Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: A

MCT detectors are used in FTIR spectrometry.

Mercury cadmium telluride is a semiconductor whose band gap is small enough to respond to mid-infrared radiation. Cooled with liquid nitrogen, MCT detectors are fast and highly sensitive, which suits FTIR spectrometers and especially FTIR microscopes used to analyse fibres, paint chips and drug traces.

Atomic absorption spectrometers use photomultiplier tubes to measure ultraviolet and visible light, and GC-MS and LC-MS detect ions with electron multipliers. None of them uses an infrared detector.

Answer: A

The common light sources are (a) and (b) only: the xenon arc lamp and the mercury arc lamp.

A fluorescence microscope needs an intense source of short-wavelength light to excite fluorescence. High-pressure mercury arc lamps (such as the HBO lamp) give strong emission lines in the ultraviolet and blue, and xenon arc lamps give a bright, fairly even spectrum across the visible range. These two have been the standard sources.

Lasers are used in specialised confocal systems rather than in the common fluorescence microscope, and a ruby laser, which emits deep red light, is unsuitable for exciting most fluorophores.

Answer: B

Pure anthracene fluoresces blue.

Anthracene, a polycyclic aromatic hydrocarbon of three fused rings, gives a strong violet-blue fluorescence under ultraviolet light. Its fluorescence is a classic example in spectroscopy and the basis of its use in scintillation counters.

Crude anthracene often looks yellowish-green under UV because of impurities such as tetracene, which is why green is a tempting wrong answer. Pure anthracene does not fluoresce yellow or orange.

Answer: A

The gas-phase ionization technique is electron impact.

In electron impact (EI) ionization, the sample is first vaporised, and the gas-phase molecules are then struck by a beam of electrons (usually 70 eV), which knock out an electron and produce molecular ions and fragments. It is the standard source in GC-MS.

The other three are desorption techniques, in which ions are formed directly from a condensed sample: fast atom bombardment ionises a sample dissolved in a liquid matrix, field desorption ionises a sample coated on an emitter in a strong electric field, and laser desorption uses laser pulses on a solid. They suit non-volatile and thermally fragile compounds that cannot be vaporised.

Answer: A

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

• Flame ionization detector: responds to carbon-hydrogen bonds burnt in a hydrogen flame, so it is the detector for hydrocarbons and most organic compounds.
• Thermal conductivity detector: senses any change in the thermal conductivity of the carrier gas, so it responds to every compound and is the universal detector.
• Electron capture detector: highly sensitive to electronegative atoms, so it is used for halogenated compounds such as organochlorine pesticides.
• Thermionic (nitrogen-phosphorus) detector: selective for compounds containing nitrogen and phosphorus.

Every other option pairs at least one detector with the wrong class, for example the electron capture detector with nitrogen and phosphorus compounds.

Answer: D

The crystal with the largest interplanar spacing is ammonium dihydrogen phosphate.

In a wavelength-dispersive X-ray spectrometer the crystal diffracts X-rays according to Bragg's law, \(n\lambda = 2d\sin\theta\), so a larger spacing \(d\) reaches longer wavelengths. The common analysing crystals have approximately these 2d values:

• Topaz: 2.71 Å
• Lithium fluoride: 4.03 Å
• Sodium chloride: 5.64 Å
• Ammonium dihydrogen phosphate (ADP): 10.6 Å

ADP is therefore used for the long-wavelength X-rays of light elements, while LiF and topaz, with small spacings, give high dispersion for the short wavelengths of heavier elements.

Answer: D

XRF is generally used for elements with atomic numbers greater than 8.

X-ray fluorescence detects the characteristic X-rays emitted when inner-shell vacancies are filled. For the lightest elements these X-rays are very soft and are absorbed by air, the detector window and the sample itself, and the fluorescence yield is tiny, because the excited atom loses its energy by Auger emission instead. In practice qualitative XRF covers elements heavier than oxygen (atomic number 8).

Limits of 2, 4 or 6 would include beryllium, boron, carbon or nitrogen, which routine XRF cannot detect reliably.

Answer: D

Mulling is associated with IR spectrometry.

To record the infrared spectrum of a solid, the finely ground sample is made into a paste, a mull, with a few drops of a heavy oil such as Nujol (liquid paraffin) or a fluorinated oil, and pressed between salt plates. The oil lowers the scattering of the infrared beam by the particles. The oil's own C-H bands must be allowed for, which is why a fluorocarbon mull is sometimes used alongside it.

Atomic emission and atomic absorption spectroscopy atomise the sample in a flame, plasma or furnace, and UV-visible spectrometry normally uses solutions, so none of them uses a mull.

Answer: D

The diode that emits the longest wavelength is gallium aluminium arsenide.

An LED emits at a wavelength set by the band gap of its semiconductor: the smaller the gap, the longer the wavelength. Roughly:

• gallium nitride: blue to near-UV, about 365-450 nm;
• gallium phosphide: green, about 555 nm;
• gallium arsenide phosphide: red to yellow, about 580-650 nm;
• gallium aluminium arsenide: deep red to near-infrared, about 660-880 nm.

GaAlAs has the smallest band gap of the four, so it gives the maximum wavelength. GaN, with the widest gap, gives the shortest.

Answer: C

The off-axis aberrations are (c) and (d) only: coma and field curvature.

Off-axis aberrations appear only for object points away from the optical axis and grow with the field angle. Coma makes off-axis points look like comet-shaped smears, and field curvature means the sharp image lies on a curved surface instead of a flat one. Astigmatism and distortion are the other off-axis aberrations.

Spherical aberration (a) occurs even for a point on the axis, because marginal rays focus nearer the lens than paraxial ones. Chromatic aberration (b) arises from dispersion and is present on axis too. Any option containing (a) or (b) is therefore wrong.