Transient isotachophoresis of highly saline trace metals under strong electroosmotic flow conditions

Asif RiazDoo Soo Chung

School of Chemistry,Seoul National University,Seoul, Korea

Transient isotachophoresis of highly saline tracemetals under strong electroosmotic flow conditions

Transient isotachophoresis (TITP) is usually performed under low-electroosmotic flow(EOF) conditions using a coated capillary or a low pH background electrolyte. We useda bare fused-silica capillary for TITP stacking of anionic complexes of some heavymetals under high-EOF conditions (pH 9.0). The sample component chloride as aleading electrolyte induced stacking by an isotachophoretic mechanism and thecomplexing agent 4-(2-pyridylazo) resorcinol (PAR) acted as a terminating electrolyte.The optimized background electrolyte was composed of 150 mM N-tris(hydrox-ymethyl)methyl-3-aminopropanesulfonic acid, 127 mM triethylamine, and 0.1 mM PARat pH 9.0. The strong EOF at pH 9.0 pulled the analytes against their mobilities towardthe outlet side, allowing a separation in the normal polarity mode. The stacking effi-ciency, reproducibility, analysis time, and sample loading capacity in coated and barecapillaries were compared. The stacking efficiency and reproducibility were higher andthe analysis time was shorter in the coated capillary. However, a larger volume of asample could be injected in the bare capillary to achieve detection limits comparable tothose for the coated one without compromising the resolution between the analytepeaks. The limits of detection (S/N = 3) were in the sub-ppb range for the selectedmetals (Fe21, 0.3 ppb; Ni21, 0.16 ppb; and Zn21, 0.8 ppb) in a standard saline samplewith 250 mM NaCl matrix. The proposed method was successfully applied to theanalysis of reference urine samples and human urine samples.

Keywords: Bare capillary / Trace metals / Transient isotachophoresis / Urine / 4-(2-Pyridylazo)resorcinol DOI 10.1002/elps.200406151

1 Introduction

Over the contemporary chromatographic techniques,capillary electrophoresis (CE) has unique advantages,such as simplicity, small volume sample/reagent require-ments, short analysis time, and low waste production [1].Since its introduction, CE has also gained special attentionfor metal ion analysis with its advantages of multielementanalysis and elemental speciation [2, 3]. However, CE suf-fers from a low concentration sensitivity for absorbancedetection because of the narrow (20–100 mm) inner capil-lary diameter being the optical path length [4]. Moreover,CE of a highly saline sample shows peak-broadening dueto a lower electric field across the sample zone, and realsaline samples are often subjected to tedious and time-consuming steps, such as preconcentration and matrix

removal. One facile way of overcoming the poor sensitivityand salt problem in CE is transient isotachophoresis (TITP)which provides on-line preconcentration and desalting [5–10]. The high concentration of highly mobile chloride ionsin a saline sample can induce isotachophoretic stacking ofsuitable anionic analytes yielding a several hundredfoldsensitivity enhancement [9, 11, 12].

Generally, TITP stacking is performed in a suppressedelectroosmotic flow (EOF) to minimize zone-broadeningdue to an EOF mismatch arising from variations in theconductivities of sample zones. In order to suppress theEOF, various strategies have been used. Krivánková andBocek [13] used a linear polyacrylamide-coated capillaryfor TITP stacking and Boden and Bachmann [12] usedcationic surfactants. For tandem isotachophoresis-CE aswell as for single-capillary TITP stacking, polytetra-fluoroethylene capillaries with a suppressed EOF are fre-quently used [11]. Although the low EOF in a coated cap-illary provides a shorter analysis time, high stacking effi-ciency, and good reproducibility, the process of coatingitself is a time-consuming and often expensive procedure.Furthermore, a coated capillary requires proper main-tenance and its lifetime is rather short compared to a barefused-silica capillary.

Correspondence: Prof. Doo Soo Chung, School of Chemistry,Seoul National University, Seoul 151-747, KoreaE-mail: [email protected]: 182-2-877-3025

Abbreviations: DCF, 2,7-dichlorofluorescein; PAR, 4-(2-pyridy-lazo) resorcinol; TAPS, N-tris(hydroxymethyl)methyl-3-amino-propanesulfonic acid; TEA, triethylamine; TITP, transient isota-chophoresis

668 Electrophoresis 2005, 26, 668–673

2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electrophoresis 2005, 26, 668–673 TITP of highly saline trace metals 669

In this report, we demonstrate that TITP stacking ofanions could be done in a bare fused-silica capillary usinga high-pH BGE. Although lower separation efficiencieswere expected, a longer sample plug could be injected toachieve detection limits similar to or better than those fora coated capillary [9]. With our method, a real salinesample could be analyzed for trace metals as their anioniccomplexes within 30 min without sample pretreatmentsteps like desalting or preconcentration.

