75 

 best separation achieved at the time for the analysis of beads, greatly improved over 



direct thermal desorption. The second point is that the peak shapes demonstrate 



that the column is overloaded and that a reduction in the number of beads analyzed 



is necessary for better chromatographic peak shapes. The components found in 



lesser abundance in the sample show less fronting, i.e. evidence of more abundant 



components overloading the column sample capacity. 



Figure 2-14 demonstrates this point by the examination of characteristic 

 cholesterol ions. These ions are the [M-H]~ ion at m/z 385, the M"* ion at m/z 386 

 and the carbon- 13 isotope of the M~' ion at m/z 387. The peak shape near scan 

 number 800 is a significant improvement over previous experiments where 

 cholesterol was detected (including the thermal desorption profiles shown in figure 

 2-5). 



Reducing the number of beads to from 12 to 5 and employing a slightly longer 

 polar (Carbowax) column further improved peak shape, as can be seen in figure 2-15. 

 A Carbowax column was effectively used previously for thermal desorption and 

 sniffing mass spectrometric fruit odor analysis [69]. This figure demonstrates an 

 analysis employing PPINICI, to allow for detection of both positive and negative ions 

 via alternating scans between positive and negative ions [70]. The complementary 

 EI analysis of 8 rubbed beads is found in figure 2-16. The data in this figure was 

 acquired with a slightly slower temperature ramp of the GC oven. Comparison 

 between the positive and negative ion CI RICs in figure 2-15 and the RIC for the 

 EI analysis in figure 2-16 shows that peaks can be clearly correlated between the two 



