Generation of Atomic and Molecular Cadmium Species from Aqueous

Anal. Chem. 2003, 75, 635-640

Generation of Atomic and Molecular Cadmium Species from Aqueous Media Yong-Lai Feng, Ralph E. Sturgeon,* and Joseph W. Lam

Institute for National Measurement Standards, National Research Council of Canada, Ottawa, K1A 0R9, Canada

Generation of atomic and molecular species of cadmium following reaction of acidified sample with sodium tetrahydroborate reductant using a modified parallel path Burgener nebulizer has been accomplished. Once generated, atomic species are stable in water, having a half-life of 2.2 min, and can be preconcentrated for subsequent quantitation. Although the initial products of the reduction reaction appear to be free atoms, subsequent interaction with nascent hydrogen gives rise to the molecular hydride in a transformation moderated by the chemistry of surface groups exposed to the reaction products. Use of a glass spray chamber results in a predominance of atomic vapor whereas a polymeric spray chamber releases the volatile molecular species. Chemical vapor generation has frequently been used to enhance, among other attributes, the sample introduction efficiency for solution-based measurements using atomic spectroscopy. The most common, and well-established, procedure is the reduction and conversion of analyte ions into an atomic vapor or a volatile hydride species. These reactions and the analytes that respond most favorably have been well studied. Recently, however, there have also been reports of the successful generation and detection of several “unconventional” volatile species arising from reaction of acidified sample solutions with tetrahydroborate(III) reagent.1-12 Among these, cadmium appears to be the most interesting, with direct detection and identification of both molecular (presumably the hydride) and atomic forms of the * Corresponding author: (fax) 613 993 2451; (e-mail) [email protected]. (1) Sturgeon, R. E.; Liu, J.; Boyko, V. J.; Luong, V. T. Anal. Chem. 1996, 68, 1883-1887. (2) Luna, A. S.; Sturgeon, R. E.; de Campos, R. C. Anal. Chem. 2000, 72, 35233531. (3) Sanz-Medel, A.; Valdes-Hevia y Temprano, M. C.; Bordel Garcia, N.; Fernandez de la Campa, M. R. Anal. Chem. 1995, 67, 2216-2223. (4) Lu, Y.-K.; Sun, H.-W.; Yuan, C.-G.; Yan, X.-P. Anal. Chem. 2002, 74, 15251529. (5) Hwang, T.-J.; Jiang, S.-J. J. Anal. At. Spectrom. 1997, 12, 579-584. (6) Vargas-Razo, C.; Tyson, J. F. Fresenius J. Anal. Chem. 2000, 366, 182190. (7) Matusiewicz, H.; Kopras, M.; Sturgeon, R. E. Analyst 1997, 122, 331336. (8) Garrido, M. L.; Munoz-Olivas, R.; Camara, C. J. Anal. At. Spectrom. 1998, 13, 295-300. (9) Pohl, P.; Zyrnicki, W. Anal. Chim. Acta 2001, 429, 135-143. (10) Matousek, T.; Dedina, J.; Vobecky, M. J. Anal. At. Spectrom. 2002, 17, 52-56. (11) Duan, X.; McLaughlin, R. L.; Brindle, I. D.; Conn, A. J. Anal. At. Spectrom. 2002, 17, 227-231. (12) Feng, Y.-L.; Lam, J. W.; Sturgeon, R. E. Analyst 2001, 126, 1833-1837. 10.1021/ac020529+ CCC: $25.00 Published 2003 Am. Chem. Soc. Published on Web 01/04/2003

