Chemical Technology October 2015

PETROCHEMICALS COVER STORY

in order to identify all of the peaks that are in the samples. The peaks are measured in relation to one another. The tallest peak is assigned 100 % of the value, and the other peaks assigned proportionate values. The total mass of the unknown compound is normally indicated by the parent peak. A ‘full spectrum’ analysis considers all the ‘peaks’ within a spectrum [1]. After identifying the peaks, the analyst will check to see if there are any matching patterns. Most volatile compounds show up first, Denver explained, and heavier compounds later. This enables very accurate identification of where that sample may have come from. Molecular masses of each compound are visible on the chromatograms. If nec- essary, the analyst can also do a computer library search, to match spectrum patterns of compounds stored in an electronic database. Denver said that quantitative methods analysis can be done at a later stage, based on the findings of the initial GC-MS scan analysis. Many test methods are available at the ERWAT Laboratories whereby a wide range of organic compounds can be quantified, depending on what has been identified in the scans. Techniques available now are extremely sensitive, indicating low parts per billion. For more information telephone +27 11 929 7014/7000; email: laboratory@erwat.co.za/mail@erwat.co.za. References 1. www.gmu/depts/SRIF/tutorial/gcd/gc-ms2.htm 2. http://www.bris.ac.uk/nerclsmsf/techniques/gcms.html

samples are compared with samples from the spill sites. ‘Full scan’ is useful in determining unknown compounds in a sample. Since the mass spectrum produced by a given chemical compound is essentially the same every time, the mass spectrum is essentially a ‘fingerprint’ for the molecule. This ‘fingerprint’ can therefore be used to identify the compound [1]. Denver explained that, in order for a compound to be analysed by GC-MS it must be sufficiently volatile and thermally stable. Samples are usually analysed as or- ganic solutions; as a result, materials of interest need to be solvent-extracted and the extract subjected to various ‘wet chemical’ techniques before GC-MS analysis is possible. For example, SVOCs such as phenols and PAHs will be subject to liquid-liquid sample extraction. The sample solution is then injected into the GC inlet where it is vaporised and swept onto a chromatographic column by the carrier gas. The sample flows through the column and the compounds comprising the mixture of interest are separated [2]. The next component (the mass spectrometer/analyser) separates the, by-then positively charged, ions according to various mass-related properties. After the ions are sepa- rated they enter a detector which sends information to a computer which, aside from controlling the operation of the MS, also records all of the data produced and converts the electrical impulses into visual and/or hard copy displays [2]. In other words, the MS produces mass spectrum pat- terns from which chromograph plots are generated. The analyst looks at each of the chromatograms, overlays the plots (from the various samples that have been analysed),

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Chemical Technology • October 2015