DESORPTION EFFICIENCIES – HOW TO KEEP AIR SAMPLING MEDIA VARIATIONS FROM AFFECTING YOUR ANALYSES
When air samples collected on the sorbent tubes air sampling media are desorbed by solvent extraction, the contaminants of concern may not be released completely by the solid sorbent. The conditions of most methods are designed so that more than 95% of the analyte is released into the desorbing solvent and is available for quantitation. However, in some cases, a significant portion of the analyte does not go into solution, and the analytical result must be adjusted for this amount.
The adjustment is made by determining the desoption efficiency (DE) – the fraction of the analyte that is removed from the sorbent by the eluant – and applying this factor to correct the results. DEs are determined by spiking tubes with the analyte(s) of interest over a range of concentrations, eluting the analytes(s), analyzing the eluant, and calculating the ratio of amount recovered to amount spiked.
A number of techniques are available for tube spiking:
Air-generated spikes – Air of known concentration is passed through the air sampling tube for a known amount of time. • Liquid – generated spikes – Tubes are spiked with known amounts of analyte(s) as neat liquids or solutions. • Phase equilibrium method – The sorbent from a tube is added to an eluant containing known concentrations of the analyte(s) of interest.
DEs depend on several factors: the air sampling media or sorbent type (even different lots of tubes can exhibit different DEs), the solvent used for desorption, the ratio of the amount of sorbent to eluant, other compounds collected on the same tube, the temperature at which desorption is performed, and the desorption time.
Though most methods are designed to produce maximum DEs, the method-specific eluant may not be usable when analysis of a group of analytes is requested. Some analytes have concentration-dependant DE values; you may see a statement to that effect on your reports. For these analytes, the higher concentrations usually yield higher DE values.
You will see the DE values listed as footnotes on your reports. Your results have been corrected for this value, but only if we have performed the DE analyses on that lot of sorbent. If you are sending in tubes that were not purchased from Galson, you may be requested to send in some additional blank tubes so that we may verify the DE values.
If you have any questions about desorption efficiencies or whether you should send unused tubes with a particular sample, please call our Client Services department before you ship the tubes. They will be happy to address any questions you may have about air sampling and air sampling media.