2 Materials and methods

2.1 Chemicals

N-Tris(hydroxymethyl)methyl-3-aminopropanesulfonicacid (TAPS) was purchased from Sigma (St. Louis, MO,USA). Highly pure NaCl (99.999%) and 4-(2-pyridylazo)resorcinol (PAR) were from Aldrich (Milwaukee, WI, USA).2,7-Dichlorofluorescein (DCF) was from Merck (Darm-stadt, Germany). Sodium fluorescein, sulfate, and nitratesalts of metals were from Junsei (Tokyo, Japan). Trieth-ylamine (TEA) was from Yakuri (Osaka, Japan). Allreagents were of analytical grade or better and used asreceived. Reference materials for trace metals in urine (ME28351 and ME 28352) were obtained from Promochem(Wesel, Germany). Stock solutions of analytes were pre-pared as described previously [9]. A standard solution wasmade by adding appropriate volumes of the analyte stocksolutions to 50 mL of the matrix solution composed of 1 M

NaCl and 2 mM PAR, and then by diluting to 200 mL withwater. Similarly, a urine sample was prepared by mixing anappropriate volume of urine and 50 mL of the matrix solu-tion then by diluting to 200 mL with water. TEA/TAPS BGEsof different concentrations at pH 9.0 containing 0.1 mM

PAR were used. All solutions were filtered through a0.45 mm syringe filter (Whatman, Clifton, NJ, USA) anddegassed by sonication prior to CE runs.

2.2 Capillary electrophoresis

CE analyses were performed on a P/ACE 5500 system(Beckman, Fullerton, CA, USA). A 57 cm bare fused-silicacapillary (50 mm ID, 365 mm OD, 50 cm to the detector)was from Polymicro Technologies (Phoenix, AZ, USA).A new bare capillary was rinsed with 1 M NaOH for 20 minat 20 psi (= 1.386105 Pa) and this procedure was repe-ated every day. A 57 cm m-Sil-FC coated capillary (50 mmID, 365 mm OD, 50 cm to the detector) was purchasedfrom Agilent (Waldbronn, Germany) and used as descri-bed previously [9]. Samples were introduced hydrody-namically at 0.5 psi. Electrophoresis was carried out in theconstant current or voltage (details in the text) mode ofnormal polarity for a bare capillary and of reverse polarityfor a coated capillary. The capillary was thermostated at

207C during electrophoresis. Absorbance at 500 nm wasmonitored for the detection of metal-PAR complexes,fluorescein, and DCF.

3 Results and discussion

3.1 TITP in a bare and a coated capillary

The formation of stable anionic metal-PAR complexes atpH . 5 gives an advantage in examining a bare capillary forTITP where the benefit of the strong EOF could be used todrive the anionic analytes to the detector. Previously, TITPstacking of highly saline metal-PAR complexes was donein a coated capillary under negligible EOF conditions [9].These conditions were used as a guideline to obtain opti-mum conditions for TITP stacking in a bare capillary, inwhich an electroosmotic mobility of 23261028 m2/V?swas observed under the experimental conditions used fora coated capillary. This electroosmotic mobility is greaterthan the electrophoretic mobilities of the analytes butlower than the value 25261028 m2/V?s of chloride. Theordering of the mobilities shown below was obtained in abare capillary which enabled the anionic analytes to bestacked by TITP and to migrate towards the detector.

umLu . umEOFu . umau . umTu (1)

where mEOF, ma, mL, and mT are the mobilities of the EOF, ananalyte, a leading electrolyte, and a terminating electro-lyte, respectively. The velocity of EOF, vEOF, under anelectric field E across a capillary filled with a liquid medi-um is given by

vEOF = Exe/Z (2)

where x is the zeta potential, e is the dielectric constant,and Z is the viscosity of the liquid medium. TITP involvesan initial ITP step with a constant velocity given by

vITP = 2mE (3)

