Thomas Schmid

Markus Himmelsbach

James D Oliver

Marianne Gaborieau

Patrice Castignolles

Wolfgang Buchberger

Investigation of photochemical reactions of saccharides during direct UV detection in capillary electrophoresis

Keywords: Capillary electrophoresis, saccharide, direct UV detection, online irradiation, malondialdehyde, reaction mechanism

 Investigation of reaction mechanisms of direct UV detection of saccharides in CE  Separation and detection of several photooxidation reaction products of saccharides  Major portion of UV absorption of different saccharides is caused by same substance  Main photooxidation product was identified as conjugate base of malondialdehyde  UV irradiation time is crucial factor as malondialdehyde can degrade further

Introduction

Analysis of saccharides can be demanding due to the great variety of this class of substances. In addition, structural similarity of representatives such as stereoisomers of monosaccharides further complicates analysis. Analytical approaches include gas chromatography after derivatizing the analytes to increase their volatility [1,2], as well as a range of high-performance liquid chromatographic (HPLC) techniques working without derivatization. In the latter field, ligandexchange chromatography based on cation-exchangers loaded with metal ions has played a major role for many years [3]. In addition, hydrophilic interaction liquid chromatography (HILIC) is widely used with amino-modified silica as the originally preferred stationary phase complemented by a variety of other HILIC stationary phases in recent years [4]. As an alternative, anion-exchange chromatography under strongly alkaline conditions has become a routine tool for separations of saccharides [5][6][7].

Detectors commonly used range from refractive index detection to pulsed amperometric detection, evaporative light scattering detection and mass spectrometric (MS) detection [8]. Sensitive fluorescence detection is applicable after post column reactions [9]. For the separation of saccharides reversed-phase liquid chromatography typically in combination with pre column derivatization can also be employed [10].

In addition to these HPLC methods nowadays available for saccharide analysis, capillary electrophoresis (CE) presents an attractive technique providing a separation selectivity different to chromatography [START_REF] Klampfl | Capillary electrophoresis of carbohydrates: From monosaccharides to complex polysaccharides[END_REF][START_REF] Campa | Capillary electrophoresis: Methods and protocols[END_REF]. Saccharides with pK a values typically around or above 12 can be turned into charged species either by complexation with borate anions [START_REF] Hoffstetter-Kuhn | [END_REF]14] or by using a high-pH background electrolyte (BGE) [START_REF] Campa | Capillary electrophoresis: Methods and protocols[END_REF]15]. Some detection techniques used in HPLC of saccharides also work for CE such as MS detection or pulsed amperometric detection, but their robustness may be poorer than in HPLC. Also a variety of derivatization methods has been developed to introduce chromophoric or fluorophoric groups [START_REF] Campa | Capillary electrophoresis: Methods and protocols[END_REF]. Direct UV detection of underivatized saccharides at 195 nm is applicable if saccharides are separated as borate complexes, but does not work for the analytes being separated as such under strongly alkaline conditions. In the latter case indirect UV detection with sorbate as UV absorbing carrier electrolyte [15] or contactless conductivity detection [16] was reported.

As non-derivatized saccharides do not possess chromophoric groups, detection strategies based on direct UV detection may be considered as less promising. Nevertheless, Rovio et al. [17] followed by others [18][19][20][21][22][23][24] reported the application of direct UV detection in CE due to the unexpected fact that under high pH conditions saccharides exhibit UV absorption around 270 nm. The reason for this spectroscopic behavior was first assumed to be related to a complexation with alkali-ions of the BGE [17], but this was afterwards shown to be inconsistent [25]. Instead, a base-catalyzed photooxidation reaction in the detection window of the separation capillary initiated by the light of the lamp of the diode array UV detector (DAD) is supposed to take place that leads to the formation of UV absorbing substances [25,26]. This phenomenon of generating UV absorbing species from saccharides upon UV irradiation under alkaline conditions had already been observed earlier, although not in CE [27,28].

Products suggested in that context included ascorbic acid, 2,3-dihydroxyacrylaldehyde, 2-ketogluconic acid, 4-desoxy-5-keto-3,6-mannosaccharolactone [27] and later malondialdehyde (MDA) [29]. Results from electron spin resonance investigations of free radicals formed from saccharides after addition of hydroxyl radicals [30,31] were adapted for explanations of the possible reaction mechanisms taking place during direct UV detection in CE, but none of the compounds mentioned above was detected by nuclear magnetic resonance (NMR) spectroscopy [26]. Also in another study based on NMR spectroscopy [32] various compounds proposed earlier as products from UV irradiation of glucose [33] were not observed. In short, although a number of studies have been carried out, the actual reaction mechanism as well as the identity of the involved UV absorbing compounds is far from being entirely resolved.

The present study contributes to further elucidating the underlying reaction mechanisms that allow direct UV detection of saccharides in CE by a more detailed investigation of the reaction products. For this purpose, an additional UV lamp used for online irradiation during CE separations is introduced.

Products formed during irradiation are separated in the following part of the capillary before they reach the detector. The use of a UV detector reveals the peaks that contribute most to the UV absorption while identification can be achieved by coupling the set-up with an MS. In addition, the stability of the compound that contributes most to the UV absorption regarding possible degradation in highly alkaline solutions and under UV light is investigated by specific CE experiments as well as by NMR spectroscopy.

Materials and methods

Chemicals and reagents

Formic acid (LC-MS grade), ammonium formate (≥ 99.995 %), sucrose, ribose, glucose (≥ 99.5 %) and 1,1,3,3-tetramethoxypropane (99 %) were obtained from Sigma-Aldrich (Steinheim, Germany). 2-Propanol, acetonitrile and methanol, all analytical grade, were purchased from VWR (Vienna, Austria). Mesityl oxide (99 %), sodium hydroxide (1 M, Titripur ® ) and potassium hydroxide (85.8 %) were obtained from Merck (Darmstadt, Germany). For NMR measurements formic acid (> 99 %) was obtained from Univar (Ingleburn, Australia), sodium hydroxide pellets (> 98 %) and 1,1,3,3tetramethoxypropane (99 %) were purchased from Sigma-Aldrich (Castle Hill, Australia) and deuterium oxide (D, 99.9 %) was sourced from Cambridge Isotope Laboratories (Andover, MA, USA). Throughout the study purified water from a Milli-Q system from Millipore (Bedford, MA, USA) was used.

