Environ. Sci. Technol. 1991, 25,123-126
Technique for Removing Water from Moist Headspace and Purge Gases Containing Volatile Organic Compounds. Application in the Purge with Whole-Column Cryotrapping (P/WCC) Method James F. Pankow Department of Environmental Science and Engineering, Oregon Graduate Institute, 19600 N.W. Von Neumann Drive, Beaverton, Oregon 97006
rn Volatile organic compounds (VOCs) are easily removed from water by use of an inert purge gas. VOCs are therefore easily determined by the purge with whole-column cryotrapping (P/WCC) method. In P/WCC, purging takes place directly from a sample to a capillary gas chromatography (GC) column while the latter is maintained a t a cryotrapping temperature. Water vapor is purged to the column along with the analytes. The water can cause variability problems with the GC retention times and responses. A water trap technique is described that reduces the amount of water in VOC-containing gas streams. The technique involves passing most of the purge gas stream through a short column of glass beads at -10 "C. Analytes collected in the trap are quantitatively transferred to the GC column in a final short purge of the trap carried out at ambient temperature. Desiccation efficiencies of 90% can be obtained without any loss of analytes. Introduction When gas chromatography (GC) is used to analyze water, soil, and sediment samples for volatile organic compounds (VOCs), it is common to include a step that separates the analytes from the relatively large amounts of water that are also usually present. Headpace and dynamic purging based methods rely on the volatility of VOCs to create a gas phase in which there is enrichment in the VOC/water ratio relative to the sample. The EPA purge and trap (P&T) method for water samples (I) provides for additional water removal during a selective collection of VOC analytes on a sorbent trap that is later thermally desorbed to the column. A disadvantage of this approach is that portions of very volatile analytes (e.g., methyl chloride) may break through the trap and be lost. In order to eliminate losses of very volatile analytes, the purge with whole-column cryotrapping (P/WCC) method (2, 3) does not carry out intermediate sorbent trapping, but rather provides for the direct passage of purge gas flow onto the column. If the column is maintained at -90 "C during the purge step, then compounds even as volatile as methyl chloride can be trapped in a narrow band, and virtually all of the EPA purgeable priority pollutants can be resolved on a 30 m long, 0.32 or 0.53 mm i.d. fused-silica capillary column of the DB-624 type. If the compounds of interest are less volatile than methyl chloride, then WCC temperatures warmer than -90 "C can be used. When the volumes of purge gas are low (e.g., 20 mL or less), then when the purge vessel is at 20 "C, P/WCC transfers less than 0.4 p L of condensed water to the column ( 3 ) . Disadvantages of too much water (e.g., more than 1pL) on the column include peak splitting as well as variabilities in the retention times and responses of compounds that elute near the boiling point of water. GC detectors that can be adversely affected by too much water include the electron capture detector and the mass spectrometer. One technique that can be used to reduce water transfer to the column during P/WCC involves a flow-through 0013-936X/91/0925-0123$02.50/0
