Direct Chromium Speciation Using Thermospray: Preliminary Studies with Inductively Coupled Plasma Mass Spectrometry

Microchemical Journal 62, 192–202 (1999) Article ID mchj.1999.1714, available online at http://www.idealibrary.com on

Direct Chromium Speciation Using Thermospray: Preliminary Studies with Inductively Coupled Plasma Mass Spectrometry Xiaohua Zhang and John A. Koropchak 1 Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901-4409 Received January 13, 1999; accepted January 19, 1999 Thermospray (TSP) sample introduction methods have been found to selectively deposit Cr(III) in the presence of Cr(VI) and thus provide a direct, nonchromatographic speciation capability for these chromium oxidation states. Previous results have shown that the sensitivity for Cr(III) could be reduced to a negligible level compared with the sensitivity for Cr(VI) by adjusting sample and thermospray operating parameters, with detection by inductively coupled plasma atomic emission spectrometry (ICP-AES). Hence a rapid, sensitive chromium speciation method with limits of detection (LODs) at the 0.5 ng/ml level was developed. In this paper, application of this approach to inductively coupled plasma mass spectrometry (ICP-MS) was intended to further reduce the LODs for Cr detection and further elucidate the kinetics and optimum conditions for efficient, quantitative Cr(III) deposition. These preliminary results show that for Cr(III) concentrations below 10 ng/ml, LODs and accurate, selective measurements of Cr(VI) down to the 50 pg/ml are possible. However, the deposition process of Cr(III) is incomplete for Cr(III) concentrations above 10 ng/ml under the currently established conditions, restricting the sub-nanogram-per-milliliter accuracy of the selective detection of Cr(VI) with these higher concentrations of Cr(III). The results also suggest that the process is not kinetically limited or at equilibrium in the present configuration. © 1999 Academic Press

INTRODUCTION Chromium has been identified both as an essential micronutrient and as a chemical carcinogen. The essentiality or carcinogenicity of chromium is a function of its chemical form: Cr(III) is considered to be essential to mammals for the maintenance of glucose, lipid, and protein metabolism, whereas Cr(VI) is a toxic and carcinogenic form (1, 2). Chromium is a ubiquitous, naturally occurring element found in rocks, minerals, and geological emissions. In addition, as chromium chemicals are widely used in metallurgical and chemical industries, they can enter the environment through industrial waste, such as from steel works and electroplating, tanning, or dying industries. For an accurate assessment of the environmental and biological impact of chromium, it is very important to accurately determine the concentrations of the different chromium species. Thus, the development of speciation techniques with sufficient selectivity and sensitivity for Cr is of great significance. The trend of using electrochemistry for speciation studies has decreased over time while hyphenated techniques have gained greater interest over the last 5 years (3). Hyphenated techniques combine a high-performance separation technique to separate the various species with an element-selective detector (an atomic or mass spectrometer) to provide specific response for the elements of interest (4). Flow injection (FI) on-line preconcentration (5–10) and HPLC (5, 11–15) are the most frequently used separation methods. 1

