Instrumental methods, forensic, environmental, clinical applications: Practice Questions

What is the primary purpose of using mass spectrometry in chemical analysis?

A forensic chemist needs to identify a trace amount of an unknown volatile organic substance found at a crime scene. The substance is volatile and a detailed molecular structure, not just its elemental composition, is crucial for identification. Which analytical technique would be most appropriate for this task?

A forensic scientist needs to identify a trace amount of a non-polar, non-volatile solid compound recovered from a seized material. The scientist performs a quick confirmatory chemical test (Test X) which indicates the presence of an aromatic ring, but a detailed analysis using Gas Chromatography-Mass Spectrometry (GC-MS) provides the complete structural identity as N-phenylacetamide (acetanilide).

Which of the following justifies the use of GC-MS instead of relying solely on the quick chemical test (Test X) in forensic investigations?

An unknown organic liquid is analyzed using infrared (IR) spectroscopy. The spectrum shows a strong, sharp absorption band at approximately \(1740 \text{ cm}^{-1}\) and distinct C-H stretching vibrations around \(2900 \text{ cm}^{-1}\). However, there is no broad absorption band between \(3200 \text{ cm}^{-1}\) and \(3500 \text{ cm}^{-1}\). Which compound is the unknown liquid most likely to be?

An unknown compound with molecular formula \(C_4H_{10}O\) is analysed by mass spectrometry. The mass spectrum exhibits a molecular ion peak (\(M^+\)) at \(m/z = 74\). Significant fragment ion peaks are observed at \(m/z = 59\) and \(m/z = 45\). Which of the following compounds is the unknown substance?

Ethyl ethanoate has a molecular formula of \(C_4H_8O_2\). A mass spectrum of this compound shows a prominent fragmentation peak at \(m/z = 43\). Identify the fragment ion responsible for this peak and describe its structural composition.

Describe how determining the melting point of a solid pharmaceutical compound in a clinical or forensic setting serves as an indication of its purity, and state the expected observation if the sample is confirmed to be significantly impure.

Describe the role of analytical chemistry in environmental protection regarding the monitoring of atmospheric pollutants like carbon monoxide (\(\text{CO}\)).

A chemist analyzes a water sample for two common synthetic dyes, Dye A and Dye B, using thin layer chromatography (TLC). A spot containing the mixture is applied to a silica plate. After elution with a suitable solvent, the solvent front traveled \(8.0 \text{ cm}\). Dye A traveled \(4.8 \text{ cm}\) and Dye B traveled \(6.4 \text{ cm}\).

a) Define the term \(R_f\) value as used in chromatography.

b) Calculate the \(R_f\) values for both Dye A and Dye B.

c) Given that the stationary phase (silica) is polar, suggest whether Dye A or Dye B is more polar, and justify your answer based on their calculated \(R_f\) values.

An unknown straight-chain ester \(\textbf{Z}\) has the molecular formula \(C_5H_{10}O_2\). The mass spectrum of \(\textbf{Z}\) shows a molecular ion peak (\(M^+\)) at \(m/z = 102\). The most prominent fragment ions observed are at \(m/z = 43\) and \(m/z = 59\).

a) Draw the structural formula of one possible straight-chain ester isomer of \(C_5H_{10}O_2\) that is not \(\textbf{Z}\).

b) Identify the chemical formula and structure of the fragment ion responsible for the \(m/z = 43\) peak, stating the type of fragmentation that occurs.

c) Deduce the structural formula and systematic name of ester \(\textbf{Z}\) based on the fragmentation pattern, and justify your answer by showing how the observed \(m/z = 59\) peak is formed.