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 t o t h e 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
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2497
where V is the volume of stripping gas passed through the vial, c ~the " initial vapor concentration (at = 01, K = CL/CG the partition coefficient, CL the equilibria concentration of volatile in solution, CG the equilibria vapor concentration, and r = VG/ V,. Burnett (6) dealing with the application of continuous gas extraction from solution to the determination of partition coefficients has derived another equation which in our notation can be rewritten as
Equation 6 shows that an increase in the volume V leads to an increase in the error, which limits the range of concentrations obtained by continuous extraction. Continuous gas extraction can also be used for the preparation of mixtures with virtually constant concentrations of the components. This is possible in those cases when the change in substance concentration in a gas stream passing through the solution is negligible. The maximum volume of the mixture depends on the permissible error in presetting concentrations 6 and on the product KVL V,,, = KVL In (1 - 6 ) (7)
where CLois the initial concentration of the volatile component in solution. The assumptions on which the derivation of eq 1and 2 is based (4-6) have not been stated clearly, and the reasons for the discrepancy have become clear only very recently. A recent detailed analysis of the continuous gas extraction of volatiles from a nonvolatile solvent has shown (11)that eq 1and 2 are based on different models of the process. Equation 1has been derived by Fowlis and Scott with the assumption that thermodynamic equilibrium is established virtually instantaneously both between the solution and the gas bubbles passing through it and between the gas volume VG and the liquid phase. Hence, a gas bubble emerging from the solution and the gas leaving the vial should have the same composition. In contrast, in Burnett's eq 2 it is assumed that exchange of volatiles occurs only in the bubbles ascending through the solution and does not take place between the solution and the headspace gas V e The real situation is between these extreme cases so that it is preferable to avoid considerable "dead" gas volumes above solution and to carry out extraction in vials with negligible values of VG. Under these conditions the simplest relationship (6,12, 13)
For example, at 6 = 1%, VL= 100 mL, and t = 30 "C for dilute hexane (K = 283) and cumene ( K = 7625) solutions in squalane, the maximum volume of the passing gas is 0.3 and 14.0 L, respectively. If eq 7 is obeyed, the concentration of a component in the vapor-gas mixture is given by CG = CLo/K (8)
v
(3) was found to hold with sufficient accuracy. The condition of the applicability of this equation (Le., the possibility of neglecting the "dead" volume VG)depends on the permissible error 6 in the predetermined concentration, value, on the partition coefficient, and on the solution volume VG 5 KVL6 (4) Preparation of vapor-gas mixtures with regularly decreasing component concentration does not require the a priori knowledge of the partition coefficients. The results of the gas chromatographic analysis of the mixture over a relatively wide concentration range make it possible to determine the value of K which, within the operating range of a linear detector, can be calculated by the expression obtained from eq 3 17
17
where A and Ao are the parameters of the chromatographic peaks corresponding to the concentrations in the gas stream, CG and CG", at the moments of passage of the gas volume V and at the beginning of measurements, respectively. When the partition coefficient is found, the concentration of the component CG in the vapor-gas mixture at any moment of time can be calculated according to eq 3. Neglecting the error in the gas volume measurement, it is possible to estimate the accuracy of presetting the concentration of a component in the mixture from the expression derived by the differentiation of eq 3
