GCMS

The GCMS is an instrument that, going into my senior year, I do not have much experience using. I hope to incorporate this instrument into my senior research so I am able to better understand how to collect and analyze data from this instrument.

Principles of Mass Spectrometry

Mass spectrometry creates gas-phase ions from the molecules or atoms of a sample and separates the ions according to their mass-charge ratio (m/z). The abundance of these ions formed is then measured. Mass spectrometry can provide information such as the structure of complex organic and biomolecules or the dmass of molecules and atoms in samples. MS is used most often for quantitative analysis but can provide qualitative analysis as well.

Electron ionization is a common method used to form ions from sample molecules or atoms. This technique involves bombarding the sample with electrons.

What is GCMS?

Gas chromatography mass spectrometry, or GC-MS, is used to separate gases or volatile compounds and detect each component in a mixture to measure quantitatively. GC-MS can quantitate hundreds of analytes from a single injected mixture.

How a GCMS Works

The diffusion pumps or turbomolecular pumps remove the effluent of the GC fast enough to maintain a low pressure through capillary GC. The end of the capillary column is introduced to the ion source. Then, a separate pump evacuates the small volume of the source, while the ionsare extracted into the mass analyzer region. This region is kept at a lower pressure by another pump. This design is called a differentially pumped system.

To interface packed columns, a jet separator preferentially removes light He or hydrogen carrier gas atoms from the effluent, allowing a smaller concentration of analyte molecules to enter the ion source at a lower total gas flow rate. The lighter gas atoms move more quickly and spread more widely in the vacuum of the separator chambers causing them not to pass through to the ion source.

How to Analyze GCMS Data

Mass spectrums can be organized in a table or plot, most commonly a histogram. The most abundant peak in the spectrum, the base peak, is scaled to 100. This is done so the y-axis represents the relative abundance of the ions of each m/z value. The molecular ion is not always the most abundant ion, one of the fragmented ions can be the most abundant as well.