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Chromatography is the best technique for identifying different chemicals within a mixture.
Rationale
Forensic chemistry is the application of chemistry, forensic toxicology, in a legal setting. A forensic chemist can assist in the identification of unknown materials found at a crime scene. Forensic Chemists have a wide array of methods used to identify unknown substances including HPCL chromatography, gas chromatography-mass spectrometry, atomic absorption spectroscopy, Fourier transform infrared spectroscopy, and thin layer chromatography. These different types of methods is important due to the critical nature of some instruments and the number of possible unknown substances that can be found at a crime scene. They prefer using non-destructive methods first, to preserve evidence and to determine which destructive methods will produce the best results. (n.d., 2019)
Chromatography is an analytical technique that enables the separation and identification for qualitative or quantitative analysis based upon their differential distribution between 2 phases: Stationary phase and mobile phase. Stationary phase – different strengths of attachment (affinity). Mobile phase – different solubilities in solvent being used. Both can be identified in a mixture and can be determined how much of each component is present. There are different types of chromatography including gas, liquid, column, paper, high-performance liquid, etc. (DayMap, 2019)
Research Question
How is gas chromatography utilised in identifying alcoholic substances such as ethanol in blood and hence crucial in criminal investigation?
Background
‘Gas chromatography in which the sample mixture is vaporized and injected into a stream of carrier gas (as nitrogen or helium) moving through a column containing a stationary phase composed of a liquid or a particulate solid and is separated into its component compounds according to the affinity of the compounds for the stationary phase.’ (Webster, 2019) First, a sample of the mixture of substances placed in a syringe and injected into the machine. The components of the mixture are heated and instantaneously vaporize. Then, carrier added which is simply a neutral gas, designed to help the gases in our sample to move through the column. The column is a thin glass or metal tube filled with a liquid that has a high boiling point. As the mixture travels through the column, it’s adsorbed and separates out into its components. Each component emerges in turn from the end of the column and moves past an electronic detector which identifies it and prints a peak on a chart. The final chart has a series of peaks that correspond to all the substances in the mixture. The process of gas chromatography can be seen in diagram below. (Woodford, 2018)
Analysis and Interpretation
In the experiment of determination of alcohol (ethanol) by using gas chromatography to the separation of a water-ethanol mixture. It is used when law enforcement agencies need to determine whether or not someone is intoxicated. High sensitivity is required since 0.1% blood alcohol is legally intoxicated in most states. Determination will deal with higher concentrations (up to 25% by volume) which are more typical of alcohol levels found in many alcoholic beverages. Gas chromatography is an effective method for the separation, identification and quantitation of components in a mixture. I
Ethanol can be monitored to determine the proof value of the beverage, while methanol and isopropanol can be quantified to determine the levels of denaturants present. While poor methanol peak shapes often are associated with columns of limited sample capacity packed column with 5% Carbowax® 20M provides an excellent peak shape for methanol, and completely resolves methanol from ethanol.
The peak of acetaldehyde (retention time: 0.36 min) was clearly separated from ethanol (0.43 min) at a column temperature of 120 °C. Ethanol, methanol, and acetone accorded at 120 °C. An almost baseline separation between acetaldehyde, methanol, acetone, and ethanol was obtained at a column temperature of 60 °C. Some forensic important congeners of ethanol are also included and showed baseline separation at 60 °C. No carryover problems were seen for ethanol, nor for any of the other volatiles studied.
Daily calibration was performed with the aqueous ethanol calibration solutions (conc.: 1–5 g/L). A quite good linear correlation was obtained between peak area and concentration). No significant differences were observed between the calibration line of ethanol in water and those of ethanol in whole blood, serum, urine, and fecal supernatant. The biological matrix did not influence the gas chromatographic analysis. Although not shown, the calibration curves were also linear in the low concentration range (0.0001–0.5 g/L).
Conclusion and Evaluation
Data use different volume, concentration and temperature in the experiment. Data 2 used the temperature of 60°C, Injection volume of 1.0 μL, concentration of 0.8g/L and output of 60mV. Data 3 used temperature of 120°C and injection volume of 2.0 μL. The sources showed how gas chromatography utilised in identifying alcoholic substances such as ethanol in criminal investigation.
In first resource, ethanol was the hardest substance to monitor, second resource shows ethanol separated (0.43 min) at a column temperature of 120 °C, and third resource shows that no significant differences were observed between the calibration line of ethanol in water and those of ethanol in blood. To truly investigate how gas chromatography used to identifying ethanol in criminal investigation. This testing would need to involve a larger sample, no. of trials, and use the same sample, concentration, volume and column temperature throughout the experiment. They would need to remain constant in all testing. The difficultly of monitoring the substances (i.e from the driver) could be investigated more.
Reference list
- Analyzing Alcoholic Beverages by Gas Chromatography. (n.d.). [ebook] U.S, p.a single page. Available at: https://chromspec.com/pdf/e/rk03.pdf [Accessed 20 Aug. 2019].
- Chromatography. (2019).
- Merriam-webster.com. (2019). Definition of GAS CHROMATOGRAPHY. [online] Available at: https://www.merriam-webster.com/dictionary/gas%20chromatography [Accessed 15 Aug. 2019].
- Tangerman, A. (1997). Highly sensitive gas chromatographic analysis of ethanol in whole blood, serum, urine, and fecal supernatants by the direct injection method. [online] Clinical Chemistry. Available at: http://clinchem.aaccjnls.org/content/43/6/1003 [Accessed 22 Aug. 2019].
- What is FORENSIC CHEMISTRY? What does FORENSIC CHEMISTRY mean? FORENSIC CHEMISTRY meaning. (2019). Available at: https://www.youtube.com/watch?v=FD_i2xKQqoM [Accessed 20 Aug. 2019].
- Woodford, C. (2018). How does chromatography work?. [online] Explain that Stuff. Available at: https://www.explainthatstuff.com/chromatography.html [Accessed 10 Aug. 2019].
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