Quantitative Gas Chromatographic Analysis of Hydrocarbons with Capillary Column and Flame Ionization Detector I. HALASZ' and W. SCHNEIDER Scholven-Chemie AG, Gelsenkirchen-Buer, Germany
b Under certain operating conditions, quantitative analyses with capillary column and flame ionization detector are feasible. By using oxygen rather than air as combustion and scavenging gas, the sensitivity of the reading was improved. For the apparatus used, the linear range of measurements comprises four orders of magnitude. With a certain bypass, the quantity of sample fed to the column deviated from the calculated value by no more than f5%. Chromatograms were evaluated by means of an integrator. The peak area ratios correspond in first approximation to the weight concentration ratios of the hydrocarbons separated, These results could be improved by applying reproducible normalization factors. In the latter case, the relative error of analysis averaged f1.5%.
I
the rapid development of gas chromatographic analytical methods based on the capillary column and the ionization detector] interest had mainly been focused on qualitative determinations. Such systems, as compared with packed columns and thermal conduc-
tivity cells, generally imply the combined advantages of shorter duration of analysis and better resolution. Previous publications (6) have reported on quantitative analyses using a packed column and a flame ionization detector and have given also several normalization factors (reciprocals of the weight response factors) for the detectors. Desty (1) was the first author who made allusions to quantitative analyses with the capillary column and flame ionization detector. The problem is further investigated in this paper. In spite of the higher sensitivity of the argon beta radiation detector, a flame ionization detector was more suitable for our particular purposes, because its design was less complicated and its range of linear relation between concentration and signal was broader. Our apparatus has been described in an earlier paper (3).
N
Present address, Institut fur Physikalische Chemie der Universitiit Frankfurt (Main), Robert-Mayer-Strasse 11, Germany. 1
The flame ionization detector (in the following called the detector) is equipped with two platinum electrodes. One of them-the combustion nozzle-is spherical (outer diameter 3 mm., inner diameter at the gas exit point 0.2 mm.). This facilitates heat transfer, which results in a reduction of the surface temperature with concomitant suppression of electron emission from the metal. The counterelectrode, installed a t a variable distance from the nozzle
AMPLIFIER
The choice of the amplifier is determined by its current sensitivity, its time constant, its linearity] and its noise. Since the width of the peaks initially eluted from capillary columns
---
.-.-.---.-.-.
5 17"10-*A k ' .
DETECTOR
electrode] consists of a tear-shaped piece of platinum (diameter 3 mm.). Experience has shown that our detector output signal is the same with an angular (bent) or a tear-shaped counterelectrode. Similarly, varying the potential difference between both electrodes within a range of 100 to 300 volts did not cause any appreciable change in the detector signal for a given sample. The detector cell is flushed with oxygen rather than air, with a consequent increase in sensitivity by a factor of three [fuel gas (hydrogen) flow rate of 6.5 cc. per minute J and a decrease in the maximum allowable electrode gap (Figure 1). An increase in the hydrogen flow rate results in an increase of the ion current that means an increase in the detector sensitivity (Figure 2). Unfortunately the noise increases also. I n our case, an electrode gap of 6 mm. and a rate of 20 cc. of Hz per minute and 1 to 3 cc. of NZper minute, respectively] was an acceptable compromise between the operating variables. Scavenging gas was oxygen with a flow rate of 500 cc. per minute. Consequently, all analyses were carried out under these optimum conditions and nitrogen was used as carrier gas for the sample.
s
5
652 ml./min: H, acceleroling . vollage'
scavengiy gas: O?
U
5 1.4,
10.86ml/min(N,+250 Vol p p M Q ) Scavenging gas: O2
lo-.
