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6, I I ) , led to variable recoveries and low values for most
elements (Table IV). Hydrofluoric acid has been used for the dissolution of geologic samples (6). There has also been a report of H F use with some biological samples, particularly when silicon analyses are needed (3). However, when H F was added to our system, unacceptable amounts of particulate matter remained after digestion of the fecal samples; there were poor recoveries of most elements except for phosphorous and zinc. Digestions seemed to be more complete with nitric acid-hydrogen peroxide or nitric-perchloric acid combinations. However, some elements, including iron and copper, were poorly recovered when these combinations were used. We have successfully used our method to digest a wide range of diet, food, and fecal samples with differing fat contents. Traditional open flask digestion procedures (9) generally require up to 20 mL of a 1:5 perchloric-nitric mixture and 8-13 h to digest a 0.5-g sample of either freeze-dried feces or diet in an open 50-mL Erlynmeyer flask on a hot plate. Compared with this open-flask, hot-plate procedure, the microwave procedure offers considerable advantages in speed and safety. Blank values are lower because less acid is required and the sample is not exposed to a hood environment for long periods of time. The closed system maintains sample integrity well and thus permits the determination of more volatile elements. In a preliminary study, we obtained 97-105% recoveries of added boron by using the closed microwave digestion method, whereas boron recoveries were nil when conventional nitric-perchloric digestion techniques (9) were
used. Thus, the microwave digestion method is relatively safe, simple, and rapid with good precision and accuracy. This procedure seems to be particularly suited for providing digested samples for elemental determinations by ICP-ES. ACKNOWLEDGMENT We wish to express our appreciation to Kathy Huot and Elaine Westerlund for their competent technical assistance. LITERATURE CITED (1) Gorsuch, T. T. The Destruction of Organic Matter: Pergamon: New York, 1970. (2) Abu-Samra, A.; Morris, J. S.;Kolrtyohann, S. R. Anal. Chem. 1975. 4 7 , 1475-1477. (3) Nadkarni, R. A. Anal. Chem. 1984. 5 8 , 2534-2541. (4) Kingston, H. M.: Jassie, L. B. Anal. Chem. 1988, 5 8 , 2534-2541. (5) Mahan, K. I., et al. Anal. Chem. W87, 5 9 , 938-945. (6) Papp, C. S. E.: Flscher, L. B. Analyst (London) 1987, 772, 337-338. (7) Barrett, P. B.; Davldowski, L. J., Jr.; Penaro, K. W.; Copland, T. R. Anal. Chem. 1978, 5 0 , 1021-1023. (8) Aysola, P.; Anderson, P.; Langford, C. H. Anal. Chem. 1987, 5 9 , 1582-1583. (9) Analytical Methods Committee Analyst (London) 1980, 8 5 , 643. ( 10) Bock, R. A Handbodc of DecMposiMw, Methods in Ana&thl Chemis fry;translated and revlewed by Man, I. L.; Wiley: New York, 1979. (11) White, R. T., Jr.: DouthiR, G. E. J . Assoc. Off. Anal. Chem. 1985, 68, 766-769.
RECEIVED for review March 4,1988. Accepted June 27, 1988. Mention of a trademark of proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.
Laser-Enhanced Ionization as a Selective Detector for the Liquid Chromatographic Determination of Alkyltins in Sediment K. S. Epler a n d T. C. O’Haver Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
G. C. T u r k * a n d W. A. MacCrehan Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
Laser-enhanced ionlzatlon, used as an element-speclflc detector for llquld chromatography (LC), Is appiled to trlalkyltln determinations In sediment. The LC separates the organotlns and resolves spectral interferences from the analytes. A rapid method for extractlng trlbutyttln (TBT) from sediment Into 1-butanol Is described. The detection limit is 3 ng/mL tin as TBT or 0.06 ng of tin.
