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)
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. (6) Burnett, M. G. Anal. Chem. 1063, 35, 1597-1570. (7) Erdos, E.; Bares, J. Collect. Czech. Chem. Common. 1064, 29, 2716-2725. (8) Scarano, E.; Forina, M.; Gay, G. Anal. Chem. 1071, 4 3 , 1310-1312. (9) Forlna, M. Ann. Chlm. (Rome) 107& 65, 491-506. (10) Vejrosta, J.; Novak, J. J . Chromatogr. 1979, 775, 261-267. (11) Vitenberg, A. G.; Kostkina, M. I. Zh. Anal. Khlm. 1970, 34, 1800- 1806. (12) Vltenberg, A. G.; Ioffe, B. V. W l . Akad. Nauk SSSR 1977, 235, 107 1-1 074. (13) Wasik, S. P. J. Chromatogr. Scl. 1074, 72, 645-648. (14) Voi’berg, N. Sh. Zavcd. Lab. 1975, 4 7 , 6-8. (15) Huber, J. F. K.; Keulemans, A. I. M. 2. Anal. Chem. 1064, 205, 263-274. (16) 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
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1984
compounds (4). Until recently applications of the static method have been limited by the lack of relatively simple and accurate techniques of measuring the partition coefficients in dilute solutions required for calculating the equilibrium vapor composition. The optimum conditions for the use of heterogeneous systems as concentration standards likewise remained obscure. The present paper deals with the theoretical basis for the application of discontinuous gas extraction to the preparation of standard gas-vapor mixtures under static conditions with particular emphasis on the possibility of obtaining a series of mixtures with the concentration of volatiles decreasing regularly by means of multiple headspace extraction. These methods do not require the a priori knowledge of the partition coefficients which can be determined in the course of the preparation and analysis of the mixtures. The advantages of the methods are demonstrated by the preparation of gas mixtures containing vapors of hydrocarbons, esters, and ketones with concentrations ranging from a few thousanths mg/L to a few tens mg/L.
THEORY Buffer Action of Heterogeneous Systems. The simplest method of preparing mixtures at a given concentration consists essentially in the introduction of the required amount of the compound into a closed volume of gas. However, if the required concentation of the mixture is too low, the fraction of the compound sorbed on the walls of the vial may be quite high. In this case the actial concentration of the mixture will not attain the calculated level and will not be constant since at high dilutions a considerable fraction of the trace compounds will be lost in chemical and photochemical transformations. The effect of these processes can be effectively eliminated by using heterogeneous systems. If the condensed (liquid) phase contains a sufficient amount of components consumed in undesirable processes, the concentrations in the gas phase can be maintained at the required level on the basis of the partition low. By analogy with the well-known concept of the buffer capacity of solutions it is convenient to introduce the concept of the “buffer action” of heterogeneous systems manifested in the resistance of one (gas) phase to a change in concentration due to its equilibrium with the other phase. If as a result of sorption or interaction with parts of the equipment or with compounds sorbed on its walls, an amount q of the component is absorbed, then its concentration in the isolated gas phase will decrease by
AC,’ = q / VG
(1)
( VGis the gas volume). If the gas (VG)and liquid (VL)volumes are in equilibrium, the disappearance of the same amount of the component from the gas phase will result in a smaller change in its concentration in this phase which is given by
AcG = q / ( K V L + VG)
2501
centration (ACG= 0) for any (finite) losses of the gas-phase component, Maximum values of B may be attained by choosing liquids with a high K and by reducing the value of
r. In systems with unknown K or r the numerical value of B can be measured as a ratio of masses (m)or peak areas ( A ) on the chromatograms corresponding to the (n+ 1)th and to the nth injections of samples from the same vial onto the chromatographic column (4)
Thus the concentration of a gas mixture maintained in equilibrium with the solution of a given component in a sufficiently large volume of a liquid with a high partition coefficient in principle can be stabilized with any degree of accuracy. The numerical value of this concentration CG is calculated proceeding from the concentration in the liquid CLo using the main equation of the quantitative headspace analysis
CLo = CG(K or substituting (K
+ r)
[see ref 6 , eq 1.201
+ r ) from eq 3 we have CG = CL’B/K
(5)