I
3{.10-" I
1
5
10
15
20
*
25rm electrode 901:
Figure 1. Influence of scavenging gas in relation to detector response
978
ANALYTICAL CHEMISTRY
Figure 2. Claps
Linearity of ion current by different electrode
1
I -
/II: 1I
6 110 ' A
b Figure 4. Ion current in relation to benzene concentration linearity to ca. 1.5 X lo4 ampere
linearity limit is approximately the same for benzene and methane, viz. 1.5 X lo-' ampere, equivxlcnt to 4 pg. of benzene per minute fed to the combustion cell. The fact that the upper margin of the linearity range for the ion current generated in the detector is not a function of the hydrocarbon type involved, can also be concluded from a publication of McWilliam (6), who found the detector signal to be proportional to the carbon concentration in the flame. The weight concentration of carbon in different hydrocarbons varies but slightly from one component to another, the average being 80 to 95% (the only major exception being methane). For this reason i t is not surprising that the detector signal, in first approximation proportional to the carbon weight concentration of the hydrocarbon introduced into the cell, is independent of the specific constitution of the individual hydrocarbon. For practical purposes i t is feasible to characterize the upper linearity h i t in terms of current intensity rather than hydrocarbon concentration. Accordingly, the maximum ion current admissible in our measurements is less than 1 X lO-@ampere. This limitation caused by the ion current not only imposes certain restrictions as to the quantity of sample injected, bqt also implies that further important parameters have to be observed, such as kind and flow rate of the carrier gas and the flame characteristics. In summary, these conditions mean that at any time the substance fed to the detector must not exceed a certain concentration in the carrier gas (linearity of the detector response). To quote just one example: For a flow rate of 2 cc. of NZ per minute, a sample concentration of 0.2 wt. % should not be exceeded (see Figure 3). Similar limits were found by Keulemans (4). A further parameter to be considered when fixing the quantity of sample to be injected is the retention time, since peaks broaden if this parameter is increased. The linearity range available for practical utilization extends from 10-1* N
Figure 3. Ion current in relation to methane concentration Linearity of ca. 1.5
+ 10-0 ampere
of our design corresponds to 2 to 4 seconds, the amplification range has to be selected so 119 to give signal delays of less than 1 second. A Knick (Berlin) Model P24 or a Friesecke and HBpfner G.m.b.H.(Erlangen) Model FH56 amplifier waa employed. The response time of these amplifiers was smaller than 1 second at a sensitivity of 3 X 10-11 ampere full scale and the noise smaller than 5 X lo-" am ere. The recorder was a 1-mv. Leeds Northrup Speedomax Type G with l-second response time. CORRELATION BETWEEN SUBSTANCE CONCENTRATION AND DETECTOR SIGNAL
A linear relationship between the concentration of a component and the resultant detector signal is necessary for quantitative gas chromatographic analysis. To verify this dependence, test mixtures consisting of varying quantities of CR( Nt and C& Nt were prepared. The methane concentration (after combustion) was determined conductometrically by an apparatus of Whthoff OHG, that of benzene by infrared spectroscopic analysis. These gmi mixtures were continuously fed directly into the detector, and the pertinent signals recorded. Results (Figure 3) reveal that up to a current intensity of about 1.5 X 10-0 ampere a linear relationship exists between hydrocarbon concentration and current intensity. This upper limit is only slightly affected by the methane concentration in the sample or the carrier gas flow rate. At the above mentioned current intensity about 5 pg. of C& per minute reach the detector. According to Figure 4, the upper
+
+
to 1O-O ampere, the lower margin in our case being set by the amplifier performance] the upper one by the onset of the nonlinearity of the detector signal. The range covers four orders of magnitude, thus being spread slightly wider than that of a thermal conductivity cell. Linearity exists even over a broader concentration range. Small concentrations] however, correspond to but small ion current intensities, the measurement of which necessitates a high working resistance in the amplifier. This in turn results in signal delays greater than the maximum allowable time of 1 second. In practice, with one single sample injection, concentrations of one component as low as 0.01% can be determined in the presence of 99.99% of the other, provided the peak width is similar in both instances. EVALUATION
OF
CHROMATOGRAMS
According to general experience, quantitative evaluation of the chromatograms-including those derived from packed columns-is more accurate if based on peak area rather than peak height. Moreover, the capillary columns, notably those with metal capillaries, normally give peaks which show a tailing effect. In view of the small peak widths in our chromatograms] peaks were evaluated with an integrator (3) of high resolution (about 2 x loe impulses per minute for full scale recorder deflection). The i n t e grator was built by Hartman and Braun (Frankfurt/Main). In addition the integrator had an extreme base line stability, and a linearity of better than =k0.2%. REPRODUCIBILITY OF SAMPLE INJECTION
The samph ,njection system used in our experiments is outlined in Figure 5, It was always a t the same temperature as the capillary column. The liquid samples (0.3 t o 1 pl.) were introduced with a No. 701 N (10-pl.) Hamilton syringe. Behind the injection chamber the carrier gas plus sample flow were split VOL. 33, NO. 8, JULY 1961
979
Sample
/’
I
I
7
1
0.I
0.05
“‘O/Q
Figure 6. Amount of benzene sample calculated by sample size injected and splitting ratio in relation to amounts measured in detector itself
S!
I
Cop.Co umn Figure 5.
