116 

 If the dimer ion at m/z 179 is selected as the parent ion and fragmented by 



CID (figure 3-7), it exhibits similar neutral losses as the [M-H]~ at m/z 89, i.e. loss 



of HjO to form m/z 161, loss of CO to form m/z 151, loss of CO, to form m/z 135, 



as well as the presence of m/z 87. The similarity in neutral losses indicate that 



identical cleavages occur from the negatively charged carboxyl terminal site. The 



formation of m/z 87 from the dimer ion at m/z 179 occurs via a loss of 92 Da, as was 



mentioned for the m/z 177 dimer ion. 



This can be confirmed by examining the parents of the [M-H-H,]" species at 

 m/z 87 (figure 3-8). Both m/z 179 and m/z 177 dimer ions will produce this daughter 

 ion; however, the m/z 175 dimer ion can also lose 88 Da (neutral lactic acid 

 molecule which has undergone H, elimination) to form the m/z 87 ion. An ion at 

 m/z 133 is also present; this is formed via the neutral loss of CO2 from the m/z 177 

 ion analogous to the formation of m/z 135 from the m/z 179 dimer ion (figure 3-7). 



These dimer ions and fragment ions discussed thus far are present in NCI 

 mass spectra as well as MS/MS daughter spectra produced by CID, provided there 

 is sufficient collision energy. An example of the variability of CID daughter spectra 

 is evident by comparing figure 3-9, acquired with nominally eV collision energy, to 

 figure 3-7. Figure 3-9 displays only the mass-selected parent ion at m/z 179 and the 

 daughter ion at m/z 89. This implies that the neutral loss of lactic acid from the 

 dimer ion to regenerate the lactate ion is a lower energy process than fragmentations 

 associated with lactate. 



