Anal. Chem. 1992, 64, 7374-1370
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Chemical Generation of Chlorine, Bromine, and Iodine for Sample Introduction into a Surfatron-Generated Argon Microwave-Induced Plasma M. Dolores Calzada, M. Carmen Quintero, Antonio Gamero,' and Mercedes Gallegot Department of Applied Physics, Faculty of Sciences, University of Cbrdoba, 14071 Cdrdoba, Spain
A new analyticalmethodthat combines on-line gas generation and MIP-Surfatron for sample lntroductlon and analysls Is reported. Chlorine, bromine, and Iodine are generated contlnualiy from aqueous solutions by uslng a lead( I V ) oxlde minlcoiumn connected to the plasma excitation source via a gas-liquld separator. This permlts lntroductlon of the vaporized sample directly Into the plasma by using the support gas flow as analyte carrier gas. The Influence of the plasma parameters was studied in order to optimize the experimentalconditions. The emlsslon from the analyteswas monitoredon the followlng spectralilnes: Cl( I ) 837.595, Br( I)827.244, and I( I ) 804.374 nm. The detection llmits for the generated Ciz, Brz, and Ip were 50 ng/mL Ci, 50 ng/mL Br, and 200 ng/mL I, respectively (aqueous solution). The method features a linear range of over 2 orders of magnltude. A series of distilled water samples spiked wtth different amounts of halides were analyzed In order to test the applicability of the method.
INTRODUCTION The use of various plasma excitation sources in atomic emission spectroscopy (AES)has grown dramatically in recent years because of the wealth of emission lines obtained, which enable interelement selectivity and permit multielements detection. Particularly, microwave induced plasma (MIPS) have some advantages as excitation sowces in plasma emission spectrometry compared to conventional excitation plasma sources, even though inductively coupled plasmas (ICP) are historically the most widely studied and applied.' One chief reason is the higher plasma excitation temperature achieved, which allows efficient excitation of halogens and other nonmetals not readily accessible to ICP detection, with adequate sensitivity. Moreover, the low power requirements of MIPS, typically 100W or less, result in both low background emission and minimal needs for cooling (the support gas flow rate can be lower than 1L/min which allows the analyte sample carrier gas to be used as a cooling gas). On the other hand, the low gas temperature in MIPS (ca. 2000 K) limits their capacity to volatilize solid or liquid samples and atomize the analyte species. In addition, most MIPs (generally of resonant cavity) are readily perturbed and even extinguished by small impedance changes. This causes major problems with sample introduction and has constrained wide implementation of MIPs. Many of these problems can be avoided if the sample is a gas or vapor, for which a variety of sample introduction techniques have been developed,2 although their most extensive use remains in + Present address: Department of Analytical Chemistry, Faculty of Sciences, University of CBrdoba, 14071 CBrdoba, Spain. (1)Barnes, R.M. CRC, Crit. Rev. Anal. Chem. 1978,7 , 203-207. (2)Matousek, J. P.; Orr, B. J.; Selby, M. Progr. Anal. A t . Spectrosc. 1984,7, 276-314.
0003-2700/92/0364-1374$03.00/0
combination with gas chromatography.3 In this work we used a MIP produced a surface-wave launcher structure (device called Surfatron4which has been thoroughly studied in the last few years.5 It seems to be a more adequate excitation source than other MIPS because surface-wave-produced plasmas are very stable and reproducible (depending on the microwave generator and flowrate stabilities). Moreover, they operate over a wide range of experimental conditions (microwavefrequency and power, gas, pressure, gas flow rate, discharge tube diameter, etc.), which allows for ready control of the discharge parameters. Capillary columns coupled to TWlo cavity,with He plasma, have been used for the determination of haloforms in drinking water6 and halogenated compounds.7 More recently, halogenated hydrocarbons have been detected by gas chromatography with a helium plasma-mass spectrometer equipped with a modified TMolo cavity.6 Also, chlorine, bromine, and iodine have been analyzed by ICP with limits of detection of 1.6 and 0.82 g/L and 0.9 mg/L, respectively? and halides and organo halogen compounds have been assayed by arc discharge (with detection limits of 100,500, and 8 pg/L for C1, Br, and I, respectivelylO). Halides in pesticides11and halogens12 were also determined by coupling gas chromatography to a MIPSurfatron using helium as gas plasma. Chemical vaporization in conjunction with MIP has been reported for the determination of chlorine and bromine by several a ~ t h o r s . ' ~ -Recently, l~ BarnetP studied the chemical generation of chlorine and bromine and their respective hydrides from aqueous solutions with a variety of reagents: KMn04-H~S04(CW, HzS04 (HCl), K2Cr20,-HzSO4 (Brz), and HzSO4 (HBr). Five milliliters of each reaction mixture was delivered into a flask furnished with a rubber septum for (3)Ebdon. L.: Hill. S.: Ward. R. W. Analvst 1986.111. 