element.2,3 It has been speculated that cadmium hydride is actually initially generated but that this intermediate product is unstable, subsequently dissociating into atomic cadmium, perhaps during the measurement process itself.3 In this study, both molecular and atomic cadmium species are generated at the point of convergence of an acidified sample stream with that of a tetrahydroborate(III) reductant in a modified Bergener parallel path nebulizer.12 Evidence is presented to show that the initial product of the reaction is atomic cadmium. Its formation, stability, and solubility in aqueous solutions, as well as the process of its conversion from the atomic to the molecular species in the liquid phase, are discussed. These processes have been found to be moderated by the chemistry of surface groups exposed to the reaction products; i.e., use of a glass spray chamber results in a predominance of atomic vapor whereas a polymeric spray chamber releases the volatile molecular species. This is the first report of the collection of cadmium atoms in the aqueous phase for subsequent enhanced signal-to-noise-ratio detection in the gas phase. EXPERIMENTAL SECTION Instrumentation. Atomic absorption measurements were performed using a Perkin-Elmer (Norwalk, CT) Aanalyst 100 atomic absorption spectrophotometer fitted with a deuterium source background corrector. A heated quartz T tube replaced the burner and was located in the optical path and supported by the resistance heater mantle. Measurements of both Cd and Hg were undertaken with the tube at room temperature (cold vapor), whereas molecular species of Cd were inferred by AAS measurements of atomic Cd with the tube heated to 900 °C. This was accomplished using a Perkin-Elmer FIAS-400 flow injection accessory to provide the power and temperature feedback for the quartz tube. The former was controlled with the use of a dedicated, separate PC. Hollow cathode lamps (HCL) for Cd and Hg (Varian Techtron Pty.) were operated at the manufacturer’s recommended currents with a nominal spectral bandwidth of the instrument set at 0.2 nm. The resonance lines of the elements were used for measurement, viz., 228.8 and 253.7 nm for Cd and Hg, respectively. The spectrometer was interfaced to a PC, and signals were manipulated and stored using commercial Perkin-Elmer software. A Perkin-Elmer Sciex (Concord, ON, Canada) model Elan 5000 ICPMS machine was operated under conditions summarized in Table 1 for detection of cadmium and mercury. A modified Burgener parallel path nebulizer (Burgener Research Inc., Mississauga, ON, Canada) was used throughout this study for the generation of volatile species. Details of its design and operation Analytical Chemistry, Vol. 75, No. 3, February 1, 2003 635

Table 1. ICPMS Operating Conditions rf power plasma gas flow rate intermediate gas flow rate “shear” gas flow rate makeup gas flow rate scanning mode replicate time dwell time sweeps/reading reading/replicate number of replicates points/spectral peak resolution skimmer/sampler cone sample flow rate NaBH4 flow rate

1000 W 15 L/min 1.1 L/min 0.4 L/min 0.45 L/min peak hop 100 ms 100 ms 1 1 variable 1 normal nickel 1 mL/min 1 mL/min

can be found elsewhere.12 The modified nebulizer was typically inserted into a standard Scott-type double-pass glass or Ryton spray chamber, these effectively serving as gas-liquid separators. Alternatively, in the “collection mode”, illustrated in Figure 1A, the nebulizer was inserted through the side wall of a 2.5-cm-i.d. glass gas sparger containing ∼10 mL of deionized, distilled water. The tip of the nebulizer protruded through a port fabricated in the wall of the vessel (sealed with Teflon) ∼5 mm from the bottom and such that a 5-mm head of water remained above the tip. The outlet from the sparger was directed to the plasma torch for introduction of any volatile forms of the metal so produced. When using ICPMS as the detector, an external mass flow controller was used to regulate the Ar flow rate through the three-way valve fitted to the sparger, thereby ensuring a constant gas flow to the plasma under optimized conditions. The Ar used for this purpose was first humidified by passage through a water bubbler in an effort to minimize changes in the excitation conditions in the plasma arising from the different modes of sample introduction used. The plasma was sampled at a constant depth of 10 mm. Reagents. Stock solutions (1000 µg/mL, 99.99% purity) of As(III), Se(IV), Cd(II), and Hg(II) were obtained from SCP Science (Montreal, PQ, Canada). Working standards of lower concentrations were prepared by serial dilution of the stocks using 18 MΩ‚cm deionized, reverse osmosis water (DIW) obtained from a mixed-bed ion-exchange system (NanoPure, model D4744, Barnstead/Thermoline, Dubuque, IA). High-purity sub-boiling distilled HCl and HNO3 were prepared in-house from reagent grade feedstocks and used for the acidification of the working standards. A 1% (m/v) solution of sodium tetrahydroborate(III) in 0.001 M NaOH was prepared daily or more frequently if required. Both reagents were purchased from Alfa Aesar (Ward Hill, MA). All solutions were prepared in a class 100 working environment, and all glassware was cleaned by soaking for at least 24 h in 1 M HNO3 and rinsing with DIW before use. Argon gas was obtained from Praxair (Mississauga, ON, Canada). A 1-L gas bottle fitted with inlet and outlet valves and a septum side port was spiked with 100 mg of liquid mercury, flushed with Ar, and permitted to equilibrate to room temperature, providing a convenient supply of gaseous mercury vapor which could be conveniently and precisely sampled with the aid of a gastight microliter syringe. Procedure. For operation in the “collection mode”, a fourchannel peristaltic pump was used to continuously supply the 636 Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

