108 



Oligomerization and attachment reactions 



Lactic acid in the gas phase is also susceptible to oligomer formation as well 

 as other associative ion-molecule reactions. Examining mass chromatograms (figure 

 3-4) for oligomer and attachment ions produced from lactic acid demonstrates the 

 presence of a dimer ion at m/z 179, a trimer at m/z 268, and a tetramer at m/z 357, 

 all of the form [M„-H]~ The time correlation of the peaks indicate that these ions 

 are related. The formation of oligomers is most prevalent at high concentrations of 

 neutral species in the ion source. Therefore, the oligomeric peaks have narrower 

 peak widths than their precursor ion. The presence of oligomers or at least 

 dimerization is predictable from knowledge of the solution-phase chemistry. Lactic 

 acid is an a-hydroxy acid. Due to the inability of this species to form a lactone ring 

 which is stable, it undergoes oligomerization and self-esterification [SO]. Further 

 examination of oligomeric ions will be addressed later in this section. 



Two additional ions yield peaks which correlate to lactic acid. These peaks 

 at m/z 125 and m/z 127 arise from chloride ion attachment to a neutral lactic acid 

 molecule. Verification of this is presented in the daughter spectrum in figure 3-5. 

 The m/z 125 ion was mass-selected and fragmented by CID to yield fragment ions 

 at m/z 35 and m/z 89. The m/z 35 ion is clearly the "Cl~ ion. By virtue of MS/MS, 

 this ion could only have arisen from the m/z 125 ion rather than being attributable 

 to background. The ion at m/z 89 is the deprotonated molecular species of lactic 

 acid (i.e. the lactate ion). This fragment is formed via the neutral loss of HCl from 



