UV-Vis (Ultraviolet-Visible) spectroscopy is an analytical technique that measures the absorption of light in the ultraviolet (190-400 nm) and visible (400-800 nm) regions of the electromagnetic spectrum. It is widely used for quantitative analysis, kinetic studies, and characterization of electronic transitions in molecules.

Principle of UV-Vis Absorption

When light passes through a sample, molecules absorb photons whose energy matches the energy gap between electronic orbitals, promoting electrons from the ground state to an excited state. The amount of light absorbed at each wavelength is recorded as an absorbance spectrum, and the wavelength of maximum absorption (λmax) is characteristic of each chromophore. The Beer-Lambert Law (A = εcl) relates absorbance (A) to molar absorptivity (ε), path length (c), and concentration (l), enabling quantification.

Instrumentation

A UV-Vis spectrometer comprises several components. A deuterium lamp provides light for the UV range and a tungsten-halogen lamp for the visible range. A monochromator (diffraction grating or prism) selects a narrow band of wavelengths. The sample compartment holds a quartz cuvette for UV measurements or a glass cuvette for visible-range measurements, and a detector (photomultiplier tube or photodiode array) converts transmitted light into an electrical signal. Double-beam instruments split the light into sample and reference beams to correct for solvent and lamp fluctuations.

Applications

UV-Vis spectroscopy is used for quantitative analysis of nucleic acids (A260), proteins (A280), and enzyme kinetics, determination of pKa values by measuring absorbance changes with pH, quality control of pharmaceuticals, food colorants, and water pollutants, and characterization of transition metal complexes and conjugated organic compounds.

Advantages and Limitations

• Advantages: Fast, non-destructive, requires small sample volumes, and is relatively inexpensive.
• Limitations: Only measures chromophoric species; samples must be transparent and free of scattering particles.

Practical UV-Vis Spectroscopy Protocol

Warm up the instrument for 15–30 minutes to stabilize the lamp. Select quartz cuvettes for UV measurements (<350 nm) since glass and plastic absorb UV light; use disposable plastic or glass cuvettes for visible-range measurements only. Handle cuvettes by the frosted sides to avoid fingerprints on the optical windows. Fill a reference cuvette with the blank solution (the solvent or buffer alone) and the sample cuvette with the analyte solution. Insert the reference cuvette in the reference beam path and the sample in the sample beam path. For single-beam instruments, collect a baseline spectrum of the blank first, then replace with the sample. Perform a wavelength scan from 200–800 nm at medium speed (200 nm/min) to determine λmax — the wavelength of maximum absorbance. For quantitative analysis at a fixed wavelength, set the instrument to the λmax of the analyte. Measure the absorbance of a series of standard solutions (5–7 concentrations spanning the expected range) and the unknown sample. Construct a calibration curve by plotting absorbance vs. concentration. Fit a linear regression — the Beer-Lambert law (A = εbc) states that the relationship should be linear with an intercept near zero, provided the absorbance is between 0.1 and 1.0 (the linear range of most instruments). If the absorbance exceeds 1.0, dilute the sample. For DNA quantification at 260 nm, use the extinction coefficient of 50 ng/µL per OD unit for double-stranded DNA and 33 ng/µL per OD unit for single-stranded DNA. Purity is assessed by the A260/A280 ratio (~1.8 for pure DNA, ~2.0 for pure RNA) and A260/A230 ratio (>2.0). For protein quantification at 280 nm, measure the A280 of a purified protein and calculate concentration using the protein’s molar extinction coefficient. For enzyme kinetics, monitor the change in absorbance over time at a fixed wavelength — for example, NADH consumption at 340 nm (ε = 6220 M⁻¹cm⁻¹) to measure dehydrogenase activity.

Real-World Application

In a study of the enzyme alcohol dehydrogenase, the oxidation of ethanol to acetaldehyde is coupled to NAD⁺ reduction to NADH, which absorbs at 340 nm. The reaction is monitored at 340 nm for 5 minutes at 30°C with 0.1–10 mM ethanol. Initial velocities are plotted vs. substrate concentration and fit to the Michaelis-Menten equation, yielding Km = 1.2 mM and Vmax = 85 µM/min.