Anal. Chem. 1997, 69, 2187-2192
Interface for Coupling Capillary Electrophoresis to Inductively Coupled Plasma and On-Column Concentration Technique Erwen Mei,*,†,§ Hideki Ichihashi,† Wenfang Gu,† and Shin-ichi Yamasaki‡
National Institute of Agro-Environmental Sciences, 3-1-1, Kannondai, Tsukuba, 305 Japan, and Faculty of Agriculture, Tohoku University, 1-1 Tsutumidori-Amamiyamachi, Aoba-Ku, Sendai, 981 Japan
A new interface for coupling capillary electrophoresis (CE) to inductively coupled plasma (ICP) was developed. The interface was built outside and independent of the nebulizer and could be easily connected with a microconcentric nebulizer (MCN) as well as conventional pneumatic nebulizers. An on-column concentration technique was used to increase the sensitivity and to enhance the resolution of the system of capillary electrophoresis-inductively coupled plasma atomic emission spectrometry (CE-ICPAES). By doing this, it was possible to analyze 1 µg/mL of total chromium (prepared with K2Cr2O7 and CrCl3) and 1 µg/mL of copper consisting of Cu2+ and Cu(EDTA)2with good spectroscopic intensity and efficient separation. Detection limits of 18 elements for MCN-ICP-AES coupled with CE were assessed by continuous sample introduction without applying high voltage and were found to be 1-4 times higher than those typically obtained by using MCNICP-AES for elemental analysis (without connection to the CE interface). When the on-column concentration technique was used, the sensitivities and separations were further improved by increasing the amount of sample. A simple electrolyte (0.05 M HNO3) and a large inner diameter capillary (150 µm) could also be used to attain efficient separation. The toxicity of metal elements is often very species dependent.1,2 This is why separation and quantification of the chemical species, together with knowledge of the total content of the elements, are gaining importance in analyses of biological and environmental samples. Although many analytical techniques such as chromatography, atomic spectrometry, and mass spectrometry3-5 have been used in these areas, researchers are continually seeking more efficient and sensitive methods to get better results. Capillary electrophoresis (CE) is a powerful and simple separation method, as it does not require expensive or complicated equipment. It has been used for analysis in a wide range of †
National Institute of Agro-Environmental Sciences. Tohoku University. § Present address: Department of Chemistry, Kansas State University, Willard Hall, Manhattan, KS 66506. (1) Vela, N. P.; Olson, L. K.; Caruso, J. A. Anal. Chem. 1993, 65, 585A. (2) Demesmay, C.; Olle, M.; Porthault, M. Fresenius J. Anal. Chem. 1994, 348, 205. (3) Shum, S. C. K.; Houk, R. S. Anal. Chem. 1993, 65, 2972. (4) Hill, S. J.; Bloxham, M. J.; Worsfold, P. J. J. Anal. At. Spectrom. 1993, 8, 499. (5) Agnes, G. R.; Stewart, I. I.; Horlick, G. Appl. Spectrosc. 1994, 48, 1347. ‡
S0003-2700(96)00950-X CCC: $14.00
© 1997 American Chemical Society
disciplines, including environmental, biological, and clinical samples.6-11 Because only a very small volume of sample is introduced in CE, it is generally difficult to obtain satisfactory detection limits in terms of concentration for most species. Accordingly, development of highly sensitive and selective detectors has been very important and challenging work since CE came into existence. Until now, on-column UV and fluorescence detectors have been widely used. However, the sensitivity of these detectors is usually between 10-5 and 10-6 M.12 Some other detection techniques such as mass spectrometry, laser-induced fluorescence, amperometry, conductometry, chemical illumination, and laser Raman spectroscopy have been investigated to provide more sensitive and selective detection of the target analytes separated by CE.13-20 Recently, inductively coupled plasma emission and mass spectrometry (ICP-AES/MS) have been combined