (E) mode with low plasma density and inductive (H) mode with high plasma density, and E to H heating mode transition occurs with external inputs. The Inductively Coupled Plasma Mass Spectrometry is operated in the H mode.

What makes Inductively Coupled Plasma Mass Spectrometry (ICP-MS) unique to other forms of inorganic mass spectrometry is its ability to sample the analyte continuously, without interruption. This is in contrast to other forms of inorganic mass spectrometry; Glow Discharge Mass Spectrometry (GDMS) and Thermal Ionization Mass Spectrometry (TIMS), that require a two-stage process: Insert sample(s) into a vacuum chamber, seal the vacuum chamber, pump down the vacuum, energize sample, thereby sending ions into the mass analyzer. With ICP-MS the sample to be analyzed is sitting at atmospheric pressure. Through the effective use of differential pumping; multiple vacuum stages separate by differential apertures (holes), the ions created in the argon plasma are, with the aid of various electrostatic focusing techniques, transmitted through the mass analyzer to the detector(s) and counted. Not only does this enable the analyst to radically increase sample throughput (amount of samples over time), but has also made it possible to do what is called "time resolved acquisition". Hyphenated techniques like Liquid Chromatography ICP-MS (LC-ICP-MS); Laser Ablation ICP-MS  (LA-ICP-MS); Flow Injection ICP-MS (FIA-ICP-MS), etc. have benefited from this relatively new technology. It has stimulated the development new tools for research including geochemistry and forensic chemistry; biochemistry and oceanography. Additionally, increases in sample throughput from dozens of samples a day to hundreds of samples a day have revolutionized environmental analysis, reducing costs. Fundamentally, this is all due to the fact that while the sample resides at environmental pressure, the analyzer and detector are at 1/10,000,000 of that same pressure during normal operation.

An inductively coupled plasma (ICP) for spectrometry is sustained in a torch that consists of three concentric tubes, usually made of quartz, although the inner tube (injector) can be sapphire if hydrofluoric acid is being used. The end of this torch is placed inside an induction coil supplied with a radio-frequency electric current. A flow of argon gas (usually 13 to 18 liters per minute) is introduced between the two outermost tubes of the torch and an electric spark is applied for a short time to introduce free electrons into the gas stream. These electrons interact with the radio-frequency magnetic field of the induction coil and are accelerated first in one direction, then the other, as the field changes at high frequency (usually 27.12 million cycles per second). The accelerated electrons collide with argon atoms, and sometimes a collision causes an argon atom to part with one of its electrons. The released electron is in turn accelerated by the rapidly changing magnetic field. The process continues until the rate of release of new electrons in collisions is balanced by the rate of recombination of electrons with argon ions (atoms that have lost an electron). This produces a ‘fireball’ that consists mostly of argon atoms with a rather small fraction of free electrons and argon ions. The temperature of the plasma is very high, of the order of 10,000 K. The plasma also produces ultraviolet light, so for safety should not be viewed directly.

The ICP can be retained in the quartz torch because the flow of gas between the two outermost tubes keeps the plasma away from the walls of the torch. A second flow of argon (around 1 liter per minute) is usually introduced between the central tube and the intermediate tube to keep the plasma away from the end of the central tube. A third flow (again usually around 1 liter per minute) of gas is introduced into the central tube of the torch. This gas flow passes through the centre of the plasma, where it forms a channel that is cooler than the surrounding plasma but still much hotter than a chemical flame. Samples to be analyzed are introduced into this central channel, usually as a mist of liquid formed by passing the liquid sample into a nebulizer.

To maximise plasma temperature (and hence ionisation efficiency) and stability, the sample should be introduced through the central tube with as little liquid (solvent load) as possible, and with consistent droplet sizes. A nebuliser can be used for liquid samples, followed by a spray chamber to remove larger droplets, or a desolvating nebuliser can be used to evaporate most of the solvent before it reaches the torch. Solid samples can also be introduced using laser ablation. The sample enters the central channel of the ICP, evaporates, molecules break apart, and then the constituent atoms ionise. At the temperatures prevailing in the plasma a significant proportion of the atoms of many chemical elements are ionized, each atom losing its most loosely bound electron to form a singly charged ion. The plasma temperature is selected to maximise ionisation efficiency for elements with a high first ionisation energy, while minimising second ionisation (double charging) for elements that have a low second ionisation energy.

Mass spectrometry
For coupling to mass spectrometry, the ions from the plasma are extracted through a series of cones into a mass spectrometer, usually a quadrupole. The ions are separated on the basis of their mass-to-charge ratio and a detector receives an ion signal proportional to the concentration.

The concentration of a sample can be determined through calibration with certified reference material such as single or multi-element reference standards. ICP-MS also lends itself to quantitative determinations through isotope dilution, a single point method based on an isotopically enriched standard. In order to increase reproducibility and compensate errors but sensitivity variation, an internal standard can be added.

Other mass analyzers coupled to ICP systems include double focusing magnetic-electrostatic sector systems with both single and multiple collector, as well as time of flight systems (both axial and orthogonal accelerators have been used).

Applications
One of the largest volume uses for ICP-MS is in the medical and forensic field, specifically, toxicology. A physician may order a metal assay for a number of reasons, such as suspicion of heavy metal poisoning, metabolic concerns, and even hepatological issues. Depending on the specific parameters unique to each patient's diagnostic plan, samples collected for analysis can range from whole blood, urine, plasma, serum, to even packed red blood cells. Another primary use for this instrument lies in the environmental field. Such applications include water testing for municipalities or private individuals all the way to soil, water and other material analysis for industrial purposes.

In recent years, industrial and biological monitoring has presented another major need for metal analysis via ICP-MS. Individuals working in factories where exposure to metals is likely and unavoidable, such as a battery factory, are required by their employer to have their blood or urine analyzed for metal toxicity on a regular basis. This monitoring has become a mandatory practice implemented by the U.S. Occupational Safety and Health Administration, in an effort to protect workers from their work environment and ensure proper rotation of work duties (i.e. rotating employees from a high exposure position to a low exposure position).

ICP-MS is also used widely in the geochemistry field for radiometric dating, in which it is used to analyze relative abundance of different isotopes, in particular uranium and lead. ICP-MS is more suitable for this application than the previously used thermal ionization mass spectrometry, as species with high ionization energy such as osmium and tungsten can be easily ionized. For high precision ratio work, multiple collector instruments are normally used to reduce the effect noise on the calculated ratios.

In the field of flow cytometry, a new technique uses ICP-MS to replace the traditional fluorochromes. Briefly, instead of labelling antibodies (or other biological probes) with fluorochromes, each antibody is labelled with a distinct combinations of lanthanides. When the sample of interest is analysed by ICP-MS in a specialised flow cytometer, each antibody can be identified and quantitated by virtue of a distinct ICP "footprint". In theory, hundreds of different biological probes can thus be analysed in an individual cell, at a rate of ca. 1,000 cells per second. Because elements are easily distinguished in ICP-MS, the problem of compensation in multiplex flow cytometry is effectively eliminated.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a powerful technique for the elemental analysis of a wide variety of materials encountered in forensic casework. (LA-ICP-MS) has already successfully been applied to applications in forensics, metals, glasses, soils, car paints, bones and teeth, printing inks, trace elemental, fingerprint, and paper. Among these, forensic glass analysis stands out as an application for which this technique has great utility to provide highly. Car hit and runs, burglaries, assaults, drive-by shootings and bombings such as these situations may cause glass fragments that could be used as evidence of association in glass transfer conditions. LA-ICP-MS is considered one of the best techniques for analysis of glass due to the short time for sample preparation and sample, small sample size of less than 250 nanograms. In addition there is no need for complex procedure and handling of dangerous materials that is used for digestion of the samples. This allows detecting major, minor and tracing elements with high level of precision and accuracy. There are set of properties that are used to measure glass sample such as physical and optical properties including color, thickness, density, refractive index (RI) and also, if necessary, elemental analysis can be conducted in order to enhance the value of an association.

