Dear Wilmer,

Peak 28 cames also from CO+ due to dissotiation of CO2!!!
To proove N2+ simply check peak N+ at mass 14, otherwise
you will see peaks from C+ and O+ at masses 12 and 16,
respectively.

Good  luck to you in identification of your peak 28!

Stan

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Professor Dr. Stanislaw Halas
Mass Spectrometry Laboratory
Maria Curie - Sklodowska University
20-031 Lublin, POLAND
tel: (48 81) 537 6275    fax: (48 81) 533 3669