18.104.22.168.1. Thermal desorption of static headspace volatiles by SPME
SPME (solid phase microextraction), is the most common thermal desorption method (Augusto and Valente, 2002). The SPME technique includes exposing a fibre to odour source headspace in a static environment with little or no air flow. SPME is ideal for rapid analysis of volatiles emitted by small and strong odour sources, preferably in a closed container. Because of its ease of use, many researchers have begun to use SPME fibres in honey bee systems (Gilley et al. 2006; Schmitt et al., 2007; Maisonnasse et al., 2010). SPME has rather uniquely been used repeatedly to sample odours in situ from single bees in the open comb environment (Thom et al., 2007).
- Insert the SPME holder in the container with the
Volatiles should reach equilibrium (which should be in trial sampling and analysis) in the closed container before the SPME fibre is exposed to the headspace.
- Expose the SPME fibre to the headspace without the
fibre touching the sample or container
The fibre should ideally be allowed to reach equilibrium with headspace volatiles – typical adsorption times ranging from a few seconds to 30 minutes.
- Retract the fibre into the protective sheath.
- Inject the fibre into a splitless GC injection port for analysis (see section 2.2.3.)
- Trapped volatiles must be desorbed rapidly after collection since the trapped volatiles are exposed to heat and carrier gas flow and desorb off the fibre onto the column head as the fibre is heated.
The selection of SPME fibre type influences the sensitivity toward specific compound groups as well as the exposure time required to reach sorption equilibrium (Augusto and Valente, 2002). Polydimethylsiloxane (PDMS) fibres have been used to sample less polar volatiles and Carbowax/PDMS to capture more polar volatiles (Zabaras and Wyllie, 2002). Researchers should try a variety of related fibres to determine what works best for their system. For queen and worker volatiles, a number of fibre compositions were tested and PDMS/divinylbenzene was selected as the best (Gilley et al., 2006). Consider the following guidelines for use of SPME:
- Use enclosed or partially enclosed
static systems to limit dissipation of headspace volatiles (see Augusto and
Valente, 2002 for a design). In general, the more static and concentrated the
headspace volatiles, the more rapidly equilibrium is achieved.
- To limit background contaminants, only expose the fibre from its sheath when you are actively collecting volatiles. Protect the exposed fibre from the bees and hive materials with a Teflon jacket perforated with holes.
- Before sampling, bake the fibre in the GC port to remove residual volatiles left on the fibre. Follow the guidelines in the instructions that come with the SPME fibre.
- If unable to detect compounds of interest, try a longer equilibrium and exposure times first and different fibre materials next.
- Once specific chemicals of interest have been identified, optimize detection with chemical standards. Test equilibration time and sensitivity by exposing the fibre to each standard’s headspace for different periods of time. Fibres have reached minimal equilibrium time when the volatile capture no longer increases with exposure time.
Pros: SPME is easy to use
Cons: Collected compounds cannot be stored on the fibre for longer period before analysis. Researchers should take caution in over-interpreting the volatile profiles obtained with SPME. Fibres are easily contaminated by background volatiles that may not be present in the target odour source. SPME is also poorly suited for volatile quantification because the fibres have different affinities for different chemical classes (Agelopoulos and Pickett, 1998). Unfortunately, the adsorbance rate of each volatile can be significantly influenced by the other compounds present in the headspace (Romeo, 2009). There are methods to quantify SPME samples, but given the variable chemical affinities, it is difficult to calculate amounts with confidence (Augusto and Valente, 2002). Volatile emission rates are often expressed as relative emission ratios rather than absolute amounts. For these reasons, researchers should use other methods to quantify volatile emission rates (see sections 22.214.171.124 and 126.96.36.199).