In Beam EI spectra of amino acids are described. To obtain IBEI spectra a, sample was loaded on an end of the quartz tip of a modified direct inlet probe and inserted until the electron beam was interrupted. Except cystine and arginine IBEI spectra of amino acids so far studied always exhibited clearly M+1peaks. Relative intensities of M and M+1 peaks in IBEI and conventional EI spectra are listed in Table1. The complete spectrum was apparently a combination of a conventional EI spectrum and the M+1peak. This technique is quite useful for detection of M+1peaks of thermally unstable compounds such as creatine or glutamic acid of which conventional E1 spectrum scarcely exhibits the M or M+1peak. The spectral features are somewhat similar to those obtained from closed source instruments. In the case of phenylalanine, effects of measuring conditions such as source temper ature, ionization voltage, sample position and time factor on the ratio of[M+1]/[M]were examined. When a sample was mixed with ammonium chloride, the M+1peak in the IBEI spectrum was completely suppressed, which must be useful for identification of the M+1peak.