Infrared Spectroscopy Questions and Answers

Answer: C

Bands in condensed phases are tens of reciprocal centimetres wide, so resolving to four already describes their shape completely and finer settings only cost time and signal-to-noise. Detector speed, source output and wavelength accuracy are not the binding constraint. A resolution better than the thing being resolved buys nothing, which is the same principle that governs slit width in a dispersive instrument.

Answer: D

Gas-phase bands carry sharp rotational fine structure that a coarse setting smears together, so resolutions of a fraction of a reciprocal centimetre are worthwhile. Solids, library matching and solution quantitation all involve broad bands where the extra resolution is wasted. Matching an instrument setting to the width of the feature being measured, rather than always choosing the finest available, is what keeps a measurement efficient as well as correct.

Answer: A

Vibrational excitation relaxes back to the ground state within picoseconds and leaves the molecule exactly as it was, so a mull or a pellet can be recovered and used again. Moving parts, absorption and dilution are not the point - the photons certainly are absorbed. That a technique returns the sample intact matters greatly when material is scarce or precious, which is why it is often run before anything destructive.

Answer: B

A pellet or an attenuated total reflectance measurement needs a milligram or less, and a microscope accessory works with far less again. Grams and hundreds of millilitres are quite unnecessary, and no oriented crystal is required except in specialised polarised work. Small sample requirement and non-destructiveness together are why the technique is so often the first run on an unknown.

Answer: C

Coupling the spectrometer to a microscope lets a single fibre, a contaminant particle or one layer of a laminate be measured in isolation. Spectral resolution, wavenumber range and the path length problem are unchanged. Bringing a spatial dimension to a technique that had none is what turned infrared into a forensic and failure-analysis tool, and the sample size falls to micrograms in the process.

Answer: D

Wavelengths of ten micrometres cannot be focused below roughly that dimension, so the spatial resolution is far coarser than a visible microscope's. Detector size, sample thickness and mirror travel affect other things. That a fundamental limit follows from the wavelength rather than from the engineering is why near-field and attenuated total reflectance objectives, which sidestep it, are used when finer detail is needed.

Answer: A

Inside a high-index crystal the wavelength is shorter by the refractive index, so the diffraction limit is correspondingly finer - a germanium crystal of index four improves it fourfold. The shallow penetration limits depth rather than lateral resolution, and neither compression nor detector position is the mechanism. Working in a medium where the wavelength is shorter is the same trick immersion microscopy uses in the visible.

Answer: B

Germanium's index of about four gives the largest improvement in the diffraction limit among practical materials, and it is transparent through the mid infrared. It is opaque in the visible, which is a practical nuisance for viewing the sample, and hardness and freedom from bands are secondary. A material chosen for one optical property, at some cost in others, is the ordinary situation in infrared optics where the choice is narrow to begin with.

Answer: C

Diamond withstands hard samples clamped at high pressure and is chemically inert, which zinc selenide and germanium are not. Its index is lower than germanium's, it has a weak absorption of its own near 2000, and it is far more expensive. A material chosen for durability rather than optical performance is the right trade when the alternative is replacing a soft crystal regularly.

Answer: D

The first two are correct and the third is the reverse: the effective path length is micrometres rather than the tens or hundreds a transmission cell offers, so sensitivity is lower. What the technique buys is freedom from sample preparation, which is usually worth far more in routine work. Naming what a technique gives up, alongside what it gains, is what makes the comparison honest.