As malondialdehyde (MDA) is an unstable substance it is usually generated by in situ hydrolysis of its bis-acetal derivatives [34,35]. In this study the procedure of Syslová et al. [36] was followed with slight modifications: a 1 g L -1 solution of 1,1,3,3-tetramethoxypropane was prepared in 0.1 M formic acid and kept at 40 °C. To monitor the progress of hydrolysis samples were taken every 10 to 30 min and diluted to 10 mg L -1 . KOH was added to a final concentration of 30 mM to increase the pH, since the absorption maximum is different for acidic and basic conditions [37]. Progress of hydrolysis was observed by UV spectroscopy at 266 nm using an Agilent 8453 UV-Vis spectrophotometer (Agilent Technologies, Waldbronn, Germany). Until use the solution was kept at -20 °C.

Instrumentation

CE-UV and CE-MS experiments

CE separations were carried out using an Agilent 3D CE system. Fused silica capillaries (50 µm ID x 360 µm OD) were purchased from Polymicro Technologies Inc. (Phoenix, AZ, USA). New capillaries were flushed with 1 M NaOH, H 2 O and BGE for 10 min each. Preconditioning was performed by flushing with BGE for 3 min.

For CE-UV experiments the capillaries were cut to a length of 50 cm. Detection was done using a DAD equipped with a deuterium lamp. In addition, a second UV lamp of the same type was installed at a distance of 12.1 cm before the detector, where the polyimide coating of the capillary was removed (Figure S1 (A)). The length of this irradiation window was 1 cm and the distance between lamp and capillary about 2 cm. CE-MS experiments were carried out by coupling the CE to an Agilent 6510 Q-TOF/MS. Detection was done in the negative ion mode using an ESI source and an Agilent G1607A coaxial sprayer. The sheath liquid was supplied by an HPLC pump (Agilent 1100 Series G1311A) using a 1:100 splitter (also from Agilent Technologies). For CE-MS experiments the capillaries were cut to a length of 65 cm. A UV lamp was installed 15.0 cm before the end of the capillary, again with 1 cm of irradiation path length and a distance of 2 cm from the capillary (Figure S1 (B)). Based on investigations of the sheath liquid composition (see section 3.3.1) 90 % (v/v) 2-propanol in H 2 O with 0.01 % (v/v) NH 3 was selected. MS related parameters were optimized using the experimental design software Design-Expert ® (version 9) from Stat-Ease (Minneapolis, USA) and resulted in a nebulizer pressure of 23 psi, sheath liquid flow of 7 µL min -1 , drying gas flow (nitrogen) of 7 L min -1 and drying gas temperature of 200 °C. Capillary voltage and fragmentor voltage were set to 3750 V and 75 V, respectively. A mass range from m/z 50 to 1000 was selected at a scan rate of 4 spec s -1 .

HPLC-UV-MS experiments

An Agilent Series 1100 HPLC instrument equipped with UV detector coupled to the Agilent 6510 Q-TOF/MS was employed for HPLC measurements. For some experiments, the UV detector of the HPLC was bypassed when MS detection was performed. For the separation of the analytes an ODS-AQ column (250 mm x 3.0 mm ID, 3 µm particle size) from YMC (Dinslaken, Germany) was used.

The mobile phase consisted of 40 % (v/v) acetonitrile in H 2 O with 0.02 % (v/v) NH 3 and was pumped at a flow rate of 0.4 mL min -1 in the isocratic mode. An ESI source was used, employing the following parameters: nebulizer pressure 50 psi, drying gas flow (nitrogen) 8 L min -1 , drying gas temperature 250 °C, capillary voltage 3750 V and fragmentor voltage 75 V. A sample volume of 5 µL was injected.

NMR spectroscopy

13 C NMR spectra were recorded at room temperature on a Bruker DRX300 spectrometer (Bruker, Alexandria, Australia) operating at 75 MHz, equipped with a 5 mm 1 H-13 C dual probe. 13 C NMR spectra were recorded with a 8.5 µs 90 ° pulse, a 3 s repetition delay and 256 to 2048 scans. The 13 C chemical shift scale was calibrated with the resonance of the methanol signal at 50 ppm, a byproduct of the hydrolysis of 1,1,3,3-tetramethoxypropane to MDA that was confirmed through spiking.

In order to obtain adequate signal intensity in NMR spectra, MDA was prepared at a higher concentration than stated in section 2.1. Briefly, 10 g L -1 of 1,1,3,3-tetramethoxypropane were hydrolyzed to MDA in 10 M aqueous formic acid at 40 °C for 120 min in a closed glass vial. After hydrolysis, NaOH in D 2 O was added to give a final calculated pH of 13.1 and a MDA concentration of 2.2 g L -1 in 50 % D 2 O.

Results and discussion

CE-UV experiments

Separate irradiation and detection points

A second UV lamp (in the following referred to as irradiation lamp) was installed 12.1 cm before of the detector (see section 2.2.1 and Figure S1). With this set-up it is possible to physically separate the point of irradiation where potential UV-induced conversions of the saccharide take place from the point of detection. As a consequence, compounds formed during irradiation can be electrophoretically separated from each other and from the residual saccharide before reaching the detector.

Figure 1 displays the electropherogram of a solution of glucose containing mesityl oxide as EOF marker. It shows that besides the peaks of EOF marker and glucose additional peaks appear. As these additional peaks are not present when the irradiation lamp is turned off, it can be assumed that the corresponding compounds originate from a UV-induced conversion of the saccharide. Based on the fact that one of the additional peaks is of dominating intensity (about 75 % of the total area of all additional peaks), it can be concluded that this substance is responsible for the major part of the UV absorption. In the following this compound is referred to as UV substance.

The electrophoretic mobility of the UV substance can be calculated using the formula

U L U v U v µ µ µ tot Mes UV EO tot EP )) ( ) ( (      , ( 1
)
where tot µ , EP µ and EO µ represent total, electrophoretic, and electroosmotic mobility, respectively, v stands for the migration velocity, tot L for the length of the capillary and U for the separation voltage.

The indices UV and Mes specify that the variable refers to UV substance or mesityl oxide used as EOF marker. As indicated by the parenthesized U , Mes v and UV v should depend only on the applied separation voltage. However, in the experiment an additional pressure p of 10 mbar was applied to avoid long runtimes due to the high mobility of the UV product in the employed counter-EOF mode.

As

) , 0 ( ) 0 , ( ) , ( p U v p U v p U v     and ) , 0 ( ) , 0 ( Mes UV p U v p U v    equation (1) can be rewritten as U L p U v p U v µ tot Mes UV EP )) , ( ) , ( (    . ( 2)
) , ( Mes p U v can be determined from its migration time and the effective length of the capillary. As the UV substance is just formed at the irradiation lamp, its velocity ) , ( UV p U v has to be calculated using the formula

) , ( ) , ( ) , ( gluc UV sep UV p U t p U t L p U v   with (3) ) ( ) , ( ) , ( ˆsep eff eff gluc gluc L L L p U t p U t    . ( 4
)
sep L is the distance between irradiation UV lamp and detector, where the compounds formed by irradiation are separated. The denominator in equation ( 3) represents the time it takes the UV product to move across this distance. Variable t stands for the time until the substance passes the detector, whereas gluc t is the point in time when glucose passed the irradiation lamp. The latter can be calculated via equation (4).

