Forensic Chemistry

Question 1

What are the 3 broad categories of tests commonly used in forensic chemistry?

Answer 1

1. Chemical properties
2. Chromatography
3. Spectroscopy/spectrophometry

Question 2

What crucial elements is forensic chemistry used to determine?

Answer 2

• identification: what is its chemical identity? Can tell a lot from look, smell and form
• classification: using the chemical and physical properties to classify and individualise
• association: how do these properties compare to other samples/evidence?

Question 3

What are common preliminary tests conducted?

Answer 3

• colour change (spot test)
• visible physical transformation (I.e. liquifies, solidifies or evolves a gas)
• temperature change

Question 4

What are the 2 broad categories of chromatography?

Answer 4

Liquid and gas

Question 5

What is chromatography in a nutshell?

Answer 5

Takes advantage of the different ways molecules interact with each other to separate the individual components (in this case chemical species) of a mixture

Question 6

How does chromatography work?

Answer 6

• the mobile phase (which can be liquid or gas) carried the sample across or through the stationary phase (solid or viscous liquid)
• the attraction of each component of the mixture to either the mobile or stationary phase will dictate how long it takes to pass through the system, thus separating components of the mixture

Question 7

What are the most common types of chromatography?

Answer 7

• high performance liquid chromatography (HPLC)
• gas chromatography (GC)
• thin layer chromatography (TLC)
• columnar chromatography

Question 8

In a nutshell what is spectroscopy?

Answer 8

Uses what we understand about how different chemicals absorb and reflect electromagnetic energy to identify and quantify individual components in a sample by examining the resultant spectrum

Question 9

What is electromagnetic radiation?

Answer 9

Is defined by its energy. A long wavelength has low energy (radio waves) and a short wavelength has high energy (X-rays)

Question 10

How does spectroscopy work?

Answer 10

• all molecules can absorb or emit EMR, but only wavelengths specific for that molecule
• because chemists have studied which wavelengths certain molecules emit and absorb, we are able to identify if a chemical substance is present by looking at the spectra of energy absorbance and emission
• the relative amount of EMR that is emitted or absorbed will also tell us how much of that chemical substance is present in the sample

Question 11

What principle is mass spectrometry based on?

Answer 11

The mass spectra are unique doe specific molecules

Question 12

Why are chemical test often done last in the line of forensic tests?

Answer 12

• they are often destructive

Question 13

Spectroscopy techniques are….

Answer 13

Non destructive

IR spectroscopy and Uv-Vis spectrophotometry do not destroy ug altering their matrix through dilution May be required

Question 14

Laser ablation and plasma injection are……

Answer 14

Completely destructive

Question 15

What are the questions asked to determine if something is fit for purpose?

Answer 15

1. What will be the use of the analytical chemical information?
2. Timing considerations?
3. What is the degree of confidence in the result required?
4. What is the nature of the evidence available?

Question 16

What are the Quality control and Quality assurance considerations?

Answer 16

• is the test free from bias and interference?
• has the analysis method been checked and accredited?
• has the lab where tests are performed been accredited?
• what is the limit of detection of the method?
• the use of certified reference materials, lab and field banks, replicate analysis

Question 17

What are some limitations of forensic chemistry?

Answer 17

1. Many techniques destroy part if not all of the sample - needs to be a sufficient quantity of a sample
2. Many techniques require time, ranging from hours to days, may prove too long in time critical incidents and there is a choice to be made about the degree of confidence required in the process (better to do a quick test immediately and other tests later?)
3. Some types of evidence may be unsuitable for chemical analysis because they can not be destroyed or divided or if a representative sample is not possible
4. Some tests may show false positives, where positive identification of a substance is made incorrectly

Question 18

Questions to ask when looking at the nature of evidence available

Answer 18

• is the evidence divisible?
• Is the evidence homogenous making a sub-sample representative or do multiple tests need to be performed?
• if there sufficient tests for multiple tests to be performed?
• can all or some of the evidence be destroyed?

Question 19

What are calibration standards?

Answer 19

A series of solutions of known concentration that are used to determine the relationship between concentration and the instrument response

Question 20

What are matrix matched standards?

Answer 20

Are used to monitor the ablation process and have a composition identical (or very similar) to the forensic samples being analysed

Question 21

What are drift standards?

Answer 21

Used to detect changes in the response of the instrument over time