Direct mass spectrometry

A ceramic electrochemical microreactor for the methoxylation of methyl-2-furoate with direct mass spectrometry coupling. 

Mass spectrometry combines exquisite sensitivity with a precision that often depends more on the uncertainties of sampling and sample preparation than on those of the method itself. Mass spectrometry is a supreme identification and recognition method in polymer/additive analysis through highly accurate masses and fragmentation patterns quantitation is its weakness. Direct mass spectrometry of complex polymeric matrices is feasible, yet not often pursued. Solid probe ToF-MS (DI-HRMS) is a breakthrough. Where used routinely, mass spectrometrists are usually still in charge. At the same time, however, costs need to be watched. 

In the following sections, the instrumental features of direct mass spectrometry based techniques (DI-MS, DE-MS and DTMS) are presented, followed by a discussion of some mass spectra of standard compounds and reference materials. Finally, a series of case studies related to the presence of resinous materials in archaeological findings and works of art are reported and discussed. 

Features of Direct Mass Spectrometry Based Techniques... 

Direct mass spectrometry based techniques such as DI-MS, DE-MS and DTMS are unique in their ability to yield complete chemical structure assignments and to identify a great variety of resinous amorphous materials. 

Direct Mass Spectrometry to Characterise Wax and Lipid Materials... 

In the present chapter, we first provide some general information concerning the chemistry of waxes and lipids currently encountered in various items from our cultural heritage and we detail the main protocols based on direct mass spectrometry that have been developed so far. We then discuss the mass spectra obtained by EI-MS on a range of reference substances and materials sampled from museum and archaeological artefacts. We then focus on the recent possibilities supplied by electrospray ionisation for the elucidation of the structure of biomarkers of beeswax and animal fats. 

Investigations based on direct mass spectrometry analysis aiming at identifying waxes and other lipids in museum or archaeological items were first carried out at the end of... 

Direct Mass Spectrometry in the Electron Ionisation Mode... 

In contrast to direct mass spectrometry used in the El mode, ESI often requires specific pretreatments of the samples to purify the components of interest, to increase their yield of ionisation and consequently to improve selectivity and sensitivity. It is thus not a preliminary step of analysis but a method that forms part of an analytical strategy that allows the presence of well preserved high molecular long chain compounds to be shown before their fine characterisation by ESI techniques (Regert et al., 2003a Mirabaud, 2007 Mirabaud et al., 2007). 

Figure 4.1 summarises analytical methodologies in which direct mass spectrometry was used, either in the El mode or by ESI. 

Figure 4.1 Analytical strategy in which direct mass spectrometry analyses using either elec tron ionisation or electrospray are used for detecting and identifying lipid substances in archaeological and museum samples...

Results obtained by Direct Mass Spectrometry using the Electron Ionisation Mode on Archaeological Samples and Wax Sculptures... 

These results show that, depending on the lipid substances present in a sample, direct mass spectrometry may allow the identification of all the lipid substances present in a material, in particular when their mass spectral fingerprints do not overlap, or to determine the main molecular constituents still preserved in the sample. 

Although most of the analytical studies focused on lipid substances encountered in archaeological objects and museum works of art are carried out using separation techniques, direct mass spectrometry represents an alternative or a complementary analytical way for identifying such complex materials. 

Hence, direct mass spectrometry techniques, either using El or ESI, appear to be powerful and innovative analytical tools for elucidating the structure of the main biomarkers present in a wide range of waxes and lipids that may be preserved in archaeological objects and in museum works of art. In most cases, they have nevertheless to be cautiously exploited in combination with other complementary analytical techniques. 

---