Pharmaceutical industry
In the pharmaceutical industry, ICP-MS is used for detecting inorganic impurities in pharmaceuticals and their ingredients. New and reduced maximum permitted exposure levels of heavy metals from dietary supplements, introduced in USP (United States Pharmacopeia) «〈232〉Elemental Impurities—Limits» and USP «〈232〉Elemental Impurities—Procedures», will increase the need for ICP-MS technology, where, previously, other analytic methods have been sufficient. Cosmetics, such as lipstick, recovered from a crime scene may provide valuable forensic information. Lipstick smears left on cigarette butts, glassware, clothing, bedding; napkins, paper, etc. may be valuable evidence. Lipstick recovered from clothing or skin may also indicate physical contact between individuals. Forensic analysis of recovered lipstick smear evidence can provide valuable information on the recent activities of a victim or suspect. Trace elemental analysis of lipstick smears could be used to complement existing visual comparative procedures to determine the lipstick brand and color.

Single Particle Inductively Coupled Plasma Mass Spectroscopy (SP ICP-MS) was designed for particle suspensions in 2000 by Claude Degueldre. He first tested this new methodology at the Forel Institute of the University of Geneva and presented this new analytical approach at the 'Colloid 2oo2' symposium during the spring 2002 meeting of the EMRS, and in the proceedings in 2003. This study presents the theory of SP ICP-MS and the results of tests carried out on clay particles (montmorillonite) as well as other suspensions of colloids. This method was then tested on thorium dioxide nanoparticles by Degueldre & Favarger (2004), zirconium dioxide by Degueldre et al (2004) and gold nanoparticles, which are used as a substrate in nanopharmacy, and published by Degueldre et al (2006). Subsequently, the study of uranium dioxide nano- and micro-particles gave rise to a detailed publication, Ref. Degueldre et al (2006). Since 2010 the interest for SP ICP-MS has exploded. Previous forensic techniques employed for the organic analysis of lipsticks by compositional comparison include thin layer chromatography (TLC), gas chromatography (GC), and high-performance liquid chromatography (HPLC). These methods provide useful information regarding the identification of lipsticks. However, they all require long sample preparation times and destroy the sample. Nondestructive techniques for the forensic analysis of lipstick smears include UV fluorescence observation combined with purge-and-trap gas chromatography, microspectrophotometry and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and Raman spectroscopy.

Metal speciation
A growing trend in the world of elemental analysis has revolved around the speciation, or determination of oxidation state of certain metals such as chromium and arsenic. The toxicity of those elements varies with the oxidation state, so new regulations from food authorities requires speciation of some elements. One of the primary techniques to achieve this is to separate the chemical species with high-performance liquid chromatography (HPLC) or field flow fractionation (FFF) and then measure the concentrations with ICP-MS.

Quantification of proteins and biomolecules
There is an increasing trend of using ICP-MS as a tool in speciation analysis, which normally involves a front end chromatograph separation and an elemental selective detector, such as AAS and ICP-MS. For example, ICP-MS may be combined with size exclusion chromatography and preparative native PAGE for identifying and quantifying metalloproteins in biofluids. Also the phosphorylation status of proteins can be analyzed.

In 2007, a new type of protein tagging reagents called metal-coded affinity tags (MeCAT) were introduced to label proteins quantitatively with metals, especially lanthanides. The MeCAT labelling allows relative and absolute quantification of all kind of proteins or other biomolecules like peptides. MeCAT comprises a site-specific biomolecule tagging group with at least a strong chelate group which binds metals. The MeCAT labelled proteins can be accurately quantified by ICP-MS down to low attomol amount of analyte which is at least 2–3 orders of magnitude more sensitive than other mass spectrometry based quantification methods. By introducing several MeCAT labels to a biomolecule and further optimization of LC-ICP-MS detection limits in the zeptomol range are within the realm of possibility. By using different lanthanides MeCAT multiplexing can be used for pharmacokinetics of proteins and peptides or the analysis of the differential expression of proteins (proteomics) e.g. in biological fluids. Breakable PAGE SDS-PAGE (DPAGE, dissolvable PAGE), two-dimensional gel electrophoresis or chromatography is used for separation of MeCAT labelled proteins. Flow-injection ICP-MS analysis of protein bands or spots from DPAGE SDS-PAGE gels can be easily performed by dissolving the DPAGE gel after electrophoresis and staining of the gel. MeCAT labelled proteins are identified and relatively quantified on peptide level by MALDI-MS or ESI-MS.

Elemental analysis
The ICP-MS allows determination of elements with atomic mass ranges 7 to 250 (Li to U), and sometimes higher. Some masses are prohibited such as 40 due to the abundance of argon in the sample. Other blocked regions may include mass 80 (due to the argon dimer), and mass 56 (due to ArO), the latter of which greatly hinders Fe analysis unless the instrumentation is fitted with a reaction chamber. Such interferences can be reduced by using a high resolution ICP-MS (HR-ICP-MS) which uses two or more slits to constrict the beam and distinguish between nearby peaks. This comes at the cost of sensitivity. For example, distinguishing iron from argon requires a resolving power of about 10,000, which may reduce the iron sensitivity by around 99%. Another solution for some interfering species is the use of a collision chamber, as explained in collision reaction cell article.

A single collector ICP-MS may use a multiplier in pulse counting mode to amplify very low signals, an attenuation grid or a multiplier in analogue mode to detect medium signals, and a Faraday cup/bucket to detect larger signals. A multi-collector ICP-MS may have more than one of any of these, normally Faraday buckets which are much less expensive. With this combination, a dynamic range of 12 orders of magnitude, from 1 ppq to 100 ppm is possible.

ICP-MS is a method of choice for the determination of cadmium in biological samples.

Unlike atomic absorption spectroscopy, which can only measure a single element at a time, ICP-MS has the capability to scan for all elements simultaneously. This allows rapid sample processing. A simultaneous ICP-MS that can record the entire analytical spectrum from lithium to uranium in every analysis won the Silver Award at the 2010 Pittcon Editors' Awards. An ICP-MS may use multiple scan modes, each one striking a different balance between speed and precision. Using the magnet alone to scan is slow, due to hysteresis, but is precise. Electrostatic plates can be used in addition to the magnet to increase the speed, and this, combined with multiple collectors, can allow a scan of every element from Lithium 6 to Uranium Oxide 256 in less than a quarter of a second. For low detection limits, interfering species and high precision, the counting time can increase substantially. The rapid scanning, large dynamic range and large mass range is ideally suited to measuring multiple unknown concentrations and isotope ratios in samples that have had minimal preparation (an advantage over TIMS), for example seawater, urine, and digested whole rock samples. It also lends well to laser ablated rock samples, where the scanning rate is so quick that a real time plot of any number of isotopes is possible. This also allows easy spatial mapping of mineral grains.

Hardware
In terms of input and output, ICP-MS instrument consumes prepared sample material and translates it into mass-spectral data. Actual analytical procedure takes some time; after that time the instrument can be switched to work on the next sample. Series of such sample measurements requires the instrument to have plasma ignited, meanwhile a number of technical parameters has to be stable in order for the results obtained to have feasibly accurate and precise interpretation. Maintaining the plasma requires a constant supply of carrier gas (usually, pure argon) and increased power consumption of the instrument. When these additional running costs are not considered justified, plasma and most of auxiliary systems can be turned off. In such standby mode only pumps are working to keep proper vacuum in mass-spectrometer.

The constituents of ICP-MS instrument are designed to allow for reproducible and/or stable operation.

Sample introduction
The first step in analysis is the introduction of the sample. This has been achieved in ICP-MS through a variety of means.

The most common method is the use of analytical nebulizers. Nebulizer converts liquids into an aerosol, and that aerosol can then be swept into the plasma to create the ions. Nebulizers work best with simple liquid samples (i.e. solutions). However, there have been instances of their use with more complex materials like a slurry. Many varieties of nebulizers have been coupled to ICP-MS, including pneumatic, cross-flow, Babington, ultrasonic, and desolvating types. The aerosol generated is often treated to limit it to only smallest droplets, commonly by means of a Peltier cooled double pass or cyclonic spray chamber. Use of autosamplers makes this easier and faster, especially for routine work and large numbers of samples. A Desolvating Nebuliser (DSN) may also be used; this uses a long heated capillary, coated with a fluoropolymer membrane, to remove most of the solvent and reduce the load on the plasma. Matrix removal introduction systems are sometimes used for samples, such as seawater, where the species of interest are at trace levels, and are surrounded by much more abundant contaminants.

