Spectroscopic Techniques Questions and Answers

Answer: A

Correct answer: \( E = h\nu \)

Planck’s relation gives the energy of a photon as

so energy rises with frequency and falls with wavelength. This single equation orders the whole of spectroscopy: radio photons are the least energetic and flip nuclear spins, infrared photons excite vibrations, visible and ultraviolet photons promote electrons, and X-rays eject them.

Answer: B

Photons of a few electronvolts promote an electron from a bonding or non-bonding orbital to an antibonding one. Vibrational and rotational changes accompany the electronic jump, which is why the bands are broad rather than sharp lines.

Answer: C

Correct answer: molecular vibrations

Infrared photons carry a few kilojoules per mole, which matches the spacing of vibrational energy levels, so absorption sets bonds stretching and bending. Electronic transitions require the much larger energies of visible and ultraviolet photons, nuclear spin transitions the much smaller energies of radiofrequency, and ejecting an electron requires more energy still.

Answer: D

The field splits the spin states of a nucleus such as the proton, and radio frequency photons of very small energy flip between them. The splitting is proportional to the field, which is why higher-field instruments give better resolution.

Answer: A

Expressed in reciprocal centimetres, it is directly proportional to energy, which makes it more convenient than wavelength for reading a spectrum. The useful infrared range runs from about 4000 down to 400 reciprocal centimetres.

Answer: B

Correct answer: path length through the sample

The Beer-Lambert law is

so absorbance is proportional to both the concentration and the path length, with the molar absorptivity as the constant of proportionality for that substance at that wavelength. It is not proportional to the incident intensity - that cancels, which is why absorbance is a property of the sample rather than of the lamp.

Answer: C

Correct answer: \( A = \log(1/T) \)

Transmittance is the fraction of light getting through, and absorbance is defined as

The logarithm is what makes absorbance, rather than transmittance, linear in concentration. Ten per cent transmittance gives an absorbance of 1, one per cent gives 2, and complete transmission gives zero.

Answer: D

Correct answer: absorbs at a given wavelength

Molar absorptivity \( \varepsilon \) is the constant in \( A = \varepsilon c l \), and it measures how efficiently the substance absorbs at the chosen wavelength. Values run from a few units for a forbidden transition to over \( 10^5 \) for a strongly allowed one, and a large value is what makes a method sensitive - it is the reason a strongly absorbing derivative is often formed before measurement.

Answer: A

Association, ionisation or a change in refractive index alters the absorbing species itself, so the calibration ceases to be linear. Stray light and a non-monochromatic beam cause similar deviations at high absorbance.

Answer: B

Correct answer: maximum absorbance

Working at the absorption maximum gives the largest signal for a given concentration, and therefore the best sensitivity; it also sits at the flat top of the band, where a small error in wavelength setting causes almost no change in absorbance. Measuring on a steep flank would make the reading acutely sensitive to the monochromator’s calibration.