Burgener generator with a 0.1 M HCl solution in one channel and a 1% (m/v) of sodium tetrahydroborate(III) solution (in 0.01 M NaOH) in a second channel, each at a flow rate of 1 mL/min. A sample containing various concentrations of Cd2+ (ranging from 5 to 50 ppb) in 0.1 M HCl was manually valved into the 0.1 M HCl carrier stream in a flow injection mode with the aid of an 80-µL sample loop. The Ar gas (500 mL/min) was admitted to the sparger vessel from the “makeup” intake position, thereby continuously sweeping the contents of the headspace above the liquid into the detection system (quartz tube or ICPMS). Normally, a waste line continuously removed solution from the sparger vessel at a rate sufficient to maintain the liquid level constant (i.e., 2 mL/min), but during the sampling procedure, pumping through the waste line was stopped. After the reaction was complete, the peristaltic pump was stopped and the Ar gas flow was manually diverted to the “carrier” position, flushing any volatile species that had been “dissolved” in the liquid phase to the detection system. This permitted room-temperature measurement of Cd atomic vapor or its high-temperature atomization products to be measured. Following the measurement, the waste line was then used to evacuate the sparger vessel. The effect of operating temperature on the relative sensitivity of the quartz tube atomizer was assessed at room temperature and at 900 °C by comparing the integrated response resulting from the introduction of a constant volume of Ar gas saturated with mercury vapor. Continuous generation and measurement of volatile species was achieved using the system shown schematically in Figure 1B. Here, a 0.1 M solution of HCl and a 1% (m/v) sodium tetrahydroborate (in 0.001 M NaOH) solution were continuously and separately pumped to the Bergerner generator. A “shear gas” flow of 0.40 L/min was maintained. The cadmium samples were introduced into the HCl stream in a flow injection format using a sampling loop, and the generated volatile species were separated in a Scott double-pass glass or Ryton spray chamber and transported to the AAS or ICPMS detection system. For ICPMS measurements, the plasma was established and the instrument permitted to stabilize for at least 30 min prior to undertaking any measurements. Hydride generation of As and Se was used to monitor and optimize the daily performance of the system, as these elements have been well-studied and their generation characteristics are both well known. For daily optimization, the hydrides were produced from a standard solution containing 10 ng/mL concentrations of As and Se in 1 M HCl. The solution was continuously introduced, along with a 1% (m/v) solution of tetrahydroborate(III) reagent, to the nebulizer. The shear gas (Figure 1B) and makeup gas flows were adjusted independently in order to attain maximum response. A shear gas flow of 0.4 L/min and makeup gas of 0.45 L/min were generally required for maximum sensitivity with concurrent minimal aerosol introduction into the plasma. Additional variables optimized included sample and reagent solution flow rate, as well as acid and reductant concentrations. In general, the ion intensities obtained after the optimization process were reproducible and varied within 5% from day to day. All data files were exported for quantitative processing using MS Excel software.

Figure 1. Schematic diagram of vapor generation AAS/ICPMS system: (A) collection mode; (B) continuous mode.

