Chromatography is a method for the examination of substance mixtures. It is based on the physical separation of substance mixtures depending on their polarity. The substances are separated in a column – depending on their chemical composition and/or volatility – and reach the detector at different times. With sufficient intensity, every separated substance corresponds to a signal-“peak” in the chromatogram along the time-axis.
Gas chromatography (GC):
In the gas chromatography, the separation takes place in a stream of carrier gas. The samples have to be completely and undecomposed vaporizable or already be in the gaseous form. All chromatographs in our laboratory are equipped with mass spectrometers (MS). Hereby, after the ionisation of the substances, characteristic mass fragments are produced and detected. We dispose of devices with both a single-quad and a triple-quadrupole-detector (two fragmentation steps in a row à higher selectivity and sensitivity). By means of characteristic fragmentation patterns, the chemical structure of a substance can be concluded with the help of spectral libraries.
A special feature is an olfactory detector. Simultaneously to the GC-MS-analysis can here be performed an olfactory perception of analytes in a so-called “sniffing port”. The olfactory-perceived substances can be overlapped and compared with the identification of compounds from the GC-MS-analysis.
Injection systems in the gas chromatography:
In order to transfer a sample in the vapour phase we dispose of the following methods for sample preparation depending on the substance mixture to be tested:
Headspace: The sample (solid or liquid) is hermetically enclosed in a glass vessel with a septum and then heated up. This way, equilibrium ceases between the sample and the vapour phase. Using an injection needle, an aliquot of the gas flow is transferred via a transfer-line into gas chromatograph.
Liquid injection: The existing liquid sample (if necessary after the sample preparation via extraction with an appropriate solvent) is transferred to the separating column via an automatic microliter syringe.
Pyrolysis: With pyrolysis, a solid substance is exposed to a defined and very high temperature for a short time and therefore decomposed. The vaporised products then get into the injector of GC. This procedure has proven successful especially in the analysis of plastics.
Thermodesorption: A solid or liquid is heated up in a desorption tube by an inert gas flow. The vaporised substances are cryo-focused in a cooling trap. After desorption phase is concluded, the cooling trap is heated up and the substances are transferred into the separating column.
Thermodesorption is especially important in the testing of air samples for indoor pollutants.
In the sampling process, ambient air is passing through a tube, which contains the adsorbent material Tenax. This material stores easily and medially volatile organic compounds. In the laboratory, the analytes are released from the Tenax-filling of the tube via thermodesorption and measured via gas chromatography.
Another application area constitutes the examination of contaminated components in the industrial area. Hereby, the organic residues of the sample are prepared and put into a small tube for the thermodesorption.
Thermal extraction: This procedure basically corresponds to thermodesorption but is designed for larger samples. The sample is heated up by the inert gas flow but the vaporising compounds are not cryo-focussed. Instead, they are initially adsorbed by adsorbent material. Subsequently to the thermal extraction follows the thermodesorption of the adsorbent material.
SBSE (Stir Bar Sorptive Extraction): This method involves a magnetic stirring rod coated with a membrane made of silicone. In a hydrophobic sorb-material, organic compounds adsorb from aqueous solutions. After the extraction, the hydrophobic membrane is dried with a cellulose cloth and the magnetic stir bar is examined in a thermodesorption procedure.
Typical application for gas chromatography are:
- Determination of pollutants in the ambient air
- Comparison of auxiliary operating materials
- Examination of additives in plastics
- Testing of metallic surfaces for contamination with organic compounds (for example production residues)
- Determination of pollutants in dust and materials
Liquid chromatography (LC):
High-Performance Liquid Chromatography (HPLC) is used for determination of non-volatile organic substances. Depending on the applied separation phase, both polar and nonpolar substances can be examined. Because the sample has to be in liquid form, firstly, a suitable extract must be prepared out of solid, liquid or gaseous (gas is passing through an absorbent material) samples. This extract is then separated in a packed column and measured by a UV/VIS-detector.
For the determination of polar and semi-volatile compounds, we dispose of a liquid chromatograph, which is linked to a triple quadrupole mass spectrometer (MS). After the separation in the column the substances gets to the MS and there they would be identified and quantified depending on their molecular and/or fragment masses. The sensitivity of the measurements can generally be increased with tandem-MS-systems.
Typical applications for liquid chromatography:
- polar biocide (i.e. carbamates, isothiazolinone)
- Aldehydes in the ambient air (DNPH)
- quaternary ammonium compounds (QAV or rather QUATs)
- polar compounds from the SVHC-list
- Indoor-relevant mycotoxins
Ion chromatography (IC):
Ion chromatography is a special form of the HPLC and serves the determination of salts (anions, cations) in aqueous solutions or the determination of ionic surface contaminations at trace amount (0,01-1 mg/l).
At ion chromatography, ionic compounds (anions or cations) are separated via a chromatography column by using a liquid eluent and then they are measured with a conductivity detector. The substances are identified and quantified through the duration (retention time) and the surface intensity (signal strength). The quantification (density determination) is evaluated by a calibration function.
Typical application areas are:
- Examination of anions in liquid solutions; for example, chloride, nitrate, sulphate, acetate and chromate
- Examination of alkali metals and alkaline earth metals in aqueous solutions; for example sodium, potassium, magnesium and ammonium
- Determination of ionic surface contamination at trace amount; for instance, aluminium cast components, assemblies