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A range of other non-carbon sorbents are available for specialised applications including:
Activated silica gel: Used primarily in UNITY and UNITY 2 cold traps for
on-line H2S monitoring. Silica gel is not usually used in sorbent tubes. It is exceptionally hydrophilic, absorbing water up to about 40% by weight.
Quartz beads: Quartz beads are completely inert and are used primarily in UNITY, UNITY 2 and TT24-7 cold traps when monitoring high boiling, chemically active compounds such as chemical warfare agents. Quartz
beads can be used in sorbent tubes in place of quartz wool if required.
Quartz wool: Quartz wool is often used in sorbent tubes as the first ‘sorbent’ in a multi-bed sorbent tube when the range of compounds of interest includes very high boiling compounds. It is important that the quartz wool fibres are of the correct diameter; coarse grade quartz wool has a high proportion of broken fibres within it and these fragments can reduce the overall inertness of the material. If total inertness is required e.g. when analysing extremely labile compounds such as mercaptans, then silanised glass wool is recommended.
Silanised glass wool: Silanised glass wool is used in sorbent tubes and traps when analysing extremely labile compounds e.g. mercaptans. However, it has a maximum recommended operating temperature of 250°C because residual silanising reagent can break down at high temperatures (>275°C), coating the instrument flow path with active degradation products and limiting its performance, especially with regard to high boilers.
Unsilanised glass wool: Unsilanised glass wool can be used in sorbent tubes where inertness is not an issue. It must be rigorously conditioned prior to use.
Activated charcoal: Activated charcoal is prone to causing analyte
degradation and is only recommended for use in split flow filter tubes and not as a sorbent within sampling tubes.
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