1113-1138. (4) M o i s i , M.; Zakrzewski, Z.; Pantel, R: J. Phys. D 'Appl. Phys. 1979. 12.219-224. (5) Mo&an,M.; Zakrzewski, 2. In Radiative Processes in discharge plasmas; Proud, J. M.; Luessen, L. H., Eds.; Plenum Publishing Corp.: New York, 1986;pp 381-430. (6)Quimby, B.D.; Delaney, M. F.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1979,51,875-880. (7)Mulligan, K.J.; Zerezhgi, M.; Caruso, J. A. Spectrochim. Acta, Part B 1983,38,369-372. (8)Mohamad, A. H.; Creed, J. T.; Davidson,T. M.; Caruso, J. A. Appl. Spectrosc. 1989,43,1127-1131. (9)Bel'baeva, N.N.; Zheleznova, A. A.; Krotkikh, A. N.; Kuzyakov, Yu. Ya. Vestn. Mosk. Uniu., Ser 2 Khim. 1988,29,381-384. (10)Zheleznova, A. A.; Bel'baeva, N. N.; Tikhonov, A. A. Zh. Anal. Khim. 1988,43,1254-1260. (11)Hannie, T.; Coulombe, S.;Moisan, M.; Hubert, J. InDeuelopments in Atomic Plasma Spectrochemical Analysis; Barnes, R. M., Ed.; Heyden: London, 1981. (12)Lauzon, C.;Tran, K. C.; Hubert, J. J . Anal. A t . Spectrom. 1988, 3. 901-905. ' (13)Abdillahi, M. M.; Tschanen, W.; Snook, R. D. Anal. Chim. Acta 1985,172,139-145. (14)Michlewicz, K.G.;Carnahan, J. W. Anal. Chim. Acta 1986,183, 275-280. (15)Webster, G. K.;Carnahan, J. W. Anal. Chem. 1989,61,790-793. (16)Barnett, N.W. J . Anal. A t . Spectrom. 1988,3,969-972. 0 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
sample injections. After the sample was injected, the reaction was allowed to proceed for 60 s with continuous stirring. The procedure includes a purge between successive samples intended to avoid carry over. Many unsegmented flow techniques are based on chemical reactions between the analyte contained in a liquid sample and a solid material. The analyte is converted into a species that can be detected more convenientlythan the analyte itself. Different types of solid reactors (e.g. ion-exchange, enzyme, redox) have been applied in conjunction with continuousflow configurations17J8 for this purpose. The analytical applications of unstable oxidants and reductants in aqueous solutions in flow analysis have been reviewed by den Boef.lg Recently, a flow reduction system including a cadmium or zinc reducing column coupled on-line to an atomic absorption spectrometer was used for the indirect determination of various drugs in pharmaceutical preparations and biological fluids.2@22 This paper reports for the first time on the joint use of a flow oxidation system including a gas-liquid separator (including liquid-solid and liquid-gas interfaces) and an online Surfatron-MIP-AES for the determination of halogens. The argon plasma parameters (microwave power, gas flow rate, excitation temperature) were studied and optimized. Inorganic halides were determined by continuous chemical generation of chlorine, bromine, and iodine by use of oxidizing columns of lead dioxide. The method thus developed is quite sensitive and permits the simultaneous determination of the halides in aqueous solutions a t a sampling rate of 5 samples/ h. EXPERIMENTAL SECTION Materials and Plasma System. Stock solutions of chloride,
bromide, and iodide (1000 mg/L) were prepared by dissolving 2.1029,1.4893, and 1.3081 g of their respective potassium salts (Merck) in 1 L of Milli-Q water. Less concentrated solutions were obtained by dilution of these stock solutions with water. Sulfuric acid and lead(1V)oxide powder (Merck)were also used. Experimental measurements were made by using the analytical system whose instrumental components are listed in Table I). The argon plasma was produced at the atmospheric pressure, within a fused silica capillary tube (1-mm inner diameter) that was inserted into the Surfatron, through the propagation of a surface wave at 2.45 GHz. The system impedance was matched by using the adjustable antenna of the Surfatron, the reflected power being about 10% of the incident power. The plasma tube protruded 1 cm from the Surfatron gap. Sample Introduction Device. A schematic diagram of the sample introduction system is shown in Figure 1. The aqueous sample was continuously introduced into the oxidizing column by means of a Gilson Minipuls-2 peristaltic pump. Chlorine, bromine, and iodine were instantaneously generated by oxidation with the lead(1V) oxide column and isolated by the gas-liquid separator described el~ewhere.