text:
Earned value management

Earned value management (EVM), earned value project management, or earned value performance management (EVPM) is a project management technique for measuring project performance and progress in an objective manner.

Overview
Earned value management is a project management technique for measuring project performance and progress. It has the ability to combine measurements of the project management triangle: scope, time, and costs.

In a single integrated system, EVM is able to provide accurate forecasts of project performance problems, which is an important aspect of project management.

Early EVM research showed that the areas of planning and control are significantly impacted by its use; and similarly, using the methodology improves both scope definition as well as the analysis of overall project performance. More recent research studies have shown that the principles of EVM are positive predictors of project success. The popularity of EVM has grown in recent years beyond government contracting, a sector in which its importance continues to rise (e.g. recent new DFARS rules ), in part because EVM can also surface in and help substantiate contract disputes.

EVM features
Essential features of any EVM implementation include:
 * A project schedule that identifies work to be accomplished. Sometimes incorrectly called a Project Plan.
 * A valuation of planned work, called planned value (PV) or budgeted cost of work scheduled (BCWS)
 * Pre-defined "earning rules" (also called metrics) to quantify the accomplishment of work, called earned value (EV) or budgeted cost of work performed (BCWP)
 * Actual Cost which is also known as Actual Cost of Work Performed (ACWP)
 * A plot of project cumulative costs vs time especially to show both early date and late date curves

EVM implementations for large or complex projects include many more features, such as indicators and forecasts of cost performance (over budget or under budget) and schedule performance (behind schedule or ahead of schedule). Large projects usually need to use quantitative forecasts associated with earned value management. Although deliverables in these large projects can use adaptive development methods, the forecasting metrics found in earned value management are mostly used in projects using the predictive approach. However, the most basic requirement of an EVM system is that it quantifies progress using PV and EV.

Application example
Project A has been approved for a duration of one year and with a budget. It was also planned that the project spends 50% of the approved budget and expects 50% of the work to be complete in the first six months. If now, six months after the start of the project, a project manager reports that he has spent 50% of the budget, one may presume that the project is perfectly on plan. However, in reality the provided information is not sufficient to come to such a conclusion. The project can spend 50% of the budget, whilst finishing only 25% of the work, which would mean the project is not doing well; or the project can spend 50% of the budget, whilst completing 75% of the work, which would mean that project is doing better than planned. EVM is meant to address such and similar issues.

History
EVM emerged as a financial analysis specialty in United States government programs in the 1960s, with the government requiring contractors to implement an EVM system (EVMS). It has since become a significant branch of project management and cost engineering. Project management research investigating the contribution of EVM to project success suggests a moderately strong positive relationship. Implementations of EVM can be scaled to fit projects of all sizes and complexities.

The genesis of EVM occurred in industrial manufacturing at the turn of the 20th century, based largely on the principle of "earned time" popularized by Frank and Lillian Gilbreth.

In 1979, EVM was introduced to the architecture and engineering industry in a Public Works Magazine article by David Burstein, a project manager with a national engineering firm. In the late 1980s and early 1990s, EVM emerged more widely as a project management methodology to be understood and used by managers and executives, not just EVM specialists. Many industrialized nations also began to utilize EVM in their own procurement programs.

An overview of EVM was included in the Project Management Institute (PMI)'s first Project Management Body of Knowledge (PMBOK) Guide in 1987 and was expanded in subsequent editions. In the most recent edition of the PMBOK guide, EVM is listed among the general tools and techniques for processes to control project costs.

The construction industry was an early commercial adopter of EVM. Closer integration of EVM with the practice of project management accelerated in the 1990s. In 1999, the Performance Management Association merged with the PMI to become its first college, the College of Performance Management (CPM). The United States Office of Management and Budget began to mandate the use of EVM across all government agencies, and, for the first time, for certain internally managed projects (not just for contractors). EVM also received greater attention by publicly traded companies in response to the Sarbanes–Oxley Act of 2002.

In Australia, EVM has been codified as the standards AS 4817-2003 and AS 4817–2006.

US defense industry
The EVM concept took root in the United States Department of Defense in the 1960s. The original concept was called the Program Evaluation and Review Technique, but it was considered overly burdensome and not very adaptable by contractors whom were mandated to use it, and many variations of it began to proliferate among various procurement programs. In 1967, the DoD established a criterion-based approach, using a set of 35 criteria, called the Cost/Schedule Control Systems Criteria (C/SCSC). In the 1970s and early 1980s, a subculture of C/SCSC analysis grew, but the technique was often ignored or even actively resisted by project managers in both government and industry. C/SCSC was often considered a financial control tool that could be delegated to analytical specialists.

In 1989, EVM leadership was elevated to the Undersecretary of Defense for Acquisition, thus making EVM an element of program management and procurement. In 1991, Secretary of Defense Dick Cheney canceled the Navy A-12 Avenger II Program because of performance problems detected by EVM. This demonstrated that EVM mattered to secretary-level leadership. In the 1990s, many U.S. Government regulations were eliminated or streamlined. However, EVM not only survived the acquisition reform movement, but became strongly associated with the acquisition reform movement itself. Most notably, from 1995 to 1998, ownership of EVM criteria (reduced to 32) was transferred to industry by adoption of ANSI EIA 748-A standard.

The use of EVM has expanded beyond the U.S. Department of Defense. It was adopted by the National Aeronautics and Space Administration, the United States Department of Energy and other technology-related agencies.

Project tracking

It is helpful to see an example of project tracking that does not include earned value performance management. Consider a project that has been planned in detail, including a time-phased spend plan for all elements of work. Figure 1 shows the cumulative budget (cost) for this project as a function of time (the blue line, labeled PV). It also shows the cumulative actual cost of the project (red line, labeled AC) through week 8. To those unfamiliar with EVM, it might appear that this project was over budget through week 4 and then under budget from week 6 through week 8. However, what is missing from this chart is any understanding of how much work has been accomplished during the project. If the project was actually completed at week 8, then the project would actually be well under budget and well ahead of schedule. If, on the other hand, the project is only 10% complete at week 8, the project is significantly over budget and behind schedule. A method is needed to measure technical performance objectively and quantitatively, and that is what EVM accomplishes.

Progress measurement sheet
Progress can be measured using a measurement sheet and employing various techniques including milestones, weighted steps, value of work done, physical percent complete, earned value, Level of Effort, earn as planned, and more. Progress can be tracked based on any measure – cost, hours, quantities, schedule, directly input percent complete, and more.

Progress can be assessed using fundamental earned value calculations and variance analysis (Planned Cost, Actual Cost, and Earned Value); these calculations can determine where project performance currently is using the estimated project baseline's cost and schedule information.

With EVM
Consider the same project, except this time the project plan includes pre-defined methods of quantifying the accomplishment of work. At the end of each week, the project manager identifies every detailed element of work that has been completed, and sums the EV for each of these completed elements. Earned value may be accumulated monthly, weekly, or as progress is made. The Value of Work Done (VOWD) is mainly used in Oil & Gas and is similar to the Actual Cost in EVM.

