Selective Removal of Aldehydes from Complex Mixtures during Gas Chromatography R. R. Allen, Anderson, Clayton & Co., Foods Division, Sherman, Texas FFAP (3) B had been described as the “best general-purpose packing available” for gasECAUSE THE LIQUID PHASE
liquid chromatography, we tried it for separating a mixture of aldehydes, ketones, alcohols, and hydrocarbons. It gave excellent separation of the components in the mixture except that all of the aldehydes were missing from the chromatogram. Subsequent work showed a short column of FFAP could be used to subtract the aldehydes from a mixture by the same method alcohols are removed by a short boric acid column (2). This technique of “subtractive chromatography” should have considerable utility for the identification of peaks that result from the chromatography of a complex mixture. The results using this technique are shown in Figure 1. The mixture was analyzed on a 12-foot column of 20% UCON Polar plus a 12-inch column of 20% Carbowax 20M a t the head of the column. The upper chromatogram shows all the materials are well separated except heptanone and heptanal which are in one peak. The sample was again chromatographed on the same UCOK column with a 12-inch 20% FFAP column replacing the short Carbowax column. The lower chromatogram shows all of the aldehydes have been removed without affecting the other classes of compounds. The heptanone peak is now revealed since the heptanal was removed. Although not shown, a 12-inch column of 2% boric acid in Carbowax ( 2 ) substituted for the FFAP column removed the alcohols. Thus chemical nature of the “missing” peaks can be assigned. Aromatic aldehydes were also removed from the gas stream by FFAP. A mixture of benzaldehyde, anisaldehyde, isosafrole, eugenol, veratraldehgde, and vanillin were chromatographed first on a 3-foot X ‘/*-inch 20% Carbowax 20RI at 220’ C., 60 ml. He per minute, then on a 3-foot, 20% FFAP under the same conditions. Comparison of the chromatograms showed only the isosafrole and eugenol peaks on the second chromatogram, and identified the other four peaks in the first chromatogram as aldehydes. The material that is responsible for the removal of the aldehydes is unknown. However, when a thermal conductivity detector is used, a water peak can be observed when aldehydes
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Figure 1.
Gas chromatogram of mixture
Column: Upper- 12-foot X ’/,-inch 2 0 % UCON Polar, 12-inch 20% Carbowax 20M, 210’ C., 6 0 rnl./min. He Lower- Same as upper except 12-inch 2 0 % FFAP instead of Carbowax Peak identification: 7 . Propanol 2. Hexonal 3. Heptanal f 2-heptanone 4. 2-Octonone 5. Nonanal 6. 1-Octanol 7. Decanal 8. Methyl caprate 9. 1-Decanol
are injected into an FFAP column. Also, the reaction must be comparatively slow; when a mixture of shortand long-chain aldehydes were injected into a 6-inch FFAP column only a small part of the short-chain, fast-moving, aldehydes came through the column whereas all of the longer-chain aldehydes were retained. Thus it is necessary to use a removal column long enough to allow complete reaction of the aldehydes. Also, with continued use, the ability of
the FFAP column to remove aldehydes is decreased (1). Thus a freshly prepared column should be used for the best effect. LITERATURE CITED
(1) Hammarstrand, Kent, Varian Aero-
graph Co., private communication, January 1966. (2) Ikeda, R. M., Simmons, D. E., Grossman, J. D., ANAL. CHEM.36, 2188 (1964). (3) Wilkens Instrument BE Research, Aerograph Research Notes, p. 3, Fall 1964.
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