Question

With the aid of a diagram explain how a thermal desorption-gas chromatography-mass spectrometer functions for the analysis of VOCs.

Answer 1

We can firstly break this into three parts, i.e. thermal desorption, gas chromatography and mass spectroscopy and then understand each one separately as they come in sequence in the diagram. Thermal desorption technique is used to concentrate the volatile organic carbons (VOCs) in gas stream prior to their injection into the gas chromatograph.

Thermal desorption fundamentally involves collecting volatile organic compounds onto a sorbent, and then heating this sorbent in a flow of gas to release the compounds and concentrate them into a smaller volume.? It can be classified into two types: single stage and two stage operation.

Early thermal desorbers used just single-stage operation, whereby the volatiles collected on a sorbent tube were released by heating the tube in a flow of gas, from where they passed directly into the GC.

Modern thermal desorbers can also accommodate two-stage operation, whereby the gas stream from the sorbent tube (typically 100–200 mL) is collected on a narrower tube integral to the thermal desorber, called the focusing trap or cold trap. Heating this trap releases the analytes once again, but this time in an even smaller volume of gas (typically 100–200 µL), resulting in improved sensitivity and better GC peak shape.

After desorption, the sample passes to the gas chromatograph. This method is used for analysing and separating compounds that can be volatalised without decomposition.A gas chromatograph uses a flow-through narrow tube known as the column, through which different chemical constituents of a sample pass in a gas stream (carrier gas, mobile phase) at different rates depending on their various chemical and physical properties and their interaction with a specific column filling, called the stationary phase. As the chemicals exit the end of the column, they are detected and identified electronically. The function of the stationary phase in the column is to separate different components, causing each one to exit the column at a different time (retention time). Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate, column length and the temperature.

From the gas chromatograph, different sample species with different retention time reach the mass spectrometer. It is a device that ionizes chemical species and sorts the ions based on their mass-to-charge ratio. In simpler terms, a mass spectrum measures the masses within a sample?.These GC and mass spectrometer,used together, allow a much finer degree of substance identification than either unit used separately. It is not possible to make an accurate identification of a particular molecule by gas chromatography or mass spectrometry alone.

A method of analysis by graphs with peaks,measures the peaks in relation to one another. In this method, the tallest peak is assigned 100% of the value, and the other peaks being assigned proportionate values. All values above 3% are assigned. The total mass of the unknown compound is normally indicated by the parent peak. The value of this parent peak can be used to fit with a chemical formula containing the various elements which are believed to be in the compound.Once a chemical formula has been matched to the spectrum, the molecular structure and bonding can be identified, and must be consistent with the characteristics recorded by GC-MS. Typically, this identification is done automatically by programs which come with the instrument, given a list of the elements which could be present in the sample.