Using these equations, the electrophoretic mobility of the major UV substance observed in Figure 1 was determined to be -4.39•10 -8 m² (V s) -1 . This relatively high mobility directed in a direction opposite to the EOF indicates a negatively charged compound.

Similar experiments were performed with solutions of ribose and sucrose. Again, for both saccharides one major UV compound and several minor compounds were formed during UV irradiation, though relative peak areas differed somewhat depending on the type of saccharide. In terms of sensitivity of the detection the minor additional peaks should not play a role, but they might explain the difference of maxima of the UV absorption spectra of different sugars that was reported earlier [22,26,27].

Calculation of the electrophoretic mobilities of the major UV substances revealed identical values for glucose, ribose and sucrose (relative standard deviation 1.8 %). Therefore, it can be concluded that in the case of all three saccharides the same substance is formed during UV irradiation, which is responsible for the major part of the UV absorption of saccharides. Hence, the following parts of this study focus on this compound.

MDA as possible UV photooxidation product

A few publications mentioned MDA as potential intermediate or end product of UV-induced saccharide conversion. Already in 1970 Scherz [29] suggested the presence of MDA due to a color reaction of 2-methylindole with solutions of various saccharides that previously had been irradiated

with UV light at 185 nm. Gilbert et al. [31] proposed different mechanisms for the reaction of saccharides with hydroxyl radicals, one of which included MDA as a side product. Furthermore, Fan [38] found MDA in saccharide solutions after treatment with γ-radiation. However, to our knowledge no definite evidence has been published that MDA is responsible for the major part of the UV absorption of saccharides separated by CE. On the contrary, a recent study did not detect MDA after UV irradiation in a CE instrument [26].

MDA in aqueous acidic solution has been observed to have a UV absorption spectrum with a maximum at 245 nm while its conjugate base malonenolate exhibits a maximum at 267 nm under aqueous alkaline condition [37], which matches previously observed spectra of UV irradiated saccharides very well [27]. Under the highly alkaline conditions used in this study, MDA with a pK a of 4.46 [35] is present as malonenolate, that is a negatively charged ion with presumably high electrophoretic mobility. Therefore, CE experiments were performed to check if the major UV substance described in section 3.1.1 can be assigned to malonenolate. For reasons of clarity, in the following both malondialdehyde and its conjugate base (malonenolate) are referred to as MDA.

Experimental conditions were kept as similar as possible to the experiment performed in section 3.1.1, except that the sample solution was spiked with MDA and the irradiation lamp was turned off. To keep also the separation length similar, the so-called short end injection technique [39] was employed, where the sample is injected at the detection side of the capillary and both voltage and additional pressure are inverted. The particular conditions were: 0.2 g L -1 glucose in H 2 O containing 1 mM mesityl oxide and 22 mg L -1 MDA (injection -50 mbar for 1 s), separation length 8.5 cm; voltage -5 kV and additional pressure -10 mbar (all other conditions as in Figure 1).

Making use of equations ( 1) to (3) the electrophoretic mobility of MDA can be calculated, taking into account that the denominator in equation ( 3) is directly given by the migration time of MDA ) , ( MDA p U t

. The resulting electrophoretic mobility of MDA was determined to be -4.40•10 -8 m² (V s) -1 and was therefore in good accordance to the mobility of the major UV substance found in section 3.1.1.

HPLC-UV-MS experiments

To confirm the finding that MDA is generated by UV irradiation, an HPLC method capable of separating MDA from saccharides was developed. This method (described in more detail in section 2.2.2) was applied to saccharide solutions that had been collected after pumping through a 60 cm capillary using a CE with its UV lamp switched on by applying 10 mbar overnight (in total 20 h). To assess if MDA had been formed as the saccharide had passed the UV lamp of the CE, HPLC experiments were also performed with identical solutions not pumped through the capillary.

In Figure 2 the results of a glucose solution are depicted, giving evidence that MDA was produced during UV irradiation. In the extracted ion chromatogram (EIC) of m/z 71.0139 a peak emerges at the retention time of MDA with a deviation between obtained MS signal and theoretical m/z of MDA below 3 ppm. In addition, the UV spectrum of this peak matches that of the MDA standard very well.

Similar experiments were also performed with solutions of ribose and sucrose, again indicating that MDA was formed during UV irradiation.

CE-MS experiments

To be able to detect and identify the major UV substance directly after its formation and therefore minimize the possibility of secondary reactions, the CE was coupled to a Q-TOF/MS with a UV lamp installed 15.0 cm before the MS entrance (see section 2.2.1).

Preliminary experiments as well as the literature [25,40] indicate that the speed of the UV-induced conversion of saccharides is high enough for UV detection only in strongly alkaline media such as diluted KOH or NaOH, whereas solutions of ammonia are not sufficient. However, as KOH or NaOH should be avoided when MS detection is employed, the following strategy was pursued: KOH was added only to the sample solution, whilst diluted NH 3 served as BGE. To ensure that the saccharide was still in contact with KOH when reaching the UV lamp, the sample plug was moved there solely by applying a pressure of 50 mbar. Shortly after the sample had passed the UV lamp the pressure was replaced by the separation voltage causing the formed substances to separate. This procedure entailed three major benefits. First, per analysis less than 20 nL of KOH containing solution (calculated with the Hagen-Poiseuille equation) were introduced to the MS. Second, ion suppression caused by KOH was avoided as the analytes were not in contact with KOH any more when reaching the MS. Third, as diluted NH 3 was used as BGE instead of KOH, a CE voltage of +30 kV could be applied without high CE currents occurring, which improved separation efficiency.

Optimization of CE and MS related parameters

As only a small amount of substance was expected to be formed during UV treatment, a careful optimization of CE and MS parameters had to be done. From the previously performed experiments it was expected that MDA is the major substance formed during UV irradiation of saccharides. Hence, for the optimization a solution containing 1 g L -1 glucose in 85 mM KOH spiked with 36 mg L -1 MDA was used. After hydrodynamic injection (50 mbar for 1 s) a pressure of 50 mbar was applied for 10.3 min moving the sample plug shortly past the UV lamp that was turned off in all optimization measurements. Afterwards, the separation voltage was applied until the MS signal of MDA was detected.

Optimization consisted of three steps including CE parameters, sheath liquid and MS parameters carried out in this order. All steps aimed at maximizing the sensitivity regarding MDA in order to lower the limit of detection for the irradiation experiments performed later.

