A Comparison of Dynamic and Static Head Space and Solvent Extraction Techniques for the Determination of Trihalomethanes in Water Rein Otson", David T. Williams, and Peter 0. Bothwell Bureau of Chemical Hazards, Environmental Health Directorate, Health and Welfare Canada, Ottawa, Ontario, Canada, K 1A OL2
T h e dynamic head space (GS),solvent (hexatit.) cslr;ic'tioii (SE),and static head space (HS) techniques utiliing 'I'enas GC columns, a 6:1Ni electron capture detector, and a l-{:iIl electrolytic conductivity detector were cumpared 1(1rthe determination of trihalomethanes (1"Ms) in water. p-'otabk water samples ranging from 29 to 150 pg/L CHCI,,.i'roin 2.:3 t o 10.1 pg/L CHBrC12, and from I J ~ C techniques, liquid-liquid extraction ( 8 , 9 ) ,direct injectioii ( 5 , I O ) , and XAD resins ( 4 ) have been developed to fill this need. A number of surveys of trihalomethanes in drinking water using head space (3, 5-7), liquid-liquid extraction ( 8 ) .and direct injection (5-10) techniques have been repcltted. In order to provide recommendations on appropriate methodology for the determination of T H M s in dritikiiig water, a comparison of the following three cOl11lll~Jll~~ uwd techniques has been made: the dynamic head space 01' g i i h sparging (GS) technique ( I I ) , which entails purging ( i t a kvdter sample with inert gas, collection of purged 'I'HIZlh o n an ndsorbent (e.g., Tenax GC), followed by thermal desurptioii; the static head space ( H S ) technique, in which equiliL)ucitioti u t ' T H M s between the water sample and air space i i i i\ cloAed vessel allows a n aliquot of the air space to be annlbzed f'or T H M s ; and liquid-liquid extraction (SE) of THMs t'roni a water sample into a small volume of appropriate orgatiic .solvent and analysis of an aliquot o f the organic phase. Experimental Apparatus. Analyses were done using a dual c0luriin gas chromatograph (Hewlett-Packard 5837) equipped with 2.2 m m i.d. stainless steel columns, 90 cm and 150 cm i t 1 length. packed with Tenax GC 60/80 mesh (Applied Science L a h ratories Inc.) and respectively connected to a '>,'Nielectron capture detector (ECD) and a Hall electrolytic conductivity detector (Tracor 310). The ECD was maintained a t 300 "(' and both injection ports were a t 200 "C. The Hall detector c'onditions were: 2-propanol-water, 50250, a t 0.12 tiiL/niin, as electrolytic solvent; furnace temperature a t 840 "C; (;yerated in the pyrolytic mode. The Hall detector signal was integrated by means of an external integrator (Hewlett-l'ackard 3370B). A 4-port, 2-way valve connected to a 5-mL spargin; device ( I I ) permitted collection of purged volatile organics on the 150-cm Tenax column when the valve was in position 1. M'hen the valve was in position 2 the sparge/carrier gas bypassed the device and directly entered the injection port of' the gas chromatograph. 936
Environmental Science & Technology
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Culture tubes ( 3 2 mL capacity) equipped with screwcaps with predrilled, 0.5 cm diameter, centered holes and Tefloncoated, silicone rubber disks (Tuf-Bond, Pierce Chemical Co.) were used throughout. All t,ubes were inverted during storage a t 25 "C to ensure formation of an additional liquid seal by the contents. Culture tubes containing 2.5 mg of Na&O:$HpO, as residue from evaporation of aqueous thiosulfate a t 70 "C, were filled completely with aqueous samples. Gas-tight, Chaney adapter equipped, Hamilton syringes were used for transferring fluid samples for analyses. Reagents. Methanol and hexanes (Caledon Laboratories Ltd.) were distilled-in-glass quality and were checked for 'I'HMs before use. Distilled, deionized water was boiled for 8 h to give THM-free water. Chloroform (Chem Service, Inc.), tirotnoform (Chem Service, Inc.), bromodichloromethane (Fluka, A.G.), and chlorodibromomethane (Fluka, A.G.) were gas chromatographically pure. Zero grade nitrogen gas (Matheson of Canada Ltd.) was passed through molecular sieve and Tenax traps before use. Stock methanolic T H M solutions, each containing all four T H M s a t known concentrations, were prepared in volumetric f'lasks and were then stored in well-sealed culture tubes. Spiked aqueous T H M solutions (Table I) were prepared by iiijecting 12.8-pL aliquots of the appropriate stock methanolic 'I'HM solutions into sealed culture tubes filled with THM-free water. T a p water samples (A-D) of suitable T H M content were prepared by mixing t a p water from different sources in Teflon-sealed, 500-mL separatory funnels. T h e contents were allowed to mix for 4 h and were then quickly distributed and sealed into culture tubes in order to minimize the loss of 'I"Ms. THM-free water stored in culture tubes was used for blank determinations. Procedures. Dynamic Head Space or Gas Sparging ( G S ) Twhniyue. Aliquots (5.0 m L ) of spiked aqueous T H M solutions and t a p water samples were purged with nitrogen a t a rate of40 mL/min, and volatile organics were collected on the 'I'enax column held a t 50 "C. Af'ter 10 min the 4-port, 2-way valve was switched to bypass the sparging device, and the trapped organics were eluted using temperature programming and nitrogen as carrier gas (34 mL/min a t 185 "C). The column oven temperature