Table 111.
Synthetic Blend
Olefin Type Trans (Irans-heptene-
Concentration (moles/liter ) DeterKnown mined
3)
Vinyl (octene-1) Vinylidene (2-methylbutene-1 ) Trisubstituted (2methylbutene-2) Cis(cis-heptene-2) Total
0.075 6.134
0.04 6.15
0.045
0.02
0.106 0.078
0.09 0.13
6.438
6.43
represent the departure of the sample from the assumed 100% a-olefin. When these difference absorbances are applied to the olefin group type absorptivity matrix (2) shown in Table 11, the concentrations of the various impurity olefin types are obtained.
The difference absorbance a t 11.OO microns will, of course, be a negative number if the sample is less than 100% pure a-olefin, but the negative sign should be carried along in the matrix calculations; a negative value will result representing the departure of the a-olefin concentration in the sample from the assumed 100% a-olefin. This value could be algebraically added to the total theoretical olefin concentration in the sample to obtain the a-olefin concentration. However, the concentration for the a-olefins determined by this procedure is not sufficiently accurate since the group type absorptivity is used. The mean error of this analysis when applied t o an aromatic free material that contains 80% or more a-olefins is for the a-olefin believed t o be &I% value and *1.5% for the other olefin groups. This is indicated by the analysis
of a synthetic blend shown in Table I11 where all of the mean errors are less than 1%. In a blend of four a-olefin compounds, the error in the total vinyl concentration was 0.13%. In some cases, diolefins and cyclomono-olefins will interfere if present in quantities greater than 3%. The diolefin and or cyclomono-olefin content may be determined by a low-voltage mass spectrometric method ( I ) . LITERATURE CITED
( 1 ) LumDkin. H. E.. ANAL. CHEM.30. ' 321 (1658)' (2) Saier, E. L., Pozefsky, A., Coggeshall, N. D., Zbid., 26, 1258 (1954).
E. L. S l I E B L. R. COUSINS ?If. R.BASILA Gulf Research and Development Co. Pittsburgh 30, Pa.
Modified Thermal Conductivity Detector for Capillary Columns SIR: Although Golay (8) initially used a thermal conductivity detector for his investigations with capillary columns, these columns have been used almost exclusively with ionization detectors. Recently, Schwartz (4) and also Camin (1) reported using primarily 0.02-inch or larger i.d. capillary columns with thermal conductivity detectors. At this laboratory, a gas chromatograph equipped with a modified thermal conductivity detector has been used for some time with 0.01-inch i.d. capillary columns.
EXPERIMENTAL
Apparatus. The gas chromatograph was fabricated along conventional lines using essentially standard components. The detector was the GowMac Micro-Cell Model JDC-015. A 2-mv. Brown recorder was used. A sample stream splitter after the design of Halasa (3) was included in the inlet system. The splitter was fabricated from a Swagelok heat exchanger tee and stainless steel tubing. The split ratio was varied with a Nupro micrometering valve. Both detector and capillary column were housed in a
Figure 2.
temperature-controlled air bath suitable for operation up to 125' C. The significant feature of the apparatus was the modification made in the sample cavity of the detector. Figure 1 shows this modification in detail. The modification consisted of positioning a 2-inch length of stainless steel, 0.010inch i d . , tubing up through the sample cavity orifice with the end of the tubing as close as possible to the thermistor bead itself. This distance was approximately 1 mm. The tubing was then silver-soldered in this position. The capillary column was then connected to the other end of the tubing.
Chromatogram of Cs through C, blend on modified cell
Column. 190-ft. X 0.01 0 In. stainless steel capillary coated with aqualane; 25' C.; helium 1.7 ml./ min. sample size, 1 &I. 1 O O : l split Componentr
1. 2. 3.
Figure 1. Modification of sample cavity in cell a. Before modification
b. After modiflcation
2220
ANALYTICAL CHEMISTRY
lsopentane n-Pentane 2,P-Dimethylbutane 4. Cyclopentane 5. 2,3-Dimethylbutane 6. 2-Methylpentane 7. 3-Methylpentane 8. n-Hexane 9. 2,2-Dimethylpentane and methylcyclopentone 10. Benzene and PA-dimethylpentane 1 1. 2,2,3-Trimethylbutane
12. 13. 14. 15. 16. 17.
1 a. 1 9. 20. 21.
3,3-Dimethylpentane Cyclohexane 2,3-dimethyl pentane 3-Methylhexone 1 ,trans-3-Dimelhylcyclopentane 3-Ethylpentane and 1 ,franr-2dimethylcyclopentane n-Heptane 1 ,cir-2-Dimethylcyclopentane Methylcyclohexane Toluene
Table 1.
