Infrared Spectroscopy Questions and Answers

Answer: C

Grinding the solid with dry potassium bromide and pressing it under high pressure produces a transparent disc in which the sample is dispersed, with no interfering bands of the solvent. It is neither a solution, a window nor a standard. Its advantage over a mull is the clean spectrum; its disadvantages are that the salt absorbs water, giving a spurious hydroxyl band, and that the pressure can alter some crystalline forms. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Potassium bromide is hygroscopic, so unless it is dried and handled quickly it takes up moisture and the water's own broad band near 3400 appears. Decomposition, intrinsic absorption and die contamination are not the usual cause. Knowing that a particular preparation technique introduces a particular artefact is what prevents that artefact being reported as a structural feature.

Answer: A

In this technique the beam undergoes total internal reflection inside a crystal, and the evanescent wave that penetrates a micrometre or two beyond the surface is absorbed by whatever is pressed against it. It is neither simple reflection, transmission nor scattering. Because the effective path length is fixed and tiny, samples need no preparation at all - which is why the technique has largely displaced mulls and pellets in routine work. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

A solid or a liquid is simply pressed against the crystal and the spectrum recorded, with no grinding, dissolving or pressing. Sensitivity is lower rather than higher, because the effective path is only micrometres, and the wavelength range is the same or narrower. Removing the preparation step removes both the labour and the artefacts that preparation introduces, which is why the trade against sensitivity is usually worth taking. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

The penetration depth is of the order of a micrometre, depending on wavelength, angle and the refractive indices, which is why the technique samples only the surface layer. That limitation is sometimes exactly what is wanted, as when a coating is to be examined without the substrate beneath it. A technique's restriction in one application becoming its advantage in another is a recurring pattern worth noticing. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Because the wave decays within a micrometre or two, anything not touching the surface is simply not sampled and the spectrum is weak or absent. Scattering, refractive index and mechanical support are not the reason. A hard, irregular solid is the difficult case, which is why these accessories carry a pressure clamp - and why a poor spectrum from one is usually a contact problem rather than an instrument fault. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

A Michelson interferometer modulates every wavelength at its own frequency as the mirror travels, and the transform recovers the spectrum from the resulting interferogram. Arrays, filters and tunable lasers are other approaches used elsewhere. Replacing a device that selects one wavelength at a time with one that encodes them all simultaneously is the change that transformed infrared practice, and both of its advantages follow from it. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

Because each wavelength contributes to every point of the interferogram, it is measured for the whole duration rather than for the instant a scanning instrument passes it, which improves the signal-to-noise ratio enormously where detector noise dominates. The aperture benefit is the separate Jacquinot advantage. Two distinct advantages with two names, and confusing them muddles the explanation of why the technique won. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

An interferometer needs no narrow entrance slit, so a large circular aperture can be used and far more energy reaches the detector. Simultaneous measurement is the Fellgett advantage and laser calibration is the Connes advantage in wavelength accuracy. Three distinct benefits, each with its own name and its own mechanism, and together they account for the technique's complete displacement of dispersive instruments. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

The interferogram must be sampled at exactly known path differences, and the fringes of a helium-neon laser provide a ruler far finer than any mechanical encoder. It does not illuminate the sample, align the optics or calibrate absorbance. Using a known wavelength as a length standard gives these instruments wavelength accuracy better than any dispersive design, which is why they need no external wavelength calibration. Most errors here come from carrying a result across from a neighbouring case where it does not hold.