Gas chromatography -
GC/MS is used to identify organic materials such as resins, fats and waxes from their chemical composition. The technique requires a sample to be taken but this can be very small, often no larger than a pin-head.
The sample is dissolved and injected into the instrument. The first part of the instrument, the gas chromatograph (GC) is an oven containing a very long, narrow glass tube with a polymer coating, called a column. The sample moves along the column in a stream of helium gas. The different chemical constituents of the sample interact with the polymer coating on the column, some more than others. This means that the different constituents take different lengths of time to reach the end of the column. Each constituent emerges from the column and enters the detector where it is recorded as a peak, the size of which is relative to its abundance in the sample. The graph produced is called a chromatogram.
The chromatogram can be used as a 'fingerprint' for the material. However, ancient materials do not always have the same composition as modern ones because they are chemically altered by the effects of aging.
In GC/MS the detector is a mass spectrometer (MS). As well as detecting the constituents separated in the GC, the MS enables them to be identified. In the MS the chemical compounds are fragmented into ions. Each compound fragments in a different way. The range of fragmented ions and their relative abundance is recorded as a mass spectrum from which the compound can be identified.
By using the mass spectra to identify peaks it is possible to identify mixtures of materials (eg. beeswax and pine resin) and to secure identifications for very degraded ancient materials.
GC/MS has been used to identify the resins and waxes used in the construction of some turquoise mosaics from Mexico and to identify binding media used for the Nebamun wall paintings.
General organic analysis of museum materials:
Mills, J. S. and White, R. (1994) The Organic Chemistry of Museum Objects, Butterworth-Heinmann, Oxford (1994).
About the technique:
McMaster, M. and McMaster, C. GC/MS. A Practical Users Guide, Wiley-VCH (1998).
About the data:
Smith, R. M. and Busch, K. L. Understanding Mass Spectra - A Basic Approach, John Wiley and Sons (1999).