Analysis of C&
Componenta Isopentane n-Pentane 2,%Dimethylbutane Cyclopentane 2,3-Dimethylbutane ZMethylpentane 3-Methylpentane n-Hexane 2,BDimethylpentane Methylcyclopentane Benzene 2,CDimethylpentane) 2,2,3-Trimethylbutane 3,3-Dimethylpentane Cyclohexane 2,3-Dimethylpentane 3-Methylhexane
}
4.2
Blend Wt. % Found 3.8
4.3 4.5 5.2 4.6 4.5 4.6 4.5 5.6 10.6
4.8 4.9 4.9 5.1 5.4 5.4 5.4 5.9 9.6
4.7 0.5
4.9
KIlOWn
expensive conversion of existing thermal conductivity instruments for capillary columns. Deviation -0.4 $0.5 +0.4 -0.3 $0.5 +0.9
+0.8
+o
.9 +0.3 -1.0
5.3
5.3
$0.2 0 0
4.6 5 .O 4.8 1 .o
-0.2 $0.3 -0.3 0
n-Heptane
4.7
Methylcyclohexane Toluene
5.4
4.9 4.6 4.7 4.5
$0.2 -0.7 -0.7 -1.4
1, trans-3-Dimethylcyclopentane
3-Ethylpentane 1,
trans-2-Dimethylc y clopentane)
1, cis-2-Dimethylcyclopentane
RESULTS AND DISCUSSION
The thermal condc ctivity detector was first evaluated without any modification using a ZOO-foot by 0.01inch i.d. squalane coated capillary column. The sensitivity of the detector was more than adequate; however, the peak shapes and resolution were very poor compared to typical results obtained on ionization detector instruments. Since the opei-ating conditions on the capillary coliimn were very similar to those empl3yed when used with ionization detectors, it was concluded that the poor 3eak shapes and poor resolution were ca ised by excessive dead volume in the detector block from the column exit to the orifice leading into the sample cavity and also by excessive diffusion in the sample cavity itself. Therefore, the detector was modified as described ribove and shown in Figure 1. -4 typicitl chromatogram from the modified detector is shown in Figure 2 of a synthetic blend run on a 190-foot by 0.01-inch i.d. squalane
4.8 4.7 5.1 1.o
5.3 5.9
0.5
coated stainless steel capillary column. The sample size was 1 pl. and split 100:1, and the carrier gas flow was 1.7 ml. per minute. Thus, using operating conditions normally employed with 0.01-inch i.d. capillary columns, the modified detector furnished curves comparable to those obtained with ionization detectors. The analysis of the blend shown in Figure 2 is given in Table I. These results were obtained using a planimeter to measure the areas and were normalized directly using no calibration response factors. The modified thermal conductivity detector instrument has been used routinely at this laboratory for the analysis of hydrocarbons in the Csand lighter range. The range and the performance can be improved by making similar modifications with the more recently developed Cow-Mac microvolume cells. Although the modified thermal conductivity detector is not as sensitive as the ionization detectors, it does permit the convenient and in-
LITERATURE CITED
(I) Camin, D. L., King, R. W., Shawhan, S. D., Pittsburgh Conference on Analytical Chemistry and -4pplied Spectroscopy, March 4-8, 1963. (2) Golay, 31. J. E., “Gas Chromatography,” J. V. Coates, ed., p. 1, Academic Press, S e w York, 1958. (3) Halasz, I., Schneider, W., Internat!onal Gas Chromatography Symposium, Michigan State University, June 13-16, 1961. ( 4 ) Schwartz, R. D., Brasseaux, D. J., Shoemake, G. R., J . of Gas Chromatography, 1, 32, (1963). JAMES A. PETROCELLI Gulf Research & Development Co. Pittsburgh 30, Pa.
Fluorometric Determination of Selenium in Plants and Animals with 3,3’-Diaminobenzidine Caution Notice SIR: Benzidine is known to be carcinogenic. Please be advised that 3,3’-diaminobenzidine, recommended as an analytical reagent in a recent issue of your journal ( I ) , is also suspect as a carcinogen. We therefore recommend that your readers be informed that the compound should be handled with extreme caution. LITERATURE CITED
(1) Dye, W.. B., Bretthauer, Erich, Seim,
H. J., Bhncoe, Clifton, ANAL. CHEM. 35,1687 (1963).
Shawinigan Resins Corp. Springfield 1, Mass.
IRVINQ SERLIN
VOL. 35, NO. 13, DECEMBER 1963
2221