Concerning CE parameters mainly BGE composition and separation voltage were investigated.

Starting point was a BGE consisting of 1 M NH 3 , where MDA appeared as an extensively broad peak.

As the peak shape improved significantly when the concentration of KOH in the sample was reduced it was concluded that the large difference in the ionic strengths of sample and BGE had caused the peak broadening. Since the ionic strength of the sample could not be decreased it was decided to increase the ionic strength of the BGE to achieve a better agreement. Experiments showed that an addition of 20 mM ammonium formate resulted in sharp and symmetrical peaks. Investigation of the separation voltage revealed that higher voltages are beneficial, so +30 kV were selected resulting in a CE current of about 30 µA.

In a second step the influence of the sheath liquid was studied. 2-Propanol, acetonitrile and methanol were investigated as organic components, whereby with 90 % (v/v) 2-propanol in water the best results were obtained. An addition of 0.01 % (v/v) NH 3 resulted in a further slight increase of the MDA peak height.

Optimization of MS parameters was done employing the design of experiment software Design-Expert ® and included sheath liquid flow, drying gas temperature, drying gas flow as well as nebulizer pressure. Three sets of experiments were performed, which are described in more detail in the supporting information. Temperature and flow of the drying gas did not show any significant effect on the signal intensity of MDA within the investigated boundaries and were set to 200 °C and 7 L min -1 , respectively. In contrast, the nebulizer pressure exhibited a large and the sheath liquid flow a medium influence. Based on the numerical optimization of the software a nebulizer pressure of 23 psi and a sheath liquid flow of 7 µl min -1 were selected for the following experiments.

Altogether, optimizing CE and MS related parameters lead to an increase of the peak height of MDA of a factor of about 40, thus significantly improving the possibility of detecting potentially formed MDA during UV irradiation. After the optimization the LOD of the obtained method was below 0.5 mg L -1 with a repeatability of below 9 % (n=5, determined with a solution of 36 mg L -1 MDA).

CE-MS experiments with online irradiation

Solutions consisting of 1 g L -1 saccharide in 85 mM KOH were analyzed employing the same experimental set-up as in section 3.3.1, except that the UV lamp was turned on. After injection of the sample, a pressure of 50 mbar was applied to move the sample plug about 2 cm behind the UV lamp.

Hence, the conversion of the saccharide took place when it passed the UV lamp driven by the applied pressure. Afterwards the pressure was replaced by a +30 kV separation voltage. At the same time the nebulizer pressure of the ESI source that had been kept switched off before to avoid suction effects [START_REF] Mokaddem | Figure 1: Electropherogram of a 0.2 g L -1 solution of glucose in H2O containing 1 mM mesityl oxide as EOF marker (injection 50 mbar for 5 sec); BGE: 130 mM KOH; separation voltage: +5 kV; additional pressure applied: +10 mbar; second UV lamp for irradiation 12[END_REF] was set to 23 psi.

Figure 3 displays a comparison of the same sample solutions with UV lamp turned on and off. From the EICs it can be clearly seen that in the case of all three saccharides irradiation by the deuterium lamp led to the formation of a substance with an m/z of 71.0139. The deviation of the theoretical m/z was below 1 ppm in all three cases and also the migration time matched that of the MDA standard.

The slight variations of the migration times of MDA in Figure 3 originates from inevitable variations of the pressure applied by the CE to move the sample plug to the irradiation UV lamp.

In order to determine the amount of MDA produced during irradiation an external calibration ranging from 3.6 to 18.1 mg L -1 was performed (R² of 0.992). Table 1 lists the concentrations of MDA produced during UV irradiation with the described set-up. From these data the fraction of saccharide converted into MDA was calculated assuming an equimolar conversion. In the case of all three investigated saccharides below 10 mg L -1 MDA were produced corresponding to a conversion of about 1 to 2 % of the saccharides. However, as the optical set-up differs significantly from the arrangement used when conventional UV detection is performed, these findings cannot be transferred directly.

Besides differences in distance between UV lamp and capillary as well as length of the window, especially the fact that no lenses were used in the presented set-up is assumed to play an important role. Additional experiments (data not shown) revealed that during conventional UV detection about 20 % of the glucose is converted assuming that MDA is the only UV absorbing reaction product.

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Stability of MDA

To gain information about the reaction kinetics, two methods were applied to saccharides as well as to standard solutions of MDA: a stop flow method in analogy to Sarazin et al. [25], consisting of flushing the capillary with sample solution followed by stopping the flow and monitoring the UV signal, and a multiple passes method similar to the one published by Oliver et al. [22]. In the latter approach, the analyte was electrophoretically separated from the H 2 O injection plug, and subsequently pushed past the detector by applying a pressure of 50 mbar that was afterwards repeatedly inverted to allow the analyte to pass the detector multiple times. The results of both methods are shown in Figure 4. When the methods were applied to saccharide solutions, similar results were obtained as published in [25] as well as [22], although in the latter case instead of pressure a separation voltage was applied and inverted. In both Figure 4 (A) and (B) a significant and relatively rapid decrease of the UV signal was observed when a solution of MDA was injected instead of the saccharide.

Based on these experiments it can be concluded that a UV-induced conversion of saccharides takes place that leads to the formation of MDA. The decreasing signals of the MDA standard in Figure 4 suggest that MDA in turn is further converted into one or more compounds without UV absorption at 266 nm. Both methods show an increase of the saccharides' signals, followed by a maximum and a somewhat slower decrease. This behavior seems reasonable, as estimating from the signal intensities and analyte concentrations it can be assumed that in Figure 4 (A) at the maximum at least 40 % of the glucose is already converted to MDA. Therefore, the speed of the reaction in which MDA is formed from glucose might be significantly reduced, whereas due to the rising MDA concentration the degradation reaction of MDA is assumed to accelerate, causing the signal to decrease.

The experiments demonstrate that the conversion of saccharides into MDA is highly sensitive regarding residence time under the UV lamp. It is reasonable to assume that depending on the experimental set-up too long irradiation times lead to a decrease of the concentration of MDA to a level where detection problems occur. This is consistent with an optimum observed in detection sensitivity upon addition of a radical photoinitiator at low concentration [26]. The addition of the photoinitiator may increase the production of MDA, but further increase of the photoinitiator concentration might lead to MDA decomposition. It is possible that this could explain why after continuous irradiation MDA was not detected by NMR spectroscopy as a final product of saccharides in an earlier study [26]. In addition, irradiation and NMR experiment together had taken nine days in that study, where the sample had been at room temperature. During this period of time, MDA may have further decomposed under the high pH of the solution.

