Gas chromatography

Gas-liquid chromatography ( GLC ) or simply gas chromatography ( GC) is both an adsorbent and an Verteilungschromatografie that for separating mixtures of chemical compounds into individual is widely used as an analytical method. The GC is applicable only for components that are gaseous or vaporized without decomposition (boiling range up to 400 ° C). In this type of chromatography, an inert gas such as nitrogen or helium is used as the mobile phase, in special cases, also be hydrogen. The carrier gas is forced through a wound curved capillary tube, the so-called column, which generally has a length of 10-200 meters.

This column consists of either a metal (for older models ), but today, mostly of quartz glass, which is coated to increase the security against fracture. It is lined with a defined stationary phase (often with viscous polyorganosiloxane ) and is located in a temperature-controlled oven.

After entry of a sample substance which is now transported by the carrier gas, tarry components depending on the polarity and the vapor pressure of the individual gas molecules of different lengths at the stationary phase of the column. A detector measures the exit date at the end of the column; with a detector attached to the writer of this time and the amount of the substance can be represented graphically and compared with standard substances. For a very quick and easy qualitative and quantitative determination of very complex mixtures is possible. In contrast to the high performance liquid chromatography ( HPLC) only sufficiently volatile substances are detectable.

• 4.1 Use in quantitative analysis

History of Technology

The development of gas chromatography was marked by fundamental work of Archer JP Martin, Erika Cremer and Fritz Prior. The first gas chromatogram of the story originated shortly after the Second World War and is derived from the laboratory of Erika Cremer. It shows the separation of air and carbon dioxide on the activated carbon. 1951 first gas chromatograph in the modern sense of AT was James and A.J.P. Martin developed. Her first published work in 1952, GC separation of carboxylic acids. The development of the electron capture detector (ECD) in 1957 by James E. Lovelock made ​​it possible to detect chlorinated pollutants such as PCBs and chlorinated pesticides such as DDT sensitive by GC. In the following years a number of companies, the commercial gas chromatograph offered as the F & M Scientific Corporation of Avondale, Pennsylvania, which was acquired in 1965 by Hewlett Packard was born. In the 1970s was the development of capillary gas chromatography, a milestone was the invention of the " Fused Silica Column" by Raymond D. Dandenau and Ernest Zerrender. It was quickly followed by the first praxistauglichem couplings between capillary gas chromatography and mass spectrometry (GC -MS). Today, the gas chromatography is one of the most important analytical techniques in chemical laboratories.

Principle of measurement

Chromatographic separation of a mixture in a gas chromatograph is carried out at a non-polar support column, in the simplest case, only due to the different boiling points of the individual substances in the mixture, with no specific interaction with the stationary phase is carried out, but "only" a ten thousand times repeated distribution. In polar columns but alcohols, esters, ketones are held more with the same boiling points similar as paraffins. The particular interaction - specifically the balance between the gas phase and the stationary phase - is known as a Raoult's law. The higher the longer the substance is the partial vapor pressure of a substance according to Raoult's law, that is, in the gas phase, the shorter the retention time.

The strength of the interactions between the sample components and the stationary phase is determined both by their structure as well as their functional groups. It occur in non-polar substances only dispersion interactions (van der Waals bonds ), while polar stationary phases can also enter into polar interactions, such as hydrogen bonds or donor -acceptor bonds. The latter separated according to the principle that opposites attract. This means that separation phase to be included hydrogen for hydrogen bonding, for example, be able to separate substances which are able to provide the hydrogen to bond (e.g., alcohols). Also, for example enantiomers, which do not differ in their boiling points and thus same retention times would have to be separated by their different strong interactions with special derivatives of cyclodextrins.

A basic requirement for gas chromatography is that the material that you want to analyze, can be evaporated without decomposition - if it is not already present in gaseous form. By means of derivatization can be otherwise difficult to access the GC analytes such as alcohols, amines, fatty acids or sugar as far as thermally stabilize that they can be separated on commercially available phases without difficulty. Possible derivatives are the methyl ester ( BF 3 and conversion with MeOH), wherein the silyl ether alcohols with carboxylic acids.

Major appliance parts

A gas chromatograph consists of three main components: injector, separation column in the GC oven and detector. In the injector, the sample dissolved in a low boiling solvent, with a piercing membrane ( septum) is injected. This injector is usually heated ( up to 450 ° C) to achieve rapid and complete vaporization of the sample. Is also the septumfreie task and slow evaporation by means of a cooled injection system (KAS / PTV) possible. The substances (usually up to 6 bar column pressure ) into the separation column ( capillary) is transported by the carrier gas, which is installed in the GC oven as above. This serves the separation column to control the temperature accurately, so by constant temperature ( isothermal) or to achieve through a controlled temperature increase just as fast as extensive separation of the mixture.

At the end of the column is followed by the detector which produces an electronic signal when a substance leaving the separation system. The electronic signal can be used as peak ( engl. summit ) are registered on the chart recorder. The signals are then processed in an integrator or today usually on a computer system with appropriate evaluation software. The time for the separation of a mixture with the view of the various peaks on a chromatogram is often between 30 and 60 minutes.

