Solid phase microextraction (SPME) is one of several important sample preparation techniques. SPME, invented in early nineties by Prof. Janusz Pawliszyn from the University of Waterloo in Ontario, Canada, integrates sampling, extraction, concentration and sample introduction into a single step. It enables solventless extraction via a fused silica or stainless steel fibre coated with a thin film polymer, which acts as the solvent during the extraction of compounds. The fibre is mounted on syringe-like device for extraction of analytes from various matrices and introduction to a chromatographic system.
The extraction principle can be described as an equilibrium process in which the analyte partitions between the fibre and the aqueous phase. This technology has been patented, US patent number 5691206, European patent number 523092, and PCT international patent application #W091/15745, filed with the University of Waterloo. SPME has been commercialized by Supelco, Varian, Leap Technologies, and Gerstel.
Volatile or semi-volatile organic compounds may be extracted by either immersing the fibres in a liquid sample or exposing them to the headspace of a solid, liquid or gas sample. The analytes partition or adsorb to the polymer coating on the fibres. The absorbed analytes may be thermally desorbed in the injector of a gas chromatograph (GC) for separation and further quantification. Intermediate to low volatility compounds may be extracted by fibres designed for use with high performance liquid chromatography (HPLC) , and either desorbed in a SPME/HPLC interface or offline into a desorption solvent, which is subsequently injected with a conventional injection interface or autosampler.
Aside from the obvious advantages of a solventless sample preparation technique, SPME is fast, amiable to automation and easy to use. Like other sample preparation techniques, however, users still need to understand the nature of the target analytes and the complexity of the matrix.
The development of new applications and methods has progressed rapidly over the years, providing the thrust behind technological advancements and overall market growth. Many applications in environmental, food, forensics, clinical and pharmaceutical laboratories have become commonplace for analysis of semi-volatile and volatile compounds in air, aqueous matrices including complex slurry and soil samples.
Theory of SPME
The most widely used form of SPME sampling consists of exposing a thin polymeric coating directly to the sample matrix for a predetermined amount of time as shown in figure below. Once the fibre is exposed to the sample matrix, the transport of analytes from the matrix to the coating begins immediately. SPME extraction is considered to be complete when the analyte concentration has reached distribution equilibrium between the sample matrix and the fibre coating. The equilibrium conditions can be described by equation (1) according to the law of mass conservation for a two-phase system (for example, the sample matrix and the fibre coating).
CoVs = C∞sVs + C∞fVf (1)
where Co : initial concentration of analyte in the sample
Vs : volume of sample
Vf : volume of fibre coating
C∞f : equilibrium concentration on the fibre
C∞s : equilibrium concentration in the sample
The distribution constant of the analyte between the fibre coating and the sample matrix is defined by equation (2):
Kfs = C∞fVf / C∞sVs (2)
The number of moles of analyte extracted n by the coating at equilibrium can be expressed in equation (3) by combining equations (1) and (2):
n = C∞fVf = ( KfsVf Vs Co) / (KfsVf +Vs) (3)
where Kfs is the distribution coefficient between fibre coating and sample matrix. For a three-phase system (e.g. where headspace is included), equation (4) can be used for equilibrium conditions:
n = C∞fVf = ( KfsVf Vs Co) / (KfsVf +KhsVh +Vs) (4)
where Khs is the distribution coefficient between fibre coating and headspace. This equation (4) states that the amount of analyte extracted is independent of the location of the fibre in the system. The fibre may be placed in the headspace or directly in the sample, as long as the volumes of the fibre coating, headspace, and sample are kept constant.
Commercially Available Fibre Coatings
PDMS - polydimethylsiloxane
PA - polyacrylate
PDMS/DVB - polydimethylsiloxane / divinylbenzene
PEG - carbowax-polyethylene glycol
CW/TPR - carbowax / templated resin
CAR/PDMS - carboxen / polydimethylsiloxane
DVB/CAR/PDMS - divinylbenzene / carboxen / polydimethylsiloxane
C18 - C18 silica particles
Thin Film Membrane Extraction
MESI (Membrane Extraction with a Sorbent Interface)
SPME Couplings in use