Chromatographic Method for the Concentration of Trace Impurities in the Atmosphere and Other Gases JOSEF NOVAK, VLADlMfR VASAK,' and JAROSLAV JANAK Czechoslovak Academy of Science, laborafory for Gas Analysis, Brno, Czechoslovakia
b A method for the concentration of trace impurities in the atmosphere and other gases, based on the theory of phase equilibrium is described. A short gas liquid chromatographic column i s used through which, under known temperature, the analyzed air is drawn to reach an equilibrium. The column is in the form of a sampling tube. The substances collected in the column are desorbed by heating into a gas chromatographic separating column and determined. A theoretical substantiation of the method is given along with the relationships required to calculate the concentrations of trace impurities in the gas phase. By the method described a high effect of concentration is obtained and the higher it is, the more the trace component is sorbed; there is a defined quantitation of the mass of the various trapped components obtained which is especially significant from the analytical point of view. Usefulness for studies in air pollution and industrial hygiene is proved by an example of selective concentration of vapors of a certain group of substances contained in a mixture. The mean error was about 570, with concentrations of 1 to 25 p.p.m. calculated for xylene. Practical applications were frequently carried out in the p.p.b. range.
F
CHROMATOGRAPHIC analysis of trace components in gases, one may use direct methods based on highly sensitive detection, or concentration methods in which analytical procedures follow concentration of the substances. The perspective of the basic methods of trace analysis of gases has been critically evaluated ( 4 ) and gas chromatographic methods for trace analysis in air pollution have been reviewed ( 1 ) . Successful concentration of trace impurities depends upon the defined trapping of all admixtures in a small volume. Quick and complete transfer of the concentrate into the sampling head of the chromatograph is necessary for suitable application of gas chromatography. For concentration an
OR
Present address, Institute of Industrial Hygiene and Occupational Diseases, Pragne, Czechoslovakia. 660
ANALYTICAL CHEMISTRY
adsorbent is used (9) or a support wetted with a liquid of low volatility ( 2 ) . Ai sampling procedure suitable in many respects for the analysis of air contaminants is described by Cropper and Kaminsky (3). The authors recommend drawing of a known volume of gas through a tube packed with a nonvolatile liquid on a grain support. Corresponding quantities of admixtures are absorbed from the gas and finally desorbed by heat and introduced into the gas chromatograph. The maximally admissible volume is indirectly proportional to the volatility of the substance in a given sorption system. If the impurities represent a mixture of varying volatilities, the value of the volume drawn is limited by the most volatile component-Le., the one which permits passage of the smallest volume. This is apparently not advantageous especially with toxic substances in the air, where the contents of the various components usually diminish as the boiling point increases. Following the theory of phase equilibria, it should be possible to draw a volume of air through the sampling tube equal to that required for reaching the concentration equilibrium between the gas and liquid phase for the component of interest. I n practice, this means that one observes not the maximum (3) but the minimum drawn volume. The concentration equilibrium in the given sorption system is given by the corresponding partition coefficient. The equilibrated masses of substances thus concentrated are usually comparable for all components of the admixture, and the gas chromatogram consists of approximately equal peaks which can be measured very precisely. By choosing the partition systems, selective concentration of components (comprising perhaps only a fraction of a per cent of the trace impurities) can be obtained within a large range and interferences can be avoided. EXPERIMENTAL
SamplingTubes. The sampling tubes
were short columns which allowed introduction of the sample to thechromatographic column after piercing the rubber septum of the sample port, in a manner similar to a n injection syringe.
Figure 1 . 7. 2. 3. 4.