Injectors

The injector is used for the object to be examined, the substance mixture on the column. Continuous injectors / methods are:

• Split / splitless injectors ( SSL)
• On-column injection ( OCI ) with direct object to the column
• Cooled Injection System ( depending on the manufacturer KAS or PTV)
• Direct task systems using valve switching

For packed columns, the optimal amount of the sample for each component between 0.1-1 ul, for capillary columns, the optimal volume of sample should by a factor of 100 to 1000 smaller. For injection of a sample, which can still be diluted by a solvent, there are special 1-10 ul syringes. It is important for the injection that no air ( bubbles ) in the syringe, this would in fact contribute to the oxidation of the substances in the furnace chamber.

Straight with Split / Splittless injectors and cold feed systems often called autosampler are used that allow the sequential processing of a variety of samples. In addition to, inter alia, also headspace autosampler, purge and trap systems and pyrolyzers used for sample application. A relatively new development is the use of solid phase microextraction ( SPME) and stir bar sorptive extraction ( SBSE ).

Separation columns used

Important characteristics of the columns are:

• The column diameter and
• The column length.

Previously packed columns were mostly used. Among them is located inside of a thin (< 1 cm), metal or glass tube, a few meters in length, the so-called column, a solid, inert carrier material. If the gas is directed to the substance to be analyzed directly on the substrate, then one speaks of a GSC ("Gas -Solid Chromatography "). The carrier is also coated with a thin layer of a high molecular weight, viscous and low-volatile liquid, it is a GLC ("Gas - Liquid Chromatography" ). This liquid takes here the function of the actual stationary phase. The inert gas is preferably used for column chromatography is nitrogen.

Today will mainly work with the 1958 Marcel JE Golay developed by capillary chromatography. The advantage is the approximately 100-1000 times better separation, ( a plate number of 300,000 ) of substances compared with packed columns, so that the analysis time can be shortened. The polyimide-coated fused silica columns usually have an inner diameter of 0.1 to 0.5 mm and a length of 10 to 60 m. For the separation of fatty acid esters combined even columns are used to 100 m. The stationary phase lines the capillary while only a thin film. The advantage is the dramatically improved separation of similar substances, compared with packed columns. The trend in the GC is currently on ever thinner and shorter columns, because this the time required for analysis can be significantly reduced.

When using columns from different manufacturers should be noted that identical stationary separation phases are provided with a variety of names. The following list of common stationary phases is separation indicates which columns from different manufacturers are comparable in composition of the assignment of the separation columns.

Detectors

The following detectors are used:

• Flame ionization detector ( FID), widely used for the quantification of organic compounds
• Thermal conductivity detector (TCD o TCD ) for permanent gases
• Photoionization detector ( PID)
• Flame-photometric detector ( FPD), element-specific
• Nitrogen - phosphorus detector (NPD, also thermionic detector, TID ), nitrogen -phosphorus specific
• Electron capture detector (ECD ), of the halogenated organic compounds
• Atomic emission detector ( AED), element-specific
• Echelle plasma emission detector ( EPED ), element-specific
• Mass spectrometer, mass selective detector ( MSD)
• Ion mobility spectrometer (IMS) for volatile organic compounds

Partly also two detectors are connected in series (tandem principle ) for specific questions. Prerequisite for this is, however, that the first detector 's measurement is not destructive performs (ie, ECD / TCD, but no FID / NPD / PID).

Electronic signals of the detector are shown as a function of the time since the injection ( retention time) as a 2D graph, called the chromatogram. This is usually done by using electronic evaluation units ( computer chromatography data ), rarely only with plotters.

Application in the analysis

Gas chromatography is a very sensitive method for the analysis of mixtures. It is possible to even minimal amounts of substance (10-9 grams) prove. You can divide with her complex mixtures into individual components. In many cases it is sufficient only the time it takes for a substance from the time of injection to the passage of the detector, the retention time, to identify a substance. In combination with a mass spectrometer, the so-called GC / MS coupling, very small amounts of substance can be detected, while structural analysis to operate.

Common applications for the gas chromatography in the analysis of agricultural products, herbicides, meat products on hormones, the study of drugs, flavors and etheric oils, carbohydrates, crude oil components and in forensic chemistry, in doping tests in air and seawater studies in environmental analysis. Low-volatility analytes must be converted before analysis may thus only derivatized in more volatile substances. This can, for example, with trimethylsulfonium or chlorotrimethylsilane done by polar groups are converted to methylated nonpolar groups.

Use in the quantitative analysis

The size of the displayed area of the detector units is rarely in direct proportion to the actual mass of the sample to be analyzed. This makes a calibration with reference materials defined content necessary. Order (random) error of the gas chromatograph exclude ( especially the injection system ), are often used just in gas chromatography internal standards. Serves as an internal standard, an additional substance, whose retention time is close to the analyte, but this is not superimposed. It is added to the analyte and to the reference material and the solutions made ​​therefrom. After the measurement, peak areas of the analyte and reference material are set to the peak area of the internal standard to the ratio and thus eliminated the error of the injection system as much as possible respectively. After calibration, the concentration cP a sample P can be determined using the following equation.

Cs is the concentration of standard weighed, A denotes the area of ​​the peak in the chromatogram, and r denotes the sensitivity of a substance (such as a lot of signal indicates the detection of each M). r can be measured once for a specific detector and then tabulated.