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There are detailed reviews of FI methods for chromium speciation, which report that LODs for Cr(III) are between 0.02 to 55 ng/ml, with a majority above 0.50 ng/ml, while LODs for Cr(VI) range from 0.02 to 20 ng/ml, with a majority above 1.0 ng/ml (6, 10). One disadvantage of this approach is that preconcentration times are typically at least several minutes, and times as long as 50 min have been reported (9) and the lowest LODs are achieved at the long preconcentration times (10). Chromatographic separation invariably introduces a significant dilution factor (10 – 1003), meaning that the LOD for the injected sample will be a comparable factor higher than that for continuous analysis. Barnowski et al. summarized the LODs for HPLC methods for chromium speciation (15). Even with highly sensitive ICP-MS detection, the best LOD reported is still above 0.1 ng/ml for both chromium species. Further, the chromatographic methods often take more than 5 min to complete a separation. Therefore, development of faster, more sensitive methods is still of great interest. Inductively coupled plasma atomic emission spectrometry (ICP-AES) and ICP mass spectrometry (ICP-MS) are the favored methods of detection used in speciation studies because of their generally higher sensitivity over atomic absorption spectrometry (AAS) (4). The sample introduction process is considered to be a hindrance to sensitivity for the ICP methods (16). Conventional sample introduction methods of pneumatic nebulization (PN) suffer from high inefficiency. Only 1–2% of the analyte is transported to the ICP, which means the loss of 98 –99% of this potential signal. To improve sample introduction efficiency, thermospray techniques have been developed leading to analyte transport efficiencies on the order of 20 –50%, which result in a typically 20 –50 times lower LOD than those for pneumatic nebulization (17–21). Since thermospray is a high-temperature and high-pressure process, a compound may have different chemical characteristics under such conditions than under normal conditions. Valence discrimination effects for certain elements have been observed with thermospray sample introduction for atomic spectrometry (11, 22, 23). The sensitivities for As(III), Sb(III), and Se(IV) were found to be significantly lower than those for As(V), Sb(V), and Se(VI), respectively (23). The mechanism identified for such results was that in the thermospray system, lower-valence elements are reduced to the nonvolatile and insoluble zero-valence metal form which is then trapped in the vaporizer. Reducing the sensitivity of the lower valence to a negligible level compared with that of the higher valence will make a nonchromatographic separation possible. Based on this idea, a method for Se speciation was developed (22). The previous valence effect approach is limited to nonmetals with easily reducible lower oxidation states. Recently, valence discrimination was also found for chromium (24). With thermospray nebulization and ICP-AES detection, the sensitivity for Cr(III) was found to be lower than that for Cr(VI) in solutions that were not acidified. The sensitivity for Cr(III) was further depressed to a negligible level by adjusting experimental conditions (pump flow rate, thermospray control temperature, solution pH, etc.), and as a result, a nonchromatographic chromium speciation method was developed. In detail, with samples buffered to pH 4.4, at the thermospray 125°C control temperature and 0.35 ml/min pump flow rate, Cr(VI) could be selectively determined in the presence of high concentrations of Cr(III). With acidic sample aliquots (1% v/v HNO 3), where the sensitivities for both species were essentially identical, the total chromium concentration could

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ZHANG AND KOROPCHAK TABLE 1 Starting Experimental Conditions ICP conditions Forward power Outer Ar flow Intermediate Ar flow Carrier Ar flow Sampling depth Mass spectrometer and detector ion-optics settings Extraction lens First lens Second lens Third lens Fourth lens Photon stop Entrance plate Exit plate Detector voltage Quadrupole resolution set Thermospray conditions Pump flow rate Control temperature for thermospray Spray chamber temperature Condenser temperature

1.3 kW 17.0 liters/min 1.3 liters/min 1.20 liters/min 5.0 mm 2349 V 2208 V 213 V 1.60 V 260 V 210.40 V 0 0 2250 V 0.8 amu 0.35 ml/min 125°C 100°C 10°C

also be determined, and the Cr(III) concentration calculated by difference. The LOD of this method for selective determination of Cr(VI) was about 0.50 ng/ml. Good accuracy and precision were demonstrated for analysis of spiked tap water and lake water samples. The method appears amenable to high-speed flow injection analysis. The mechanism for selective determination of Cr(VI) was thought to result from the precipitation of Cr(III) to form Cr(OH) 3 inside the thermospray vaporizer. In this work, we describe our preliminary efforts to apply this direct speciation approach to ICP-MS with the primary goals of lowering LODs for the method and further investigation of the Cr(III) deposition mechanism. EXPERIMENTAL Instruments and Operating Conditions The inductively coupled plasma mass spectrometer used for this work was a Varian Ultramass 700 (Victoria, Australia), consisting of a plasma source powered by a crystalcontrolled high-frequency generator operating at 40.68 MHz. The operating conditions are described in Table 1. All data were collected at a replication time of 3.27 s with five replicates for standards and blanks. The sample introduction system is shown in Fig. 1. A fused silica capillary thermospray nebulizer (18), was employed for aerosol generation in this work. Briefly, the thermospray vaporizer consists of a resistively heated stainless-steel tube (1.6-mm o.d., 0.5-mm i.d., 50-cm length), into which a fused silica capillary (0.36-mm o.d., 50-mm i.d.) was inserted