Sampling tube
GLC packing Glass wool Capillary tube Part of hypodermic syringe
The sampling tube is shown in Figure 1. The tube itself, 1 , packed with the sorbent, was of 0.5-cm. i.d. and 4.5 cm. in length. This tube was open on one end, and the other end !vas fused to the thick walled capillary tube, 3, of 0.1-cm. i.d. and 1 cm. in length. 4 part of a hypodermic syringe, 4, with a holder for the injection needle was fused to the other end of the capillary tube. Sorbent was held by a small plug of glass wool, 2. Silicone elastomer E-301 (Imperial Chemical Ind., Ltd., G. B.) [30Y0 w./w. on Celite 545 (Johns Manville Ltd., London, G.B.)] of 0.125 to 0.25 mm. grain size was used for the stationary phase. Polyethyleneglycol 400 (Union Carbide Corp., New York, CSA) under the same conditions was also used. Chromatograph. Czechoslovakian apparatus CHRON I1 ( N . E. Laboratory Equipments, Prague) s i t h a flame ionization detector was used. The scheme of the pneumatic system for the carrier gas can be seen in Figure 2. The carrier gas flows gradually through the needle valve, 1 , the security valve, 2, through the device for the stabilization of pressure, 3, and a drying tube with silica gel, 4. The gas then flows through the needle valve, 5 , for the regulation of the flow rate, the rotameter, 6, and then passes through the stopcock, 8 , either directly into the thermostated column, 9 , or through the sampling tube, 10. For heat desorption there is a heater, 11, fixed above the sampling port. A heating element with an input of 250 watts was used as the heater. The sampling tube was connected to the stopcock, 8 , by a copper capillary, 12, which provided for comparatively easy handling of the tube when inserting or removing it from the heater. The chromatograms of substances desorbed from the sampling tubes and chromatograms of samples injected directly in the parallel analy,'> (xN,!.V,) ( 8 p , / & V , ) , This is usually sure drop along the column, and g is; the fulfilled with small concentrations of mass fraction of the stationary phase in sorbed components as in trace analysis, the column. because the equilibrium is expressed The mass, m, of the substance -Vi,Thus it is evident that as S, >> trapped in the sampling tube can be the sorption properties of the stationary determined by gas chromatography. liquid are practically uninfluenced by I t is best for the method described here the presence, CATt, of the dissolved subto use direct detector calibration either with the substances being determined, stances. I t is apparent t'hat values of the derivatives bpL,jdS, and dpc,~diZrj or with standard substances if the ratios of molar or mass response of the play a role, but if the liquid phase is determined components and of the suitably chosen these derivatives have standards are known. I t is convenient equivalent values, and therefore balance to express the relationship between the each other. mass, m, of a certain component of the Froin that it follows that the range analyzed mixture and the corresponding for using the method is limited by a peak area, F,in the form certain maximum concentration of the estimated substances in the gas. This limit as a rule corresponds to considerably higher concentrations than
662
ANALYTICAL CHEMISTRY
DIRECUON Of ASPIRATION L
I = va+ Yo 2dCsAY
,
Figure 4. Representation of analogy between parameters of frontal zone and corresponding elution zone I AI
= Length of sampling tube
= Width of zone = Width a t half height (VR Vo), AV, and AV1 a r e corresponding retention volumes Af1,*
+
where k is a factor to change area units into molar units, Jf is the molecular weight, and r is the sensitivity factor of the inktrument. The values indicated by the index s correspond to values obtained by injection of mass ma of the standard substance. If the instrument is calibrated with the same substances as are being determined, it is apparent that k, = k , .\I, = Jf, To determine the mass of the components trapped in the sampling tube, it is advantageous to use a flame ionization detector because of its high sensitivity and because it is possible to determine fairly easily (8) the calculation factor as the reciprocal values of the in the sum of effective carbons molecule of the substance,
EC,,
When using this method of detection, Equation 9 becomes
Minimum Volume of Gas Drawn Through Tube. When drawing the analyzed gas through the tube, the average linear velocity of the front of a certain component is the same as the concentration maximum velocity would be of the zone of the same component with elution chromatography in the same system under the same conditions. I n this respect we can also speak of the same velocity of the broadening of the frontal zone, considering the inflection point of its concentration curve and the elution zone, and considering the concentration maximum, which is characterized by the standard deviation = V,il/; These relationships are illustrated in Figure 4. The figure shows the case when the inflection point of the concentration flow a t the top of the frontal zone has passed distance 1 (equalling the length
of the sampling tube) which corresponds V o ) . For to a passed volume ( V R almost complete saturation of the sampling tube, it is, however, required that the above mentioned point should in addition pass the distance 1/2 AlLe., volume 1/2 AV. Considering the form of the Gaussian curve, l j 2 AI and lj2AT' can to a good approximation be replaced by All,* and AVliz (Figure 4). The values AV1/2 can be determined from the relationship
+
Table
I.
