Direct Determination of Chromium(lll) and Chromium(VI) with Ion Chromatography Using Direct Current Plasma Emission as Element-Selective Detector*

Transcription

1 Direct Determination of Chromium(lll) and Chromium(VI) with Ion Chromatography Using Direct Current Plasma Emission as Element-Selective Detector* I.T. Urasa** and S.H. Nam Department of Chemistry, Hampton University, Hampton, Virginia Abstract A method is described that utilizes direct current plasma atomic emission spectrometry as an element-selective method of detection for ion chromatographic determination of chromium(lll) and chromium(vi) species. The eluting chromium-containing species are detected on the basis of the atomic emission of chromium without any species conversion. Both anion and cation separator columns give similar results when used with varying sample matrices. By employing an on-column preconcentration procedure, the detectable concentrations of the chromium species are reduced to less than 1.0 ppb. This method is applied to the determination of chromium species in human serum, natural water, and industrial process stream samples. Introduction The development of analytical methods for trace metal speciation is an area that still presents a great challenge. This applies particularly to transition metals, some of which not only exist in more than one valence state, but can be present as anionic, cationic, and neutral species. The basic requirement for trace metal speciation is the ability to quantitatively determine each of the forms of the metal present independently and without interference from the other forms. One way of doing this is by physically separating the species present followed by their determination using an appropriate physicochemical measurement procedure. A second way of speciating metals is by employing a physicochemical analysis procedure that converts all forms of the metal to a specific species, which is then measured. Clearly the conversion of metal species from one form to another can have serious drawbacks including incomplete conversion, introduction of contaminants, interference from other metals present, and generally a complex sample pretreatment procedure. Thus, the preferred approach is one which physically separates the individual species present followed by direct quan- * Presented in part at the Pittsburgh Conference, New Orleans, LA, February 1988, paper ** Author to whom correspondence should be addressed. titation. With the development of high-performance liquid chromatography (HPLC) and other chromatographic methods, a variety of separation modes, via chelate formation, have been employed to separate various metal ions followed by colorimetric, spectroscopic, or electrochemical detection of the separated species (1 3 and references therein). For metals such as chromium, however, which can exist as anionic (Cr(VI)) and cationic (Cr(III)) species, speciation presents a more challenging problem. Several approaches have been tried. One method employed flow injection procedures requiring a two-step process during which Cr(VI) is converted to Cr(III) followed by the formation of a colored complex. This complex is then measured spectrophotometrically (4). Subramanian combined complex formation using ammonium pyrrolidinecarbodithioate (APDC) with graphite furnace atomic absorption to selectively determine Cr(III) and Cr(VI) (5). Because both species complex with APDC, selective complexationextraction was achieved by careful optimization of the ligand concentration, solution ph, and extraction time. Similar complex formation-extraction procedures have been employed by others by using dithiocarbamate-mibk systems (6). By selectively volatilizing chromium(iii) acetylacetonates at 400 C and Cr(VI) at 1200 C, Arpadjan et al. (7) were able to quantitatively determine the two chromium species separately by atomic absorption measurements. Similar selective volatilization aproaches have been employed by Wolf (8) and Lloyd et al. (9). Batley and Matousek developed a procedure involving electrodeposition of electrochemically separated Cr(III) and Cr(VI) on a graphite furnace tube followed by atomic absorption measurement (10). Van Loon et al. (11) and Naranjit et al. (12) developed and employed an ion exchange procedure by which Cr(VI) was selectively determined in the presence of Cr(III). This procedure has been used by others as a means of preconcentrating the chromium species (13,14). Krull et al. (15,16) employed paired-ion reversed-phase HPLC in combination with inductively coupled plasma emission spectrometric detection and direct current plasma emission spectroscopy to speciate chromium in chemical and environmental samples and reported very promising results. This paper reports an ion chromatographic procedure that utilizes either an anion or a cation exchange column coupled with direct current (dc) plasma atomic emission detection (IC- DCPAES) to separate and quantify the two chromium species directly without species conversion. The ability of dc plasma 30 Reproduction (photocopying) of editorial content of this journal is prohibited without publisher's permission.