Although the presence of EOF in a bare capillary does notdisturb the formation of ITP zones [14], the term E inEq. (3), which is inversely proportional to the mobility m,induces a dissimilar vEOF for each analyte zone that resultsin longitudinal zone-broadening [15, 16]. Thus, the effi-ciency of an analyte stacked in a bare capillary is expect-ed to be lower than that in a coated capillary. In conven-tional ITP analysis the local vEOF cannot be neglected inrelation to the vITP, regardless of the self-correcting prop-erties of the ITP steady state [15]. However, in TITP, theinitial ITP steady state lasts for a limited time providingnarrower zones of analytes compared to the originalsample plug length and the local EOF disturbances canbe neglected for practical reasons. Peak widths of twosimple model compounds, fluorescein and DCF, stackedby TITP were compared in a coated and a bare capillaries


CE

and

CE

C

670 A. Riaz and D. S. Chung Electrophoresis 2005, 26, 668–673

using a BGE composed of 95 mM TAPS and 73 mM TEA atpH 9.0. For this system, chloride in the sample acted asthe leading electrolyte for the model compounds, whilethe BGE co-ion TAPS2 acted as the terminating electro-lyte with the least mobility (ca. 21261029 m2/V?s) [9, 10].Electrophoresis was performed with a constant drivingcurrent of 25 mA through the capillary. The separationconditions were the same except that a neutral surfactantFC-PN (0.02 vol%) was added to BGE and a reversepolarity electric field was applied across the coated cap-illary. Figure 1a shows that the peak width increased withthe sample plug length in the bare capillary and de-creased in the coated capillary. Note that TITP stackingdoes occur in the bare capillary in spite of the peaksbroader than those in the coated capillary for longersample plugs. The resolution between the peaks of thetwo model compounds decreased with the sample pluglength in the coated capillary. However, in the bare capil-lary, the resolution decreased up to a sample plug one-third of the effective length but changed little afterwards(Fig. 1b). When the plug length increased from 7% to 50%of the effective length, the resolution decreased from 13to 0.6 for the coated capillary and from 12 to 5.5 for thebare capillary. It is because the resolution R in CE of twoclosely eluting analytes is proportional to [1]:

R / 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

mAVE þ mEOFj jp (4)

Figure 1. Relative peak widths of DCF (squares) andfluorescein (circles), and the resolution between DCF andfluorescein peaks (triangles), with respect to the sampleplug length in a coated (open symbols) and a bare (closedsymbols) capillary. 1 mM of each analyte dissolved in250 mM NaCl. BGE, 95 mM TAPS, 73 mM TEA, pH 9.0;driving current, 25 mA.

where mAVE is the average mobility of the two analytes. Thus,for a sufficiently long sample plug, which leaves a shortenedseparation length, the resolution for anionic analytes havingnegative mAVE can be higher in a bare capillary than in acoated capillary. Therefore, in a bare capillary, despite theless efficient peaks, a longer sample plug could be injectedto improve the detection sensitivity to be comparable to oreven better than that in a coated capillary.

3.2 TITP of anionic complexes in a bare capillaryand optimization

Although PAR forms anionic complexes with a number ofmetal ions, our system selectively stacked Fe-PAR, Ni-PAR,and Zn-PAR whose mobilities fell in between the mobilitiesof the leading and the terminating electrolytes. The leadingtype of stacking effect of chloride ions in the sample on themetal-PAR complexes and the terminating behavior of free-PAR in the sample had been shown previously [9]. Theconcentrations of PAR in all the standard and urine sampleswere maintained at 0.5 mM to ensure the maximum com-plexation while 0.1 mM PAR was added to the BGE to mini-mize the dissociation of the PAR complexes during electro-phoresis. Excluding PAR from the BGE resulted in lowerpeak heights and distorted peak shapes (data not shown).Figure 2 shows TITPstacking of 1mM metal-PAR complexesin a 250 mM NaCl matrix containing 0.5 mM PAR, with a BGEcomposed of 95 mM TAPS, 73 mM TEA, and 0.1 mM PAR atpH 9.0, using a bare capillary. The three complex peakswere poorly separated, while they were baseline-separatedin a coated capillary under similar conditions. In order toimprove the separation, the TAPS concentration inBGE andthe BGE pH were optimized.