with CE and proved to be a very useful detector.21-24 The challenge when working with CE-ICP-AES/MS has been to design an efficient interface to connect CE with ICP. Although interfaces connecting with conventional pneumatic concentric nebulizers (CPCN), highefficiency nebulizers, and direct injection nebulizers have been developed, all of these are complicated and are not always compatible with other kinds of nebulizers. In this report, a simple interface which is compatible with both CPCN and microconcentric nebulizers (MCN) was described. The interface is outside and independent of the nebulizer and could be connected to the sampling tube of the nebulizer and/or directly (6) Monnig, C. A.; Kennedy, R. T. Anal. Chem. 1994, 66, 280R. (7) St. Claire, R. L., III Anal. Chem. 1996, 68, 569R. (8) Jorgenson, J. W.; Lukacs, K. D. Science 1984, 222, 266. (9) Chen, M.; Cassidy, R. M. J. Chromatogr. 1993, 640, 473. (10) Johansson, I. M.; Pavelka, R.; Henion, J. D. J. Chromatogr. 1991, 559, 515. (11) Rush, R. S.; Cohen, A. S.; Karger, B. L. Anal. Chem. 1991, 63, 1346. (12) Smith, R. D.; Wahl, J. H.; Goodlett, D. R.; Hofstadler, S. A. Anal. Chem. 1993, 65, 574A. (13) Foret, F.; Thompson, T. J.; Vouros, P.; Karger, B. L.; Gebauer, P.; Bocek, P. Anal. Chem. 1994, 66, 4450. (14) Jansson, M.; Roeraade, J.; Laurell, F. Anal. Chem. 1993, 65, 2766. (15) Mank, A. J. G.; Yeung, E. S. J. Chromatogr., A 1995, 708, 309. (16) Roberts, R. E.; Johnson, D. C. Electroanalysis 1994, 6, 269. (17) Zhou, J.; Lunte, S. M. Anal. Chem. 1995, 67, 13. (18) Kar, S.; Dasgupta, P. K.; Liu, H.; Huang, H. Anal. Chem. 1994, 66, 2537. (19) Huang, B.; Li, J.; Zhang, L.; Cheng, J. Anal. Chem. 1996, 68, 2366. (20) Kowalchyk, W. K.; Walker, P. A., III; Morris, M. D. Appl. Spectrosc. 1995, 49, 1183. (21) Olesik, J. W.; Kinzer, J. A.; Olesik, S. U. Anal. Chem. 1995, 67, 1. (22) Liu, Y.; Lopez-Avila, V.; Zhu, J. J.; Wiederin, D. R.; Beckert, W. F. Anal. Chem. 1995, 67, 2020. (23) Lu, Q.; Bird, S. M.; Barnes, R. M. Anal. Chem. 1995, 67, 2949. (24) Kinzer, J. A.; Olesik, J. W.; Olesik, S. V. Anal. Chem. 1996, 68, 3250.
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to the nebulizer by a capillary. The sample was first separated by CE and then introduced into the nebulizer by self-aspiration. In CE, several on-column concentrating techniques have been used to improve the detection sensitivity.25 One of these techniques is to introduce large amounts of sample into electrophoresis capillary and then apply high electrophoresis voltage to focus the analytical sample at the end of the sample plug. In this report, this technique also was used, as it had led to improved sensitivity. A large amount of sample results in low conductivity; therefore, it was possible to use a large inner diameter capillary to obtain good sensitivity and efficient separation. Also, the longer sample plug allowed the use of 0.05 M HNO3 as an electrophoresis electrolyte. EXPERIMENTAL SECTION Reagent and Materials. Sodium acetate trihydrate (99.0% pure, Wako Pure Chemicals Industries, Ltd.) and nitric acid (EL, Grade 1.38, Kanto Chemical Co, Inc.) were used as electrolytes. K2Cr2O7 (99.5% pure, Wako), CrCl3‚6H2O (99.5% pure, Wako), CuSO4‚5H2O (99.5% pure, Junsei Chemical Co., Ltd.), and disodium dihydrogen ethylenediaminetetraacetate dihydrate (99.5% pure, Junsei) were used as analyte species. All solutions were prepared by dissolving the proper amount of material in deionized water. Capillary Electrophoresis. Fused silica capillaries (GL Science Inc., Japan) with inner diameters (i.d.) of 100 (120 cm long) and 150 µm (160 cm long) and an outer diameter of 375 µm were used as electrophoresis capillary, and a 150 µm i.d. capillary (40 cm long) was used as auxiliary capillary. The HCZE30PN0.25 high-voltage dc power supply provided by Matsusada Precision Inc. (Japan) was used. Nebulizers. CPCN (Glass Expansion Pty. Ltd., Australia) and MCN (CETAC Technologies, Inc., Omaha, NE) were