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Samples which are to be analyzed by gas chromatography are usually filled in so-called autosampler vials, which are sealed with a crimp lid. For sampling, an SPME needle is pierced through the septum into the sample vial. An SPME needle looks similar to a normal cannula because it is also made of stainless steel. However, this does not serve to remove or inject fluids, but helps in penetrating the septum. There is a fused silica (fused silica) or glass fiber (StableFlex) in the needle, coated on the end. This coated fiber absorbs analyte molecules from the sample. The sample can either be taken directly from the liquid phase or from the gas phase above the liquid, the so-called head-space. If the sample is to be taken out of the gas phase, a volatility of the analyte has to be given, so that enrichment from the gas phase is possible. More information about Headspace analysis can be found here.
Following the extraction of the analyte from the liquid sample, is the desorption in order to release the analytes from the coated fiber of the SPME needle. Different methods are suitable depending on the choice of the desired analytical method. If sample is getting analyzed by of gas chromatography, the desorption can be carried out without further intermediate steps within the sample inlet of the gas chromatograph. In this case, the so called thermal desorption is used, using the high temperature within the chromatograph to dissolve the absorbed analyte molecules from the fiber and to convert them into the gas phase. The steps of thermal desorption of the analytes and transfer into the gas phase for the gas-chromatographic separation are connected and thus analyte loss is avoided or at least minimized. The temperatures of the Inlet depend on the respective temperature during the chromatographic separation, but usually are between 100 and 300 ° C.
The absorbed analyte molecules can also be dissolved from the fiber with the aid of a solvent. This method is used if later analysis is to be carried out by HPLC (high pressure liquid chromatography).
Through the use of different coating materials, substance groups can be selectively absorbed. The commercially available adsorbing materials which differ greatly in their polarity include: carbowax, polydivinylbenzene, polyacrylate, carboxes and PDMS (polydimethylsiloxane). Depending on the analyte, combinations of different layer thicknesses and adsorbing materials are used.
The following table lists the fibers recommended by Merck for various types of analytes (MW = molar mass):
|Analyt Type||Recommended Fiber|
|Gases and low molecular weight compounds (MW 30-225)||75 µm/85 µm Carboxen/polydimethylsiloxane|
|Volatiles (MW 60-275)||100 µm polydimethylsiloxane|
|Volatiles, amines and nitro-aromatic compounds (MW 50-300)||65 µm polydimethylsiloxane/divinylbenzene|
|Polar semi-volatiles (MW 80-300)||85 µm polyacrylate|
|Non-polar high molecular weight compounds (MW 125-600)||7 µm polydimethylsiloxane|
|Non-polar semi-volatiles (MW 80-500)||30 µm polydimethylsiloxane|
|Alcohols and polar compounds (MW 40-275)||60 µm Carbowax (PEG)|
|Flavor compounds: volatiles and semi-volatiles, C3-C20 (MW 40-275)||50/30 µm divinylbenzene/Carboxen on polydimethylsiloxane on a StableFlex fiber|
|Trace compound analysis (MW 40-275)||50/30 µm divinylbenzene/Carboxen on polydimethylsiloxane on a 2 cm StableFlex fiber|
|Amines and polar compounds (HPLC use only)||60 µm polydimethylsiloxane/divinylbenzene|
Advantages of SPME:
For further information on SPME we recommend the How to get started guide by Merk!