Earned value (EV)
$ \begin{align} \mathrm{EV} & = \sum_\mathrm{Start}^\mathrm{Current} \mathrm{PV(Completed)} \quad \mathrm{or} \quad \mathrm{EV} = \mathrm{budget\, at\, Completion\,(BAC)} \times \mathrm{Actual\%\, Complete} \end{align} $

EV is calculated by multiplying %complete of each task (completed or in progress) by its planned value

Figure 2 shows the EV curve (in green) along with the PV curve from Figure 1. The chart indicates that technical performance (i.e. progress) started more rapidly than planned, but slowed significantly and fell behind schedule at week 7 and 8. This chart illustrates the schedule performance aspect of EVM. It is complementary to critical path or critical chain schedule management.

Figure 3 shows the same EV curve (green) with the actual cost data from Figure 1 (in red). It can be seen that the project was actually under budget, relative to the amount of work accomplished, since the start of the project. This is a much better conclusion than might be derived from Figure 1.

Figure 4 shows all three curves together – which is a typical EVM line chart. The best way to read these three-line charts is to identify the EV curve first, then compare it to PV (for schedule performance) and AC (for cost performance). It can be seen from this illustration that a true understanding of cost performance and schedule performance relies first on measuring technical performance objectively. This is the foundational principle of EVM.

Scaling EVM from simple to advanced implementations
The foundational principle of EVM, mentioned above, does not depend on the size or complexity of the project. However, the implementations of EVM can vary significantly depending on the circumstances. In many cases, organizations establish an all-or-nothing threshold; projects above the threshold require a full-featured (complex) EVM system and projects below the threshold are exempted. Another approach that is gaining favor is to scale EVM implementation according to the project at hand and skill level of the project team.

Simple implementations (emphasizing only technical performance)
There are many more small and simple projects than there are large and complex ones, yet historically only the largest and most complex have enjoyed the benefits of EVM. Still, lightweight implementations of EVM are achievable by any person who has basic spreadsheet skills. In fact, spreadsheet implementations are an excellent way to learn basic EVM skills.

The first step is to define the work. This is typically done in a hierarchical arrangement called a work breakdown structure (WBS), although the simplest projects may use a simple list of tasks. In either case, it is important that the WBS or list be comprehensive. It is also important that the elements be mutually exclusive, so that work is easily categorized into one and only one element of work. The most detailed elements of a WBS hierarchy (or the items in a list) are called work packages. Work packages are then often devolved further in the project schedule into tasks or activities.

The second step is to assign a value, called planned value (PV), to each work package. For large projects, PV is almost always an allocation of the total project budget, and may be in units of currency (e.g. dollar, euro or naira) or in labor hours, or both. However, in very simple projects, each activity may be assigned a weighted "point value" which might not be a budget number. Assigning weighted values and achieving consensus on all PV quantities yields an important benefit of EVM, because it exposes misunderstandings and miscommunications about the scope of the project, and resolving these differences should always occur as early as possible. Some terminal elements can not be known (planned) in great detail in advance, and that is expected, because they can be further refined at a later time.

The third step is to define "earning rules" for each work package. The simplest method is to apply just one earning rule, such as the 0/100 rule, to all activities. Using the 0/100 rule, no credit is earned for an element of work until it is finished. A related rule is called the 50/50 rule, which means 50% credit is earned when an element of work is started, and the remaining 50% is earned upon completion. Other fixed earning rules such as a 25/75 rule or 20/80 rule are gaining favor, because they assign more weight to finishing work than for starting it, but they also motivate the project team to identify when an element of work is started, which can improve awareness of work-in-progress. These simple earning rules work well for small or simple projects because generally, each activity tends to be fairly short in duration.

These initial three steps define the minimal amount of planning for simplified EVM. The final step is to execute the project according to the plan and measure progress. When activities are started or finished, EV is accumulated according to the earning rule. This is typically done at regular intervals (e.g. weekly or monthly), but there is no reason why EV cannot be accumulated in near real-time, when work elements are started/completed. In fact, waiting to update EV only once per month (simply because that is when cost data are available) only detracts from a primary benefit of using EVM, which is to create a technical performance scoreboard for the project team.

In a lightweight implementation such as described here, the project manager has not accumulated cost nor defined a detailed project schedule network (i.e. using a critical path or critical chain methodology). While such omissions are inappropriate for managing large projects, they are a common and reasonable occurrence in many very small or simple projects. Any project can benefit from using EV alone as a real-time score of progress. One useful result of this very simple approach (without schedule models and actual cost accumulation) is to compare EV curves of similar projects, as illustrated in Figure 5. In this example, the progress of three residential construction projects are compared by aligning the starting dates. If these three home construction projects were measured with the same PV valuations, the relative schedule performance of the projects can be easily compared.

Making earned value schedule metrics concordant with the CPM schedule
The actual critical path is ultimately the determining factor of every project's duration. Because earned value schedule metrics take no account of critical path data, big budget activities that are not on the critical path have the potential to dwarf the impact of performing small budget critical path activities. This can lead to gaming the SV and Schedule Performance Index (SPI) metrics by ignoring critical path activities in favor of big-budget activities that may have more float. This can sometimes even lead to performing activities out-of-sequence just to improve the schedule tracking metrics, which can cause major problems with quality.

A simple two-step process has been suggested to fix this:
 * 1) Create a second earned-value baseline strictly for schedule, with the weighted activities and milestones on the as-late-as-possible dates of the backward pass of the critical path algorithm, where there is no float.
 * 2) Allow earned-value credit for schedule metrics to be taken no earlier than the reporting period during which the activity is scheduled unless it is on the project's current critical path.

In this way, the distorting aspect of float would be eliminated. There would be no benefit to performing a non-critical activity with many floats until it is due in proper sequence. Also, an activity would not generate a negative schedule variance until it had used up its float. Under this method, one way of gaming the schedule metrics would be eliminated. The only way of generating a positive schedule variance (or SPI over 1.0) would be by completing work on the current critical path ahead of schedule, which is in fact the only way for a project to get ahead of schedule.

Advanced implementations
In addition to managing technical and schedule performance, large and complex projects require cost performance to be monitored and reviewed at regular intervals. To measure cost performance, planned value (BCWS) and earned value (BCWP) must be in the same currency units as actual costs.

In large implementations, the planned value curve is commonly called a Performance Measurement Baseline (PMB) and may be arranged in control accounts, summary-level planning packages, planning packages and work packages.

In large projects, establishing control accounts is the primary method of delegating responsibility and authority to various parts of the performing organization. Control accounts are cells of a responsibility assignment (RACI) matrix, which is the intersection of the project WBS and the organizational breakdown structure (OBS). Control accounts are assigned to Control Account Managers (CAMs).

Large projects require more elaborate processes for controlling baseline revisions, more thorough integration with subcontractor EVM systems, and more elaborate management of procured materials.

In the United States, the primary standard for full-featured EVM systems is the ANSI/EIA-748A standard, published in May 1998 and reaffirmed in August 2002. The standard defines 32 criteria for full-featured EVM system compliance. As of the year 2007, a draft of ANSI/EIA-748B, a revision to the original is available from ANSI. Other countries have established similar standards.

