Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: B

The method is collision induced dissociation (CID).

In tandem mass spectrometry (MS/MS) the first analyser selects a precursor ion. That ion is accelerated into a collision cell containing an inert gas such as argon, nitrogen or helium, where collisions convert part of its kinetic energy into internal energy and break it into product ions, which the second analyser then separates. The product ion pattern identifies the molecule with high specificity, which is why MS/MS is used to confirm drugs and toxins.

The other names are not the standard technique. Fragmentation is caused by collision with gas atoms, not by mass, charge or a generic 'impact'.

Answer: A

The quantity is polarizability.

An electric field distorts the electron cloud of a molecule and induces a dipole moment. For moderate fields the induced dipole is proportional to the field, and the constant of proportionality is the polarizability (alpha):

\[ \alpha = \frac{\mu_{\text{ind}}}{E} \]

A change in polarizability during a vibration is what makes it Raman active. The total dipole moment includes any permanent dipole, 'unit dipole moment' is not a defined quantity, and depolarization (as in the Raman depolarization ratio) describes scattered light, not this ratio.

Answer: A

The key's answer is ethylene.

A vibration absorbs infrared radiation only if it changes the molecule's dipole moment. Hydrochloric acid and carbon monoxide are polar diatomic molecules whose single stretching vibration changes the dipole, so both absorb strongly in the IR, and nitric acid is a polar molecule with many IR bands.

Ethylene is a non-polar, centrosymmetric molecule. Its characteristic C=C stretching vibration is symmetric, causes no change in dipole moment, and is therefore IR inactive (it is seen in the Raman spectrum instead). Ethylene does have some other IR-active vibrations, but of the four it is the one whose key vibration is IR inactive, which is the reading the key takes.

Answer: B

The precolumn is a scavenger column.

In HPLC a scavenger (or saturator) column is placed between the pump and the injector. It is packed with silica, which dissolves slightly into the mobile phase, so the mobile phase is conditioned (pre-saturated) before it reaches the analytical column and does not dissolve that column's packing.

A guard column sits after the injector, just before the analytical column, and protects it by trapping particles and strongly retained sample components; it does not condition the mobile phase. 'Conditioning column' and 'mobilizing column' are not standard terms.

Answer: B

Time-of-flight mass spectrometry operates in pulse mode.

In a TOF analyser, ions are accelerated by the same voltage so that they all gain the same kinetic energy, and their mass is found from the time they take to cross a field-free flight tube: lighter ions arrive first. For the timing to be meaningful all ions must start together, so they are produced or extracted in short pulses, for example by a pulsed laser in MALDI or by orthogonal pulsed extraction.

A continuous beam would give no start time, and 'direct' and 'syntactic' are not operating modes of a TOF instrument.

Answer: B

The correct sequence is entrance slit, collimating lens, prism, focusing lens, exit slit.

Light enters a monochromator through the entrance slit. A collimating lens (or mirror) makes the rays parallel, the prism (or grating) disperses them into their wavelengths, the focusing lens forms an image of the slit for each wavelength, and the exit slit lets through only a narrow band of wavelengths.

Placing the focusing lens before collimation, or the prism before the collimating lens, would disperse non-parallel light, and the third option runs the light path backwards.

Answer: C

LOD and LOQ are obtained from the signal to noise ratio.

The limit of detection is the smallest amount of analyte that gives a peak clearly distinguishable from baseline noise, conventionally a signal-to-noise ratio of about 3:1. The limit of quantitation is the smallest amount that can be measured with acceptable precision and accuracy, conventionally about 10:1.

Retention time, column efficiency, peak asymmetry and dead time describe chromatographic separation and peak shape, not the sensitivity limits of a method.

Answer: A

The source is the Coolidge tube, spelled 'Collidge' in the paper.

An X-ray fluorescence (XRF) spectrometer, which the paper calls 'XRE', excites a sample with primary X-rays and measures the secondary (fluorescent) X-rays it emits to identify its elements. The primary X-rays usually come from a Coolidge-type X-ray tube, in which electrons from a heated filament are accelerated onto a metal anode such as rhodium or tungsten.

A Crookes tube was the early cold-cathode discharge tube, a hollow cathode lamp is the line source used in atomic absorption spectroscopy, and an incandescent bulb gives visible light, not X-rays.

Answer: B

A column with 2 µm particles is the most suitable.

Ultra-high-performance liquid chromatography uses columns packed with very small particles, typically below 2 micrometres, which give much higher efficiency and faster separations. Back-pressure rises steeply as particle size falls, so these columns need pumps that work at around 15,000 psi or more; a pressure of 14,000 psi is exactly what a sub-2 micrometre column generates.

Particles of 5 and 10 micrometres are the sizes of conventional HPLC, run at a few thousand psi, and 20 micrometres is used for preparative work. None of them needs UHPLC pressure.

Answer: D

X-rays are generated with a Coolidge tube.

The Coolidge tube, introduced by William Coolidge in 1913, is the standard X-ray tube. A heated tungsten filament (cathode) emits electrons, which are accelerated through a high vacuum by a high voltage and strike a metal target (anode); their sudden deceleration and the excitation of the target atoms produce X-rays.

The Anger camera is a gamma camera for detecting radiation in nuclear medicine, not a source of X-rays, and 'West tube' and 'Mayer tube' are not X-ray tubes.