Sample injection and bypass system
in a proportion 1:lOOO to 1:2000. For constant flow rate and temperature, the quantity of sample fed into the system is reproducible to within 2 to 5%. It was, however, suspected that the quantity of sample actually reaching the separation column was smaller than that calculated from the volume injected and the bypass ratio in the splitting system. The plot (Figure 4) allows a calculation of the relation between the quantity of benzene (9) introduced continuously into the detector and the corresponding quantity of electricity (coulomb) delivered by the detector. In a subsequent series of experiments with samples containing constant quantities of benzene, the splitting ratio of the gas into the main stream und bypass stream was varied. The quantities of sample actually reaching the detector were in each case calculated from the peak areas. The correlation between the calculated and measured quantities of benzene in the sample is plotted in Figure 6. The deviation is about =k5%, thus lying within the analytical error of the benzene concentration determinations and giving evidence of the fact that the quantity of sample passing through the separation column is, in fact, calculable. The main drawback associated with bypass systems is the possible falsification of the original concentration ratios. For the time being the problem whether 980
~~~
ANALYTICAL CHEMISTRY
such falsifications really occur was deferred. Prime importance was, instead, attributed to the question of its reproducibility-Le., its independence from the quantity of sample injected, the concentration of the components in the mixtures, the flow rate in the bypass, and the temperature level. NORMALIZATION FACTORS AND QUANTITATIVE ANALYSIS
The reproducibility of the normalization factors (area factors) was established. The area factors may be defined by the following equation: f = -
F.ci Fi.c
where F , is the peak area and c( the weight concentration of substance i in the sample mixture, whereas the symbols
Table 1.
without indices refer to the standardization component. (The area factors are reciprocals of the detector response figures.) The internal standard chosen for our experiments was n-heptane, hencef;heptane = 1. As will be shown later, the dispersion of the area factors is smaller when concentrations are expressed in terms of weight per cent rather than mole per cent. The mixtures used for the determination of the normalization factors and for the analyses are listed in Table I. The reproducibility of the area factors is shown in Table 11. Separations were carried out in a 60meter capillary column, internal diameter 0.25 mm., pretreated with a 10% solution of squalane in petroleum benzene. Column and bypass temperatures were always 70” C. The chromatogram is shown in Figure 7. Within every test series, operating conditions, qualitative composition of the feed mixture, sample size, and splitting ratio of the gas stream were hcld constant. Standard deviation from the average of the data listed in the vertical column of Table I1 amounts to *0.11 to 0.39% of the average. The normalization factors are given to four decimal places. Although the accuracy of the last place in the normalization factors is surely inadequate, it was, nevertheless, given to show a notion of the reproducibility. The normalization factors listed in Tables I11to V are all average values. Table 111 gives the results of euperi-
-
Mixtures Used in Analyses
Mixture No. 51
52
13.34 17.55 16.99 22.48 29.64
4.69 4.53 6.38 79.05 5.35
54
55
56
58
3.63 3.85 84.11 3.94 4.47
4.26 4.32 49.45 4.34 37.63
Per cent by weight A . 2,PDimethylbutane
B. %Methyl-1-pentone C. Benzene
D. n-Heptane Methylcyclohexane
E.
4.44 4.66 6.05 4.45 80.40
16.38 18.02 24.47 18.68 22.45
Mixlure NO. 55 7OoC. 60m. 1 0.26mm.
Figure 7. No. 55
Chromatogram of Mixture 1. 2.
3. 4. 5.
2,2-Dlrncthylbutane 2-Methyl-1 -pentene Benzene n-Heptane Merhylcyclohexane
ments in which the bypass ratio was varied within the range of 1:360 to 1:4320. In every case the quantity of sample injected was controlled to correspond to the maximum allowable, the other operating conditions being kept unchanged. The deviations decrease with increasing retention times. Probably the response of our recorder waa not quick enough for the first peak of the chromatogram (2,Mimethylbutane). Although the normalization factors increase with increasing bypass velocity, they remain constant to within *l%. Under normal conditions we worked with a bypass velocity of 2 to 6 liters per minute. In this range the deviations are smaller than *0.5%. Finally the percentage share of components was modified so that the concentration of certain substances varied between 3 and 80 wt. %. Results are summarized in Table IV. Deviations do not exceed *1,5%. Measured normalization factors for some hydrocarbons-straight chain, branched, and cyclic paraffins aa well aa aromatics-are listed in Table V. Considering the normalization factors for the individual hydrocarbons-except benzene and some low boiling paraffins-these do not deviate essentially from unity. Some of the deviations-except benzene-may have been caused by the impurities of the substances. Most of the applied hydro-
carbons were, however, purer than 99 mole %. It follows that the error committed when calculating without normalization factors is not too great. In first approximation, therefore, the ratio of the peak areas is proportional to the concentration ratio by weight of the components in the injected sample. The accuracy of sbch an approximative quantitative analysis may be seen from Table VI. The peak area valuesmeasured with the integrator-were divided by 10 and are tabulated in the u p p r part of Table VI. In the lower part, of this table the uncorrected and thc vnlues corrected by applying the normdimtion factors are listed. The maximnl absolute errors of the uncorrected vnlues were smaller than 1.2%. the rchtivc errors smaller than 5%. The corresponding maximal errors of the corrected values were smaller than 0.37
~~
Table
II.
