2490
Anal. Chem. lQ04, 56, 2490-2496
Estimation of Vapor Pressures for Nonpolar Organic Compounds by Capillary Gas Chromatography Terry F. Bidleman' Special Analytical Laboratory, National Swedish Environmental Protection Board, Box 1302, S-171 25 Solna, Sweden
The accuracy of a method based on capillary gas chromatography (GC) for estlmatlng vapor pressures of nonpolar compounds was tested by comparing vapor pressures measured by GC (P,) with llterature values for the subcooled llquld (P,). Two 1.0 m long fused silica columns were used, a poly(d1methylslloxane) bonded phase column (BP-i) and a WCOT hydrocarbon phase column (Apolane-87). GC measurements were carried oul by using n-C,, and n-C,, hydrocarbons as vapor pressure reference compounds. Plots of log P,, vs. log PLwere made for 24 organochlorines and polycyclic aromatic hydrocarbons to establish a correlation between measured and llterature values. Systematic errors between P,, and P , were observed whose dlrectlon and magnltude depended on the vapor pressure range and the column used for measurements. On the BP-1 column P,, was too low by a factor of 2.3 at P , = 10-1 torr, and too hlgh by a factor of 3.5 at P , = torr. The Apolane-87 column showed more nearly Ideal behavlor. On thls column P,, underestimated P , by a factor of 1.9 at P , = lo-' torr and the error decreased for lower PL. Corrections for these Systematic errors were made by relatlng P,, to P , through regression equations. The GC method was applled to 30 PCB lsomerlds contalnlng one to seven chlorlnes. The average vapor pressure ( P , ) decreased by a factor of 4.5 for each chlorine added to the biphenyl core, although large differences In vapor pressure were measured for Isomers of a particular chlorlnatlon level. Wlthln each chlorlnatlon level Isomers havlng the greater number of ortho chlorlnes had the higher vapor pressures.
Developing the ability to predict the transport, distribution, and fate of organic pollutants is an important and rapidly expanding area of environmental science. A wide variety of chemicals are in use today and new ones are introduced to the marketplace each year. The task of assessing the environmental effects of these substances is enormous, and much effort has gone into determining ways to explain pollutant behavior in terms of basic thermodynamic and kinetic concepts. Essential to these models is a knowledge of pollutant chemical and physical properties, two of the most fundamental being water solubility and vapor pressure. From these two properties a number of others necessary for predicting pollutant cycling can be derived. Bioconcentration factors (BCF), octanol-water partition coefficients (Kow),and partition coefficients between sediment organic carbon and water (K,) are closely correlated (inversely) with solubility (1,2). From water solubility and.vapor pressure the Henry's law constant (H) can be calculated. H is needed to estimate vapor exchange rates across an air-water interface and the importance of precipitation scavenging in removing vapors from the atmosphere (3,4). Vapor pressure also governs the adsorption of On leave from the Department of Chemistry and Marine Science Program, University of South Carolina, Columbia, SC 29208. Use the U.S.A. address for correspondence. 0003-2700/84/0356-2490$01.50/0
organic pollutants to airborne particles (5-8) and pesticide volatilization from soils (9). Methods for determining physical properties of high molecular weight organics are time-consuming and require careful experimental technique, and rapid methods are needed to estimate these properties for existing chemicals and new substances. Recen, advances in this area have been made for the solubility-correlated properties, e.g., the use of reversedphase thin-layer chromatography and high-performance liquid chromatography to estimate BCF, KO,, and KO,(2, 10-12). Similarly, gas chromatography (GC) has been used to estimate vapor pressures of herbicides (13, 141, PCB isomerids ( I @ , and PCB replacement fluids (16). GC is an attractive technique for vapor pressure measurement. It is fast and tolerant of impurities and only minute quantities of chemical are needed. The latter aspect is especially important for chemicals that are highly toxic or produced in limited quantities for research purposes, e.g., carcinogens and pesticide metabolites.
Conventional and GC Methods for Vapor Pressure Determination. Two techniques are common for measuring vapor pressures of low-volatility chemicals. Gas saturation (17-20), the most widely used method, is carried out by passing a slow stream of nitrogen or air through a column of sand or glass beads coated with the compound of interest. The saturated vapor is passed through an impinger, a cold trap, or a solid adsorbent column which quantitatively strips the compound from the gas stream. The trap contents are analyzed, and the vapor pressure is calculated using the ideal gas law. The effusion or vapor balance method (17,20,21) is based on the effusion rate of material through a small orifice into a vacuum. This rate is measured by the weight loss of a small vessel containing the chemical or by the twisting force produced on a fine quartz fiber. Since these are nonspecific measures of material flux, the method is sensitive to the presence of volatile impurities (17). Gas saturation and effusion have official recognition. The saturation method is recommended by the US. Environmental Protection Agency (19), and both methods are accepted by the Organization for Economic Cooperation and Development (OECD) (20,22). Both techniques require great care to obtain accurate and reproducible results. Precisions of a few percent are attainable in a single laboratory, but agreement among laboratories is less satisfactory. In one study (23) two independent gas saturation values for a pyrethrin insecticide agreed within 45%. However, during an OECD intercalibration exercise on physicochemical properties determinations (22) agreement between saturation and effusion values for benzoic acid, hexachlorobenzene, and di-2-ethylhexyl phthalate was no better than a factor of 2-3. Spencer and Cliath (17)quote several dieldrin vapor pressures which span more than an order of magnitude. The GC method is based on the simple concept that retention volumes (or times) are inversely related to solute vapor pressure. Jensen and Schall(l3) used GC to estimate vapor pressures for several phenoxy herbicide esters. The method of data treatment was improved by Hamilton (14) to reduce extrapolation errors and his approach was subsequently used by Westcott and Bidleman (15) for five PCB isomerids and 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2401
Table I. Test Compounds for Vapor Pressure Determinations
no.