In addition to using BCWS and BCWP, implementations often use the term actual cost of work performed (ACWP) instead of AC. Additional acronyms and formulas include:

Budget at completion (BAC)
According to the PMBOK (7th edition) by the Project Management Institute (PMI), Budget at Completion (BAC) is the "sum of all budgets established for the work to be performed."

It is the total planned value (PV or BCWS) at the end of the project. If a project has a management reserve (MR), it is typically not included in the BAC, and respectively, in the performance measurement baseline.

Cost variance (CV)
According to the PMBOK (7th edition) by the Project Management Institute (PMI), Cost variance (CV) is a "The amount of budget deficit or surplus at a given point in time, expressed as the difference between the earned value and the actual cost." Cost variance compares the estimated cost of a deliverable with the actual cost.

$$ \begin{align} CV = EV - AC \end{align} $$

CV greater than 0 is good (under budget).

Cost performance index (CPI)
According to the PMBOK (7th edition) by the Project Management Institute (PMI), Cost performance index is a "measure of the cost efficiency of budgeted resources expressed at the ratio of earned value to actual cost."

$$ \begin{align} CPI = {EV\over AC} \end{align} $$

CPI greater than 1 is favorable (under budget).

CPI that is less than 1 means that the cost of completing the work is higher than planned (bad).

When CPI is equal to 1, it means that the cost of completing the work is right on plan (good).

CPI greater than 1 means that the cost of completing the work is less than planned (good or sometimes bad).

Having a CPI that is very high (in some cases, very high is only 1.2) may mean that the plan was too conservative, and thus a very high number may in fact not be good, as the CPI is being measured against a poor baseline. Management or the customer may be upset with the planners as an overly conservative baseline ties up available funds for other purposes, and the baseline is also used for manpower planning.

Estimate at completion (EAC)
According to the PMBOK (7th edition) by the Project Management Institute (PMI), Estimate at completion (EAC) is the "expected total cost of completing all work expressed as the sum of the actual cost to date and the estimate to complete."

EAC is the manager's projection of total cost of the project at completion.

$$ \begin{align} EAC = AC + {(BAC-EV)\over CPI} = {BAC \over CPI} \end{align} $$

This formula is based on the assumption, that the performance of the project (or rather a deviation of the actual performance from a baseline) to date gives a good indication of what a performance (or rather deviation of a performance from a baseline) will be in the future. In other words, this formula is using statistics of the project to date to predict future results. Therefore, it has to be used carefully, when the nature of the project in the future is likely to be different from the one to date (e.g. performance of the project compare to baseline at the design phase may not be a good indication of what it will be during a construction phase).

Estimate to complete (ETC)
According to the PMBOK (7th edition) by the Project Management Institute (PMI), Estimate to complete (ETC) is the "expected cost to finish all the remaining project work."

ETC is the estimate to complete the remaining work of the project. ETC must be based on objective measures of the outstanding work remaining, typically based on the measures or estimates used to create the original planned value (PV) profile, including any adjustments to predict performance based on historical performance, actions being taken to improve performance, or acknowledgement of degraded performance. While algebraically, ETC = EAC-AC is correct, ETC should never be computed using either EAC or AC.

In the following equation:

$$ \begin{align} EAC = AC + ETC \end{align} $$

ETC is the independent variable, EAC is the dependent variable, and AC is fixed based on expenditures to date. ETC should always be reported truthfully to reflect the project team estimate to complete the outstanding work. If ETC pushes EAC to exceed BAC, then project management skills are employed to either recommend performance improvements or scope change, but never force ETC to give the "correct" answer so that EAC=BAC. Managing project activities to keep the project within budget is a human factors activity, not a mathematical function.

To-complete performance index (TCPI)
To-complete performance index (TCPI) is an earned value management measure that estimates the cost performance needed to achieve a particular management objective. The TCPI provides a projection of the anticipated performance required to achieve either the BAC or the EAC. TCPI indicates the future required cost efficiency needed to achieve a target BAC (Budget At Complete) or EAC (Estimate At Complete). Any significant difference between CPI, the cost performance to date, and the TCPI, the cost performance needed to meet the BAC or the EAC, should be accounted for by management in their forecast of the final cost.

For the TCPI based on BAC (describing the performance required to meet the original BAC budgeted total):

$$TCPI_{BAC} = { BAC - EV \over BAC - AC }$$

or for the TCPI based on EAC (describing the performance required to meet a new, revised budget total EAC):

$$TCPI_{EAC} = { BAC - EV \over EAC - AC }$$

This implies, that if revised budget (EAC) is calculated using Earned Value methodology formula (BAC/CPI), then at the moment, when TCPI based on EAC is first time calculated, it will always be equal to CPI of a project at that moment. This happens because when EAC is calculated using formula BAC/CPI it is assumed, that cost performance of the remaining part of the project will be the same as the cost performance of the project to date.

Independent estimate at completion (IEAC)
The IEAC is a metric to project total cost using the performance to date to project overall performance. This can be compared to the EAC, which is the manager's projection.

$$IEAC = \sum AC + { \left( BAC - \sum EV \right) \over CPI }$$

Limitations
Proponents of EVM note a number of issues with implementing it, and further limitations may be inherent to the concept itself.

Because EVM requires quantification of a project plan, it is often perceived to be inapplicable to discovery-driven or Agile software development projects. For example, it may be impossible to plan certain research projects far in advance, because research itself uncovers some opportunities (research paths) and actively eliminates others. However, another school of thought holds that all work can be planned, even if in weekly timeboxes or other short increments.

Traditional EVM is not intended for non-discrete (continuous) effort. In traditional EVM standards, non-discrete effort is called "level of effort" (LOE). If a project plan contains a significant portion of LOE, and the LOE is intermixed with discrete effort, EVM results will be contaminated. This is another area of EVM research.

Traditional definitions of EVM typically assume that project accounting and project network schedule management are prerequisites to achieving any benefit from EVM. Many small projects don't satisfy either of these prerequisites, but they too can benefit from EVM, as described for simple implementations, above. Other projects can be planned with a project network, but do not have access to true and timely actual cost data. In practice, the collection of true and timely actual cost data can be the most difficult aspect of EVM. Such projects can benefit from EVM, as described for intermediate implementations, above, and earned schedule.

As a means of overcoming objections to EVM's lack of connection to qualitative performance issues, the Naval Air Systems Command (NAVAIR) PEO(A) organization initiated a project in the late 1990s to integrate true technical achievement into EVM projections by utilizing risk profiles. These risk profiles anticipate opportunities that may be revealed and possibly be exploited as development and testing proceeds. The published research resulted in a Technical Performance Management (TPM) methodology and software application that is still used by many DoD agencies in informing EVM estimates with technical achievement. The research was peer-reviewed and was the recipient of the Defense Acquisition University Acquisition Research Symposium 1997 Acker Award for excellence in the exchange of information in the field of acquisition research.

