Technical Notes Anal. Chem. 1994,66, 572-575
Dynamic Headspace Analysis of Volatile Organic Solvents in Water Suzanne Lesage' and Susan Brown National Water Research Institute, Environment Canada, Burlington, Ontario L 7R 4A6
A dynamic headspace analysis method to measure volatile organic solvents in water was developed. The system consists of a flow cell with a constant headspace, sampled automatically and analyzed using a portable gas chromatograph. The method is applied to the assessment of the effectiveness of surfactants on the dissolution of organic solvents in water. This totally automated method simplifies the measurement of volatile organic compounds in flowing systems. Volatile organic compounds are the major pollutants found in groundwater' and are very commonly found in municipal and industrial effluents. In groundwater, the solvents often form pools of nonaqueous phase liquids (NAPL), which continue to dissolve slowly and contaminate aquifers. Groundwater scientists use columns loaded with gasoline or solvent to study the dissolution p r o ~ e s s . *The ~ ~ commonly used method to monitor the columns requires that the effluent be collected and then analyzed separately by gas chromatography. Not only is this very time consuming, but losses of volatiles during sample collection can be substantial. Different sampling devices have been devised to minimize sampling 10sses."~ A commercial continuous water analyzer* purges the effluent continuously, but requires large sample flows (1-2 gal/h) and is therefore not suited to laboratory-scale experiments. The use of a portable gas chromatograph with an automatic air sampling mechanism was investigated. A flow cell, which allows a constant-volume headspace sample to be withdrawn at regular intervals and analyzed by gas chromatography, was constructed. The use of surfactants is one of the remedial techniques being investigated to remove nonmiscible solvents from soils and aquifers, because they can enhance the solubilization process. Different surfactants are not as effective for this purpose, and many different ones must be screened before ( 1 ) Plumb, R. H., Jr. Groundwater Contamination and Analysis at Hazardous
(8)
Waste Sites; Lesage, S., Jackson, R. E., Eds.; Marcel Dekker Inc.: New York, 1992; Chapter 7. Billington, J. W.; Huang, G. L.; Seto, F.; Shiu, W. Y.; Mackay, D. Enuiron. Toxicol. Chem. 1988, 7, 117-124. Mackay, D.; Shiu, W. Y.; Maijanen, A,; Feenstra, S. J . Contam. Hydrol. 1991, 8 , 23-42. Zalidis, G. C.; Annable, M. D.; Wallace, R. B.; Hayden, N. J.; Voice, T. C. J . Contam. Hydrol. 1991, 8, 143-156. Borden, R. C.; Piwoni, M. D. J . Contam. Hydrol. 1992, 10, 309-323. Burris, D.R.; MacIntyre, W. G. Enuiron. Toxicol. Chem. 1985.4.371-377, Tancrede, M. V.; Yanagisawa, Y. J . Air Waste Manage. Assoc. 1990, 40, 1658-1 663. Siemens Chromatographs, ES Industries, Berlin N.J. 08009.
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(2) (3)
(4) (5) (6) (7)
they are used in the field. The current technique used for the assessment of the ability of surfactants to dissolve solvents contaminating soils and groundwater has been described by Fountain et al.:9 It involves shaking a flask containing the organic solvent and the surfactant solution on a rotor for several successive periods of 24 h and analyzing the content. Because of the propensity of surfactant to form emulsions, the process of analyzing the mixture is difficult and at times impossible because some emulsions will not break even after several days of standing. The experimental setup described here can be used advantageously for this purpose.