Many approaches to metal speciation involve the combination of chromatographic separation with element-specific atomic spectroscopic detection (1-11). Two factors determine the success of such combinations. First, the effluent from the chromatographic system must be compatible with the method of atomization employed by the atomic spectroscopic detector. Second, the spectroscopic detector must be sufficiently sensitive to compensate for the dilution of analyte resulting from chromatographic dispersion. For many environmental or clinical applications, this will require sub-parts-per-billion detection capability. The combination of liquid chromatography (LC) with laser-enhanced ionization (LEI) spectroscopy, using :I flame as an atom reservoir, meets these criteria. The
suitability of flame spectroscopy as a detector for LC is seen in a number of successful applications of atomic absorption spectroscopy (AAS)(2,6,10, 11). A flame is easily interfaced with an LC column and is compatible with a wide variety of LC mobile phases a t normal LC flow rates. In the area of sensitivity, LEI is generally at least 100 times more sensitive than flame AAS. Berglind et al. have demonstrated the feasibility of LC-LEI with a separation of organometallic forms of iron and chromium (12). We present here a further demonstration of LC-LEI as applied to the determination of trialkyltin compounds in water and sediment. The environmental significance of alkyltin compounds stems from the widespread use of tributyltin (TBT) as the primary toxicant in marine antifouling paints used for pleasure and commercial water craft. Recently, such use of TBT coatings has been implicated in contributing to the loss of economically important nontarget organisms such as fish and mollusks. TBT concentrations in water in the sub-partper-billion range have been shown to cause mutations in juvenile oysters (13). Because of its organophilicity, TBT tends to bioaccumulate in marine organisms and has also been found concentrated in sediments, particularly in harbors. Sediment TBT originates from both the controlled-release paint on the
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watercraft as well as from paint chips that fall into the water. A lack of reliable, highly sensitive analytical methods for TBT determination in sediments has impeded the investigation of its role in the marine environment. Various methods for the species-specific determination of organotins are currently in use. Gas chromatography (GC) has been used for TBT determinations prior to flame photometric detection (14-ZO),electron-capture detection (21,22), and flame-ionization detection (23,24).GC-mass spectrometry (MS) has also been used (17,18,25). Most frequently volatile organotin derivatives have been prepared for separation by GC. One approach has been hydride generation or directly preceding injection into the GC (16,19,20,22,23) on-line (26).Hydride generation has also been used in conjunction with analysis by flame AAS, in which case the organotin hydrides are volatilized with a headspace apparatus, concentrated with a cold trap, and separated by a thermal temperature program according to their boiling points (20, 27-29). However, with the hydride generation methods, unpredictable interferences from organic substances have been reported (30,31).This is a particularly serious problem for the analysis of sediments. Grignard alkylation has also been used to prepare organotin derivatives prior to GC determination (14-18,25). This approach has been hampered by contamination of commercial Grignard reagents with TBT. In addition, there has been some concern about the possibility of molecular rearrangement of organotin compounds accompanying the Grignard reaction (32).Some recent GC work has avoided the use of preparing organotin derivatives (21, 24). LC has been interfaced with various atomic spectroscopic detectors for organotin determinations. LC-flame AAS (33, 34),LC-direct current plasma (DCP) spectrometry (35),and LC-hydride generation-DCP (35)have been used for organotin determinations. LC-inductively coupled plasma (ICP) spectrometry, it appears, has not been used for determining organotins. Flame and plasma detectors lack the sensitivity necessary for most chromatographic applications (2)unless a preconcentration step is used. An additional problem with interfacing an ICP with a chromatograph is the possibility of extinguishing the plasma with an organic mobile phase. This limitation on the choice of eluents also applies to ICP-MS, a technique with very high sensitivity that has been successfully utilized as an LC detector for other metals (36,37). Organotin compounds may be separated by cation exchange LC (34,38)without preparing derivatives, providing the potential advantages of simplicity and accuracy, and this is the separation technique that we have chosen to interface with LEI. A variety of methods has been used for extracting TBT from sediment. These range from extraction with such varied diethyl ether (la, a mixture solvents as hydrochloric acid (B), of hydrochloric acid and methanol (22),tropolone in benzene (14,15,25), and hexane (18,39,40). The choice of extraction procedure is generally dictated by the compatibility of the procedure with the subsequent derivatization and preconcentration. In this paper, we describe a method for rapidly extracting T B T from sediment into 1-butanol. In the LEI technique used here, two pulsed lasers are used to perform a double-resonance electronic excitation of tin atoms in the flame. The resulting excited tin atoms undergo rapid collisional ionization, which is then detected by electrodes in the flame (41-43).The high sensitivity of LEI arises from the ability to ionize virtually every atom irradiated by the laser under the proper conditions (e.g., flame temperature, laser energy) and from the high detection efficiency. A difficulty encountered in LEI is the potential interference from easily ionized elements. One approach to the elimination
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Eluent
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Amplifier
Schematic diagram of LC-LEI.
59232 cm-l 55187 c m - l 603.8 nm7 3 8 6 2 9 cm-l I
284.0 nm 3428 cm-l Figure 2.
Energy level diagram showing scheme for excitation of tin.
of this type of interference is to provide chromatographic resolution of the analytes from these elements in the sample. In the alkyltin analysis discussed here, matrices contained high concentrations of sodium and calcium. A high concentration of an easily ionized element like sodium causes a space charge to form around the LEI cathode probe, which causes a multiplicative interference with LEI detection (44).Additive spectral interferences may also occur. In the present analysis, sodium and calcium were found to give rise to broad-band spectral interference. A wavelength modulation procedure was developed to distinguish such broad-band interfering signals from narrow-band tin LEI.