RESULTS AND DISCUSSION Formation of gaseous cadmium species for analytical purposes has stimulated considerable interest, not only because of the detection of a “cold” vapor but also because of the inference that it is initially generated as an unstable, volatile molecular species which then decomposes to yield the atomic vapor.3 It is well known that mercury can be readily reduced in aqueous solutions and liberated as the cold vapor. Because of its finite solubility in water, the vapor can be literally collected and then released for subsequent measurement, provided it is not oxidized in the interim in this medium. Figure 2A illustrates several signal “transients” that resulted from collection of the newly generated cold vapor under a column of room-temperature water with its subsequent liberation in a single pulse by sparging the water column with a transfer gas of Ar and directing the released vapor to the ICPMS. It is evident that the solubility of Hg0 in the aqueous phase is rather low at room temperature, as it is not possible to accumulate this element for any significant period of time without evidence of its continual ‘leakage” from the aqueous phase and escape to the headspace, whereupon it is immediately conducted to the detection system. Nevertheless, it is clear from the rapid spike achieved that a collection or preconcentration of the sample has been accomplished. It is well known that the solubility of a gas in water is temperature dependent, the solubility increasing as the temperature decreases. Such is the case for both mercury and cadmium in the present system, as was concluded by conducting measurements at the extreme ranges of temperature available with this aqueous system. Figure 2B presents the results from an experiment identical to that described above but with the temperature of the aqueous phase used for collection of mercury being reduced to 2 °C in an ice bath. It is clear from this comparison that the efficiency with which mercury is collected in the cooler water is enhanced, leading to improved response. This characteristic is consistent with the dissolution of a gas in water.

When this same experiment was repeated using cadmium as the analyte, no such bleeding of the product into the headspace was detected at room temperature and a single, well-defined spike was measured, as illustrated by the data in Figure 2C. This suggests that the volatile reaction product is more “soluble” in the aqueous phase than was elemental mercury. Conversely, when this experiment was repeated at a temperature of 80 °C, there is evidence of significant “bleeding” of the species from the liquid phase, as shown in Figure 2D, and the intensity of the resulting peaks was diminished, consistent with the reduced solubility of a gas in water at elevated temperatures. In an effort to identify the volatile cadmium species generated by this procedure, the system was connected to the quartz tube atomizer for AAS detection (cf. Figure 1). As such, it was possible to ascertain whether the species was atomic or molecular in nature by simply noting the effect of the tube temperature on response. Atomic absorption signals were achieved with the quartz tube at room temperature, leading to the conclusion that the species collected in and released from the aqueous phase was, in fact, atomic cadmium. Figure 3 illustrates data that support the conclusion that the atomic Cd0 is stable in this solution (i.e., initially pure DIW, final pH 6-12 after reaction). The ability to concentrate the generated cadmium as Cd0 over the duration of the generation step as well as the measurement process attests to the stability of the species. Further, it is evident that its solubility limit has not been exceeded with the concentrations of analyte investigated here, i.e., total of 15 ng in a 10-mL volume of collection phase. Such stability permitted construction of a linear calibration curve based on the integrated absorbance response from the preconcentrated analyte, illustrating that this approach can be applied to the practical determination of cadmium. An estimate of the half-life of the Cd0 in solution was made by generating and collecting a constant 4 ng of the analyte under water while permitting the resultant solution to stand undisturbed Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

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Figure 2. (A) 202Hg intensity-time scans for ICPMS detection of mercury following generation of the cold vapor by continuous merging of a 0.1 M HNO3 containing 10 ng/mL mercury (or blank) with a 1% (m/v) solution of sodium tetrahydroborate(III) at a flow rate of 1 mL/min. The resulting vapor was collected in a 10-mL volume of DIW at room temperature and subsequently sparged from the liquid phase and transported to the plasma torch in a 0.85 L/min flow of Ar: (a) 2-, (b) 4-, and (c) 8-min generation/collection period. (B) 202Hg intensity-time scans for ICPMS detection of mercury following generation of the cold vapor by continuous merging of a 0.1 M HNO3 containing 10 ng/mL mercury (or blank) with a 1% (m/v) solution of sodium tetrahydroborate(III) at a flow rate of 1 mL/min. The resulting vapor was collected in a 10-mL volume of DIW at a temperature of 2 °C and subsequently sparged from the liquid phase and transported to the plasma torch in a 0.85 L/min flow of Ar: (a) 1-, (b) 2-, (c) 4-, and (d) 8-min generation/collection period. (C) 114Cd intensity-time scans for ICPMS detection of cadmium following generation of the cold vapor by continuous merging of a 0.1 M HCl containing 10 ng/mL cadmium (or blank) with a 1% (m/v) solution of sodium tetrahydroborate(III) at a flow rate of 1 mL/min. The resulting vapor was collected in a 10-mL volume of DIW at room temperature and subsequently sparged from the liquid phase and transported to the plasma torch in a 0.85 L/min flow of Ar: (a) 2-, (b) 4-, and (c) 8-min generation/collection period. (D) 114Cd intensity-time scans for ICPMS detection of cadmium following generation of the cold vapor by continuous merging of a 0.1 M HCl containing 10 ng/mL cadmium (or blank) with a 1% (m/v) solution of sodium tetrahydroborate(III) at a flow rate of 1 mL/min. The resulting vapor was collected in a 10-mL volume of DIW at a temperature of 80 °C and subsequently sparged from the liquid phase and transported to the plasma torch in a 0.85 L/min flow of Ar: (a) 2-, (b) 4-, and (c) 8-min generation/collection period.