Laser ablation is another method. While being less common in the past, is rapidly becoming popular has been used as a means of sample introduction, thanks to increased ICP-MS scanning speeds. In this method, a pulsed UV laser is focused on the sample and creates a plume of ablated material which can be swept into the plasma. This allows geochemists to spacially map the isotope composition in cross-sections of rock samples, a tool which is lost if the rock is digested and introduced as a liquid sample. Lasers for this task are built to have highly controllable power outputs and uniform radial power distributions, to produce craters which are flat bottomed and of a chosen diameter and depth.

For both Laser Ablation and Desolvating Nebulisers, a small flow of Nitrogen may also be introduced into the Argon flow. Nitrogen exists as a dimer, so has more vibrational modes and is more efficient at receiving energy from the RF coil around the torch.

Other methods of sample introduction are also utilized. Electrothermal vaporization (ETV) and in torch vaporization (ITV) use hot surfaces (graphite or metal, generally) to vaporize samples for introduction. These can use very small amounts of liquids, solids, or slurries. Other methods like vapor generation are also known.

Plasma torch

The plasma used in an ICP-MS is made by partially ionizing argon gas (Ar → Ar+ + e−). The energy required for this reaction is obtained by pulsing an alternating electric current in load coil that surrounds the plasma torch with a flow of argon gas.

After the sample is injected, the plasma's extreme temperature causes the sample to separate into individual atoms (atomization). Next, the plasma ionizes these atoms (M → M+ + e−) so that they can be detected by the mass spectrometer.

An inductively coupled plasma (ICP) for spectrometry is sustained in a torch that consists of three concentric tubes, usually made of quartz. The two major designs are the Fassel and Greenfield torches. The end of this torch is placed inside an induction coil supplied with a radio-frequency electric current. A flow of argon gas (usually 14 to 18 liters per minute) is introduced between the two outermost tubes of the torch and an electrical spark is applied for a short time to introduce free electrons into the gas stream. These electrons interact with the radio-frequency magnetic field of the induction coil and are accelerated first in one direction, then the other, as the field changes at high frequency (usually 27.12 MHz or 40 MHz). The accelerated electrons collide with argon atoms, and sometimes a collision causes an argon atom to part with one of its electrons. The released electron is in turn accelerated by the rapidly changing magnetic field. The process continues until the rate of release of new electrons in collisions is balanced by the rate of recombination of electrons with argon ions (atoms that have lost an electron). This produces a ‘fireball’ that consists mostly of argon atoms with a rather small fraction of free electrons and argon ions.

Advantage of argon
Making the plasma from argon, instead of other gases, has several advantages. First, argon is abundant (in the atmosphere, as a result of the radioactive decay of potassium) and therefore cheaper than other noble gases. Argon also has a higher first ionization potential than all other elements except He, F, and Ne. Because of this high ionization energy, the reaction (Ar+ + e− → Ar) is more energetically favorable than the reaction (M+ + e− → M). This ensures that the sample remains ionized (as M+) so that the mass spectrometer can detect it.

Argon can be purchased for use with the ICP-MS in either a refrigerated liquid or a gas form. However it is important to note that whichever form of argon purchased, it should have a guaranteed purity of 99.9% Argon at a minimum. It is important to determine which type of argon will be best suited for the specific situation. Liquid argon is typically cheaper and can be stored in a greater quantity as opposed to the gas form, which is more expensive and takes up more tank space. If the instrument is in an environment where it gets infrequent use, then buying argon in the gas state will be most appropriate as it will be more than enough to suit smaller run times and gas in the cylinder will remain stable for longer periods of time, whereas liquid argon will suffer loss to the environment due to venting of the tank when stored over extended time frames. However, if the ICP-MS is to be used routinely and is on and running for eight or more hours each day for several days a week, then going with liquid argon will be the most suitable. If there are to be multiple ICP-MS instruments running for long periods of time, then it will most likely be beneficial for the laboratory to install a bulk or micro bulk argon tank which will be maintained by a gas supply company, thus eliminating the need to change out tanks frequently as well as minimizing loss of argon that is left over in each used tank as well as down time for tank changeover.

Helium can be used either in place of, or mixed with, argon for plasma generation. Helium's higher first ionisation energy allows greater ionisation and therefore higher sensitivity for hard-to-ionise elements. The use of pure helium also avoids argon-based interferences such as ArO. However, many of the interferences can be mitigated by use of a collision cell, and the greater cost of helium has prevented its use in commercial ICP-MS.

Transfer of ions into vacuum
The carrier gas is sent through the central channel and into the very hot plasma. The sample is then exposed to radio frequency which converts the gas into a plasma. The high temperature of the plasma is sufficient to cause a very large portion of the sample to form ions. This fraction of ionization can approach 100% for some elements (e.g. sodium), but this is dependent on the ionization potential. A fraction of the formed ions passes through a ~1 mm hole (sampler cone) and then a ~0.4 mm hole (skimmer cone). The purpose of which is to allow a vacuum that is required by the mass spectrometer.

The vacuum is created and maintained by a series of pumps. The first stage is usually based on a roughing pump, most commonly a standard rotary vane pump. This removes most of the gas and typically reaches a pressure of around 133 Pa. Later stages have their vacuum generated by more powerful vacuum systems, most often turbomolecular pumps. Older instruments may have used oil diffusion pumps for high vacuum regions.

Ion optics
Before mass separation, a beam of positive ions has to be extracted from the plasma and focused into the mass-analyzer. It is important to separate the ions from UV photons, energetic neutrals and from any solid particles that may have been carried into the instrument from the ICP. Traditionally, ICP-MS instruments have used transmitting ion lens arrangements for this purpose. Examples include the Einzel lens, the Barrel lens, Agilent's Omega Lens and Perkin-Elmer's Shadow Stop. Another approach is to use ion guides (quadrupoles, hexapoles, or octopoles) to guide the ions into mass analyzer along a path away from the trajectory of photons or neutral particles. Yet another approach is Varian patented used by Analytik Jena ICP-MS 90 degrees reflecting parabolic "Ion Mirror" optics, which are claimed to provide more efficient ion transport into the mass-analyzer, resulting in better sensitivity and reduced background. Analytik Jena ICP-MS PQMS is the most sensitive instrument on the market.

A sector ICP-MS will commonly have four sections: an extraction acceleration region, steering lenses, an electrostatic sector and a magnetic sector. The first region takes ions from the plasma and accelerates them using a high voltage. The second uses may use a combination of parallel plates, rings, quadrupoles, hexapoles and octopoles to steer, shape and focus the beam so that the resulting peaks are symmetrical, flat topped and have high transmission. The electrostatic sector may be before or after the magnetic sector depending on the particular instrument, and reduces the spread in kinetic energy caused by the plasma. This spread is particularly large for ICP-MS, being larger than Glow Discharge and much larger than TIMS. The geometry of the instrument is chosen so that the instrument the combined focal point of the electrostatic and magnetic sectors is at the collector, known as Double Focusing (or Double Focussing).

If the mass of interest has a low sensitivity and is just below a much larger peak, the low mass tail from this larger peak can intrude onto the mass of interest. A Retardation Filter might be used to reduce this tail. This sits near the collector, and applies a voltage equal but opposite to the accelerating voltage; any ions that have lost energy while flying around the instrument will be decelerated to rest by the filter.

Collision reaction cell and iCRC
The collision/reaction cell is used to remove interfering ions through ion/neutral reactions. Collision/reaction cells are known under several names. The dynamic reaction cell is located before the quadrupole in the ICP-MS device. The chamber has a quadrupole and can be filled with reaction (or collision) gases (ammonia, methane, oxygen or hydrogen), with one gas type at a time or a mixture of two of them, which reacts with the introduced sample, eliminating some of the interference.

The integrated Collisional Reaction Cell (iCRC) used by Analytik Jena ICP-MS is a mini-collision cell installed in front of the parabolic ion mirror optics that removes interfering ions by injecting a collisional gas (He), or a reactive gas (H2), or a mixture of the two, directly into the plasma as it flows through the skimmer cone and/or the sampler cone. The iCRC removed interfering ions using a collisional kinetic energy discrimination (KED) phenomenon and chemical reactions with interfering ions similarly to traditionally used larger collision cells.

