Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with ethers

Chemical ionization mass spectra of several ethers obtained with He/(CH₃)₄Si mixtures as the reagent gases contain abundant M + 73]⁺ adduct ions which identify the relative molecular mass. For the di-n-alkyl ethers, these M + 73]⁺ ions are formed by sample ion/sample molecule reactions of the fragment ions, M + 73 ? C_nH_2n]⁺ and M + 73 ? 2CnH_2n]⁺. Small amounts of M + H]⁺ ions are also formed, predominantly by proton transfer reactions of the M + 73 ? 2CnH_2n]⁺ or (CH₃)₃SiOH₂]⁺ ions with the ethers. The di-s-alkyl ethers give no M + 73] ⁺ ions, but do give M + H]⁺ ions, which allow the determination of the relative molecular mass. These M + H]⁺ ions result primarily from proton transfer reactions from the dominant fragment ion, (CH₃)₃SiOH₂]⁺ with the ether. Methyl phenyl ether gives only M + 73]⁺ adduct ions, by a bimolecular addition of the trimethylsilyl ion to the ether, not by the two-step process found for the di-n-alkyl ethers. Ethyl phenyl ether gives M + 73]⁺ by both the two-step process and the bimolecular addition. Although the mass spectra of the alkyl etherr are temperature-dependent, the sensitivities of the di-alkyl ethers and ethyl phenyl ether are independent of temperature. However, the sensitivity for methyl phenyl ether decreases significantly with increasing temperature.