Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: B

The technique is NAA, neutron activation analysis.

In NAA a sample is bombarded with neutrons, usually in a nuclear reactor. Some of its nuclei capture neutrons and become radioactive, and as they decay they emit gamma rays whose energies identify each element and whose intensities show how much is present. It is extremely sensitive and was used in forensic work on gunshot residue, hair and bullet lead.

ICP-MS ionises the sample in a plasma and sorts the ions by mass, and AAS (atomic absorption spectroscopy) measures light absorbed by atoms in a flame or furnace. Neither uses neutrons. 'XBF' is not a standard technique (X-ray fluorescence is XRF).

Answer: B

The sequence describes HPLC, high performance liquid chromatography.

In HPLC the liquid mobile phase is drawn from its reservoir and delivery system and forced by a high-pressure pump through the injector, where the sample enters, into a packed column, where the separation happens. The separated components pass through a detector (often UV), and its signal goes to a data system that produces the chromatogram.

Gas chromatography uses a carrier gas from a cylinder, not a pump. HPTLC separates on a plate with no pump or column. Capillary zone electrophoresis drives separation by an electric field in a capillary, not by a pump.

Answer: B

WCOT and PLOT are examples of tubular columns, the open tubular (capillary) columns of gas chromatography.

In a WCOT (wall-coated open tubular) column, a thin film of liquid stationary phase coats the inner wall of a fused-silica capillary. In a PLOT (porous-layer open tubular) column, the wall is coated with a layer of porous solid adsorbent. A third type, SCOT, has a support-coated wall. All have an open centre through which the carrier gas flows.

They are column designs, not peak shapes or injection ports, and UPLC is a liquid chromatography technique with packed columns.

Answer: D

R_f is the distance travelled by the solute divided by the distance travelled by the solvent front.

Both distances are measured from the origin line where the sample was spotted:

\[ R_{f} = \frac{d_{\text{solute}}}{d_{\text{solvent front}}} \]

Because the solute can never move further than the solvent, R_f always lies between 0 and 1. Option A is the reciprocal, which would always be 1 or more. The ratios using a standard spot define a relative value (R_x or R_st), used to compare a component with a reference, not R_f.

Answer: B

The detector is the electron capture detector (ECD).

An ECD contains a radioactive source (usually nickel-63) that ionises the carrier gas to give a steady current. Molecules with electronegative atoms, especially halogens, capture electrons and reduce that current. It is therefore extremely sensitive to chlorinated pesticides such as DDT and lindane, halogenated herbicides and PCBs, at picogram levels.

Ultraviolet and refractive index detectors are used in liquid chromatography, not gas chromatography, and electrochemical detectors are mainly used in HPLC for oxidisable or reducible compounds.

Answer: C

It is not called the migration time.

The time an unretained substance, travelling with the mobile phase, takes to pass through a chromatographic column from injector to detector is called the dead time, also the void time or hold-up time (written \(t_{M}\) or \(t_{0}\)). It is subtracted from retention times to give adjusted retention times.

Migration time is the term used in capillary electrophoresis for the time an analyte takes to reach the detector under the electric field. So dead time, void time and hold-up time are all names for the quantity, and migration time is not.

Answer: B

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

• A UV spectrophotometer measures the absorption of ultraviolet light (d).
• Chromatography separates by distribution between phases; in its classic forms (column, thin layer) the stationary phase holds compounds by adsorption (b).
• NMR places nuclei in a strong magnetic field and records their resonance (c).
• Atomic absorption spectrometry uses a hollow cathode lamp of the element being measured as its light source (a).

The codes that pair NMR with the hollow cathode or AAS with the magnetic field reverse the last two items, and those pairing UV with adsorption reverse the first two.

Answer: C

The cause is electrostatic interaction between species reducing absorption.

The Beer-Lambert law assumes that absorbing molecules act independently. At high concentrations, usually above about 0.01 M, they are close enough for electrostatic interactions between them to alter their charge distribution and so their ability to absorb at a given wavelength. The molar absorptivity changes and the plot of absorbance against concentration curves away from a straight line.

Dissociation and reaction with the solvent are chemical deviations caused by equilibria, which can occur at any concentration and are not the high-concentration effect. Fluorescence is an instrumental or sample interference, not the cause of non-linearity at high concentration.

Answer: B

In capillary electrophoresis we measure the migration time of the analyte.

In capillary electrophoresis, analytes move through a narrow fused-silica capillary under a high voltage, driven by their own electrophoretic mobility and by electro-osmotic flow. The time each takes to travel from the inlet to the detector is its migration time, which identifies it and from which its mobility is calculated.

Retention time is the corresponding term in chromatography, where compounds are retained by a stationary phase. 'Electrophoretic time' and 'anodic time' are not standard terms.

Answer: C

The answer is (i) and (ii) both are correct.

A flame ionisation detector burns hydrogen in an excess of air, a lean, oxygen-rich flame in which organic compounds are burnt completely and form ions that are measured as a current. A flame photometric detector uses a hydrogen-rich flame, which reduces sulphur and phosphorus compounds to excited species (S₂* and HPO*) that emit light at characteristic wavelengths.

Since both descriptions are accurate, the options marking either one wrong, or both, are incorrect.