An alternative approach is to use t

An alternative approach is to use tubes containing single sorbents of increasing strength connected together in series with the tube containing the weakest sorbent first in line. However, this is an inefficient approach with regard to the resources required for sampling and analysis.Pre-packed as well as pre-packed and preconditioned sorbent tubes are available commercially. Alternatively, sorbent tubes can be filled in the laboratory as specified in 6.1.D.4.2 Capillary GC column. Follow the specifications given in 6.8. Thicker film and/or longer capillary columns may be required if VVOCs are of interest.D.4.3 Thermal desorption apparatus. Simultaneous analysis of compounds covering a wide volatility range, VVOCs, VOCs, and SVOCs, is facilitated by using more than one sorbent These are used in series in order of increasing strength, both in the sample tube and in the focusing trap, and by using backflush desorption. Backflush means that the flow of gas used to desorb the sample from the sorbent tube or focusing trap have to be in the reverse direction to that used during sampling or trapping. In this way, higher boiling compounds are trapped on and released (desorbed) from the front (weaker) sorbents and never come into contact with the rear (stronger) sorbents.