69 



are not present. However, this figure also illustrates that the thermal desorption 



profile is still less distinct than from the solids probe. In contrast to the solids probe, 

 where volatiles are desorbed directly into the ion source, the enclosed sample 

 container permits greater head space above the sample. This space allows for 

 volatiles to be desorbed and remain in the enclosed sample container, mixing with 

 additional desorbed volatiles as the temperature increases. Furthermore, the beads 

 in the vial are probably not at a uniform temperature. As a result, the distinction 

 between desorption profiles for different compounds appears simply as differences 

 in the initial appearance of masses, similar in appearance to frontal chromatography. 

 This point is illustrated in figure 2-12. 



Figure 2-12 depicts the mass chromatograms for four ions associated with 

 lactic acid. These representative ions shown are the loss of H, from the [M-H]~ ion 

 at m/z 87, which may also correspond to the [M-H]~ ion of pyruvic acid, the [M-H]~ 

 ion of lactic acid at m/z 89, the [M2-H]~ lactic acid dimer ion at m/z 179, and the 

 [M4-3H]^ lactic acid tetramer ion at m/z 357. The profiles of these ions are clearly 

 disbursed over most of the run once the temperature is sufficient for the desorption 

 of lactic acid. The wide profile makes identification of molecular and fragment ions 

 with identical m/z values virtually impossible without a reduction in the profile width 

 by either more efficient loading of desorbed species onto the column or by re- 

 focusing the sample bands either prior to or directly on the column. 



