Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: C

The applied voltage is usually not more than 30 kV.

Capillary electrophoresis is a form of high-voltage electrophoresis. Commercial instruments apply up to about 30 kV across a capillary typically 25-100 μm in internal diameter. The narrow capillary dissipates the heat well enough to allow such high fields, which give fast, high-resolution separations, as in forensic DNA fragment analysis.

10 kV and 15 kV are within the working range but are not the upper limit, and 60 kV is beyond what the instruments use.

Answer: A

Molarity is the moles of solute present in 1 liter of solution.

\[ M = \frac{\text{moles of solute}}{\text{litres of solution}} \]

For example, 58.5 g of sodium chloride (one mole) made up to 1 litre of solution is a 1 M solution.

Moles of solute per kilogram is molality, and it is defined per kilogram of solvent, not solution. Equivalents of solute per litre is normality. Grams of solute per litre is a mass concentration (g/L), which ignores the molar mass and so is not molarity.

Answer: A

In reverse phase chromatography A will elute followed by B.

Reverse phase uses a non-polar stationary phase (typically C18 bonded silica) and a polar mobile phase such as water with methanol or acetonitrile. A polar compound is attracted to the polar mobile phase and spends little time on the non-polar packing, so it comes out first. The less polar compound B is held longer by the stationary phase and elutes after A.

B first would be the order in normal phase chromatography, where the stationary phase is polar. Two compounds of different polarity do not co-elute, and one compound does not prevent the elution of another.

Answer: D

The technique is electrophoresis.

In electrophoresis an electric field is applied across a buffer, and charged species move towards the electrode of opposite charge at rates that depend on their charge, size and shape. Differences in that rate of migration separate them, which is how DNA fragments, serum proteins and isoenzymes are separated in forensic work.

The chromatographic options separate by partition or exchange between a stationary and a mobile phase. Ion chromatography uses an ion-exchange resin, paper chromatography a liquid held on paper, and micellar liquid chromatography a surfactant mobile phase; none of them is driven by an applied field.

Answer: B

Small solvent capacities is not an advantage of the reciprocating pump.

Reciprocating pumps are the standard pumps of HPLC and UHPLC. The advantages usually listed for them are a small internal volume, high output pressure, ready adaptability to gradient elution and a constant flow rate. Their main drawback is a pulsed flow, which needs a pulse damper.

A small solvent capacity describes the displacement or syringe pump, whose limited reservoir has to be refilled. The other three options are the textbook advantages of the reciprocating design, so they are not the answer.

Answer: A

Collisions of absorbing atoms with other atoms and molecules cause Lorentz broadening.

In a flame the absorbing atoms collide continually with gas molecules. Each collision shortens the lifetime of the excited state and slightly shifts its energy, so the absorption line widens. This collisional or pressure broadening is named after Lorentz.

Doppler broadening has a different cause: the random thermal motion of atoms towards and away from the detector. Resonance lines are the lines themselves, between the ground state and an excited state, not a broadening effect, and 'Walsh phenomena' is not a line-broadening mechanism; Walsh is known for introducing atomic absorption spectroscopy.

Answer: C

The key reads both (A) and (R) are correct but (R) is not the correct explanation of (A).

The Assertion is true: commercial spectrophotometers use lamps such as tungsten-halogen and deuterium, which are cheap, stable and give a continuous range of wavelengths that a monochromator can scan. A laser gives only one or a few fixed lines.

The Reason is also accepted: lasers are used where a very intense, narrow beam has to be focused into a tiny volume, as in detectors coupled to capillary electrophoresis or micro-column separations. But that does not explain why lamps are preferred; the preference comes from cost and the need for a continuous spectrum. So the options calling either statement wrong do not fit.

Answer: B

The statement that is not correct is Synthesis of labelled compound is easy.

Preparing a compound labelled with a radionuclide is one of the recognised drawbacks of radiochemical methods: the synthesis is often difficult and costly, and it must be done quickly with short-lived isotopes and with radiation safeguards.

The other statements are true. Radioactive decay is a nuclear process, so the signal does not depend on temperature, pressure or the chemical form of the atom. Because counts follow known statistics, the counting precision can be fixed in advance by choosing the counting time. And a sample made radioactive or mixed with a tracer cannot simply be returned to normal use afterwards.

Answer: B

The correct match is a-v, b-iv, c-ii, d-iii.

• N, the number of theoretical plates, measures column efficiency.
• B/A, the ratio of the back and front half-widths of a peak at 10% of its height, is the asymmetry factor.
• K (k') is the capacity factor or retention factor, how long a compound is retained relative to the dead time.
• RT is the retention time.

Dead time (t_M or t₀) is the time an unretained compound takes and is the extra item in List II. Every other option gives N or K the dead time or swaps B/A and N, which is why they fail.

Answer: B

The correct match is a-iii, b-ii, c-iv, d-i.

• A low pressure mercury vapour lamp gives a few intense lines and is the usual source of the simple filter fluorimeter.
• A high pressure xenon arc lamp gives an intense continuum from the ultraviolet to the visible and is the standard source of the scanning spectrofluorimeter.
• An Nd:YAG laser is used to excite photoluminescence, for example in laser detection of latent prints.
• An ordinary tungsten filament lamp radiates strongly in the near infrared and is the source for that region.

Atomic absorption uses hollow cathode lamps, not any of these sources, so options using (v) are wrong.