EXPERIMENTAL SECTION The main expressions and the accuracy with which gas mixtures can be prepared under real conditions were checked making use of vapor mixtures of aromatic and paraffin hydrocarbons and diethyl ether with nitrogen. Volatile hydrocarbons and diethyl ether of "chemically pure" and "chromatographically pure" grades manufactured by the Kharkov Chemical Reagents Factory were used. The solvents were squalane (Ferak, d426 = 0.805), Carbowax-300 (Merk, d:' = 1.127), and Carbowax-400 (Loba-Chemie, ddZ0= 1.128). The solvents were previously purified from volatile impurities by bubbling nitrogen through them at room temperature over 1day. Solutions were prepared by weight (0.01-0.15 g in 10-50 g of solvent) with an error not exceeding 0.5%. Measurementswere performed with a Tsvet-136 chromatograph with a FID and a 2 m metal column of 3 mm i.d. The packing was 15% Apiezon L on Chromosorb W (60-80 mesh), the column temperature was 130 "C, and the carrier gas (nitrogen) flow rate was 30 mL/min. The vapor-gas mixtures were injected onto the column with a gas valve at the detector thermostat 130 "C. The detector signals were traced with a SERVOGOR RE 511 recorder (Goerz) over the 2 to 100 mV range. The peak areas were measured with a TAKEDA RIKEN 2213 electronic integrator. Preparation of Vapor-Gas Mixtures at a Regularly Decreasing Component Concentration, The scheme of an apparatus for continuous gas extraction is shown in Figure 1. Nitrogen was employed to extract the volatiles. The gas flow rate was produced and maintained constant to within 1%with a standard gas flow controller of the Tsvet-100chromatograph. The rate and volume of the stripping nitrogen were determined with an IRG-110 digital gas meter with a special integrating attachment. The procedure of vapor-gas mixture preparation was as follows. The initial solution was introduced with a 5-mL syringe into a thermostated (i0.2 "C) saturator provided with a 23 mm diameter glass filter with pore diameter 10-16 pm (Figure 1). The liquid phase volume was calculated from the mass and density of introduced solution tking into account the expansion coefficients and 0.734 X respecof squalane and Carbowax (6.4 X tively). The phase volume ratio in the saturator, P, was 0.2-0.4, so that condition 4 was satisfied. First a slow gas flow (1-3 mL/min) was passed through the solution to prevent the liquid from seeping through the filter, and after the desired temperature had been reached (in 30 min), the gas flow rate was raised to the required level (20-55 mL/min). After 25-30 min the first headspace samples were injected oqto the chromatographic column. Figure 2 presents the chromatograms of a gas mixture with a decreasing concentration of benzene, toluene, p-xylene, and cumene vapors in nitrogen. Absolute Detector Calibration. As an independent check of the preparation of vapor-gas mixtures containing microcomponents in the 5 mg/m3 to 1.5 mg/L concentration range, diffusive
2408
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 NITROGEN FROM
Table I. Liquid-Gas Partition Coefficients
compound
concn range, CL, % by temp, mass O C
re1 std dev S, K
%
ref
Squalane-Nitrogen benzene
0.06-0.01 1.2-0.04
50.5
410 215"
1.6 0.4
223 (50 "C)
n- hexane
0.03-0.001
30
280
0.6
64.4 (80 "C)
cyclohexane toluene
1.0-0.01 0.3-0.01
50 30
275' 1380
0.9 2.9
ethylbenzene n-xylene cumene diethyl ether
0.9-0.09 0.4-0.1 0.5-0.2 0.2-0.001
30 4200 30 4390 30 7600 65.5 27.0'
277 (4, 5) 1775 (25 "C) (16)
30
3.9 2.8 5.5
(15)
(6)
1.8
Carbowax-300-Nitrogen benzene toluene Figure 1. Equipment for continuous gas extraction: (1) glass filter, (2) rubber gasket, (3)Teflon cap, (4) clamping devlce, (5) glass caplllary tube, (6) 0.2 mm Ld. steel capillary tube, (7) copper capillary tube for addltional flow stabilization (0.2 mm Ld., 8 m long), (8)flow rate controller of Tsvet-100 chromatograph.
1.0-0.02
25 25
1.0-0.1
600' 115On
1.7
1.8
Carbowax-400-Nitrogen benzene n-hexane
0.2-0.0001 0.7-0.002 0.08-0.001
25 35 20
585' 340' 31.5'
0.6 2.3 2.2
'Concentration determined by detector directly in the gas flow at the outlet of saturator (bypassing the chromatographic column). RESULTS AND DISCUSSION
TIME(MIN)
I
I
I
I
I
90
75
60
45
30
Figure 2. Chromatograms illustrating variation of concentration during contlnuous gas extraction (mlxture no. 2, Table 11): (1) benzene, (2) toluene, (3)p-xylene, (4) cumene. Nitrogen flow rate was 54.8 M m i n ; measurement range was 0.2 nA. Dashed llnes connect peaks of the same component on different chromatograms. samplingof volatiles from capillaries was used ( I ) . Glass capillaries 0.9-1.2 mm in diameter were filled with a standard 10 ML microsyringe, and those of still smaller diameter (0.3-0.5 mm) were fded under vacuum. Subsequentlythe capillaries were centrifuged (2-3 min at 6OOO rpm) and maintained for not less than 1.5 h until steady-state diffusion conditions were reached. The height of the column of evaporated liquid (0.8-7 mm) was measured with a comparator to within 0.005 mm. Injection onto the chromatographic column was started only 1-1.5 h after the beginaing of the experiment. The chromatographic data thus obtained were used to calculate the calibration factor (CDis the component concentration in the gas stream determined by the actual diffusion conditions and AD is the area of the corresponding chromatographic peak.)
