Question 1

What happens when light passes through a sample in UV-Vis spectroscopy?

Accepted Answer

When light passes through a sample, some wavelengths are absorbed by molecules while others are transmitted. Transmission is the ratio of transmitted light intensity to incident light intensity. The absorbance, calculated as A = -log(T), indicates how much light the sample absorbed. This absorbance measurement forms the basis for both identifying compounds and quantifying their concentrations.

Question 2

How does Beer's Law relate absorbance to sample concentration?

Accepted Answer

Beer's Law states that absorbance equals the molar attenuation coefficient multiplied by path length and concentration. The molar attenuation coefficient reflects a compound's ability to absorb light at a specific wavelength. Standard cuvettes have a 1 cm path length. This relationship allows you to calculate sample concentration if the absorptivity is known, or you can use calibration curves principles and applications to determine unknown concentrations.

Question 3

Why does each molecule have a unique absorbance spectrum?

Accepted Answer

Each molecule's unique absorbance spectrum results from its distinct chemical structure, which determines the band gap—the energy difference between ground and excited states. When a photon's energy exactly matches this band gap, the molecule absorbs it and transitions to a higher energy state. Different molecular structures have different band gaps, producing characteristic absorbance patterns at specific wavelengths.

Question 4

What equipment components select specific wavelengths in a UV-Vis spectrophotometer?

Accepted Answer

A monochromator separates wavelengths of light spatially and positions an exit slit where the desired wavelength emerges. Alternatively, filters can select specific wavelengths. Most spectrophotometers use a deuterium lamp for the UV range (170–375 nm) and a tungsten filament lamp for the visible range (350–2,500 nm). The monochromator can scan across wavelength ranges to generate complete absorbance spectra.

Question 5

How do you prepare a sample and blank for UV-Vis measurement?

Accepted Answer

First, prepare a blank by filling a clean cuvette with sample solvent and wiping the outside with lint-free paper. Insert it into the spectrophotometer with grooved sides out of the beam path and secure the lid. Measure and record the blank's absorbance. Then rinse the cuvette twice with sample, fill it about three-quarters full, wipe it clean, and measure the sample's absorbance at the same wavelength. Subtract the blank measurement from the sample.

Question 6

How can UV-Vis spectroscopy measure reaction kinetics?

Accepted Answer

Reaction kinetics are measured by taking repeated UV-Vis absorbance measurements at the absorbance maximum over time. Record an initial absorbance reading before the reaction begins. After adding the reagent and mixing, measure absorbance at regular intervals. As the reaction proceeds, changes in absorbance reflect changes in compound concentration, allowing you to track reaction progress and determine when completion occurs.

Question 7

What are common laboratory applications of UV-Vis spectroscopy?

Accepted Answer

UV-Vis spectroscopy quantifies proteins using Bradford assays with Coomassie blue dye at 595 nm or direct protein absorbance at 280 nm. It measures bacterial cell density via optical density at 600 nm, where OD600 of 1 indicates approximately 8 × 10⁸ cells per mL. It also quantifies environmental pollutants like nitrogen oxides by measuring colored azo dye complexes. These applications demonstrate UV-Vis versatility across biological, environmental, and analytical fields.

Ultraviolet-Visible (UV-Vis) Spectroscopy - Assessment Quiz