Figure 2. Electropherogram of 1 mM Zn21 (2), Fe21 (3),Ni21 (4), and free-PAR (1) in a matrix of 250 mM NaCl and0.5 mM PAR in a bare capillary. Injection, 252 s at 0.5 psi;BGE, 95 mM TAPS, 73 mM TEA, 0.1 mM PAR pH 9.0;applied voltage, 25 kV.



Figure 3a shows that the efficiencies of the complexpeaks improved substantially with the TAPS concentra-tion up to 150 mM. Further increase in the TAPS con-centration did not show a significant effect and ratherdegraded the efficiency, which fact can be attributed tothe Joule heating effect. Figure 3b demonstrates that theresolution improved with the TAPS concentration, but theresolution between the Fe-PAR and Ni-PAR peaksshowed little improvement beyond 175 mM TAPS. Con-sidering the analysis time as well as the resolution, 150 mM

TAPS was chosen as an optimal concentration. As the pHof BGE containing 150 mM TAPS was increased from 8.3to 9.3, the resolution between the Zn-PAR and Fe-PARpeaks improved while the resolution between the Fe-PARand Ni-PAR peaks decreased (data not shown). Theoverall optimum resolution was obtained at pH 9.0. The

Figure 3. (a) Peak widths at the half height of Zn-PAR(open diamonds), Fe-PAR (open squares), and Ni-PAR(open circles), and (b) the resolution between the complexpeaks: RZn-PAR,Fe-PAR (closed triangles) and RFe-PAR,Zn-PAR

(open triangles) peaks with respect to the TAPS con-centration in a BGE at pH 9.0. (c) Resolution between thecomplex peaks with respect to sample plug length: RZn-

PAR,Fe-PAR (closed triangles) and RFe-PAR,Zn-PAR (open trian-gles) with all BGEs containing 0.1 mM PAR at pH 9.0.Other conditions as in Fig. 2.

peak efficiencies at pH 9.0 were 2- to 6-fold higher thanthose at pH 8.3. Figure 3c shows the resolution betweenthe closely eluting complex peaks with respect to thesample plug length under the optimized conditions usingthe BGE composed of 150 mM TAPS, 127 mM TEA, and0.1 mM PAR at pH 9.0. We found that sample plugs up toone-third of the effective length could be injected whilemaintaining the baseline resolution (R . 1.5) between thecomplex peaks.

3.3 Sensitivity enhancement by TITP

Figure 4 compares electropherograms obtained in twodifferent modes of electrophoresis. Figure 4a is a conven-tional CE electropherogram of 20mM Fe21, Ni21, Zn21, DCF,and fluorescein, and 0.5 mM PAR in BGE injected for theshortest possible plug length of 1 s at 0.5 psi with ourinstrument. Figure 4b shows TITP stacking of 0.2 mM Fe21,Ni21, Zn21, DCF, and fluorescein in a matrix of 250 mM

NaCl. The injected sample plug was one-third of theeffective length. The sensitivity enhancement was nearly600-fold for Zn-PAR, 1000-fold for Fe-PAR, 430-fold for Ni-PAR, 550-fold for DCF, and 500-fold for fluorescein. Thepeak heights for the Zn-PAR and Fe-PAR complexes werequite a bit higher compared to other analytes in the sampledue to zinc and iron impurities in the reagents used to pre-pare 250 mM NaCl and 0.5 mM PAR. This was confirmedwith TITP stacking of blank standard samples, whichshowed substantial peak heights/areas for the two com-plexes (data not shown). The peak areas, after subtractingoff the blank peak areas, were used for quantification.

Figure 4. Electropherograms for (a) the normal CE modeinjecting a 20 mM sample in BGE for 1 s at 0.5 psi and (b)TITP stacking mode injecting a 0.2 mM sample in a 250 mM

NaCl and 0.5 mM PAR matrix for 480 s at 0.5 psi. Peaks:(2) Zn21, (3) Fe21, (4) Ni21, (5) DCF, (6) fluorescein, and(1) free-PAR. BGE containing 150 mM TAPS, 127 mM TEA,and 0.1 mM PAR at pH 9.0. Other conditions as in Fig. 2.


