1.1 Sample Preparation in Analytical Process
Typically, the analytical process for complex samples always contains several steps, including sampling, sample pretreatment, separation, identification and quantitation, data processing, and decision making []. Each step may cause significant effects on the accuracy of the final result.
The sampling step means to choose the representative sampling points and the appropriate sampling method. In most cases, the complex samples are not able to be handled by most analytical instruments directly. So the sample pretreatment step is conducted to separate the analytes from the complex matrix and obtain a suitable concentration of the target analytes. While conducting the separation step, it means to divide the isolated mixture which contains the target analytes into single components by means of chromatography or electrophoretic techniques. The identification and quantitation step can confirm the unknown analytes and determine its amount by retention time and peak area combined with selective detection. Nowadays, gas chromatography or liquid chromatography coupled with mass spectrometers which can provide more specific information is widely used to improve the accuracy of the detection. Finally, the data processing step can provide the analytical results of the target analytes which support the following decision making. It is noted that the analytical steps mentioned above follow one from another, which means the latter step cannot begin before the former one has finished. Hence, the slowest step determines the final speed of the whole analytical procedure and the errors of each step will affect the accuracy of the whole process.
The miniaturization and integration of the analytical instrument have brought great improvement in analytical science. However, the automation of the analytical process and the on-site analysis are still hot research topics in the instrumentation technology, which will lead to more rapid, accurate, and precise results. Presently, the gas chromatography/mass spectrometers (GC/MS) and the liquid chromatography/mass spectrometers (LC/MS) have successfully realized the automatic separation, quantitation, and data processing of the complex samples. Nevertheless, the sample pretreatment techniques always contain multi-steps and organic solvents consumption, which make it tough to integrate the sampling and sample pretreatment steps into automatic procedures. Therefore, almost 80% of the analysis time is spent on the sampling and the sample pretreatment steps.
As for complex samples, it is believed that it will not be possible to improve the extraction efficiency by the reasonable design and the optimization of the extraction process []. Therefore, the development of sample pretreatment technique is indistinctive because it is considered as a problem of technique instead of science. The exhaustive techniques tend to completely remove the target analytes from the complex samples to the extraction phase, which do not require calibration during the transfer of the analytes. Consequently, although it will consume much more resources, exhaustive techniques are still more popular than the non-exhaustive techniques among the researchers and regulatory agencies.
1.2 Solid-Phase Microextraction (SPME)
The non-exhaustive microextraction techniques possess many excellent advantages. It will not change the chemical components and the concentrations of the analytes because only a very small amount of the target analytes is removed from the samples. Hence, using microextraction techniques will result in a more representative information and lead to a more accurate characterization of the analytical system or process compared with the exhaustive extraction [].
Solid-phase microextraction (SPME) is a technique developed for the rapid pretreatment of laboratory samples and on-site samples [].
Taking advantage of equilibrium extraction and selective adsorption, the analytes are moved from the sample matrix onto the coating by conducting SPME procedures. In the first step, the coating is exposed to the sample to selectively extract the analytes by the strong affinity of the analytes and the extraction phase. And then, all of the components extracted by the coating are desorbed into an analytical instrument.
A degree of selectivity is required for every kind of sample pretreatment techniques, and it is impractical to lead all the compounds in the sample into the analytical instrument. An ideal sample pretreatment technique is supposed to remove the compounds incompatible with the instrument including the matrix compounds. It is also desirable to remove the unwanted compounds as many as possible to avoid the interference. Generally, the sensitivity, selectivity, and reproducibility of the SPME technique are mainly determined by the properties of the adsorbent coated on SPME device []. Therefore, choosing a proper coating can simplify the analytical procedures and save plenty of time.
1.3 SPME Devices and Coatings
1.3.1 Historical Retrospect
In the original work of SPME, fused silica optical fibers, which were uncoated or coated with liquid or solid polymer, were dipped into the aqueous sample containing the target analytes and desorbed into a GC injector. It needs to open the injector during the insertion and movement of the fiber, which results in the loss of head pressure at the column [].
Then, the combination of coated fiber into a microsyringe tremendously accelerates the development of SPME technique, resulting in the first SPME devices []. Another SPME sampler is based on a piece of microtube with coating inside. Such a tube can be installed inside a needle, or it can just be the needle of a syringe. Heating and cooling the air in the upper part of the tube can push the liquid or gas samples into and out of the microtube, which will accelerate the mass transport of analytes from the sample to the coating and realize the active sampling.
Some other samplers have been reported too, including the coating interior of vessels, the coating exterior of the magnetic stirring bars, and even the pieces of poly(dimethyl)siloxane (PDMS) tubes and thin films. Although SPME devices are mainly used in laboratory presently, the recent research has paid more attention to the remote monitoring, clinical, and field environmental applications [].