2 to excite all forms of a given element with equal efficiency (17 20) makes it particularly suitable as an element-selective detector for this speciation procedure. Important factors in the development of this method and the approach used to improve detection will be discussed, as well as several areas of application. Experimental Instrumentation The equipment used in this work is listed in Table I. Chemicals and reagents American Chemical Society certified chromic nitrate and potassium chromate were obtained from Fisher Scientific Co. Double-distilled nitric acid and redistilled hydrochloric acid were obtained from GFS Chemicals. Deionized water (18.3 ΜΩ) was prepared by using Nanopure II water purification system from Sybron Corp. Special materials Human serum, in freeze-dried form, and natural water were obtained from the National Bureau of Standards (NBS), as Standard Reference Materials (SRM) 909 and 1643(b), respectively. Waste water was obtained from an industrial processing plant waste stream. Procedure The ion chromatograph was interfaced with the dc plasma by inserting the chromatographic column exit tubing into the dc plasma sample tube, thereby aspirating the chromatographic effluents directly into the plasma excitation zone. The mobile phase flow rate was fixed at 2.0 ml/min. This solution flow rate was within the optimum range of the sample uptake rate for the dc plasma. Chromatographic peaks were obtained as analog signals monitored continuously by tracing the photomultiplier tube current output of the dc plasma spectrometer at nm. These peaks corresponded to the atomic emission Table I. Equipment Used Equipment and accessories Description Source Ion chromatograph Model 2010i Dionex Corp. Columns HPIC-AS 7 HPIC-CS 2 DC Plasma-echelle spectrometer Strong base anion exchange resin polystyrene-divinylbenzene copolymer with quaternary ammonium function groups. Anion separator for polyvalent anions (250 mm 4 mm); 15 μm particles Strong acid cation exchange resin polystyrene-divinylbenzene copolymer with sulfonic acid functional groups. Cation separator for transition elements (250 mm 4 mm); 15 μm particles. Model Spectraspan IV, three-electrode configuration Dionex Corp. Dionex Corp. ARL signals resulting from the excitation of the chromium species in the chromatographic effluents. The chromatographic equipment used is listed in Table I. The anion column was used with an eluent consisting of 0.05 Μ HNO 3. The eluent for the cation column consisted of 7.5 mm trilithium citrate and 10.0 mm oxalic acid. The eluent concentrations specified here represented the optimum values determined after evaluation of several eluent compositions. The anion and cation columns were used separately for Cr(III)/Cr(VI) species separation. Chromium(III) standard solution was prepared as 1000 μg/ml Cr(III) using reagent-grade Cr(NO 3 )3. A similar standard of Cr(VI) was prepared from reagent-grade K 2 CrO 4. Analytical samples were prepared by dilution of appropriate volumes of these standards. One of the goals of the study was to determine the chromium species in varying sample matrix. In order to evalute potential matrix effects, chromatographic studies of the two species were also done in varying sodium chloride and acid concentrations. The method was applied to the determination of the chromium species in human serum, natural water, and industrial process stream samples. In order to improve measurement sensitivity, and thus lower the detection limit, sample preconcentration was done by multiple injections of the sample onto the analytical column followed by a rapid regeneration of the analytical column using an appropriate counterion. Sample injections were done with a 1.0-mL injection loop. Results and Discussion Behavior of chromium(lll) and chromium(vi) in chromatographic columns In water, chromium(iii) exists as the hydrated hexaaqua ion [Cr(H 2 O) 6 ] 3+, while Cr(VI) exists as the chromate species 2 HCrO 4 or the dithromate species Cr 2 O 7, depending on solution ph. Therefore, theoretically, the two valence states can be separated with a cation column in which the anionic species would elute as an unretained peak while the cationic species would be retained, and vice versa. Figure 1 shows the chromatogram obtained when a mixture of Cr(III) and Cr(VI) (1.0 ppm each) was separated with the cation column. While Cr(VI) eluted as expected, Cr(III) was highly retained in this column and never eluted. Figure 2 shows the chromatogram obtained for the same mixture, but