Sampling using sorbent tubes – Diffusive or pumped?
Friday, 14 December 2012 at 4:27:PM
Sampling of VOCs and SVOCs using sorbent tubes for analysis by thermal desorption–gas chromatography is ideal for a wide range of applications from monitoring industrial air to odour profiling of foods. However, for those new to the area, all the possible sampling options can make it difficult to decide how best to proceed.
In the first of a series of blog posts about sorbent-tube sampling, Dr Nicola Watson, Environmental Monitoring Specialist at Markes, considers the pros and cons of diffusive and pumped sampling.
Speaking generally, pumped (or active) sampling is both quicker and applicable to a wider range of situations than diffusive (or passive) sampling. However, diffusive sampling using axial tubes has long been popular for occupational and environmental monitoring, as the timescale of sampling fits in with the need to identify exposure to chemicals over longer periods.
In addition, diffusive samplers are easy to set up and require no intervention during sampling, making them a relatively low-cost option. One upshot of this is that diffusive monitoring has changed from being a niche technique primarily used for occupational exposure monitoring, to being applied in a number of large-scale urban/rural air-monitoring projects, as well as longer-term indoor air measurements.
It may be that standard procedure (or previous experience) dictates which method to use. However, if this is not the case, then it is usually best to approach the decision by considering the following points.
Question 1: Do you know the compounds that you’re looking for?
Knowing this is essential if using diffusive sampling, as you need to decide on the most appropriate sorbent. For quantitative diffusive monitoring, you will also need to know the validated uptake rate for the compound on that particular sorbent. If you don’t know the compounds you’re looking for, then it is best to use pumped sampling with a multi-bed sorbent tube. This way, analytes of the widest possible volatility range will be collected at a known flow rate and sample time – enabling quantitation to be carried out.
Question 2: Do you know the expected concentration of the compound(s) in the test atmosphere?
Whether pumped or diffusive sampling is chosen, it is essential that the expected concentration of the analyte in the atmosphere will result in a mass on the tube that is above the limit of detection of the TD–GC(MS) method. In addition, for diffusive sampling, the concentration will help determine the sorbent to use and the exposure time of the sampler – although it’s worth noting that diffusive sampling is not really suitable for trace concentrations (below about 1 part per billion).
Question 3: Are the compounds of different volatility?
Finally, for diffusive sampling the compounds you want to detect mustn’t differ too widely in volatility, as they have to be trapped on a single sorbent bed (multi-bed sorbent tubes cannot be used for diffusive sampling). If you’re interested in compounds spanning a wide range of volatilities, then pumped sampling using multi-bed sorbent tubes is the better option.