Electrolyte system strategies for anionic isotachophoresis with electrospray-ionization mass-spectrometric detection. 1. Regular isotachophoresis and free-acid isotachophoresis

3072 Electrophoresis 2013, 34, 3072–3078

Zdena MalaPetr GebauerPetr Bocek

Institute of Analytical Chemistry,Academy of Sciences of theCzech Republic, Brno, CzechRepublic

Received June 27, 2013Revised July 17, 2013Accepted July 17, 2013

Research Article

Electrolyte system strategies for anionicisotachophoresis with electrospray-ionization mass-spectrometric detection.1. Regular isotachophoresis and free-acidisotachophoresis.

The subject of this work is the definition of a simple model based on general ITP theorythat allows describing and predicting the behavior of ITP systems compatible with ESI-MSdetection. The model is exemplified by anionic ITP of weak acids that represent an interest-ing potential application field of ITP-ESI-MS. Suitable ESI-compatible electrolyte systemsof very simple composition are proposed including a special free-acid ITP arrangement.The properties of these systems are discussed using illustrative diagrams of their stackingwindows. The use of anionic ITP-ESI-MS in negative-ion ESI mode is reported for thefirst time and its suitability for sensitive trace analysis is demonstrated. The presentedITP-ESI-MS application example comprises a free-acid ITP system formed of formic andpropionic acids and direct injection analysis of ibuprofen and diclofenac in waters withquantitation limits of the order 10−10 M.

Keywords:

Diclofenac / ESI-MS detection / Ibuprofen / Isotachophoresis / Water analysisDOI 10.1002/elps.201300292

1 Introduction

The combination of CE and MS detection has already becomea routine technique with available commercial instrumenta-tion. Its high analytical importance is confirmed by the num-ber of published successful applications [1–3], despite the factthat its basic sensitivity is lower compared to LC. The mostcommonly used arrangement is CZE with ESI-MS detection.The selection of operational electrolytes for this combinationis not easy because the entire system should be composedonly of volatile substances that are compatible with ESI-MS.In practice, the composition of the electrolyte systems is re-stricted to simple organic acids and their ammonium salts.This is also a limitation for potential stacking procedures thatcan be used for preconcentration before the CZE step that isnaturally a dilution/dispersion one.

Our recent results [4] indicate that the problem has asuccessful solution, consisting in the application of the ITPtechnique in a permanent ITP-ESI-MS arrangement. Thereare many applications that have used transient ITP as an

Correspondence: Dr. Petr Gebauer, Institute of Analytical Chem-istry, Academy of Sciences of the Czech Republic, Veverı 97, CZ-602 00 Brno, Czech RepublicE-mail: [email protected]: +420-532290238

Abbreviations: Dic, diclofenac; HFor, formic acid; HPro, pro-pionic acid; Ibu, ibuprofen

online preconcentration method in CZE-ESI-MS [1, 2]. Theadvantage of permanent ITP is given by the fact that the dis-persion step is avoided and the analytes remain stacked whenthey reach the detector. The ITP-MS technique has been al-ready applied although the number of respective papers re-mained low. Most of them were aimed at cationic ITP-MSand the analyzed substances were quaternary phosphoniumions [5], peptides and proteins [6–10], vitamins [10], and pes-ticides [4]. Anionic ITP-MS was reported only by Zhao etal. [11–13] who used a reversed (EOF driven) arrangementin the positive-ion ESI mode for the analysis of monophos-phate nucleosides, dinucleotides, and similar compounds.The reached sensitivities of the individual published appli-cations cannot be directly compared as they strongly dependon the used instrumentation and applied procedure but theresults with cationic ITP allow the conclusion that the LODmay typically reach the 10−9–10−10 M range.