EXPER I MENTAL SECTION Chemicals. An equimolar mixture of tetrachloroethene (PERC), 1,1,2-trichloro-1,2,2-trifluroethane (CFC- 1 13), 1,1,1trichloroethane (TCA), and toluene (TOL) was prepared, and a 40-pL aliquot was applied to the generator column. The column was a 2-cm, 5-mm-i.d. glass column (Omni, Mandel Scientific Company Ltd., Guelph, ON, Canada) filled with 340 mg of silica gel of 50-pm particle size (Waters/Millipore, Mississauga, ON, Canada). Thecolumn was eluted with water purified by reverse osmosis and a Millipore Milli-Q system at a flow rate of 0.6 mL/min. To study surfactants, purified water, to which was added 1% of either Tween 20 (polyoxyethylene(20)sorbitan monolaurate), Tween 40 (polyoxyethylene(20)sorbitan monopalmitate), or Tween 80 (polyoxyethylene(20)sorbitan monooleate) (Aldrich, Milwaukee, WI), was used to elute the column. Apparatus. A schematic of the experimental setup for the dissolution of solvents is shown in Figure 1. The column was flushed continuously with an eluent (water or water with surfactant) delivered through a stainless steel metering pump. The system was constructed entirely of stainless steel and glass to prevent losses through adsorption. The eluent from the column was monitored using a dynamic headspace flow cell (Pat. Pend.). The flow cell, as shown in Figure 2, has two main distinctive features: a make-up air line and a vent/ splitter. The headspace above the flow of water must be flushed after a sample is taken to prevent carryover from one sample to the next and replenished to allow the next sample to be (9) Fountain, J. C.; Klimek, A,; Beikirch, M. G.; Middleton, M.J . Hazard. Mater. 1991, 28, 295-311.
0003-2700/94/03860572$04.50/0 Published 1994 by the Amerlcan Chemical Society
DATA STORAQE
-1
GAS CHROMATOGRAPH
2 3 0 0 200 100 n
I
WASTE
Septum
t
vent \ Splitter i i a " 0.d.
view tube
+ to
water inlet thouoh 1/10' 66 tube
+
REP2
*
REP3
I
Table 1. Center of Mass*
make-up gas (air)
,,I
REP1
Figure 3. Dissolution curve for toluene: 100 pL of toluene on a silica column.
Figure 1. Schematic view of experimental setup.
headspace sampling line
0
waste
114" SS steel T
Figure 2. Dynamic headspace analysis flow cell (Pat. Pend.).
taken. This is achieved by continuously supplying a stream of air just above the air/water interface. The excess air can exit through a vent, which also acts as a supplement of air during sampling, to prevent water from being drawn in the sampling loop. The positive air pressure eliminates the need for a membrane to separate the aqueous and gas phases. The analysis was conducted by a Photovac Model 10s portable gas chromatograph equipped with a PID, 10.6-V lamp energy. The chromatograph was set to sample automatically every 8 min a 1-mL sample of the headspace. Each experiment was conducted in triplicate. Calibration. The flow cell may be simply calibrated by pumping through a standard solution of the compound of interest, kept in a Tedlar bag with no headspace. The precision of the analysis was measured by pumping a 100mg/L solution of toluene and sampling it every 8 min. The response was 3.5 f 0.07 V-s (2% RSD, n = 23). An alternate technique was also used here. Because the total mass of solvent applied to the generator column was known, the sum of the detector response over the total elution time was equated to the mass applied. The mass eluted at each point was calculated as a fraction of the total. The concentration in micrograms per liter was calculated by dividing the mass by the amount of eluent collected between each data point.
RESULTS AND DISCUSSION The success of the method rests on being able to analyze the solution of volatiles eluting from the test column, while minimizing sample handling and losses due to volatilization. This was achieved by constructing a flow-through cell where
water 1%Tween 20 1%Tween40 1%Tween80 a
CFC-113
PERC
TOL
TCA
51.2 f 1.9 12.9 f 1.6 7.3f0.8 5.6h0.9
52.7 f 4.1 15.7 f 0.9 7.2f 1.6 6.2f 1.7
23.5 f 1.8 11.9 f 0.4 5.9f0.5 5.2f 1.0
13.0 f 1.4 8.6 f 0.5 5.1 f0.4 4.5f0.5
Expressed in pore volumes f standard deviation (n = 3).
