Identification

With conventional GC detectors, such as the FID and ECD, identification of the most commonly encountered acids is based on comparison of the retention times obtained with authentic standards. For unidentified acid peaks in general, retention index values or, for FAMEs, equivalent chain lengths (ECL) from the literature may be helpful. The preferred solution is, however, MS detection in view of the more definitive structural information it provides. Especially for car-boxylic acids, the usual data (e.g. molecular weights, fragmentation patterns, isotopic peak patterns) afforded by MS are supplemented by additional struc tural information, the most useful being the degree of unsaturation.

The presence of a double bond can be deduced from the molecular weight of an ester but its location cannot be ascertained due to migration during fragmentation. Hence, for reliable identification of positional isomers by GC-MS, two methods are employed: the on-site method of fixing the location of the double bond through its chemical modification, or the remote group method in which the carboxylic group is derivatized to a nitrogen-containing product which restricts double-bond migration. The remote group method is more convenient and versatile.

Chemical modification involves the addition of a reagent across the double bond of the acid ester to generate a product which gives diagnostic fragment ions. Dimethyl disulfide is a widely used reagent since it adds to a double bond in a single step at room temperature and enables identification of positional and geometrical isomers after separation on an appropriate column. But the picture is less clear with polyenoic acids, especially when the double bonds are in close proximity, and with acids containing other structural features such as cyclopropane rings. Diels-Alder reactions with cyclopentadiene derivatives can be applied similarly. The double-bond site may also be established by treating the unsaturated acid with OsO4 and converting the resulting diol to the bis-TMS ethers for GC-MS analysis. Although this method is suitable for locating the double-bond sites of polyunsaturated acids, their fragmentation patterns are more complex and careful interpretation is necessary.

In derivatizing the carboxylic group, the picolinyl and DMOX compounds are the most commonly generated nitrogen-containing products. In the mass spectra of these derivatives, the saturated segments of the molecules are indicated by the regular separation of successive peak clusters by 14 amu (corresponding to the cleavage of a CH2 group), whereas at doublebond sites the gap is only 12 amu. Furthermore, fragmentation on either side of the double bond gives two ions which are separated by 26 amu. In a branched acid derivative, the site of branching is shown by a similar gap of 28 amu.

Geometrical isomers and ring structures are more reliably identified by infrared (IR) spectrometry, which underscores the utility of GC-Fourier transform IR (FTIR)-MS in the structure elucidation of acids. However, the inherently lower sensitivity of IR requires larger sample sizes and columns with a higher load capacity.

Quantitative analysis of acids by GC-MS is carried out most sensitively by selected ion monitoring (SIM) employing an isotopically labelled analogue or a derivative of a structurally similar acid as internal standard. The desired sensitivity of detection is a critical factor in the choice of the derivative. For increased sensitivity ion currents must be intensified by reducing fragmentation. Hence, TBDMS derivatives are preferred to those of TMS. Moreover, TBDMS de-rivatizes the amino acids arginine and glutamine, whereas TMS fails to do this. (However, the preferred method for quantification of amino acids involves the butyl perfluoroacyl derivatives.) Fragmentation may also be reduced by increasing molecular stability via cyclic derivatives, as illustrated by quinoxalinol compounds utilized in the GC-MS analysis of 2-oxo-acids. An excellent method of augmenting sensitivity is performing negative ion mass spectrometry via derivatives (e.g. p-nitrobenzyl, pentafluorobenzyl) with high electron affinity.

These methods have allowed the determination of a variety of acids by GC-MS at pg levels. Even mixtures of acids can be analysed quantitatively by monitoring several characteristic ions. Programmable SIM, which optimizes the selectivity at various points in a chromatogram and the desired sensitivity of analysis, has been invaluable in this regard.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment