167 

 This approach provided positive compound identification; however, it should 



be noted that analyses in this manner are tedious. Daughter spectra, once acquired, 



need to be interpreted to determine the suspected identity of the compound. To 



confirm the identity by comparison to standard, the standard which may not be 



readily available is necessary. Once the comparison is conducted, the daughter 



spectra may not match, leading to further examination and interpretation with 



subsequent comparisons to additional standards. 



Another disadvantage to this method of compound identification, as stated 

 earlier, is the presence of multiple compounds at any point in the desorption. It is 

 therefore possible that a particular parent ion m/z may correspond to ions from more 

 than one component (just as two or more components may coelute during GC 

 operation). An example where the daughter spectrum includes ions from the parent 

 ions of two components is shown in figure 4-2. Selection of negative ion at m/z 72 

 by Ql and CID fragmentation produced a daughter spectrum revealing the presence 

 of ^^Cl~ and ^^Cl~ isotope peaks, as well as losses of 30 Da, 28 Da, and 1 Da. The 

 parent ion at m/z 72 thus includes both Clj" and one (or more) other compounds. 



There are some cases where spectral interpretation alone was used to identify 

 compounds; the presence of most of these compounds was later confirmed by the 

 GC/MS data contained in Chapter 5. The presence of propanoic acid (figure 4-3) 

 and propenoic acid (figure 4-4) is easily determined. The [M-H]~ ions of both 

 compounds undergo a neutral loss of CO2 (44 Da), which is characteristic of 

 carboxylic acids (characteristic neutral losses will be discussed further in the next 



