In trace materials analysis, concentration of compounds in air, soil, slurry, aqueous solutions and other complex matrices are extracted. Limitations of trace detection for target analytes are usually of the order of parts per billion (ppb). Scientists often encounter problems in accurate, timely and reliable preparation of samples in both laboratories and at sample sites. Preparing samples for trace analysis can take up more than 80% of the total analysis time. As a result, effective sample preparation is important for high throughput projects.
Solid Phase Micro Extraction Method
Solid phase micro-extraction (SPME) is a low cost adsorption/desorption method that provides a suitable solution for preparing samples in trace analysis. SPME fiber sampling process has been demonstrated to be effective for a wide range of complex matrices, including food product flavors, chemical warfare agents in environmental samples, forensics investigations of post-blast explosions and fire accelerants, environmental pollution in water and soil, and toxicology cases involving drugs and blood alcohol in serum and urine.
SPME also offers a quick and flexible means of extracting compounds from samples. This method combines extraction, sampling, concentration and sample introduction into a single step, allowing solvent-free extraction using a stainless steel fiber coated with a thin film polymer, or a fused silica. The fiber, which serves as solvent during compound extraction, is placed on a syringe-like device to extract analytes from complex matrices and introduce into a chromatography system. Figure 1 illustrates how the SPME fiber sampling process works.
Figure 1. SPME fiber sampling process
Compounds can be extracted either by submerging the fibers in a liquid sample or exposing those with considerable vapor pressure to the headspace of a liquid, solid or gas sample. The analytes, which are adsorbed to the polymer coating on the fibers, are thermally desorbed in the injector of a gas chromatograph (GC) for separation.
Choosing the most suitable type of SPME fiber for analytes depends on the molecular weight (MW), volatility as well as the polarity of each target analyte. Fibers with polar phases, such as Carbowax and polyacrylate coatings, are suitable for polar analytes. These fibers can even be utilized for compounds that are less volatile using longer extraction times. Small molecule analytes can be effectively obtained with porous fibers with divinylbenzene (DVB) or Carboxen (CAR) coatings, while large molecule analytes can be collected using thin film fibers with 7µm and 30µm polydimethylsiloxane (PDMS) coatings.
Another aspect to consider is the thickness of the fiber coating. Diffusion of an analyte from the headspace or sample matrix into the coating on the fiber is proportional to the coating’s thickness. A thicker film is capable of retaining volatile compounds and transferring them to the GC injection port without any major loss. In case of compounds with higher boiling point, a thin film ensures rapid diffusion and release of the analyte during thermal desorption. Although a thick film can effectively remove high boiling compounds from the sample matrix, the rate of desorption will be longer and analytes may be carried over to the next extraction.
SPME Multi-fiber Conditioner
Although SMPE fibers are costly, they can be easily reconditioned between applications. The injector of a GC unit is recommended for desorbing contaminants and residues from a fiber. In certain SPME fibers, a high temperature is required to desorb high boiling residues completely. Sometimes the GC injector cannot be set to a high temperature owing to the presence of a column with low temperature limit. Since SPME fibers take up considerable amount of time to condition and recondition, using the GC injection port would be a sheer waste of time and resources. Figure 2 depicts a SPME fiber conditioner for reconditioning different types of fibers simultaneously.
Figure 2. Field Forensics SPME multi-fiber conditioner
By means of the Field Forensics SPME Conditioner, fibers can be reconditioned separately from the GC, with the suggested specifications for each. During this cleaning procedure, a fiber is heated up to 350°C and cleaned with an inert gas flow. Then, in a second heated and purged port, a new fiber can be made offline for following analyses. The SPME Conditioner X1 Model CN303R unit can support up to four separate ports to maintain a timely flow of different reconditioned fibers for high throughput trace materials analysis. The table below shows a summary of reconditioning settings for different SPME fibers.
Table 1. Conditioning temperatures and times for SPME fiber types.
|Analyte (Polarity and Molecular Weight)
||SPME Fiber Coating Type
|Gasses and low MW compounds MW range 30-225
|Non-polar compounds MW range 125-600
||2 – 4 hours
|Non-polar semi volatile compounds MW range 80-500
|Polar semi-volatiles MW range 80-300
|Trace compound analysis MW range 40-275
|Volatile polar alcohols MW range 40-275
|Volatile polar amines MW range 50-300
|Volatiles and low MW compounds MW range 60-275
Featuring a precision furnace with programmable temperature controller, the SPME Fiber Conditioner is easy to use and deploy. The unit has been successfully used for high throughput trace materials applications involving chemical warfare agent detection, arson, explosives forensics, etc.
For safe and secure sample collection and storage of SPME fibers, Field Forensics offers two accessories: SafePorter Model SP200 and TuffSyringe Model TS100. The former is a field transport container for SPME syringes, sample holders and other samplers, and features a hermetic seal to preserve and protect SPME samples. The latter is a rugged SPME manipulator that can be easily carried in a shirt or coat pocket.
SMPE techniques can be effectively used for high throughput trace materials analysis. Field Forensics offers compact, portable, and low-cost SPME multi-fiber conditioners and storage accessories that use different processing settings to recondition one or more fibers simultaneously. These units operate independently of a GC-MS unit and can be configured for optimum and concurrent reconditioning of different types of SPME fibers. They also eliminate the necessity to tie up a GC-MS with maintenance tasks.
This information has been sourced, reviewed and adapted from materials provided by Field Forensics.
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