The commercial SPME device manufactured by Supelco (Belle- fonte, USA) is presented in Figure 1. The fi ber glued into a piece of stainless steel tubing is mounted on a special holder. The holder is equipped with an adjustable depth gauge, which makes it possible to control repeatedly, how far the needle of the device penetrates the sample container or the injector.
This is important, as the fi ber can break if it hits an obstacle. The movement of the plunger is limited by a small screw that moves in the z-shaped slot of the device. For protection during storage or septum piercing, the fi ber is with- drawn into the needle of the device, with the screw in the uppermost posi- tion. During extraction or desorption, the fi ber is exposed by depressing the plunger. The plunger is moved to its lowermost position only for replacement of the fi ber assembly. Each type of fi ber has a hub of a different color.
Figure 1: The SPME device
If the sample is in a vial, the septum of the vial is fi rst pierced with the needle (with the fi ber in the retracted position), and the plunger is lowered, which exposes the fi ber to sample. The analytes are allowed to partition into the coating for a pre-determined time, and the fi ber is then retracted back to the needle. The device is then transferred to the SPME instrument. When gas chromatography is used for analyte separation and quantifi cation, the fi ber is inserted into a hot injector, where thermal desorp- tion of the trapped analyte takes place.
For spot sampling, the fi ber is exposed to a sample matrix until partitioning equilibrium is reached between sample matrix and the coating material. In the time average approach, on the other hand, the fi ber remains in the needle during exposure of the SPME device to the sample. The coat- ing works as a trap for the analytes that diffuse into the needle, resulting in integral concentration over time measurements.
SPME sampling can be performed in three basic modes: direct extraction, headspace trapping, and extraction with membrane protection.
Figure 2: Modes of SPME operation: direct extraction (a), headspace trapping (b) and membrane-protected SPME (c)
In direct extraction, the coated fi ber is inserted into the sam- ple and the analytes are transported directly from the sample matrix to the extracting phase. To facilitate rapid extraction, some agitation is required to transport the analytes from the bulk of the sample to the vicinity of the fi ber. For gaseous samples, natural fl ow of air (e.g. convection) is usually suffi cient to facilitate rapid equilibrium for volatile analytes.
In headspace mode, the analytes are extracted from the gas phase equilibrated with the sample. The primary reason for this modifi ca- tion is to protect the fi ber from the adverse effects caused by non-volatile, high molecular weight substances present in the sample matrix (e.g. human acids or proteins). Here, the amount of an analyte extracted by the fi ber coating does not depend on the location of the fi ber, in the liquid or gas phase; therefore, the sensitivity of headspace trapping is the same as that of direct sampling as long as the volumes of the two phases are the same in both sampling modes. When no headspace is used in direct extraction, a signifi cant sensitivity difference between direct and headspace trapping can occur only for very volatile analytes. However, the choice of sampling mode has a signifi cant impact on the extraction kinetics. When the fi ber is in the headspace, the analytes are removed from the headspace fi rst, followed by indirect extraction from the matrix.
In general, the equilibration times for volatile compounds are shorter for headspace SPME than for direct extraction under similar agita- tion conditions, because of the following reasons: a substantial portion of the analytes is present in the headspace prior to the beginning of the ex-
traction process; there is typically a large interface between sample matrix and headspace; and the diffusion coeffi cients in the gas phase are typi- cally higher by four orders of magnitude than in liquids. The concentration of semivolatile compounds in the gaseous phase at room temperature is small, and headspace extraction rates for these compounds are substan- tially lower. They can be improved by using effi cient agitation or by increasing the extraction temperature.
In the third mode (SPME extraction with membrane protection), the fi ber is separated from the sample with a selective membrane, which lets the analytes through while blocking the interferences. The main purpose for the use of the membrane barrier is to protect the fi ber against adverse effects caused by high molecular weight compounds when dirty samples are analyzed. While headspace trapping serves the same purpose, mem- brane protection enables the analysis of less volatile compounds. Use of thin membranes and an increase in extraction temperature result in shorter extraction times.