The stability of MDA under highly alkaline conditions was thus monitored by 13 These results indicate that MDA is not stable for the time period preceding the NMR experiments done in the previous study [26]. Combined with the fact that MDA was only produced in minimal amounts then (at least 20 times less than in the present study assuming a maximum of 20 % conversion of glucose to MDA) this may explain why it was not detected then.

Conclusion

The presented study contributes to elucidating the photochemical reaction taking place during direct UV detection of saccharides in CE by investigating the formed substances. The use of an additional UV lamp made it possible to separate the compounds formed in this reaction from each other and showed that several substances are generated. However, among them one substance is responsible for the major part of the UV absorption of the saccharide during direct UV detection in CE and this substance was shown to be identical for the investigated saccharides glucose, ribose and sucrose.

Moreover, a comparison of the electrophoretic mobilities as well as additional HPLC experiments could demonstrate that this substance is MDA or more precisely its conjugate base malonenolate due to the alkaline conditions. In addition, it was possible to identify this substance directly after its formation by coupling the CE to MS detection. Investigations of the reaction kinetics revealed that MDA is further degraded by UV irradiation, demonstrating that the length of irradiation (presumably strongly affecting the generation of radicals initiating the reaction) is of central importance. Further insight would be required to know whether the initiation is due to water photolysis generating hydroxyl radicals or the direct photolysis of the saccharides. Moreover, it was shown that MDA slowly decomposes under highly alkaline conditions which complicates analysis further.

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Figure 1 :

 1 Figure 1: Electropherogram of a 0.2 g L -1 solution of glucose in H2O containing 1 mM mesityl oxide as EOF marker (injection 50 mbar for 5 sec); BGE: 130 mM KOH; separation voltage: +5 kV; additional pressure applied: +10 mbar; second UV lamp for irradiation 12.1 cm before the detector turned on.

Figure 2 :

 2 Figure 2: MS results (EIC at m/z 71.0139) of HPLC measurements with bypassed HPLC-UV lamp of 1 g L -1 glucose in 85 mM KOH which had undergone UV treatment (solid line) compared to an identical solution without UV treatment (dashed line) and a 1 g L -1 glucose solution in 85 mM KOH spiked with 36 mg L -1 MDA (dotted line). The peak at 3.4 min belonging to non-reacted glucose is due to its fragmentation in the ESI source. The insert displays the UV spectra at 2.3 min obtained from additional experiments where the UV detector of the HPLC was used.

Figure 3 :

 3 Figure 3: EICs at m/z 71.0139 of CE-MS experiments of 1 g L -1 saccharide in 85 mM KOH (50 mbar for 15 s). The UV lamp used for irradiation was either turned on (solid lines) or turned off (dashed lines with offset); samples: glucose (a), ribose (b), sucrose (c); see text for measurement conditions.

Figure 4 :

 4 Figure 4: Stop flow method (A): continuous introduction of 0.18 g L -1 glucose in 130 mM KOH (solid line) and 0.018 g L -1MDA in 130 mM KOH (dashed line) by +50 mbar; after 1 min the flow was stopped and the signal monitored. Multiple passes method (B): samples prepared in H2O (injection 50 mbar for 2 sec); BGE 130 mM KOH; voltage of +13 kV was applied until the analyte was about 5 min in front of the detector (estimated from preliminary runs); afterwards a pressure of +50 mbar was applied that was inverted about 0.5 min after the analyte had passed the detector. Capillary length was 60 cm for both (A) and (B).

Figure 5 :

 5 Figure 5: 13C NMR spectrum of MDA (A), with zoom onto the aldehyde carbon signal at 193.5 ppm (B) at 0 days (dotted line), 1 day (dashed line), 15 days (solid line) and 19 days (dash-dotted line), and the observed 13C chemical shifts (in ppm) for MDA and MDA enolate (C). The signal at 50 ppm corresponds to methanol, at 110 ppm to MDA (carbon atom in the middle position in enolate form) as well as 115 ppm (carbon atom in the middle position in neutral enol form) and 193.5 ppm (aldehyde carbon atom) and 173 ppm to the carboxyl carbon of sodium formate.

  The signal of the aldehyde carbon atom of MDA, visible at 193.5 ppm, decreased by 61 % in terms of peak height after 19 days which is consistent with MDA degradation. In accordance, the signals at 110 ppm and 115 ppm, corresponding to the carbon in the middle position of malonenolate and presumably its neutral enol form, declined with a similar pattern to that of the aldehyde carbon. However, no new signals and no increase in any other signal were observed. Instead, a dark orange precipitate formed and settled at the bottom of the NMR tube.

C NMR spectroscopy.

A solution containing 2.2 g L -1 MDA was monitored over the course of 19 days by NMR spectroscopy. The results are displayed in Figure

5

.

Table 1 :

 1 Concentrations of MDA produced during UV irradiation of 1 g L -1 saccharide solutions.

		Concentration of MDA produced
	Glucose	9.2 mg L -1
	Sucrose	3.8 mg L -1
	Ribose	5.9 mg L -1

Supporting Information for

Investigation of photochemical reactions of saccharides during direct ultraviolet absorbance detection in capillary electrophoresis

Thomas Schmid 1 , Markus Himmelsbach 1 , James D. Oliver 2,3 , Marianne Gaborieau 2,3 , Patrice

CE-UV experiments with online irradiation

In order to ensure that the additional peaks present in Figure 1 originate from a UV-induced conversion of glucose, further experiments were performed (Figure S2 (A) to (F)). As the additional peaks are not present when the irradiation UV lamp is turned off (Figure S2 (A)), a UV-induced reaction is confirmed to take place. To rule out that the additional peaks originate from a degradation of mesityl oxide that is used as EOF marker, measurements were repeated with solutions of mesityl oxide alone (Figures S2 (E) and (F)). Though mesityl oxide is partly degraded, no degradation products with UV absorption are observed. Measurements of glucose alone (Figures S2 (C) and (D)) clearly show that the additional peaks indeed originate from a reaction of glucose.

A B

Optimization of MS parameters

MS parameters were optimized using the software Design-Expert ® . Variables to be optimized were sheath liquid flow, drying gas temperature, drying gas flow and nebulizer pressure.

In a first set of experiments a screening was done to identify the relevant factors. The applied fractional factorial design (2 4-1 design) revealed that gas temperature and flow had no significant influence on the peak height of MDA, whereas the nebulizer pressure exhibited a large and significant influence. The sheath liquid flow exhibited only a weak influence. As a consequence, nebulizer pressure and sheath liquid flow were studied in more detail at a large experimental space reaching from 10 to 60 psi nebulizer pressure and 1 to 10 µL min -1 flow of sheath liquid. Peak heights increased with rising nebulizer pressure. However, above 40 psi no MS signals were obtained, probably because no stable ESI spray could be formed. Again, the sheath liquid flow showed only a minor influence with slightly better results at higher flow rates. Numerical optimization done with the Design-Expert ® software revealed an optimum at 7.8 µL min -1 sheath liquid flow and a nebulizer pressure of 19.4 psi.

