Sampling using sorbent tubes – Choosing tubes and sorbents

Use of sorbent tubes

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 to proceed.

What type of tube should I use?

Stainless steel tubes are suitable for the vast majority of compounds analysed by thermal desorption, but if you’re analysing highly reactive compounds (such as methanethiol and isobornyl methacrylate), make sure they’re inert-coated.

Glass tubes are also classed as inert, but because of their thicker walls, they have a narrower internal diameter and therefore contain less sorbent, which can reduce the safe sampling volume. They are also only suitable for pumped sampling, firstly because the diffusive path length is not fixed, and also because they have a small amount of quartz wool at the front end, so the sorbent bed itself is not exposed to the test atmosphere. Therefore stainless steel tubes are the only option for axial diffusive sampling.

What should I consider when deciding on the sorbent?

Analyte boiling point is the key factor when deciding on which sorbent type to use, with the following being a rough guide:

• Use a weak sorbent, such as a porous polymer sorbent, when working with compounds boiling above 100°C.
• For compounds boiling between 30°C and 100°C, use a medium-strength sorbent, such as a graphitised carbon black.
• Always use a strong sorbent, such as a carbon molecular sieve, for compounds boiling in the range –48°C to 30°C.
• Finally, compounds with boiling points below –48°C are typically too volatile for ambient-temperature sorbent sampling.

However, you also need to bear in mind the compatibility of compound and sorbent. For example, reactive species such as methanethiol tend to require the more inert sorbents (so avoid graphitised carbon blacks), while polar species are more compatible with porous polymer sorbents. If you’re unsure, it is a good idea to seek the advice of the manufacturer to see what would work best for the particular compounds you’re interested in.

Can I use more than one sorbent in a tube?

Yes – with pumped sampling, a common practice is to combine up to three sorbents, to increase the volatility range that can be sampled. In such cases, a good choice for reliably sampling a wide analyte range (about C₃to n-C₃₂) would be separate beds of porous polymer, graphitised carbon black and carbonised molecular sieves.

If using multiple beds, the weakest sorbent must always be at the front (sampling) end of the tube, so that the less volatile compounds don’t become stuck on the strongest sorbent. Naturally, the thermal desorption system must be capable of desorbing compounds in the opposite direction to which they were sampled (so-called backflush operation) – this is standard with Markes systems.

Multi-bedded tubes able to screen a wide range of compounds are definitely growing in popularity, especially for indoor air applications. It is worth remembering, though, that your GC column and conditions should be optimised to ensure maximum separation. Even if you end up with some co-elution, however, GC/MS deconvolution software packages like TargetView can greatly aid compound identification.

I’ve got lots of tubes to keep track of. What options are there?

There are three main options for keeping track of sorbent tubes, the most basic of which is of course the serial number. However, many commercial sorbent tubes now come supplied with barcodes, which allow simple tracking of the tubes, for example using a laboratory information management system (LIMS). Just make sure that the barcode is permanently etched on rather than simply applied as a sticky label – these can degrade during handling and analysis, and the VOCs in the glue can contaminate your sample (especially if sampling diffusively).

A more versatile approach is to use radio-frequency identification (RFID) tags. These attach to the sorbent tube and hold comprehensive information on its ‘life history’ – for example, the date it was packed, the sorbent type, and the number of thermal cycles it has been subjected. They can also hold data about the current sample, such as its name, the sampling method, and details of flow rates and sampling times. Such tags can be read by the tube autosampler, and offer the additional advantage of allowing the analyst to keep an audit trail.