dryer constructed from Nafion tubing ( 4 ) . The highly polar nature of the permeable Nafion allows water to diffuse out of the purge gas stream, but retains the majority of the nonpolar analytes in the stream. While simple, a Nafion dryer may not allow for the quantitative transfer of VOCs and can also lead to certain memory effects in the method (5). In order to facilitate the use of P/WCC with GC/MS, and to provide an alternative to the Nafion approach, this paper describes a simple water trap that avoids memory effects as well as the losses of analytes that may occur in P&T and in P/WCC with a Nafion dryer. The trap makes use of the fact that the vapor pressure of water decreases significantly as temperature decreases. The trap (Figure 1) incorporates a short column packed with glass beads that can be maintained at a temperature such as -10 "C during the bulk of the purge step. Extremely volatile analytes will pass through the trap where they can be focused on the column at WCC temperatures such as -90 "C. A portion of the less volatile analytes and the bulk of the water will be collected on the trap. If the trap is warmed to -20 "C for the final 30-60 s of the purge step, essentially 100% of any trapped analytes can be purged from the trap and focused on the column. The small amount of water collected on the trap allows very high purging efficiencies. At the same time, the short duration of the period in which the water trap is warm minimizes the amount of water transferred to the column. The GC temperature program begins at the conclusion of the purge step. Theory When a total volume V (mL) of gas is bubbled incre(K) through a sample volume mentally at temperature V,, (mL), a volatile analyte can be purged no faster than (6) (1) C l I C 1 , O = exp t-(H/RT,) Vg1/ V,,l
'fi
where cl,ois the initial concentration of the analyte in the sample, c1 is the concentration remaining after passage of Vgl, H (atm-m3/mol) is the Henry's law constant of the m3, analyte at T1, and R is the gas constant (8.2 X atm/mol.K). If F (mL/min) is the flow rate at TI and the total pressure in the purge vessel, and tl (min) is the duration of the overall purge step, then V,, = Ft,. Table I summarizes the nomenclature. The maximum possible fractional efficiency epl (0 5 epl C 1) of the sample purging is given by ep1
= (1 - Cl/Cl,O)
(2)
The value of epl will tend to approach 1.0 as H and Vgl/ V,, increase. Equation 1gives the maximum purging rate and eq 2 gives an upper estimate of epl because bubbles with a finite lifetime in the purge vessel will never be able to become fully equilibrated with the liquid. However, available evidence for typical purging conditions suggests that eqs 1 and 2 provide very good approximations for all
0 1990 American Chemical Society
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123
purge flow (reg. 1)
GC run flow (reg. 2)
t
then assuming that the purge gas leaving the purge vessel at this stage is essentially free of analyte leads to c2/c2,0 = exp[-(H/RT1)Vg2/ Vn21 (3) and ep2 =
to detector t
Flgure 1. Schematic diagram of the apparatus used in PlWCC with incorporation of a water trap. Swageiok fittings are marked with
cross-hatching.
Table I. Nomenclature
(4)
(1- C Z / C Z . O )
where cZpis the concentration of the analyte in V,, at the beginning of purging of the warmed trap. If t2 (min) is the time during which the warmed trap is purged, then V,, = Ft,. Since we have taken T,to describe the temperature of both purge steps, for any given analyte, the same value of H will describe both purge steps. Equations 3 and 4 assume a uniform liquid concentration in the warmed trap. Modeling the purging of the condensed water in this manner is reasonable since all of the water in the trap is likely to be condensed within a few millimeters, and the chromatographic efficiency of the packed bed in the trap is likely to correspond to a theoretical equilibrium plate height of a t least 2 mm. By application of the ideal gas law
c,,~