after injecting a 1.0 Μ HC1 aliquot onto the column following the Cr(VI) elution. The injection of 1.0 Μ HC1 introduced a relatively high concentration of the counterions (hydronium ions) on the column, rapidly regenerating the column by displacing the Cr(III) ions, which in turn eluted as a sharp peak. It will be shown below that in addition to being able to elute a strongly retained species in this way, rapid column regeneration lends itself as an effective method for on-column analyte preconcentration, and this, coupled with the elementselective nature of the dc plasma detector, facilitates chromium and other trace metal speciation determinations considerably. In Figure 3 are the chromatograms obtained for Cr(III) and Cr(VI) using the anion separator column. The hexaaqua chromium(iii) ion would be expected to elute as an unretained peak in this case, but as can be seen in Figure 3A, this was not the case. Instead, Cr(III) was retained almost as long as, and was not separated from, Cr(VI). Upon acidification of the sample, however, an unretained Cr(III) peak that was well resolved from 31

3 Figure 1. Ion chromatographic separation of chromium(iii)/chromium(vi) with cation column. Eluent: 7.5 mm trilithium citrate/10.0 mm oxalic acid. Sample: 1.0 ppm Cr(lll)/1.0 ppm Cr(VI). SI=sample inject. the Cr(VI) peak was obtained, as shown in Figure 3B. There is no logical explanation for the retention of the Cr(III) on the anion exchange column. In any event, it appears that Cr(III) and Cr(VI) can be separated with this particular column, which is commonly used for polyvalent anions, only if the solution is acidified, as depicted in Figure 3B. Even though acidification of Cr(VI) favors the formation of Cr 2 2 O 7 species or the chlorochromate ion CrO 3 C1 _ (if HCI is used), its retention characteristics in distilled deionized water and in acid solution are essentially similar, as depicted in Figures 3A and 3B, respectively. On the other hand, sample acidification reduces the retention of Cr(III) on both the anion and cation exchange columns. Whether separation is done with an anion or cation column, the measured chromatographic peak areas for the two chromium species are practically identical. This is important for quantitative measurements and underscores the uniqueness of dc plasma as an element-selective detector in which the analytical signals obtained are only a function of the concentration of a targeted metal, in this case chromium. The analytical curves prepared in this way for the two chromium species were linear over 3 orders of magnitude (0.1 to 100 ppm) and practically identical, as depicted in the analytical curve data in Table II. Thus, with this method, only one analytical curve is necessary for the quantitation of various species containing a targeted element. Effects of acid and analyte concentrations on Cr(III) elution Rapid column regeneration, as discussed above, and sample acidification have a similar effect on Cr(III) retention. In both cases, hydronium ions effectively prevent the interaction of Cr(III) ions with the exchange site. For chromium concentration of 1.0 ppm or less, acid concentration of 1.0 Μ was found to be optimum for total displacement of the Cr ions from the exchange site. This is illustrated in Table III, which compares the peak areas measured for peaks obtained via rapid column regeneration with those obtained when samples were aspirated into the dc plasma directly via loop injection without a column. Figure 2. Ion chromatographic separation of Cr(lll)/Cr(VI) with cation column: Elution of Cr(III) by rapid column regeneration. Eluent: 7.5 mm trilithium citrate/10.0 mm oxalic acid. Sample: 1.0 ppm Cr(III)/1.0 ppm Cr(VI). Regenerating eluent: 1.0 Μ HCI. SI=sample inject. Al=1.0 Μ HCI inject. Figure 3. Ion chromatographic separation of Cr(III)/Cr(VI) with anion column: (A) 1.0 ppm Cr(III)/1.0 ppm Cr(VI) in deionized water; (B) 1.0 ppm Cr(III)/1.0 ppm Cr(VI) in 1.0 Μ HCI. Eluent: 0.05 Μ HNO 3. 32