~3 In this manner, the generation, separation, and desiccation of the halogens took place in a continuous manner. The oxidation minicolumn was made by packing a glass capillary (5-cm length and 2-mm i.d.) with lead dioxide powder. The column packing was critical inasmuch as every two PbO2 segments (ca. 0.5 cm long) must be separated by (17)Burguera, J. L., Ed. Flow Injection Atomic Spectroscopy; Dekker: New York, 1989. (18)Valchcel, M.; Luque de Castro, M. D. Non-chromatographic Continuous Separation Techniques; The Royal Society of Chemistry: Cambridge, 1991. (19)Den Boef, G. Anal. Chim. Acta 1989,216,289-297. (20)Montero, R.; Gallego, M.; Valchcel, M. Anal. Chim. Acta 1990, 234,433-437. (21)Montero, R.; Gallego, M.; ValcCcel, M. Talanta 1990,37,11291132. (22)Montero, R.; Gallego, M.; Valchcel, M. Analyst 1990,115,943946. (23)Menhdez Garcia, A.; Sanchez Uria, J. E.; Sanz Medel, A. J.Anal. At. Spectrom. 1989,4,581-585.
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Table I. Equipment Specifications
instrument component microwave generator microwave launcher
monochromator
optical arrangement
read-out system argon flow control discharge tubes
description and specification Microtron 200 Mark 111,2450 i 25 MHz, 0-200 W (ElectroMedical Supplies, Wantage, U.K.) Surfatron;the microwave power is supplied to the Surfatron through a coaxial line using an N-type coonector Jobin-Yvon THR-lWs, 1-m focal length, Czerny-Turnermount with a grating of 1200 grooves/ mm. Reciprocal linear dispersion of 0.83 nm/ mm in the range 500-1000 nm Surfatron and monochromator mounted on the same optical rail. Plasma viewed axially and imaged 1:l on the entrance slit by using a fused silica lens R-212 Hamamatsu PMT; Jobin-Yvon Spectralink system controlled by a computer Brooks R-2-25C fused silica capillary, 1-mm i.d.
MIP
A
'atron
11
L&J/
desiccator
sample !$waste
Flgure 1. Flow diagram of the sample introduction system.
one of glass beads (0.85-1.19-mm 0.d.) in order to avoid abrupt changes in the column compacity that would stop the solution flow and disengage the system connections. Small glass wool beds on each end prevented material losses. Method. The sample solution,' containing 0.25-25,0.30-35, and 0.70-75 pg/mL of chloride,bromide, and iodide,respectively, in 12N H2S04, was directlyintroduced into the generating column at a flow rate of 3.0 mL/min. The redox reactor was thermostated at 60 "C. The oxidation of the halides was instantaneous and the volatile species formed (chlorine, bromine, and iodine), together with the aqueousphase, reached the gas-liquid separator (filled up with glass beads). A mild stream of argon (plasma gas at a flow rate of 0.125 L/min) carried the volatile species while the solution was drained to waste. The gas phase was desiccated by bubbling through concentrated sulfuric acid before insertion into the discharge tube. The atomic emission lines of chlorine, bromine, and iodine selected for condition optimization and obtainment of the calibration curves were 837.595,827.244, and 804.374 nm, respectively. The optimum instrumentalconditions are summarized in Table 11. The atomic emissionof the halogens were measured 10 min after the sample was introduced.
RESULTS AND DISCUSSION Of the most intense lines usually reported in the literature, we selected those a t 837.595 nm for chlorine(1) and 827.244 nm for bromine(1). However, we chose the line at 804.374 nm for iodine(1) even though this is not the most intense line generally employed (206.163 nm24)in order to accomplish the simultaneous determination of the three halogens with the same phototube over the range 500-1000 nm. (24)Quintero, M. C.; Sbez, M.; Cotrino, J.; Menhdez, A.; Sanchez Uria, J. E.; Sanz Medel, A. Spectrochim. Acta, Part B 1992,47,79-87.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
Table 11. Optimum Operating Conditions for the Determination of Halides Plasma forward power reflected power generator frequency inner diameter (plasma tube) argon flow rate Monochromator photomultiplier volatge integration time slit width (entrance and exit) sequential scanning (bandpass)
100
w