The error in the determination of K provides a predominant contribution to the total error of presetting the concentration CG and essentially determines its accuracy. Hence, the knowledge of the accurate values of K is an indispensable condition for the successful use of the vapor phase in heterogeneous systems as a concentration standard. At present, convenient gas chromatographic methods for the determination of K are available (6-8). If the buffer action is relatively strong (large K and small r), repeated use of the same solution becomes possible by replacing the headspace with pure gas. This can easily be done with the aid of thermostated 50-100 mL glass syringes (6). The relative change in the gas-phase concentration caused by multiple extraction should not exceed the permissible error 6 in the preset concentration, and therefore the number of replacements should be less than or equal to -6/ln B. For example, for equal phase volumes, K = 1000 and 6 = 1%, the number of gas-phase replacements may be as high as 10. Discontinuous Gas Extraction. Multiple replacement of the headspace by pure gas can also be used to prepare series of gas mixtures with regularly decreasing concentration of components. In this case systems with relatively low values of K and moderate buffer action should be used (see eq 3, otherwise too many replacements must be made to attain the desired degree of reduction of the component concentration in the mixture. After each subsequent extraction CG (eq 5) decreases by a factor of B so that after (n+ 1)extractions (nth replacement of the gas phase) the concentration in the gas phase is given by
(2)
( K = CL/CGis the partition coefficient). The relative concentration change related to the partition coefficient and phase volume ratio (r = V,/VL) by the equation (ACG’ - ACG)/ACG’ = K / ( K r) =B (3)
+
may serve as the measure of the buffer action of a heterogeneous system (5). B is the buffer coefficient or the fraction of the component left in the condensed phase, a quantity which by definition may vary from zero to unity. The value of B = 0 corresponds to the absence of the buffer action in the system because of the zero partition coefficient or the zero liquid phase volume. The extreme value B = 1indicates the maximum buffer effect, i.e., the absolute stability of a con-
Multiple headspace replacement resulting in the regular variation in component concentration in the gas phase does not require the a priori knowledge of the partition coefficients for the gas mixture preparation. The results of determination of the component concentration before and after a replacement can readily yield the value of K (7) which in the linear range of detector operation is related to the parameters of the corresponding chromatographic peaks by the equation where CG and CG‘are the component concentrations in the gas phase before and after its replacement by pure gas, respectively; A and A’are the areas or heights of the chromatographic peaks. Substitution of K from eq 7 into eq 6 gives
2502
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
an expression for the calculation of component concentration in the gas phase when systems with unknown values of K are used
However, as the number of extractions increases, the error in the preset concentration will increase by the sum of the relative errors in the measurements of peak parameters ( A and A? each time a new mixture is prepared. This factor determines the maximum permissible number of gas-phase replacements in this version of standard mixture preparation. Among the disadvantages of using multiple headspace extraction for the preparation of a series of gas mixtures with a predetermined variation in component concentration are the possible errors associated with incomplete replacement of the headspace by pure gas, which are difficult to reveal (9), and the difficulty of automating the procedure. These drawbacks are eliminated if partial rather than complete replacement of the headspace by pure gas is carried out. This procedure can easily be performed with a high accuracy by collecting a sample of pressurized gas from the system and can readily be combined with pneumatic gas injection from the sample vial onto the chromatograph.
EXPERIMENTAL SECTION The equations derived from the above theoretical considerations as well as the accuracy which can be achieved in practice were tested by taking as examples mixtures of oxygenated compounds and aromatic hydrocarbons from air and nitrogen. For these practical investigations "chemically pure" and "chemically pure for chromatography" samples of several volatile hydrocarbons, ketones, ethanol, ethers, and esters from the Kharkow Factory of Chemicals were used. Distilled water and Carbowax-300 (Merck, d t O= 1.127) were used as solvents. Solutions of concentrations of 100 mg/L were prepared by weighing with an analyticalbalance (0.05-0.16 g in 30-100 g of the solvent). Lower concentrations, all weightlweight, were prepared by further dilution with a precision of better than 1%. Gas chromatographic analyses and detector calibration were carried out with the same instruments and under the same conditions as reported in ref 1. Preparation of Gas Mixtures with Constant Component Concentration. A 100-mL syringe thermostated to f0.1 "C (5) and calibrated with water by weight was employed. Aqueous solutions of oxygen-containing compounds were introduced into this syringe with a graduated 5-mL syringe. When using viscous solutions in Carbowax-300, the syringe was weighed before and after the introduction of solution and the volume