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
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