Anal. Chem. lB88, 60,250-254
250
Liquid Chromatography-Photolysis-Electrochemical Detection for Organobromides and Organochlorides Carl M. Selavka, Kai-Sheng Jiao, and Ira S. Krull*
Barnett Institute and Department of Chemistry, Northeastern Uniuersity, 341 Mugar Building, Boston, Massachusetts 02115 Paul Sheih, Wing Yu, and Martin Wolf
Cambridge Analytical Associates, 1106 Commonwealth Avenue, Boston, Massachusetts 02215
An Improved Mgh-performance ilquld chromatogrephlc detectlon method has been developed for the determlnatlon of alkyl and aryl organobromldes and aromatlc Chlorinated campmds. Thedetectbnapproachusearon#ne,~cokmn photoiysia wlth UV Ight to Hberate anlonlc bromlde or chkrhle from halogenated compounds, which is then detected amperometrkally by uslng a sliver electrode at modest oxldative potentlals. Under optimized condltlons, the detectlon approach Is llnear over 3 orders of magnitude, offering limits of detectbn In the 0.1-7range for organokomkles and In the 15-40range for aromatk chlorinated analytes. Enhanced selectivity Is demonstrated through the use of chromatographk retention tlmes, dual electrode response ratios, and qualitatlve lamp odoff responses for analyte ldentiflcation. The method Is validated for trace analysts by uslng shgbbhd determlnatlons of 1,2-dlbromo-3thloropropane and Aldrin.
Organohalogens are widely employed in industrial and agricultural applications, which has led to contamination problems in the environment. Since many of these compounds are toxic or carcinogenic (11, analytical methods have been devised to detect traces in foods and water. Generally, gas chromatographic (GC) methods are employed, due to the volatility of many of these compounds, with selective detection obtained by using electron-capture detection, the Coulson electrolytic conductivity detector, or the Hall detector (2-4). However, there is a need for alternative methods that allow for the determination of nonvolatile organohalogens. In addition, an alternate method is needed for laboratories wishing to confirm GC analyses by using a second method, or in cases where extensive sample preparation or cleanup are required prior to GC. Liquid chromatographic (LC) methods generally allow for less complex extraction and cleanup procedures and have been used to advantage for thermally sensitive halogenated organics. However, the detectors available in LC are less sensitive and selective for organohalogens than those used in GC. Our efforta to improve the detectability of these compounds in LC led to the development of a postcolumn reaction detection approach, which we recently reported for the trace determination of organoiodides (5,6). This method incorporates an on-line, postcolumn photochemical derivatization step prior to oxidative electrochemical (EC) detection, in which the organohalogen is photolyzed to form halide anion, which is oxidized electrochemically. It was also determined that the formation of phenolic species, arising from hydrolysis of cationic intermediates generated during photolysis of aromatic precursors, could contribute to the oxidative EC response (6). We have now extended this method, named LC-photolysisEC (LC-hv-EC), to allow for the determination of brominated and chlorinated compounds.
EXPERIMENTAL SECTION Apparatus. The LC-hu-EC apparatus has been described previously (5). The only change from this earlier description involved the use of a dual Ag working electrode cell (Bioanalytical Systems, Inc., West Lafayette, IN) in the EC detector, which replaced the dual glassy carbon electrodes used for the organoiodides. The design of the knitted open tubular postcolumn reactor has also been described (7); in the present work, a knitted reactor composed of 0.8 mm i.d. X 1.6 mm 0.d. Teflon tubing was used. One experiment also supplanted the Hg lamp (primary output 254 nm) used in the photolysis apparatus with a Zn lamp (having an intense band at 214 nm and additional bands at 274, 307,330,334,468,472,481,and 636 nm) and accompanying power supply (BHK, Inc., Monrovia, CA). The gradient LC-UV system, ion chromatography-conductivity detection system, UV spectrophotometer and photolysis-cyclic voltammetry apparatus used for mechanistic experiments have also been described previously in detail (6, 8). Standards and Reagents. Inorganic and organic standards were obtained in the highest available purity from Aldrich Chemical Co. (Milwaukee, WI). Reagent grade acids and electrolytes were obtained from J, T. Baker (Phillipsburg,NJ). Aldrin was obtained from Alltech Associates (Deerfield, IL), and 1,2dibromo-3-chloropropane(DBCP) was obtained from the Environmental Protection Agency (Research Triangle, NC) as the standard reference material. Procedure. The