The method considered here was checked by taking as examples the prepartion of vapor mixtures of volatile hydrocarbons and diethyl ether with nitrogen. The necessary partition coefficients were found from the results of the gas chromatographic analysis of the mixtures (eq 5) by the least-squares method (Table I). The substance concentration in the gas stream was calculated from eq 3. The gas mixture concentrations were checked chromatographically, the absolute calibration of the detector being carried out with reference mixtures prepared by the diffusion method (Table 11). The observed discrepancies did not exceed the errors of the methods used. The total errors of chromatographic analysis and gas mixture preparation over the 2 mg/L to 1.4 mg/m3 range did not exceed 8%,with the average about 4%. The accuracy of the diffusion method modification employed by us (14) was 1-2%. It is not necessary to carry out continuous gas extraction under strictly equilibrium conditions, it is only important to reach a constant ratio of the vapor to liquid concentations, which can be obtained by stabilizing the gas flow, choosing an optimum value of r, and eliminating sorption in the gas circulation system. The constancy of the volatile concentration ratio in the gas and liquid phases and the accuracy of presetting the concentration were verified by checking that the condition
V , In A1 = A0
v2In A2 A
=
... = Vi In Ai = const
(9)
A0
following from eq 5 is obeyed. The required degree of flow stabilization depends on the conditions of the continuous gas extraction procedure. Under equilibrium conditions it is not necessary to maintain a stable flow; only the total volume of the stripping gas passing through the solution should be known. Under nonequilibrium conditions the gas flow stability affects the concentration ratio in the vapor and liquid phases. The experimental data showed that in the instrument used the equilibrium character of continuous gas extraction was ensured at the flow rate up to 55 mL/min. This fact was
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 24QQ Table 11. Accuracy of Gas Mixture Preparation by Continuous Gas Extraction (Solvent: Squalane) detector calibration' CD, 10y mg/L mg/L.pV.s
compound
no. of measurmts
preset by dynamic method ACG/CG f volume of stripping concn range, 0, %, gas, V, L CG,mg/ma calcd by eq 6
initial concn, C'L, g/L
Mixture No. 1, VL = 3.94 mL, 30 "C hexane benzene toluene ethylbenzene
0.643 1.22 0.910 0.574
10 14 16 16
7.21 4.97 5.41 5.12
4.30 7.60 8.25 8.25
0.220 0.500 2.26 7.10
3.9 4.2 4.7 3.2
190-4.0 580-3.0 590-280 510-279
4.5 1.2 4.6 f 1.1 3.4 f 0.4 5.5 0.1
300-1.4 210-3.7 660-30.2 460-120 290-143
5.7 6.5 6.1
Mixture No. 2, VL = 3.87 mL, 30 "C 0.256 0.300 0.287 0.087
benzene toluene p-xylene cumene
9 14 15 16
0.444 0.489 0.471 0.394
0.230
8.50
1.12
17.60
3.00 4.14
18.40 19.55
Mixture No. 3, VL = 4.02 mL, 30.5 "C hexane benzene toluene ethylbenzene cumene
0.0342 0.0843 0.0585 0.0266 0.0050
8 9 22 23 23
0.265 0.253 0.235 0.147 0.050
7.30 8.00 20.40 20.90 20.90
0.215 0.175 1.10
1.82 2.10
* 0.9 ** 0.6 0.6 * 0.1
560-15.7 970-11.0 1520-350 1870-1150
*
*
** 1.1 1.8 * 1.0 5.0 * 0.5 5.4 * 0.4
ec
a, %
-3.1 f 0.6 +6.2 2.0 -5.0 f 0.6 -0.1 0.5
*
* *
3.3 -3.6 1.6 -7.8 0.4 +6.8 0.5 -0.2
-3.7 6.3 t 1 . 2 2.2 +3.1 1.4 -0.7
* 0.7
+3.8 f 0.8
= (CG- CG")/C&' is the relative 'Mixture no. 1, 1 nA range; no. 2, 0.2 nA range; no. 3, 50 nA range. b0.95 confidence interval. discrepancy between concentrations preset by gas extraction techniques, CG,and those found from absolute calibration, C G =~ f A , Table 111. Continuous Gas Extraction with Direct Introduction of Vapors into Detector
compound benzene
solvent Carbowax-300
benzene
Carbowax-400
benzene toluene
squalane Carbowax-300