672 A. Riaz and D. S. Chung Electrophoresis 2005, 26, 668–673

3.4 Reproducibility and quantification

In a bare capillary, to obtain good reproducibility inmigration times, peak areas, and peak heights, a moremeticulous conditioning was required in contrast to theTITP analysis in a coated capillary. Each day a bare cap-illary was preconditioned with 1 M NaOH for 20 min at20 psi followed by 30 min rinsing with water at 20 psi.Before injecting a sample, the capillary was conditionedby rinsing at 20 psi using the following sequence: Waterfor 1 min, 0.1 M NaOH for 3 min, water for 3 min, and BGEfor 4 min. Since the large volumes of highly saline samplesmight change the composition of the BGE in the inlet andthe outlet vials, the BGE was replenished after eachrun. Figure 5 shows a trace of three overlaying electro-pherograms obtained in sequence. Relative standarddeviations (RSDs) in migration times were 0.1–0.3%, peakareas 0.3–4.0%, and peak heights 0.3–2.6% (n = 3). Day-to-day RSDs in migration times were 0.5–1.2%, peakareas 3.9–5.9%, and peak heights 1.1–4.9% (n = 3). Thusthe reproducibility obtained in a bare capillary with acareful conditioning was comparable to or better thanthose reported previously in a coated capillary [9], whereday-to-day RSDs in migration times and peak areas were0.7–1.4% (n = 3) and 3.7–9.6% (n = 4), respectively. Underthe optimized conditions the separation efficiencies in abare capillary (NFe-PAR, 140 000; NNi-PAR, 150 000; NZn-PAR,100 000; NDCF, 150 000) were lower than those in a coatedcapillary (NFFe-PAR, 380 000; NNi-PAR, 430 000; NZn-PAR,190 000; NDCF, 550 000) as expected. However, injecting alonger sample plug, the LODs (S/N = 3) obtained in a barecapillary with 480 s injection of a standard sample (Fe21,0.3 ppb; Ni21, 0.16 ppb; Zn21, 0.8 ppb; DCF, 2 ppb) couldbe improved up to 2.5-fold compared to those obtained in

Figure 5. Three overlaying electropherograms obtainedin sequence. Samples; 1 mM Zn21, Fe21, Ni21, DCF, andfluorescein in 250 mM NaCl, and 0.5 mM PAR. Injection,360 s at 0.5 psi. Other conditions as in Fig. 4b.

Table 1. LOD, dynamic range, linear correlation coeffi-cient (r), reproducibility in migration times (MT)and peak areas (PA), and efficiency

LODa) Dynamicrangeb)

r % RSD(n = 3)

Efficiencyc)

MT PA

DCF 2 ppb 5–2000 ppb 0.9998 0.15 0.3 1.56105

Ni-PAR 0.16 ppb 0.5–300 ppb 0.9998 0.10 0.3 1.56105

Fe-PAR 0.3 ppb 1–250 ppb 0.9980 0.08 3 1.46105

Zn-PAR 0.8 ppb 2–300 ppb 0.9990 0.08 4 1.06105

a) Sample matrix, 250 mM NaCl, 0.5 mM PAR; injection,480 s at 0.5 psi; other conditions as in Fig. 5

b) Injection of 180 s at 0.5 psi as a plug length for realsamples

c) Calculated for a 100 nM standard sample

a coated capillary with 60 s injection (Fe21, 0.7 ppb; Ni21,0.4 ppb; Zn21, 1.2 ppb; DCF, 3.6 ppb) [9]. Table 1 lists theLODs, the dynamic ranges, the linear correlation coeffi-cients (r), the reproducibility in migration times and peakareas, and the separation efficiencies in a bare capillary.

3.5 Application to real samples

Since excessive amounts of Zn21 and Fe21 in urine sam-ples overloaded the capillary with longer sample plugs, ashorter sample plug (10% of the effective length) waschosen for the analysis of certified/real urine samples.First, certified reference materials for high level(ME28351) and low level (ME28352) trace metals in urinewere analyzed. For quantification an internal standardDCF (0.1 mM) was also added to the sample. Figures 6a

Figure 6. Electropherograms of various urine samplescontaining 250 mM NaCl, 0.5 mM PAR, and 0.1 mM DCF.(a) 1:20 diluted ME 28351, (b) 1:10 diluted ME 28352, and(c) 1:10 diluted real urine sample. Injection, 180 s at0.5 psi. Other conditions as in Fig. 4b.



