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Gas chromatography

Gas chromatography is a technique where the sample is in gas phase or volatilises once injected. The sample is transported to a chromatographic column using a carrier gas (mobile phase). It is a technique of great importance as it allows the separation, detection and quantification of mixtures of a large number of compounds using retention time and peak area.

Gas Chromatography with Flame Ionization Detector (GC-FID) enables the separation, detection and quantification of mixtures of volatile or semi-volatile products. The FID detector is selective for molecules with C-H bonds in their chemical structure. Since practically all substances that are analysed for gases are organic, the FID detector is almost universal.

Gas chromatography with thermal conductivity detector (GC-TCD) allows to separate, detect and quantify mixtures of inorganic gases and small hydrocarbon molecules.

Job request

In order to carry out one analyses, put in touch with the responsible for the UEA.

In the case of samples to be analysed through the FID detector, all the components of the mixture must be volatile at the temperature of the injector (300ºC, approx.), unless the sample is to be analysed using the headspace technique.

In the case of samples to be analysed through the TCD detector, they must be in the gas phase at room temperature.

It be necessary to inform of the toxicity and precautions of storage of the sample. Once the report has been given, if people do not come to search the remainder of the sample after a week, this will be destroyed.

The equipment

Agilent 7890A gas chromatograph

Year:
2013
Characteristics:

Equipped with two types of detector: an FID and a TCD.

The equipment has an autosampler for the injection of liquid samples with 50 positions.

The injection of gas samples is done manually, using a gas tight syringe.

Agilent 7820A gas chromatograph

Characteristics:

Equipped with two types of detector: an FID and a TCD.

The equipment has an autosampler for the injection of liquid samples with 16 positions.

The injection of gas samples is done manually, using a gas tight syringe.


How does it work?

Gas chromatography is a technique where the sample, once injected, volatilises and is transported to a chromatographic column by means of a carrier gas (mobile phase). The carrier gas is an inert gas that does not react with the molecules of the analyte, but only transports it through the column.

The chromatographic column is filled with a stationary phase, which retains the analytes of interest in the sample. The choice of the suitable stationary phase for each type of analyte is an essential factor in gas chromatography.

Due to the selective retention of the analytes in the stationary phase of the column, they leave the column separately. At the exit of the column, these pass through a detector that gives a response in the form of a signal, and allows its quantification.

Applications / Practical cases

The FID is a very versatile detector.

En el cas del cromatògraf 7890A, la columna instal·lada és la DB-FFAP d’Agilent J&W (30m x 0,32mm, 0.50 µm), que permet la detecció d’àcids grassos volàtils i alcohols en mostres líquides.

Specifically, the analysed compounds are:

  • Methanol, ethanol, propanol, butanol, isopropanol and hexanol.
  • Acetic acid, propionic acid, isobutyric acid, butyric acid, isovaleric acid, valeric acid and hexanoic acid.

The sample volume required for analysis is 2 mL of sample filtered to 0.2 µm. 

In the case of the chromatograph 7820A, two capillary columns are available for the determination of different compounds:

  • Agilent HP-5 column (25m x 0.2mm, 0.33 µm): non-polar, generalist column with a wide range of applications.
  • Agilent GS-Alumina (30m x 0.53mm): Polar column, excellent for the analysis of light hydrocarbons (C1 to C8), including C1 to C4 saturated and unsaturated hydrocarbons.

The TCD is used for the analysis of gaseous samples. The analysis is done with a double packed column (PPU and MS5A) and allows the detection of permanent gases in gaseous samples.

In particular:

  • Hydrogen, oxygen, nitrogen, methane, carbon dioxide and carbon monoxide.

The sample volume required for analysis is 5 mL.

 

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