Analytical Spectroscopy

In the context of the Beer-Lambert Law, what is the primary advantage of using the specific absorbance value, A 1cm1% , instead of molar absorptivity, ϵ?

• It provides a more accurate measurement for highly concentrated solutions above 10 mM.

• It is useful when the molecular weight of the analyte is unknown or for analysing mixtures.

• It is a universal constant for all substances, unlike molar absorptivity.

• It is independent of the path length of the cuvette, simplifying calculations.

What is the 'Stokes shift' in fluorescence spectroscopy?

• The time delay between the absorption of a photon and the emission of a fluorescent photon.

• A reduction in fluorescence intensity due to high sample concentration.

• The difference in wavelength between the maximum of the absorption spectrum and the maximum of the emission spectrum.

• The 90-degree angle between the excitation source and the detector in a spectrofluorimeter.

In a spectrofluorimeter, why is the detector typically placed at a 90-degree angle to the incident light source?

• To ensure the path length for both excitation and emission is exactly 1 cm.

• To allow for the simultaneous measurement of both absorbance and fluorescence.

• Because fluorescent light is only emitted at a perfect right angle to the excitation beam.

• To minimise the amount of high-intensity excitation light reaching the detector.

What is the fundamental difference in the role of the flame in Atomic Absorption (AA) spectroscopy compared to Flame Photometry (Atomic Emission)?

• The flame temperature in AA is significantly higher than in AE to ensure complete atomisation.

• AA uses an air-acetylene flame, whereas AE exclusively uses a natural gas/compressed air flame.

• In AA the flame atomises the sample, while in AE it both atomises and excites the atoms.

• In AA the flame excites atoms, while in AE it only atomises them.

What is the primary advantage of a photodiode array (PDA) detector compared to a photomultiplier tube (PMT) in a UV/Vis spectrophotometer?

• It can measure the entire spectrum simultaneously, allowing for very rapid analysis.

• It is a key component of a double-beam instrument, whereas a PMT is only used in single-beam instruments.

• It provides much higher spectral resolution, typically better than 0.1 nm.

• It has significantly higher sensitivity for detecting very low light levels.