Forensic Instrumentation and Spectroscopy Questions and Answers

Answer: A

Nuclear magnetic resonance is a tool to investigate nuclear structure in the sense of the key: atomic nuclei with spin are placed in a magnetic field and absorb radio frequency radiation, and the signals show the environment of the nuclei and so the structure of the molecule.

Electron structure is studied by methods such as ESR or photoelectron spectroscopy. Scattering of light is the basis of Raman or turbidity measurements. Reflection of light belongs to reflectance spectroscopy. None of them is the working principle of NMR.

Answer: A

An MCT detector is used in FTIR spectrometry. MCT is mercury cadmium telluride, a photoconductive semiconductor that is cooled with liquid nitrogen and gives a very sensitive and fast response across the mid infrared range, which suits a Fourier transform instrument and an FTIR microscope.

AAS uses a photomultiplier tube for visible and ultraviolet light, GC-MS uses an electron multiplier for ions, and LC-MS likewise detects ions. These are not infrared detectors, so they do not use MCT.

Answer: C

In a scanning electron microscope the incident electron beam is focussed by electromagnets, called electromagnetic lenses. Coils carrying current create a magnetic field that bends the path of the electrons and brings them to a fine spot on the specimen.

Glass lenses cannot focus electrons, because electrons are charged particles and are absorbed by glass. Mirrors are used for light, not for an electron beam. Slits are apertures that limit the beam and do not focus it. So the key accepts electromagnets.

Answer: C

The instrument that can view a particle under a microscope while a light beam passes through it to give an absorption spectrum is the micro spectrophotometer. It joins an optical microscope to a spectrophotometer, so a single fibre, a paint chip or an ink particle can be seen and its visible or UV spectrum recorded.

An atomic force microscope maps a surface by a probe and gives no absorption spectrum. XRF and XRD spectrographs use X rays to give the elements and the crystal structure.

Answer: D

Spin-spin coupling is observed in nuclear magnetic resonance spectroscopy, option D. In an NMR spectrum, the magnetic field of one nucleus acts on the neighbouring nuclei through the bonding electrons and splits each signal into a multiplet, from which the number of neighbouring hydrogens can be read. Raman spectroscopy (A) measures scattered light, neutron activation analysis (B) measures radiation from activated nuclei, and mass spectrometry (C) separates ions by mass to charge ratio, so none of them shows spin-spin coupling.

Answer: B

In X-ray fluorescence spectrometry the sample is excited by X-rays from an X-ray tube, option B. The primary X-rays knock out inner shell electrons of the atoms of the sample, which then emit secondary, characteristic, X-rays whose energies identify the elements present. An electron gun (A) is the exciting source in an electron microscope or electron probe analyser, a proton gun (C) in PIXE, and a neutron gun (D) in neutron activation, so they are rejected. The wavelength dispersive and energy dispersive versions of the technique differ only in how the secondary X-rays are separated; both use an X-ray tube as the primary source.

Answer: A

The electrodeless discharge lamp is used in the atomic absorption spectrometer, option A. In atomic absorption the light source must emit the narrow line spectrum of the element being measured, and the hollow cathode lamp does this for most elements, but for volatile elements such as arsenic, selenium and antimony an electrodeless discharge lamp gives a more intense output. Atomic emission (B), infra-red (C) and X-ray fluorescence (D) instruments use other sources, such as a plasma, a heated source or an X-ray tube.

Answer: D

The basic requirements of a monochromator include resolution, spectral range and dispersion, so the answer is all of the above, option D. A monochromator isolates a narrow band of wavelengths from a source and should be able to separate close wavelengths (resolution), cover the range of wavelengths needed for the analysis (spectral range), and spread the wavelengths widely apart (dispersion). Since each of A, B and C is a requirement, none alone is the complete answer.

Answer: A

Infra-red spectrometry gives a unique fingerprint of a chemical structure, option A. The absorption of infra-red radiation by the vibrations of the bonds of a molecule, especially in the region from about 1500 to 400 wave numbers, is so characteristic that this region is called the fingerprint region, and it is used to identify a compound by comparing its spectrum with a library. Ultraviolet (B) and visible (D) spectra show broad bands that are shared by many compounds, and the near infra-red (C) shows overtone bands, which are less specific. The region below about 1500 wave numbers, in which the bending and skeletal vibrations occur, is where two different compounds almost never give the same pattern, even if their functional groups are alike.

Answer: B

Mass spectrometry requires the sample to be in the vapour state, option B. The sample must be vaporised and ionised before the ions are separated by their mass to charge ratio, so liquids and solids are first heated or otherwise changed to vapour, or ionised from the liquid or solid by special techniques. The key takes the vapour state as the basic requirement; the liquid, solid and semisolid states (A, C and D) are not accepted, because they would need conversion to gas phase ions before analysis. Gas chromatography coupled with mass spectrometry makes use of this, since the compounds are separated in the vapour state in the column and then pass into the mass spectrometer.