4 The close agreement between the two sets of data suggests that practically all the Cr(III) is eluted. Both hydrochloric and nitric acids gave similar results. Elution of chromium(vi) by rapid regeneration of the anion column Highly retained anionic species can be eluted from an anion exchange column in a manner similar to that described above by using an appropriate strong eluting ion. This was demonstrated for Cr(VI) by using the AS 7 column with water as eluent. Water was used here as the weakest mobile phase, thereby ensuring maximum retention of Cr(VI) species in this column. The sample consisted of a mixture of Cr(III) and Cr(VI), 1.0 ppm each, in 1.0 Μ HCI. As shown in Figure 4, Cr(III) eluted as an unretained peaks as expected. Immediately after that, a 0.5 Μ NaOH aliquot was injected onto the column; this caused the Cr(VI) to elute as a sharp peak, as shown. For this anion separator column, the 0.5 Μ NaOH appeared to be optimum for a 1.0 ppm analyte concentration. On-column sample preconcentration Strong retention of the analyte in the column coupled with rapid regeneration of the column can serve as a simple and effective means of preconcentrating the analyte. By making multiple injections of the dilute sample, the highly retained analyte essentially builds up on the column. If this is then followed by the injection of a relatively high concentration of an appropriate eluting ion as discussed above, the built-up analyte will elute as a sharp peak whose concentration is now higher than in the original sample. In this way, detection capability can be improved considerably, allowing the determination of ultratrace analyte concentrations. This aproach was applied to a solution consisting of 0.1 ppm each of Cr(III) and Cr(VI) in deionized water. A 1 ml aliquot of solution was injected onto the cation separator column using water as mobile phase. Following the elution of Cr(VI), which eluted as an unretained peak, a 1.0 Μ HCI aliquot was injected to elute the Cr(III) species. Sample injection was done in one, two, and four steps before injecting the acid, but in each case, the Cr(VI) was allowed to elute first before a subsequent injection. As depicted in Figure 5, not only did the analytical signals obtained for Cr(III) increase with the number of sample injections made, but this increase was linear with the injection steps. In order to verify that the number of injection steps correlated with solutions containing equivalent chromium concentration, a solution of 0.1 ppm Cr(III) was used for 1, 2, 5, and 10 injection steps as described above. The peak areas obtained were compared with those obtained with solutions of 0.1, 0.2, 0.5, and 1.0 ppm, respectively. As shown in the data in Table IV, the correlation was almost a perfect 1 to 1. The same thing was found to be true for Cr(VI). Thus, this method of sample preconcentration is indeed a viable one and should be suitable for trace determination of chromium species, as will be shown below. The detectable concentration measured in this way was below 1.0 ppb for both Cr(III) and Cr(VI). Effects of other ions on the elution and on-column preconcentration of chromium(iii) Theoretically, a preconcentration procedure increases the concentration of not only the analyte of interest but also other ions Table II. Analytical Curve Data for Cr(lll) and Cr(VI) Obtained with Anion Exchange Column Peak area (V. min) Concentration (ppm) Cr(III) Cr(VI) ± ± ± ± ± ± ± ± ± ±14.00 Note: ± values are standard deviations of three replicate measurements. Table III. Elution of Different Cr(lll) Concentrations by Rapid Cation Column Regeneration with 1.0 Μ HCI. Comparison with Direct Loop Injection Cr(III) Concentration (ppm) Direct loop injection Signal magnitude (peak area) Rapid column regeneration ± ± ± ± ± ±0.30 Note: ± values are standard deviations of three replicate measurements. Figure 4. Ion chromatographic separation of Cr(III)/Cr(VI) with anion column: Elution of Cr(VI) by rapid column regeneration. Eluent: deionized water. Sample: 1.0 ppm Cr(III)/1.0 ppm Cr(VI). Regenerating eluent: 0.5 Μ NaOH. SJ=sample inject. OH =0.5 Μ NaOH inject. 33

5 of a similar charge (anions or cations) that may be present. Therefore, it is important to know how the presence of other ions might affect the determination of the analyte of interest. Because the speciation method developed in this study would be applicable to the analysis of environmental and clinical samples, which might be high in sodium chloride content, the effect of Na + on chromium(iii) preconcentration was evaluated. Figure 6 shows the data obtained when 0.1 ppm Cr(III)/0.1 ppm Cr(VI) mixture was separated on the cation column in the presence of increasing concentrations of NaCl as Cr(III) was eluted via rapid column regeneration, as discussed earlier. It can be seen that, whereas the Cr(III) signal changed very little over a NaCl concentration range of 0 to 1%, the Cr(VI) signal increased by a factor of 1.5 to 2. This increase is attributed to salt effect that the chromium atoms can experience in the dc plasma