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Use of SEM in trace evidence analysis
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using SEM To analyze Gun shot residue, pollen , diatoms hair and even fibres
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5102FSBMOL Report Emma Burns PriestleyThe use of SEM in the analysis of trace evidence samples
SEM which stands for Scanning electron microscope is used vastly over the forensic science field to
identify types of trace evidence in this report we are going to be looking at the uses of it to analyse
gunshot residues, pollen, diatoms, hairs and fibres. Due to SEM being a non-destructive method
meaning the evidence will not be destroyed and only tiny samples are needed in order to create
suitable data that will help identify whether a firearm was used, pollen fibres, hair types and even if
a victim was drowned or not. The SEM works by using a beam of high energy electrons which
generate signals that hit the surface of the specimen giving a detailed 3D image of the sample which
shows information about the sample including external morphology, chemical composition and the
type of species.
When there is an acceleration in the number of electrons present when using an SEM there becomes
significant amounts of kinetic energy, and this kinetic energy is dispersed as a variety of signals
produced by electron-sample interactions when the electrons slowly make their way in the solid
sample. The signals which are present include secondary electrons (that produce SEM images),
backscattered electrons. Secondary electrons and backscattered electrons are commonly used for
creating images of the trace evidence samples. Secondary electrons are mostly important in showing
morphology and topography for the samples (Susan Swapp, n/a). However, backscatter can help
identify the types of material in question. Energy dispersive x ray also provides us with information
on the composition of a sample and what chemicals are present this happens because when a
sample is being hit with the electron beam electrons bounce of the surface of the sample creating an
x ray to be emitted which balances the two electron states. This x ray which is emitted is a
characteristic of the sample’s chemical composition. Every element has their own electrons and
when more electrons are given the element has to rebalance itself which is where the x-ray comes
from.
Gunshot residue:
GSR is the key evidence which is found when a crime has involved a firearm this is because the
residue can be found on the clothes and the hands of the person in suspicion of using a firearm. The
residue can be analysed using an SEM. The scanning microscope works by focusing a beam of
electrons on the samples that were gathered from the suspect. The electrons then interact with the
atoms in the samples they get detected through the detector. The detector collects and produces
signals which contain information about the GSR sample’s surface topography, composition of the
sample and other elemental properties like the conductivity (Thompson, 2018). Due to SEM being
non-destructive samples don’t get ruined meaning results can be achieved. The particles which have
a high mean atomic number are detected by the SEM backscattered electron signals. These particles
will show up brighter than those of lower mean atomic number composition. As the beam current
increases the amount of signal each particle from the sample will also increase.
The image displayed below is a scanning electron microscope image of a gunshot residue showing all
the detected elements as we can see there is oxygen, carbon, aluminium, antimony, lead and barium
present. So, the backscatter SEM signals detected barium, lead and antimony as they have a high
atomic number.
, 5102FSBMOL Report Emma Burns Priestley
GSR Screenshot EDS. [online]. [15th November 2021]. Available from:
https://www.azom.com/equipment-details.aspx?EquipID=6867
Pollen:
“Plant evidence can be used to determine whether a crime scene is a primary or secondary scene
and to locate missing bodies” (Coyle HM, 2005). Because pollen can get everywhere i.e., victims
nose/ clothing and even in air filters of cars and the tyres palynologist can gather samples from all
these possible areas and use them in an SEM. If samples from the victim don’t match the samples
collected from the crime scene, then detectives will know that the body has been moved. There are
two types of pollen 1) wind transported pollen much lighter in weight 2) insect transported pollen is
much heavier and stickier than wind transported. Pollen is non-conductive because of this using an
SEM a very detailed image of each pollen sample can be obtained allowing us to be able to identify
the species of plant that the pollen has come from. For example, pollen that gets transported by
wind can be identified by the shape and whether or not air sacs are present and pollen that has like
spiky outer texture is pollen which insects transport.
Diatoms:
Diatoms are microscopic single-celled, golden-brown algae and their main applications in forensic
science and theses currently are assisting in the diagnosis of drowning as a cause of death (Kirstie R.
Scott, 2014) and whether it was in salt water or fresh water. Diatom species vary depending on the
water type they also vary in shape this is how examiners can identify the type of diatom whether its
fresh water or seawater. Furthermore, this will help in identifying where the possible crime scene is.
SEM has allowed us to be able to create a three-dimensional structure of diatom valves and show
the structural design of the silica shell. Due to the vast amount of diatom species a normal light
microscope can’t be used as it won’t be easy to identify the diatom which is why a SEM is used as
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