EXPERIMENTAL SECTION Instrumentation. The experimental setup is shown in Figure 1. The LEI instrumentation has been described previously (41-45). Briefly, the system consists of a frequency-doubled neodymium:YAG laser operated at 10 Hz,which pumps two dye lasers whose beams are directed into an analytical flame. In this study, the first dye laser contained Rhodamine 6G and was tuned to 568.0 nm. This light was frequency-doubled to give ultraviolet light at 284.0 nm, the wavelength necessary to excite a tin atom from the 5p level at 3428 cm-I to the 6s level at 38 629 cm-', with an energy of 0.086 mJ/pulse (Figure 2). The second dye laser is tuned to a tin transition that further excites the tin atom from the 6s level to a level just below the ionization limit, from which rapid collisional ionization occurs. We have used the 603.77-nm line, with a laser energy of 1.8 mJ/pulse, which connects the 6s level with the 7p level at 55 187 cm-', which is 4045 cm-' below the tin ionization limit. In this case Rhodamine 640 is the dye used, with 0.1% sodium hydroxide in ethanol. The flame is a premixed air-acetylene flame generated at a 5-cm single-slot burner head. A water-cooled cathode (44)at a potential of -1000 V is immersed in the flame, 2 cm above the burner head, which acts as the anode. The two laser beams are aligned to be collinear at a position 3 mm below the cathode. The signal detection system is comprised of a 1-MHz ac-coupled lo6 V/A preamplifier connected to the anode, a 10 kHz-1 MHz active band-pass filter, and a gated integrator interfaced to a microcomputer. The computer can also control the wavelength of the dye lasers via stepper motors that vary the angle of the laser cavity gratings. The gated integrator is set for a minimum exponential time constant of 1shot
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(0.1 s), and the output is read into the microcomputer after each laser shot. The computer stores successive 10-shotaverages. The stored data is subjected to a digital smoothing routine, which was adjusted to improve the signal-to-noise ratio without affecting chromatographic resolution. A 10-point moving window smoothing function was utilized for all data presented here. LC Separation. An ion-exchangechromatographicseparation of alkyltin cations was performed by using a 20-pL injection volume onto a Whatman Partisil-10 SCX cation exchange column with a 75:25 methanol-water eluent, 0.05 M in ammonium acetate buffer (pH 5.1) pumped at 2 mL/min. A 30-cm piece of polyethylene tubing (0.3 mm id.) connected the output of the column with the pneumatic nebulizer of the LEI burner. The nebulizer was set to draw solution at a rate of 8.8 mL/min when not connected to the column. No problems were observed to be caused by the mismatch of this rate with the 2 mL/min chromatographic flow rate. Poor sensitivity was achieved when pneumatic solution uptake rates less than chromatographicflow rates were used. The methanol in the mobile phase has no noticeable effect on either the thermal or laser-induced background ionization normally encountered in the flame. Sediment Extraction. Sediment samples were collected by a Ponar grab sampler at the mouth of the York River-from Sarah's Creek (which contains several recreational marinas) and Mobjeck Bay (a more pristine area). The top 4 cm of sediment was used. Samples were frozen in polycarbonate bottles. For some studies, 1-g samples of the uncontaminated sediment were weighed into 10-mL glass centrifuge tubes, and were spiked with small volumes (150 pL) of TBT (20 pg/mL) in 60:40 methanol-water solvent. The spiked samples were mixed on a vortex mixer and were deliberately equilibrated under refrigeration for 24 h to facilitate incorporation of the spike into the sediment. Standards of TBT were prepared in 1-butanol containing 100 rg/mL butylated hydroxytoluene (BHT, a preservative) and refrigerated immediately. The procedure for extraction of TBT from sediment samples was as follows: 1 g of wet sediments was weighed into 10-mL centrifuge tubes, 3 mL of 1-butanolwas added, and the 1-butanol was thoroughly mixed into the sample with the aid of a vortex mixer. The samples were placed in a 200-W ultrasonic cleaning bath for 30 min and then centrifuged. Supernatants were passed through 0.50-pm nylon syringe filters. Extracts were stored in glass vials in the refrigerator and were stable for several days.
RESULTS AND DISCUSSION A separation of three trialkyltin components in a water sample by using LC-LEI is shown in Figure 3a. The sample is a 60:40 mixture of methanol and tap water that has been spiked with low level TBT, tripropyltin (TPT),and triethyltin (TET). After 6 min the column flow rate can be increased from 2 to 3 mL/min, to elute trimethyltin from the column with a retention time of 10.6 min. Chromatographic peaks a t 3.4 min and 6.0 min, which correspond to sodium and calcium, respectively, are also observed. These ions are present in the tap water and are being observed as a result of spectral interference of these elements with the tin second step transition at 603.77 nm. Also, the presence of a small amount (
