Anal. Chem. 1986, 58, 1822-1826
1822
Association, Water Pollution Control Federation: Washington DC, 1980. (7) Chrlswell, Colin D.; Ericson, Rhonda L.; Junk, Gregor A,; Kenneth W.; Fritz, James S.; Svec, Harry J. J. A m . Water Assoc. 1977, 69, 669-674. (8) Van Rossum, Peter; Webb, Ronald G. J . Chromatogr. 1978, 750. 381-392. (9) Chen, Abraham S.; Larson, Richard A,; Snoeyink, Vernon L. Environ. Sci. Technol. 1982, 16, 268-273. (10) McCreary, John J.; Snoeyink, Vernon L.; Larson, Richard A. Environ. Sci. Technol. 1982, 16. 339-344. (11) Voudrlas, E. A.; Dielmann, L. M.; Snoeyink, Vernon L.; Larson, Richard A.; McCreary, John J.; Chen Abraham S. C. Water Res. 1983, 17, 1107-1 114. (12) Coyle, Gerry T.; Maloney, Stephen W.; Gibs, Jacob; Suffet, Irwin H. I n Water Chlorination : Environmental Impact and Health Effects, Joky, Robert L., Brungs, William A., Cotruvo, Joseph A,, Cumming, Robert E., Mattice, Jack S., Jacobs, Vivian A,, Ed.; Ann Arbor Science: Ann Arbor, MI, 1983; Vol 4, pp 421-443. (13) Alben, Katherine T.; Shpirt, Eugene Environ. Sci. Technol. 1983, 17, 187- 192. (14) Alben, Katherine T.; Shpirt, Eugene; Kaczmarczyk, Joan H. Proc. AWWA Annu. Conf., Conf. Proc. Am. Water Works Assoc. 1984, 1555-1571. (15) Grob, Kurt J. Chromatogr. 1973, 8 4 , 255-273. (16) White, Lowell D.; Taylor, David G.; Mauer, Patricia A,; Kupel, Richard E. Am. Ind. Hyg. Assoc. J. 1970, 31, 225-232. (17) Coleman, W. Emile; Melton, Robert G.; Slater, Robert W.; Kopfler, Frederick W.; Voto, Stephen J.; Allen, Wendy K.; Aurand, Theresa A. Proc. Am. Water Works Technol. Conf. V I I , Am. Water Works Assoc, 1980, 93-111. (18) Sawicki. E. Health Lab. Sci. 1975, 72. 407-414. (19) Golden, C. Sawicki. E. Anal. Letf., PartA 1978, A l l , 1051-1062. (20) Swanson, Donald H.: Walling, Joseph F. Chromatogr. Newsl. 1981, 9 , 25-26. (21) Cooke, N. E.; Gaikwad, R. P. Can. J . Cbem. Eng. 1983, 67, 697-702. (22) Jackson, W. Roy; Larkins, Frank P.; Thewlis, Paul; Watkins, Ian Fuel 1983, 62, 606-607. (23) Koh, Tee-Siaw Anal. Chem. 1983, 55, 1814-1815.
