Mobility spectra for organic compounds at temperatures of similar to 50 deg
reesC and 175-250 degreesC were categorized by chemical class using back-pr
opagation neural networks with the successful classification even of chemic
als not familiar to the networks. Network performance suggested that chemic
al class information in spectra at similar to 50 degreesC differed from tha
t in spectra at high temperatures, prompting a detailed analysis of regions
where chemical class information was located. These regions, or drift time
s in the mobility spectra, were identified at each temperature using a meth
od of incrementally removing portions of spectra so that the value or struc
tural content of the subtracted region could be seen in comparisons of netw
ork performance. At high temperatures (175-250 degreesC), chemical class in
formation was contained in fragment ions located in a narrow region of the
spectra with reduced mobilities of 3.06-2.11 cm(2) V-1 s(-1) corresponding
to the drift times near and encompassing the reactant ions peaks. In contra
st, spectra at low temperature (similar to 50 degreesC) were classified thr
ough fragment ions that resided in a broad region of drift time between pro
tonated water clusters and product cluster peaks. This corresponded to redu
ced mobilities of 1.8-1.2 cm(2) V-1 s(-1). These findings suggest that frag
mentation in ion mobility spectrometry and other atmospheric pressure chemi
cal ionization based methods, with moisture <1 ppm, may be more common than
previously understood. Class specific fragmentation reactions for ions at
low temperature have never been described in LMS and became evident only wh
en mobility spectra were formatted with a logarithmic axis for ion intensit
y. <(c)> 2001 Elsevier Science B.V. All rights reserved.