tested. The interface was connected directly to the sampling tube of the nebulizer. The PETFE free-aspiration sampling tube of the MCN of 175 µm i.d. × 1.5 mm o.d. was used as sampling tube for both MCN and CPCN. The length of the sampling tube should be as short as possible so as to reduce the dead volume between the interface and the nebulizer. Different carrier gas flow rates were used with different nebulizers to get optimal sensitivity and resolution. Inductively Coupled Plasma Emission Spectrometer. An Analytical Research Laboratory (ARL) Maxim III optical emission spectrometer (provided by Fisons) was used. A standard Maxim ICP torch was used with an intermediate gas flow rate of 0.8 L/min and an outer gas flow rate of 13 L/min. A power of 1.1 kW was used. Spray Chambers. A conical, locally constructed spray chamber, with volume of 78 mL and without impact bead inside, was used for both MCN and CPCN. Interface. The interface was made from quartz, and its principle is shown in Figure 1. The first arm of the interface was connected with the outlet end of the electrophoresis capillary, and the second arm was connected with the auxiliary capillary used to draw up the makeup solution. The other end of the auxiliary capillary was dipped into 0.05 M HNO3. The third arm of the interface was connected directly to the sampling tube of the nebulizer. All three arms of the interface were tapered so that the capillary could be directly inserted into the arms of the (25) Chien, R. L.; Burgi, D. S. Anal. Chem. 1992, 64, 489A.
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Figure 1. Diagram of interface for coupling capillary electrophoresis to inductively coupled plasma spectrometry: 1, interface, total dead volume of the interface, ∼19 µL; effective dead volume of the interface after all three taper arms are connected with the 375 µm o.d. capillary, ∼1 µL; effective dead volume is the key of the interface, it should be made as small as possible; 2, electrophoresis capillary; 3, auxiliary capillary; 4, makeup electrolyte; 5, buffer electrolyte; 6, sampling tube of nebulizer; 7, dc supply; 8, to nebulizer.
interface without using any connector. By opening the carrier gas valve, the makeup solution and the buffer electrolyte or sample solution were respectively pulled into the auxiliary and the electrophoresis capillaries by self-aspiration. These solutions were mixed in the junction and aspirated into the nebulizer. While the makeup solution was contacted to the ground of the dc supply, the buffer electrolyte solution was connected to the negative polarity of the dc supply. The negative high voltage was used to form an opposite direction electroosmotic flow against the selfaspirating flow. Sample Injection. Sample was introduced into the electrophoresis capillary directly by self-aspiration. Before and after sampling, the auxiliary capillary was disconnected from the interface to avoid the entry of air into the electrophoresis capillary. Although a smaller amount of air was introduced into the plasma and a short air plug was observed in the outlet end of the auxiliary capillary, no severe effect was found on the plasma, and the air plug was easily removed when the auxiliary capillary was connected to the interface again. Sample volume was calculated by measuring the sampling time or by weighing the sample and was respectively 0.00314 or 0.00328 mL/min. The above measurements were made using the 150 µm i.d. (160 cm long) electrophoresis capillary, and MCN was operated at 0.25 L/min carrier gas flow rate. Under these conditions, the flow rate of makeup solution was 0.0136 mL/min, which was more than 4 times the sample flow rate. RESULTS AND DISCUSSION Characteristics of the Interface. The interface described here is similar to those developed by Barnes et al.23 and Olesik et al.24 in principle. But as it was placed outside and independent of the nebulizer, this interface was more