However, due to the large influence of the nebulizer pressure a third set of experiments was performed to find the optimum at higher precision. Nebulizer pressure and sheath liquid flow were varied over


Kamal Kasmaoui

Farid Makhlouf

Refk Selmi

The Decision to Remit: Is it a Matter of Interpersonal Trust?

Keywords: Interpersonal Trust, Remittances, Social Capital. JEL classification: F24, D71, A13, Z13

This article seeks to assess the role of the level of interpersonal trust in a country in the remittance landscape. Using historical data from the 2010-2014 wave of the World Value Survey (WVS) for interpersonal trust, our findings underline the substitution role played by interpersonal trust with remittances. More accurately, remittances tend to drop when the rate of interpersonal trust in the country of origin is high. Overall, a rise in trust is likely to underpin social cohesion, limiting therefore the need for remittances. Potential elements including human capital, cultural factors, the quality of institutions, the financial development and the inequality have been advanced to explain the obtained finding.

Introduction

Social cohesion and trust among individuals have long been documented in the existing literature. In retrospect, Ibn Khaldun, the well-known 14th century historian, economist and the father of sociology, borrowed in his book Al Muqaddimah (1377) the concepts of social cohesion and solidarity among groups in society. He re-considered the original meaning of social cohesion (i.e., people related to each other by blood ties), and underscored that it rather implies solidarity, group feeling as well as awareness. He added that social cohesion is one of the crucial elements that influences the power and weakness of a group or tribe. It must be pointed out here that there is no common definition of social cohesion. A variety of indicators have been employed to investigate social cohesion but the most frequent indicators incorporate measures of trust and common social norms. Indeed, the trust can be perceived as "the expectation that arises within a community from regular, honest and cooperative behaviour, based on commonly shared norms, on the part of other members of that community" (Fukuyama, 1995, pp. 36).

The link between trust and economic outcomes have been largely studied. On the one hand, the trust's impact on economic development has been the subject of many economic researches (Guiso et al. 2006;Algan and Cahuc, 2010, among others). On the other hand, some empirical studies have tried to assess the extent to which the trust is linked to the income inequality (for example, Alesina and La Ferrara, 2000;[START_REF] Jordahl | Inequality and Trust[END_REF]. Nevertheless, the effect of trust on remittances has not yet been well discussed and/or explored, although there was a wider policy concern regarding the effect of immigration on social cohesion. Given this consideration, this study attempts to address whether the level of interpersonal trust in a country has a significant impact on the remittances made by its diaspora. It must be stressed at this stage that interpersonal trust is hugely perceived as a key factor for development and social cohesion (see inter alia, [START_REF] Knack | Does social capital have an economic payoff?[END_REF][START_REF] Porta | Trust in Large Organizations[END_REF][START_REF] Zak | Trust and Growth[END_REF][START_REF] Kasmaoui | Does Trust Influence Economic Growth? Evidence from the Arab World[END_REF]. Moreover, [START_REF] Fukuyama | Trust: The Social Virtues and the Creation of Prosperity[END_REF] stated that trust can be also regarded as the basis of the virtuous circle of the economy. Building trust is not easier in any country, and it seems likely to be much more difficult in developing countries. Based on trust data from the latest World Value Survey (WVS, 2010(WVS, -2014) ) covering 60 countries in the world, the average of trust in developed countries is 42.17%, while it is about 17.90% in developing countries. In some countries like Norway and Sweden, more than 60% of respondents deeply believe that people can be trusted. In other countries such as Brazil, only 10% of respondents agreed that people can be trusted. Overall, the level of trust is relatively low in developing countries (see Figure A1 in Appendix).

At the same time, the majority of developing countries are considered as emigration countries. Generally speaking, the lower the country's income, the more likely its citizens will tend to migrate, and in turn the greater will be its remittances flows. Owing to its size, its marked stability and its countercyclical behavior tending to rise during times of distress and heightened uncertainty, this issue has devoted a tremendous attention. Given the growing evidence on the huge role of remittance flows relative to other flows in developing countries, it is highly anticipated that the last decade was marked by increasing interest of policymakers and academics devoted to their developmental role. More particularly, the contribution of remittances to the overall development has been largely documented (for instance, [START_REF] Chami | Are immigrant remittance flows a source of capital for development?[END_REF]Ratha, 2007;[START_REF] Ruiz | Remittances, Institutions and Growth: A Semiparametric Study[END_REF]Fayissa and Nsiyah, 2008;2010;[START_REF] Rao | Are Direct and Indirect Growth Effects of Remittances Significant?[END_REF][START_REF] Makhlouf | Transferts de fonds vers le Maroc, enjeux, comportement et impacts[END_REF]. In fact, migrant remittances are viewed as a source of foreign funds of paramount importance for several developing countries. Remittances continue to rise to over $500 billion in 2018 (World Bank). In some countries including Nepal, Tajikistan and Tonga, the share of remittances attained more than 30% of GDP, according to the World Bank.

At the present, remittances are more than double the official aid received by these economies.

According to Ratha (2007), "remittances directly augment the income of recipient households.

In addition to providing financial resources for poor households, they affect poverty and welfare through indirect multiplier effects and also macroeconomic effects".

In addition to the remittances' contribution to the economic development, prior research tried to address what drives these inflows and what causes developing countries to receive different levels of remittances. Remittances are driven by social, economic and behavioral factors in receiving countries (for instance, [START_REF] Makhlouf | Transferts de fonds vers le Maroc, enjeux, comportement et impacts[END_REF][START_REF] Edelbloude | Do remittances respond to revolutions? The Evidence from Tunisia[END_REF]. Remittances are also determined by cultural and political situations in home countries (see, for example, [START_REF] Makhlouf | Voting with The Wallet: The Response of Remittances to Political Systems[END_REF]. Since remittances are affected by the economic situation in countries of origin, one can expect that the level of remittances might be affected by the behavior of the people who receive them. In that case, trust is a major issue. In fact, remittances may be strengthened by ties between migrants and their families and friends in the country of origin.

The present research incorporates a reflection of the association between social capital and economic development by discussing the question whether interpersonal trust rise or lessen the need for remittances. More specifically, this study goes beyond the various usual determinants of remittances by seeking to test if the trust heterogeneity in developing countries exert a significant impact on remittance inflows. To this end, we use the WVS trust data for most emigration countries1 and remittances from the World Bank. Our findings reveal that interpersonal trust has a negative effect on remittances. These results are still fairly solid and unambiguous after controlling for confounding factors and possible reverse causality.