Relationship for Calculating Retention Volumes for Different Temperatures
Stationary phase Silicone elastomer E 301
Benzene Toluene p-Xylene
(1760/T) - 3.40 (2070/T) - 4.01 (2460/T) - 4.84
Polyethyleneglycol 400
Acetone Methanol Toluene
(1920/T) - 4.07 (2250/T) (2070/T) - 34.66 .94
Table II.
EquilibPartial rium Flow Flow Presumed vapor rate in Temp. of pressure rate concn. of satura- of satu- concn. in satura- through vapors in tion rated saturator, tion dilution sample Saturation li$uid, vapors, (grams/ml.) line, line (gram/ml.) liquid C. mm. Hg 10-4 ml./min liters/min. 10-6
which follows from the discussion of the symmetrical Gaussian curve. The aspiration, minimum volume V,,,, therefore is
V,,,
VR
4- Vo
+
AViin
Series
(13)
By combining relationships 12 and 13, and neglecting V o against V R , we obtained after adjustment
V,,,
EV
+
R [ ~ d(8In 2)lnl
n = l/H
1/Cu
1
Benzene
21.5
75.4
3.2
1.44
1.40
0.329
2a
Benzene Toluene p-Xylene
21.5
25.10 7.32 2.19
1.065 0.370 0.127
1.44
1.40
0.110 0.038 0.013
3a
Benzene Toluene p-Xylene
20 5
25 00 7 25 2 17
1 055 0.366 0 126
1 80
5 20
0 0365 0 0127 0 0044
4*
Acetone hlethanol
24.0
...
...
. . .
(14)
From relationship 14 it is apparent that V,,, depends on the resolving power of the sampling tube; in the region of greater aspiration velocities than those which would correspond to the optimum flow rate (which is usually the case), V,,, increases with increasing velocity of aspiration because the value n decreases. At greater aspiration velocities, the terms A and B/u in the van Deemter equation can be neglected relative to the term Cu, and one can write
.
a Mixture of benzene, toluene, and p-xylene was equimolar; when calculating partial vapor pressure of components ideal behavior is presumed. * Mixture of acetone, methanol, and toluene was eqnimolar; partial pressures of components were not calculated because solution is nonideal.
Table 111.
Calibration of Detector
5'01ume of
standard soln., Series pl.
A value of 0.014 seconds was found for C a t 25' C. using Silicone elastomer E
Standard substance
rr
Fs,
mm. (ZC,f),
M,
ma gram X 10-6
Notes
1
5
Cyclopentane
100
296
5
70.13
1.845
2
5
Cyclopentane
100
296
5
70.13
1.845
3
5
Cyclopentane
200
340
5
70,13
1.845
Smaller diameter of burner jet
4
10
Acetone Methanol Toluene
100
154 35 581
2 0.4 7
58.08 32.04 92.13
2.65 1.45 4.21
Values (ZC.,) were not used for calculation
301. The length of the packing in the tube was 4.5 em., and the inner diameter
Table IV.