There is the difficulty inherent for any periodic monitoring of synchronizing data timing: actual deliveries, actual invoicing, and the date the EVM analysis is done are all independent, so that some items have arrived but their invoicing has not and by the time analysis is delivered the data will likely be weeks behind events. This may limit EVM to a less tactical or less definitive role where use is combined with other forms to explain why or add recent news and manage future expectations.

There is a measurement limitation for how precisely EVM can be used, stemming from classic conflict between accuracy and precision, as the mathematics can calculate deceptively far beyond the precision of the measurements of data and the approximation that is the plan estimation. The limitation on estimation is commonly understood (such as the ninety–ninety rule in software) but is not visible in any margin of error. The limitations on measurement are largely a form of digitization error as EVM measurements ultimately can be no finer than by item, which may be the work breakdown structure terminal element size, to the scale of reporting period, typically end summary of a month, and by the means of delivery measure. (The delivery measure may be actual deliveries, may include estimates of partial work done at the end of month subject to estimation limits, and typically does not include QC check or risk offsets.)

As traditionally implemented, EVM deals with, and is based in, budget and cost. It has no relationship to the investment value or benefit for which the project has been funded and undertaken. Yet due to the use of the word "value" in the name, this fact is often misunderstood. However, earned value metrics can be used to compute the cost and schedule inputs to Devaux's Index of Project Performance (the DIPP), which integrates schedule and cost performance with the planned investment value of the project's scope across the project management triangle.

Darling & Whitty (2019) conducted an ethnographic study to see how EVM is implemented, applying Goffman's Dramaturgy (sociology), they found there is a sham act occurring through impressionable acts presenting statistical data as fact even when the data may be worthless. Findings include sham progress reporting can emerge in an environment where senior management’s ignorance of project work creates unworkable binds for project staff. Moreover, the sham behaviour succeeds at its objective because senior management are vulnerable to false impressions. This situation raises ethical issues for those involved, and creates an overhead in dealing with the reality of project work. Further the Darling & Whitty study is pertinent as it provides sociological insight to how a scientific management technique has been implemented.

title: Earned value management


text:
Talk:Ether

As a Drug
It is used as a drug. Any information about that? —The preceding unsigned comment was added by 203.97.178.2 (talk) 00:03, 3 January 2007 (UTC).
 * Yeah I was searching for information on ether as a drug, but all I found was its chemical properties. Someone should add it. CheesePlease NL 11:38, 26 September 2007 (UTC)

Agreed. There should be some information regarding its use as a general anesthetic. —Preceding unsigned comment added by 70.162.18.223 (talk) 07:18, 21 July 2008 (UTC)

 * Yeah I came here to find out more about it's use as an anaesthetic, but it's just a page of scientific forumula and information only useful to scientists. -NeF (talk) 01:13, 25 July 2008 (UTC)

 * There's a very brief mention of ether used as an anesthetic in the article Chloroform. There's also a not very satisfactory discussion of ether at History of general anesthesia, and a somewhat better discussion at Anesthesia. None of these mention that ether was still used occasionally for surgery until the mid-20th century. All of these articles badly need some knowledgeable help. Milkunderwood (talk) 20:52, 1 May 2016 (UTC)

Well, I know of a site called erowid.org that has information on pretty much every type of drug that you could imagine, and ones you couldn't. It has some stories about trips on Ether and the like, that I've been currently reading up on. Here's the link to the Ether section of the site:

http://www.erowid.org/chemicals/ether/

Along with the first time recreational use stories that people have posted:

http://www.erowid.org/experiences/subs/exp_Ether_First_Times.shtml

The first link has a link to the general trip stories, as well as links to info about the history of the drug and the like. The first story I read about Ether usage was this (and another one listed below it to show others ways it effects people):

http://www.erowid.org/experiences/exp.php?ID=10737 http://www.erowid.org/experiences/exp.php?ID=42973

Some pretty interesting stuff, but I don't see why people would use stuff like Ether and Dizocilpine for recreational uses. They seem way too dangerous and unstable for that sort of usage. KiwiMaddog (talk) 03:08, 14 March 2009 (UTC)
 * Probably because they just read or watched Fear and Loathing in Las Vegas —Slicing (talk) 03:42, 22 July 2012 (UTC)

 * I'm suprised no one has mentioned this, but ether refers to a whole class of organic compounds. The drug known in colloquial use as "ether" refers specifically as diethyl ether i.e. the one with medical and recreation drug use. This is mentioned at the start of the article, though it might not have been when this discussion started. — Preceding unsigned comment added by J1812 (talk • contribs) 04:59, 10 January 2013 (UTC)

Various remarks
How does it smell?

In the sidebar, furan is shown as an aromatic compound that is not an ether. Furan is the prototypical aromatic ether and is the unsaturated form of THF (hence tetrahydrofuran)

The funny thing about the Michaelson-Morely experiment is that Einstein was already formulating relativity before he knew anything about it.

Also, wasn't the root of the english word "ether" the Latin word "aether", which means "air" as in "the sky".

You sure aether doesn't refer specifically to the upper air, which corresponds to the fifth element that fills all of space? Its Greek equivalent does, at least.

 * Despite their comparably attenuated reactivity, ethers can act as Lewis acids (see Acid-base reaction theories). For instance, diethyl ether forms a complex with boron compounds, such as boron trifluoride etherate, F3B:O(CH2CH3)2

Wouldn't the ether be the Lewis base here? Josh Cherry 00:59, 23 Oct 2003 (UTC)

The article states that the "x y ether" nomenclature is official IUPAC nomenclature; I don't want to edit the article as I may be wrong, but I'm sure that I was taught (at degree level) that this was trivial nomenclature and the only IUPAC nomenclature was the "x-oxy-y" type, such as "methoxyethane" - please correct me if I'm wrong, but I'm going to do a bit of research and find out, and possibly edit the article if I'm right - ta :) edd 08:33, 5 Jul 2004 (UTC)

should this be a disambig page?
what do people think? a discussion of diethly ether belongs on a separate page to discussion of ether's in general... jsut a thought... Erich 06:38, 15 Jul 2004 (UTC)

Williamson ether synthesis
The equation given for the Williamson ether synthesis shows an alkyl halide as a reactant, but what follows gives sulfonate esters as an example of a leaving group. Either the equation is not general enough or this example is incorrect. Josh Cherry 15:53, 26 Dec 2004 (UTC)

Space theory
Should some mention be made of early astronomical/astrological claims of space consisting of heavenly bodies suspended in "ether?" 153.104.16.114 17:29, 14 November 2005 (UTC)

I was wondering the same thing, I found it under "aether theories" — Preceding unsigned comment added by 72.67.68.139 (talk) 21:13, 23 March 2014 (UTC)

What came first?
If the known chemical was named after the ancient concept of "ether" or "aether", being what fills the void, or the universal solvent, then shouldn't this be the main entry for the word "ether"? All other entries may branch from this one. The article makes no mention of the ethymology of the chemical. Odraxir 00:17, 30 December 2005 (UTC)

The problem is that most of the article is relevant to chemistry alone and to include every association with "ether" will make it confusing to read. As wil alcohol, maybe a "Chemistry of.." page is better and to help produce a whole new specialist chemical section.

