Characteristics of in-beam electron impact ionization mass spectra of several amino sugars, which give barely detectable molecular ion peaks by the use of conventional direct inlet probes, are described. The spectra of these compounds almost always exhibited the M + l peak and the spectral features were apparently a combination of those of the conventional electron impact spectrum and some major peaks originating from the M + l ions. In the case of neamine, effects of the measurement conditions such as the source temperature, ionizing voltage, the position of the sample and time factors on the intensity of the M + 1 peak were examined. When the samples were mixed with ammonium chloride, the M + l peaks in the in-beam spectra were completely suppressed, while with aqueous ammonia, the abundance of the M + 1 peaks increased by several times.

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