cyclohexane squalane hexane Carbowax-400 diethyl squalane ether
vol of stripping initial gas, V , L signal no. range of concns (ACG/CG concn, solution before temp, measurmt measprepared, a)," -5% CLO, g/L v01, VL, mL measurmt total O C range, nA urmts CG,mg/L (calcd by eq 6)
*
9.25
5.84
4.10
10.88
20
100-4
21
4.8-0.91
0.50 0.97 0.40
6.83
2.60
2.56 2.91 2.82
12.44
25 30 25
400-10 400-25 504.1
20 26 50
10.84.18 9.5-0.46 4.04.0023
19.20 9.95 12.35
3.96 3.45 2.59
0.13 1.46 0.93
8.23 6.05 7.59
35 50.5 25
200-10 200-10 400-20
61 17 19
46.8-0.20 7.3-0.013 7.9-0.85
1.12
2.06 1.31 1.40 0.03
12.82 10.07 5.98 0.41
30 35 50 20
400-4 400-25 200-10 100-4
22
8.10
2.27 2.65 3.78 2.45
23 37
4.9-0.079 8.4-0.12 7.7-0.093 22.4-0.28
1.57
2.62
0.08
0.49
65.5
1000-10
20
18.9-0.058
6,OO
21
* 0.1 2.8 * 0.5 3.2 * 0.4 1.5 * 0.2 2.1 * 0.2 1.3 * 0.2 0.9
2.1
zk
0.3
4.0 h 0.6 1.0 0.2 1.8 0.2 1.0 f 0.1
** 4.3 * 0.8
0.95 confidence interval. confirmed by the constancy of the CL/CG ratio at the rates ranging from 20 to 55 mL/min and by the agreement between the partition coefficients measured in this work and the published data (Table I). As the flow rate rises from 55 to 115 mL/min, the magnitude of CL/CGfor benzene in Carbowax-300 increases by 22% (with an error of CL/CG 5 4% in individual determinations). If there is only one volatile component in the solution, i t is possible to bypass the chromatographic column and connect the detector to the outlet of the vial with solution. In this case the recorder will record continuously an exponent
where i and io are detector signals corresponding to the volume V and the beginning of measuring the volume of the stripping gas passed, respectively. This scheme is very simple and was used in testing the sensitivity and the range of detector within which the linear
relationship In (io/$ = f ( V ,was obeyed. The detector readings were measured from the "background" level corresponding to the vapor concentration of a pure solvent. This background was usually measured in special experiments. However, when working with solutions characterized by low liquid-gas partition coefficients (e.g., diethyl ether in squalane), the background was determined at the end of the experiment after practically all the compound had been extracted from the liquid, and the signal was constant and corresponded to the solvent background (Figure 3). The measurements were started after passing through the solution a volume of gas required for displacing air from the saturator and the tubes directing the flow to the detector. As seen from Figure 3, the part of the chart corresponding to this period of time exhibits a maximum. The advantage of this modification of the method is the shortened path of the gas flow to the detector leading to a decreasing probability of errors due to substance sorption in the gas circulation system. A drawback of this scheme is the
2600
Anal. Chem. W 4 , 56, 2500-2503 0.491
A
TIME (MIN)
10 n A
J-
20
15
50nA
I
-I_
0.2pA 10
_I--
$1 c?
18.9
11.66
I
0.058
In conclusion is should be noted that an important advantage of continuous gas extraction is the possibility of obtaining vapor-gas mixtures at microconcentrations of the components by using solutions containing these substances at much higher concentrations, the preparation of which is fairly easy. It is also possible to calculate simultaneously the partition coefficients from the curves of dependence of substance concentration in the gas flow on the volume of the stripping gas, and, hence, liquid solvents (including polymer solvents), the properties of which vary from batch to batch, can be used. Registry No. Hexane, 110-54-3; benzene, 71-43-2; toluene, 108-88-3; ethylbenzene, 100-41-4;cumene, 98-82-8; cyclohexane, 110-82-7;hexane, 110-54-3;diethyl ether, 60-29-7;p-xylene, 106-42-3.