The basic feature of ITP is concentration and permanentstacking of analytes. It can enhance the sensitivity of CE-ESI-MS analyses by orders of magnitude and allow its use forsuccessful trace analysis. This is particularly important foranalyses of anionic substances that may require working inthe negative-ion ESI mode that is generally considered to beless sensitive and reproducible [11]. An interesting exampleof a suitable analytical problem is the determination of phar-maceuticals in water. It is well known that large amountsof pharmaceuticals are widely used for treatment of humansand animals. Residues of them are getting to wastewaters,where a part is eliminated by sewage treatment plant and

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Electrophoresis 2013, 34, 3072–3078 CE and CEC 3073

a part browses to surface water. This represents a seriousenvironmental problem and requires methods that allowsufficiently sensitive monitoring. The concentration of in-dividual drugs in surface waters ranges from units to sev-eral hundreds of nanograms per milliliter. Examples are di-clofenac (Dic) and ibuprofen (Ibu), anti-inflammatory drugsfrequently used in many fields according their analgesic andantipyretic effects. They belong to analytes that occur in wa-ters regularly and in considerable concentrations. For theirdetermination various separation methods were used as GC-MS [14–16], LC-MS [17–19], CE-UV [20, 21], CE-MS [22], andCE-C4D [23]. These methods are usually combined with off-line concentration steps as solid-phase or liquid-phase extrac-tion to reach the required LODs.

Prerequisite of a successful ITP-ESI-MS application is tofind an electrolyte system providing a suitable and sufficientlyselective window for ITP concentration and stacking of theanalytes of interest. Moreover, the electrolytes used shouldcomply with ESI requirements and therefore the optimiza-tion procedure of an ITP separation with ESI-MS detectionis a quite complex task. A comprehensive analysis of thisproblem that would allow, using the general ITP theory, todescribe and predict the behavior of suitable systems includ-ing selected model examples and assessment of their ITPstacking windows for practice, is missing so far. The solutionof these tasks, oriented to anionic ITP-ESI-MS, is the subjectof this work.

2 Materials and methods

2.1 Chemicals

All chemicals, ibuprofen, diclofenac, formic acid (HFor), pro-pionic acid (HPro) (all from Sigma, St. Louis, MO, USA),acetic acid (from Fluka, Buchs, Switzerland) were of analyti-cal reagent grade and methanol (Fisher Chemical, Loughbor-ough, UK) was optimal LC/MS grade. Deionized water froman Aqua Purificator G7749 (Miele, Gutersloh, Germany) wasused for the preparation of stock solutions of the BGE compo-nents and for the preparation of stock solutions of Ibu and Dicas the samples. The working solutions were prepared freshevery morning. All solutions were filtered prior to use withCronus nylon 25 mm, 0.45 �m syringe filters (SMI-LabHut,Churcham, UK). Drinking and river water were used withoutany pretreatment.

2.2 Instrumentation

The ITP experiments were performed using a 7100 CE sys-tem (Agilent Technologies, Waldbronn, Germany). A barefused-silica capillary (100 �m id, 85 cm length, Agilent Tech-nologies) was installed in the cartridge and kept at constanttemperature of 20�C. The samples were injected by pressure.The capillary was flushed and filled with the leading elec-trolyte and terminating electrolyte was placed into the inlet

vial. The EOF velocity was not measured, as it was not neces-sary to know its exact value. Its constant magnitude was mon-itored by watching the reproducibility of detection times thatwas very good in all experiments (the intraday variability of de-tection times was below 2%). All separations were performedusing a running voltage of –20 kV. No additional pressurewas applied to the capillary to compensate ESI suction.

The CE-MS coupling was done via ESI using a com-mercial CE-ESI-MS interface (Agilent Technologies). A coax-ial sheath liquid flow, composed of 1% acetic acid in 50%methanol, was provided during analyses. It was supplied viasplitter by a system based on a 1200 series isocratic pump(Agilent Technologies) at a flow rate of 8 �L/min. The 6130single quadrupole mass spectrometer (Agilent Technologies)was operated in single ion monitoring negative mode, usingthe following operation parameters: capillary voltage 3500 Vnegative; nebulizer pressure 10 psi; drying gas flow 10 L/min;drying gas temperature 200�C. [M-H]− molecules were mon-itored and the optimum fragmentor value was determinedexperimentally. The peak width was set to 0.120 min, pro-viding a cycle time of 0.72 s. Nitrogen gas was supplied byan N110DR generator (Peak Scientific, Frankfurt, Germany).The CE and MS instruments were controlled and data acqui-sition performed by ChemStation software (Agilent).

2.3 Calculations

The pKa and mobility values used for calculations (see Table 1)were taken from the database of the newest version of thefreeware program Simul 5 Complex by Gas et al. [24, 25]that also served for the simulations of the behavior of theinvestigated systems. The pKa values [26] of Dic and Ibu andthe ionic mobility of Ibu [27] were taken from literature. Theionic mobility of Dic was estimated from that of ibuprofenand from the ratio of their molecular masses.