The remainder of the article is organized as follows: Section 2 discusses the factors determining remittances dynamics. Section 3 presents the conducted methodology and the data. Section 4 reports and discusses the main findings. Section 5 concludes and provides some policy implications.

Methodology and data collection

We use cross sectional data to explore the relationship between trust and remittances.

Data for the variable of interest "trust" comes from the World Value Survey (WVS). We obtain data for the dependent variable (i.e., remittances) and the rest of independent variables from the World Bank. We expect that a high interpersonal trust in home countries may counteract remittances in countries of origin. The relationship between remittances and trust may be substitutable: When trust is very high in countries of origin, people could easily access financial assistance and informal loans. Trust could encourage exchanges between individuals within countries. In contrast, if individuals do not trust each other, they will tend to be more likely to involve their relatives abroad. In this second case, remittances will undoubtedly increase.

Identification Strategy

As mentioned at the outset, the main objective of this study is to provide some fresh insights on the role of interpersonal trust in remittance dynamics. However, remittances as external income can also influence the level of trust. While trying to deal with possible endogeneity bias, we conduct an instrumental variable method. Inspired by [START_REF] Knack | Does social capital have an economic payoff?[END_REF] and [START_REF] Zak | Trust and Growth[END_REF], we instrument trust with the proportion of Muslims and Christians in each country as also used in [START_REF] Kasmaoui | Does Trust Influence Economic Growth? Evidence from the Arab World[END_REF]. The latter may influence the level of trust in the country of origin but do not have a direct effect on remittances. A way to correct for the endogeneity problem is to perform a two-stage least squares (2SLS) or GMM using lag of the explanatory variables as instruments. In the current study, we conduct a 2SLS model.

Baseline Model and variables definition

The model performed throughout the rest of our analysis can be denoted as log(𝑅𝑒𝑚𝑖𝑡 𝑖,10-14 ) = 𝛽 1 log (𝑇𝑟𝑢𝑠𝑡 𝑖,10-14 ) + 𝑋 𝑖 𝜗 + 𝜀 𝑖 (1)

Where, Remit represents the Remittances variable or our dependent variable. Remittances comprise personal transfers and compensation of employees; it is the average annual remittances over the 2010-2014 period, where i represents the countries under study.

Remittances inflows are in millions of U.S. dollars.

Trust corresponds to the percentage of people in each country giving the response "most people can be trusted" to the question "Generally speaking, would you say that most people can be trusted, or that you can't be too careful in dealing with people?". Respondents not answering the question on trust were excluded from the dataset. The lowest trust value recorder in our sample is 3.2 % in the Philippines, and the highest rate is 60.3 % in China. The countrywise distribution of non-respondents is summarized in Table A1 (Appendix).

X refers to the matrix of control variables supposed to significantly influence remittances including per capita income, inflation and unemployment in the origin countries. Values of control variables are taken at the beginning of the period considered.

GDP per capita, PPP (lgdp)

is the Gross Domestic Product converted into international dollars using purchasing power parity rates (PPP).

Inflation measured by the consumer price index, reflecting the annual percentage change in the cost to the average consumer of acquiring a basket of goods and services that may be fixed or changed at specified intervals (for our case, yearly). Data for this variable are collected from the World Bank.

Unemployment refers to the share of the labor force that is without work but available for and seeking employment. Data is downloaded from the World Bank. Ethnic Fractionalization is an ethnic fractionalization index. Data for this variable are collected from the Historical Index of Ethnic Fractionalization Dataset (HIEF).

Proportion of Muslims and

The variables consist of data on 41 developing countries taken from the 2010 -2014 wave of the WVS. We have transformed the variables by taking natural logarithms to correct for heteroskedasticity and dimensional differences between the time series under consideration. Table A2 (in the Appendix) displays the descriptive statistics of the variables used in our analysis. Figure 1 relates the levels of remittances to trust; it shows that countries with high levels of trust also display lower levels of remittances.

Discussion of results

Table 1 reports the findings of the Baseline model (Column 1) and shows that interpersonal trust in home countries are likely to lower remittances. This outcome is still fairly robust to the incorporation of additional potential control variables (Column 2) and after controlling for possible endogeneity bias (2SLS results, Columns 3, 4, 5, 6, 7 and 8).

Columns 3, 4, 5 and 6 report the first and second-stage estimations of the 2SLS model.

Instrumenting for trust with the proportion of Muslims and Christians in the population, the effect of trust on remittances remains significant at the 5% and 10% level respectively.

Instrument validity tests show that the instruments are jointly significant in the first stage (as suggested by the value of F test (10.42 and 11.05) and the partial R2 Shea test (0.37 and 0.44).

The first stage F-statistic for excluded instruments is greater than ten suggesting that our instruments satisfy the relevance condition. The instruments validate the Hansen J-test of overidentifying restrictions implying that the instruments are not correlated with the error term.

Columns 7 and 8 report the first and second-stage estimations of the 2SLS model where using Ethnic Fractionalization as instrument for trust. In doing so, the effect of trust on remittances becomes insignificant. Validity tests of the instruments both the F-statistic, partial R2 Shea and the Hansen J-test show that the instrument (Ethnic Fractionalization) is not valid in the case of our study. This deeply highlights that international interpersonal trust in countries of emigration is a significant determinant of remittances. Remittances could substitute (i.e., negative correlation) trust between individuals. More precisely, when trust among members of the same community is relatively high, they trust each other, help each other, and lend each other money. In this case, they will have less need of individuals living abroad. However, when trust is weak in countries of emigration, individuals will turn to their relatives or friends living abroad for more assistance. In this case, we may think that migration in general and remittances in particular are a response to the lack of trust among individuals and the trust in institutions. [START_REF] Knack | Does social capital have an economic payoff?[END_REF] assume that ethnic homogeneity is one of the key determinants of trust. They argue that ethnic homogeneity strengthens "social capital" or "trust", which are, in turn, associated with faster growth and high per capita income. Moreover, these authors suggest that Polarization2 may increase rent-seeking activities that erode trust and undermine norms of cooperation. The effect of trust on remittances can be explained by several elements. These incorporate among others:

Table 1. The regression of remittances on trust and other potential control variables

Inequality: Several studies have found an association between increased inequality and decreased interpersonal trust [START_REF] Knack | Does social capital have an economic payoff?[END_REF][START_REF] Zak | Trust and Growth[END_REF][START_REF] Rothstein | All for all: Equality and Social Trust[END_REF]Bjørnskovk, 2007;[START_REF] Wilkinson | The Spirit Level -Why Equality is Better for Everyone[END_REF][START_REF] Stiglitz | Le prix de l'inégalité[END_REF]. Hence, the remittances may pass through its effect on inequality.

