3.2.3 Chromatographic methods

High Performance Liquid Chromatography (HPLC-PDA) and combined techniques with mass spectroscopy.

HPLC is used with a variety of detection methods, photodiode array (PDA), ultra-violet (UV), refractive index (RI) and mass spectroscopy (MS). It is a chromatographic technique, and since its early development in the late 1960s it has become one of the most commonly used techniques for both qualitative and quantitative analysis. It makes use of the interactions of molecule(s) with the mobile liquid phase and solid column phase. This allows separation of complex mixtures based on their retention time in the column. Identification of the molecules and derivatives can then be achieved, based on retention time, molecular weight etc. Frequently software packages contain extensive libraries which greatly assist in identification. Increasingly improved detectors allow for successful identification of samples in the nano, pico and even femto-scale range. In archaeological and other heritage science it has been used for the analysis of a number of materials, for example to identify wine residues in ceramics, identification of dyes in textiles and pigments in paintings.

Used at: NMS

Gas Chromatography Mass Spectrometry (GC/MS)

GC is an extensively used chromatographic technique which was developed in the 1950’s. It is used for the qualitative and quantitative analysis of organic materials. A liquid sample is injected into a port, which is held at a high temperature (150-250°C,) the sample is then vaporized and moved by the inert carrier gas across a solid column phase. Like HPLC, components within a mixture are interact and then are eluted from the column with differing retention times, these then flow into the mass spectrometer detector. Compounds and derivatives can be identified by using a spectral library as each compound has an unique fragmentation pattern. Successful GC/MS depends on samples having suitable vapour pressures and thermally stabilities. It is an extremely sensitive technique and is therefore able to identify the constituents of very small amounts of sample material. The major archaeological applications of this technique are in organic residue analysis, but it is also used for the identification of organic materials such as bog butter, waxes, resins and adhesives either used in the construction of an object or later conservation treatments, particularly in paintings.

Used at: Glasgow University Chemistry and Earth Sciences Departments, Bristol (Chemistry), Bradford (Archaeological Sciences) and York (Archaeology)

3.2.2 Inductively-coupled Plasma techniques (ICP)

Inductively Coupled Plasma Spectroscopy (ICPS) Plasma Spectroscopy covers a family of techniques used for the analysis of a wide range of elements, particularly trace elements. All the techniques initially require the transformation of the sample from a solid sample into a plasma, effectively its vaporisation into positive and negative ions, electrons and molecular fragments. The composition of the plasma can then be measured by several methods.

Inductively-coupled Plasma Atomic Emission Spectroscopy (ICP-AES)

Atomic emission spectroscopy measures spectral emission arising from energy changes within an atom to determine which elements are present, and in what concentration, within the sample. Detection limits are or can be very low, down to parts per million (ppm) and even parts per billion (ppb), depending on the element. Conventional AAS requires samples to be in solution prior to analysis. This technique has been extensively used in archaeological science for provenance studies, characterising trace element compositions of ceramics, metals, vitreous materials, lithics and soils.

Used at: SUERC, Stirling (Environmental & Biological Sciences), Edinburgh Geosciences

Inductively-coupled Plasma Mass Spectrometry (ICP-MS)

Mass spectrometry uses the mass and charge of ions (charged particles) to separate the component constituents form the plasma which can then be quantitatively detected to determine each element present. This is a highly sensitive technique with detection limits in the parts per billion (ppb) range for some elements. It has been employed in provenance studies as for ICP-AES.

Used at: SUERC, Edinburgh Geosciences

Laser Ablation Inductively-coupled Plasma Mass Spectrometry (LA-ICP-MS)

Here a laser is used to ablate a tiny amount of material from the sample/object into the plasma; a mass spectrometer then identifies and quantifies the elements present as in the technique above. This technique is particularly useful in archaeological science as it is minimally destructive and so can be used on entire objects (albeit with considerable limitations due to the size of the required vacuum chamber). It is extremely sensitive for trace and rare earth elements with detection limits in the ppb range. This technique is increasingly being used in provenance studies of a range of materials but has significant potential for more. With a multi-collector (LA-MC-ICPMS) it can also be used to characterise radiogenic isotope ratios such as lead, which can be used to provenance metals.

Used at: Aberdeen Geosciences, St Andrews

Thermal ionization mass spectrometry (TIMS)

TIMS is another mass spectrometry technique which utilises a heated filament to ionize a sample prior to separation in the mass spectrometer. While radiogenic isotopic ratio analysis has been extensively applied to geological materials, the archaeological applications of TIMS have used lead isotope ratios to source metals, glasses and pigments. It requires the removal and dissolution of small samples (usually in hydrofluoric acid), as well as knowledge of the sample’s bulk chemistry.

Used at: SUERC

Secondary ion mass spectrometry (SIMS)

By subjecting the sample to a focused beam of high energy ions SIMS can be used to examine the composition and charge state of materials. It is the most sensitive technique available for the analysis of surfaces, determining the elemental, isotopic, or molecular composition of the surface, and used for example for the examination of depletion layers in historical glass .

Used at: Department of Materials at Oxford and Imperial College London