(apart from continuously sweeping the headspace above the liquid free of any volatile product) for increasing amounts of time prior to sparging the remaining element from the aqueous phase to the detector. Assuming a first-order loss of product, a simple logarithmic fit of the data to a first-order rate equation resulted in a calculated half-life of ∼2.2 min. Loss of the analyte may occur 638 Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

as a result of its diffusive release from the solution in a slow but undetectable manner, oxidation of the atoms to an ionic form soluble in the DIW, and adsorption onto the vessel walls. Using solution nebulization ICPMS, it was verified that 95% of the Cd, which could be preconcentrated and sparged from this vessel, remained in the solution phase if the analyte was permitted to

Figure 3. Dependence of background-corrected absorbance at 228.8 nm on volume of 30 ng/mL Cd2+ solution processed: (A) 80µL loop; (B) 500-µL loop.

stand for 30 min undergoing “decay”. It is therefore clear that reoxidation of the Cd0 species occurs and is the principal mechanism of loss. If the generated cadmium species (using a glass spray chamber) are passed through various absorber media comprising pure water, 0.5 M NaOH, or 0.5 M HCl, the signal recorded using a quartz tube atomizer at room temperature is attenuated by approximately 50%, 50%, and 80%, respectively. Clearly, the resulting species have good solubility in DIW or basic solutions and are easy oxidized in acidic media, consistent with the above mechanism of loss that occurs on “long-term” storage. Mechanism of Vapor Generation. The atomic absorption response arising from the introduction of cadmium vapor following its direct generation with the use of the Burgener nebulizer fitted with a double-pass spray chamber was examined using the quartz tube atomizer. Free atoms of atomic vapor can be detected at room temperature whereas molecular species do not contribute to the signal, necessitating the cell be heated to a sufficient temperature (900 °C) to ensure their dissociation. When a double-pass glass spray chamber (Figure 1B) was used, the signal recorded with the quartz cell at room temperature is higher than that obtained when the tube was heated to 900 °C. This effect has been reported earlier by Sanz-Medel et al.3 and is a consequence of the increased rate of loss of analyte vapor from the quartz cell by increased diffusion at the higher temperature. This effect was quantified using atomic mercury vapor to experimentally determine the ratio of signals at room temperature and 900 °C. For this purpose, a small volume (20 µL) of Ar gas that had been equilibrated with liquid mercury at room temperature in a septum sealed vessel was injected into the carrier gas line with the aid of a gastight syringe. A value of 2.0 ( 0.1 was obtained for the ratio of integrated response at room temperature and 900 °C, similar to that reported by Sanz-Medel et al.3, i.e., 2.6-2.8 (in this case, a larger ratio was obtained as the QTA was heated by a flame as

Figure 4. Comparison of background-corrected AAS response for cadmium at 228.8 nm in a (A) room-temperature and a (B) 900 °C quartz tube atomizer with flow injection mode of cadmium vapor generation from a generator fitted with a double-pass glass spray chamber. Sample volume: 80-µL loop containing a 50 ng/mL solution of cadmium.

opposed to electrically and the possibility for decreased response due to turbulence may have occurred). From Figure 4, it can be seen that the experimental ratio for cadmium generated in a system fitted with a double-pass glass spray chamber is somewhat less than 2; quantitatively, a ratio of 1.64 ( 0.09 was determined. By contrast, if a double-pass Ryton spray chamber is used under the same generation conditions, it is significant that the signal for cold vapor Cd completely disappears, as illustrated in Figure 5. It is well known that sulfur can readily bind “soft” metals, and thus, the sulfur functional groups present on the surface of the Ryton [poly(p-phenylene) sulfide] spray chamber may react with newly generated atomic cadmium, prohibiting their escape from the generator system. Vapor generation of mercury by solution reduction is generally accepted to result in the direct production of the atomic species. However, the data presented in Figure 6, characterizing the experimental system used here, suggest a rather unusual conclusion. When the Ryton spray chamber was used, the ratio of coldto-hot signal for mercury in the quartz tube was determined to be 0.87 ( 0.05, whereas when a glass spray chamber is used this signal ratio is significantly different, i.e., 1.8 ( 0.1 (i.e., equivalent to the expected effect of the enhanced diffusion at the higher temperature). These results suggest that, even for mercury, a molecular form of the element appears to be generated, although its relative amount is very small compared to the atomic species. The implication is that when a conventional glass vapor generation Analytical Chemistry, Vol. 75, No. 3, February 1, 2003

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tube is heated to 900 °C, the AAS signal can be observed. Interestingly, the Ryton surface seems to function as an extraction device that selectively removes atomic species, permitting that fraction present as a molecular species to be released and subsequently detected. These observations find explanation with a model for vapor generation which requires that the atomic form of the element is initially generated followed by the formation of the molecular compound (presumably CdH2). An alternative explanation is that a fraction of the analyte is initially generated as a molecular species but with much lower efficiency than its atomic counterpart, since the cadmium signal arising from a glass spray chamber is much higher than that from a Ryton spray chamber. The generation efficiency of molecular Cd species relative to atomic Cd was determined to be ∼5%, based on the integrated relative responses from 100 and 1000 ng/mL solutions of Cd2+ in systems fitted with glass and Ryton spray chambers, respectively. Figure 5. Comparison of background-corrected AAS response for cadmium at 228.8 nm in a (A) room-temperature and a (B) 900 °C quartz tube atomizer with flow injection mode of cadmium vapor generation from a system fitted with a double-pass Ryton spray chamber. Sample volume: 80-µL loop containing a 5 µg/mL solution of cadmium.

Figure 6. Comparison of background-corrected AAS response for mercury at 253.7 nm in a room-temperature and a 900 °C quartz tube atomizer with flow injection mode of mercury vapor generation from systems fitted with double-pass (A) Ryton and (B) glass spray chambers. Sample volume: 80-µL loop containing a 1 µg/mL solution of mercury. Signal intensities for cold/hot are (A) 0.87 ( 0.05 and (B) 1.8 ( 0.1.

system is used, the majority of the atomic mercury species is easily released, with the consequence that the molecular species cannot be efficiently observed. In the case of cadmium (Figure 5), the vapor arising from use of a generator fitted with a Ryton spray chamber does not give rise to a detectable Cd0 cold vapor AAS signal; when the quartz 640

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CONCLUSION A possible two-step mechanism accounting for the generation of volatile Cd species may be described by the following set of reactions:

BH4- + 3H2O + H+ ) H3BO3 + 8H*

(1)

H* + Cd2+(aq) ) Cd0(ad, g) + H2(excess)

(2)

H* + Cd0(ad) ) CdH2(g)

(3)

In a system fitted with a glass spray chamber, a thin film of water is present on the surface from which the cadmium atomic species are easily released and contribute to a cold vapor signal. In a system fitted with a Ryton spray chamber, the cadmium atomic species appear to be removed by formation of an adsorbed or immobilized species bound to the sulfur groups on the surface, with the result that only molecular cadmium species are released from the surface. In such case, only a high-temperature quartz tube atomizer results in an AAS signal. The influence of the surface of the spray chamber suggests that the generation reaction may comprise a two-step process, wherein the second step of the reaction is significant because it occurs on the surface and is mediated by its chemistry. The use of both AAS with the heated quartz atomizer and ICPMS provided a synergistic approach to the elucidation of the vapor generation step, which could not have been accomplished with the latter alone. ACKNOWLEDGMENT We thank J. Burgener of Burgener Research Inc. for his generous support in providing the modified nebulizer used in this study and P. L’Abbe of the NRC glassblowing shop for his technical assistance in the fabrication of the sparger unit. Y.-L.F. is grateful to NSERCanada and the NRC for financial support in the form of a postdoctoral fellowship. Received for review August 15, 2002. Accepted November 14, 2002. AC020529+