in the presence of NaCl at concentrations comparable to those used in this experiment. These phenomena have previously been reported in the literature (21). To verify the effect in this particular work, the same solutions were aspirated directly into the dc plasma and the emission intensity was measured. The presence of sodium caused an enhancement of the chromium emission signal by factors comparable to those observed above. Chromium(VI) was affected more than Cr(III) because with this particular column, Na coeluted with the unretained Cr(VI). The salt effects observed here imply that matrix matching between samples and standards could be required when samples are high in sodium content. No similar matrix effects were observed when acid (HCI or HNO 3 ) was used instead of NaCl. The effect of high NaCl concentration on sample preconcentration via multiple injections, as presented in Figure 5, was investigated by repeating the study in the presence of 1% NaCl. The results are shown in Figure 7. Again, not only do the results show that the linear relationship between the analytical signal and the number of preconcentration steps done is observed, but also that the Cr(VI) peak is unaffected, as before, by the degree of preconcentration. Therefore, it is apparent that the presence of high NaCl concentration (up to 1%) does not affect the preconcentration of Cr(III). The only effect observed is the salt effect discussed above for Cr(VI). Again, no similar effects were observed when samples were in acid matrix (up to 2 Μ HCI or HNO 3 ) instead of NaCl matrix. Application to the analysis of clinical and environmental samples The speciation method described above was applied to the analysis of clinical and environmental samples for chromium species. The clinical sample used consisted of a human serum material obtained from NBS as SRM 909. Environmental samples included both natural water and waste water. Natural water was also obtained from NBS as SRM 1643(b) in 5% Table IV. Correlation Between Peak Areas Obtained with Sample Preconcentration and Those Obtained with Different Standard Solutions (1.0 ml loop used) Preconcentration of 0.1 ppm Cr(III) Different solutions Corresponding Sample Number of mass Peak area concn Mass Peak area Concn steps (μg) (V. min) (ppm) (μg) (V. min) ± ± ± ± ± ±0.1 Note. ±values are standard deviations of three replicate measurements. water. Sample: 0.1 ppm Cr(III)/0.1 ppm Cr(VI). (A) one injection; (B) two in- jections; (C) four injections. SI=sample inject. AI=1.0 Μ HCI inject. Figure 5. Preconcentration of Cr(III) by multiple column injection followed by rapid column regeneration. Column: cation separator. Eluent: deionized 34

6 HNO 3. Waste water samples were obtained from an industrial waste stream. Analytical standards were prepared to mimic as closely as possible the known sample matrix. Thus, for example, because NaCl was a major component (about 134 mm) of the human serum materials, standards of the chromium species were prepared in approximately 1% NaCl. In the case of the industrial waste water, the sample matrix was determined to be predominantly NaCl (about 200 mm). Therefore, standards for this analysis were prepared in similar NaCl concentrations. The data obtained for SRM 909 and 1643(b) are shown in Table V. The certified concentrations are for total chromium; however, with the IC-DCPAES method, both Cr(III) and Cr(VI) were found in SRM 909 at approximately equal concentrations as shown. For SRM 1643(b), the chromium present was mainly in the Cr(III) form. A determination of total chromium content was done for both SRMs by direct analysis of the samples and Figure 6. Effects of NaCl on Cr(III) elution by rapid column regeneration. Column: cation separator. Eluent: deionized water. Sample: 0.1 ppm Cr(III)/0.1 ppm Cr(VI). (A) in deionized water; (B) in 0.2% NaCl; (C) in 0.5% NaCl; (D) in 1% NaCl. Figure 7. On-column preconcentration of Cr(III) in the presence of 1.0% NaCI. Column: cation separator. Eluent: deionized water. Sample: 0.1 ppm Cr(III)/0.1 ppm Cr(VI). (A) one column injection; (B) two column injections; (C) four column injections. 35