Routine maintenance
As with any piece of instrumentation or equipment, there are many aspects of maintenance that need to be encompassed by daily, weekly and annual procedures. The frequency of maintenance is typically determined by the sample volume and cumulative run time that the instrument is subjected to.

One of the first things that should be carried out before the calibration of the ICP-MS is a sensitivity check and optimization. This ensures that the operator is aware of any possible issues with the instrument and if so, may address them before beginning a calibration. Typical indicators of sensitivity are Rhodium levels, Cerium/Oxide ratios and DI water blanks. One common standard practice is to measure a standard tuning solution provided by the ICP manufacturer every time the plasma torch is started. Then the instrument is auto-calibrated for optimum sensitivity and the operator obtains a report providing certain parameters such as sensitivity, mass resolution and estimated amount of oxidized species and double-positive charged species.

One of the most frequent forms of routine maintenance is replacing sample and waste tubing on the peristaltic pump, as these tubes can get worn fairly quickly resulting in holes and clogs in the sample line, resulting in skewed results. Other parts that will need regular cleaning and/or replacing are sample tips, nebulizer tips, sample cones, skimmer cones, injector tubes, torches and lenses. It may also be necessary to change the oil in the interface roughing pump as well as the vacuum backing pump, depending on the workload put on the instrument.

Sample preparation
For most clinical methods using ICP-MS, there is a relatively simple and quick sample prep process. The main component to the sample is an internal standard, which also serves as the diluent. This internal standard consists primarily of deionized water, with nitric or hydrochloric acid, and Indium and/or Gallium. The addition of volatile acids allows for the sample to decompose into its gaseous components in the plasma which minimizes the ability for concentrated salts and solvent loads to clog the cones and contaminate the instrument. Depending on the sample type, usually 5 mL of the internal standard is added to a test tube along with 10–500 microliters of sample. This mixture is then vortexed for several seconds or until mixed well and then loaded onto the autosampler tray. For other applications that may involve very viscous samples or samples that have particulate matter, a process known as sample digestion may have to be carried out, before it can be pipetted and analyzed. This adds an extra first step to the above process, and therefore makes the sample prep more lengthy.


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Die Walküre

Die Walküre (The Valkyrie), WWV 86B, is the second of the four epic music dramas that constitute Richard Wagner's Der Ring des Nibelungen (English: The Ring of the Nibelung). It was performed, as a single opera, at the National Theatre Munich on 26 June 1870, and received its first performance as part of the Ring cycle at the Bayreuth Festspielhaus on 14 August 1876.

As the Ring cycle was conceived by Wagner in reverse order of performance, Die Walküre was the third of the four texts to be written, although Wagner composed the music in performance sequence. The text was completed by July 1852, and the music by March 1856.

Wagner largely followed the principles related to the form of musical drama, which he had set out in his 1851 essay Opera and Drama under which the music would interpret the text emotionally, reflecting the feelings and moods behind the work, using a system of recurring leitmotifs to represent people, ideas, and situations rather than the conventional operatic units of arias, ensembles, and choruses. Wagner showed flexibility in the application of these principles here, particularly in Act III, when the Valkyries engage in frequent ensemble singing.

As with Das Rheingold, Wagner wished to defer any performance of the new work until it could be shown in the context of the completed cycle, but the 1870 Munich premiere was arranged at the insistence of his patron, King Ludwig II of Bavaria. Die Walküre has achieved some popularity as a stand-alone work and continues to be performed independently from its role in the tetralogy.

The story of Die Walküre is based on the Norse mythology told in the Völsunga saga and the Poetic Edda. In this version, the Volsung twins Sieglinde and Siegmund, separated in childhood, meet and fall in love. This union angers the gods, who demand that Siegmund must die. Sieglinde and the couple's unborn child are saved by the defiant actions of Wotan's daughter, the title character, Valkyrie Brünnhilde, who as a result faces the gods' retribution.

Background and context
Wagner began work on what became his Ring project in October 1848, when he prepared a prose outline for Siegfried's Death, based on the legendary hero of Germanic myth. During the following months, he developed the outline into a full "poem" or libretto.

After his travels to Switzerland in May 1849, Wagner continued to expand his project, having decided that a single work would not suffice for his purposes. He would, therefore, create a series of music dramas, each telling a stage of the story, basing the narrative on a combination of myth and imagination; Siegfried's Death would provide the culmination. In 1851, he outlined his purposes in his essay "A Communication to My Friends": "I propose to produce my myth in three complete dramas, preceded by a lengthy Prelude (Vorspiel)". Each of these dramas would, he said, constitute an independent whole, but would not be performed separately. "At a specially appointed Festival, I propose, some future time, to produce those three Dramas with their Prelude, in the course of three days and a fore-evening".

In accordance with this scheme, Wagner preceded Siegfried's Death (later Götterdämmerung (The Twilight of the Gods) with the story of Siegfried's youth, Young Siegfried, later renamed Siegfried. This was, in turn, preceded by Die Walküre (The Valkyrie), dealing with Siegfried's origins, the whole tetralogy being fronted by a prologue, Das Rheingold. Because Wagner prepared his texts in reverse chronological sequence, Die Walküre was the third of the dramas to be conceived and written, but appears second in the tetralogy.

Prior history
During the lengthy time that has passed since the gods entered Valhalla at the end of Das Rheingold, Fafner has used the Tarnhelm to assume the form of a dragon, and guards the gold and the ring in the depths of the forest. Wotan has visited Erda seeking wisdom, and by her has fathered a daughter, Brünnhilde; he has fathered eight other daughters, possibly also by Erda. These, with Brünnhilde, are the Valkyries, whose task is to recover heroes fallen in battle and bring them to Valhalla, where they will protect the fortress from Alberich's assault should the dwarf recover the ring. Wotan has also wandered the earth, and with a woman of the Völsung race has fathered the twins Siegmund and Sieglinde, who have grown up separately and unaware of each other. From the Völsungs Wotan hopes for a hero who, unencumbered by the gods' treaties, will obtain the ring from Fafner.

Act 1

Prelude to Act I

Scene 1

As a large storm rages, Siegmund finds shelter from his enemies in a large dwelling built around a massive ash tree. Unarmed and wounded, he collapses with exhaustion. Sieglinde enters; she gives Siegmund some water and some honeyed mead, and tells him that she is the wife of Hunding, and that he may rest there until Hunding's return. As they talk, they look at each other with growing interest and emotion. Siegmund gets ready to leave, telling Sieglinde that misfortune follows him, and he does not want to bring it on her; she replies that misfortune dwells with her already.

Orchestral Interlude

Scene 2

Hunding returns and questions Siegmund's presence. Calling himself Wehwalt ("woeful"), Siegmund explains that he grew up in the forest with his parents and twin sister. One day he found their home burned down, his mother killed and his sister gone. Recently he fought with the relatives of a girl being forced into marriage. His weapons were destroyed, the bride was killed, and he was forced to flee. Hunding reveals that he is kin to one of Siegmund's pursuers; Siegmund may stay, he says, but they must fight in the morning. Before leaving, Sieglinde gives a meaningful glance to a particular spot on the tree in which, the firelight reveals, a sword is buried to the hilt.

Scene 3

Sieglinde returns, having drugged Hunding's drink. She reveals that she was forced into the marriage and that during their wedding feast, an old man appeared and plunged a sword into the trunk of the ash tree which neither Hunding nor any of his companions have been able to remove. She is longing for the hero who will draw the sword and save her. When Siegmund expresses his love for her, she reciprocates, and when he speaks the name of his father, Wälse, she recognises him as Siegmund, and realises that the sword was left for him. Siegmund then draws the sword from the tree. She reveals herself as Sieglinde, his twin sister. Siegmund names the sword "Nothung" and declares that it will be her protection. The two sing of their passionate love for each other, as the act ends.