Aminal
The illustration of the aminal does not have the same structure on this page, as on the aminal page... (Which one is correct?) ChristianB 13:44, 18 August 2007 (UTC)

 * the image on the ether page is technically a hemiaminal but the distinction is not always made. I have no objections against the current image V8rik 17:54, 18 August 2007 (UTC)
 * I think what is shown is NOT a hemiaminal but an aminal-acetal --kupirijo (talk) 01:09, 22 October 2008 (UTC)
 * Actually according to IUPAC it is an α-aminoether and it is recommnded that the word aminal should not be used AT ALL.--kupirijo (talk) 13:03, 9 November 2008 (UTC)
 * Recomienda... 190.51.183.15 (talk) 13:03, 8 March 2012 (UTC)

Needs a message that directs people to the Ether of classic physics
I think there needs to be a message that says something along the line of "for the Ether in classic physics that is believed to be the medium of which light travels, see [link]" at the top. RegalStar (talk) 21:28, 16 January 2008 (UTC)

 * Isn't it enough with the link to the disambiguation page that is already at the top of the article? --Itub (talk) 09:34, 17 January 2008 (UTC)

Origin of name
Why are they called ethers? Did early chemists believe these newly discovered substances were the "aether" (either the luminifarious one or the Fifth Element), or is it more complicated than that? Daibhid C (talk) 21:22, 6 June 2008 (UTC)

 * The name was introduced (though not necessarily coined) in 1730 (1729 O.S.) by German chemist August Sigmund Frobenius either for the compound's purity, transparency, lightness and lack of color   or its high volatility . --84.41.34.154 (talk) 21:29, 28 April 2017 (UTC)

That picture is massive
we need to find a different illustration, the one currently one the site is needlessly large--66.210.54.83 (talk) 19:54, 26 September 2008 (UTC)

 * Which one? There are sixteen images in this article at present.

 * Ben (talk) 12:34, 28 September 2008 (UTC)

The one that appeared in previous versions of the article such as here:http://en.wikipedia.org/w/index.php?title=Ether&direction=next&oldid=240861814. Worth a look. You can't miss it. Meltyman (talk) 23:59, 2 October 2008 (UTC)

 * I think this edit was enough to fix the problem.

 * I'd prefer to undo this revision by Meltyman because the image is now too small to read easily.

 * Ben (talk) 12:02, 3 October 2008 (UTC)

I re-sized the image in the commons and then put it in a frame rather than a thumbnail in the article, i.e. pic on page is now actual size but that actual size is a fraction of the actual size of the previous image. This edit was OK but it bled into the next section a bit, I thought. Meltyman (talk) 19:54, 3 October 2008 (UTC)

 * It's legible now but it's the aspect ratio has changed so the image is stretched. Ask the original author to recreate it in the required shape.

 * Ben (talk) 22:12, 3 October 2008 (UTC)

The aspect ratio has changed and the image has been stretched, but neither change is important to the understanding of the article or to the ease with which it can be read. It could look better but it's far from the mess that prompted me to edit this article in the first place. If the current edit bothers you, you might try following your own orders. Meltyman (talk) 23:00, 3 October 2008 (UTC)

Dibutyl Ether
If Dibutyl Ether is going to redirect to this page, then there should be more info on it in the article. Boiling point, structure, etc. —Preceding unsigned comment added by 98.173.141.129 (talk) 14:50, 29 July 2009 (UTC)

Possible error in page, at least an inconsistency?
In the 'Structure and bonding' section the C-O-C bond angle is said to be about 120 degrees. In the 'Physical Properties' section it is said the C-O-C bond angle is about 110 degrees. I believe it's exactly 109.5 degrees because of the sp3 hybridization, but I'm just a first semester o-chem student so wanted someone to verify this error before correcting it. —Preceding unsigned comment added by Jusmai77 (talk • contribs) 18:39, 21 January 2010 (UTC)
 * The average bond angle in sp3 hybridization is 109.5 degrees, but in practice this is not always the case. Note that water is 104.5 degrees even though it is sp3 hybridized. This is due to the fact that the lone pairs take up more space than the hydrides. Intermolecular forces and intramolecular forces must also be considered. As it happens, I modeled a couple molecules with ether groups on avogadro, and found that the optimization produced angles between 110 and 115 degrees. CH3CH2OCH2CH3 had an angle of 113.2, and CH3CH2OCH2CH2OCH2CH3 had an angle of 111.4. Adam.braley (talk) 16:55, 28 October 2010 (UTC)

Rather than computational models that are hard to cite, can we use some real world data? --Rifleman 82 (talk) 16:42, 28 October 2010 (UTC)

That's a good point. I'll see if I can't get a better citation, but any first semester organic student can tell that 120 degrees is at least unlikely.Adam.braley (talk) 17:05, 28 October 2010 (UTC)

Yes, 120 ° is certainly strange. I was just thinking about how best to cite this source. --Rifleman 82 (talk) 00:49, 29 October 2010 (UTC)

Hybridization
I'm not a specialist but afaik 120deg angle is for sp2-hybridization, not for sp3. sp3-hybridization has a 109deg angle. Am I wrong? Wizardist (talk) 06:50, 4 June 2010 (UTC)

Polyethers: is it fair to refer to crown ethers as polyethers?
I'm not at all confident that the initial explanation of polyethers is accurate. Polyethers are definitely NOT molecules with multiple ether groups, they are polymers that contain repeating ether groups from their propagated monomers. What I'm not sure of is whether it's fair to call crown ethers polyethers. My instinct is no, but I can't back it up since I don't "know" that large crown ethers aren't synthesized by polymerization techniques. I doubt it, since neither step reaction polymerization nor chain reaction polymerization makes any sense for them as far as I can see. Anyway, I submit the question to you my fellow Wikipedians.Adam.braley (talk) 16:51, 28 October 2010 (UTC)

R'
I changed the general formula to R-O-R'. This is consistent with the picture but the online iupac goldbook reference uses R-O-R. A different references that alludes to both R-O-R and R-O-R' is http://docs.google.com/viewer?a=v&q=cache:C8gFvmb-ls8J:chemistry.csudh.edu/faculty/jim/ethers.ppt+ethers+ror&hl=en&gl=us&pid=bl&srcid=ADGEESgZXYBidUi-rktMmIlMZnRmQLeFV_-PZPgdYkNNV-PeAPl7RpK7vAu9kLE9cHGRYmdNCH7413us4yOepqxtM0JwQcOqISSD6Q4KpJr3cyRhmGAOPW0zYSiQraRmmifCPB_hqOke&sig=AHIEtbTm3E1O2QEz8jxkVdlvAayTicHpGQ which is a google view version of http://chemistry.csudh.edu/faculty/jim/ethers.ppt

Peroxide formation
I think some clarification is needed about how the tendency of ethers to form explosive peroxides depends on their structure. The Sept 20th, 2011 version of this section in the ether article contained the statement "Primary and secondary ethers with a CH group next to the ether oxygen form peroxides....For this reason, diisopropyl ether and THF are often avoided as solvents.".