flexible and could be easily connected to various types of nebulizers. The makeup solution provided a continuous and stable electrical contact at the exit end (grounding) of the electrophoresis capillary so that the complete electrophoresis circuit could be established. The pH of the mixed solution, the electrophoresis current, and the voltage distribution between the electrophoresis capillary and the auxiliary capillary could be adjusted by changing the kind of makeup solution and its concentration. The increase of the inner diameter and the decrease of the length of auxiliary capillary resulted in an increase of the uptake volume of the makeup solution, but the uptake volume of the buffer solution (or sample solution) changed very little. Accordingly, this resulted in an increase of the total uptake volume and a decrease of sample transport efficiency and,
Table 1. Effect of Auxiliary Capillary (AC) on Signal Intensity and the Uptake Volume per Minute of Both Buffer Electrolyte Solution and Makeup Solution i.d. of AC (µm)
uptake volume of makeup solution (µL) uptake volume of buffer solution (µL) signal intensity (counts/s)
50a
75a
100a
150a
200a
31
92
100b
1.0
2.8
7.7
2.1
2.3
2.8
2.1
2.3
2.3
28.9
26.7
20.0
17.8
13.1
21.6
4.3
a The length of AC was 35 cm. b The length of AC was 70 cm, CPCN was used, and 100 µg/mL chromium was used as sample. The carrier gas flow rate was 0.4 L/min.
Figure 3. Electropherogram of chromium obtained by CE-MCNICP-AES. Total concentration, 1 µg/mL; chemical species, Cr3+ and Cr2O72- (1:1); buffer electrolyte solution, 0.05 M HNO3; high voltage, 8 kV; inner diameter of capillary, 150 µm; length of sample plug, 30% of capillary (∼8.4 µL sample). S/N based on peak height: Cr3+, 1.65; Cr2O72-, 5.89. S/N based on peak area: Cr3+, 195.2; Cr2O72-, 647.75.
Figure 2. Electropherogram of chromium obtained by CE-CPCNICP-AES. Total concentration, 100 µg/mL; chemical species, Cr3+ and Cr2O72- (1:1, the ratio is in terms of element concentration; those below are the same); buffer electrolyte solution; 0.04 M sodium acetate (pH 7.2); high voltage, 15 kV; inner diameter of capillary, 100 µm; length of sample plug, 25% of capillary (∼2.4 µL sample). S/N based on peak height: Cr3+, 22.2; Cr2O72-, 12.3. S/N based on peak area: Cr3+, 1573.3; Cr2O72-, 1152.4.
hence, induced the decrease of the signal intensity, as shown in Table 1. Therefore, a smaller inner diameter auxiliary capillary could be used to obtain a higher signal intensity. But this might decrease the liquid flow rate between the interface and nebulizer and thereby broaden the peak. As a compromise between the above contradictory factors, the capillary of 150 µm i.d. was chosen as auxiliary capillary in this experiment. Evaluation of the Interface. Preliminary tests were carried out to evaluate the interface and the system. Figures 2 and 3 respectively show the electropherograms obtained by CECPCN-ICP-AES and CE-MCN-ICP-AES for the test solution containing Cr2O72- and Cr3+. In both cases, separation of Cr2O72from Cr3+ was observed, showing the suitability of the interface. The signal to noise ratio (S/N) was calculated on the basis of peak height and peak area respectively for evaluating the sensitivity of CE-MCN-ICP-AES and CE-CPCN-ICP-AES. For the calculation of peak height, the 10 adjacent data points on each side of the biggest datum point, including the biggest datum, for a total 21 data were chosen. Peak area was calculated by summing the data points across the peak. The background signal was subtracted from both peak height and peak area. The baseline signal before the peak was used as background signal, and 300 baseline points were used to calculate the standard deviation of