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FIG. 1. Diagram of the sample introduction system used in this work.

and placed such that the capillary tip was just 3–5 mm beyond the end of the stainless-steel tube. In this case, the capillary was of constant internal diameter and not terminated with an exit aperture as used before (18). Liquid samples were pumped through the fused silica capillary. Solvent vaporization and aerosol generation result from the energy transfer from the heated stainless-steel tube through the fused silica wall and the surrounding annular space to the flowing liquid stream. The particular thermospray system used herein was a prototype constructed by Leeman Labs, Inc. This system employs a fused silica thermospray nebulizer and glassware arrangement as described by Koropchak et al. (18), but uses digital controllers to monitor and control the operating temperatures as opposed to the analog triac controllers used in the prior publications. Specifically, two CN 9000A digital temperature controllers from Omega Engineering, Inc. (Stamford, CT) were used to monitor and regulate the thermospray control and spray chamber temperatures. A Varian 2510 HPLC pump continuously delivered a carrier flow to the nebulizer. The sample solutions were introduced in a flow injection manner using a Rheodyne (Cotati, CA) Model 7125 metal-free injector following a 1-ml PEEK injection loop. Finally, the Ar

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carrier gas flow rate was monitored and controlled using a mass flow controller (Model FC-280, Tylan General, Torrance, CA). Thermospray aerosols were input into a heated cylindrical Pyrex glass chamber. Desolvation of the aerosols was accomplished with a Friedrichs condenser cooled to 10°C. The condenser temperature was regulated using a refrigerated recirculating bath (NESLAB Instruments, Inc., Newington, NH). The dry aerosol from the condenser was introduced into the ICP. The thermospray conditions are also listed in Table 1. The ICP operating conditions, the control temperature for thermospray, the condenser temperature, and the spray chamber temperature were optimized to provide the maximum signal-to-background ratio for Cr(VI) at m/z 52. These conditions were used throughout this work, unless stated otherwise. Chemicals and Reagents Analytical-grade chemicals and deionized distilled water were used for the preparation of all the solutions used in this study. A Cr(III) stock solution (1000 mg/ml) was prepared using chromium nitrate [Cr(NO 3) 3 z 9H 2O] from MC/B (Norwood, OH). A Cr(VI) stock solution (1000 mg/ml) was prepared using potassium dichromate (K 2Cr 2O 7) obtained from GFS (Columbus, OH). Acetic acid/sodium acetate buffer was prepared by mixing acetic acid stock solution and sodium acetate stock solution in different volume ratios to achieve different pHs. The acetic acid stock solution (0.20 M) was prepared from acetic acid (glacial) obtained from J. T. Baker Chemical Company (Phillipsburg, NJ). Sodium acetate solution (0.20 M) was prepared from anhydrous sodium acetate from E.K. Industries, Inc. (Addison, IL). A three-acid mixture (TH 1)/NaOH buffer was prepared by mixing the three-acid mixture stock solution and sodium hydroxide stock solution in different volume ratios to obtain different pHs. The TH 1 stock solution was a mixture of 0.04 M boric acid, 0.04 M phosphoric acid and 0.04 M acetic acid. All the acids and the sodium hydroxide are from Fisher Scientific (Fair Lawn, NJ). The pH meter used was a Model 955 Accumet mini pH meter from Fisher Scientific, which was calibrated with pH 4.0 and 10.0 buffer solutions that also came from Fisher Scientific. The working solutions were prepared daily by diluting the stock solutions. RESULTS AND DISCUSSION ICP-MS Instrument Setting There are five basic experimental variables associated with ICP-MS: (1) outer gas flow rate, (2) intermediate gas flow rate, (3) injector gas flow rate, (4) forward power, (5) sampling depth (25). The critical parameters were found to be the injector gas flow rate and the forward power. The injector flow rate here is the thermospray system carrier gas flow rate. The forward power was changed from 0.7 to 1.5 kW by ICP-MS software and the sensitivities for nonacidified 100 ng/ml Cr(VI) solution were measured using time scans, which suggested that 1.3 kW was the optimum forward power. Under this forward power, we investigated different carrier gas flow rates, using time scans to collect data. Since at the optimum carrier gas flow rate the sensitivity for 100 ng/ml Cr(VI) was too high and out of the instrument detection range, Cr m/z 53 data were collected instead of m/z 52 data. The optimum sensitivity was observed for a carrier gas flow rate of 1.2