Act 2

Prelude to Act 2

Scene 1

On a high mountain ridge, Wotan instructs Brünnhilde, his Valkyrie daughter, to protect Siegmund in his forthcoming battle with Hunding. Fricka arrives, and in her role as goddess of family values demands that Siegmund and Sieglinde be punished for their adultery and incest. She scorns Wotan's argument that he requires Siegmund as a "free hero", who can further his plans to recover the ring from Fafner, uninhibited by Wotan's contracts. She retorts that Siegmund is not free but is Wotan's pawn, whose every move the god seeks to direct. Defeated by Fricka's argument, Wotan reluctantly agrees that he will not protect Siegmund.

Scene 2

After Fricka leaves, the troubled Wotan gives Brünnhilde the full story, and with great sorrow rescinds his earlier instruction; he orders her to give the victory to Hunding and then departs.

Orchestral Interlude

Scene 3

Siegmund and Sieglinde are running away from Hunding but are forced to stop. With a vision of Hunding's dogs scratching Siegmund's face Sieglinde faints, consumed with guilt and exhaustion.

Orchestral Interlude

Scene 4

Brünnhilde suddenly appears to Siegmund and tells him of his impending death; he refuses to follow Brünnhilde to Valhalla when she tells him Sieglinde cannot accompany him. Siegmund still believes that his father's sword will assure him of victory over Hunding, but Brünnhilde tells him it has lost its power. Siegmund threatens to kill both Sieglinde and himself. Much moved, Brünnhilde decides to defy her father and grant victory to Siegmund.

Scene 5

Hunding's horn is heard; he arrives and attacks Siegmund. Under Brünnhilde's power, Siegmund begins to overpower Hunding, but Wotan appears and shatters Siegmund's sword with his spear. Hunding stabs Siegmund to death. Brünnhilde gathers up the fragments of the sword and flees on horseback with Sieglinde. Contemptuously, Wotan kills Hunding with a simple wave of hand, and sets out in pursuit of Brünnhilde, vowing to punish her harshly for her disobedience.

Act 3

Prelude to Act 3 - Walkürenritt (The Ride of the Valkyries)

Scene 1

The Valkyries congregate on the mountain-top, each carrying a dead hero and chattering excitedly. Brünnhilde arrives with Sieglinde, and begs her sisters for help, but they dare not defy Wotan. Sieglinde tells Brünnhilde that without Siegmund she no longer wishes to live. Brünnhilde tells Sieglinde that she is pregnant by Siegmund, and urges her to remain alive for her child's sake, and to name the child Siegfried. Brünnhilde gives the fragments of the sword Nothung to Sieglinde, who thanks her for her loyalty and comfort, and resolves to save the child. As she departs, Wotan is heard approaching with great wrath.

Scene 2

When Wotan arrives, the Valkyries vainly try to hide Brünnhilde. He faces her and declares her punishment: she is to be stripped of her Valkyrie status and become a mortal woman, to be held in defenceless sleep on the mountain, prey to any man who finds her. The other Valkyries protest, but when Wotan threatens them with the same, they flee.

Orchestral Interlude

Scene 3

In a long discourse with Wotan, Brünnhilde explains that she decided to protect Sieglinde knowing that this was Wotan's true desire. Wotan consents to her request that he surround her resting place with a circle of fire that will protect her from all but the bravest of heroes. He bids her a loving farewell and lays her sleeping form down on a rock. He then invokes Loge, the demigod of fire, and creates a circle of perpetual fire around her. Before slowly departing, Wotan pronounces that anyone who fears his spear shall never pass through the fire.

Text, sources, characters
Wagner's original title for the work was Siegfried und Sieglind: der Walküre Bestrafung ("Siegfried and Sieglinde: The Valkyrie Punished"), but he quickly simplified this to Die Walküre. Prose sketches for the first two acts were prepared in November 1851, and for the third act early in the following year. These sketches were expanded to a more detailed prose plan in May 1852, and the full libretto was written in June 1852. It was privately printed, with the other Ring libretti, in February 1853. Wagner constructed his Die Walküre libretto from a range of ancient Norse and Germanic sources, principally the Völsunga saga, the Poetic Edda, the Prose Edda,  the Nibelungenlied and other fragments of Teutonic literature. From this plethora of material he selected particular elements and transformed them, to create his own narrative through the compression of events, the rearrangement of chronology and the fusion of characters. For example, in the Völsunga saga Siegmund is not Wotan's son, although he arranges the latter's conception by a Völsung woman. Sigurd (Siegfried) is not the child of Siegmund's incestuous marriage to his sister, but of a later wife who preserves the sword fragments. Likewise, in the sagas Sieglinde is a somewhat different character, Signy; she is Siegmund's twin sister, but the son she bears him is not Siegfried, and the manner of her death is quite different from that depicted by Wagner. Hunding is a conflation of several characters in the sagas, notably Siggeir who is wedded to Signy, and the villainous King Hunding who is Siegmund's mortal enemy in the Poetic Edda.

Wotan (Odin) appears in the northern sagas as the god of all life as well as of battles, although he is by no means omnipotent. Fricka (Frigg) has most of the hallmarks of her counterpart in the Poetic and Prose Eddas, as wife of Wotan and goddess of family values. Brünnhilde is a less central figure in the sagas than she is in the Ring cycle. In an early lay, she is sought as a wife by Gunther, who seeks the help of Siegfried in overcoming her superhuman strength. Certain aspects of her Ring character appear in the Eddas and the Nibelungenlied, such as her encirclement by Wotan in a ring of fire, and her rescue by a hero without fear.

The Valkyries have a basis in historical fact, within the primitive Teutonic war-cult. According to Cooke, originally they were "grisly old women who officiated at the sacrificial rites when prisoners were put to death." They became entwined in legend: in the Poetic Edda they emerge as supernatural warrior maidens carrying out Odin's orders as to who should die. In the Poetic Edda the Valkyries are given names: Skuld, Skogul, Gunn, Hild, Gondul and Geirskogul. Some of these names differ in other sources. The names that Wagner gave to his Valkyries were his own invention, apart from Brünnhilde and Siegrune.

Wagner effected a number of changes between his original draft and the final text. For example, in the first sketch, Wotan appeared in person in Act I to drive the sword into the tree. Siegmund withdrew the sword much earlier in the act, and in Act II Hunding was not slain by Wotan, but left alive to follow Wotan's instruction: "Get hence, slave! Bow before Fricka."

Composition
Apart from some rough sketches, including an early version of what became Siegmund's "Spring Song" in Act I of Die Walküre, Wagner composed the Ring music in its proper sequence. Having completed the music for Das Rheingold in May 1854, he began composing Die Walküre in June, and finished the full orchestral score nearly two years later, in March 1856. This extended period is explained by several concurrent events and distractions, including Wagner's burgeoning friendship with Mathilde Wesendonck, and a lengthy concert tour in London at the invitation of the Royal Philharmonic Society, when he conducted a full season amid some controversy, although his own Tannhäuser overture was well received.

The system of leitmotifs, integral to the Opera and Drama principles, is used to the full in Die Walküre; Holman numbers 36 such motifs that are introduced in the work. The well-known "Valkyrie" motif, used to introduce Brünnhilde in Act II, forms the basis of the famous Ride of the Valkyries that opens Act III. Wagner wrote the concert version of the Ride in 1862, for performance in concerts at Vienna and Leipzig.

Premiere, Munich, 26 June 1870
As with Das Rheingold, many years elapsed between the completion of the Die Walküre score and its first performance. Seeing little chance of the Ring project coming to any immediate fruition, and in need of money, in August 1857 Wagner abandoned work on it and concentrated instead on Tristan und Isolde and Die Meistersinger von Nürnberg, and on a revised version of Tannhäuser. However, King Ludwig of Bavaria, to whom Wagner had sold the copyrights to the Ring works, was insistent that the two completed Ring operas be staged, and over Wagner's bitter protests arranged for them to be performed at the Munich Hofoper, Das Rheingold on 22 September 1869 and Die Walküre on 26 June 1870. The core of Wagner's objection to a Munich performance was his relationship with the married Cosima von Bülow, with whom he was cohabiting in Switzerland; he could not return to Munich without provoking scandal, and therefore could not directly control the performances.

