Chemistry Labs

Analytical chemistry

UV-Vis, infrared (IR), Raman

Electronic absorption, molecular vibrations and inelastic light scattering provide complementary molecular fingerprints.

1 Lower secondary2 Upper secondary3 Undergraduate4 Advanced5 Research

IntuitionIntuition: the measurement idea

A spectrum records how much light is absorbed or scattered at each wavelength. Different spectral regions isolate different molecular motions, so one molecule yields several complementary fingerprints.

Explore the UV-Vis, infrared (IR), Raman measurement concept in this interactive signal.

SchoolSchool level: signal and result

UV-Vis quantifies electronic absorption by Beer–Lambert. IR identifies bond vibrations; Raman identifies Raman-active vibrations and often complements IR selection rules.

Definition:

Electronic absorption, molecular vibrations and inelastic light scattering provide complementary molecular fingerprints.

First identify the measurand, choose a signal that responds to it, and compare the sample with a calibrated standard or a validated model.

A=εbc,E=hν=hcλA=\varepsilon bc,\qquad E=h\nu=\dfrac{hc}{\lambda}

Example: Worked analytical example

Calculate the analyte result from the stated measurement and method relation.

Solution

At 525 nm, a 1.00 cm cell with 1.0×10⁻⁴ mol/L solute and ε=1.5×10⁴ L mol⁻¹ cm⁻¹ has A=1.50.

Method checkpoints
StagePurpose
PrepareControl matrix and contamination
MeasureAcquire a calibrated response
ValidateCheck recovery and uncertainty

UndergraduateUniversity: quantitative method

Instrument bandwidth, stray light, solvent cut-off, path length and matrix absorption control quantitative accuracy. Peak positions identify chromophores or functional groups; band shape gives concentration and environment information.

Calibration, selectivity, sample preparation and uncertainty belong to the method itself, not to afterthoughts. Report units, conditions and the calibration range.

A=εbc,E=hν=hcλA=\varepsilon bc,\qquad E=h\nu=\dfrac{hc}{\lambda}

References