Question

Discuss the relationship between a gas chromatogram and mass spectrometry data. How do they correspond to identify the compounds present?

Answer 1

Gas chromatography–mass spectrometry (GC-MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry by which complex mixtures of chemicals may be separated, identfied and quantified.

The GC-MS is composed of two major building blocks- the gas chromatograph and the mass spectrometer. The difference in the chemical properties between different molecules in a mixture and their relative affinity for the stationary phase of the column will promote separation of the molecules as the sample travels the length of the column. The molecules are retained by the column and then elute from the column at different retention times and this allows the mass spectrometer downstream to capture, ionize, accelerate, deflect, and detect the ionized molecules separately. The mass spectrometry process normally requires a very pure sample while gas chromatography using a traditional detector such as Flame ionization detector cannot differentiate between multiple molecules that happen to take the same amount of time( retention time) to travel through the column, which results in two or more molecules that co-elute. Combining the two processes reduces the possibility of error, as it is extremely unlikely that two different molecules will behave in the same way in both a gas chromatograph and a mass spectrometer. Therefore, when an identifying mass spectrum appears at a characteristic retention time in a GC-MS analysis, it typically increases certainty that the analyte of interest is in the sample.

In otherwords sample components emerge from the GC column , flowing into the capillary column interface connecting the GC column and the MS. Identification of the compound based on its mass spectrum relies on the fact that every component has a unique fragmentation pattern. The computer drives the MS, records the data and converts the electrical impulses to visual displays and hard copy displays. As each solute exits the GC column, it is diverted into mass spectrometer which is capable of monitoring the amount of and identifying the chemical nature of the solute. In this way both qualitative and quantitative information about the mixture can be obtained.