Anal. Chem. 2007, 79, 6324-6333
Chemical Vapor Generation of Arsane in the Presence of L-Cysteine. Mechanistic Studies and Their Analytical Feedback Emanuela Pitzalis,† Dele` Ajala,† Massimo Onor,† Roberto Zamboni,‡ and Alessandro D’Ulivo*,†
Institute of Chemical and Physical Processes, Laboratory of Instrumental Analytical Chemistry, National Research Council of Italy, Research Area, Via G. Moruzzi, 1, 56124 Pisa, Italy, and Department of Chemistry and Industrial Chemistry, University of Pisa, Via Risorgimento, 35, 56126 Pisa, Italy
The complex reactivity of the system As-AH-RSH-THB (As ) As(III), As(V); AH ) HCl, HClO4, CH3COOH; RSH ) L-cysteine (Cys); THB ) NaBH4) was investigated using continuous flow (CF) hydride generation (HG) coupled either with atomic absorption (AAS) or atomic fluorescence spectrometry (AFS). AsH3 generation was examined in the presence of Cys by varying acidity and the type of acid, the mixing sequence, and the reaction time of reagents. The strong depression of arsane generation, which is typically observed in the range of acidity of 0.2-2 M HCl, can be addressed to the low reaction rate of thiolborane, hydroboron complexes, or both toward those As(III) substrates that are formed in the same reaction environment. The simultaneous presence of Cys-borane and As(III)-Cys species is at the origin of the gap of the arsane generation efficiency in the 0.2-2 M HCl acidity range. The selective formation of Cys-borane complexes, which are formed faster than As(III)-thiol complexes, can be achieved by a careful choice of the mixing sequence of the reagents. The simultaneous mixing of sample, Cys, and THB is able to reduce substantially the gap of the arsane generation efficiency in the 0.2-2 M HCl acidity range. These properties were employed to implement a simple method for selective determination of As(III) in samples containing inorganic arsenic: (i) Total inorganic arsenic is determined by sample treatment with 0.2 M Cys for 30 min, acidity 0.1 M HCl, followed by CF-HGAFS; (ii) As(III) is selectively determined in 0.005 M CH3COOH in the presence of Cys using a chemifold setup allowing the simultaneous mixing of sample, 0.2 M Cys and 0.1 M THB. The selectivity, measured from the ratio between the slopes of calibration graphs As(III)/As(V), is 220. The interference effects of Cu(II), Fe(III), Ni(II), Co(II), Ag(I), Pd(II), and Pt(IV) can be kept under control using the simultaneous mixing of all the reagents. The tolerance toward the interferences was almost the same as that obtained by allowing the formation of As(III)-Cys complexes (offline sample pretreatment with Cys for 30 min). The method was tested with the application to the natural waters and mineral well waters analysis employing CF-HG-AFS. * To whom correspondence should be addressed. E-mail:
[email protected]. † National Research Council of Italy. ‡ University of Pisa.
6324 Analytical Chemistry, Vol. 79, No. 16, August 15, 2007
The determination and speciation of simple arsenic species dissolved in water samples, as for example inorganic As(III), As(V), monomethylarsonic acid (MMAA), and dimethylarsinic acid (DMAA), is conveniently performed by hydride generation techniques (HG).1 It takes into consideration mainly two different analytical approaches: (i) separation techniques coupled with atomic spectrometric detectors2-4 and (ii) selective generation of the different arsanes.5-9 The second approach is based on the complex reactivity of different arsenic species, As(III), As(V), MMAA, and DMAA with aqueous tetrahydroborate(III) (THB). The generation efficiency of AsH3 (from As(III) and As(V)), CH3AsH2 (from MMAA), and (CH3)2AsH (from DMAA) can be controlled by using different reaction conditions in terms of pH, type of acid or buffer, THB concentration, and additives. The careful choice of reaction conditions has been conveniently employed for selective determination of As(III), As(V), MMAA, and DMAA without the use of any separation techniques.5-9 Concerning total arsenic (As(III) + As(V) + MMAA + DMAA) determination, it can be achieved by HG using pretreatment with Cys at low-acidity reaction conditions with THB10 or at high concentration of THB and HCl.11 In particular, the effect of L-cysteine (Cys)12-14 or other thiolic compounds7,15 in combination with different acids and pH conditions is interesting because it can change dramatically the reaction efficiency of the different arsenic species in the HG apparatus employed. The most popular (1) Deˇdina, J.; Tsalev, D. L. Hydride Generation Atomic Spectrometry; Wiley: Chichester, 1995; Chapter 2. (2) Le, C.; Cullen, W. R.; Reimer, K. Talanta 1994, 41, 495-502. (3) Gong, Z.; Lu, X.; Ma, M.; Watt, C.; Le, C. Talanta 2002, 58 77-96. (4) Karadjova, I. B.; Lampugnani, L.; Onor, M.; D’Ulivo, A.; Tsalev, D. L. Spectrochim. Acta, Part B 2005, 60, 816-823. (5) Shraim, A.; Chiswell, B.; Olszowy, H. Talanta 1999, 50, 1109-1127. (6) Shraim, A.; Chiswell, B.; Olszowy, H. Analyst 2000, 125, 949-953. (7) Carrero, P.; Malave´, A.; Burguera, J. L.; Burguera, M.; Rondo´n, C. Anal. Chim. Acta 2001, 438, 195-204. (8) Sigrist, M. E.; Beldome´nico, H. R. Spectrochim. Acta, Part B 2004, 59, 10411045. (9) Serafimovski, I.; Karadjova, I. B.; Stafilov, T.; Tsalev, D. L. Microchem. J. 2006, 83, 55-60. (10) Le, X. C.; Cullen, W. R.; Reimer, K. J. Anal. Chim. Acta 1994, 285, 277285. (11) Cabon, J. Y.; Cabon, N. Anal. Chim. Acta 2000, 418, 19-31. (12) Brindle, I. D.; Le, X. C. Anal. Chem. 1989, 61, 1175-1178. (13) Chen, H.; Brindle, I. D.; Le, X. C. Anal. Chem. 1992, 64, 667-672. (14) Welz, B.; Sˇ ucmanova´, M. Analyst 1993, 118, 1417-1423. (15) Anderson, R. K.; Thompson, M.; Culbard, E. Analyst 1986, 111, 11431152. 10.1021/ac070513p CCC: $37.00
