72 

 Thermal Desorption from Multiple Beads/Cryo-focused GC Separation 



Cryo-focusing GC/MS has been previously used in the detection of volatile 

 emanations from humans [68]. The collection and desorption of human emanations 

 was achieved by the use of charcoal, and re-focusing accomplished via the use of 

 three successively colder traps. Most of the experiments reported in this dissertation 

 employed a single stage of cold-trapping. TTie use of on-column cryo-focusing solved 

 the problem of wide desorption profiles due to inefficient compound removal from 

 the headspace above the beads in the previous series of experiments. Due to the 

 similarity in size of the 3" x 1/4" glass tube (use for multiple bead studies above) and 

 the glass injector insert in the GC injection port, experiments were conducted using 

 the insert as the sample holder. By reversing the injection insert, beads could be 

 placed into the insert. Using an insert with a glass frit allowed the beads to be 

 supported above the column entrance and more importantly, allowed free passage 

 of volatiles past the beads. Additionally the design of the injection port allows for 

 a directional sweep of helium over the beads and out of the injector through the 

 column entrance; the enclosed vials for multiple beads have less directional flow. 

 The additional advantage of using the injection port to initially desorb volatiles is 

 that the column could then be held at ambient room temperature with a section of 

 it cryo-focused while the beads were heated external to the GC oven. 



The injection insert, when inserted reversed, held up to 12 beads. Figure 2-13 

 is the RIC for the first such experiment involving the maximum number of beads 

 (12). Two points are evident when examining this figure. First, this was by far the 