As the date for the Die Walküre premiere approached, Wagner grew more distressed and sullen; a letter from the critic Franz Müller reporting that everything was going well did nothing to console him. Cosima wrote in her diary that his distress "pierces my heart like a dagger, and I ask myself whether this disgraceful act will really go unavenged?". The premiere was attended by leading figures from the musical world, including Liszt, Brahms, Camille Saint-Saëns, and the violinist Joseph Joachim. The reception from audience and critics was much more positive than had been the case a year earlier with Rheingold, although Osborne quotes one dissenting voice, from the critic of the Süddeutsche Presse. Having described the first act as "for the most part, drearily long-winded", this critic thought that the second act dragged and came to life only occasionally. He went on: "The third act begins so deafeningly that total stupor would be ensured even if the rest were less long-winded ... The overall effect of the music is not agreeable...". Cosima kept all communications from Munich away from Wagner, and tore up the more critical newspaper reviews.

After a second performance, Die Walküre was performed at the Hofoper three further times, alternating with Das Rheingold in a Ring semi-cycle. King Ludwig, who was absent from the premiere, attended one of the later performances. The Munich festival had taken place amid a mounting war fever, as relations between France and the German states rapidly deteriorated; the Franco-Prussian War broke out on 19 July.

Bayreuth premiere, 14 August 1876
In May 1872, Wagner laid the foundation stone for the Festspielhaus in Bayreuth, Northern Bavaria. He had originally envisaged holding the first Bayreuth Festival there in 1873, but delays in the building work, and in completing the Ring music, led to multiple postponements. Finally, the festival was scheduled for August 1876; Die Walküre would be performed on 14 August, the second day of the festival. Wagner was involved in every stage of the preparations; according to Ernest Newman's biography he was "a far better conductor than any of his conductors, a far better actor than any of his actors, a far better singer than any of his singers in everything but tone". Heinrich Porges, a contemporary chronicler, describes Wagner demonstrating to Amalie Materna, as Brünnhilde, how to sing the scene in which she tells Sieglinde of the impending birth of Siegfried: "He sang [the final words] with truly thrilling force". The singer Lilli Lehmann (Helmwige), in her 1913 memoirs, remembered Wagner acting the role of Sieglinde in rehearsals: "Never yet has a Sieglinde known how to approach him, even approximately".

The Die Walküre performance on 14 August was free from the mechanical problems that had affected Das Rheingold the day before, and was generally well received by the distinguished audience that included the Kaiser Wilhelm I, Emperor Pedro II of Brazil, representatives from various European royal houses and many of  Europe's leading composers. Wagner, however, was far from pleased. He was unnerved by an incident involving the kaiser, when the 79-year-old Wilhelm stumbled and almost fell over a doorstep, and was very critical of two of his main singers, Niemann and Betz, whom he deemed "theatre parasites" and said he would never employ again – a view he later revised. Among the scenes that he felt had not come off were those on the mountain-top: "I'll change that some day when I produce Walküre in heaven, at the right hand of God, and the old gent and I are watching it together".

Three Ring cycles were performed in the first Bayreuth Festival. The stage designs used in Die Walküre, and the other operas, were based on sketches by Josef Hoffman which were converted to stage sets by the Brückner brothers from the Coburg State Opera. These designs, and Carl Döpler's costumes, influenced  productions well into the 20th century.

Revivals
After the 1876 festival, Die Walküre was not seen again at Bayreuth for 20 years, until Cosima revived the Ring cycle for the 1896 festival. It was quickly taken up by other opera houses: Vienna and New York in 1877, Rotterdam in 1878 and London in 1882. The New York performance, on 2 April 1877, was conducted by Adolf Neuendorff as part of a Wagner festival organised by the Academy of Music; it preceded the Metropolitan Opera premiere by nearly eight years.

The London performance, at Her Majesty's Theatre on 6 May 1882, was the first Ring cycle to be performed anywhere, after the 1876 Bayreuth premiere. It was conducted by Anton Seidl, who had been an assistant to Wagner at Bayreuth, and had Albert Niemann in his Bayreuth role as Siegmund. The Musical Times, in a long review, mixed approval with criticism: its reviewer noted a number of empty seats in the auditorium, thought the stage sets compared unfavourably with those at Bayreuth, and deemed the orchestra "inefficient". However, the critic praised individual performances, and judged that the music and the drama had held the attention of an audience whose enthusiasm far exceeded that shown to Das Rheingold the previous day. The correspondent for The Era newspaper was distressed by the incestuous nature of the story, which it described as "brutal and degrading", despite the quality of the music: "A composer must have lost all sense of decency and all respect for the dignity of human nature who could thus employ his genius and skill".

The New York Met performance, on 30 January 1885, was part of a Wagner festival conducted by Leopold Damrosch – no other Ring operas were staged. Amalie Materna, Bayreuth's original Brünnhilde, reprised the part here. The stage designer, Wilhelm Hock, recreated the original Bayreuth designs. The performance was received with great enthusiasm by the audience, who demanded numerous curtain calls. Damrosch fell ill just before the festival ended, and died on 15 February 1885.

During the 1880s and 1890s, Die Walküre was shown in many European cities, sometimes as part of a Ring cycle but often as an independent work: Brussels, Venice, Strasbourg and Budapest in 1883, Prague in 1885, St Petersburg in 1889, Copenhagen in 1891 and Stockholm in 1895. By then it was travelling worldwide: to Mexico in 1891, Argentina in 1899, Australia in 1907, South Africa in 1912 and Brazil in 1913. These productions often followed the Bayreuth line of staying close to the staging and costumes that Wagner had approved for the original festival, although there were some deviations, such as Adolphe Appia's productions which replaced  pictorialism with stylised sets using colours and lights.

At Bayreuth, no significant changes in presentation occurred until after the Second World War, when Wieland Wagner revealed his "New Bayreuth" style of largely bare stages and unadorned costumes. Although traditional productions continued outside Bayreuth, many performances followed the new trend. From the 1970s onwards there was increasing innovation; the Leipzig Die Walküre began the trend of placing the opening scene of Act II within Valhalla rather than on a mountain-top; Harry Kupfer at Bayreuth in 1988 set the whole cycle in a post-nuclear dystopia; Jürgen Flimm's Bayreuth 2000 production had Wotan as a corrupt businessman, sitting in an office surrounded by 21st-century paraphernalia, including a paper-shredder used to cover his tracks. A few prominent productions adhered to traditional staging, including Otto Schenk's at the New York Met in 1989, which was described by the New York Times as "charmingly old fashioned", and as "a relief to many beleaguered Wagnerites".

According to Charles Osborne, Die Walküre is sufficiently self-contained to be quite frequently staged on its own. The 2018 Bayreuth Festival broke new ground when for the first time it scheduled Die Walküre as a stand-alone, outside the context of the Ring cycle.

Music
Osborne writes that, like Das Rheingold, Die Walküre is primarily a work for solo voices, but with better integration of the vocal parts into the overall musical structure. As with its predecessor, Wagner composed Die Walküre under the principles he had defined in his book-length 1851 essay Opera and Drama, eschewing the traditional operatic norms of chorus, arias and vocal "numbers". There is, however, some division of opinion as the extent that these principles were fully observed. The critic Barry Millington opines that of all Wagner's works, Die Walküre is the fullest embodiment of the Opera and Drama precepts, achieving a complete synthesis of music and poetry. This, he says, is achieved without any notable sacrifice in musical expression". In his analytical essay The Perfect Wagnerite, Bernard Shaw praises the synthesis of music and drama: "There is not ... a note in it that has any other point than the single direct point of giving musical expression to the drama". Gutman's view, however, is that this only applies to the first two acts; the "apogee" of this style, he says, is found in the later opera Tristan and Isolde. Roger Scruton refers to deviations in Die Walküre such as the "Spring Song" (Winterstürme), in which Siegmund holds up the action to declare his love for Sieglinde in what is to all intents and purposes an aria, while Osborne notes the "impressive ensembles" in Act III, as the Valkyries sing together.

Act I
The act opens in the key of D minor, which frames the music until Siegmund's death in Act II. The short prelude depicts a storm; a stamping rhythm in the basses rises to a climax in which "Donner's Call" from Das Rheingold is heard. As the scene proceeds, several new motifs are introduced; that representing Siegmund, derived from the "Spear" motif from Das Rheingold; that for Sieglinde, a gentle melody on strings, which Holman says "conveys at once Sieglinde's inner beauty and misfortune"; and the motif which Newman names "The Dawning of Love", which will recur in the final love duet of the act. These three motifs, and their developed variants, are prominent throughout the act. Among other motifs, the aggressive brass staccato that identifies Hunding is a particularly striking phrase, "as dark and dour as the man himself".