(24) Alben. Katherine T.; Kaczmarczyk, Joan H. J. Chromatogr. 1986, 357 , 497-500. (25) Fed. Regist. 1979, 4 4 , 68624. (26) Activatd Carbon Product Bulletin: Flkrasorb 300 and 400 Grsnular Activated Carbons for Potable Water Treatment; Calgon Corp.: Plttsburgh, PA, 1976. (27) Fisher, James L., Calgon Corp., Pittsburgh, PA, private communication to K. Alben, June 13, 1985. (28) Basic Concepts of Adsorption on Activated Carbon: Calgon Corp.: Plttsburgh, PA, 1985. (29) Glaze, William H.;Lin, '2.4. Optlmization of Liquid-LiqvM Extraction Methods for Analysis of Organics ln Water, National Technlcal Information Service: Springfield, VA, Oct 1963. (30) Glaze, William H.; Rawley, R.; Burleson, J. L.; Mapel, D.; Scott, D. R. I n Advances in the Identlkatlon and Analysis of Organlc Pollui%nts in Water; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Vol. 1, pp 267-280. (31) Richard, John J.; Junk, Gregor A. J. Am. Water Works Assoc. 1977, 69,62-64. (32) Mleure, James P. J. Am. Water Wwks Assoc. 1977, 69, 60-62. (33) Veith, Gilbert D.; Macek, K. J.; Petrocelli, S. R.; Carroll, John I n Aquatic Toxic&@; Eaton, J. G., Parrish, P. I?.,Hendricks, A. C., Eds.; ASTM: Philadelphia, PA, 1980; pp 116-129. (34) Banerjee, Sufi; Yalkowsky, Samuel H.; Valvani, Shri Environ. Sci. Technol. 1980, 14, 1227-1229. (35) Hansch, Corwin; Quinlan, John E.; Lawrence, Gary L. J. Org. Chem. 1968, 33, 347-350. (36) Barbari, Timothy A.; King, C. Judson Envlron. Sci. Technol. 1982, 76, 624-627. (37) Junk, Gregor A.; Ogawa, I.; Svec, Harry J. I n Advances in the Identification and Analysis of Organic Pollutants; Keith, Lawrence H., Ed.; Ann Arbor Sclence: Ann Arbor, MI, 1981; pp 281-292. (38) Dixon, Wilfrid J.; Massey, Frank J. Introduction to Statistlcal Ana/@; McGraw-Hill: New York, 1957; pp 109, 124-127, 384. (39) Glaser, John A.; Foerst, Dennis L.; McKee, G. Ed.; Quave, Stephen A,; Eudde, William L. fnviron. Sci. Techno/. 1981. 75, 1426-1435.
RECEIVED for review November 25, 1985. Accepted March 18, 1986.
Magnitude of Artifacts Caused by Bubbles and Headspace in the Determination of Volatile Compounds in Water James F. Pankow Water Research Laboratory, Department of Chemical, Biological, and Environmental Sciences, Oregon Graduate Center, 19600 N . W. Von Neumann Dr., Beauerton, Oregon 97006
The formation of bubbles in a water sample or the presence of headspace above a water sample wlU cause losses of volatlle analytes. Bubbles may occur when ground waters high in dlssdved gases are kougM to the M a c e . Equations are derived to predict the magnitudes of both types of artlfacts. When 1.0 atm is the lowest pressure that a sample sees, the crlterlon for bubble formatbn wly be pb > 1.0 pw, where p,, Is the equivalent In situ parHal pressure (atm)of the dissolved gas and p w Is the vapor pressure of water. I f bubbles form, the percent error (worst case) at 20 OC (293 K) WW be 100(exp[-1024 H (293 K) K, (293 K) @, 0.97711 - 1). H (293 K) (atm-m3/mol) and K , (293 K) (Watm) are the Henry's law constants of the analyte and bubble-forming gas, respectively, at 293 K. The percent error due to a headspace at 20 OC will be -100(41.6H (293) VdV,)/(41.6 H (293) V , / V , 1). V , (mL) Is the volume of the headspace, and V , Is the volume of the sample. Bubbles and headspace can in some cases cause problematic artifacts; in many cases, however, the arklfads they cause wlll be smaH for many of the commonty determined organlc compounds.