Reproducibility of Normalization Factors (n-heptane = 1) Mixture No. 51; sample size: 1.4 pl * splitting ratio: 1:1440; rarrier gaa: 1:i
cc. NS per minute; column inlet pressure: 2.5 kg. per sq. ~. . cm. 2Meth2,2-Di- MethylYlCYmethyl1Benclobutane pentene zene hexane 1. 2.
0.9644 0.9591 3. 0.9532 4. 0.9594 0.9029 5. 0.9674 6. 7. 0.9615 8 . 0.9616 0.9616 9. Av. 0.9628
Std. dev.,
70 0.39
1.0049 0.9977 0.9915 0.9987 1.OOO1 1.0050 1.0008 1.OOO1 0.9994 0.9998
0.9503 0.9474 0.9433 0.9475 0.9492 0.9542 0.9503 0.9437 0.9492 0.9496
1.0107 1.0068 1.0071 1.0057 1.0073 1.0091 1.0082 1.0073 1.0070 1.0077
0.28
0.30
0.11
Table 111.
Effect of Bypass Ratio and Sample Size on Normalization Factors (n-heptane = 1) Mixture No. 51; all conditions, except bypaas ratio and sample size, see Table I1
ByPam Sample Rate, Size, Liter per pl. Minute 0.2 0.5 0.7 1.4 4.0
1 .o 2.0 6.0
Area Factora Splitting Ratio
2,2-Dimethylbutane
2-Methyl-lpentene
Benzene
Methylcycle hexane
1 :360 1 :720 1 : 1440 1:4320
0.9350 0.9482 0.9628 0.9676
0.9873 0.9954 0.9998 1.0083
0.9372 0.9400 0.9493 0,9525
1.0030 1.0035 1.0077 1.0005
Table IV.
Normalization Factors for Variation of Concentration Ratios of Test Mixtures (n-heptane 1) Splitting ratio: 1 : 1440; sample size: 0.5 pl.; all other conditions see Table II
-
Normalization Factora B C E 2,2-DiMethylMixture methyl- %MethylcycloNo. butane pentane Benzene hexane A
51 52 54 56 58
0.9628 0.9359 0.9503 0.9438 0.9538
0.9998 0.9891 1.0183 0.9993 0.9823
0.9493 0.9413 0.9329 0.9427 0.9504
absolute % and 1.5 relative %, respectively. Small peaks arising from a certain impurity content of the parent substance were cointegrated with the corresponding main peak. From these resulta i t may be concluded that quantitative analyses with a capillary column-flame ionization detector system are feasible within the normal magnitude of error, provided that working temperature, bypass system, and flame ionization detector remain unchanged. It has still to be clarified whether modifications in the dimensions of the bypass, its temperature, and the design of the flame ionization detector deet the reproducibility of the results to any measurable extent.
1.0077 0.9777 1.0113 0.9746 1.0066
A 13 5
4 3 4
Per Cent by Weight B C D E 18 5 5 4 4
17
6 6 84 50
22 79 5 4 4
30 5
80 5
38
Table V. Weight % Normalization Factors of Some Hydrocarbons.
-
(area factors) (n-heptane 1) Pentane Hexane Heptane Nonane 2,Z-Dimethylbutane 2,3,4-Trimethylpen t ane 2-Met hyl- 1-pent ene %Methylcyclohexane Benzene Ethylbenzene pXylene +Xylene
0.995 0.993 1 .Ooo 1 .Ol6 0.963 1.006 1 .ooo 1.008 0.949 0.986 0.986 0.992
VOL 33, NO. 8, JULY 1961
981
Table VI.
Quantitative Analyses with Capillary Column and Flame Ionization Detector
Mixture No. 55 Integrated Area Values
D. *Heptane
E. Methylcyclohexane
Weighed in Per Cent b.w.