of the solution was calculated from the density data (under the conditions used the difference in the solvent and solution densities could be neglected, being less than 0.2%). The thermostated syringe was sealed and periodically shaken for -30 min for establishing the equilibrium. Subsequently, samples of headspace gas were injected onto the chromatographic column and the peak parameters ( A ) measured. The content of the vapor of the compound under study present in the mixture was calculated by using eq 5. Discontinuous Gas Extraction. The equipment used here was the same, but after headspace sampling the gas phase above the solution was totally displaced and pure gas was sucked into its initial volume. After equilibration, the headspace gas was again analyzed. The replacement of the headspace by pure gas was repeated 4-6 times. The concentrations at each extraction step were calculated according to eq 8. RESULTS AND DISCUSSION The static method of preparing standard gas mixtures with a constant component concentration was tested for vapor-air mixtures of acetone, methyl ethyl ketone, ethanol, dioxane, ethyl acetate, and n-butyl acetate obtained from their aqueous solutions, as well aa those of benzene and toluene from solutions in Carbowax-300. The ethanol concentration was calculated by using the partition coefficient K = 7970 f 330 at
Table I. Liquid-Gas Partition Coefficients concn range, CL, temp, compound % by mass O C acetone n-butyl acetate
1.0-0.001 1.0-0.1 0.7-0.06
re1 std dev, K
S,, %
lit. data
3.4 3.2 1.6
1140 (11) 750 (11)
Water-Nitrogen 15 1170 20 750 15 190 25 15
87
2.3
1.0-0.001
770
2.2
440 at 25 O C (6)
1.0-0.1
15
10650
3.8
5396 at 25
ethyl acetate
1.0-0.04
15
300
1.0
methyl acetate methyl propionate methyl butyrate n-butyl acetate dibutyl ether
0.03-0.001
25
190a
2.6
213 (13)
0.03-0.001
25
130a
3.1
141 (13)
0.06-0.001
25
9oa
3.3
119 (13)
0.05-0.001
25
86a
2.3
0.05-0.001
25
methyl ethyl ketone dioxane
0.7-0.02
oc (12)
Water-Air
5.3a
2.8
Carbowax-300-Nitrogen benzene toluene
1.0-0.0001 1.0-0.1
25 25
BOOb
1.7 1.8
1150b
a Measured by static method (discontinuous gas extraction). All the other data obtained by continuous gas extraction (7). *Concentration determined by detector directly in the gas flow at the outlet of exchanger (bypassing the chromatographic column).
Table 11. Accuracy of Vapor-Air Mixtures Preparation by Static Method as preset gas mixture
initial concn, cL0
mg/L
C O ~ C CG ~ ,
mg/L
S,, 70
as found from peak areas peak heights CGD S,, %
mg/L
CGD
mg/L
S,, 70
Acetone (solvent, water; 20 "C)" 1025 97.0 51.5 10.2 6.2
1.37 1.32 0.7 1.33 0.5 0.6 0.127 0.125 0.6 0.125 0.645 1.7 0.2 0.0656 0.0666 0.0133 1.5 0.0131 0.0133 2.2 Benzene (solvent, Carbowax-300; 25 oC)b
0.4
0.0100
0.7
0.0100
1.7
2.7
0.0101
0.6 0.7
2.9
" B = 0.970; r = 18.5; m = 1, n = 10. Here and in Tables I11 and IV m is the number of analyzed mixtures and n is the number of reDeated measurements. b B = 0.962, r = 23.5. m = 2, n = 10.
15 OC (11). In all other cases the calculations were carried out with the values of K measured by the dynamic method and listed in Table I. 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 were within experimental errors (the accuracy of the diffusion method modification employed by us (10) was 1-29'0). Three criteria were used for evaluating the constancy of the mixtures: reproducibility of sampling from the same container
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2503
Table 111. Constancy of Vapor-Air Mixtures Prepared by the Static Method
mixture components
B at 25 "C
mixture no. 1" acetone methyl ethyl ketone ethanol n-butyl acetate mixture no. 2" dioxane ethyl acetate mixture no. 3" acetone methyl ethyl ketone ethanol n-butyl acetate mixture no. 4e dioxane ethyl acetate mixture no. 5* benzene toluene
concn, mg/L initial CL in gas Co
0.984 0.977 0.998 0.911
1210 1335 7450 595
0.998 0.942
8295 280
reproducibility S,C % of mixture of repeated preparation (m = 3, sampling (n = 10) n = 7)
after 8 h
of storage (n = 10)
0.92 2.86
1.8 0.1 2.1 0.8
2.7 1.6 3.3 1.0
2.4 1.1 3.2 0.9
0.80 0.88
0.6 0.3
1.3 1.3
1.1 1.4
1.02
1.70
0.984 0.977 0.998 0.911
12.1 13.3 74.5 5.9
0.0102 0.0169 0.0092 0.0286
1.1 0.3 3.1 1.0
2.9 0.9 4.0 1.3
2.9 2.0 6.0 1.0
0.998 0.942
82.9 2.8
0.0080 0.0088
3.7 1.6
4.0 7.0
3.1 0.9
0.962 0.980
6.2 12.2
0.0099 0.0104
0.4 0.6
0.4 0.6
" Solvent, water; r = 18.5. *Solvent,Carbowax-300; r = 23.5. Concentrations of mixtures 1 and 2 were derived from peak areas. For mixtures 3, 4, and 5 peak heights were used (of the order of 1.5-10 cm). Here n is the number of samplings and m is that of repeated mixture preparations. Table IV. Vapor-Air Mixtures Preparation by Discontinuous Gas Extraction (Solvent, Water; 25 "C; r = 8.71)
compound
B
methyl acetate
0.956
methyl propionate
0.937
methyl butyrate
0.912
n-butyl acetate
0.908
dibutyl ether
0.378
gas mixture initial concn, concn range," CL",mg/L CG, mg/L 290 6.1 325 6.8 560 11.5 470 9.9 530 11.0
1.45-1.15 0.031-0.025 2.3-1.65 0.050-0.035 5.3-3.2 0.115-0.075 5.0-2.8 0.105-0.065 37-0.31 0.79-0.016
For six to seven extractions; reproducibility of SI, 0.4-2.2% (n = 6).
