Question 35 — Forensic Chemistry (25 marks) Answer parts (a) and (b) of the question on pages 2–4 of the Section II Writing Booklet - HSC - SSCE Chemistry - Question 35 - 2018 - Paper 1

Mass spectrometry offers several advantages:

1. High Sensitivity: It can detect very low concentrations of compounds, making it ideal for forensic analysis.
2. Speed: The analysis can be performed quickly, providing timely results during investigations.
3. Quantitative and Qualitative Data: It provides both the amount and identity of compounds simultaneously, which is useful in complex mixtures.
4. Versatility: Applicable to a wide range of substances, whether volatile or non-volatile.

A suitable chemical test involves using Benedict's reagent. When Benedict's reagent is added to a solution containing a reducing sugar such as glucose and heated, a color change will occur, forming a red/orange/brown precipitate.

In contrast, a non-reducing sugar like sucrose will not produce a color change.

Reducing sugars such as glucose contain a free aldehyde or ketone group that can be oxidized. When Benedict's reagent is introduced, the aldehyde group reduces the Cu²⁺ ions to Cu⁺, producing the characteristic color change. This can be represented by the reaction:

$R-CHO + 2Ag^+ + 2H_2O \rightarrow 2Ag(s) + R-COOH + 2H^+$

In contrast, non-reducing sugars like sucrose do not contain free aldehyde groups; hence they do not react with Benedict's reagent and do not produce a precipitate.

Differences in the structure of DNA, specifically the sequence of nucleotides, can be used to create a unique DNA profile for individuals. Variations in short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs) can serve as genetic markers, allowing forensic scientists to match biological samples from a crime scene to a suspect with high accuracy.

Electrophoresis is a technique used to separate DNA fragments based on their size. When an electric current is applied, negatively charged DNA fragments migrate through a gel matrix towards the positive electrode. Smaller fragments move faster than larger ones, leading to a separation based on size. This resulting pattern of bands can be compared to known samples, allowing for identification and matching to an individual.

Both techniques are used in forensic science but have different applications.

Similarities:

• Both require very small sample sizes.
• Both provide qualitative and quantitative data.

Differences:

• Emission Spectroscopy: Analyzes by exciting atoms to emit light, used for identifying elemental compositions, can identify only components in a mixture.
• Chromatography: Separates components in a mixture based on their distribution between stationary and mobile phases, providing detailed information about mixture compositions. It can separate both organic and inorganic substances.