The properties of ITP systems were taken from simu-lations performed the newest version of the freeware pro-gram Simul 5 Complex by Gas et al. [24, 25]. All calcula-tions and simulations were performed without ionic strengthcorrections that were proved not to have major qualitativeeffects on the results.

Table 1. pKa and ionic mobility values of used electrolytecomponents and model analytes (for details, seeSection 2.3)

pKa Ionic mobility(10−9 m2V−1s−1)

Formic acid 3.75 56.6Acetic acid 4.76 42.4Propionic acid 4.87 37.1Lactic acid 3.86 36.5Ammonium 9.25 76.2Ibuprofen 4.38 20.9Diclofenac 4.24 15


3074 Z. Mala et al. Electrophoresis 2013, 34, 3072–3078

Figure 1. Setup schemes of ITP systems: (A) regular ITP, (B) free-acid ITP. For explanation see text.

3 Results and discussion

3.1 Theoretical model

A functional ITP electrolyte system must comprise a per-manently sharp (self-sharpening) boundary migrating in thesame direction as the potential analytes. To create such asystem for anionic analysis of weak acids, we need a pair ofanionogenic substances, typically also weak acids. Figure 1Abrings the scheme of a regular anionic ITP system. The lead-ing zone L is formed by weak acid HA, the terminating zone Tcontains the weak acid HB and the counter-ionic speciespresent in all zones is a weak base R ionizable to RH+. Thecondition to form an ITP system is equivalent to that of for-mation of a permanently sharp (self-sharpening) boundarybetween zones L and T and can be expressed in the simpleform [28]:

uB,T < uA,T and uB,L < uA,L (1)

where ui,j is the effective mobility of substance i in zone j(unsigned quantity). The velocity of the ITP boundary vITP

expresses equal migration velocity of zones L and T, vITP =uA,L i/�L = uB,T i/�T, where i is the electric current densityand �j is the specific conductivity of zone j.

For application with MS we consider ITP to be par-ticularly useful when operated in the so-called peak modewhere minor sample components are concentrated andpermanently stacked at the system’s sharp ITP boundary.The condition for an analyte X (weak or strong acid HX) tobe stacked at the ITP boundary is based on comparison ofeffective mobilities:

uB,T < uX,T and uX,L < uA,L (2)

By expressing the effective mobilities of monohydricweak acids as ui,j = ui Ki/(Ki + cH,j), where ui is the mo-bility of ion i, Ki is the dissociation constant of acid i and cH,j

is the H+ concentration in zone j, we get condition 2 in theform:

uB1 + cH,T/KHX

1 + cH,T/KHB< uX < uA

1 + cH,L/KHX

1 + cH,L/KHA(3)

that defines the stacking window for analysis of weak orstrong acids HX in a given ITP system. All values for theleading zone are known as they are given by the selectedcomposition. The concentrations of all components in theterminating zone are completely determined by the compo-sition of the leading zone as they are automatically adjustedaccording to the Kohlrausch regulating law [29]. They can becalculated and accessed using publicly available simulationprograms (see Section 2.3).

As is seen from condition 3, it is predominantly the pH(H+ concentration) that controls the system’s operational re-gion (mobility window) for stacking of weak acids. The pres-ence of the (buffering) counterion allows here to play with thepH of the leading zone depending of course on the particularpK values of both HA and R. When assuming equality inboth relationships of conditions 2 and 3 we get two equationsexpressing the dissociation constant of analyte HX as func-tions of its ionic mobility and of parameters of the leading orterminating zone:

KHX = fL(uX,uA,L,cH,L) (4a)

KHX = fT(uX,uB,T,cH,T) (4b)

Using these equations we can calculate two curves thatare formed by points [pKHX, uX] of weak acids for whichthe stacking conditions 2 are at the limits of validity towardthe leading or terminating zones, respectively. In a graphicalpresentation using a pKHX versus uX network, these curves(stacking contours) demarcate the area of stackable monohy-dric weak acids in a given system.