All the common ethers, including diethyl ether, tetrahydrofuran (THF) and diisopropyl ether contain a CH group adjacent to the oxygen. The first two are primary ethers containing CH2 groups adjacent to the oxygen, while diisopropyl ether is a secondary ether. THF and diethyl ether are very widely used as solvents, with precautions recommended such as storing in the dark and not distilling to dryness because of the danger of peroxides. The article diisopropyl ether says it is more prone to peroxide formation than diethyl ether, and less so than methyl t-butyl ether (MTBE). To totally avoid a CH group adjacent to the oxygen one would need to go a highly hindered (crowded) compound such as bis (t-butyl) ether, which may be too hindered to even exist as a stable compound.

Pending further clarification of the relation of peroxide formation to structure, I will change the text in the main article to say simply that ethers are susceptible to forming explosive peroxides, unless appropriate precautions are taken.CharlesHBennett (talk) 20:49, 20 September 2011 (UTC)

Actually MTBE has 3 primary Hydrogens, diethyl ether has 2 secondary Hydrogens, and 2 tertiary Hydrogens in diisopropyl ether. The C-H bond strength is tertiary<secondary<primary which applies to free radical reactions like Bromination and Peroxide formation. Peroxide formation, like combustion, is a chain reaction. (Triplet) Oxygen removes a Hydrogen to form Hydroperoxide and Carbon radical that reacts with Oxygen, and that Peroxide radical reacts with and takes the Hydrogen (from the lowest energy bond) and the chain continues. Isooctane has high flash point (compared diesel's cetane number of secondary C-H bonds) because it is mostly higher energy primary C-H bonds and hindered Tertiary and Secondary C-H bonds which don't propagate the combustion chain reaction. Historically, we were cleaning out a lab connected to an EPA Superfund site and found a clear bottle labeled "isoPropyl Ether" with large clear (peroxide) crystals in it. We gingerly took that bottle to a vacant area and detonated it. Shjacks45 (talk) 12:01, 4 July 2019 (UTC)

By the way, Eucalyptol (natural oil) structure is basically a di-tert-Butyl Ether, Shjacks45 (talk) 12:09, 4 July 2019 (UTC)

Trivial Names: "X Y ether" or "Y X ether"?
Sometimes, as if following IUPAC rules, we put X and Y in alphabetical order, as in e thyl m ethyl ether, tert- a myl m ethyl ether (TAME). But then sometimes the reverse order seems more usual, as in m ethyl tert- b utyl ether (MTBE), e thyl tert- b utyl ether (ETBE). I know these are not systematic names and as such anything could happen, but is there any (historical?) reason behind this inconsistency? It would be greatly appreciated if this was clarified in the article(s), since these names are rather confusing for average (non-advanced) readers including myself. Textbook says that butyl-, ethyl-, methyl- must be said in this (alphabetical) order, but then we have a name like methyl tert-butyl ether, which may mislead you into believing that tert-butyl is alphabetized under “t” (a wrong assumption which will only confuse you even more). — Gyopi (talk) 09:39, 3 July 2015 (UTC)

Typical Wiki
This is what I hate about wikipedia. Saw the term ether on a TV show, wanted to find out what it was, come here and the entire article is chemistry mumbo jumbo. The average person isn't going to understand any of this. How about explaining what kind of chemical it is and its uses, in a way that isn't chock full of chemistry terms? Wiki articles tend to be way too literal. — Preceding unsigned comment added by 24.137.115.238 (talk) 04:46, 12 November 2015 (UTC)
 * AN unsigned commenter is actually right for once. You morons need to quit being so smart and realize that your article for ether contains virtually none of the information that the general public would come here to find... like uses and history and stuff. Of course that is kinda contained in the article for diethyl ether, but is little help to the average person who arrives here by searching "Ether" wanting to find out about civil war medicine. Try to view the article from the vantage point of someone who has seen a member of the opposite sex without clothes on. Nerds. 206.113.15.122 (talk) 21:09, 11 January 2018 (UTC)

Assessment comment
Substituted at 14:43, 29 April 2016 (UTC)

R?
what kind of element is R? 108.65.81.190 (talk) 17:42, 2 November 2016 (UTC)
 * In typical organic chemistry nomenclature, R is used to denote an alkyl or aryl group, or other substituent. I have added the missing definitions of R and R' to the article.  -- Ed (Edgar181) 18:18, 2 November 2016 (UTC)

Not everyone writes that that way.
Example: in English, we say pentyl phenyl ether. But here we say phenyl pentyl ether (fenil-pentil éter). Alfa-ketosav (talk) 14:31, 27 April 2018 (UTC)

Add MTBE to list of ethers
Methyl T-Butyl Ether was a controversial gasoline additive competing to "Oxygenate" gasoline, but Ethanol (and Iowa) won. Also more electrophilic that Ethyl ether, advantages for complexing Grignard reagents and Lithium compounds. Shjacks45 (talk) 11:04, 4 July 2019 (UTC)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Talk:Ether


text:
Electron microscope

An electron microscope is a microscope that uses a beam of electrons as a source of illumination. They use electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing them to produce magnified images or electron diffraction patterns. As the wavelength of an electron can be up to 100,000 times smaller than that of visible light, electron microscopes have a much higher resolution of about 0.1 nm, which compares to about 200 nm for light microscopes. Electron microscope may refer to: Additional details can be found in the above links. This article contains some general information mainly about transmission electron microscopes.
 * Transmission electron microscopy (TEM) where swift electrons go through a thin sample
 * Scanning transmission electron microscopy (STEM) which is similar to TEM with a scanned electron probe
 * Scanning electron microscope (SEM) which is similar to STEM, but with thick samples
 * Electron microprobe similar to a SEM, but more for chemical analysis
 * Ultrafast scanning electron microscopy, version of a SEM that can operate very fast
 * Low-energy electron microscopy (LEEM), used to image surfaces
 * Photoemission electron microscopy (PEEM) which is similar to LEEM using electrons emitted from surfaces by photons

History
Many developments laid the groundwork of the electron optics used in microscopes. One significant step was the work of Hertz in 1883 who made a cathode-ray tube with electrostatic and magnetic deflection, demonstrating manipulation of the direction of an electron beam. Others were focusing of the electrons by an axial magnetic field by Emil Wiechert in 1899, improved oxide-coated cathodes which produced more electrons by Arthur Wehnelt in 1905 and the development of the electromagnetic lens in 1926 by Hans Busch. According to Dennis Gabor, the physicist Leó Szilárd tried in 1928 to convince him to build an electron microscope, for which Szilárd had filed a patent.

To this day the issue of who invented the transmission electron microscope is controversial. In 1928, at the Technical University of Berlin, Adolf Matthias (Professor of High Voltage Technology and Electrical Installations) appointed Max Knoll to lead a team of researchers to advance research on electron beams and cathode-ray oscilloscopes. The team consisted of several PhD students including Ernst Ruska. In 1931, Max Knoll and Ernst Ruska successfully generated magnified images of mesh grids placed over an anode aperture. The device, a replicate of which is shown in the figure, used two magnetic lenses to achieve higher magnifications, the first electron microscope. (Max Knoll died in 1969, so did not receive a share of the 1986 Nobel prize for the invention of electron microscopes.)