-
-
+
The need to determine organic compounds accurately in water samples is growing at a rapid pace. As such, the attention given to sample acquisition, handling, and storage is
increasing (I). With volatile compounds, there has been particular interest in the artifacts that result if (1)bubbles form in the water during sampling and/or (2) headspace is present in the sample vial after sampling. The former may occur when ground water high in dissolved gases is removed from the overlying hydrostatic pressure and brought to the surface. For volatile compounds, both bubble formation and the presence of headspace will cause a negative bias since they cause losses. While some uncertainty in the concentration of a contaminant will not greatly alter the perception of the potability of a given water, artifacts must nevertheless be kept within limits. Firstly, measured levels must be compared in a confident manner to specific water quality standards. Secondly, it is frequently important to know whether the water quality is indeed improving or worsening. Finally, when the processes controlling the fates of contaminants are studied, artifacts can become very troublesome when trying to assess the relative importance of various natural fate processes such as volatilization, biodegradation, sorption, and dispersion. Although it is not always possible to offer simple equations for the expected magnitudes of artifacts, it can be done for the cases when volatile compounds partition to bubbles and headspace. The equation for when bubbles form due to sample depressurization has not yet been presented, and is derived here. Although the equation for headspace-related
0003-2700/86/0358-1822$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
artifacts is derivable within the context of known headspace analysis principles ( 2 , 3 ) ,it is also presented here, since there are many similarities between the cases of bubble- and headspace-related artifads. It should be noted that this paper deals only with bubbles and headspace as sources of artifacts; any conclusions drawn concerning the suitability of different types of sampling devices remain subject to considerations of other sources of artifacts (e.g., sorption, degradation, and leaching-related contamination).
EFFECTS OF BUBBLES DUE TO SAMPLE DEPRESSURIZATION For Henry's law partitioning of a gas to water cdg = KHP (1) where Cdg is the concentration of the dissolved gas (M), K H i s the Henry's law constant for the dissolved gas (M/atm), and p is the partial pressure of the gas (atm). When the partial pressure of the gas changes then for a nonreactive gas Acd, = KHAP
(2)
Equation 2 is also true to a good approximation for COz (reactive) for water of both zero and nonzero alkalinity since COP is only a weak acid in water. Equation 2 remains approximately true for COz even when the degassing leads to the precipitation of calcium or magnesium carbonate. A dissolved gas may be thought to exert a hypothetical, in situ vapor pressure, pis, even when there is no gas phase present Pis
=
Cdg/KH
(3)
'This pisis the chemical activity of the dissolved gas in units of atmospheres. This pressure equals the fugacity which Mackay and Paterson (4)have used in the modeling of the fates of environmental contaminants. When sampling ground water, a common perception is that bubbles of a gas will form whenever the sample is brought up to the level of the water table (i.e., the pressure is reduced to atmospheric levels), and the sample is supersaturated with respect to the ambient atmospheric level of that specific gas (pmbient),i.e., whenever cdg
= K#is
'
Kflambient
(4)
In fact, bubbles will form only when the water is brought to the ground surface and pw+
Xpi,is I
> 1 atm
(5)