with the mixture, constancy of mixture concentration during one working day, and reproducibility of the mixture preparation procedure. The results of tests for the gas mixture constancy are presented in Table I11 demonstrating the possibility of preparing mixtures in the trace (0.8-3.0 mg/L) and microconcentration (8.0-29 mg/m3) range. Tests for component concentrations below 10 mg/m3 are difficult, these concentrations being close to the FID sensitivity threshold. The mean relative standard deviation was 2%. The preparation of gas mixtures with a regularly decreasing concentration by the static method is demonstrated taking as an example mixtures with ether vapors (Table IV). Different techniques of data treatment (calculation of concen-
trations from the areas of each pair of chromatographic peaks corresponding to consecutive extractions, from mean values of K for the whole series of extractions, and from the first two values of peak areas) yield results which agree within the experimental error. The calculations based on two consecutive peak areas seem to be less reliable, their accuracy dropping gradually as the number of extractions increases. However, even in this case the reproducibility of the data is not inferior to 2%. Registry No. Acetone, 67-64-1; methyl ethyl ketone, 78-93-3; ethanol, 64-17-5; n-butyl acetate, 123-86-4; dioxane, 123-91-1; ethyl acetate, 141-78-6; benzene, 71-43-2; toluene, 108-88-3; methyl acetate, 79-20-9; methyl propionate, 554-12-1; methyl butyrate, 623-42-7; dibutyl ether, 142-96-1.
LITERATURE CITED Kostkina, M. I.; Ioffe, 8. V. Anal. Chem., precedlng paper in this issue. Burnett, M. G.; Swoboda, P. A. T. Anal. Chem. 1062, 3 4 , 1162-1163. Field, T. G.; Gillbert, J. B. Anal. Chem. 1086, 38, 628-629. Ronkalnen, P.; Denslow, J.; Leppanen. 0. J . Chromatogr. Sci. 1071,
(1) Vltenberg, A. G.;
(2) (3) (4)
11, 384-390. (5) Ioffe, B. V.; Reznlk, T. L. Zh. Anal. Khim. 1081. 36, 2191-2198. (6) Ioffe,B. V.; Vltenberg, A. G. "Head-space Analysls and Related Methods In Gas Chromatography";Wiley-Interscience: New York, 1984 p 276. (7) Vltenberg, A. G.; Ioffe, B. V. Dokl. Akad. Nauk SSSR 1077, 235, 1071-1074. (8) McAullffe,C. D. Chem. Techno/. 1071, 1 , 46-51. (9) Ioffe,B. V.; Vitenberg, A. Q.; Reznlk, T. L. Zh. Anal. Khim. 1082, 37, 902-907. ( 1 0 ) Vol'berg, N. Sh. Zavod. Lab. 1075, 47, 6-8. (11) Ioffe, B. V.; Vltenberg, A. 0. Chromatosra~hla1078, 1 1 , 282-286. (12) Rohrschnelder, L. Anal. Chem. 1973, 4 5 , 1241-1247. (13) Buttery, R. 0.; Ling, L. C.; Guadagni, D. C. J . Agric. food Chem. 1080, 17, 385-389.
RECEIVED for review April 2,1984. Accepted May 24,1984.
