Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: C

The correct matching is a-(iii), b-(iv), c-(i), d-(ii).

Kohlrausch's regulating function (1897) is the theoretical basis of isotachophoresis. Shapiro and colleagues (1967) showed that proteins in SDS run by molecular weight, the basis of SDS-PAGE. Harry Svensson (Rilbe) developed the theory of isoelectric focusing in the early 1960s. Pierre Grabar, with Williams, introduced immunoelectrophoresis in 1953.

Only the third code gives all four pairs; the first, for example, wrongly gives Kohlrausch immunoelectrophoresis.

Answer: D

With crossed polarizer and analyzer, no light will penetrate.

The polarizer lets through light vibrating in one plane only. If the analyzer is set at 90 degrees to it, it blocks exactly that plane, so by Malus's law the transmitted intensity, which varies as the square of the cosine of the angle, falls to zero and the field is dark.

Light passes only if something between them, such as a birefringent fibre or crystal, changes the plane of polarisation. That is why crossed polars are used in fibre and soil examination, but on their own they show no light, no bands and no halo.

Answer: B

The fluorescent group in a molecule is called a fluorophore.

A fluorophore is the part of a molecule that absorbs light of one wavelength and re-emits it at a longer wavelength. Fluorophores underlie fluorescent fingerprint powders, DNA dyes and fluorescence detectors.

A chromophore is the part of a molecule that absorbs light and gives colour, but need not fluoresce. Quantum yield is a measure of how efficiently a fluorophore emits light, not a group in the molecule, and 'photophore' is a light-emitting organ in some animals.

Answer: D

The size exclusion methods are (b) and (d) only: gel filtration and gel permeation.

Size exclusion chromatography separates molecules by size as they pass through a porous gel. Small molecules enter the pores and take longer; large ones are excluded and elute first. When the mobile phase is aqueous (as for proteins) it is called gel filtration chromatography, and when it is an organic solvent (as for synthetic polymers) gel permeation chromatography.

Gel adsorption separates by adsorption onto a surface, not by size, and gel diffusion usually refers to immunodiffusion techniques such as Ouchterlony's, where antigens and antibodies diffuse through agar and precipitate.

Answer: D

The highest refractive index belongs to toluene.

Approximate refractive indices at 20 degrees C are: water 1.333, ethanol 1.361, chloroform 1.446 and toluene 1.497. Aromatic compounds such as toluene, with their polarisable pi electrons, have higher refractive indices than water, alcohols or chlorinated alkanes.

Refractive index matters in liquid chromatography because the refractive index detector responds to the difference between the eluent and the analyte. Water is the lowest of the four, and chloroform, though dense, is still below toluene.

Answer: A

The carrier gases are (a) and (c) only: hydrogen and nitrogen.

In gas-liquid chromatography the carrier gas must be inert towards the sample and the stationary phase and suit the detector. Helium, nitrogen, hydrogen and argon are used; nitrogen is cheap, and hydrogen gives fast analyses with good efficiency.

Acetylene and nitrous oxide are reactive gases. They are used as fuel and oxidant in the flame of atomic absorption spectroscopy, not as carriers in GLC, which is why any option containing (b) or (d) is wrong.

Answer: D

A mass analyser's performance is measured by its resolving power.

The resolving power of a mass analyser is its ability to separate ions of nearly equal mass-to-charge ratio, usually expressed as \(m / \Delta m\). High-resolution analysers such as time-of-flight, Orbitrap and FT-ICR instruments can distinguish ions differing by a small fraction of a mass unit.

The retardation factor (Rf) belongs to thin-layer chromatography, the retention factor to column chromatography, and a separation coefficient to partitioning between phases. All describe chromatographic separation, not the performance of a mass spectrometer.

Answer: C

A silicon drift detector is a semiconductor transducer.

The silicon drift detector, now standard in energy-dispersive X-ray spectrometers attached to SEMs and XRF instruments, is a silicon diode in which an X-ray photon creates electron-hole pairs in proportion to its energy. The charge drifts to a small collecting anode, giving fast, high-resolution counting with only modest cooling.

Ionization chambers and Geiger tubes are gas-filled detectors, and scintillation detectors convert X-rays into flashes of light in a crystal that a photomultiplier then counts. None of these is a semiconductor device.

Answer: C

The answer is (c) and (d) only: oxygen and silicon.

Oxygen and silicon have very small cross-sections for thermal neutron capture and give no useful activation products that way. With fast (14 MeV) neutrons, however, they undergo threshold reactions: \(^{16}\mathrm{O}(n,p)^{16}\mathrm{N}\) and \(^{28}\mathrm{Si}(n,p)^{28}\mathrm{Al}\), whose short-lived products are easily counted. Fast-neutron activation is therefore the standard way of determining oxygen and silicon.

Chlorine and nickel are measured more sensitively with thermal neutrons, through capture reactions such as \(^{37}\mathrm{Cl}(n,\gamma)^{38}\mathrm{Cl}\), so options including (a) or (b) are wrong.

Answer: D

The key gives solids and liquids both.

In inductively coupled plasma spectrometry (ICP-OES or ICP-MS) the sample is usually introduced as a liquid, nebulised into the argon plasma. Solid samples such as glass, soil, paint or gunshot residue are analysed either after acid digestion into solution or directly by laser ablation, which vaporises a small spot of the solid into the plasma.

Because both liquid and solid samples are routinely analysed, an answer naming only solids or only liquids is incomplete. Gases are not the normal sample form for ICP analysis.