When the concentration of the counterion is zero, weobtain a special case of ITP where the only counter-cationicspecies in all migrating zones is H+ released by the disso-ciation of the acid(s). The scheme of such a system is inFig. 1B where the leading zone L is formed by weak acid HAand the terminating zone T contains only the weak acid HB.Systems of this kind were described already long ago [30]and later classified as buffer free [31], based on the conceptthat the counter-ionic species that is considered to ensurethe buffering of the system is missing. It has been howeversimultaneously shown [31] that such arrangement providesstable ITP systems, indicating that buffering (which is noth-ing else that self-protection against unwanted migrating pHdiscontinuities) is ensured despite the missing counterion.We therefore believe that the most concise way to talk aboutsuch a system according to present knowledge is to call itfree-acid ITP.

A drawback of a free-acid system is its rigidity wherethe only parameter that can be changed is the concentrationof HA in the leading zone (the concentration of HB in the



terminating zone is automatically adjusted). Therefore, onlycH,L and by this also cH,T can be somewhat varied that doesnot allow much effective changes of the system’s mobilitywindow and thus tuning of its selectivity. But free-acid ITPsystems have a great benefit given by their extreme simplicityand this makes them well suitable for use with ESI-MS.

3.2 Systems for analysis of weak acids by anionic

ITP-ESI-MS

Most typical anionic analytes in CE are weak acids of low-molecular mass, with pKa values in the acidic to neutralregion. For their ITP analysis, the leading and terminatingsubstances should be sought for among substances of samekind but requirement of compatibility with ESI-MS dramat-ically narrows the spectrum of available species. Our basicselection represents the most common substances that wereused in the majority of papers on CZE-ESI-MS, CZE-transientITP-ESI-MS, and ITP-ESI-MS [1–3] and comprises acetic acidand HFor with ammonium as the potential counterion. Toget more flexibility, we included also HPro and lactic acid aspotential system components. From the pKa and ionic mo-bilities of the four acids (given in Table 1) it is seen that theoperational region of ITP systems composed of these acidsstarts at approximately pH 2.5 and that all are fully ionizedabove approximately pH 6. Simultaneous use of acetic andlactic acid between pH 5 and 6 should be avoided becausehere their mobility reversal [32] proceeds that brings the dan-ger of potential formation of mixed zones [33] and/or hybridboundaries [34, 35].

Figure 2 shows the stacking contours for ITP zones ofall four acids for the system with 10 mM HFor as the leadercontaining 5 mM NH4

+ at pH 3.78 as the counterion. Notethat the contours are calculated for the steady-state zone con-centrations as adjusted behind the indicated leading zone (inFig. 2, e.g. the values are: 7.34 mM and pH 3.83, 8.67 mM andpH 4.41, and 7.89 mM and pH 4.45 for the zones of Lac, Ac,and Pro, respectively). Each contour has two potential mean-

Figure 2. Calculated stacking contours of propionate (Pro), ac-etate (Ac), lactate (Lac), and formate (For) in an ITP system with10 mM HFor and 5 mM NH4

+ in the leading zone. The contoursexpress the pKHX versus uX (in 10−9 m2V−1s−1 units) dependencefor equality in condition 2 as explained in detail in the text.

ings: when the respective zone is considered as the leader,then the area above the contour marks the points of analytesthat are stacked behind this leading zone. Vice versa, if thezone should serve as the terminator, then the points of ana-lytes that are stacked in front of this zone lie in the area belowthe contour. For a particular system formed by a selected pairof leading and terminating zones, the ITP stacking windowis defined by the area between the contours of both zones.The selection must be in practice done so to get the pointsof analytes of interest inside of the stacking window. For thesystem of zones in Fig. 2, the contours do not intersect andtherefore it is possible to set up an ITP system using any pairof the four acids. At somewhat higher pH zone values (whenincreasing, e.g. the ammonium concentration in the leadingzone to 8 mM) there is a shift of all contours towards higherionic mobilities and a pronounced change of the system’sselectivity (data not shown). The use of the zones of Lac asleader and Ac as terminator is not advisable here as the con-tours of Lac and Ac are crossing and therefore the stackingwindow is limited only to the area between the two contoursabove the intersection point.