two flowing phases, aqueous and gas, are in contact with each other. The solventspartition themselves between the aqueous effluent and the headspace, which is monitored automatically at regular intervals by the gas chromatograph. The potential for losses is virtually eliminated. The sensitivity is adjusted by the size of the headspace, the flow rate of the sweep gas, and the volume of headspace sampled. The cell is constructed of stainless steel and glass. It contains no moving parts and no membranes that could be fouled. The elution profiles from a generator column containing 100 pL of toluene are shown in Figure 3. The maximum concentration obtained using this column is representative of the aqueous solubility of the compound. In this case a maximum concentrationrange of 450-540 mg/L was obtained for toluene. This compareswell with the range of values from 470 to 627 mg/L (at 20-25 O C , in freshwater) published in the literature.1° The system could be used advantageously to study the dissolution of solvent mixtures. The elution profiles for a mixture of four solvents, applied to a generator column using water as an eluent, are shown in Figure 4a-d. The dissolution experiments were conducted in triplicate, and as can be seen from the figures,the replicationwas good. Because no simple mathematical expression describes these curves, it is difficult to compare them. One mean of comparison is to calculate the volume necessary to elute half of the solvent applied (center of mass). This allows averaging of the results over several measurements and improves the precision of the comparisons. The results are shown in Table 1. With water as an eluent, replicate measurements were better than 8% RDS. The technique was applied to measuring the solubilization enhancement of organic solvents by surfactants. The elution curves with a 1% solution of Tween 80 are shown on Figure 4e-h. The maximum concentratiofiachieved is measured from (10) Montgomery, J. H.; Welkom, L. M. Groundwater Chemical Desk Reference; Lewis Publishers: Chelsea, MI, 1990; p 502.
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573
r
I
1%lWEEN80
cFc-113
1
MLS
1,1,1-TCA
l,l,I-TCA
1% WEEN 80
WATER ELUENT
0
TOLUENE 1% TWEEN 80
1
-I1
E
MLS -1
I
PERCHLOROETHYLENE
PERCHLOROETHYLENE
1o/' TWEEN 80
WATER ELUENT
50 0
0 MLS
MLS
I
REP1 A REP2
X
REP3
1
~~
Flgure 4. Expertmental dissolution curves in water (a-d) and 1 % Tween 80 (e-h).
the first few data points where there is sufficient NAPL to generate a saturated solution. Theefficiency of the surfactants can be assessed by comparing the maximum concentration obtained with water or with the surfactant solutions. For example, the solubility of CFC-113 was increased from 125 to 1500 pg/mL (Figure 4a and e) using a 1% solution of Tween 80. It must be kept in mind that the compounds were applied together as an equimolar mixture and that the maximum aqueous concentrations achieved would be a quarter of the pure component solubility, assuming ideal behavior. The center of mass of the elution profiles for the three surfactants used in this experiment are summarized in Table 1. Tween 80 was the most effectivein enhancing the dissolution of the four test solvents. Although the solubility of all the 574
AnaiyticaiChemistry, Vol. 66,No. 4, February 15, 1994
solvents was increased, the effect was most pronounced for the least soluble compounds, namely, CFC-113 and PERC. The purpose of the experiment was not so much to find a suitable surfactant for the mixture, but to demonstrate the usefulnessof the technique to assessthe solubilizationcapacity of surfactants. The effect of the surfactant on the partitioning of the solvent to the headspace was studied. With a 1% Tween 60 solution, the response of 100 mg/L toluene was reduced by 10%. Therefore, it is important to calibrate the cell using a solution in the same matrix as the samples of interest. This method is a steady-state measurement. It is not necessaryfor the solvent to reach completeequilibriumbetween the water and the headspace as long as the same degree of partitioning occurs for the standard and the sample. The
temperature and flow rates, both aqueous and gas, are the critical parameters that must be kept constant. The sensitivity of the analysis depends on the type of gas chromatograph, the amount of headspace sampled, and the air/water volume ratio in the flow cell. Under the conditionsreported above, detection limits of 50 pg/L were obtained for toluene. There are many potential applications for this system. In addition to soil washing studies as described here, the apparatus can be used to monitor bioreactors and process effluents or in any instance where intensive monitoring of volatile organic compounds in aqueous solutions is required. It could advantageously replace the cumbersome zero-headspace sample collector used in hazardous waste leaching tests.
CONCLUSION This automated sampling system allows for continuous effluent monitoring without sample transfer, minimizing losses of volatiles. It also permits more frequent sampling and thus
better definition of column elution. Another advantage is the ability to deal with emulsion-forming compounds. The emulsions are kept to a minumum when a generator column is used to dissolve the compounds of interest and headspace analysis can still be conducted in the presence of emulsions, should they occur.
ACKNOWLEDGMENT This work was presented at the 23rd International Symposium on Environmental Analytical Chemistry in Jekyll Island, GA, June 16, 1993. It was funded by Environment Canada. Received for review August 8, 1993. Accepted December 1, 1993.@
* Abstract
published in Aduance ACS Absrracrs, January 15, 1994.
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