A. B.
C. D. E.
16.38 18.02 24.47 18.68 22.45
2 6427 6768 9814 6942 8348
1 6318 6643 9615 6776 8141
A . 2,ZDimethylbutane B. %Methyl-1-pentene C. Benzene
Found Uncorrected Per Cent b.w. 1 16.85 17.72 25.65 18.07 21.71
2 16.78 17.67 25.62 18.13 21.80
3 16.78 17.71 25.63 18.11 21.77
4 16.79 17.71 25.62 18.10 21.78
5 16.84 17.71 25.68 18.05 21.72
3 6398 6749 9773 6905 8298
Area Factors 0.963 1.ooo 0.949 1.000 1.008
4 6287 6626 9593 6776 8156
5 6570 6910 10022 7043 8477
Found Corrected Per Cent b.w. 1 16.52 18.03 24.77 18.40 22.28
The question whether the concentration ratios are in fact falsified if a bypaw is included in the system, is not answered by this paper, which restricted its findings t o the statement that under constant operating conditions the normalization factors are affected with practically the same errors as those obtained with packed columns
model mixtures gave analogous results. I t may, therefore, be presumed that our bypass system complies with the basic requirements for quantitative analyses with a capillary column and flame ionization detector. It should, nevertheless, be attempted to develop an unbranched system permitting a reproducible introduction of samples aa
and thermal conductivity cells. Detailed investigations are underway to find whether such falsifications do, in fact, occur. A similar series of experiments on
small as 0.05 to 1 pg.
2 16.45 17.99 24.75 18.45 22.36
3 16.45 18.02 24.76 18.44 22.33
4 16.46 18.02 24.75 18.42 22.35
5 16.50 18.03 24.81 18.37 22.29
W. Scott, ed., p. 46, Butterworths, (2)London, Halhz, 1960. I., Schneider, W., 2. anal. C h m G 175,94(1960); in “Gas Chroma-
tography,” R. P. W. Scott, ed., p. 104, Butteworth, London, 1960. (3)Tedr. 32, 675 I*,(1960f er, G*, Ins* (4) InformalSymposium on Gas Chr+ matogra hy, Atlantic City, N. J., ANAL.(%EM. 32,339 (1960). (5) McWilliam, I. G., J . Appl. Chem. (London)6, 379 (1959).
LITERATURE CITED
e.
(1) Dqsty, D. H., Geach, J., Goldu A., in ‘‘Gas Chromatography,” R.
8;
RECEIVED for review December 20, 1960. Accepted March 24, 1961.
Compact Two-Stage Gas Chromatograph for Flash Pyrolysis Studies STANLEY B. MARTIN and ROBERT W. RAMSTAD
U. S. Naval Radiological Defense Laboratory, b A compact, portable, two-stage gas chromatography system which permits the direct measurement of the complex reaction products o f flash pyrolyses has been constructed and successfully operated as part of a program dealing with the high temperature behavior o f materials. A unique feature of this system i s that the flash pyrolysis reaction i s carried out directly in the carrier gas stream just prior to its entering the first of two initially coupled stages. Products of the reaction mixed with the helium carrier go directly onto the liquid partition column of the first stage, and pass onto the absorption column of the second stage. The two stages are then uncoupled. The first stage proceeds to analyze the higher molecular weight products while the second stage concurrently analyzes the fixed gases. To achieve the pressure balance necessary to perform such a valving 982
ANALYTICAL CHEMISTRY
San Francisco 24, Calif.
operation, a precise electrical flow control was developed. Maximum component density was achieved by constructing each stage to fit inside o f a small Dewar flask. Details of construction, performance, and utility o f the instrument are described.
A
ideally suited for investigating the mechanisms of some reactions is a combined reactorchromatograph in which the reactants and/or products are carried through the reacting zone by the chromatography carrier gas (1). This approach lends itself particularly well to the study of the rapid degradation of nonvolatile solids or liquids when these reactions can be induced by some external stimulus as in the case of flash pyrolysis. The subject of flash pyrolysis is really beyond the scope of this paper. The interested reader should TECHNIQUE
refer to the work of Nelson and Lundberg (3). The application of chromatographic techniques to such a reacting system has been used with some succew and has been reported elsewhere by the author (9). This application made use of adsorption chromatography to resolve and determine the amounts of the fixed gases and liquid partition chromatography to analyze the condensable substancea which had been trapped out of the line ahead of the adsorption column. This procedure was necessarily laborious, time-consuming, and not completely reliable. The principle of staging two or more chromatography systems seemed the most natural solution to this problem, but the multiple-stage systems commercially available were much too large. Compactness and portability were qualities deemed essential to the program which involves the use of several different sources of intense radiant energy,