Tube Analyzed temp., vo0( TI, Series components O C. ml./gram 25 320 1 Benzene 335 Benzene 24 930 2 Toluene 2730 p-Xylene 360 Benzene 22 1025 3 Toluene 3050 p-Xylene 273 Acetone 780 4 hlethanol 25 1030 Toluene
, ,
Toluene
(15)
where u is the average linear velocity of the gas flow in centimeters per second and C is the constant, in seconds, characterizing the resistance to mass transfer. By combining relationships 14 and 15, we obtain
Preparation of Samples
r
500 100 50
200
Results of Analyses Using Sampling Tube
F, mm.2 312 646 579 319 1016 873 631 171 188 266
ZC,f 6 6 7 8 6 7 8 2 0.4 7
m, gram
M
x 10-6
78.11 78 11 92 13 106 16 78.11 92 13 106.16 58.08 32 04 92 13
9.03 3 74 3 40 1 89 1.282 1.113 0.810 5.9 15.8 3.9
c,, gram/
ml. ) 10 -6 Sotes 0.287 0.114 0.037 0.0071 0.037 Smaller diameter of 0.012 burner jet 0,0027 0.254 ZC,f were not used for 0.021 calculation 0.038 ~
VOL. 37, NO. 6, M A Y 1965
663
of the tube was 0.5 cm. The velocity of the aspiration was around 5 ml. per second which, presuming that the free cross sectional area equals about half of the whole area corresponds to a linear flow rate of 8.1 cm. per second. Substituting these terms under the radical in Equation 16, we obtain
d ( 8 ln2)Cull
=
h h
A 2M
0.936
From this it is evident that one must aspirate about the double volume of the analyzed gas that one would expect when considering the retention volume L'R.
100
The required specific retention volumes were measured a t 30°,40°, and 50' C. using a glass column 80 cm. long and 0.7-cm. i.d. From the calculated retention volumes, empirical equations in the form log V,O = (-4/!f) B were constructed which held for the measured range of temperature and its close vicinity. The equations for the studied systems are in Table I. Each value represents the average of about 10 determinations. For each series the samples were prepared, the detector calibrated, and analysis performed both by using sampling tubes and by direct injection of sample. The data which are required for substitution into relationship 7 and 11, together with the results of the analysis, are contained in Tables I1 to V. Comparison of the final results by using sampling tubes, together with the results of the analysis by direct injection and the calculated values, are in Table VI. The agreement between the results is in all cases very good. For benzene and toluene the results of the analysis also correspond well to the assumed concentrations, although larger deviations are found in the case of p-xylene. This is apparent from the insufficient rate of saturation of the vapors during dynamic preparation of the gas mixture. Figure 5d and B shows a pair of chromatograms from the 3rd series and Figure 6 d and B a pair from the 4th series of experiments. Chromatograms -1 were obtained by direct injection a t sensitivity 1 / 5 0and chromatograms B were obtained by using a sampling tube a t the same sensitivity. An example proving the high concentration effect for less volatile substances is the determination of naphthalene in clean lighting gas, shown in Figure 7.4 and B. The sampling tube contained Silicone E 301 as a liquid phase.
+
5
0
Figure 5.
10
\[mnl
0
5
10
lIMl
Chromatograms of traces of benzene, toluene, and xylene in air A-Direct injection of 1 0-ml. gas sample B-Sampling in tube with 0 . 3 gram of Silicone E 3 0 1 packing a t 2 4 ' C. Sensitivity restriction 1 /50 I-Benzene 2-Toluene 3--p-Xylene
h tam1
8
A
206
p
1
100
I
r
I 0
DISCUSSION
To check the method, four series of eyierinimts were performed with samples of air containing benzene vapors, vapor mixtures of benzene, toluene, and p-xylene of higher and loner concentrations, and vapor mix664
ANALYTICAL CHEMISTRY
Figure 6.
I
ffnctl
Chromatograms of traces of acetone, methanol, and toluene A-Direct injection of gas sample B-Sampling in tube with 0.3 gram PEG 400 packing a t 2 4 . 3 ' C. Sensitivity restriction 1 /50 I . Acetone 2 . Methanol 3. Toluene
Results of Analyses by Direct Injection
Table V.