where p , is the vapor pressure of water (0.023 atm at 20 "C), Cipi,isis the sum of the in situ vapor pressures for all of the dissolved compounds, and 1atm is the atmospheric pressure a t the ground surface at sea level. Equation 5 and the subsequent equations in this section are easily modified for use at elevations other than sea level. (The satisfaction of the inequality in eq 4 without satisfaction of eq 5 is capable of causing artifacts, but only by volatilization from an interface between the water and surface air.) Thus, in a manner completely analogous with the boiling of water, steam distillation, and other bubble-formation processes, bubbles will only form when the vapor pressures of all of the constituents (including water) s u m to a value that is greater than the overlying pressure. After a given ground water is sampled, this pressure will be 1 atm so long as the water is not brought to the surface with a suction pump. With such pumps, the minimum pressure seen by the sample can be as low as -0.37 atm as the limits of "suction lift" are reached. (Since 0.37 atm > p,, though, the water will not boil at this point.) Under low-pressure conditions, bubble formation and concomitant volatilization losses can be exacerbated greatly. Thus, while they remain useful in sampling
1823
when the water table is near ground surface, suction pumps have in general fallen from favor for sampling ground water for volatile compounds (5). Alternative sampling devices include (1)bailers that can be activated at any depth, (2) submersible pumps (which will be taken in this paper to include devices such as bladder pumps), and (3) samplers that maintain the sample at the in situ hydrostatic pressure (6). With bailers, the minimal sample pressure is the 1 atm that the sample experiences at the surface. Submersible pumps will in general be similar in this regard. Some pumps may, however, expose the sample to low pressures in the pump itself. While any bubbles so formed may be able to recollapse before the sample reaches the surface (the pump outlet pressure will be greater than the in situ water column pressure since the water must still be lifted to the surface), it seems possible they may not all collapse completely, and subsequent bubble nucleation at the surface may be facilitated if eq 5 is satisfied. When the minimal sample pressure is 1 atm, eq 5 is the appropriate criterion for bubble formation. As such, eq 5 will be taken as the basis for further analysis here. Assuming that only one dissolved gas is present at a time (situations when two or more dissolved gases are important may be considered in a similar manner), the minimal criterion for bubble formation is then (6) P i s > 1.0 - ~w In the worst case, bubble formation will continue so long as eq 6 applies. This is the worst case since, (1)pisvalues substantially larger than 1atm are usually required to nucleate bubbles either heterogeneously (as on particles of suspended silt and clay) or homogeneously (7-9) and (2) a sample can probably be sealed in its container before the bubble formation process is complete. Therefore, by eq 2, the worst-case change in the dissolved concentration of the gas due to bubble evolution alone will be ACdg,b = KH(Pis - 1.0 + P w )
(7)
If V, (mL) is the volume of bubbles that leaves the sample volume Vs (mL), then vg/vs(mL/mL) = (1000RT/(1.0 - Pw))ACdg,-, (8)
V g / V s(mL/mL) = [1000RT/(1*0 - Pw)lKH(pis - 1.0 + pw) (9) where R is the gas constant (8.2 X m3.atm/mol.deg), T is the temperature (K), and the factor (1000 RTI(1.0 - p , ) ) is the volume in liters occupied per mole of the volatilized gas at its partial pressure of (1.0 - p,) atm. When a gas bubbles incrementally through a volume V, that contains a second, volatile compound, i.e., an analyte of interest, the differential equation governing the equilibrium rate of loss of the analyte is Vs dc = -(Hc/RT) dVg (10) where c is the concentration of the analyte (mol/m3) and H is the Henry's law constant of the analyte (atm.m3/mol). Equation 10 integrates to yield
= exp[-(H/RT)Vg/VsI percent error = (c/co - 1 ) l O O C/CO
(11) (12)
where c is the concentration of the analyte after Vg bubbles through V,, and co is the initial, artifact-free concentration. Equations 9 and 11 yield C / C o = eXp[-lOOOHKH(pis - 1.0 pw)/(l.o - p , ) ] (13)
+
Two different conventions have been used in the above discussion to express Henry's law constants. Following common practice, for the dissolved gases (e.g., COz, 02, N2, and
1824
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Table I. Henry's Law Constants K B and B f o r Four Common Gases for Water of Low Ionic Strength"
KH,M/atm gas
10 O C
20 "C
30 OC
40 "C
10 O C
H, atm.m3/mol 20 "C 30 OC
N2 CH4
10-3.09 10-2.73 10-2.77 10-1.27
10-3.17 10-2.83
10-3.23 10-2.91 10-2.93 10-1.53
10-3.28 10-2.97 10-2.98 10-162
1.22 0.535 0.589 0.0187
1.47 0.676 0.720 0.0256
0 2
COZ
10-2.M
10-1.41
1.69 0.807 0.855 0.0335
40 "C 1.91 0.935 0.963 0.0419
"Data from NRC (10).