Figure 3 demonstrates the dependence of the ITP stack-ing window on pH (given by the concentration of the counte-rion in the leading zone), for systems with 10 mM HFor andvarying concentration of NH4

+ in the leading zone and HProas the terminator. For 5 mM ammonium (pHL = 3.78), arelatively broad stacking window is obtained (gray area be-tween the contours, Fig. 3B). When the ammonium concen-tration in the leading zone is increased to 8 mM (pHL =4.37, Fig. 3C), the stacking window moves toward highermobilities and this continues together with a shift to higherpKa values when proceeding to 10 mM ammonium (pHL =6.50, Fig. 3D). For the latter value the stacking window

Figure 3. Calculated ITP stacking windows in ITP systems with10 mM HFor in the leading zone and HPro in the terminatingzone, for (A) 0 mM, (B) 5 mM, (C) 8 mM, and (D) 10 mM NH4

+

in the leading zone. The stacking contours (curves expressingequality in condition 2) for the leading and terminating zone aremarked For and Pro, respectively. The stacking windows (areas ofstackable monohydric weak acids of given pKHX and uX) betweenthe stacking contours are marked in gray. Mobility values are in10−9 m2V−1s−1 units.



indicates separation at full ionization (i.e. according to ionicmobilities) and remains similar with further increasing of theammonium concentration (data not shown). Change of theconcentration of formate in the leading zone has only slighteffect on the stacking window.

Figure 3A shows the stacking window for a free-acidITP system where the leading zone contains only 10 mMHFor and no ammonium. As can be seen, the stacking win-dow of this system is narrower than in the preceding casesand shifted toward lower ionic mobility values. It can beonly modestly changed in the low-pKa-area by change of theHFor concentration in the leading zone (data not shown).Despite this, its high concentration power and very simplecomposition make this system useful for particular separa-tion tasks by ITP-ESI-MS.

3.3 Application example: Analysis of

pharmaceuticals in waters

To demonstrate the practical applicability of the theoreticalmodel and proposed systems to sensitive analysis of anionsby ITP-ESI-MS, we selected the particular analytical problemmentioned in the Introduction, namely the determination ofsome pharmaceuticals in water. The model analytes selectedin this work were diclofenac and ibuprofen. Both substancesare weak acids and their pKa values (see Table 1) indicatethat they are partially or fully ionized at pH values above4. Also both Ibu and Dic have low ionic mobilities and thecourse of their mobility curves indicates that only HPro atpH values below 4 seems to be a suitable terminator. Basedon preliminary experiments, we selected for their analysisthe free-acid system consisting of 10 mM HFor as the lead-ing zone and HPro as the terminator. The effective mobilityof propionate in the terminating zone is very low (uPro,T =1.6 × 10−9 m2V−1s−1, pHT = 3.53) due to the acidic natureof the system and this value is sufficient to ensure stackingof both analytes despite their low ionization degree. Figure 4shows the stacking-window diagram of the system that in-cludes pKHX–uX points of both analytes as well as those ofthe leading and terminating substances (lying naturally onthe respective stacking contour). According to the diagram,ITP stacking of both analytes is ensured in this system astheir points are in the gray area. The analytes are stacked atthe same ITP boundary and their zones therefore enter thedetector at the same time. This fact, however, does not repre-sent a problem in this case as their signals could be monitoredsimultaneously in the single ion monitoring mode.

Figure 5 presents orientation analyses performed for a10−7 M standard solution of Ibu and Dic (panel A), drink-ing water (panel B), and drinking water spiked with 10−7 MIbu and Dic (panel C). It can be seen that the used systemprovides effective stacking and well evaluable sharp peaks ofthe analytes both in standard solution and in drinking waterdespite the fact that detection was performed in the less rec-ommended negative-ion ESI mode, necessary here to get Ibuionized [22].

Figure 4. Calculated ITP stacking window of the free-acid ITP sys-tem with 10 mM HFor in the leading zone and HPro in the termi-nating zone. The shown pKHX–uX points relate to the substancesas indicated. The meaning of the diagram is the same as in Fig. 3.