background signal. The S/N was defined here as the signal intensity divided by 3 times the standard deviation of the background signal. Comparing the S/N of the two systems, it was clear that the sensitivity of CE-MCN-ICP-AES is much higher than that of CE-CPCN-ICP-AES, because MCN has much higher nebulizing efficiency than CPCN does. In CE-MCN-ICP-AES, the forced flow was much smaller than that in CE-CPCN-ICP-AES; therefore, complete separation could be obtained with 8 kV in about 12 min. But in CE-CPCN-ICPAES, only nearly baseline separation could be obtained with 15 kV in about 4 min. The height of two peaks was also related to the forced flow rate. Lowering of the forced flow rate induced good separation and decreased the peak height of Cr3+, but the peak height of Cr2O72- remained almost unchanged. This is because the highly charged cation (Cr3+) is easily trapped by the electric double layer,26 and the lower forced flow rate causes the sample to stay a longer time in the electrophoresis capillary. This may cause more severe adsorption of highly charged cations to the electric double layer. When positive voltage was used as an electrophoresis voltage, Cr3+ eluted from the capillary before Cr2O72- did. The signal intensity of Cr3+ was higher than that obtained by negative voltage, because Cr3+ stayed in the capillary for a shorter time in positive voltage than in negative voltage. On-Column Concentration and Separation. In CE, the sample volume is a very important factor affecting the efficiency of separation and sensitivity. In general, sampling volume is less than 10% of the total length of the electrophoresis capillary.27 But in this report, an on-column concentration technique was used, where a large amount of sample solution was introduced into capillary. Figure 4 shows the electropherogram of copper with different sampling volume. Comparing parts a and b of Figure 4 for Cu2+ and Cu(EDTA)2-, it is clear that increasing injection volume increases both signal intensity and resolution of two peaks. Table 2 shows the effect of sampling volume on signal intensity and separation. As the increase in sampling volume increased (26) Schure, M. R.; Lenhoff, A. M. Anal. Chem. 1993, 65, 3024. (27) Oda, R. P.; Landers, J. P. In Handbook of Capillary Electrophoresis; Landers, J. P., Ed.; CRC Press: Boca Raton, FL, 1994; p 10-4.
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a
b
Figure 5. Effect of sampling volume and electrolyte on electrophoresis current. (a) Capillary, 150 µm i.d.; MCN; 8 kV; 10 µg/mL mixed copper solution (Cu2+:Cu(EDTA)2- ) 1:1); buffer solution, 0.05 M HNO3. (b) Capillary, 150 µm i.d.; CPCN; 15 kV; 100 µg/mL mixed chromium solution (Cr3+:Cr2O72- ) 1:1); buffer solution, 0.04 M sodium acetate. In both cases, complete separations were obtained, and very similar phenomena could be obtained whenever mixed copper or chromium was injected. Figure 4. Electropherogram of copper obtained by CE-MCN-ICPAES. Total concentration, 10 µg/mL; chemical species, Cu2+ and Cu(EDTA)2- (1:1); buffer electrolyte solution, 0.05 M HNO3; high voltage, 8 kV; inner diameter of capillary, 150 µm; length of sample plug, (a) 5% of capillary (∼1.4 µL sample), (b) 30% of capillary (∼8.4 µL sample). Table 2. Effect of Sampling Volume on Signal Intensity and Separationa peak heigth × 104
5% 10% 20% 30% 40%
peak area × 107
Cu2+
Cu(EDTA)2-
Cu2+
0.83 1.30 2.30 3.40 6.50
1.81 3.80 11.20 9.50 10.50
0.098 0.14 0.28 0.35 0.81
fwhp (s)
CuCu(EDTA)2- Cu2+ (EDTA)20.20 0.40 1.31 1.68 4.14
39 39 39 33 36
36 33 33 48 132
∆T (s) 90 99 189 222 243
a Signal intensity is presented by peak height and peak area respectively in arbitrary units; sampling volume is presented by the ratio of the lengths of sample plug and electrophoresis capillary. fwhp, full width at half-maximum peak height. ∆T, the difference of two peaks in migration time. All these data were obtained under the same conditions: concentration of mixed copper solution, 10 µg/mL (Cu2+: Cu(EDTA)2- ) 1:1); buffer electrolyte solution, 0.05 M HNO3; electrophoresis voltage, 8 kV; electrophoresis capillary, 150 µm i.d.; nebulizer, MCN. 5% of sample plug length is equal to ∼1.4 µL sample volume.
