Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: B

The near-infrared region is 0.80 to 2.50 micrometres.

The infrared part of the spectrum is conventionally divided into three regions:

• Near IR: about 0.78 (0.80) to 2.5 micrometres (12,800 to 4,000 cm^-1).
• Mid IR: 2.5 to 50 micrometres (4,000 to 200 cm^-1), the region of fundamental molecular vibrations used for identification.
• Far IR: 50 to 1,000 micrometres.

The range 0.25 to 0.80 micrometres is ultraviolet and visible light. Near-infrared photography and imaging are used in document examination to distinguish inks that look alike in visible light.

Answer: B

Gratings made photographically with lasers are holographic gratings.

A holographic grating is made by exposing a photoresist coating to the interference pattern of two laser beams, which forms a set of fine, perfectly regular lines. After development the pattern is coated with metal. Such gratings have very few periodic errors, so they produce far less stray light and fewer 'ghost' lines than ruled gratings, which is valuable in spectrophotometers.

Ruled gratings are cut mechanically with a diamond tool. Echelle gratings are coarse ruled gratings used at high orders, and Littrow and Ebert are names of monochromator mountings, not of a way of making gratings.

Answer: B

The highest resolution is given by the TEM, the transmission electron microscope.

Resolution is limited by the wavelength of the illumination. Electrons accelerated through high voltages have wavelengths far shorter than light, so electron microscopes resolve much finer detail. In a TEM the beam passes through an ultra-thin specimen, giving resolution down to about 0.1-0.2 nm, enough to image atomic lattices.

The SEM scans a focused beam over the surface and builds an image from secondary electrons; its resolution is a few nanometres, coarser than the TEM, but it gives three-dimensional surface views and, with EDX, elemental analysis of gunshot residue. Stereo and fluorescence microscopes use light and are limited to about 200 nm.

Answer: C

The molecular mass spectrometry technique is LC-MS.

Liquid chromatography-mass spectrometry separates compounds and ionises them gently (for example by electrospray), so it measures intact molecules and their fragments. It is widely used for drugs, poisons and metabolites.

ICP-MS (inductively coupled plasma) and GD-MS (glow discharge) are atomic or elemental methods: they break the sample down into atoms and measure elements and isotopes. Laser microprobe mass spectrometry is likewise used mainly for elemental analysis of small particles.

Answer: C

The most sensitive optical detector for HPLC is the fluorescence detector.

A fluorescence detector measures the light emitted by excited molecules against a dark background, so it is often 10 to 1,000 times more sensitive than UV absorbance detection, and very selective, for compounds that fluoresce naturally or after derivatisation.

UV and photodiode array detectors measure absorbance, which is less sensitive. A mass detector can be very sensitive, but it is not an optical detector, so it does not answer the question.

Answer: B

Electrospray is not a detector for gas chromatography.

Electrospray ionisation is a method of producing ions from a liquid, used to couple liquid chromatography to a mass spectrometer. It needs a liquid stream, so it has no place in GC, where the sample is a gas.

The electron capture detector (sensitive to halogenated compounds), the thermal conductivity detector (universal) and the flame ionisation detector (for organic compounds) are all standard GC detectors.

Answer: A

The Kovats retention index is used in gas chromatography.

Devised by Ervin Kovats in 1958, it expresses a compound's retention time relative to those of n-alkanes run under the same conditions: an alkane with n carbons has an index of 100n. Because it depends less on column and instrument settings than raw retention time, it helps identify compounds across laboratories.

HPLC, capillary electrophoresis and paper chromatography use other measures of retention or migration, such as retention factors and Rf values.

Answer: A

The highest flame temperature comes from acetylene + oxygen.

The oxy-acetylene flame reaches about 3,100 degrees Celsius, which is why it is used for welding and cutting steel.

Acetylene with nitrous oxide reaches about 2,700 to 2,950 degrees, used in atomic absorption for elements that need a hot flame. Hydrogen with nitrous oxide is a little cooler, and acetylene with air reaches only about 2,300 degrees, because the nitrogen in air absorbs heat.

Answer: D

A low signal to noise ratio is not an advantage of diode array spectrophotometers.

A low signal-to-noise ratio is a disadvantage in any instrument, because it means the signal is hard to distinguish from noise. A diode array spectrophotometer in fact gives a good signal-to-noise ratio, because it can average many rapid spectra.

Its real advantages are the other three: it measures many wavelengths simultaneously across the whole spectrum, scans in a fraction of a second, and has high wavelength precision because there are no moving parts.

Answer: D

When a neutron turns into a proton, negative beta rays are produced.

In beta-minus decay a neutron in the nucleus changes into a proton, emitting an electron (a negative beta particle) and an antineutrino. The atomic number rises by one, while the mass number stays the same.

Positive beta (positron) emission is the reverse, a proton becoming a neutron. Alpha decay emits a helium nucleus, and gamma rays are photons released when a nucleus drops to a lower energy state; neither involves a neutron becoming a proton.