initial Concentration of analyte in sample concentration of analyte remaining in sample after passage of V,, c ~ , ~concentration of analyte in trap at beginning of purging of warmed trap cz concentration of anslyte remaining in trap after passage of v, ep, maximum possible fractional efficiency of the sample purging ep2 fractional efficiency of the purging of analytes in the water trap Ed, desiccation efficiency (%I of the trap E, overall efficiency (%) for purging of analytes from a sample to the GC column F flow rate (mLjmin) at T,and the total pressure in the purge vessel Henry's law constant ( a t m d / m o l ) for the analyte at TI Henry's law constant (atm.ms/mol) for water at TI(4.2 X 1 0 . ' atm.ms/mol at 20 "C and 1 atm total pressure) vapor pressure (atm) of liquid water at the temperature of the warmed trap [0.0231 atm at T,= 293 K (20 "C)] vapor pressure (atm) over ice st the trapping temperature [0.00257 atm at T2 = 263 K (-10 "C)] gas constant (8.2 X m3.atm/mol~K) time (min) duration of the overall purge step time ( m i d duration of purging of the warm trap time (min) duration that the trap is cold temperature (K) in the purge vessel and in the warm trap temperature (K) in the cold trap volume (mL) of gas bubbled at temperature TI(K) through sample (Ft,) volume (mL) of gas passed at temperature TI(K) through warm water trap (Ft,) volume (mL) of sample volume (mL) of liquid water in the trap molar volume (mL) of liquid water at TI(18 mL/mol) volume of water that actually reaches the column volume of water that could have reached the column for a direct PjWCC purge at TIlasting t , minutes volume (uL)of water actuallv transferred to the column e,
of the purgeahle priority pollutants under typical purging conditions (3). Let us assume that all of whatever analyte purged from a water sample according to eqs 1 and 2 is retained in a cold-zone water trap at temperature T2.If the trap is warmed rapidly from T2to T,,the continuing flow of purge gas from the purge vessel will begin to effect a repurging, and the trapped analyte will continue onto the column. If the total volume of gas purging the warmed trap is V (mL) and if the volume of trapped liquid water is V,, (mLf 124
Environ. Sci. Technol., Voi. 25, No. 1, 1991
where Vis the molar volume of liquid water (18 mL/mol), t , (min) is the duration of the overall purging, t2 (min) is the length of time during which the trap is warm, (tl - t z ) is the duration of time that the trap is cold, p1(atm) is the vapor pressure of liquid water a t the temperature of the warmed trap [0.0231 atm at T,= 293 K (20 " 0 1 , and p z is the vapor pressure over ice at the trapping temperature [0.00257 atm at Tz = 263 K (-10 "C)]. The volume of water V ,, (pL) actually transferred to the column is given by
Reducing T2will not allow Vw,c,lto be reduced below the limit given by eq 6 with p 2 = 0. The desiccation efficiency of the trap is given by (7) Edes = (1- Wact/wpd x 100% where W, is the amount of water that actually reaches the column, and W,, is the amount that could have potentially reached the column for a direct P/WCC purge at TI lasting t, minutes. Therefore
Although VW.-,will increase as t, increases, since p , > pz, Edeswill also increase. Figure 2 illustrates this behavior as a function o f t , for t2 = 0.5 min, TI = 293 K, and Tz = 263 K. When t, >> t2, Ed- approaches the asymptotic limit Ed, E (1- p2/p1) X 100% (9)
For TI= 293 K and Tz = 263 K, this limit is 89% (Figure 2). Equations 5-8 all allow for the fact that the purge flow rate denends on the temnerature. heine eaual to F a t T.. and FF2JT,at T,. With a cold-zone water trap, the overall efficiency of PIWCC in transferrinc analvtes from a samde to the GC column will he given 6y (10) E,,,II = (eplep2)X 100% I
.
L.
Equation 10 is only an approximation. First, we assumed above that all of the analyte removed from the purge vessel is collected in the water trap. In fact, some will pass
WATER ON COLUMN. V w 8 COI (VL) 0.43 0 53 0.64
W A T E R ON COLUMN, Vw,col (VL)
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Figure 2. E, as a function of t l for t , = 0.5 min, F = 20 mLlmin, T , = 293 K, and T , = 263 K. When t , >> t , , Edesapproaches the asymptotic limit (1 - p,/p ,) X 100%. For the conditions of this figure, this limit equals 89%.