procedures used in method development, optimization, characterization of analytical performance, and mechanistic studies were essentially identical with those used in reports of LC-hv-EC for organoiodides ( 5 , 6 ) . The test analytes chosen for use in these experiments were: 1-bromopentane, 2bromopentane, 1-chloropentane,1,2-dibromoethane(EDB), 1,2dichloroethane, 3-bromo-1-propene (allyl bromide), 3-chloro-lpropene (allyl chloride), bromobenzene, chlorobenzene, 1,4-dibromobenzene, 1,4-dichlorobenzene,1,2-dibromo-3-chloropropane (DBCP), and 1,2,3,4,10,10-hexachloro1,4,4a,5,8,8a-hexahydro1,45,&dimethanonaphthalene(Aldrin). Validation of the method was performed by using a single-blind protocol, with DBCP spiked into 50:50 MeOH-water at levels between 50 ppm and 100 ppb and Aldrin spiked into methanol at levels between 10 ppm and 100 ppb. A study was performed to determine the EC and hv-EC behavior of various electrolytes when using glassy carbon, Pt, Ag, or Au electrodes. This experiment utilized a flow injection (FI)-hv-EC apparatus having a 2-min irradiation time when a flow rate of 1 mL/min was used. The mobile phases prepared for these experiments were composed of 50:50 (v/v) methanol (MeOH) with 0.2 M solutions of sodium acetate (NaOAc),sodium citrate (Na2C6H507), disodium phosphate (Na2HP04),or sodium sulfate (Na,SO,). Finally, the ion chromatographicdetermination of bromide and chloride liberated during batch photolysis of organobromides and organochlorides utilized a mobile phase of 2.5 mM phthalic acid (unadjusted pH 2.6) at 2 mL/min.
RESULTS AND DISCUSSION Selection of Initial Operating Conditions. All of the test analytes could be adequately resolved by using a CI8 reversed-phase column and mobile phases containing 20-50%
0003-2700/88/0380-0250$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
(by volume) MeOH. The greater hydrophobicity of Aldrin required the use of 75-85% MeOH to effect reasonable capacity factors (k’= 4-6). The UV spectra collected for the test analytes displayed the following ,A, values (in nm): 1-bromopentane(212), 2-bromopentane (211),l-chloropentane (205), allyl bromide (230), allyl chloride (214), EDB (225), 1,Bdichloroethane (209), bromobenzene (255, 228), chlorobenzene (263,223), 1,4-dibromobenzene (263,231), and 1,4dichlorobenzene (263,218). From these data, it was expected that the low-pressure Hg lamp used in the photolysis apparatus (having primary output at 254 nm) would not provide for highly efficient photolytic conversion of these compounds, especially for aliphatic chlorinated compounds. However, it was postulated that through the use of longer residence times for analytes in the postcolumn photolyzer, sufficient photolytic conversion efficiency could be achieved for brominated compounds and aromatic chlorides. The EC and hv-EC behavior of electrolytes on several electrode surfaces (glassy carbon (+1.1 V), Pt (+1.1 V), Au (+1.0 V), and Ag (+0.3 V)) were examined. It was found that NaOAc, Na2C6HSo7,N%HPO,, and Na2S04could all be used for conventional LC-EC on any of the electrode materials. However, when the lamp was switched on in the FI-hv-EC system, high background currents were obtained for Na2C6H607and NazHP04on all surfaces except glassy carbon. In addition, NaaO., displayed limited solubility in MeOH/water mixtures having MeOH content greater than 50%; this fact would limit the choice of mobile-phase compositions available for optimum chromatographic resolution of analytes from matrix materials. Therefore, NaOAc was used as the supporting electrolyte in the remainder of the experiments. Cyclic voltammetry (CV) indicated that Br- could be detected on glassy carbon at potentials above +1.2 V, on Pt above +1.1V, on Au above +0.7 V, and on Ag at potentials above +0.25 V. Chloride could only be detected on Au at potentials above +1.2 V, or on Ag above +0.3 V. It was noted that the background currents on Au were greatr than 1 wA, which would preclude the use of this surface for high-sensitivity analyses. Also, although improved selectivity could be gained through the use of Pt electrodes for Br- and Ag electrodes for Cl-, the sensitivity of detection for Br- on Ag was roughly lOOX that on Pt, so Ag was chosen for use in LChv-EC for both organobromides and -chlorides. Photolysis-cyclic voltammetry (hv-CV) was performed on oxygenated anions of bromide and chloride, in order to ascertain the necessity of rigorous mobile-phase deoxygenation. It was found that 1 part per thousand (1 ppth) solutions of BrOf, C103-, and C10, exhibited no inherent electroactivity on Ag, in the potential range +0.4 to -0.6 V. However, photoreduction of these oxygenated species to Br- or Cl- occurred with good efficiency, even for photolysis times