Wagner uses other Rheingold motifs to deliver key information. Thus, the parentage of Siegmund and Sieglinde is revealed to the audience as Wotan, when the Valhalla music plays softly on trombones. The same theme references Wotan again, when Sieglinde recounts the visit of the old man at her wedding. A repeated falling octave in G♭, extracted from the "Sword" motif, illustrates Siegmund's desperate desire for the sword, and recurs at the end of the act, together with the full "Sword" motif in triumphant brass, as he draws the sword from the tree.

Act II
The second act opens exuberantly, with a short prelude that prefigures the celebrated Valkyrie motif that in the following act will form the basis of the "Ride of the Valkyries" in Act III. This motif was first sketched in 1851, for intended use in Siegfried's Tot, before the full plan of the Ring cycle was developed. The first scene of the act introduces Brünnhilde's energetic "Hojotoho!", as she answers Wotan's summons, expressing what Holman describes as her "manly enthusiasm" for her role as warrior maiden. The Wotan-Fricka dialogue that follows is illustrated by motifs that express Fricka's sour disillusion with her marriage, and Wotan's bitterness and frustration as he is unable to answer his wife's forceful arguments.

In the colloquies between Wotan and Brünnhilde, several soundings of the "Woman's Worth" motif are heard. The "Annunciation of Death" motif is the crucial point, where the two narratives (Wotan/Brünnhilde and Siegmund/Sieglinde) come together. Wagner chose the tonality of F♯ minor for this scene, eventually modulating to B minor in preparation for the Valkyries' entry in Act III.

Act III
The act opens with the famous sequence known as the "Ride of the Valkyries", formed by combining the Valkyries' own belligerent theme with Brünnhilde's war cry from act II. The Ride has achieved lasting popularity as an orchestral concert piece outside the framework of the Ring cycle; according to Newman, in the orchestral version and sometimes within the opera itself, the basis staccato phrasing is corrupted by eliding the second and third notes and emphasising the fourth rather than the first as originally written.

At the midpoint of the act, prior to Wotan's vengeful entrance and the long denouement with Brünnhilde, we hear the "Reconciliation" motif ("Redemption by Love" per Newman) in which Sieglinde praises Brünnhilde for her rescue: "O highest of wonders! Noblest of maids!". The motif will next be heard at the very end of the Ring cycle, bringing the entire tetralogy to a close on a note of reconciliation and optimism.

The final section of the act is marked by what Millington describes as "a succession of carefully controlled climaxes", of which the most affecting is that of Wotan's farewell to his errant daughter. The music is eventually dominated by the five falling notes of Brünnhilde's "Sleep" motif which, when Wotan has used his spear to summon Loge, is transformed into the "Magic Fire" music as Brünnhilde is encircled in the ring of fire, and Wotan sadly departs.

Orchestral forces
Die Walküre is scored for the following instrumental forces:
 * Woodwind: piccolo; 3 flutes with 3rd doubling 2nd piccolo; 3 oboes; cor anglais doubling 4th oboe; 3 clarinets; bass clarinet; 3 bassoons
 * Brass: 8 horns (5-8 doubling Wagner Tubas in Bb and F); 3 trumpets; bass trumpet; 2 tenor trombones; bass trombone; contrabass trombone (doubling bass trombone); contrabass tuba
 * Percussion: 2 sets of timpani; cymbals; triangle; gong; tenor drum; glockenspiel
 * Strings: 16 first violins; 16 second violins; 12 violas; 12 cellos; 8 double basses; 6 harps
 * Off-stage: cow horn, thunder machine

Critical assessment
The first Munich performances of Die Walküre were generally hailed as successes by audiences and critics; leading composers who were present greeted the work with acclaim, recognising in it evidence of Wagner's genius. One dissident voice was presented by the critic of the Süddeutsche Presse, who was scathing about the dearth of moral standards expressed in the story and furthermore found the whole experience tedious: the first act was, for the most part, "wearyingly long-winded"; the second act only occasionally sprang to life, while in the third it was "barely possible to hear isolated shrieks from the singers through the tumult of the orchestra". The overall effect was "not agreeable ... permeated with what one can only call pagan sensuality, and ... produces finally nothing but an enervating dullness". This harsh, if isolated judgement, found some echo six years later, when Die Walküre was first performed at Bayreuth as part of the Ring cycle. Critics could now form relative views on the merits of the four operas. Although there was general admiration for the first act, Die Walküre emerged as the least-liked of the four, in particular on account of the second act, deemed "a great failure" and an "abyss of boredom".

Many modern critics of Die Walküre have recorded much more positive opinions. To Charles Osborne it is "marvellously rich ... Wagner has found a way to integrate his voice parts into the overall structure without sacrificing their lyrical independence". It is, he says, the opera that stands up most strongly outside the tetralogy, and is popular enough to be staged frequently on its own, even within Bayreuth festivals.

Writing in 2006, Millington thought that, notwithstanding the liberal use of ensembles in the third act, Die Walküre showed the greatest fidelity of the four operas to the theoretical principles expressed by Wagner in Opera and Drama: "A thoroughgoing synthesis of poetry and music is achieved without any notable sacrifice in musical expression". The modern view is that, of the Ring operas, Die Walküre is both the most approachable and the one that can most successfully be performed in extracts.

title: Die Walküre


text:
Siegfried (opera)

Siegfried, WWV 86C, is the third of the four epic music dramas that constitute Der Ring des Nibelungen (The Ring of the Nibelung), by Richard Wagner. It premiered at the Bayreuth Festspielhaus on 16 August 1876, as part of the first complete performance of The Ring cycle.

The autograph manuscript of the work is preserved in the Richard Wagner Foundation.

Background and context
The libretto of Siegfried was drafted by Wagner in November–December 1852, based on an earlier version he had prepared in May–June 1851 and originally entitled Jung-Siegfried (Young Siegfried), later changed to Der junge Siegfried. The musical composition was commenced in 1856, but not finally completed until 1871.

The libretto arose from Wagner's gradual reconception of the project he had initiated with his libretto Siegfrieds Tod (Siegfried's Death) which was eventually to be incarnated as Götterdämmerung, the final section of the Ring cycle. Having sketched music and worked with the text for Siegfrieds Tod in 1851, he realized that it would need a 'preface'. At this point he conceived that the prefatory opera, Der junge Siegfried, could act as a comic foil to the tragedy of Siegfrieds Tod. Preliminary musical sketches for Der junge Siegfried in 1851 were however quickly abandoned, although Wagner had written to his friend Theodor Uhlig that "the musical phrases are making themselves for these stanzas and periods, without my even having to take pains for them. It's all growing out of the ground as if it were wild." Shortly afterwards he wrote to Uhlig that he was now planning to tell the Siegfried story in the form of "three dramas, plus a prologue in three acts"—a clear prefiguring of the Ring cycle.

Full work was finally commenced on the music of Siegfried, as the composer henceforth referred to it, in 1856, when Wagner prepared concurrently two drafts, a complete draft in pencil and a version in ink on up to three staves in which he worked out details of instrumentation and vocal line. The composition of Acts I and II was completed by August 1857. Wagner then left off work on Siegfried to write the operas Tristan und Isolde and Die Meistersinger. He did not resume work on Siegfried until 1869, when he composed the third act. The final revision of the score was undertaken in February 1871. Performance was withheld until the first complete production of the Ring cycle, at Bayreuth in August 1876.