the total quantity of analyte entering the plasma, the peak area was also increased with the increase in injection volume for both Cu2+ and Cu(EDTA)2-. As Cu2+ could stay longer in the capillary and share longer electrophoresis time, the peak width of Cu2+ remained almost unchanged, while the peak height continuously 2190 Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
increased with the increase of sample volume from 5% to 40%. In contrast, the peak width of Cu(EDTA)2- increased dramatically when the injection volume exceeded 30% of the total length of capillary, and the peak heights with 30% and 40% injection volume were lower than that with 20% injection volume. Because Cu2+ and Cu(EDTA)2- moved to the two ends of the sample plug after high volume was applied, the difference in migration time of the two peaks was expected to increase with the increase of the length of the sample plug. Reduction of the carrier gas flow rate allows the analyte to stay a longer time in the capillary, and the increase of the electrophoresis voltage will result in reducing the peak width of Cu(EDTA)2-. However, low carrier gas flow rates also broaden the peak width, and higher voltage induces strong adsorption of cations to the electric double layer and depresses signal intensity. Consequently, a compromise should be made, again taking the above contradictory factors into consideration. In the on-column concentrating method,25 a relatively long and low-concentration sample plug was considered to be advantageous for separating two kinds of particles with opposite polarity because these charged particles move to the opposite ends of sample plug when high voltage is applied. Electrophoresis Current, Electrolyte, and Inner Diameter of the ELectrophoresis Capillary. Figure 5 shows the relation of sampling volume and electrophoresis current when (a) 0.05 M HNO3 and (b) 0.04 M sodium acetate were used as buffer solution. The observed difference can be explained from the differences in mobilizing velocity of ions in buffer solution. The charged particles in the sample plug move out of the sample plug and focus in the boundary between the sample plug and buffer solution when high voltage is applied. At the same time, a certain amount of
Table 3. Effect of Electrophoresis Voltage on Signal Intensity and Separationa peak height × 104 voltage (kV) 0 4 6 8 10
Cu2+
Cu(EDTA)2-
1.40 1.70 1.65 1.25 1.10
1.40 2.60 3.45 3.30 3.45
peak area × 106
fwhp (s)
Cu2+
Cu(EDTA)2-
Cu2+
Cu(EDTA)2-
4.64 1.29 1.13 1.06 0.77
4.34 4.00 3.33 3.00 3.22
68 24 24 27 27
66 54 24 27 27
a The data were obtained by electrophoresis of 1 µg/mL Cu2+ and 1 µg/mL Cu(EDTA)2-, respectively. Experimental conditions: buffer electrolyte, 0.05 M HNO3; capillary, 150 µm i.d.; nebulizer, MCN; sampling volume, 20% of electrophoresis capillary (∼5.6 µL).