Series
Analyzed components Benzene Beiizeiie Toluene p-Xylene Beiizene
Injected sample volume, ml. 10
F, r
ZC,I 6 6
mrn.2 470
*til
78 11 78.11 213 92 13 2a 10 100 64 7 106 16 (12.5) 8 278 6 78 11 0 035 3* Toluene 10 50 7 92 13 p-Xylene 8 106 16 292 2 51 0 251 Acetone 2 58.08 0 10 LIethaiiol 89 10 50 0 4 32 04 0 019 4 136 0 38 Toluene 8 92 13 0 . 038 a Values in brackets are calciilated from ratio of peak areas of toluene and p-xylene ( F t / F , = 5.1) measured when feeding large sample volume (50 ml.) at sensitivity 1/20. b Conceiitrat,ioii of toluene and p-xylene was too low for analysis by direct injection. 1
tures of acetone, methanol, and toluene. Besides analyses with sampling tubes, parallel analyses were performed siniultaneously by direct injection. In cases where it was possible to define the composition of the vapors over the liquid in the sat'urator, the content of the admixture was also calculated from the relationship of the flow rate in the saturation and dilution stream. Two cases were studied: when the sorbed substances as well as the sorbent are nonpolar, and sorption of mixtures of polar substances with nonpolar ones on a polar sorbent. In addition, the influence of the concentration of coniponents was also observed. In the case of the nonpolar systeni the admixtures were either benzene itself, or a mixture o f benzene, toluene, and p-xylene. The packing of the sampling tube weighed 0.3 gram, which corresponds to a liquid phase w i g h t of 0.09 gram (30% Silicone elastomer E 301 on Celite). The temperature of the heater was 200' C. A solution of cyclopentane in benzene (0.369 X gram per ~ 1 . ) was used t o calibrate the detect,or. The recalculations of benzene, toluene, and xylene was done using sum of effective carbons. I n case of the polar system, the coniponents were acetone, methanol, and toluene. The packing of the sampling tube (0.3 gram) consisted of 30% polyethyleneglycol 400 on Celite. The temperature of the heat'er was 150" C. The detector was calibrat'ed in this case with an equimolar solution of acetone, methanol, and toluene in carbon disulfide (0.265 X 10-6 gram of acetone, 0.145 x 10-6 grain of methanol, and 0.421 x 10-6 gram of toluene per PI. of solution). The calibration of the detector was in both cases done bj- injecting a known volume of solution by niicrosyringe. The samlile of artificially polluted air used in the direct analysis was injected with a normal syringe that had been flushed several times with sample. The
100
C,, (grani/inl. ) 10 -6 0 273 0 124 0 037 ( 0 006)
m, grams x 10-6 2 73 1 236 0 368 ( 0 063) 0 350
Table VI.
Series 1
2
3 4
Comparison of Final Analysis with Calculated Values
Substance Benzene Benzene Toliiene p-X y lene Benzene
Calculated 0 329 0 110 0 038 0 013 0 036 0 013
Tolriene
p-Xylene hcetone Llethaiiol Tollieiie
0 004
'
C,, (gram/ml.) 10-6 Sampling tube 0 0 0 0 0 0 0 0 0 0
___
l?irect
i n j ec ti on
0 273 0 124 u OJ7 ( 0 006) 0 035
287 114 057 007 037
012 003
2,54 021
0 251 0 019 0 03';
038
--
!