Table 2. Analysis of the reference material for urine

Assignedconcentration

Confidence range(2s)

Concentrationby our method

(a) ME28351Ni 24 ppb 18–30 ppb 24 6 2* ppbFe 700 ppb 530–870 ppb 745 6 70* ppbZn 1700 ppb 1300–2100 ppb 1550 6 99* ppb

(b) ME28352Ni 14.8 ppb 11.2–18.4 ppb 14.0 6 0.3* ppbFe 204 ppb 154–254 ppb 209 6 21* ppbZn 830 ppb 630–1030 ppb 790 6 93* ppb

* Standard deviation (s) of three aliquots analyzedsequentially

and b show electropherograms of 1:20-diluted ME 28351and 1:10-diluted ME 28352, respectively. The reproduci-bility for inter-day and intra-day urine analyses in migra-tion times were in the respective ranges of 0.3–0.5% RSDand 0.9–1.0% RSD (n = 3). The amounts of the threemetals obtained experimentally were in agreement withthe assigned values as listed in Table 2. TITP stacking in abare capillary was applied to human urine as Fig. 6cdepicts an electropherogram of a 1:10-diluted urinesample. The reproducibility in migration times of theidentified peaks was 0.5–0.7% RSD. The trace metal ionswere spiked (1–50 ppb) in aliquots of a selected humanurine sample and the amount of the recovered metals wasdetermined under the conditions described in Fig. 6. Thespiked amounts were successfully recovered in the 91–106% range.

4 Concluding remarks

More than 400-fold sensitivity enhancement by TITPstacking of anionic complexes of Fe21, Ni21, and Zn21

with PAR in a highly saline sample was demonstrated in abare fused-silica capillary. A BGE of high pH (9.0) pro-vided a sufficient EOF to pull the anionic analytes towardthe capillary outlet under a normal polarity, where theanalytes were detected by absorbance. Optimizing the

separation conditions, the LODs (S/N = 3) in the sub-ppbrange were obtained for the selected metal complexes(Fe21, 0.3 ppb; Ni21, 0.16 ppb; Zn21, 0.8 ppb). The meth-od was shown to be applicable to the analysis of theselected metal ions in real samples of human urine with-out desalting or off-line preconcentration, whereby anal-ysis could be done within 30 min with the advantage ofusing a bare fused-silica capillary.

This work was supported by the R&D Program for FusionStrategy of Advanced Technologies.

Received July 6, 2004

5 References

[1] Jorgenson, J. W., Lukacs, K. D., Anal. Chem. 1981, 53,1298–1302.

[2] Liu, B.-F., Liu, L.-B., Cheng, J.-K., J. Chromatogr. A 1999,834, 277–308.

[3] Timerbaev, A. R., Analyst 2001, 126, 964–981.[4] Baker, D. R., Capillary Electrophoresis, John Wiley, New

York 1995.[5] Schoots, A. C., Verheggen, T. P., Vries, P. M. D., Everaearts,

E. M., Clin. Chem. 1990, 36, 435–440.[6] Gebauer, P., Thormann, W., Bocek, P., J. Chromatogr. 1992,

608, 47–57.[7] Krivánková, L., Pantucková, P., Gebauer, P., Bocek, P.,

Caslavska, J., Thormann, W., Electrophoresis 2003, 24,505–517.

[8] Bocek, P., Deml, M., Gebauer, P., Dolník, V., Analytical Iso-tachophoresis, VCH Verlagsgesellschaft, Weinheim 1988.

[9] Riaz, A., Kim, B., Chung, D. S., Electrophoresis 2003, 24,2788–2795.

[10] Shim, S. H., Riaz, A., Choi, K. W., Chung, D. S., Electropho-resis 2003, 24, 1603–1611.

[11] Krivánková, L., Gebauer, P., Bocek, P., J. Chromatogr. A1995, 716, 35–48.

[12] Boden, J., Bachmann, K., J. Chromatogr. A 1996, 734, 319–330.

[13] Krivánková, L., Pantucková, P., Bocek, P., J. Chromatogr. A1999, 838, 55–70.

[14] Thomann, W., J. Chromatogr. 1990, 516, 211–217.[15] Reijenga, J. C., Aben, G. V. A., Verheggen, T. P. E. M., Ever-

aerts, F. M., J. Chromatogr. 1983, 260, 241–254.[16] Caslavska, J., Thormann, W., J. Chromatogr. A 1997, 772,

3–17.


Transient isotachophoresis of highly saline trace metals under strong electroosmotic flow conditions

Documents

Transcript of Transient isotachophoresis of highly saline trace metals under strong electroosmotic flow conditions