Apparently independent of this effort was work at Siemens-Schuckert by Reinhold Rüdenberg. According to patent law (U.S. Patent No. 2058914 and 2070318, both filed in 1932), he is the inventor of the electron microscope, but it is not clear when he had a working instrument. He stated in a very brief article in 1932 that Siemens had been working on this for some years before the patents were filed in 1932, claiming that his effort was parallel to the university development. He died in 1961, so similar to Max Knoll, was not eligible for a share of the 1986 Nobel prize.

In the following year, 1933, Ruska and Knoll built the first electron microscope that exceeded the resolution of an optical (light) microscope. Four years later, in 1937, Siemens financed the work of Ernst Ruska and Bodo von Borries, and employed Helmut Ruska, Ernst's brother, to develop applications for the microscope, especially with biological specimens. Also in 1937, Manfred von Ardenne pioneered the scanning electron microscope. Siemens produced the first commercial electron microscope in 1938. The first North American electron microscopes were constructed in the 1930s, at the Washington State University by Anderson and Fitzsimmons and at the University of Toronto by Eli Franklin Burton and students Cecil Hall, James Hillier, and Albert Prebus. Siemens produced a transmission electron microscope (TEM) in 1939. Although current transmission electron microscopes are capable of two million times magnification, as scientific instruments they remain similar but with improved optics.

In the 1940s, high-resolution electron microscopes were developed, enabling greater magnification and resolution. By 1965, Albert Crewe at the University of Chicago introduced the scanning transmission electron microscope using a field emission source, enabling scanning microscopes at high resolution. By the early 1980s improvements in mechanical stability as well as the use of higher accelerating voltages enabled imaging of materials at the atomic scale. In the 1980s, the field emission gun became common for electron microscopes, improving the image quality due to the additional coherence and lower chromatic aberrations. The 2000s were marked by advancements in aberration-corrected electron microscopy, allowing for significant improvements in resolution and clarity of images.

Transmission electron microscope (TEM)
The original form of the electron microscope, the transmission electron microscope (TEM), uses a high voltage electron beam to illuminate the specimen and create an image. An electron beam is produced by an electron gun, with the electrons typically having energies in the range 20 to 400 keV, focused by electromagnetic lenses, and transmitted through the specimen. When it emerges from the specimen, the electron beam carries information about the structure of the specimen that is magnified by lenses of the microscope. The spatial variation in this information (the "image") may be viewed by projecting the magnified electron image onto a detector. For example, the image may be viewed directly by an operator using a fluorescent viewing screen coated with a phosphor or scintillator material such as zinc sulfide. A high-resolution phosphor may also be coupled by means of a lens optical system or a fibre optic light-guide to the sensor of a digital camera. Direct electron detectors have no scintillator and are directly exposed to the electron beam, which addresses some of the limitations of scintillator-coupled cameras.

The resolution of TEMs is limited primarily by spherical aberration, but a new generation of hardware correctors can reduce spherical aberration to increase the resolution in high-resolution transmission electron microscopy (HRTEM) to below 0.5 angstrom (50 picometres), enabling magnifications above 50 million times. The ability of HRTEM to determine the positions of atoms within materials is useful for nano-technologies research and development.

Scanning transmission electron microscope (STEM)
The STEM rasters a focused incident probe across a specimen. The high resolution of the TEM is thus possible in STEM. The focusing action (and aberrations) occur before the electrons hit the specimen in the STEM, but afterward in the TEM. The STEMs use of SEM-like beam rastering simplifies annular dark-field imaging, and other analytical techniques, but also means that image data is acquired in serial rather than in parallel fashion.

Scanning electron microscope (SEM)

The SEM produces images by probing the specimen with a focused electron beam that is scanned across the specimen (raster scanning). When the electron beam interacts with the specimen, it loses energy by a variety of mechanisms. These interactions lead to, among other events, emission of low-energy secondary electrons and high-energy backscattered electrons, light emission (cathodoluminescence) or X-ray emission, all of which provide signals carrying information about the properties of the specimen surface, such as its topography and composition. The image displayed by SEM represents the varying intensity of any of these signals into the image in a position corresponding to the position of the beam on the specimen when the signal was generated.

SEMs are different from TEMs in that they use electrons with much lower energy, generally below 20 keV, while TEMs generally use electrons with energies in the range of 80-300 keV. Thus, the electron sources and optics of the two microscopes have different designs, and they are normally separate instruments.

Electron diffraction
Transmission electron microscopes can be used in electron diffraction mode where a map of the angles of the electrons leaving the sample is producdd. The advantages of electron diffraction over X-ray crystallography are primarily in the size of the crystals. In X-ray crystallography, crystals are commonly visible by the naked eye and are generally in the hundreds of micrometers in length. In comparison, crystals for electron diffraction must be less than a few hundred nanometers in thickness, and have no lower boundary of size. Additionally, electron diffraction is done on a TEM, which can also be used to obtain many other types of information, rather than requiring a separate instrument.

Sample preparation for TEM

Materials to be viewed in a transmission electron microscope may require processing to produce a suitable sample. The technique required varies depending on the specimen and the analysis required:
 * Chemical fixation – for biological specimens this aims to stabilize the specimen's mobile macromolecular structure by chemical crosslinking of proteins with aldehydes such as formaldehyde and glutaraldehyde, and lipids with osmium tetroxide.