compound
temp range ("C) GC measurements
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
naphthalene 1-methylnaphthalene 1,2,3,4-tetrachlorobenzene biphenyl 2-chlorobiphenyl 4-chlorobiphenyl fluorene hexachlorobenzene phenanthrene aldrin anthracene 7-hexachlorocyclohexane 2,4,6-trichlorobiphenyl dieldrin pyrene 2,2',5,5'- tetrachlorobiphenyl fluoranthene 2,2',4,5,5'-pentachlorobiphenyl p,p'-DDE o,p'-DDT benz[a]anthracene D.D'-DDD ~+-DDT benzo[a]pyrene
40-80 40-90 40-90 40-90 40-100 40-100 70-110 70-110 70-120 70-110 70-110 70-110 70-110 70-120 70-120 70-120 70-110 70-110 70-120 80-120 90-130 80-130 80-130 90-130
17
18 19 20 21 22
23 24
by Addison e t al. (16) for chlorodiphenyl ethers. GC measurements are based on relative retention volumes of test and reference substances, and two criteria are necessary for accuracy: Vapor pressures of the reference compounds must be accurately known over the temperature range of the GC measurements and at the final extrapolation temperature (usually 20-25 "C). Partitioning of the compounds between the carrier gas and stationary phase must be controlled by volatility alone. Column selectivity is undesirable. In this second requirement lies most of the uncertainty in the GC method. Previous GC vapor pressure work has been done on nonpolar silicone oil phases such as SE-30(13, 14) and OV-1 (16), or the normal hydrocarbon phase Apolane-87 (15). Di-n-butyl phthalate (DBP) was used as a reference substance, and in one case (15) a herbicide ester was used whose vapor pressure had been measured vs. DBP. The assumption of separations based purely on volatility was not tested. Also, many heavy organics are solids a t room temperature, and there is a question as to whether GC responds vapor pressure. to the solid (Ps) or subcooled liquid (PL) This study was undertaken to answer some of the questions about the accuracy of the GC method for estimating ambient temperature vapor pressures of nonpolar compounds. The approach was to compare GC-measured vapor pressures to literature values determined by some other technique, usually gas saturation or effusion. Organochlorines and polycyclic aromatic hydrocarbons (PAH) were chosen because they are important pollutants and because vapor pressures for many of these are available.
EXPERIMENTAL SECTION Chromatography. Two 1.0-m fused silica capillary columns were used: a 0.22-mm i.d. BP-1 bonded phase column (polyand (dimethylsiloxane), equivalent to SE-30 or OV-1, SGE, LM.); a 0.25-mm i.d. WCOT Apolane-87 column, 0.25-pm film thickness (Quadrex Corp.). Columns were installed in a Varian 3700 chromatograph with a flame ionization detector and were operated under the following conditions: carrier gas, He, 2-4 mL/min; injector, 150-200 "C; detector, 300 "C. An approximately 100-800-ng sample in 1 pL of hexane or pentane was injected at a 101 to 20:l split ratio. Isothermal runs were made at four to seven temperatures spaced 10" apart over different ranges depending on sample volatility (Table I). Re-
ref alkane octadecane
eicosane
-log PGc,torr (25 "C) BP-1 Apolane-87 0.770 1.254 1.521 1.600 2.085 2.353 2.450 2.924 3.079 3.764 3.123 3.095 3.188 4.399 4.071 3.851 3.936 4.423 4.693 4.917 5.094 4.914 5.206 6.076
0.674 1.352 1.582 1.592 1.959 2.371 2.545 3.040 3.287 3.603 3.320 3.309 3.148 4.349 4.435 3.907 4.301 4.625 4.804 5.016 5.647 5.330 5.453 6.948
-'pi3 PL, torr, lit. (25 "C) 0.503 1.179 1.222
1.376 1.860 1.975 2.226 2.643 2.999 3.104 3.188 3.383 3.648 3.792 3.926 4.108 4.190 4.627 4.719 5.002 5.390 5.487 5.625 6.754
Table 11. Vapor Pressures (P2, torr) of the Reference Alkanes Octadeoane and Eicosanea temp, O C 130 120 110 100 90 80 70 60 50 40 25*
octadecane 1.384 0.728 0.371 0.182 8.59 X 3.89 X 1.68 X 6.89 x 2.68 x 9.80 x 1.91 x
lo-' lo-' lo-' 10-3 10-3
10-4 10-4
eicosane 0.362 0.178 8.44 X 3.84 X 1.68 X 6.99 x 2.76 x 1.03 x 3.64 x 1.20 x 1.97 x
lo-' 10-3
10-3 10-3 10-4 10-4 10-5
Calculated from In P = A + B/T. Parameters determined by MacKnick and Prausnitz (24) over the indicated temperature ranges were as follows: octadecane, A = 25.548, B = -10165 (45-88 "C); eicosane, A = 26.849, B = -11230 (71-107 "C). *Vapor pressures are for the subcooled liquid (PL), since extrapolations were made from above the melting points (28 "C for octadecane, 37 "C for eicosane). tention times of test compounds were expressed relative to those of two vapor pressure standard alkanes. Eicosane (n-C,) was used as a reference for fluorene and less volatile compounds, whereas relative retention times of more volatile compounds were calculated vs. octadecane (n-C18). Vapor pressures of the reference alkanes have been measured by gas saturation (24) and are given in Table 11. Pesticides were obtained from the U.S.Environmental Protection Agency, Pesticides and Industrial Chemicals Repository, Research Triangle Park, NC. PCB isomerids were synthesized in this laboratory (25,26).All other chemicals were analytical reagent quality. Data Treatment. The equations for calculating vapor pressures from GC retention data have been derived by Hamilton (14). Vapor pressures of two substances at the same temperature are related through In Pl = L 1 / L zIn Pz + C (1) where 1and 2 refer to test and reference compounds and L is the heat of vaporization. The parameters L,/L2and constant C can be obtained from GC data using In (VR)I/(~R)Z = (1- Ll/L,) In PZ- C (2)
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Table 111. GC Data for Vapor Pressure Determinations, Apolane-87 Column retention time ( t ~(min) ) and In re1 tR (vs. eicosane) 120 O C 110 OC 100 O C 90 "C 80 OC 70 O C
compound 2,4-DCB
eq 2 parameters
m
-C
L,/L2
r2
Poc, torr (25 "C)'
0.71 -2.170
1.15 -2.394
1.89 -2.642
3.54 -2.838
7.35 -3.058
0.2593
-1.548
0.7407
0.996
1.54 X
phenanthrene
0.94 -1.234
1.57 -1.377
2.68 -1.548
4.68 -1.735
8.74 -1.934
18.43 -2.139
0.2193
-0.843
0.7807
0.999
4.93 X
2,2',5,5'-TCB
1.81 -0.579
3.15 -0.680
5.75 -0.784
10.87 -0.893
21.73 -1.023
49.13 -1.159
0.1387
-0.335
0.8613
0.999
1.24 X
dieldrin
4.14 0.248
7.32 0.163
13.46 0.066
26.54 0.000
53.23 -0.128
121.7 -0.251
0.1181
0.457
0.8819
0.995
4.48
p,p'-DDE
6.54 0.705
12.05 0.661
23.46 0.622
47.87 0.590
102.8 0.531
253.5 0.482
0.0528
0.796
0.9472
0.996
1.57 X
8.98 1.023
16.69 0.987
32.91 0.960
68.19 0.944
150.2 0.910
0.0332
1.074
0.9668
0.982
9.64 X lo4
3.23
6.22
12.60
26.54
60.47
0.000
0.000
o,p'-DDT eicosane (reference)
0.000
0.000
0.000
X
lo-'
156.5 0.000
'Calculated using eq 1 and In P2for eicosane at 25 OC (-10.835). Example for 2,4-DCB: In PGC= 0.7407(-10,835)
+ 1.548 = -6.477.
, . . 0,P-DDT
*-*-• P,P-DDE
io
20
30
MINUTES
Flgure 1. Chromatogram of several vapor pressure test compounds and elcosane reference, Apolane-87 column, 100 OC. Chromatographed quantities calculated from lnjectlon amounts and the split ratio (13:l) were 6-18 ng.
I
-6
In this equation (V&/(V& is the relative retention volume (or time) of the test to the reference compound. The left side of eq 2 was plotted vs. In Pz(from Table 11) at four to seven temperatures and the slope (1- L1/L2) and intercept (-C) were obtained by linear regression. Using these parameters, the test compound vapor pressure (PI)at 25 O C was calculated by using eq 1. In subsequent discussions, this GC-determined vapor pressure is referred to as PGc. Sample data for one run are shown in Table I11 and Figures 1 and 2.
RESULTS AND DISCUSSION Characteristics of the Short Capillary Columns. An advantage of capillary over packed column GC for vapor pressure work is that analytical times are much shorter (15). This is because capillary columns only 1m long (and perhaps less) can be used and still provide enough resolution to separate several compounds (Figure 1). However even with a short column run times can be a few hours when low volatility substances are chromatographed at low temperatures, and it is desirable to use a fairly high flow rate to decrease retention times as much as possible. At the 2-4 mL/min flows used here, the 1.0-m columns provided 900-2000 theoretical plates,
I
-5
I
1
-4
-3
I
I
-2
-1
L n P2
Flgure 2. Plots of In ( V R ) , / ( V & vs. In P , (from Table 11). Data are glven in Table 111.
calculated from the eicosane peak. Sample quantities should be kept small to avoid overloading the column. The effect of sample size on peak shape is shown in Figure 3, overload being indicated by leading peaks with an increased retention time. Of the two columns studied, the BP-1 had the higher sample capacity. Good peak shape was obtained on the BP-1 with 130 ng sample, while the Apolane-87 column showed signs of overload above about 8-16 ng. These are actual on-column quantities, calculated from amounts injected and the split ratio. Because of this limited sample capacity, it would be advantageous to use a more sensitive detector (e.g., electron capture) whenever possible. Compounds with long retention times gave broad peaks and it was sometimes difficult to discern the peak when small sample quantities were used. Even at the highest electrometer sensitivity, some overload was unavoidable when chromatographing the lowest vapor pressure compounds a t low tem-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1984
2493
0-
170 ng
A
Flgure 3. Effect of sample quantlty on peak shape, BP-1 column. The
attenuation has been changed to give approximately the same sized peaks. Indicated are chromatographed quantitles of elcosane, calculated from inJectionamounts and the split ratio. Temperature and flow conditions were as follows: 90 "C, 4.1 mL/mln, 11:l split.
-1
-
-2
-
-7
d -7
-6
-5
-4
-3
-2
-1
0
Flgure 4. Comparison of P, with subcooled liquid vapor pressures (PJ from the literature, BP-1 column, eicosane and octadecane standards. Numbers refer to compounds In Table I. SolM line indicates a 1: 1 correspondence: dashed line is the least-squares relationship.
peratures. This produced some peak distortion of the type shown in Figure 3. In these cases the retention time was not taken at the peak maximum, but instead was estimated at the midpoint between the beginning and the end of the peak. Tests with normal and overloaded peaks (as in Figure 3) showed that the error using this method was no more than 2-370 of the retention time.
Comparison of GC and Literature Vapor Pressures. PCcwas determined for 24 organochlorines and PAH having known vapor pressures. Duplicate or triplicate runs were carried out for about two-thirds of the compounds in Table I spanning the entire vapor pressure range, each experiment consisting of a data set at four to seven temperatures as in Table 111. The average precision was 9% (relative standard deviation for triplicates, range/mean for duplicates). Selection of the test compounds was made after a literature search for vapor pressures of high molecular weight nonpolar organics. Only one literature value was found for some compounds, while for others several values were retrieved. Different reports for a single compound sometimes agreed very well and at other times poorly. Uncertainties in the dieldrin vapor pressure were mentioned in the Introduction. We also found 11 values for anthracene spanning a factor of 8. On the other hand, five reports of the p,p'-DDT vapor pressure agreed within 21% RSD. When literature values were selected for comparison with PGC,some judgement was necessary. In most cases two or more values agreeing within a factor of 2 were averaged and an outlier was excluded. Also, we favored more recent results obtained by gas saturation or effusion. Space does not permit a presentation of all the literature vapor prmures that were examined, but this information is included as supplementary material in the microfiche version of this paper and is also available from the author. The literature values in Table I are thus either from single reports or averages of two or more values differing by about 2-fold. GC and literature vapor pressures are compared in Figures 4 and 5. From the results for high-melting compounds (e.g., anthracene, hexachlorobenzene) it is clear that GC responds vapor to the subcooled liquid (PL)not the solid phase (Ps) pressure. This is true despite the fact that these compounds
L o g PL
Flgure 5. A s In Figure 4 for the Apolane-87 column, eicosane and octadecane standards.
were chromatographed far below their melting points. With a few exceptions, the literature vapor pressures were for the crystalline solids. For these, PL was calculated from Psusing In (PL/Ps)= 6.8(Tm- 298)/298
(3)
where T, is the absolute melting point and 6.8 is an average empirical constant related to the entropy of fusion (27). The variation in this constant from compound to compound may be a source of error in estimating PL from literature Ps. Differences between PLand PSare very large for high melting compounds. For example, PL/PS for hexachlorobenzene and anthracene (mp 230 and 216 "C,respectively) are 108 and 78. Even for p,p'-DDT, mp 109 "C,PLis seven times higher than
PS.
log PGcwas correlated with log PLon both columns with
1.2 = 0.97-0.98,but on neither column was Pw equal to PLover
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ANALYTICAL CHEMISTRY, VOL. 56,
-7
-6
-5
-4
-3 Log PL
NO. 13, NOVEMBER 1984
-2
-1
0
Figure 6. As in Flgure 4 for the BP-1 column, DBP standard. Points 0 and E represent vapor pressures of octadecane and elcosane, measured vs. DBP.
