208 

 Cryo-focusing GC 



Cryo-focusingwas employed for the majority of analyses in this chapter. The 

 need for cryo-focusing to narrow sample component bands is due to the wide 

 desorption profiles from simple thermal desorption of components. Liquid nitrogen 

 (LN2, b.p. -196°C) is used to prevent further migration of component bands beyond 

 the section of column immersed in LNj. In actuality, some of the higher boiling 

 components may reside in the first 15 cm of column above the LNj. The column 

 oven is set at 25°C; therefore, the high boiling components may be focused in this 

 section. These will, however, proceed through the column upon the ramping the 

 oven. No apparent broadening of peaks due to this effect was observed. The 

 advantage of using cryo-focusing is that it does not introduce discrimination of 

 sample compound classes. Essentially, the observed compounds are those which 

 were transferred to the glass beads and simply heated off without loss of particular 

 compound classes. The drawback to this method are contaminant peaks. These 

 possibly arise from the septum of from the breakdown of the stationary phase at the 

 column entrance located in the injection port. These contaminants may be cryo- 

 focused and detected along with the sample components. The high abundance of 

 acids dominate the mass spectra and obscure some of the trace components, making 

 identification of the trace components difficult. 

 Purge and trap GC 



The use of microscale purge and trap discriminates against acids and other 

 more polar compounds, but this is beneficial when used to complement information 