1pA
0.r
1 I I
-!-2pA:
5
I
0
Flgure 3. Typical chart recorded under continuous gas extraction without chromatographic column: diethyl ether in nitrogen (seeTable HI).Nitrogen flow rate was 21.5 mL/min. Vertical lines indicate time of switching to specified measurement ranges. Arrows indicate the beginning of signal measurement and concentrations calculated by eq 3, in mg/L.
necewity for stabilizing the flow even for the equilibrium type of continuous gas extraction and the impossibility of carrying out calibration for more than one substance. A simplified modification of continuous gas extraction with direct introduction of the vapor of one volatile component only into the detector (bypassing the chromatographic column) makes it possible to preset the vapor-gas mixture concentration with a relatively high accuracy (Table 111). The average deviation from the exponential law of concentration variation over 2 to 3 orders of magnitude is only 2.4%.
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11)
(12) (13) (14) (15) (16)
Loveiock, J. E. Anal. Chem. 1061, 33, 162-178. Ritter, J. J.; Adams, N. K. Anal. Chem. 1076, 48, 612-619. Barratt, R. S. Analyst (London) 1981, 706, 817-849. Fowlls, I . A.; Scott, R. P. W. J. Chromatogr. 1963, 7 7 , 1-10. Fowlls, I. A.; Maggs, R. G.:Scott, R. P. W. J. Chromatogr. 1064, 75, 471-481. Burnett, M. G. Anal. Chem. 1063, 35, 1597-1570. Erdos, E.; Bares, J. Collect. Czech. Chem. Common. 1064, 29, 2716-2725. Scarano, E.; Forina, M.; Gay, G. Anal. Chem. 1071, 4 3 , 1310-1312. Forlna, M. Ann. Chlm. (Rome) 107& 65, 491-506. Vejrosta, J.; Novak, J. J . Chromatogr. 1979, 775, 261-267. Vitenberg, A. G.; Kostkina, M. I. Zh. Anal. Khlm. 1970, 34, 1800- 1806. Vltenberg, A. G.; Ioffe, B. V. W l . Akad. Nauk SSSR 1977, 235, 107 1-1 074. Wasik, S. P. J. Chromatogr. Scl. 1074, 72, 645-648. Voi’berg, N. Sh. Zavcd. Lab. 1975, 4 7 , 6-8. Huber, J. F. K.; Keulemans, A. I. M. 2. Anal. Chem. 1064, 205, 263-274. Rohrschneider, L. Anal. Chem. 1973, 45, 1241-1247.
RECEIVEDfor review September 19,1983. Accepted May 24, 1984.
Preparation of Standard Vapor-Gas Mixtures for Gas Chromatography: Discontinuous Gas Extraction B. V. Ioffe,* M. I. Kostkina, and A. G. Vitenberg Chemistry Department, Leningrad State University, 199164 Leningrad, USSR
A static methods theory Is presented for the preparatlon of standard vapordas mixtures based on a concept of the buffer action of heterogeneous systems malntalnlng the mlcroconcentratlons of the gas-phase components at a stable level. A posslblllty of worklng wlth systems wlth unknown partitlon coefficients Is explored. These methods were tested for solutions of volatile aromatlc hydrocarbons, ketones, alcohols, and esters in Carbowax and water. The concentratlons of the vapor-gas mlxtures prepared from them ranged from 0.003 to 37 mg/L.
Dynamic methods of preparation of vapor-gas mixtures for the calibration of gas chromatographs based on continuous gas extraction of volatile components from solutions have been considered in the preceding paper (I). Technically simpler static methods permitting the preparation of mixtures stable 0003-2700/S4/0356-2500$01.50/0
for long periods of time based on the principle of partition of volatile Components between the liquid and the gas phases have hardly been used up to the present. They include both the equilibration in the liquid-gas system in a single stage with subsequent headspace analysis (the simplest version) and the repeated replacement of the equilibrium gas phase: methods of discontinuous (repeated or multiple) gas extraction. The idea of using the partition law in the liquid-vapor systems for the preparation of gas mixtures of predetermined concentration dates as far back as the beginning of the 1960s. In the first publication on this problem a static method was described. It is based on Henry’s law in its simplest form and employed for the calibration of an argon ionization detector with aqueous solutions of acetone and ethanol (2). However, the vapor-liquid equilibrium under static conditions was also employed on a very limited scale. The available papers describe the calibration of a FID with mercaptans (3) and calibration of a flame photometric detector with sulfur-containing 0 1984 American Chemical Society