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DIRECT Cr SPECIATION USING THERMOSPRAY TABLE 2 Comparison of TH 1/NaOH Buffer and HAc/NaAc Buffer System for Cr(VI) Measurements

Blank signal (counts/s)

m/z 5 52 m/z 5 53 Ratio b a b

Sensitivity (counts/s z ppb)

Blank RSD (%)

LOD (ng/ml)

DD H 2 O a

HAc/NaAc

TH 1 /NaOH

HAc/NaAc

TH 1 /NaOH

HAc/NaAc

TH 1 /NaOH

HAc/NaAc

TH 1 /NaOH

8650 778 11.12

33620 2550 13.18

17714 1320 13.40

51094 6034

37395 4429

2.3 5.1

2.8 6.0

0.045 0.065

0.040 0.054

Deionized distilled water. Ratio of sensitivity at m/z 52 to sensitivity at m/z 53 for blank signals.

liters/min. Other ICP parameters, as well as ion lens voltages, were adjusted to give the maximum sensitivity for Cr. After ICP-MS instrument parameters were optimized, the short-term stability of the entire system was measured for five injections of a 10 ng/ml Cr(VI) solution. The resultant RSDs were 3.8% at m/z 52 and 5.7% at m/z 53. These values are higher than those obtained from a conventional pneumatic nebulizer. The result could likely be improved with the addition of a membrane desolvation system (19). Spectral Interference Chromium has 4 isotopes at m/z 50, 52, 53, and 54. 52Cr 1 (83.8%) and 53Cr 1 (9.55%) are the isotopes that are relatively free of isobaric interference which can be selected as the masses for chromium measurement. However, 52Cr 1 suffers from the interference of the background species of 40Ar 12C, especially when buffer solutions, which always contain some carbon species, are added. To study the buffer effects on the background level and LOD, a low-carbon-content buffer, TH 1/NaOH, was employed to compare with the HAc/NaAc buffer that was used previously for ICP-AES experiments. Two sets of Cr(VI) standard solutions, each with concentrations of 0.10, 1.0, and 10 ng/ml, were prepared. A buffer of 2 3 10 24 M HAc/NaAc was added to one set of solutions and 2 3 10 24 M TH 1/NaOH buffer was added to the other set of solutions to adjust solution pH to 4.4, which was the optimum chromium speciation pH determined by previous experiments. For each set of solutions, two calibration curves were plotted for both m/z 52 and m/z 53. All the calibration curves are of good linearity with the relative linearity coefficient R greater than 0.999. The results for the blank sensitivity, RSD, and LOD are shown in Table 2. The blank signals for both buffers are higher than that of deionized distilled water (DD H 2O), owning to some spectral interference from the buffer materials. The signal ratios for two Cr isotopes ( 52Cr/ 53Cr) of buffer blank solutions are all larger than that of deionized distilled water, which confirms that the carbon species from buffer materials forms 40Ar 12C and increases 52Cr background more than 53Cr background. As expected, the TH 1/NaOH buffer gave a lower blank signal than NaAc/HAc buffer because of its lower concentration of carbon species. Comparing the LODs for both buffers and both m/z values, we found that m/z 52 provided slightly better LODs than m/z 53, because the abundance of m/z 52 overcomes the disadvantages of its higher background level. The

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ZHANG AND KOROPCHAK TABLE 3 Control Temperature (°C) Effect on Sensitivity and Signal-to-Background Ratio (S/B) of Chromium in Unacidified Solution Control temperature (°C)

Sensitivity (counts/s)

S/B

Blank

Cr(VI)

Cr(III)

Cr(VI)

Cr(III)

90 110 120 125 130 135

5,023 5,891 11,760 13,198 14,972 15,315

1,383 16,925 1,609,080 2,675,639 2,104,660 1,949,749

249 412 24,253 24,715 34,989 44,900

0.28 2.87 136.82 202.73 140.28 127.31

0.05 0.07 2.06 1.87 2.37 2.93

Note. All figures are based on the average of five replicates for samples and five replicates for blanks. The RSDs for all the measurements are within 10%.

