Facilities & Resources

Analytical Instruments

MCI has a wide range of analytical methods available for the examination and analysis of materials from museums and ethnographic or archaeological contexts. These techniques range from optical microscopy and digital image analysis to more recent methods such as ICP-MS (Inductively Coupled Plasma Mass Spectroscopy). The following describes the types of techniques used at MCI, how they work and their application in museum studies.

AOM - Applied Optical Microscopy

What is it?

The optical microscope is an instrument designed to make fine details visible. Zacharias and Hans Janssen built the first recorded microscope in the 1590's. The image of an object, or its parts, are magnified when viewed through a simple lens. Modern optical microscopes combine many simple lenses and mirrors together in proper alignment, to yield clear, detailed images at relatively high magnification. From the inception of the Smithsonian Institution, optical microscopy has played a prominent role in the description, analysis and identification of objects from museum collections as well as materials from ethnographic and/or archaeological contexts. Optical microscopy provides unique information about the structure and state of preservation of objects and the identity of their component materials not possible with high-tech instrumentation. Analyses are done nondestructively, when possible, or by taking very small samples, when destructive means are the only option. The emphasis of MCI's Optical Microscopy Laboratory is to answer pertinent collection-based and archaeological questions using optical microscopy and digital image analysis.

How does it work?

Light (visible, infra-red, ultraviolet, etc.) is either reflected off an object or transmitted through a thin section of it. The light is transmitted through a series of lenses & mirrors to a detector (usually our eyes, but also to a photographic film, digital camera or other detectors). The image is then stored in our memory, on film or in a digital file on a computer. The image can then be scanned to a digital format and processed using image analysis software.

XRF - Energy dispersive X-ray fluorescence

What is it?

XRF is a non-destructive tool for determining the elemental make up of an object. It is especially useful for identifying inorganic compounds such as metal alloys, glass, ceramics and pigments.

How does it work?

An x-ray beam is aimed at a spot on the surface of an object, causing the chemical elements in the material to emit characteristic "fluorescent" radiation. Measuring this fluorescent radiation provides identification and determination of the elements present.

FTIR - Fourier transform infrared spectroscopy

What is it?

FTIR is an analytical tool that produces a "fingerprint" spectrum of the different chemical compounds that make up an object. FTIR is useful for characterizing organic materials such as coatings, adhesives, and paint binders. When attached to a microscope, the sample size is drastically reduced.

How does is work?

A sample is radiated in the infrared region of the electromagnetic spectrum. The manner in which it absorbs infrared radiation is characteristic of the sample's molecular structure. Identification can be achieved by comparing the wavelength spectrum of the absorbed IR light with spectra of known compounds.

ICP-MS - Inductively Coupled Plasma Mass Spectrometer

What is it?

Inductively Coupled Plasma - Mass Spectrometry is a technique for elemental analysis with sensitivities in the parts-per-billion range. ICP-MS provides qualitative and quantitative information along with isotopic composition. Applications include rocks, soils, sediment, water, air, plant and animal tissues. It allows for rapid scanning for up to 72 elements in less than 5 minutes per sample.

How does it work?

An argon plasma gas is obtained by allowing a free flowing stream of argon into a torch with a high frequency current flows in an induction coil with varying magnetic fields. The plasma has temperatures greater that 10,000 K. A liquid sample is introduced into the system as an aerosol by nebulization. Once the sample hits the plasma, ions from the sample are excited and spontaneously revert to a lower energy state and emit a photon of energy. For quantitative purposes, it is assumed that the emitted energy is proportional to the concentration of ions.

Liquid Chromatography

What is it?

Liquid chromatography is used to separate, detect, and quantify the individual chemical components of a mixture.

How does it work?

The mixture is typically injected at the top of a column packed with solid particles of a medium that acts as the stationary phase. Liquids of increasing strength are then passed through the column, and the mixture components separated by virtue of their ability to differentially adhere to the stationary phase. Those components that adhere weakly exit the column first; those more strongly retained exit later. The separated components then pass through a detector for quantification.

XRD - X-ray diffraction

What is it?

XRD is an analytical tool that permits "definitive" identification of crystalline inorganic pigments, using very small samples.

How does it work?

X-rays are diffracted (bent) or reflected in a manner determined by a material's crystalline structure. The angles and intensity of the diffractions and reflections are recorded and interpreted by comparison with references.

SEM-EDS - Scanning electron microscopy-energy dispersive x-ray spectroscopy

What is it?

SEM is a tool that allows image magnification up to 100,000 times. Through an analytical attachment that functions similarly to the already described XRF, it can provide the elemental analysis of a sample's surface. It can, for example, be used to identify pigments in cross-sectional paint layers.

How does it work?

A sample is exposed to a beam of electrons in a vacuum system. Recording the interactions of electrons with the sample create an image that allows the observer to see extremely small surface features. The XRF-like attachment allows for the simultaneous analysis of the elemental composition.

Ultraviolet/visible (UV-VIS) spectrometry

What is it?

UV/VIS spectrometry is a tool that measures the wavelength-dependant absorption of light in the visible or ultraviolet region. It is very useful for identifying a variety of materials, for example dyestuffs.

How does it work?

A sample in solution is irradiated or reflected in the UV-Visible range. Certain wavelengths are selectively absorbed by the sample producing a characteristic pattern which can help identify the material.

GC-MS - Gas chromatography with mass spectrometry

What is it?

GC-MS gives "definitive" identification of organic materials such as paint binders.

How does it work?

GC-MS first separates dissolved samples or derivatives (chemically modified samples), into fractions according to their volatility (and polarity). Each fraction is then ionized so that electrically charged fragments are formed. These ions are separated according to mass and counted. Interpretation of the resulting "mass separation" provides the identification and chemical makeup of the sample.

X-ray Radiography

What is it?

X-radiography is used for the structural examination of art and artifacts, as it is used by doctors in hospitals. For art works, it helps to reveal losses, replacements, and methods of construction that may not be visible to the naked eye.

How does it work?

X-rays are a form of high energy electromagnetic radiation that travel like ordinary light but can penetrate through most objects. The amount of x-ray intensity absorbed by the object depends on the density and thickness of the material. Thus, exposing an x-ray sensitive film to the transmitted x-rays provides a recorded image of the interior of the object. Since denser areas absorb more x-rays, the film receives less exposure and the corresponding areas appear lighter.

Xeroradiography

What is it?

Xeroradiography is an alternative method of recording X-ray images.

How does it work?

Instead of using a photographic film to record the image, x-rays travel through objects and are received on an x-ray sensitive metal plate. The plate is then processed through a unique photocopying-type machine. The recorded image of the interior of the object is then transferred onto plastic-coated paper. The denser elements appear darker by xeroradiography. Xeroradiography is more sensitive than film-based x-ray imaging, and for painted wood, it helps to reveal attachments, insect damage, and construction. The final image is a mirror image of the object (different from X-ray film images), so it is important to avoid any confusion in interpretation.