216 

 process competes with fragmentation in the negative ion mode; therefore, simple 



bond cleavages will be favored over more complex rearrangements. Due to 



autodetachment, only ions with low energy and relatively long lifetimes will be 



recorded [72,107]. Positive ions can retain this charge for longer times and can only 



lose a positive charge via abstraction of an electron or proton from collision with a 



neutral [107]. 



Despite these aforementioned limitations, ECNCI can still provide 



information complementary to or not accessible with PCI. The CI reagent gas can 



function in three ways for negative ion analyses [72]. It can produce thermal 



electrons for EC processes described above, produce charge transfer reactions (Eq 



5-13) provided the EA of the sample molecule is greater than the EA of the reagent 



gas molecule, and it can allow for true CI reactions (Eq 5-14 and Eq 5-15) between 



the reagent ions and sample molecules to occur: 



X-' + M ^ M-' + X charge transfer 5-13 



[X-H]~ + M -> [M-H]~ + X proton abstraction 5-14 



X~ + M ^ MX- adduct formation 5-15 



The term NCI is commonly used to indicate ion/molecule reactions between 



the reagent gas and sample. Electron capture reactions are usually referred to as 



ECNCI since these involve a different process than NCI ion/molecule reactions [76]. 



The problem with many reagent gases, including methane and isobutane, is that the 



spectra produced are difficult to classify as either ECNCI or NCI [76]. When using 



lower energy electrons, an [M-H]~ ion is typically observed in place of MX . In 