Human capital and cultural factors: Another mechanism by which remittances could be indirectly influenced by interpersonal trust are human capital and cultural factors [START_REF] Coleman | Social Capital in the Creation of Human Capital[END_REF][START_REF] Knack | Does social capital have an economic payoff?[END_REF][START_REF] Zak | Trust and Growth[END_REF][START_REF] Makhlouf | Voting with The Wallet: The Response of Remittances to Political Systems[END_REF]. [START_REF] Coleman | Social Capital in the Creation of Human Capital[END_REF] argued that social capital is a key factor in explaining educational outcomes. Furthermore, Whiteley (2000) stated that investment in education may not have a pronounced impact in a society with low trust level. [START_REF] Knack | Does social capital have an economic payoff?[END_REF] added that interpersonal trust is most pronounced in countries with a well-educated population. Likewise, [START_REF] Zak | Trust and Growth[END_REF] concluded that interpersonal trust could be enhanced by improving education.

Institutions:

The effect of trust on remittances can be achieved through the quality of institutions because the link between remittances and the quality of institutions is demonstrated in the existing literature (see for example, Putnam et al.1993;Fukuyam, 1995;[START_REF] Knack | Does social capital have an economic payoff?[END_REF]Zack and Kanck, 2001;[START_REF] Abdih | Remittances and institutions: Are remittances a curse?[END_REF][START_REF] Berdiev | Remittances and corruption[END_REF]. [START_REF] Knack | Does social capital have an economic payoff?[END_REF] pointed out that institutions have an influence on growth through trust. Zak and Kanck (2001) also concluded that trust depends on the social, economic and institutional environment in which the transaction is conducted. In addition, for [START_REF] Putnam | Making Democracy Work. Civic Traditions in Modern Italy[END_REF], institutional quality may exert a significant impact on the linkage between social capital and economic efficiency. [START_REF] Fukuyama | Trust: The Social Virtues and the Creation of Prosperity[END_REF] argued that a high level of trust between citizens might affect the performances of all institutions in society, including businesses.

Likewise, La [START_REF] Porta | Trust in Large Organizations[END_REF] provide evidence of the existence of a positive relationship between government effectiveness and trust. Remittances would facilitate cooperation and reduces the budgetary constraint of recipient families, and this could help to strengthen trust.

An increase in interpersonal trust is likely to underpin social cohesion in the country of origin, thus limiting the need for remittances.

Informal vs formal remittances: The negative result between formal remittances and trust may be also explained by the fact that informal remittances are higher in low trust countries because migrants in countries where the quality of institutions is low bring money home directly or send money with family or friends visiting the country of origin. Another possibility that may explain why formal remittances are less important in countries with relatively low levels of trust is the fact that migrants bring with them durable goods such as refrigerators, TV and other consumer goods instead of sending money to buy them in their country of origin.

Unfortunately, the lack of data on informal remittances makes it impossible to estimate its effect.

Conclusions and some policy implications

Nowadays, all societies are heavily concerned about collapsing trust. The present research goes beyond particularized trust which refers to trust in the family and close friends, and investigates the economic implications of the trust that people have in other members of society where uncertainty prevails-often referred to a generalized trust [START_REF] Putnam | Tuning in, tuning out: The strange disappearance of social capital in America[END_REF][START_REF] Putnam | Bowling Alone: the Collapse and Revival of American Community[END_REF].

In this context, [START_REF] Fukuyama | Trust: The Social Virtues and the Creation of Prosperity[END_REF] claims that when trust does not extend beyond the family, the supply of capital and skilled managers will be limited, thus constraining the development of private enterprises. When trust among individuals within the same community is higher, it would be easier to carry out financial transactions as loans and financial aids; hence the need to migrate will be lowered. In the same way, when trust among individuals in a community is greater, mutual aid increases accordingly and people bear the cost of this mutual aid. This implies a low demand for money from members of the same community who work and live abroad as this operation could have a higher opportunity cost than it would from individuals who live in the same country.

Remittances are a complex phenomenon that respond to economic factors in both host and home countries. This evidence has been largely documented in the economic literature.

Throughout the present study, the drivers of remittances are extended to account for behavioral factors (in particular, the interpersonal trust). Based on data from the World Value Survey and the World Bank, we show that interpersonal trust exerts a significant and negative impact on remittance inflows. Remittances can compensate for the lack of trust.

This article' outcomes provide robust evidence that remittances can be perceived as individuals' reactions to socio-economic conditions in countries of origin. An increase in interpersonal trust strengthens social cohesion in the country of origin, reducing therefore the need for remittances. To our best knowledge, this is the first work that explores the interpersonal trust as a determining factor of remittances. Other factors that may influence remittances, including among others the institutional trust, deserve to be meticulously studied.

It may be also quite interesting to effectively address how policymakers could improve such a prominent type of social capital. If trust exerts a significant impact on economic and social outcomes, it seems of utmost importance to identify the institutions and public policies required for it to develop. A major driving force of interpersonal trust is the belief that others will act in equitable and communal way. The role of institutions is vital in this framework.

This is an evident policy lever to boost trust by enhancing the integrity and transparency of institutions.

Last but not least, understanding and enhancing the economic implications of social cohesion necessitate robust and unambiguous evidence that can appropriately inform policymakers; hence the need to have proper and effective social measures. It must be mentioned at this stage that the statistical community has made noticeable progress in this matter. However, certain topics have not yet devoted a particular attention. Trust is one of these topics. Given the trust's role on the overall development, effective measures of trust at higher frequencies are highly required to be able to assess how trust respond to possible unforeseen shocks 3 , how it can be safeguarded, and whether relevant policies can restore or strengthen it.

3 The stock of trust can be promptly depleted after the occurrence of an unforseen shock [START_REF] Algan | The European trust crisis and the rise of populism[END_REF]).

  Proportion of Christians are respectively the proportion of Muslims and the proportion of Christians in each country. Data are collected from Pew Research Center.

Figure 1 .

 1 Figure 1. Remittances and Interpersonal trust

The countries under study include Algeria, Armenia, Azerbaijan, Belarus, Brazil, Colombia, Cyprus, Ecuador, Egypt, Georgia, Ghana, Iraq, Jordan, Kazakhstan, Kuwait, Kyrgyzstan, Lebanon, Malaysia, Mexico, Morocco, Nigeri, Pakistan, Palestine, Peru, Romania, Russian Federation, Rwanda, Slovenia, South Africa, Philippines, Poland, Thailand, Trinidad and Tobago, Tunisia, Turkey, Ukraine, Uruguay, Uzbekistan and Yemen.

"Polarization is typically maximized when there are two groups of equal size, while fragmentation is high when there are several small groups".

Figure A1: Level of trust in developed and developing countries

Source: Authors' calculations using WVS.