7 standard solutions with the dc plasma. The data obtained (Table V) were in close agreement with the total species concentration found. For the industrial process stream samples, the analyses were done in three modes. In the first mode, the unfiltered sample was aspirated directly into the dc plasma system. The chromium concentration obtained in this way was for total chromium present, including ionic, particulate, and other unfilterable forms. In the second mode, the sample was first filtered with a 0.2 μm membrane filter before aspiration into the dc plasma system. This provided information on the unfilterable fraction of the total chromium present. In the third mode, the sample was filtered and then injected on the cation separator column. This provided information on the Cr(III) and Cr(VI) species present. The results are summarized in Table VI. The power of the IC- DCPAES system to speciate chromium is well depicted in these data. speciation. In this regard, the chromatography can employ either anion or cation exchange columns, depending on sample ph. If the sample is in acid solution, an anion exchange column gives better chromatography. On the other hand, if the sample is in neutral or basic solution, a cation exchange column will give better results in the separation of Cr(III) and Cr(VI). In both cases, however, quantitation of both chromium species is done with equal efficiency. Acknowledgment This research was supported by a grant from U.S. Department of Energy, Division of Chemical Sciences, Grant Number DE-FG05 86ER Conclusions The work reported in this paper demonstrates that the scope and applicability of ion chromatography can be extended considerably by using it with an element-selective method of detection such as dc plasma atomic emission spectrometry. Because this detector responds only to a specific element in the chromatographic effluent, it is particularly suitable for trace metal Table V. Results of the Determination of Chromium Species in Standard Reference Materials Certified Experimental results Standard chromium Total Cr* reference concn (NBS) Cr(lll) Cr(VI) determined material, NBS μg/ml μg/ml μg/ml μg/ml SRM ± ± ± ±0.01 (human serum) SRM 1643(b) 0.019± ±0.005 Not detected 0.020±0.00S (natural water) Note: ±values are standard deviations of three replicate measurements. * Total chromium was determined by direct aspiration of the sample into the dc plasma and analyzing in the integration mode. Table VI. Results of the Determination of Chromium Species in Industrial Process Stream Samples Analysis mode, Cr concn (ppm) DC plasma a, DC plasma b, IC-DCP C, direct injection, direct injection, column injection, Sample unfiltered filtered filtered I 111.6± ±4.3 Cr(VI)= 99.4± 5.0 Cr(lll)= II 450.0± Cr(VI)=367.8±11.0 Cr(III)= III ±3.0 Cr(VI)= 93.6± 5.0 Cr(III)= 2.6± 0.1 Note: ±values are standard deviations of the mean of three replicate measurements, a Sample aspirated directly into the dc plasma, b Sample aspirated into the dc plasma after filtraton with 0.2 μm filter, c Filtered sample injected onto cation column; effluent aspirated directly into the dc plasma. References 1. J.C. Van Loon. Metal speciation by chromatography/atomic spectrometry. Anal. Chem. 51: 1139A 50A (1979). 2. W.S. Gardner and P.F. Landrum. Fractionation of metal forms in naural waters by size-exclusion chromatography with inductively coupled argon plasma detection. Anal. Chem. 54: (1982). 3. B.D. Karcher and I.S. 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8 Smith, Jr. Speciation of Cr(III) and Cr(VI) via reversed-phase HPLC with inductively coupled plasma emission spectrometric detection (HPLC-ICP). Anal. Lett. 15 (A3): (1982). 16. I.S. Krull, K.W. Panaro, and L.L. Gershman. Trace analysis and speciation for Cr(VI) and Cr(lll) via HPLC-direct current plasma emission spectroscopy (HPLC-DCP). J. Chromatogr. Sci. 21: (1983). 17. R.K. Skogerboe and I.T. Urasa. Evaluation of the analytical capabilities of a dc plasma-echelle spectrometer system. Appl. Spectrosc. 32: (1978). 18. I.T. Urasa. Determination of arsenic, boron, carbon, phosphorus, selenium, and silicon in natural waters by direct current plasma atomic emission spectrometry. Anal. Chem. 56: (1984). 19. I.T. Urasa and F. Ferede. Use of direct current plasma as an element-selective detector for ion chromatographic determination of trace elements species. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1987, paper I.T. Urasa and F. Ferede. Use of direct current plasma as an element selective detector for simultaneous ion chromatographic determination of arsenic(iii) and arsenic(v) in the presence of other common anions; Anal. Chem. 59: (1987). 21. M. Miller, E. Keating, D. Eastwood, and M.S. Hendrick. Measured and modeled enhancement of transition metal emissions in the dc plasma jet. Spectrochim. Acta. 40B: (1985). Manuscript received May 13, 1988; revision received September 15,