Infrared Spectroscopy Questions and Answers

Answer: A

Resolution is not unlimited: it is set by the mirror's travel, so a higher resolution needs a longer and more precisely engineered scan. The other three are the genuine advantages, named after Fellgett, Jacquinot and Connes respectively. A claim that a technique has no limit at all is worth doubting immediately, since every instrument's resolution is bounded by something physical.

Answer: B

It is a convention: infrared spectra are traditionally shown as per cent transmittance, so bands dip towards the baseline rather than rising from it. Transmittance is not proportional to concentration, which is why quantitative work converts to absorbance. Absorbance is computable for any sample with a known path, and the detector measures intensity either way. An inherited convention that differs from the neighbouring technique's is worth noting explicitly when two spectra are compared.

Answer: C

On a transmittance plot the baseline is at or near a hundred per cent, meaning the light passes unabsorbed at that wavenumber. Complete absorption would be at zero, the bottom of the plot. Dark current and resolution are instrument properties not shown on the axis. Because the axis runs the opposite way from an ultraviolet plot, checking which end is which before reading a spectrum saves a misinterpretation.

Answer: D

Particles scatter more strongly at shorter wavelengths, so a poorly ground solid gives a baseline that rises steadily towards high wavenumber. Detector drift and source variation divide out in the background ratio, and decomposition would change the bands rather than the baseline. Grinding the sample more finely is the remedy, and recognising the tilt as a preparation fault rather than a sample property is what prompts it.

Answer: A

Scattering is worst when particle size is comparable with the wavelength, so grinding below a couple of micrometres suppresses it. Absorption per unit mass is unchanged by particle size, contact is not the mechanism, and excessive grinding can change a crystal form rather than coarse particles doing so. Reducing a particle far below the wavelength makes it optically invisible, which is the same principle behind index-matching in a mull.

Answer: B

Mechanical energy can convert a metastable polymorph into a different one, whose infrared spectrum differs, so the analysis reports a form that was created rather than found. Heating, moisture and contamination are lesser risks that also exist. That a preparation step can change the very property being measured is a hazard worth knowing in any solid-state technique, and it is one reason attenuated total reflectance, which needs no grinding, is preferred.

Answer: C

Different packings alter the intermolecular interactions and hence the exact positions of many bands, particularly in the fingerprint region. The molecular formula is identical, which is what makes the distinction subtle, and both position and intensity change. That an infrared spectrum can distinguish two forms of the same compound is genuinely useful in pharmaceutical work, where the two may dissolve at quite different rates.

Answer: D

Subtracting a stored spectrum removes the contribution of a solvent, a mull oil or a coating substrate, leaving the analyte's own bands. Wavelength drift, dark current and normalisation are handled elsewhere. The technique works because absorbances add, so a known component's contribution can be taken away - but only if it is present in exactly the same amount, which is why the subtraction factor usually needs adjusting.

Answer: A

Subtracting more of the reference than is actually present drives those regions below the baseline, producing bands that point the wrong way and can be mistaken for real features. Baseline position, resolution and the wavelength scale are unaffected. That an over-corrected spectrum announces itself with negative features is fortunate, and checking for them is the standard way to judge whether a subtraction factor was set correctly.

Answer: B

Resolution is inversely proportional to the maximum path difference:

\[ \begin{aligned} \Delta\tilde{\nu} &\propto \frac{1}{d} \\ &\Rightarrow 2 \ \mathrm{cm^{-1}} \end{aligned} \]

giving about 2 reciprocal centimetres, a finer figure since a smaller number means better resolution. Eight would be worse rather than better, which is the direction error to watch for whenever a resolution is quoted as a width.