CHI), the Henry's law constant has been expressed in terms of KH with units of M/atm. For the volatile organic compounds, the Henry's law constant has been expressed in terms of H with units of atmm3/mol. The two type of constants may be interconverted according to KH = (H.1000 L/m3)-'. Considering the inverse nature of this relationship, a high volatility from water obtains when KH is small, i.e., H is large. H and KH values are temperature and ionic strength (I) dependent. As indicated in Table I, the KH values for the common gases are available as a function of temperature a t low I (i.e., fresh water); they increase by a factor of 1.1-1.4 for every 10 "C decrease in temperature. While many of the H values currently available for the volatile organic compounds pertain primarily to 20-25 "C at low I (Table 11), it is known that numerous of these H values decrease by a factor of -2 with every 10 "C decrease in temperature; this general rule may be used with the Table I1 data to estimate artifacts according to eq 13. At constant temperature, the general effect .of increasing I is to cause increases in H values, Le., decreases in KH values. When T = 293 K c/co (293 K) = exp[-1024H (293 K)KH (293 K) (pi,- 0.97711 (14) Unless pis is close to (I - p w ) ,a condition which would subject eq 14 to considerable error if p w # 0.023 atm, then by virtue of the temperature dependencies of H for organic analytes and KH for common dissolved gases, eq 14 will typically give overestimates of the error at temperatures lower than 20 "C (such as are typical for ground waters) and underestimates of the error at temperatures higher than 20 "C (such as may occur at the surface while sampling on a warm day). Nevertheless, since (1)one will usually only be interested in determining the general magnitude of the possible error and (2) eq 13 will in any event provide the exact value of the error only when the bubbling process goes to completion, eq 14 will often be useful when considering the possible magnitudes of bubble-related artifacts. If one of the Table I gases should bubble and cause the loss of a second Table I gas (present at comparatively lower concentrations), then eq 13 and 14 may still be applied with H being that for the second gas; this is the rationale for including H values in Table I. For the gas(es) predominantly responsible (recall eq 5) for the bubble formation, eq 2 can be used to predict artifact magnitudes. For an initial perspective on bubble-related artifacts for volatile compounds, if Nz bubble formation is to cause a -10% artifact (c/co = 0.90) for H = 0.11 atmm3/mol (e.g., trichlorofluoromethane), then by eq 14 a p i s value of 2.4 atm would be required at 20 "C. With the atmospheric level of Nz being 0.78 atm, this corresponds to about 3 times the surface level saturatiop concentration. While this degree of supersaturation has been found in natural ground waters (15), it probably represents an upper limit for most natural situations. By eq 9, it corresponds to a V g /Vsvalue of only 0.023. If Vg/Vs= 0.023 is a rough upper bound for most natural waters for bubbles due to Nz-degassing,the range for general consideration can probably be limited to V,/ V , I0.20 (1mL of bubbles/5 mL of sample). Figure 1 indicates how the
Table 11. Henry's Law Constants ( E I ) for Selected Organic Compounds at 20-25 OC and Low Ionic Strength, and Artifact Errors for Bubbles and Headspace for V J V , = 1 mL/39 mL at 20 "C"
compd halog, nated nonaromatics methyl chloride methyl bromide methylene chloride chloroform bromodichloromethane dibromochloromethane bromoform dichlorodifluoromethane trichlorofluoromethane carbon tetrachloride chloroethane 1,l-dichloroethane 1,2-dichloroethane l,l,l-trichloroethane 1,1,2-trichloroethane 1,1,2,2-tetrachloroethane hexachloroethane vinyl chloride 1,l-dichloroethene 1,2-trans-dichloroethene trichloroethene tetrachloroethene ethylene dibromide (EDB)b 1,2-dichloropropane trans-1,3-dichloropropene hexachlorocyclopentadiene hexachIoro-1,3-butadiene chlorinated ethers bis(chloromethy1) ether 2-chloroethyl vinyl ether bis(2-chloroisopropyl) ether 4-chlorophenyl phenyl ether 4-bromophenyl phenyl ether monocyclic aromatics benzene chlorobenzene o-dichlorobenzene m-dichlorobenzene p-dichlorobenzene toluene ethylbenzene o-xylene m-xylene p-xylene
neg artifact error a t 20 OC headH, atm.m3/mol bubbles space
0.04 0.20 0.002 0 0.002 9 0.002 4 0.00099 0.OOO 56 3.0 0.11 0.023 0.15 0.004 3 0.000 91 0.03 0.000 74 0.OOO 38 0.002 5 0.081 0.19 0.067 0.009 1 0.015 3 0.000 82 0.002 3 0.001 3 0.016 0.026
4 19
4 18
