Dear Peter,
as far as I understand it, the main problem in negative ion mode is not a
lower sensitivity but a lower S/N ratio originating from a much higher
electronic background caused by electrons. Therefore the unambigious
assignment of a signal will become much harder in comparison to positive
mode if the sample amount is limited. If the amount of sample is high
enough, signal intensity should be comparable in both modes (I had a sample
of a 9 kDa oligo last week, which yielded almost identical spectra in
positive as well as in negative mode on MALDI using DHB as matrix). The
only reason for different sensitivity I could imagine is a variation in ion
transmission efficiency when switching from positive to negative mode
(maybe there are other reasons as well, but I am a biochemist not a
physicist...), but I think this is rather unlikely.
Yours sincerely,
Marcus Macht
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Dr. Marcus Macht
University of Cologne
Centre for molecular medicine - Service laboratory
Joseph-Stelzmann-Str. 52
50931 Cologne, Germany
Tel.: +49 221 478-6995
Fax: +49 221 478-6977
e-mail: Marcus.Macht@uni-koeln.de
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