the entire vapor pressure range. Positive or negative systematic errors were observed, as can be seen by the deviation of the regression lines from the ideal 1:l relationship (Figures 4 and 5). On the BP-1 column the slope of log PGc vs. log PL was only 0.85 instead of unity, and the intercept was -0.51 rather than zero. Thus P m underestimated or overestimated PL at the high- and low-volatility ends of the scale. PGC was 2.3 times too low at PL = 10-1 torr and 3.5 times too high at PL = IO-’ torr. The Apolane-87 column showed more nearly ideal behavior, with a slope of 0.95 and an intercept of -0.33. On this column, PGc underestimated PL by a factor of 1.9 at PL = lo-’ torr, and the error decreased for lower Pb The systematic error can be eliminated and the accuracy improved by using Figures 4 and 5 as calibration plots to correlate PGc to PL. The regression equations for these two figures are given in Table IV. Choice of Vapor Pressure Reference Compounds, DBP has been used as a vapor pressure standard in previous work (13-16). At the beginning of this study we measured P m vs. DBP on the BP-1 column for 14 of the compounds in Table I as well as for octadecane and eicosane. Vapor pressures for DBP were those determined by Small et al. (28),and used by Hamilton (14).Although the slope of log PGCvs. log PL was about the same as that in Figure 4, the intercept was more negative and all points fell below the theoretical line (Figure 6). Alternatively, PGCof DBP, other phthalates (diethyl, dimethyl), and some organophosphate pesticides measured against the eicosane standard were 3-4 times higher than the literature values. Thus the behavior depicted in Figure 4 does not appear to be caused by an inaccuracy in the DBP vapor pressure, but to a general tendency for polar compounds to elute faster on the BP-1 column than nonpolar compounds of the same volatility. A few trials on the Apolane-87 column produced similar results. As a result, vapor pressure measurements of nonpolar compounds vs. DBP are likely to have a negative error, for example, the PCB vapor pressures determined in previous work (15)are probably low by a factor of 3. It should still be possible to use DBP as a reference if PL is calculated from PGC using an appropriate correlation equation. This relationship, based on 16 compounds run on the BP-1 column, is given in Table IV. For work with nonpolar compounds the n-alkane standards are preferred over DBP because of the lower systematic error
between PW and PL. Only two alkanes were used in this study, but it would be convenient to introduce a couple additional ones to reduce the standard-sample retention volume differences. A lighter alkane (Cl4or C16)would be useful for the PL = 10-L10-2 torr range, and CZzor Ca for PL below lo+ torr. Application to PCB Isomerids. PCB are a well-known class of pollutants for which physical properties data are incomplete. Although solubilities for many PCB have been much less is known about their vapor determined (29,301, pressures. Albro et al. (31)determined the composition of the Aroclor fluids 1248,1254, and 1260, and published a list of the PCB isomerids with their chlorine substitution patterns, retention indexes on a Dexsil 410 column, and molar percentages in the commercial mixtures. From this list we selected 20 PCB containing four to seven chlorines for vapor pressure determinations. In choosing the isomerids, we tried to select some of the more abundant ones as well as to represent the volatility range (as indicated by retention index) for a particular level of chlorination. In addition, we measured the vapor pressures of 10 other PCB containing one to three chlorines. PGCfor the isomerids were measured using octadecane and eicosane standards, as described in the Experimental Section. Since PL is the property of environmental interest, PL was calculated from PGC using the regression equations in Table IV. All PCB were run on the BP-1 column, and 18 were also run on the Apolane-87 column. The average difference between PL on the two columns was 25.6% (of the BP-1 value), with Apolane-87 giving the higher result in most cases (Table VI. Vapor pressure of PCB isomerids as a function of chlorine number is shown in Figure 7. With each additional chlorine the average PL decreased by a factor of 4.5 (0.655 log unit), a lower increment than Bopp’s estimate of 0.776 (3).This trend in vapor pressure with chlorine number is approximate because only 30 out of 209 PCB are represented. The important point is that vapor pressure variations for isomers of the same chlorine content are large compared to the changes produced by adding chlorines. Within each chlorination level, isomers with the greater number of ortho chlorines had the higher vapor pressures. Jensen and Sundstrom (26)also noted that isomers with more ortho chlorines eluted earlier from charcoal columns, and recently Rapaport and Eisenreich (11) reported an “ortho effect” in using HPLC to estimate KO,for PCB isomerids. Considering the rather large differences in volatilities of PCB having the same chlorine content, it would be better to use individual isomer properties when modeling the environmental fate of PCB rather than simply the average values for a particular chlorination level.