untrapped onto the column. This will tend to make ep2 underestimate the transfer efficiency of the region downstream of the purge vessel and make eq 10 underestimate Eoverau. Second, we know that eq 3 assumes that the purge gas flowing through the warmed water trap is free of analyte. In actuality, that gas will always continue to remove some portion of analyte from the purge vessel. This second assumption will tend to cause eqs 3 and 4 to overestimate ep2' However, this tendency will only be very slight as long as H for the analyte is significantly greater than the H for water itself. Indeed, even if all of the water in the purge gas is condensed into Vpiz,all analytes that satisfy this criterion will exert a partial pressure that is much higher than that from the original sample, and so the purge gas for V,, will in fact be relatively low in analyte. With Hwak, = 4.2 X atmm3/mol at 20 "C and 1atm total pressure, all of the purgeable priority pollutants are characterized by H values that are much larger than this value (see Table I in ref 3). On the basis of the above discussion, we conclude that eq 10 will provide a lower bound on Eoverall for volatile organic compounds. Importantly, for the types of conditions used with P/WCC, ep2will almost always be extremely close to 1.0. For example, for F = 20 mL/min, t , = 15 min, t2 = 0.5 min, Tl = 293 K, and Tz = 263 K, then eq 5 gives Vs2as only 0.0046 mL. Thus, even when H is extremely small, say 2 x atm.m/mol, eq 4 gives ep2 = 0.97. We conclude that eq 10 will provide a very good estimate of Eoverd in P/WCC, and moreover, that to a very good approximation we will usually have (11) Eovera11 N epl X 100% Figures 3-5 give Eoverau vs tl calculated according to eq 10 with Vsl ranging from 2 to 10 mL, and for the same values of F , t2, T1,and T 2 discussed in the preceding paragraph. In all cases, the bottom x axes begin at tl = 1 min, since in P/WCC it seems likely that -0.5 min represents (1)a lower bound for useful values of the period during which the water trap is cold (tl - tz)and (2) a value for t , that can provide ep2 1 while still keeping Vw,col
I
1
1
I
I
I '
I
9 101112131415 (mid
Figure 3. E,,,, as a function of t l for V,, = 2 mL, F = 20 mL/min at T , , t , = 0.5 min, F = 20 mL/min, T , = 293 K, and T , = 263 K. WATER ON COLUMN, Vw,col (VL)
0 75
7
0
1
1
2
1
3
l
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l
l
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Figure 4. E,v,,,,, as a function of t l for V,, = 5 mL. Other cond%ions as in Figure 3.
small. Assuming t 2 = 0.5 min, the top x axes give Vw,col as it depends on tl. When tl is small, a large fraction of Vw,colis transferred to the column during the short t 2time period. As a result, Vw,colincreases only slowly as tl increases upward from tl 2 min. H values for VOCs range from 0.2 atmm3/mol for methyl bromide to 0.0001 atm-m3/molfor the chlorinated ethers (3). At T , = 293 K, and with V,, = 2 mL, Figure values approaching 30% are easily 3 shows that Eoverall obtained even for H as small as 0.0001 atm.m3/mol. Figure 5 indicates that most VOCs of interest can be purged efficiently with Vw,colvalues of C0.5 FL even when V,, = 10 mL. By comparison, assuming standard P&T values of 440 mL for V,, and 5 mL for Vsl, eqs 1 and 2 indicate that conventional P&T with an 11-min purge will yield a
-
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WATER ON COLUMN, V w 8 c o ~ (pL)
0 32
0 21 100
c 53
0 43
3 64
0 75
90I
/
/
H = 0.0001
t,
(rnin)
Figure 5. EWml as a function of f , for V,, = 10 mL. Other conditions as in Figure 3.
purging efficiency of 31% for H = 0.0001 atm.m3/mol. Experimental Section The P/WCC system in Figure 1 is a modified version of one described elsewhere ( 3 ) . The differences involve (1)use of the water trap, (2) placement of the waste port (D) on the side of the purge vessel, and (3) addition of both a '/I6 in. 0.d. drain line and a '/I6 in. stainless steel (SS) 0.d. flush line through port D. Modification 2 prevents the portion of sample that might seep through the frit prior to the purge step from becoming isolated from the purge gas. Modification 3 allows the purge vessel to be both drained and flushed when pressurized. The glass bead column inside the aluminum block water trap unit was constructed with a 6.5 cm long piece of 1/8 in. o.d., 0.085 in. i.d. SS tubing. The 0.45-0.50-mm beads were held in place by a small amount of glass wool on each end. The 1/8-in.tube was connected to the 1/16-in.SS gas lines with Swagelok reducing unions. The aluminum block was wrapped with a small amount of fiberglass insulation to help it achieve -10 "C during the initial portion of the purge step. During that portion, liquid coolant (water containing 30% ethanol) at -10 "C was circulated through the block. To prevent the fittings on the glass bead column from becoming isolated cold spots, a small fan was aimed at the block at all times. When the 100-W cartridge heater in the block was activated to achieve -20 "C during the t2 (1 min) portion of the purge step, coolant flow to the bIock was haIted, and any coolant remaining in the line was blown out by using a compressed air line connected to the upstream side of the coolant line through a snap valve. (As an convenient alternative to the water/ethanol mixture, cold nitrogen gas from a liquid nitrogen dewar could be used as the coolant.) For all analyses, the value of tl used was 7 min. To achieve a purge flow F of 20 mL/min, a pressure of 30 psi was set on the purge flow gas regulator. During all phases of the purge step, SV1 was open, SV2 was closed, and the column was maintained at a WCC temperature of -90 "C. A t the conclusion of each purge, SV2 was opened to provide carrier gas at the GC run pressure (5 psi). The GC temperature program used was ballistic to -30 " C 126
Environ. Sci. Technol., Vol. 25, No. 1, 1991
and then at 10 "C/min to 215 "C. Data acquisition was begun at -30 "C. The column used was a 30 m long, 0.32 mm i.d. DB-624 (1.8-pm film thickness) fused-silica column from J&W Scientific (Folsom, CA). As described elsewhere (31, the inlet of the column was fitted with an "ice trap" and the exit end was interfaced to a Finnigan 4000 GC/MS equipped with conventional diffusion pumps. A t the conclusion of each GC run, the three-way valve was rotated to select the purge pressure again, SV3 was opened, and the trap block was heated to 100 "C for 2 min. This provided a back-flush cleaning of the trap as well as a draining and flushing of the purge vessel through the two in. 0.d. lines inserted through port D. As with the P&T method, P/WCC with the water trap can be automated through the use of two multiport gas valves. Results and Discussion The water trap described above was tested by analyzing a 4 pg/L standard solution containing three VOC internal standard (IS)compounds: 1,2-dichloro-l,l-difluoroethane (ISl), 2-bromo-1-chloropropane (IS2), and ethylbenzene-d, (IS3). ISl-IS3 eluted at 38,93, and 110 "C, respectively, i.e., before, during, and after the elution of the water transferred to the column. A total of 34 replicate standards were run over several days without the water trap. Another 34 replicates were then run over several days with the water trap. Use of the water trap reduced Vw,colto -0.4 YL. For each analysis, the GC/MS areas for IS1 and IS2 were normalized to the area for IS3 (IS3 would be expected to be least affected by the eluting water). Without the water trap, the relative standard deviations (CVs) for the normalized areas for IS1 and IS2 were 17.0 and 49.3%, respectively. With the water trap, the two CVs were reduced substantially to just 11.1and 11.8%, respectively. The retention times of the comopunds also became much more reproducible. Without the water trap, the absolute retention time window for IS2 was 740 f 30 s; with the water trap, the variation was reduced to only f 2 s. The reduction of Vw,eolto -0.4 pL achieved with the water trap thus provided large improvements in response and retention time reproducibility. Therefore, use of the water trap allows the P/WCC method to be used very effectively with large purge gas volumes. Acknowledgments The assistance of Lorne M. Isabelle, Bruce A. Tiffany, and John M. E. Storey in a portion of this work is gratefully acknowledged. Literature Cited (1)
(2) (3) (4) (5) (6)
US.EPA
Test Methods for Organic Chemical Analysis of Municipal and Industrial Wastewater; EPA-60014-82057; Longbottom, J. E., Lichtenberg, J. J., Eds.; U.S. EPA: Washington, DC, 1982. Pankow, J. F. HRC&CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1987, 10, 409-410. Pankow, J. F.; Rosen, M. E. Environ. Sci. Technol. 1988, 22, 398-405. Cochran, J. W. HRC&CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1988, 11, 663-665. Rosen, M. E. Ph.D. Thesis, Oregon Graduate Center, Beaverton, OR, 1988. Pankow, J. F. Anal. Chem. 1986, 58, 1822-1826.
Received for reuiew December 1, 1989. Revised manuscript received July 11, 1990. Accepted July 25,1990. This work was made possible with federal funds from the U.S. Geological Survey ( U S G S ) under Grant 14-08-0001-Gl235. T h e contents do not necessarily reflect the views or policies of U S G S , nor does mention of trade names constitute endorsement for use.
