Determination of Volatile Halogenated Hydrocarbons in Water by Gas Chromatography Klaus L. E. Kaiser* and Barry G. Oliver Environment Canada-Canada
Centre for Inland Wafers, Burlington, Ontario, Canada L 7R 4A 6
A rapid method for the quantltatlve determination of volatile halogenated hydrocarbons, such as CHC13, CHBrCI2, CHBr2CI, and CCI4 in water is described. This method is based on the equilibrationof the dissolved compounds in water with a small volume of gaseous headspace under reduced pressure at elevated temperature. Headspace samples, so equilibrated, are directly injected into conventional gas chromatograph Inlets for rapld quantification of the volatile compopnds present. With a 63Nielectron capture detector, quantitative determinations of chloroform and similar compounds in the 0.1 to 10 pg/l. range in water samples of less than 60 ml are easily performed in approximately 0.75 h. Several equilibrations can be performed simultaneously wlthout difficulty.
Volatile halogenated hydrocarbons, notably haloforms, have been found in surface and drinking waters throughout North America (1-4) and Europe (5, 6). In many cases, such contaminants are the result of the chlorination of raw and waste waters. Because of their toxic and possibly carcinogenic properties (7), nationwide surveys have been initiated. Such programs demand a simple, rapid, and reliable method for the sample analysis. At present, the most widely used method appears to be the gas stripping technique (8). The volatile trace contaminants are stripped off a small water sample with helium or nitrogen from which they are separated by adsorption on macroreticular resins or other adsorbents. The adsorbents then, usually with special precautions and inlet modifications, are transferred into gas chromatographs for analysis of the subsequently desorbed contaminants. More recently, a direct aqueous injection gas chromatographic technique (9) has been applied for the determination of such compounds in water. Both methods have similar limits of detection and both have severe drawbacks with respect to simplicity of operation. In addition, a recent critique (10) points out their deficiencies with respect to the insufficient distinction between the haloforms actually present in water and those formed from haloform precursors during the determination. The application of headspace methods for the determination of hydrocarbons in water has been demonstrated by Mackay et al. (11) and by McAuliffe (12). Their techniques involved equilibration at ambient pressure, the latter with additional purge gas. Because of the large headspace volume used, subsequent trapping of some compounds was necessary (11).Repetitive determinations of the signal area from several equilibrations of one sample, allowed calculation of the original contaminant concentrations. Cowen et al. (13)described an evacuated gas sampling valve for the determination of volatile organic compounds in water using 2-ml sample solutions. This paper describes a simple, new method which allows rapid quantitative determinations of haloforms and other volatile organochlorine compounds by a modified headspace technique. I t is based upon the considerable vapor pressure of such compounds which is made use of by the equilibration of the water samples with a small amount of supernatant gas
(headspace) under reduced pressure at elevated temperatures and subsequent normalization to ambient pressure. Samples of this headspace gas can be directly introduced into any available gas chromatograph with sensitive detector for a rapid, quantitative determination of the volatile contaminants.
EXPERIMENTAL A normal laboratory separatory funnel with Teflon stopcock (2-mm bore) 60-ml volume, was filled with the (cold) water sample to leave an airspace of approximately 2 ml after closing with the stopper. The funnel was then inverted and the small airspace was quickly evacuated to approximately 10 Torr by the suction of a water vacuum pump and the stopcock closed again. The entire funnel was then submersed in the upside-down position into a thermostat-controlled water bath a t 30, 50, 70, or 90 "C. Small gas bubbles developed in the sample and rose to the surface. After sufficient time, usually 30 min (vide infra) had elapsed, the headspace was quickly returned to atmospheric pressure by turning the stopcock once for a half turn. Immediately following, a 5-p1 sample of this headspace was withdrawn with an appropriate gas-tight syringe by inserting the syringe needle through the opened stopcock into the headspace after which the stopcock was elosed again. Several consecutive headspace samples withdrawn by the same technique allowed multiple determinations of the same water sample. For the gas chromatographic analysis of the gas samples, a Tracor 550 gas chromatograph with a 1.8-m, 6-mm o.d., and 4-mm i.d. glass column with 10% OV-1 Gas Chrom Q, 80/100 mesh and 63Nielectron capture detector was used. The column temperature was kept a t 50 "C, the injection port a t 185 "C, and the detector at 330 "C, respectively. A Tracor recorder with 1-mV full scale deflection was used.
RESULTS AND DISCUSSION Our experimental results suggest the usefulness and ease of this headspace technique for the determination of volatile halogenated hydrocarbons in water samples of various origin. While our investigations were primarily focused on CHC13, other haloforms, such as CHBrCl2 and CHBr2C1, as well as CCld and CHBr3 can easily be determined by this method. Figure 1 shows the gas chromatograms of four consecutive injections of the headspace of a solution of 10 pgh. chloroform in water after equilibration at 70 OC. At an attenuation of X4, the peak heights of the four CHC13 peaks ranged from 196 to 211 mm with a mean of 205 mm and a mean deviation of f 6 mm or f3%. These samples had been equilibrated for approximately 1h and showed good reproducibility, quite satisfactory in comparison to other methods ( I , 9). In order to minimize the time required for the equilibration, a series of experiments were performed under the same conditions except for headspace sampling at various equilibration times. As can be seen from the plot in Figure 2, constant CHC13 concentrations in the vapor phases were obtained a t equilibration times of approximately 30 min. Therefore, subsequent analyses were undertaken with a 30-min equilibration time. This quick equilibration is thought to be due to the "self-purging" conditions imposed on the water samples under reduced pressure (water pump vacuum). Normally, water contains a certain amount of dissolved gases, in particular oxygen, nitrogen, and carbon dioxide, which evolve from the water under the conditions described and which probably enhance the equilibration considerably. Other
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
T I M E (MINUTES)
Flgure 1. Gas chromatograms of four 5-p1 headspace injections of water samples containing 10 &g/I. CHC13 (attenuation X4)
E I E 60r
CHLOROFORM (pg / a )
Figure 3. Plot of chloroform peak height vs. chloroform concentration at several equilibration temperatures a
115 20 25 30 EQUILIBRATION TIME ( m i n ) a t
Figure 2. Plot of chloroform peak height vs. equilibration time; at 10 pg/l. CHC13
purging methods, for example the method by Bellar et al. (81, which operates at atmospheric pressure, use external gas such as Nz or He to purge the haloforms from the water samples. Obviously, the disadvantage of such a procedure compared to the described technique is the great dilution of the haloforms in the large volume of purge gas. This resulted in the necessity of trapping the contaminants from the purge gas on adsorptive resins, for example XAD-2 (IO)or on stationary phases for gas chromatography ( 8 ) . While such a procedure has the advantage of allowing storage of “fixed samples” over a period of time, the introduction of such resins, etc., into the gas chromatograph requires modification to the inlet system
A series of measurements was undertaken to determine the influence of the equilibration temperature. For this purpose, solutions of 1to 10 pgh. CHC13 in distilled water were analyzed by the headspace technique at equilibration temperatures of 30, 50, 70, and 90 O C . From the results given in Figure 3, a strong dependence of the chloroform concentration in the vapor phase on the bath temperature is evident. Obviously the distribution of compound between water and gas phases de2208
Figure 4. Gas chromatogram of 5-pl headspace of chlorinated lake water (attenuation X8)
pends on several parameters, such as solubility in water and the vapor pressure of the compound at the equilibration temperature, provided there is no saturation in either phase. The chloroform concentration in the headspace at 30 “C was approximately one tenth of that at 90 “C. Although the
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table I. Retention Times ( R T )and Relative Detector Responses ( D T )for Halogenated Methanes at GC Column Temperatures of 50 and 100 "C GC temperature 50 "C
Compound RT (s) DTa RT(s) DTa CHC13 130 1 57 1 cc14 190 5 72 6 CHBrClz 240 0.8 79 1.4 CHBrzCl 450 0.1 119 0.3 ... ... 187 0.1 CHBr3 Arbitrary units of peak heights (CHC13 = l),values obtained from headspace samples of equimolar aqueous solutions at 70 "C.