CONCLUSIONS Capillary GC is capable of providing accurate vapor pressures for nonpolar compounds if the method is properly calibrated. The use of n-alkanes as vapor pressure reference compounds is preferred over the commonly used DBP, because nonpolar stationary phases appear to show some selectivity for DBP. The GC method responds to the subcooled liquid vapor pressure, but plots of log P ~ vs. c log PL show some deviation from ideal behavior and a measurement of P w may under- or overestimate PL depending on the vapor pressure range and the GC column used. Corrections for these systematic errors can be made by relating PQCto PL through regression equations. Parameters for these equations have been determined on two nonpolar capillary columns, BP-1 and Apolane-87. These columns should be useful to estimate PL of other nonpolar organics.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
ACKNOWLEDGMENT
Table IV. Parameters for the Equation log P m = log PL b
+
JIJ
column
vapor pressure ref
BP-1 BP-1 Apolane-87
n-alkanes DBP n-alkanes
b
m
r2
0.849 -0.516 0.967 0.841 -1.118 0.897 0.950 -0.326 0.976
std error of regression
biphenyl monochlorobiphenyls 2-CB 3-CB 4-CB dichlorobiphenyls 2,4-DCB 2,5-DCB 3,3’-DCB 4,4’-DCB trichlorobiphenyls 2,4,6-TCB 2,4,5-TCB 2,4’,5-TCB tetrachlorobiphenyls 2,2’,5,6’-TCB 2,2’,5,5’-TCB 2,2’,3,3’-TCB 2,3’,4,4’-TCB 2,3’,4’,5-TCB 3,3’,4,4’-TCB pentachlorobiphenyls 2,2’,4,5,5’-PCB 2,2’,4,4’,5-PCB 2,2’,3,4,5’-PCB 2,3’,4,4’,5-PCB 2,3,3’,4,4’-PCB hexachlorobiphenyls 2,2’,3,4’,5’,6-HCB 2,2’,4,4’,5,5’-HCB 2,2’,3,4,4’,5’-HCB 2,2’,3,3’,4,4’-HCB 2,3,3‘,4,4‘,5-HCB heptachlorobiphenyls 2,2’,3,4’,5,5’,6-HCB 2,2’,3,3’,4,4’,6-HCB 2,2’,3,4,4’,5,5‘-HCB 2,2’,3,3’,4,4’,5-HCB
--5 6
t
I
-log PL,torr (25 “C) BP-1 Apolane-87
ortho chlorines
s
I
I
0
1
2
1.277
1.331
1 0 0
1.848 2.134 2.162
1.717 2.119 2.151
1 1 0 0
2.860 2.860 3.188 3.272
2.618
2
3.145 3.478 3.592
1 1
3 2 2
3.688 3.924 4.134 4.335 4.389 4.785
1 1 0
4.599 4.658 4.770 5.047 5.185
2 2 2 1 1
3
5.074 5.293 5.421 5.592 5.793
2 2 2 1
3 3
5.640 5.854 6.010 6.202
2 2
I
I
3 4 CHLORINES
W. F. Spencer provided vapor pressure literature information, and he and D. Mackay offered valuable comments and suggestions during the experimental stages of this work. Registry No. p,p’-DDE, 72-55-9; o,p’-DDT, 789-02-6; p,p’DDD, 72-54-8; p,p’-DDT, 50-29-3; 2,4-DCB, 33284-50-3; 2,2’,5,5’-TCB, 35693-99-3; 3-CB, 2051-61-8; 2,5-DCB, 34883-39-1; 3,3’-DCB, 2050-67-1;4,4’-DCB, 2050-68-2;2,4,5-TCB, 15862-07-4; 2,4’,5-TCB, 16606-02-3;2,2’,5,6’-TCB, 41464-41-9; 2,2’,3,3‘-TCB, 38444-93-8; 2,3’,4,4’-TCB, 32598-10-0; 2,3’,4’,5-TCB, 32598-11-1; 3,3’,4,4’-TCB, 32598-13-3; 2,2’,4,4‘,5-PCB, 38380-01-7; 2,2’,3,4,5’-PCB, 38380-02-8; 2,3‘,4,4’-5-PCB, 31508-00-6; 2,3,3’,4,4’-PCB, 32598-14-4; 2,2’,3,4’,5’,6-HCB, 38380-04-0; 2,2’,4,4’,5,5’-HCB, 35065-27-1; 2,2’,3,4,4‘,5’-HCB, 35065-28-2; 2,2’,3,3’,4,4’-HCB, 38380-07-3; 2,3,3’,4,4‘,5-HCB, 38380-08-4; 2,2’,3,4’,5,5‘,6-HCB, 52663-68-0; 2,2’,3,3’,4,4’,6-HCB, 52663-71-5; 2,2’,3,4,4’,5,5’-HCB, 35065-29-3; 2,2’,3,3’,4,4’,5-HCB, 35065-30-6; 2-CB, 2051-60-7; 4-CB, 2051-62-9; 2,4,6-TCB, 35693-92-6; 2,2’,5,5’-TCB, 35693-99-3; 2,2’,4,5,5’-PCB, 37680-73-2;naphthalene, 91-20-3;1-methylnaphthalene,90-12-0; 1,2,3,4-tetrachlorobenzene, 634-66-2;biphenyl, 92-52-4; fluorene, 86-73-7;hexachlorobenzene, 118-74-1; phenanthrene, 85-01-8; aldrin, 309-00-2; anthracene, 120-12-7; y-hexachlorocyclohexane, 58-89-9; dieldrin, 60-57-1; pyrene, 129-00-0; fluoranthene, 206-44-0; benz[a]anthracene, 56-55-3; benzo[a]pyrene, 50-32-8; octadecane, 593-45-3; eicosane, 112-95-8.
