MASS SPECTRA OF SOME MISCELLANEOUS ARTEFACTS AND ADDITIVES
Extraneous substances can be introduced into lipid extracts from innumerable sources. Of course, some may be added deliberately - for example, 2,6-di-tert-butyl-p-cresol (BHT) to minimize autoxidation in samples. Others may be introduced by accident. All solvents, including from time to time those grades that are nominally of high purity, can contain contaminants, and as large volumes of solvent may be used to obtain small amounts of lipids, any such impurities can be troublesome. Other extraneous lipid-like materials can be introduced accidentally into lipid samples from a variety of sources. Plastic ware of all kinds (other than that made from Teflon™) can be especially troublesome and is best avoided, since plasticisers (diesters of phthalic acid usually) or elements of the polymers per se are very easily leached out. These compounds tend to co-chromatograph with lipids, so they may spread confusion and obscure compounds of interest in chromatograms.
Manufacturers of fine chemicals, like all human kind, are fallible, and all laboratory reagents can on occasion contain impurities that may cause problems in analytical procedures. It is necessary to exercise vigilance to detect and eliminate these at an early stage. Further contaminants can arise from fingerprints and from a host of materials in everyday use in laboratories, including cosmetics, hair preparations, hand creams, soaps, polishes, the exhausts from vacuum pumps, lubricants and greases, if they are used carelessly.
Some of those contaminants encountered in our work that have interfered with our mass spectrometry are described below. I have concentrated on the fingerprint spectra, not on the details of interpretation, as this is not usually relevant. You may also wish to consult our pages dealing with mass spectra of miscellaneous lipophilic components.
2,6-Di-tert-butyl-p-cresol (BHT)
BHT is widely used by lipid analysts to limit autoxidation in tissue extracts or in lipid preparations of various kinds. It tends to elute just before the methyl ester of 14:0 from most GC stationary phases, and it will inevitably be encountered in mass spectrometric analysis of fatty acid derivatives. Its mass spectrum is -
Other Antioxidants
Innumerable other antioxidants may be found in lipid extracts, derived from plastics, packaging materials, etc., for example. Two produced by Ciba that we have found in our lipid preparations are illustrated below (with thanks to Claire Fernie). We have probably encountered more, but these two are listed in the commercial mass spectral library used in our laboratory so were easily identified.
Irganox 1076™ or octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate -
Irgafos 168TM or tris(2,4-ditert-butylphenyl)phosphite
No interpretations of the spectra are offered here, but I can recommend a useful paper (Buchert, T., Gruner, A. and Palibroda, N. Rapid analysis of polymer homologues and additives with SFE/SFC-MS coupling. Packaging Technol. Sci., 7, 139-154 (1994)).
Phthalates (Plasticisers)
Phthalate esters are widely used in the manufacturer of a wide range of plastics, and they are readily leached from these by contact with solvents, lipid extracts or even fresh tissues. They are especially troublesome in lipid analysis as they tend to elute with fatty acid derivatives from many of the common GC stationary phases. They are affected only slowly by transesterification reagents, although some basic reagents can give rise to sufficient partially or fully methylated phthalates to further confuse chromatograms (Shantha, N.C. and Ackman, R.G. J. Chromatogr. A, 587, 263-267 (1991)). Unfortunately, they are not easily removed from lipid extracts or derivatives by adsorption or other forms of chromatography. Two representative spectra are illustrated below -
Dibutylphthalate -
Dioctylphthalate -
No interpretation of the spectra is offered here, other than that the key diagnostic feature in both is the base peak at m/z = 149.
Polysiloxanes
Polysiloxanes are easily leached from silicone rubbers, such as those found in composite Teflon™-lined caps for vials. For example, they will appear in chromatograms if the septum is inserted into a GC sample vial the wrong way round, or if septa are damaged by over vigorous use. They appear as as a long series of sharp peaks (20 or more components) that are regularly spaced in the chromatogram. Some representative spectra are illustrated below (with thanks to Tom Shepherd).
Again, no detailed interpretation is offered, but the key diagnostic features are a series of ions spaced approximately 70 amu apart, especially in the lower mass range, e.g. at m/z = 73, 147, 221, 295, 355, etc.
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Updated: 23/10/2007 |
Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland
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