was raised at a rate of 30 "C/min to 130 "C, a t 5.0 min the program was continued at 30 "C/min to 185 "C, a t 9.5 niin cooling was begun, and at approximately 14 min purging of a fresh aliquot was initiated. In addition to spiked aqueous T H M solutions in culture tubes, another type of solution (GS-x),commonly used for GS technique calibration, was also analyzed. Type GS-x was prepared 10 min prior to analysis by injecting a 2.0-pL aliquot ~f the appropriate stock met,hanolic T H M solution into 5.0 mL of THM-free water in the sparging device. T h e T H M concentrations in these solutions were essentially identical with those in Table I. Static Head Space ( H S ) Technique. T a p water samples and spiked aqueous T H M solutions which had been used in the GS technique analyses were immediately prepared for H S analyses by removing and discarding an additional 5.0 mL of the liquid contents of the culture tubes. After replacing the reriioved liquid with air, the tubes were quickly sealed and were stored overnight a t 25 "C. Aliquots (50 p L fur ECD, then 1.0 m L for Hall detector) of the gas phase were then with-
0013-936X/79/0913-0936$01.00/0
Published 1979 American Chemical Society
Table 1. Trihalomethane Concentrations in Spiked Aqueous Solutions standard
I II Ill IV
concn, p g / L CHBrC12 CHBr2CI
CHC13
20 40 80 120
1.6 3.2 6.4 9.9
CHBr3
0.35 0.70 1.40 2.10
0.40 0.80 1.6 2.4
drawn through the disk seal a n d were injected onto the appropriate Tenax column. The gas chromatographic conditions were: nitrogen carrier gas, Hall, 34 mL/min measured a t 185 "C, EC, 54 mL/min measured a t 185 "C; the column oven was held a t 140 "C for 2 min, then programmed a t 30 "C/min to 185 "C, a t 6.2 min cooling to 140 "C was begun, and a t approximately 10 min analysis of a fresh aliquot was initiated. Liquid-Liquid or Solvent Extraction ( S E ) Technique. Aliquots (2.0 mL) of liquid were removed from sealed culture tubes filled with t a p water samples or spiked aqueous T H M solutions and were replaced with aliquots (1.0 mL) of hexanes. T h e sealed tubes were agitated (wrist action shaker) for 30 min a n d were then stored overnight a t 25 "C. Aliquots of 1.0 and 5.0 pL of the hexane layer were then respectively analyzed by the ECD and the Hall detector. Chromatographic conditions were identical with those used for H S analyses. Effect of Sample Distribution. Bulk spiked aqueous T H M solutions were prepared in 500-mL, Teflon-sealed separatory funnels by injecting 202-pL aliquots of stock methanolic T H M solutions into 500-mL portions of THM-free water. The T H M concentrations in the four bulk solutions were essentially identical with those listed in Table I. After 4 h. the bulk solutions were quickly distributed and sealed into culture tubes and after 24 h the tube contents were analyzed by the GS technique. T H M Determination. For each analytical technique, the contents of three tubes a t each concentration (solutions I-IV and tap water samples A-D) were analyzed once. T h e relative standard deviation (RSD = (standard deviation/niean] X 100%)was calculated from triplicate peak area values. T H M concentrations in t a p water samples were determined by the use of calibration curves plotted from the data for spiked aqueous T H M solutions analyzed by the same technique. T h e calibration curves were prepawd by plotting mean peak area against T H M concentration. Results and Discussion T h e solvent extraction (SE) and static head space ( H S ) techniques were essentially the same as those used by other workers (6-9). T h e gas sparge (GS) technique was based on t h a t of Bellar and Lichtenberg ( 2 2 ) except that the gas chromatography column was used Foth to trap and to separate the trihalomethanes. Appropriate temperature programming allowed the same Tenax column to be used for all three techniques and the column perforriance was not significantly al-
tered even after several hundred analyses. Electron capture and Hall detectors were both used to allow confirmation and comparison of analytical results. Improved sensitivity for trihalomethanes was achieved by operating the Hall detector in the pyrolytic mode. Typical detection limits a n d retention times for the T H M s are summarized in Table 11. As expected, the EC detector gave considerably lower method detection limits than the Hall detector. Data for bromoform are absent from Tables 111-V since none could be detected in the tap water samples and bromoform levels in the spiked aqueous T H M solutions were generally below technique detection limits. T o facilitate the comparison of the three techniques, the same type of tube, sealed with screw cap and Teflon-coated silicone disk, was used throughout for the analysis and/or storage of samples and spiked solutions. These tubes were chosen because of their suitability for sampling, shipping, storage, and ease of manipulation. T h e use of sodium thiosulfate to stop T H M formation has been demonstrated (5) to allow storage of samples a t 4 "C for a t least 2 months without noticeable change in T H M concentration. Preliminary investigation of the three techniques demonstrated no interfering peaks from methanol, hexane, or THM-free water. However, a constant trace peak for chloroform (