0.262 0.276 0.246
Table V. Vapor Pressures of PCB Isomerids
compound
2495
3.146 3.202 2.967 3.449
Supplementary Material Available: a listing of literature vapor pressures that were examined (8 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th Street, N.W., Washington, D. C. 20036. Orders must state whether for photocopy or microfiche and give complete title of article, names of authors, journal issue date, and page numbers. Prepayment, check or money order for $13.50 for photocopy ($15.50 foreign) or $6.00 for microfiche ($7.00 foreign), is required and prices are subject to change.
3.765 4.317 4.833 4.521
5.154
LITERATURE CITED
4.863 5.268 5.373
5.650 6.016
I
I
5
6
I I
7
Figure 7. Vapor pressures of FCB isomerids as a function of chlorine number. Within each chlorination level the range of P L and the arithmetic mean is given. Regression equation: log (mean PL)= -0.655(chiorine number) - 1.431, r 2 = 0.993.
(1) Mackay, D. Envlron. Scl. Techno/. 1982, 16, 274-278. (2) Briggs, G. C. J . Agric. Food Chem. 1981, 29, 1050-1059. (3) Bopp, R. F. J . Geophys. Res. 1983, 8 8 , 2521-2529. (4) Sllnn, W. G. N.; Hasse, L.; Hicks, B. B.; Hogan, A. W.; Lal, D.; Liss, P. S.;Munnich, K. 0.; Sehmel, G. A.; Vlttorl, 0. Atmos. Environ. 1978, 12, 2055-2087. (5) Yamasakl, H.; Kuwata, K.; Mlyamoto, H. Environ. Sci. Technol. 1982, 16, 189-194. (8) Keller, C. D.; Bldleman, T. F. Atmos. Envlron. 1984, 18, 837-845. (7) Junge, C. E. In “Fate of Pollutants in the Air and Water Environments”; Suffet, I. H., Ed. Wiley: New York, 1977, Vol. I, pp 7-26. ( 8 ) Eiceman, 0. A.; VanDlver, V. J. Atmos. Environ. 1983, 17, 461-465. (9) Spencer, W. F.; Farmer, W. J.; Cllath, M. M. Residue Rev. 1973, 49, 1. (10) Renberg, L.; Sundstrom, 0. Chemosphere 1979, 7 , 449-459. (11) Rapaport, R. A.; Elsenrelch, S.J. Envlron. Scl. Technol. 1984, 18, 163-1 70. (12) Bruggeman, W. A.; Van der Stem J.; Hutzinger, 0. J . Chromatogr. 1982, 238, 335-348. (13) Jensen, D. J.; Schall, E. D. J . Agrlc. Food Chem. 1968, 14, 123-126. (14) Hamilton, D. J. J . Chromatogr. 1980, 195, 75-83. (15) Westcott, J. W.; BMleman, T. F. J . Chromatogr. 1981, 210, 331-336. (18) Addison, R. F.; Paterson, S.;Mackay, D. Chemosphere 1983, 12, 827-834. (17) Spencer, W. F.; Cilath, M. M. Residue Rev. 1983, 85, 57-71. (18) Sonnefeld, W. J.; Zoller, W. H.; May, W. E. Anal. Chem. 1983. 55, 275-280. (19) Fed. Reglst. 1980, 45 (227), 77345-77348. (20) “OECD Guldellnes for Testing of Chemicals”; OECD: Paris, 1981; Sect. 104-105. (21) Murray, J. J.; Pottie, R. F.; Pupp, C. Can. J . Chem. 1974, 52, 557-583. (22) Klein, A. W.; Harnisch, M.; Poremskl, H. J.; Schmidt-Bleek,F. Chemosphere 1981, IO, 153-207. (23) Grayson, B. T.; Langner, E.; Wells, D. festic. Sci. 1982, 13, 552-556. (24) MacKnlck, A. 8.; Prausnltz, J. M. J . Chem. Eng. Data 1979, 2 4 , 175-178. (25) Sundstrorn, G. Acta Chem. Scand. 1973, 27, 600-604.