charged particles in the buffer solution also migrate toward the sample plug. Consequently, the electrophoresis current is dependent on the ratios of the moved-out and the moved-in ions. As the conductivity of 0.04 M sodium acetate solution is much lower than that of 0.05 M HNO3, the electrophoresis current for 0.04 M sodium acetate was lower than that for 0.05 M HNO3 when no sample was injected. This means that 0.04 M sodium acetate can be used to replace the 0.05 M HNO3 in the case when the electrophoresis current is too big to obtain good resolution. In experiments, it was also found that, when 0.05 M HNO3 was used, the adsorption of Cu2+ and Cr3+ to the electric double layer was more severe than when 0.04 M sodium acetate was used. The effects of the inner diameter of the electrophoresis capillary on separation efficiencies and sensitivities were also examined. When the concentration of the sample solution was low and a relatively long sample plug was introduced into capillary, the electrophoresis current was dominated by low-conductivity sample plug, as shown in Figure 5a. Therefore, efficient separation could be obtained with 150 µm i.d. capillary. A reduction in the length of the sample plug or an increase in the concentration resulted in an increase of the electrophoresis current and poor separation of Cr3+ from Cr2O72- and Cu2+ from Cu(EDTA)2-. For the solution containing 10 µg/mL of total copper or chromium, complete separation of Cr3+ from Cr2O72- and of Cu2+ from Cu(EDTA)2- was not possible with the 150 µm i.d. capillary when the length of sample plug was shorter than 1% of the total length of capillary and 0.05 M HNO3 was used as electrophoresis electrolyte. Under the same conditions, the 100 µm i.d. capillary or 0.04 M sodium acetate can be used to replace respectively the 150 µm i.d. capillary or 0.05 M HNO3 to obtain better separation. But when the interface is coupled with MCN, 0.05 M HNO3 should be used to avoid the clogging of MCN often caused by the buffer solutions with high electrolyte concentrations. Effect of Electrophoresis Voltage on Peak Height, Peak Area, and Peak Width. The effects of electrophoresis voltage on peak height, peak area, and peak width for cations were different from those for anions, as listed in Table 3. When lowconcentration sample was introduced into the capillary, the peak height was affected by both on-column concentration and the adsorption of ions to the electric double layer, while the peak area was only affected by the adsorption of ions. Thus, the peak area decreased with the increase of electrophoresis voltage for both Cu2+ and Cu(EDTA)2-. In contrast, the peak height of Cu2+ increased with electrophoresis voltage up to 4 kV and then decreased with the electrophoresis voltage, suggesting that the
Figure 6. Electropherogram of copper obtained by CE-MCN-ICPAES. Chemical species, 0.5 µg/mL Cu2+ and 0.5 µg/mL Cu(EDTA)2-; buffer electrolyte solution, 0.05 M HNO3; high voltage, 8 kV; inner diameter of capillary, 150 µm; sample plug, 20% of capillary (∼5.6 µL sample). Table 4. Detection Limits Obtained by CI-MCN-ICP-AES and MCN-ICP-AESa element
wavelength (nm)
CI-MCN-ICP-AES (µg/mL)
MCN-ICP-AES (µg/mL)
Mo Cr Zn B P Pb Ni Fe Co Si Mn Mg V Cu Ca Al Ba Na
204.6 205.6 206.2 209.0 213.6 220.4 221.6 238.2 238.9 251.6 257.6 279.6 292.4 324.8 393.4 396.2 455.4 589.6
0.0138 0.0123 0.0051 0.0551 0.0518 0.0717 0.0053 0.0035 0.0047 0.0608 0.0029 0.0024 0.0047 0.0050 0.0057 0.0155 0.0012 0.0078
0.0074 0.0074 0.0017 0.0485 0.0257 0.0471 0.0027 0.0020 0.0039 0.0240 0.0009 0.0008 0.0048 0.0033 0.0029 0.0198 0.0008 0.0093
a The uptake rates of sample solution and makeup solution for CIMCN-ICP-AES were 3.1 and 13.6 µL/min, respectively. The uptake rate of sample for self-aspiration-MCN-ICP-AES was 33.6 µL/min. The mixed standard solution of 5 µg/mL for every element was used for both systems.