LOD with the TH 1/NaOH buffer is slightly better than that for the HAc/NaAc buffer. Therefore, the former buffer was chosen as the working buffer in ICP-MS and m/z 52 as the detection mass for all remaining studies. Chromium Speciation Conditions Since a new buffer was chosen, the conditions for Cr(III) deposition efficiency were reoptimized. Thermospray control temperature is one primary factor (1). The sensitivity and signal-to-background ratio for both species of chromium with a concentration of 100 ng/ml are shown in Table 3, which indicates that 125°C is still the best control temperature for deposition of Cr(III) while maintaining high Cr(VI) sensitivity. A second factor is pump flow rate, which also affects the thermospray system behavior and the Cr(III) deposition process (1). We changed the pump flow rate from 0.20 to 0.40 ml/min in 0.05 ml/min increments, and the optimum flow rate at which Cr(III) was deposited most completely was found to be 0.25 ml/min. The most important factor affecting Cr(III) deposition is solution pH (1). We finely adjusted buffer pH from 4.0 to 5.6 and measured the sensitivity for both Cr(III) (100 ng/ml) and Cr(VI) (10 ng/ml). “Background residual signal” (BRS) was defined as the residual concentration value for Cr(III) calculated using the calibration curve plotted for Cr(VI). From Table 4, we can find that for Cr(III) at pH 4.8, the signal background ratio is the lowest and the BRS is the minimum. Based on the results above, under conditions optimized so far (0.25 ml/min pump flow rate, 125°C control temperature, and pH 4.8), the lowest BRS for 100 ng/ml Cr(III) was 0.30 ng/ml, while the LOD calculated from the Cr(VI) calibration curve based on 33 background standard deviation was 0.050 ng/ml. Deposition Mechanism Since the background residual signal for Cr(III) limits the accuracy of the determination of Cr(VI), especially at picogram-per-milliliter concentrations, we investigated the effects of new parameters on BRS. Considering the high-temperature, high-pressure character-

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DIRECT Cr SPECIATION USING THERMOSPRAY TABLE 4 pH Effect on the Response of 100 ng/ml Cr(III) at Control Temperature 125°C and 0.25 ml/min Pump Flow Rate pH

S/B a BRS b (ng/ml) a b

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

6.81 3.07

2.52 1.47

2.05 1.29

0.99 0.47

0.59 0.30

1.24 0.61

3.45 1.81

4.24 1.53

4.5 1.95

Signal-to-background ratio. Background residual signal (ng/ml).

istics of the thermospray system, we changed the pressure of the system to see how that affected BRS. The length of fused silica capillary was doubled from its original length of 70 cm to 140 cm, increasing the pressure from 80 to 200 kg/cm 2. The BRS for 100 ng/ml Cr(III) solution from this longer thermospray vaporizer was found to be 2.89 ng/ml which was more than eight times higher than the result from the original thermospray vaporizer. Because the length or the pressure of the fused silica capillary vaporizer is the only difference between these two vaporizers, we assumed that the pressure inside the fused silica capillary may play a role in the Cr(III) deposition process, which is an interesting topic under continuing study. The control temperature was reoptimized for this longer vaporizer. From Fig. 2, it can be found that the sensitivities for 1 ng/ml Cr(VI) declined slowly with increasing control temperature, while the sensitivities for 100 ng/ml Cr(III) decreased to a minimum and then increased. The lowest sensitivity was achieved at a control temperature of 145°C and with the minimum BRS of 0.26 ng/ml, which is comparable to the value obtained with the shorter-length vaporizer. To determine if the deposition process is kinetically limited, we increased the heated length of the fused silica capillary by moving the power supply connection point from the inlet of the fused silica capillary backward in 15-cm intervals, thus heating longer and longer lengths of the fused silica capillary. The original total heated length was 55 cm, and the fused silica capillary used here was 140 cm long. The results in Table 5 show that with increasing heated length, the control temperatures at which the optimum BRSs are obtained are also increasing, which can be explained by heat input efficiency calculations (26). With the control temperature measurement position fixed, the longer the length between the inlet power supply connection and the control temperature measurement point (L1), the less power is coupled into the flowing fluid when the control temperature is set the same. The control temperature set value has to be increased to maintain the same amount of power coupled into the flowing fluid with longer L1. As a result, for different heating lengths, it is possible to reproduce a similar chemical environment for Cr(III) inside the fused silica capillary vaporizer, where the Cr(III) deposition chemical reaction takes place, by changing the control temperature. Since increasing the pressurized and/or heated length within the vaporizer increased the residence time of the Cr(III) within the deposition environment, the fact that such increases have no obvious effect on the BRS, and therefore deposition efficiency, suggests that the process is not kinetically limited with the current thermospray configuration and operating conditions.