2496
Anal. Chem. 1984, 56,2496-2500
(26) Jensen, S.. Sundstrom, 0. Ambio 1074, 3, 70-76. (27) Mackay, D.; Bobra, A.; Chan, D. W.; Shiu, W. Y. Envkon. Sci. Techno/. 1082, 16, 645-649. (28) small, P. A.; Small, K. W.; Cowley, P. Trans. Faraday SOC. 1048,44, 810-8 16. (29) Mackay, D.; Mascarenhas, R.; Shiu, w. Y. Chemosphere 1080, 9 , 257-264. (30) Yalkowski, S. H.;Valvanl, S.C.; Mackay, D. Residue Rev. 1083, 85, 43-55. (31) Aibro, P. W.; Corbett, J. T.; Schroeder, J. L. J . Chromatogr. 1081,
205, 103-111.
RECEIVED for review ~ ~17, 1984. ~ i~~~~~~~d l ~~l~ 2, 1984. I am grateful to the Chemistry Department and Marine Science Program at USC, and tothe Swedish Environmental Protection Board for their encouragement and financial support.
Preparation of Standard Vapor-Gas Mixtures for Gas Chromatography: Continuous Gas Extraction A. G. Vitenberg,* M. I. Kostkina, and B. V. Ioffe
Chemistry Department, Leningrad State University, 199164 Leningrad, USSR
The possibilities of using gas extractlon under dynamic conditlons in preparing gas mlxtures of known concentratlons for equipment testing and the development of analytical technlques are discussed. Conditions are specified for the preparation of mixtures in whlch the concentration of volatile components decreases regularly by contlnuous gas extraction from solutions. These methods widen the possibilities of using the exlstlng dynamic variatlons of Preparation vapor-gas mixtures and make it possible to operate with hlgh boiling, high polar, and unstable substances. Proposed methods were tested for hydrocarbon solutions in squalane or poly(ethylene glycol) from whlch vapor-gas mixtures at concentratlons ranging from 1 mg/ma to 50 mg/L were prepared.
The increasing interest in the determination of trace amounts of noxious compounds in the environment and in agricultural and industrial products requires development of methods for the preparation of standard gas mixtures with precisely known ultrasmall quantities of components. These reference mixtures are needed both for the testing and calibration of analytical equipment (chromatographs and gas analyzers) and for the development of various analytical techniques. Equipment testing can be carried out not only with standard mixtures at a known and strictly constant concentration but also with mixtures the composition of which changes in the course of operation according to a definite law. The exponential dilution according to Lovelock (1) may serve as the best known example because it has been employed for more than 20 years with many modifications (2). Dynamic and static methods differ in the technique of gas mixture preparation (3). Dynamic methods serve for obtaining gas mixture streams used immediately. Although these methods are time consuming and require fairly complex equipment, they are more frequently employed since the simpler static versions are of little use in the preparation of stable mixtures of low concentration in homogeneous systems. The levels of theoretical and technical development of these methods differ greatly, the less known and less frequently employed being those based on phase equilibria. In our opinion, these methods, making it possible to use the headspace analysis technique (improved considerably in recent years), are very promising and these methods are the most effective in many cases. In principle, the liquid-gas equilibria can be employed in all the modifications of these methods 0003-2700/84/0356-2496$01.50/0
under static and dynamic conditions. and at constant or varying concentrations. Of the four possible combinations, three have already been proposed and developed to various extents: (a) a static method of preparing vapor-gas mixtures with constant composition, (b) a dynamic method of preparing mixtures at a regularly decreasing concentration, and (c) dynamic methods of preparing mixtures at constant concentration. Dynamic variations of headspace analysis:gas extraction of volatile hydrocarbons from solutions in squalane by a stream of nitrogen have first been recommended in the early 1960s for the calibration of gas chromatographic detectors over a wide concentration range (4-6). The expressions derived in these publications were somewhat different, and these discrepancies, which had not found explanation at the time, as well as the absence of precise data on the partition coefficients accounted for the delay in the application of this promising method. The publications on the dynamic headspace method of preparing standard gas solutions which appeared much later dealt only with the problem of obtaining mixtures with a constant composition (7-10). The present paper deals with the theoretical basis of the application of continuous gas extraction to the preparation of standard vapor-gas mixtures under dynamic conditions. Some obscure points present in the literature are elucidated with particular emphasis on the possibility of obtaining a series of mixtures with the regularly decreasing concentration of volatiles. These methods do not require the a priori knowledge of the partition coefficients which can be determined in the course of preparation and analysis of the mixtures. The advantages of the methods are demonstrated by the preparation of gas mixtures containing vapors of hydrocarbons and diethyl ether with concentrations ranging from a few thousandths mg/L to a few tenths g/L.
THEORY The dynamic version of standard mixture preparation is based on continuous gas extraction carried out by bubbling a stream of pure gas through a liquid of volume VL contained in a vial with a gas volume VGabove it. Regular variations in component concentrations in the gas stream at the vial outlet have been considered independently and almost simultaneously in two papers (4-6) with markedly different results. Indeed, Fowlis and Scott (4, 5 ) have proposed to calculate the concentration of the volatile component according to the expression 0 1984 American Chemical Society
