Gas Chromatographic Analysis of Polycyclic Aromatic Hydrocarbons in Soot Samples SIR:In connection with the exchange of letters between Thomas and Monkman (1968) on the one hand and Long and Chakraborty (1968) on the other concerning an unknown “derivative of pyrene” isolated from soot by the latter authors, I wish to report that an apparently identical compound has been isolated from a coal tar pitch. The absorption spectrum of this substance (Figure 1) has the seemingly characteristic peaks at 378 and 358 mp found by Chakraborty and Long (1967). A general shift of 1 to 2 mp to lower wavelengths of all the maxima in the present spectrum is not a serious discrepancy and, in any case, is probably accounted for by the use of isooctane (2,2,44rimethylpentane) as a spectral solvent rather than cyclohexane. However, the small peak at 287 mp must at present be considered to be due to an impurity. Based on this spectrum, on the chromatographic behavior, and, especially on the molecular weight as determined by mass spectrometry (Figure 2), it is proposed that the compound in question is cyclopenta[c,dpyrene (or acepyrene). The structure of this compound is shown in Figure 2.
Figure 1. Ultraviolet absorption spectrum in isooctane of a substance Isolated from the benzofluorenefraction of coal tar pitch
Acepyrene was isolated from a coal tar extract after paper partition chromatography (N,N-dimethylformamide-isooctane system) of alumina column chromatography fractions containing, preponderantly, a mixture of benzofluorenes. It is eluted, therefore, well after the methylpyrenes and just before benz[a]anthracene and -chrysene. This behavior is entirely consistent with that to be expected from a compound of the structure shown. Although gas chromatography has not been used in this isolation, it is evident that a compound of molecular weight 228 could easily be incompletely resolved from the chrysene group, which is of the same weight. In the paper chromatogram, acepyrene forms the leading edge of a band containing mostly benzofluorenes. The relatively fast movement of this substance (with a greater R, than shown by monomethylpyrenes) is also expected 6f a polycyclic aromatic hydrocarbon alkyl-substituted in two positions. In fact, were the molecular weight not so clearly 228, one would be strongly tempted to say the unknown is a dimethylpyrene. As shown in Figure 2, the mass spectrum possesses peaks at m/e values greater than 228. Those greater than 229 are almost certainly due to impurities. In our experience the parent ion peak in the mass spectrum of polycyclic aromatic hydrocarbons and their simple alkyl derivatives is always strong. The relative prominence of the 226 and 227 mass peaks is not unexpected, since the loss of one hydrogen (from the alicyclic ring) should produce a fairly stable carbonium ion, while the loss of a second hydrogen would give another compound of presumably great stability. The height of the 229 peak is close to what would be expected from the 1sC contribution to a Clg hydrocarbon. Cyclopenta[c,dlpyrene is the only compound of reasonable structure that can have a molecular weight of 228 and the absorption spectrum and chromatographic behavior of an alkyl pyrene derivative. It is by no means a trace component, for 5 to 10 pg. were isolated from 5 mg. of coal tar pitch, indicating a concentration of the order of 0.1 %. Literature Cited Chakraborty, B. B., Long, Ronald, ENVIRON. SCI. TECHNOL. 1, 828 (1967). Long, R., Chakraborty, B. B., ENVIRON. SCI. TECHNOL. 2, 217 (1968). Thomas, R. S., Monkman, J. L., ENVIRON.SCI. TECHNOL. 2, 217 (1968).
5 I50
200
m/e 250
3 00
Figure 2. Ma& spectrum and proposed structure of a substance isolated from benzofluorenefraction of coal tar pitch 948 Environmental Science & Technology
Lawrence Wallcave
Eppley Institute for Research in Cancer Omaha, Neb. 68105 Received for review March 24, 1969. Accepted July 15, 1969.
