Gas chromatography - mass spectrometry(GC/MS)
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.
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.
Spectrum
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 is being used to identify the resins and waxes used in the
construction of some turquoise mosaics from Mexico.
Further Reading
General organic analysis of museum materials:
Mills, J. S. and White, R. (1994) The Organic Chemistry of
Museum Objects (Butterworth-Heinmann, Oxford)
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)