© 2007 American Chemical Society Published on Web 07/18/2007
and widely employed thiol is, without any doubt, Cys, which was first proposed by Brindle and Le.12 The presence of Cys changes dramatically the optimal range of acidity at which arsane is generated, from ∼1 M acidity in the absence of Cys down to 0.020.1 M acidity in the presence of Cys, depending on the type of HG apparatus.14,13 Further studies, reported later by other authors,5-7 demonstrated that arsane can be efficiently generated also at very high acidities (5-10 M HCl) in the presence of Cys. Therefore, in the presence of Cys, arsane can be efficiently generated either at low acidities (below 0.1 M HCl) or very high acidities (above 5 M HCl), while there is a gap of generation efficiency in an intermediate range around 0.2-2 M HCl. This peculiar reactivity of arsenic compounds has received little attention7 and still remains to be clarified with hypotheses, which are not in contrast with the reactivity of borane complexes in aqueous solution. Carrero et al.7 appears the only group who formulated hypotheses on the mechanism of arsane generation in Cys-HCl media. They concluded that at elevated acidities the arsane formation could be ascribed to atomic or “nascent” hydrogen (H•), formed by THB hydrolysis, while at low acidities the arsane is formed by the action of BH4-. This mechanism is in contrast with much evidence reported recently that clarifies many controversial aspects of the mechanism of HG by THB derivatization. The evidence can be summarized as follows: (i) THB is hydrolyzed stepwise forming hydroboron intermediates of the type [LnBH4-n]m (L ) OH-, H2O, etc., n ) 0-3, m ) 0, (1 is the charge depending on L);16 (ii) atomic, or “nascent hydrogen”, cannot be formed by THB hydrolysis;17-19 (iii) all hydroboron intermediates [LnBH4-n]m containing at least one B-H bond can be active in the HG, but some elements react selectively only with some of them;16 (iv) the hydrides are formed (pH 0-12) by direct transfer of hydrogen from boron to analyte atom most likely through concerted mechanism taking place via an analyte-borane complex intermediate.20-22 The above-reported evidence also gives an opportunity for a better understanding of the reactivity of different elements forming volatile derivatives by aqueous THB reaction, as in the case of transition and noble metals recently discussed.23 In order to clarify fundamental aspects related to the role of additives in HG, a study is under development in the author’s laboratory investigating the reactivity of the As-AH-RSH-THB reaction system. The attention is here focused on the role of Cys in arsane generation from As(III) and As(V), and on those results that also present interesting implications in selective arsenic(III) determination in the presence of inorganic As(V), with no sensitivity losses, high selectivity ratio, and interference-free conditions. (16) D’Ulivo, A.; Onor, M.; Pitzalis, E. Anal. Chem. 2004, 76, 6342-6352. (17) Laborda, F.; Bolea, E.; Baranguan, M. T.; Castillo, J. R. Spectrochim. Acta, Part B 2002, 57, 797-802. (18) D’Ulivo, A. Spectrochim. Acta, Part B 2004, 59, 893-825. (19) D’Ulivo, A.; Onor, M.; Pitzalis, E.; Spinello, R.; Lampugnani, L.; Cristoforetti, G.; Legnaioli, S.; Palleschi, V.; Salvetti, A.; Tognoni, E. Spectrochim. Acta, Part B 2006, 61, 797-802. (20) D’Ulivo, A.; Baiocchi, C.; Pitzalis, E.; Onor, M.; Zamboni, R. Spectrochim. Acta, Part B 2004, 59, 471-486. (21) D’Ulivo, A.; Mester, Z.; Sturgeon, R. E. Spectrochim. Acta, Part B 2005, 60, 423-428. (22) D’Ulivo, A.; Mester, Z.; Meija, J.; Sturgeon, R. E. Anal. Chem. 2007, 79, 3008-3015. (23) Feng, Y. L.; Sturgeon, R. E.; Lam, J. W.; D’ Ulivo, A. J. Anal. At. Spectrom. 2005, 20, 255-265.
EXPERIMENTAL SECTION Chemicals. The 1 M NaBH4 (THB) stock solutions were prepared by dissolving the solid (BDH, pellets, reagent for AAS, assay 96%) in 0.5 and 0.025 M NaOH, respectively. The solutions were then microfiltered on a 0.45-µm membrane and stored in the refrigerator; working solutions of 0.1 M THB in 0.005 M NaOH were prepared daily by proper dilution of the stock solutions and adjustment in NaOH concentration. L-Cysteine (powder, assay >99.0%) was purchased from Fluka. Stock solutions of As(III) and As(V) were prepared by dilution of commercial Fluka 1000 mg L-1 AAS standard solutions. MMAA and DMAA stock solutions (1000 mg L-1 As) were prepared from CH3AsO(ONa)2‚6H2O (Carlo Erba, Codex, assay >99.4%) and (CH3)2AsO(ONa)‚3H2O (Carlo Erba, Codex, assay >96%), respectively. Purity checks on MMAA and DMMA stock solutions were performed by HPLCHG-atomic fluorescence spectroscopy (AFS)4 and indicated that inorganic As(V) was the main impurity, while As(III) was not detectable (