Act 1
Prelude to Act 1

Scene 1

A cave in rocks in the forest. An orchestral introduction includes references to leitmotifs including themes relating to the original hoard plundered by the Nibelung Alberich, and one in B-flat minor associated with the Nibelungs themselves. As the curtain rises, Alberich's brother, the dwarf Mime, is forging a sword ("Zwangvolle Plage!"). Mime is plotting to obtain the ring of power originally created by his brother Alberich. He has raised the human boy Siegfried as a foster child, to kill Fafner, who obtained the ring and other treasures in the opera Das Rheingold and has since transformed himself from a giant to a dragon. Mime needs a sword for Siegfried to use, but the youth has contemptuously broken every sword Mime has made. Siegfried returns from his wanderings in the forest driving before him a large bear that terrifies Mime, and immediately breaks the new sword. After a whining speech by Mime about ingratitude, and how Mime has brought him up from a mewling infant ("Als zullendes Kind"), Siegfried senses why he keeps coming back to Mime although he despises him: he wants to know his parentage. Mime is forced to explain that he encountered Siegfried's mother, Sieglinde, when she was in labor; she died giving birth to Siegfried. He shows Siegfried the broken pieces of the sword Nothung, which she had left in his custody. Siegfried orders him to reforge the sword; however, Mime is unable to accomplish this. Siegfried departs, leaving Mime in despair ("Aus dem Wald fort in die Welt zieh’n").

Scene 2

An old man (Wotan in disguise) arrives at the door and introduces himself as the Wanderer ("Heil dir, weiser Schmied!"). In return for the hospitality due a guest, he wagers his head on answering any three questions Mime may ask. The dwarf asks the Wanderer to name the races that live beneath the ground, on the earth, and in the skies. These are the Nibelungs, the Giants, and the Gods, as the Wanderer answers correctly. When Mime still refuses hospitality, the Wanderer demands that Mime answer his three questions, or yield his own head. The Wanderer asks Mime to identify: the race most beloved of Wotan, but most harshly treated; the name of the sword that can destroy Fafner; and the person who can repair the sword. Mime can answer only the first two questions: the Wälsungs (Siegmund and Sieglinde whose tale is told in the opera Die Walküre) and the sword Nothung. Wotan tells him that only "he who does not know fear" can reforge Nothung, and abstains from taking Mime's head, leaving it for that person.

Scene 3

Mime despairs as he imagines the ferocity of the dragon Fafner, while "the orchestra paints a dazzling picture of flickering lights and roaring flames" ("Verfluchtes Licht!"). Siegfried returns and is annoyed by Mime's lack of progress. Mime realizes that Siegfried is "the one who does not know fear" and that unless he can instill fear in him, Siegfried will kill him as the Wanderer foretold. He tells Siegfried that fear is an essential craft; Siegfried is eager to learn it, and Mime promises to teach him by taking him to Fafner ("Fühltest du nie im finst’ren Wald"). Since Mime was unable to forge Nothung, Siegfried decides to do it himself ("Nothung! Nothung! Neidliches Schwert!"). He succeeds by shredding the metal, melting it, and casting it anew. In the meantime, Mime brews a poisoned drink to offer Siegfried after the youth has defeated the dragon. After he finishes forging the sword, Siegfried demonstrates its strength by chopping the anvil in half with it ("Hoho! Hoho! Hohei! Schmiede, mein Hammer, ein hartes Schwert!" - Siegfrieds Schmiedelied - Siegfried's Forging Song).

Act 2
Prelude to Act 2 - Fafners Ruhe (Fafner's Rest)

Scene 1

Deep in the forest. The Wanderer arrives at the entrance to Fafner's cave, near which Alberich secretly keeps vigil by a rocky cliff ("Im Wald und Nacht"). The two enemies recognize each other. Alberich boasts of his plans to regain the ring and rule the world. Wotan states that he does not intend to interfere, only to observe. He even offers to awaken the dragon so that Alberich can bargain with him. Alberich warns the dragon that a hero is coming to kill him, and offers to prevent the fight in exchange for the ring. Fafner dismisses the threat, declines Alberich's offer, and returns to sleep. Wotan mysteriously advises Alberich that all things follow their own necessary ways which no one will change. He then rides away on his horse, leaving Alberich alone. Alberich withdraws and hides himself again in the rocks.

Orchestral Interlude

Scene 2

At daybreak, Siegfried and Mime arrive. After assuring Siegfried that the dragon will teach him what fear is, Mime withdraws. As Siegfried waits for the dragon to appear, he hears a woodbird singing from the trees (Waldweben - Forest Murmurs). He attempts to mimic the bird's song using a reed pipe, but is unsuccessful. He then plays a tune on his horn (Siegfrieds Hornruf - Siegfried's Horn Call), which unintentionally wakes Fafner in his cave. After a short exchange, they fight; Siegfried stabs Fafner in the heart with Nothung. Regretful about his own life Fafner in his last moments learns the boy's name and tells Siegfried to beware the might of the curse, which condemns every lord of the Ring to death, just as it has now brought death to him. When Siegfried withdraws his sword from Fafner's body, his hands are burned by the dragon's hot blood and he puts his finger in his mouth. On tasting the blood, he finds that he can understand the woodbird's song ("Hei! Siegfried gehört nun der Niblungen Hort!"). Following its instructions, he takes the ring and the magic helmet Tarnhelm from Fafner's hoard.

Scene 3

Outside the cave, Alberich and Mime meet and quarrel over the treasure ("Wohin schleichst du eilig und schlau"). Alberich hides as Siegfried comes out of the cave. Siegfried contemplates the ring but doesn't know what could be its use, viewing it just innocently as a valueless object of nature ("Was ihr mir nützt, weiß ich nicht"); nevertheless, on the forest bird's advice he decides to keep it. Then he complains to Mime that not even the dragon Fafner has taught him the meaning of fear. Mime congratulates him on having won his battle, and offers him the poisoned drink; however, the magic power of the dragon's blood allows Siegfried to read Mime's treacherous thoughts, and he stabs him to death ("Willkommen, Siegfried!"). Hidden nearby, Alberich is heard laughing spitefully at his brother's death. Siegfried puts Mime's body into the treasure cave and places Fafner's body in the cave entrance to block it. The woodbird now sings of a woman sleeping on a rock surrounded by magic fire ("Nun sing! Ich lausche dem Gesang"). Siegfried, wondering if he can learn fear from this woman, follows the bird towards the rock.

Act 3
Prelude to Act 3

Scene 1

At the foot of Brünnhilde's rock. The Wanderer summons Erda, the earth goddess ("Wache, Wala!"). Erda, appearing confused, is unable to offer any advice ("Männerthaten umdämmern mir den Muth"). Wotan informs her that he no longer fears the end of the gods; indeed, he wills it, because he now recognizes that through his own demise the true heritage of his life will be left to the independent pair free from envy in their love, to Siegfried the Wälsung, who took Alberich's ring without succumbing to its corrupting influence thanks to his fearlessness, and Brünnhilde (Erda's and Wotan's child), who will work the deed that redeems the World ("Dir Unweisen ruf’ ich’s in’s Ohr"). Dismissed, Erda sinks back into the earth.

Scene 2

Siegfried arrives, and the Wanderer questions the youth ("Mein Vöglein schwebte mir fort"). Siegfried, who does not recognize his grandfather, answers insolently and starts down the path toward Brünnhilde's rock. The Wanderer blocks his path, but Siegfried mocks him, laughing at his floppy hat and his missing eye, and breaks his spear (the symbol and source of Wotan's authority and power) with a blow from Nothung. Wotan, accepting his fate, calmly gathers up the pieces and vanishes ("Zieh’ hin! Ich kann dich nicht halten!").

Orchestral Interlude

Scene 3

Thanks to his fearlessness Siegfried passes through the ring of fire, emerging on Brünnhilde's rock ("Selige Öde auf sonniger Höh’!"). At first, he thinks the sleeping armored figure is a man. However, when he removes the armor, he finds a woman beneath. At the sight of the first woman he has ever seen and struck with the feeling of love, Siegfried at last experiences fear. In desperation, he kisses Brünnhilde, waking her from her magic sleep. Upon waking she hails the Sun and greets Siegfried as the World's Light ("Heil dir, Sonne! Heil dir, Licht!"). Afraid and hesitant at first to do so, Brünnhilde is eventually won over by Siegfried's love and renounces through her love for him the world of the gods and with it her own powers ("Ewig war ich, ewig bin ich"). Together, they solemnly sing praises to love, in comparison with which the glory of the gods itself seems dimmed and in which even dying can be jubilant: "radiant love, laughing death!" ("Leuchtende Liebe, lachender Tod!")

title: Siegfried (opera)