Use of Two or
More Internal Standards in Gas Chromatography
Emerson H. Lee and George
D. Oliver,
Plastics Division, Monsanto Chemical Co., Texos City, Tex.
N quantitative analyses for Iraphy, minor components by gas chromatoga known weight of an internal MAKING
standard is sometimes added to the sample for a reference [Ray, N. H., J. A p p l . C h . 4, 21 (1954)l. This is often necessary for good accuracy because small variations in sample size and CJperating mnditions cause the absolute heights or areas of peaks to vary slightly. This in evident on repeated injection of the same mmple. If in a given analysis the time between peaks is large, accuracy may be reduced when one ref-
erence peak is used for all unknowns. An obvious solution to this problem, which does not appear to be in general use, is the addition of two or more internal standards to the sample to be analyzed. These standards and their concentrations may be chosen so that their magnitude and peak position a p proach those of the desired unknowns, as shown in Figure 1. The internal standards may be quantitatively mixed so that only one weight of internal standard is added to each sample. Because each unknown has to be calculated
n Figure 1. Determination of minor concentrations of styrene, benzene (Bz),and toluene (Tol), in ethylbenzene (EB), using n-decane and 2,2,5-trimethylhexane (TMH) as internal standards
anyway, the only additional work involved is that of memuring the peak heights or areas of the additional references. The above scheme was applied to the determination of small amounts of styrene (0.1 to 3%), benzene, and toluene (0.02 to‘0.5%) in ethylbenzene, using helium as a carrier gas. The ’ Narcoil column used was 25 weight % 20 on Chromosorb, 2 meters in length, and operated a t 120” C. The internal standards used were n-decane for styrene and 2,2,5trimethylhexane for both benzene and toluene. The styrene peak was normally 5 to 10 times the area of the benzene and toluene peaks, and in elution sequence it followed them by about 30 minutes. A typical chromatogram is shown in Figure 1. In the middle of the concentration ranges given above, the relative error of determination was about, 1 to 2% of the true value, As more accurate detecting devices are developed, trace analysis wiil become more feasible, and use of multiple internal standards should be an effective aid to accuracy. With many present types of equipment such use will undoubtedly aid in determination of several minor components. ACKNOWLEDGMENl
Thanks are given to Nina Hadden and E. A. Hinkle for helpful suggestions.
Determination of Primary Amino Alkyl Alkoxy Silane Coating on Glass Substrates Howard B. Bradley and Donald J. Neal, Research Laboratory, Linde Co., Division of Union Carbide Corp., Tonawanda, N. Y. MINOGILANES
are used extensively
A as coupling agents on siliceous materials such as glass cloths, glass fibers, and “mineral wools.” Examples are: Type 1 . R,,Si(NH&-, Type 2. [H,N(CHdzIn Si(ORIen Type 3. [HsF(CI**)z NH(CHz),]nSl(OR)-r Type 4. Rrn Si[O(CHt), NH,],,
Methods for determining the amount of silicone that has adhered to the
substrate are necessary for control purposes. If it were possible to leach the aminosilanes quantitatively from the substrates, the extract could then be analyzed by infrared spectroscopy or by chemical methods (1,S), but ex-
perience has shown that such quantitative leaching can probably not be accomplished. In trying to solve a similar problem, Petty (2) was unable to leach all the silicone from textiles. The manner of attachment of the silanic compounds to the substrates is not clearly understood, but it is known that each of these silanes, through contact with water or atmospheric moisture during the treatingcuring-aging process, loses its easily hydrolyzable functional groups; thus, the amino groups of Type 1, the alkoxies of Types 2 and 3, and the amino alkoxics of Type 4 are eventually replaced by oxygen or hydroxy groups.
Ultimately, compounds of Types 1 and 4 leave an aEyl siloxane deposit on the substrates; Type 2 leaves a primary amino alkyl siloxane; and Type 3 leaves an (N-amino alkyl) amino alkyl siloxane. Because the substrate is mostly silica, a total silicon determination would have no value. A total nitrogen determination would not tell the silicone content of substrates treated with Type 1 or 4 compounds, but should prove adequate for Types 2 and 3. A total carbon determination would probably have to be used for Types 1 and 4. Actually both Type 2 and Type 3 can be determined adequately with a modiVOL 31, NO. 1 1 , NOVEMBER 1959
* 1925