equilibrations at 90 "C resulted in the highest sensitivy of the method, most of the subsequent investigations were performed at 70 "C as sufficient sensitivity was obtained at this temperature together with good reproducibility (Figure 1). Even at an equilibration temperature of 90 "C, only a small percentage of the haloform in the sample will move into the vapor phase. The ratio of the concentration of chloroform in the vapor phase to the concentration of chloroform in solution is 0.4 at 30 "C, 0.6 at 50 "C, 1.0 a t 70 OC, and 3.7 a t 90 "C. An investigation of the applicability of this method to the determinations of lower concentrations of CHC13 in water resulted in a detection limit of approximately 0.1 pg/l. CHCl3 a t 70 "C equilibration temperature and 5 - ~ headspace 1 injections. At this low concentration, blank determinations become quite important. It was found that ordinary distilled water from a large central Barnstead still contained approximately 0.6 pgP. @HC13.After treatment of this distilled water with a Millipore Super Q system, the concentration had dropped to 0.1 gg/l. CHC13. The sensitivity of the technique can be increased, however, by simply increasing the injection volume. Injections of 100-gl headspace over a 10 MgP. CHC13 solution resulted in a 20-fold increase in peak height as compared to a 5-pl injection. In addition to chloroform, several other halogenated methane derivatives can easily be determined by the headspace technique. Particularly CC14, CHBrC12, CHBr2C1, and CHBr3 are commonly observed in environmental samples containing CHC13. Figure 4 shows a gas chromatogram of the headspace of Lake Ontario water after treatment with 10 mg/l. chlorine for 24 h. By comparison with standard solutions of authentic compounds, the following residues were observed: CHC13, 43 pg/l.; CC14, 0.3 pg/l.; CHBrC12, 15 Fg/l., and CHBr2C1, 20 gg/l. Quantitative measurements of standard solutions of 10 pg/l. each of CCl4, CHC13, CHBrC12, CHBrzCl, and CHBr3 showed a marked decrease in the overall sensitivity from CC14to CHBr3 as measured by the peak height of the detector response. This however, could be expected and was, in part, rectified by a strong temperature programming (20 "C per min up to 100 "C) of the GC immediately following the injection or by isothermal GC operation at 100 "C. Table I gives relative retention times and responses of five halogenated methane derivatives at GC column temperatures
of 50 and 100 "C. These data were obtained from headspace analysis of solutions of 10 pg/l. chlorocarbon each with equilibration a t 70 "C. The relative sensitivities in Table I, therefore, reflect overall sensitivity of the method and detector. As is shown, bromoform does not elute from the column a t 50 "C, but produces a sharp peak a t 100 "C GC temperature. Also, the response to CHBr2C1 improved markedly at the higher column temperature. It has recently been argued (10) that much of the data on chloroform in drinking water may be in error because the samples were not analyzed quickly enough. As a result of the delay between sampling and analysis of the water, haloform precursors may have been transformed to the respective haloforms and, therefore, haloform concentrations higher than present in the samples at the time of sampling may have been reported. To determine the effect of the equilibration temperature and time on the CHC13 concentration derived from a chloroform precursor, solutions of 14 kg/l. chloral (CCl3CHO.H20) were buffered a t pH 4,7, and 10, respectively, and kept a t 70 "C. At pH 4, chloral was slowly decomposed to chloroform. At pH 10, the chloroform concentration reached approximately 80%of the theoretical value in 2 h. At room temperature, the acidic chloral solution did not show any significant decomposition over the period of three days. If chloral can be assumed as a representative chloroform precursor (an environmentally observed precursor was identified as hexachloroacetone (25)),no significant changes of samples should be expected under such conditions. ACKNOWLEDGMENT The technical assistance of Karen Bothen is gratefully acknowledged. LITERATURE CITED (1)T. A. Bellar, J. J. Lichtenberg.and R. C. Kroner, J. Am. Water Works Assoc., 66,703 (1974). (2)W. W. Bunn, B. B. Haas, E. R. Deane, and R. D. Kleopfer, Environ. Lett., 10, 205 (1975). (3)J. M. Symons, T. A. Beilar, J. K. Carsweil, J. DeMarco. K. L. Kropp, G. G. Robeck, D. R. Seeger, C. J. Siocum, B. L. Smith, and A. A. Stevens, J. Am. Water Works Assoc., 67,634 (1975). (4)P. D. Foley and G. A. Missingham, J. Am. Water Works Assoc., 68, 105 (1976). (5) J. J. Rook, Water Treat. Exam., 23,'234 (1974). (6)J. J. Rook, J. Am. Water Works Assoc., 68, 168 (1976). (7)Anonymous, Chem. Eng. News, 54 (lo),6 (1976). (8) T.A. Beiiar and J. J. Lichtenberg, J. Am. Water Works Assoc., 66,739 (1974). (9)A. A. Nicholson and 0. Meresz, Bull. Environ. Contam. Toxicol., 14, 453 (1975). (10)Anonymous, Chem. Eng. News, 54 (16),35 (1976). (11) D. Mackay, W. Y. Shiu, and A. W. Woikoff, in "Water Quality Parameters", ASTM STP 573,American Society for Testing and Materials, 1975,pp 251-258. (12)J. McAuliffe, Chem. Techno/., 46 (1971). (13)W. F. Cowen, W. J. Cooper, and J. W. Highfill, Anal. Chem., 47, 2483 (1975). (14)W. V. Ligon, Jr.. and R . L. Johnson, Jr., Anal. Chem., 48,481 (1976). (15) W. H.Glazeand J. E. Henderson IV. J. WaterPollut. ControlFed., 47,2511 (1975).
RECEIVEDfor review July 1,1976.Accepted August 19,1976. This paper was presented in part at the 59th Chemical Conference and the 23d Spectroscopy Society Symposium, London, Ontario, Canada, June 7,1976.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976