 * Cryofixation – freezing a specimen so that the water forms vitreous (non-crystalline) ice. This preserves the specimen in a snapshot of its native state. Methods to achieve this vitrification include plunge freezing rapidly in liquid ethane, and high pressure freezing. An entire field called cryo-electron microscopy has branched from this technique. With the development of cryo-electron microscopy of vitreous sections (CEMOVIS) and cryo-focused ion beam milling of lamellae, it is now possible to observe samples from virtually any biological specimen close to its native state.
 * Dehydration – replacement of water with organic solvents such as ethanol or acetone, followed by critical point drying or infiltration with embedding resins. See also freeze drying.
 * Embedding, biological specimens – after dehydration, tissue for observation in the transmission electron microscope is embedded so it can be sectioned ready for viewing. To do this the tissue is passed through a 'transition solvent' such as propylene oxide (epoxypropane) or acetone and then infiltrated with an epoxy resin such as Araldite, Epon, or Durcupan; tissues may also be embedded directly in water-miscible acrylic resin. After the resin has been polymerized (hardened) the sample is sectioned by ultramicrotomy and stained.
 * Embedding, materials – after embedding in resin, the specimen is usually ground and polished to a mirror-like finish using ultra-fine abrasives.
 * Freeze-fracture or freeze-etch – a preparation method  particularly useful for examining lipid membranes and their incorporated proteins in "face on" view.  FreezeFracture final.jpg External face of bakers yeast membrane.jpg fresh tissue or cell suspension is frozen rapidly (cryofixation), then fractured by breaking (or by using a microtome) while maintained at liquid nitrogen temperature. The cold fractured surface (sometimes "etched" by increasing the temperature to about −100 °C for several minutes to let some ice sublime) is then shadowed with evaporated platinum or gold at an average angle of 45° in a high vacuum evaporator. The second coat of carbon, evaporated perpendicular to the average surface plane is often performed to improve the stability of the replica coating. The specimen is returned to room temperature and pressure, then the extremely fragile "pre-shadowed" metal replica of the fracture surface is released from the underlying biological material by careful chemical digestion with acids, hypochlorite solution or SDS detergent. The still-floating replica is thoroughly washed free from residual chemicals, carefully fished up on fine grids, dried then viewed in the TEM.
 * Freeze-fracture replica immunogold labeling (FRIL) – the freeze-fracture method has been modified to allow the identification of the components of the fracture face by immunogold labeling. Instead of removing all the underlying tissue of the thawed replica as the final step before viewing in the microscope the tissue thickness is minimized during or after the fracture process. The thin layer of tissue remains bound to the metal replica so it can be immunogold labeled with antibodies to the structures of choice. The thin layer of the original specimen on the replica with gold attached allows the identification of structures in the fracture plane. There are also related methods which label the surface of etched cells and other replica labeling variations.
 * Ion beam milling – thins samples until they are transparent to electrons by firing ions (typically argon) at the surface from an angle and sputtering material from the surface. A subclass of this is focused ion beam milling, where gallium ions are used to produce an electron transparent membrane or 'lamella' in a specific region of the sample, for example through a device within a microprocessor or a focused ion beam SEM. Ion beam milling may also be used for cross-section polishing prior to analysis of materials that are difficult to prepare using mechanical polishing.
 * Negative stain – suspensions containing nanoparticles or fine biological material (such as viruses and bacteria) are briefly mixed with a dilute solution of an electron-opaque solution such as ammonium molybdate, uranyl acetate (or formate), or phosphotungstic acid. This mixture is applied to an EM grid, pre-coated with a plastic film such as formvar, blotted, then allowed to dry. Viewing of this preparation in the TEM should be carried out without delay for best results. The method is important in microbiology for fast but crude morphological identification, but can also be used as the basis for high-resolution 3D reconstruction using EM tomography methodology when carbon films are used for support.
 * Sectioning – produces thin slices of the specimen, semitransparent to electrons. These can be cut using ultramicrotomy on an ultramicrotome with a glass or diamond knife to produce ultra-thin sections about 60–90 nm thick. Disposable glass knives are also used because they can be made in the lab and are much cheaper. Sections can also be created in situ by milling in a focused ion beam SEM, where the section is known as a lamella.
 * Staining – uses heavy metals such as lead, uranium or tungsten to scatter imaging electrons and thus give contrast between different structures, since many (especially biological) materials are nearly "transparent" to electrons (weak phase objects). In biology, specimens can be stained "en bloc" before embedding and also later after sectioning. Typically thin sections are stained for several minutes with an aqueous or alcoholic solution of uranyl acetate followed by aqueous lead citrate.

EM workflows
In their most common configurations, electron microscopes produce images with a single brightness value per pixel, with the results usually rendered in greyscale. However, often these images are then colourized through the use of feature-detection software, or simply by hand-editing using a graphics editor. This may be done to clarify structure or for aesthetic effect and generally does not add new information about the specimen.

Electron microscopes are now frequently used in more complex workflows, with each workflow typically using multiple technologies to enable more complex and/or more quantitative analyses of a sample. A few examples are outlined below, but this should not be considered an exhaustive list. The choice of workflow will be highly dependent on the application and the requirements of the corresponding scientific questions, such as resolution, volume, nature of the target molecule, etc.

For example, images from light and electron microscopy of the same region of a sample can be overlaid to correlate the data from the two modalities. This is commonly used to provide higher resolution contextual EM information about a fluorescently labelled structure. This correlative light and electron microscopy (CLEM) is one of a range of correlative workflows now available. Another example is high resolution mass spectrometry (ion microscopy), which has been used to provide correlative information about subcellular antibiotic localisation, data that would be difficult to obtain by other means.

The initial role of electron microscopes in imaging two-dimensional slices (TEM) or a specimen surface (SEM with secondary electrons) has also increasingly expanded into the depth of samples. An early example of these ‘volume EM’ workflows was simply to stack TEM images of serial sections cut through a sample. The next development was virtual reconstruction of a thick section (200-500 nm) volume by backprojection of a set of images taken at different tilt angles - TEM tomography.

Serial imaging for volume EM
To acquire volume EM datasets of larger depths than TEM tomography (micrometers or millimeters in the z axis), a series of images taken through the sample depth can be used. For example, ribbons of serial sections can be imaged in a TEM as described above, and when thicker sections are used, serial TEM tomography can be used to increase the z-resolution. More recently, back scattered electron (BSE) images can be acquired of a larger series of sections collected on silicon wafers, known as SEM array tomography. An alternative approach is to use BSE SEM to image the block surface instead of the section, after each section has been removed. By this method, an ultramicrotome installed in an SEM chamber can increase automation of the workflow; the specimen block is loaded in the chamber and the system programmed to continuously cut and image through the sample. This is known as serial block face SEM. A related method uses focused ion beam milling instead of an ultramicrotome to remove sections. In these serial imaging methods, the output is essentially a sequence of images through a specimen block that can be digitally aligned in sequence and thus reconstructed into a volume EM dataset. The increased volume available in these methods has expanded the capability of electron microscopy to address new questions, such as mapping neural connectivity in the brain, and membrane contact sites between organelles.

Disadvantages

Electron microscopes are expensive to build and maintain. Microscopes designed to achieve high resolutions must be housed in stable buildings (sometimes underground) with special services such as magnetic field canceling systems.

The samples largely have to be viewed in vacuum, as the molecules that make up air would scatter the electrons. An exception is liquid-phase electron microscopy using either a closed liquid cell or an environmental chamber, for example, in the environmental scanning electron microscope, which allows hydrated samples to be viewed in a low-pressure (up to 20 Torr) wet environment. Various techniques for in situ electron microscopy of gaseous samples have been developed.

Scanning electron microscopes operating in conventional high-vacuum mode usually image conductive specimens; therefore non-conductive materials require conductive coating (gold/palladium alloy, carbon, osmium, etc.). The low-voltage mode of modern microscopes makes possible the observation of non-conductive specimens without coating. Non-conductive materials can be imaged also by a variable pressure (or environmental) scanning electron microscope.

Small, stable specimens such as carbon nanotubes, diatom frustules and small mineral crystals (asbestos fibres, for example) require no special treatment before being examined in the electron microscope. Samples of hydrated materials, including almost all biological specimens, have to be prepared in various ways to stabilize them, reduce their thickness (ultrathin sectioning) and increase their electron optical contrast (staining). These processes may result in artifacts, but these can usually be identified by comparing the results obtained by using radically different specimen preparation methods. Since the 1980s, analysis of cryofixed, vitrified specimens has also become increasingly used by scientists, further confirming the validity of this technique.

title: Electron microscope