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FIG. 2. Effects of thermospray control temperature on sensitivities for 1 ng/ml Cr(VI) and 100 ng/ml Cr(III).

For different concentrations of Cr(III), Table 6 shows that the BRS increases with increasing concentration, suggesting that equilibrium is not achieved. The data in Table 6 also suggest that Cr(III) concentrations up to 1 mg/ml will not significantly affect the

TABLE 5 Dependence of Thermospray-Heated Length on 100 ng/ml Cr(III) Deposition Efficiency Control temperature (°C) Heated length (cm)

115

125

135

145

155

165

175

185

195

0.33 6.73

0.55 2.41 5.82

0.31 0.42

0.45 0.38

1.85 1.38

BRS a (ng/ml) 55 70 85 a

7.89

2.89

Background residual signal.

0.81 17.74

0.26 9.26

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DIRECT Cr SPECIATION USING THERMOSPRAY TABLE 6 Background Residual Signal (BRS) for Different Cr(III) Concentrations Cr(III) concentration (ng/ml)

BRS (ng/ml) Deposition efficiency (%) a b

0.1

1.0

10

100

1000

nd 100

nd 100

0.15 98.5

0.30 99.7

0.51 99.95

Conditions: heated length, 55 cm; control temperature, 145°C. Not detectable.

accuracy of selective Cr(VI) measurements in the nanogram-per-milliliter range. This observation is consistent with our previous ICP-AES study. However, for accurate Cr(VI) measurements at the picogram-per-milliliter level, the Cr(III) concentration must be less than 10 ng/ml. To fully take advantage of the lower LODs of ICP-MS with this method, further reduction in BRS is required. CONCLUSIONS This work has applied the thermospray direct, nonchromatographic separation method to ICP-MS detection. The high sensitivity of ICP-MS unveiled a background residual signal of the Cr(III) deposition process that could not be detected with ICP-AES. The BRS will limit the accuracy of sub-nanogram-per-milliliter Cr(VI) measurements when the Cr(III) concentration is larger than 10 ng/ml. To take advantage of the high sensitivity of ICP-MS, with an LOD of 0.050 ng/ml for Cr(VI) using thermospray ICP-MS, reductions in the residual signal are required. Although the deposition process appears to be a straightforward hydrolysis process resulting in the deposition of Cr(OH) 3 inside the thermospray vaporizer, the efficiency of the process exceeds expectations based on equilibrium considerations alone (1). The only comparable studies of which we are aware are related to solubility of minerals in hydrothermal fissures and recently suggest that hydrolysis of metal ions is increased at high temperatures and high pressures (27). These observations may explain the deposition of Cr(III) in thermospray capillaries. Further investigation of this unique chromium deposition process, with the goal of decreasing the BRS for Cr(III), is under continuing study. ACKNOWLEDGMENTS This research was sponsored by the SIUC Office of Research Development & Administration. The authors thank Varian Instruments and Gerald Shkolnik for the Liberty 220 ICP spectrometer and Leeman Labs, Inc., and John Leeman for the thermospray nebulizer system used.

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