A
I
I 0
IO
Figure 7.
irmi
o
10
1r-1
I
Chromatograms of trace components of lighting gas
A-Direct injection of 10 ml. B-Sampling in tube with Silicone E 301 a t 2 5 ' C. Column: 2 5 % Apiezon 1, 1.5 meters, temp., 1 5 0 ' C., sensitivity restriction 1 /50 I . Tri- and tetramethyl benzenes 2. Indene 3. Naphthalene
sample was aspirated through the tube a t a rate of about 5 nil. per second. The capacity of the packing of the sampling tube, given by the content of the liquid phase and by the partition coefficient, can be regulated within wide
limits by choosing the liquid iihast. By this method a selectivity of concentration for some suhtancp in the) mixture, or for a group of hubstance>, can be obtained. The chromatogranii in Figure 8 show the estimation or VOL. 37, NO. 6, M A Y 1965
665
hfm 1
A 2oL
B
10
d
Figure 8.
Chromatogram of gasoline vapors in air (gasoline contents of
570 benzene and 570 toluene)
A-Sampling in tube with PEG 400 6-Sampling in tube with Silicone E 301, calumn as described in Figure 7, temperature 7 . Benzene 2. Toluene
aromatic hydrocarbons contained in gasoline vapors. Part -4 demonstrates the composition of traces of gasoline vapors containing 10% of aromatic hydrocarbons in air, trapped with a sampling tube packed with polyethyleneglycol 400. Part B demonstrates the composition of the substances trapped from the same sample by a tube with Silicone elastomer E 301. Both separations were made on the column with Apiezon L. Attention should be drawn to the fact that the influence of water vapor can be eliminated by choosing a nonpolar packing of the sampling tube. This interference is the main source of difficulties in present methods. When choosing a polar (hydrophilic) stationary phase, however, the packing of the tube must be protected from moisture. .Ilarge excess of moisture in the stationary phase would distort the sorption properties of the packing. If interference from moisture is ewluded, the use of solid adsorbents as packing of the sampling tubes could be considered, and instead of the partition coefficient one would consider adsorption isotherms. Thus, a sufficient effect of concentration could be obtained for very volatile substances, even for gases. 666
ANALYTICAL CHEMISTRY
CONCLUSION
The method described is applicable to estimation of trace impurities in the air (in p.p.b. range) and for evaluation of the degree of air pollution. It can be used without changes also for the analyses of trace components in various technical gases, natural gases, exhaust gases, expiratory gases, and others. From the analytical point of view the method has some important advantages; for example during the sampling it is not necessary to measure exactly the volume of the gas to be analyzed. One need only ensure that a sufficient quantity of air is drawn through the sampling tube and that the exact temperature measured. Maximum use is made of the packing of the tube for all trapped components, and full advantage is taken of the effect of concentration. Because the contents of the admixtures are very often in indirect proportion to their volatilities and the values of the partition coefficients of the admixtures can be chosen so that they would also be in indirect proportions to the volatilities of these admixtures, quantitation of the mass of the various components can be reached a t the sorption. By appropriately choosing the packing of the
60’ C.
sampling tube, it is possible to concentrate selectively those substances which interest us and suppress the influence of those which interfere.
LITERATURE CITED
J. Gas Chromatog. 1, (7) 6 (1963). (2) Colson, E. R., ANAL.CHEM.35, 1111 i1963). ., F. R., Kaminsky, S., Ibid.,
(1) Altshuller, A. P.,
tography,” p. 149, Academic Press, New York, 1962. (6) dal Nogare, S., Juvet, R. S., Jr., “Gas-Liquid Chromatography,” p. 365, Interscience, New York, 1962. (7) Prigogine, I., Defay, R., “Chemische Thermodynamik,” p . 100, V’EB Deutscher Terlag f . Grundstoffindustrie, Lepzig, 1962. (8) Sternberg, J.(,C., Gallaway, W. S., Jones, I). T. L., Gas Chromatography,’’ P;. Brenner, J. E. Callen, AI. D. Weiss, eds., p. 231, Academic Press, Ken7 York, 1962. ( 9 ) Widmark, K., Widmark, G., Acta Chem. Scand. 16,575 (1962). ‘
RECEIVED for review September 16, 1964. Accepted January 26, 1965.
