Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: D

Tenax TA is based on 2, 6-diphenylene oxide.

Tenax TA is a porous polymer of 2,6-diphenyl-p-phenylene oxide. It is hydrophobic and stable at high temperatures, so it is widely used in sorbent tubes to trap volatile and semi-volatile organic compounds from air and then release them by thermal desorption into a gas chromatograph; in fire investigation it is used to collect ignitable liquid residues.

2,4-dinitrophenylhydrazine is a reagent for aldehydes and ketones, 2,4,6-tribromoaniline is a bromination product of aniline, and Asahipak NH2P-50 is an HPLC column packing, not the basis of Tenax.

Answer: A

MRM stands for multiple reaction monitoring.

In tandem mass spectrometry with a triple quadrupole, the first quadrupole selects a precursor ion, the second fragments it by collision, and the third selects a specific product ion. Monitoring several such precursor-to-product transitions at once is multiple reaction monitoring.

Because each transition is specific to a compound, MRM gives very sensitive and selective quantitation, and it is widely used in forensic toxicology to confirm drugs and poisons in blood and urine. 'Multiple range', 'multiple ratio' and 'magnetic resonance' monitoring are not what the term means.

Answer: C

The parameter that is not part of bioanalytical method validation is retention.

The US FDA's guidance on bioanalytical method validation for chromatographic methods such as LC-MS lists the parameters to be established: selectivity, accuracy, precision, sensitivity (the lower limit of quantification), the calibration curve, recovery, reproducibility, stability of the analyte, and for LC-MS also matrix effect, dilution integrity and carry-over.

Accuracy, precision and carry-over are therefore all validation parameters. Retention time is used to identify the analyte in a chromatogram, but it is a property of the analysis rather than one of the validation characteristics the guidance requires.

Answer: A

Lead at microgram level in groundwater is best determined by AAS, atomic absorption spectroscopy.

AAS measures the absorption of a characteristic wavelength (283.3 nm or 217.0 nm for lead) by free atoms of the element in a flame or graphite furnace. It is element-specific, sensitive to microgram and, with a graphite furnace, sub-microgram levels, inexpensive and routinely used for heavy metals in water, as in the BIS and APHA standard methods.

NMR studies molecular structure and is not a trace-metal technique. Neutron activation analysis is sensitive but needs a nuclear reactor and is poor for lead. GC-MS analyses volatile organic compounds, not metals.

Answer: D

The source not used in LC-MS/MS is field ionization.

An LC-MS interface must ionise analytes coming out of a liquid stream at atmospheric pressure. The standard atmospheric pressure ionisation sources are electrospray ionisation (ESI), for polar and large molecules; atmospheric pressure chemical ionisation (APCI), for less polar, thermally stable compounds; and atmospheric pressure photoionisation (APPI), for non-polar compounds.

Field ionisation produces ions from gas-phase molecules in a very high electric field under vacuum. It is used with gas chromatography or direct inlets, not as an LC-MS source, so it is the odd one out.

Answer: A

The correct sequence is nebulisation, desolvation, volatilisation, dissociation.

In flame atomic absorption the sample solution goes through these steps:

1. Nebulisation: the solution is converted into a fine aerosol mist.
2. Desolvation: in the flame the solvent evaporates, leaving tiny solid particles.
3. Volatilisation: the solid particles vaporise into gaseous molecules.
4. Dissociation (atomisation): the molecules break into free ground-state atoms that absorb the lamp's radiation.

Each step depends on the previous one, so the other sequences, which put volatilisation or dissociation before the aerosol is formed or the solvent removed, are physically impossible.

Answer: C

Scattering with no change in wavelength is Rayleigh scattering.

When light strikes molecules, most of the scattered light is elastic: the photons keep the same energy and so the same wavelength as the incident light. This is Rayleigh scattering, whose intensity varies as the inverse fourth power of wavelength, which is why the sky is blue.

A very small fraction is scattered inelastically, the Raman effect, with a shift in energy. Stokes lines have longer wavelengths than the incident light (the molecule gains vibrational energy), and anti-Stokes lines shorter wavelengths (the molecule loses energy). Raman spectroscopy uses these shifted lines, which is why it is valuable for identifying drugs, inks and explosives.

Answer: B

The transducer used in a mass spectrometer is the electron multiplier.

After ions are separated by their mass-to-charge ratio, they must be counted. In an electron multiplier, each ion striking the first dynode releases several secondary electrons, which are accelerated to the next dynode and multiplied again, giving a gain of about a million and a measurable current for even single ions. It is the standard detector in GC-MS and LC-MS instruments.

The electron capture detector is a gas chromatography detector for halogenated compounds, the bolometer is a thermal detector for infrared radiation, and the phototube detects light in UV-visible spectrophotometers.

Answer: A

The technique is isotope dilution analysis.

In isotope dilution, a known amount of the analyte labelled with a radioactive or stable isotope (a spike) is added to the sample and mixed thoroughly with the unlabelled analyte already present. A portion of the analyte is then isolated and its isotopic ratio, or specific activity, is measured. The change in that ratio caused by dilution with the natural analyte gives the amount originally present, even if recovery of the isolated portion is incomplete.

Reverse isotope dilution works the other way, adding unlabelled material to determine a labelled compound. Isotope exchange is a different phenomenon in which isotopes swap between species.

Answer: D

The pyroelectric material used in infrared detectors is triglycine sulfate.

Pyroelectric detectors use crystals whose electrical polarisation changes with temperature. When modulated infrared radiation warms the crystal, the change in polarisation produces a current. Triglycine sulfate (TGS), often deuterated (DTGS), is the classic pyroelectric material and is used as the detector in many FTIR spectrometers.

3-aminophthalate is the light-emitting product of the luminol reaction used to detect blood. Nd:YAG is a laser crystal, and 1,2-dimethoxyethane is an organic solvent.