Comparison of marks found at scenes of crime with test marks made by implements thought to be responsible for them is a well-established and valuable source of scientific evidence. If the marks are small or show fine detail they may be examined by optical microscopy using a comparison bridge to display the images of the unknown and test marks side by side.
Optical microscopy is simple, though the process of comparison requires a great deal of skill. The samples can be manipulated easily and since comparisons are mostly made at low magnification the resolution is more than adequate. Because of the limited depth of field however, examination of large areas of rough or curved surface is difficult and photographs do not show corresponding points clearly.
Those not familiar with comparison microscopy cannot interpret the pictures and this may be a serious shortcoming when they are presented in court.

The Scanning Electron Microscope has a great depth of field and can produce striking images over a wide range of magnifications on rough or curved surfaces.
In order to compare two samples simultaneously the reference image from one is held in a digital image store. The second sample is then scanned in real time whilst a segment of the stored image is overlaid and by a combination manipulating the positional controls and magnification the markings on each image can be accurately compared.

CamScan supply special sub-stages to give the operator the ability to rotate or tilt the sample in addition to the standard 5-axis stage controls on the fully eucentric specimen stage.

This very comprehensive stage replaces the standard door mounted unit and can be easily changed in minutes.
It is deigned to accept a range of shell cases, bullet and cartridge cases and allows two samples to be mounted side by side for detailed comparison. Each sample can be independently rotated through 360º and their relative heights adjusted. Both samples can be tilted from 0 to 120º to view underneath shell flanges for extraction marks.

Comparisons are made by image overlay techniques after which the results can be archived for subsequent recall for future inspection and comparisons of further evidence.

By removing the comparison sub stage the ultra long working distance platform can be used for very low magnification work such as document investigations, fingerprints or complete postage stamps.

The X and Y movements are controlled through high-speed motors.

Introduction
When a gun is fired some of the hot gas produced by the detonation and ignition of the cartridge may escape from the side of the weapon. The gases cool quickly and materials that are solid at normal temperature condense and are deposited as a particulate residue on the person who is firing and on surfaces in the vicinity of the firing point.

The deposits contain the products of decomposition of the propellant and primer and also a contribution from the bullet or cartridge case.

Particles Unique to Gun Shot Residue (GSR)
Pb, Sb, Ba (Lead, Barium and Antimony)
Sb and Ba (Antimony and Barium)

Particles indicative of GSR
Ba,Ca and Si
Pb and Sb
Pb and Ba
Sb (with S)
Sb (without S)
Ba
Pb

Other particles of interest include:
Mn and Al (Found in lead free ammunition)
Sb and Zn (Found in Sintox explosive)

Various methods of detecting firearms residue in samples taken from suspects who have fired a gun have been devised.
Colour test-indicating presence of particles of lead and barium
Atomic Absorption Spectrophotometry (AA)
Neutron Activity Analyser (NAP)

The later two methods can measure amounts of lead, barium and antimony but although they are very quick to perform they lack specificity and this uncertainty can reduce the value of scientific evidence presented in court.

SEM/EDX Analysis
The technique employing a Scanning Electron Microscope fitted with an Energy Dispersive X-ray Analyser has long been accepted. Both chemical composition and morphology can be ascertained and the combination is unique to those found in Gun Shot Residue.

Procedure
Particles are collected from the skin and clothing with adhesive tape lift off or by filter concentration.
The tape or filter is mounted on a stub, carbon coated, then searched by imaging in the SEM. As each particle is located it is analysed by the EDX analyser. Images are obtained from the backscattered detector so particles that contain high atomic number elements can be easily distinguished from the generally low atomic number background.

Manual Searching
The sample is searched using a combination of beam shift and stage movement however it is not an attractive proposition.
Although particles from Gun Shot Residue vary from 0.5 to 50µm the larger ones are rapidly lost, probably within a few minutes of being deposited, and those left will be generally less than 10µm in size.
If you consider a typical search area to be 1cm x 1cm then the problem becomes clear when you realise that this area equates to 100,000,000 square microns in which there may be only two or three particles, each a few microns in diameter.

Disadvantages of Manual Searching
Very time consuming.
Tiring and cannot be carried out for more than a few hours at a time.
Open to operator lapses.
Ties up both instrument and operator from other important work.

Automatic Searching
The CamScan system speeds up this operation enormously by searching each sample automatically.

Specimen stage
The specimen stage on all CamScan SEMs have a proven ability to withstand the rigours of operating day and night year by year without requiring any service attention.

They are equipped with Fastrac High Precision Stage Control offering extremely fast, accurate stage positioning and precise recall in X,Y and Z axes.
Directly coupled linear on-stage encoders allow the true sample position
to be continuously monitored.
Closed loop software control ensures extremely rapid and precise stage
positioning without overshoot.
Stage position readout to ±0.5µm resolution.

Auto Search Routine
Up to 15 x 12mm dia. sample stubs can be loaded into the chamber for auto search or 11 x 25mm square stubs and left unattended - this leaves a spare position for the calibration standard.

During auto search a continually updated table of found particles is displayed showing:
Positional co-ordinates.
Elemental composition / classification.
Particle size.

During auto search an image and spectra of each verified particle is displayed as it is being analysed and then this information is stored and can be recalled later.

Particles are classified using the program developed by the FSS, London Metropolitan Forensic Laboratory in conjunction with CamScan.