negative effect of the adsorption of Cu2+ on peak height was larger than the positive effect of on-column concentration. The peak height of Cu(EDTA)2- continued to increase until 6 kV, after which the peak height and peak width showed little change with increasing electrophoresis voltage. The adsorption of Cu2+ to the electric double layer seemed to be more severe than that of Cu(EDTA)2- when 10 kV was applied, as the recovery rates were ∼17% for Cu2+ and ∼72% for Cu(EDTA)2- in terms of peak area. Because Cu2+ can stay in the capillary longer than Cu(EDTA)2-, for Cu2+ 4 kV was enough to obtain good peak shape, while 6 kV was needed for Cu(EDTA)2-. As the effect of high voltage on cations was different from that on anions, calibration must be done to get accurate results. Figure 6 shows the electropherogram of 0.5 µg/mL Cu2+ and Cu(EDTA)2-. It can be seen that the peak height and peak area of Cu2+ are much smaller than those of Cu(EDTA)2-. Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
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Table 5. Detection Limits of Elements Obtained by CE-MCN-ICP-AES and CE-CPCN-ICP-AESa CPCN (µg/mL)
Cr3+ Cr2O72Cu2+ Cu2+ Cu(EDTA)2Cu(EDTA)2-
MCN (µg/mL)
based on peak height
based on peak area
based on peak height
based on peak area
2.6 4.1
0.032 0.044
0.31 0.085 0.58b 0.11c 0.22b 0.039c
0.0025 0.00075 0.0033b 0.00093c 0.0017b 0.00020c
a The data were obtained under the following conditions. CPCN: 100 µm i.d. capillary, 15 kV; buffer solution, 0.04 M sodium acetate; 100 µg/mL mixed chromium solution (Cr3+:Cr2O72- ) 1:1) of ∼2.4 µL was injected. MCN: 150 µm i.d. capillary, 8 kV; buffer solution, 0.05 M HNO3; 1 µg/mL mixed chromium solution (Cr3+:Cr2O72- ) 1:1) of ∼8.4 µL was injected; 10 µg/mL mixed copper solution (Cu2+: Cu(EDTA)2- ) 1:1) was injected. b Injected volume of ∼1.4 µL. c Injected volume of ∼8.4 µL.
Detection Limits. The detection limit is defined here as the concentration that produces a net signal equivalent to three standard deviations (3σ) of the background intensity at the analyte wavelength. The detection limits of 18 elements were obtained with a conventional self-aspiration MCN-ICP-AES and with a capillary interface (CI)-MCN-ICP-AES system when no voltage was applied and sample was introduced into plasma continuously. These are listed in Table 4. With CI-MCN-ICP, 150 µm i.d. capillaries were used as both electrophoresis capillary and auxiliary capillary. Because a much smaller volume was introduced into plasma in CI-ICP-AES than in conventional ICP-AES, the detection limits obtained by CI-ICP-AES were 1-4 times higher than those of conventional ICP-AES. The elemental detection limits of copper and chromium, where these elements were in the form of Cu2+, Cu(EDTA)2-, Cr3+, and Cr2O72-, were also calculated and are listed in Table 5. The detection limits based on peak heights were ∼100 times higher than those based on peak areas. The detection limits obtained by MCN were lower
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than those obtained by CPCN because the nebulizing efficiency was much higher in the former. A much lower elemental detection limit can be obtained for the elements existing in negative ion form than for those in positive ion form when 0.05 M HNO3 was used as the buffer electrolyte solution. Injecting larger amounts of sample and on-column concentration could decrease the detection limits, as shown in Table 5. CONCLUSIONS Initial results show that the interface described in this paper is efficient to couple capillary electrophoresis to ICP-AES. It can be easily and directly connected to a conventional pneumatic nebulizer and to a microconcentric nebulizer. By using this CEICP-AES system, efficient separation and satisfactory sensitivity were obtained for samples containing Cu2+ and Cu(EDTA)2-, and Cr3+ and Cr2O72-, respectively. The on-column concentration technique could improve the sensitivity and separation efficiency of CE-ICP-AES. Furthermore, 0.05 M HNO3 (pH about 1.3) can be used as electrolyte to separate Cu2+ and Cu(EDTA)2-, and Cr3+ and Cr2O72-, when the on-column concentration technique was used. ACKNOWLEDGMENT Support for this research was provided by the Japan International Science & Technology Exchange Center STA Program. We thank Dr. Akito Tsumura of the National Institute of AgroEnvironmental Sciences for his kind assistance and Miss Jennifer A. Pugin of University of Alaska Fairbanks for correcting English in the manuscript. Dr. Don McCune is acknowledged for providing the software for time-resolved measurements in Maxim. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. Received for review September 17, 1996. Accepted March 25, 1997.X AC960950W X
Abstract published in Advance ACS Abstracts, May 1, 1997.
