Solid-phase microextraction

Solid-phase microextraction (english solid phase microextraction, SPME ) is a method of sampling and Analytenanreicherung which proved in some areas of chemical analysis to be advantageous over traditional methods such as " purge and trap " or SPE ( solid phase extraction, solid phase extraction ) has. It was developed by Janusz Pawliszyn since 1990.

Worldwide, the sole licensee for solid-phase microextraction technology is Supelco ® (U.S. Patent 5,691,206; European Patent EP523092 ).

Construction of a sampler

A schematic illustration (bottom, not to scale ) shows the structure of a SPME sampler again.

The sampler consists mainly of four parts:

• One guide for a piston. The guide has a bayonet fitting around the plunger to lock in the depressed condition. The two separate parts of the guide can be adjusted against each other by means of a thread, so as to enable a more or less great penetration depth of the needle into a protective sample vessel.

• A piston for extending the sampler. At the end of the piston there is a spring which pushes the piston back again when the locking is released.

• The actual, screwed to the piston sampler. It consists of a stainless steel needle, to which a quartz fiber ( "fused silica " ) or fiberglass ( " StableFlex " ) is attached. These quartz fiber is coated with a thin layer of adsorbent.

• A protective cannula, through which the fiber is guided, due to their low mechanical stability.

Sampling

First, the membrane is pierced piercing ( septum ) of the sample vessel with the protective cannula. Then the fiber is extended by depressing the plunger. Once the adsorbent is exposed, the analyte molecules are adsorbed. This leads to a strong enrichment of the analytes relative to the sample. Here, the adsorbed amount in a first approximation linearly proportional to the concentration in the sample. In the figure below, the adsorption from the gas phase (" Headspace Sampling" ) is shown. Analog but this is also possible with the adsorption from the liquid phase ( "Immersion Sampling" ).

After equilibration, the locking of the piston is achieved and the fiber is retracted into the needle guard, so that the adsorption will be interrupted. Then the sampler is removed. The adsorbed amount hardly changes even in the retracted state in the course of hours, which allows easy transportation of the sample to a laboratory. This is followed by the actual analysis. For this, the probe is pushed through the septum into the injector of a gas chromatograph. Then the fiber is extended. Due to the high temperature in the injector, it is then for the desorption of the analytes. Then the fiber is again retracted and the sampler away. Desorption can alternatively be carried out by extraction in a modified injector port of a HPLC unit.

Analogy to nature

In nature, the sense of smell of the snakes works very similar: the tongue snaps out of his mouth, odors are adsorbed by the moist surface, the tongue is pulled back into the mouth, the odors are transferred to the Jacobson'sche organ and analyzed only there.

Reusability

With proper treatment, a fiber can be used without problems for at least 50 analyzes. This assumes that the fiber is not subjected to mechanical load (over), it is regularly cleaned by heating in the injector of a gas chromatograph or a suitable conditioning station and they do not come into contact with aggressive chemical solvents that attack the adsorbent.

Sampling conditions

The optimization of the sampling conditions requires the variation of different parameters. Always apply the general principle that good results can be achieved only by careful adherence to good practices.

Adsorbent

Commercially, five adsorbents ( in decreasing polarity: polyacrylate, Carbowax, polydivinylbenzene, Carboxen and PDMS, polydimethylsiloxane) available in multiple combinations and layer thicknesses for different applications. The rule of thumb is that polar analytes should be analyzed with polar adsorbents and non-polar analytes using non-polar adsorbents. For high molecular weight analytes lower layer thicknesses are recommended.

Adsorption time

Time adsorbate curve is not linear, but asymptotically. For reproducible measurement results should be used near the equilibrium. If different analytes occur in highly variable or very high concentrations, can cause displacement phenomena on the fiber. Under these circumstances, can also be carried out in the pre - equilibrium, one must then but definitely pay attention to precise time control. When trace analysis is needed longer adsorption times usually.

Adsorption

For headspace analysis of semi-volatile compounds from solid or semi-solid samples, the sample vessel is heated to a temperature-controlled heating block in the rule. Temperature data in the literature vary from 40-65 ° C. In the SPME analysis of liquid samples, the extraction can be improved by setting a suitable pH value, and by saturation with sodium chloride. Is absolutely necessary in intensive agitation of the sample ( 250 rpm ) to compensate for the lower compared to the gas phase diffusion coefficient. The stirring speed must be kept constant also. This applies both to measurement by headspace as well as by immersion.

Desorption

Desorption takes place as a rule in the injector of a gas chromatograph at a temperature between 200 and 280 ° C. In this case, five minutes is usually sufficient. A check can be made by extending the desorption under otherwise constant adsorption and applying the integrals of analyte peaks against the desorption time. One obtains a line parallel to the X-axis, the desorption is complete. For desorption in the injector port of an HPLC system, proceed analogously.

Benefits

Although one might think differently after the above, the time required for the optimization of the sampling conditions by drastically shortening the time required for sampling is more than compensated. Moreover, automation is possible. Overall, this significant time savings (> 70% per sample) is obtained for routine analysis.

The complete absence of organic solvents is opposite the liquid / liquid extraction a significant ecological and economic advantage, since no subsequent disposal of the solvent is necessary. To the risk of cross-contamination is eliminated by a solvent.

The avoidance of complex sample preparation means that samples are less altered. This is particularly the case of sensitive analyte of significance. SPME can be used both for gaseous and liquid samples. In width, this does not apply to " purge and trap " or liquid-liquid extraction. A similar process of SPME, the Stir Bar Sorptive Extraction dar.

Disadvantages

Due to the small quantities that are adsorbed by the fiber, this method is suitable, unlike, for example, the SPE, not for the preparative isolation of the analytes. Similarly, the recoverable enrichment factors compared to the SPE or SBSE are lower.

Areas of application

• Environmental analysis ( eg determination of herbicides in drinking water )
• Food analysis ( eg determination of photogenerated degradation products in milk)
• Aroma analysis ( eg determination of the odorants in fragrance of flowers. In a special case literally capture the scent of a single flower on board the Space Shuttle )
• Forensics ( eg determination of amphetamines in urine)
• Sample introduction in gas chromatography (special headspace technology )