Figure 6 demonstrates the sensitivity limits of the methodwith real samples. Drinking water spiked with 10−10 M of Ibu(20 ng/L) and Dic (30 ng/L) (panel A) provided well-developedpeaks and a sensitivity around the LOQ. The analysis of an un-treated river water sample (panel B) showed presence of bothIbu and Dic in concentrations not far below 10−10 M, whereasin a sample of water taken from a small stream (panel C) theamount of Ibu and Dic found was approximately ten timeslower, being near the LOD of the method. Untreated drinkingwater (panel D) did not show detectable amounts of any of theanalytes. We cannot fully exclude that the waters containedalso other weak acids with identical m/z values that contributeto the detected signals. This was however most probably notthe case here as the ratios of the signals obtained at twoconsiderably different fragmentor values were same for bothmodel and real samples. The reached sensitivity indicatesthat the described method is suitable for fast monitoring ofdrug levels in surface waters and output water from wastew-ater treatment plants. We have not performed full validationof the method at the present stage but the results indicate agood potential to get satisfactory reproducibility at the shownLOQ levels. Although the analyte peaks are quite narrow dueto efficient ITP stacking, even with the simple MS instru-ment used we were able to get 20–30 points per peak withoutusing peak width/cycle time at the limits of the instrument.This does not guarantee extreme reproducibility [36] but theproblem is very strongly compensated by the high sensitiv-ity enhancement of ITP and should become less importantwhen using devices with high sampling frequency.

4 Concluding remarks

The stacking capability of ITP systems allows, under prop-erly selected conditions, to increase the sensitivity of elec-trophoretic analyses by several orders of magnitude. Withefficient ITP stacking much larger sample amounts may be



Figure 5. Experimental records ofnegative single ion monitoring sig-nals for ibuprofen (Ibu, m/z 205,upper trace) and diclofenac (Dic,m/z 294, lower trace) of ITP-MSanalyses in a system with 10 mMformic acid in the leading zone and10 mM propionic acid as the ter-minating electrolyte. The samplewas: (A) a 10−7 M solution of Ibuand Dic in 2 mM propionic acid, (B)untreated drinking water, and (C)drinking water spiked with 10−7 MIbu and Dic. Sample injection pro-ceeded by a pressure of 10 mbarfor 5 s, the fragmentor was 130 forIbu and 100 for Dic.

introduced while the analytes still reach the detector focusedto narrow peaks. This offers the principal possibility to de-crease the concentration LOD in any CE-MS instrumentationwithout the necessity of changing the hardware or workingconditions. The combination of ITP with ESI-MS is a verypromising technique, e.g. in the analysis of weak acids. Theselection of operational electrolytes for this method is noteasy as the entire system should consist of volatile substancescompatible with ESI. An efficient strategy based on theo-retical predictions is therefore of essential importance. Thetheoretical definition of ITP stacking windows and stackingcontours defined by Eq. (4a, b) offers an effective way to theprediction of experimental results. Based on the theory, func-tional electrolyte systems are proposed that can be used forITP-ESI-MS analysis of anions in acidic to neutral medium.The designed electrolyte systems have very simple composi-tion and are based on formic, acetic, propionic, and/or lacticacids with ammonium as counterion or even without thiscounterion (free-acid ITP mode). The applicability of the pro-

posed electrolytes is demonstrated on the example of analysisof ibuprofen and diclofenac in various water samples usinga free-acid ITP system composed of formic and propionicacids as the leader and terminator, respectively. With directsample injection, the method provides LOQs on the level10−10 M, corresponding to concentrations of 20 ng ibuprofenand 30 ng diclofenac per liter. This makes this method not re-quiring laborious sample pretreatment suitable for fast moni-toring of drug levels in surface waters and output waters fromwastewater treatment plants. The shown experiments repre-sent the first published analysis of anions by ITP-ESI-MS inthe negative ESI mode and demonstrate the potential of thistechnique for sensitive trace analysis. The work on advanceddesigns of systems for anionic ITP-ESI-MS is in progress.

We gratefully acknowledge support by the Grant Agency ofthe Czech Republic (P206/13/5762) and by Institutional supportRVO:68081715 of the Academy of Sciences of the Czech Republic.

The authors have declared no conflict of interest.

Figure 6. Experimental records of negative single ion monitoring signals for ibuprofen (Ibu, m/z 205, upper trace) and diclofenac (Dic, m/z294, lower trace) of ITP-MS analyses in a system with 10 mM formic acid in the leading zone and 10 mM propionic acid as the terminatingelectrolyte. The sample was: (A) drinking water spiked with 10−10 M Ibu and Dic, (B) untreated river water, (C) untreated water from asmall stream, (D) untreated drinking water. Sample injection proceeded by a pressure of 100 mbar for 150 s, the fragmentor was 130 forIbu and 100 for Dic.



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Electrolyte system strategies for anionic isotachophoresis with electrospray-ionization mass-spectrometric detection. 1. Regular isotachophoresis and free-acid isotachophoresis