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Reference

Spectral methods

Six spectral methods supported by Evolve Sensing instruments. Each entry describes what the method measures, where it performs well and where it has limitations, because choosing the wrong technique is a more expensive mistake than choosing the wrong instrument.

01

Absorption spectroscopy

Measures the light a sample removes at each wavelength.

Beer-Lambert relates absorbance to concentration and path length, which makes absorption the workhorse for quantitative chemistry. It requires a stable source, a stable reference, and a spectrometer whose baseline does not shift between the reference and the sample.

Suited to
Clear liquids and gases; quantitative concentration.
Limitations
Turbid or scattering samples, where scattering masquerades as absorbance.
02

Transmission spectroscopy

Measures the fraction of light that passes through a sample.

Closely related to absorption and often used interchangeably, but the distinction matters for the optics: transmission is the measured quantity and absorbance is the calculated one. Useful from the UV through the near-IR on films, windows, filters and solutions.

Suited to
Films, coatings, optical components, solutions.
Limitations
Anything opaque, or so thin the signal is dominated by surface reflection.
03

Reflectance spectroscopy

Measures light returned from a surface rather than through it.

Diffuse reflectance is the practical route into solids and powders that transmission cannot reach. It brings its own complications: particle size, packing density and surface finish all change the spectrum without any change in chemistry.

Suited to
Powders, solids, coatings, agricultural products, ore.
Limitations
Comparing samples with different physical presentation.
04

Fluorescence spectroscopy

Measures light re-emitted after a sample is excited.

Highly sensitive, because the measurement is of emitted photons against a dark background rather than a small change in a bright one. That sensitivity makes it the technique of choice for trace detection and for labeled biological assays.

Suited to
Trace detection, biological labels, contamination screening.
Limitations
Quantification, because quenching and inner-filter effects are not linear.
05

Raman spectroscopy

Measures the small wavelength shift from inelastic scattering.

Gives a molecular fingerprint from vibrational modes, works through glass and water, and requires no sample preparation. The scattered signal is very weak, so the practical constraints are fluorescence background and the collection of enough light from the sample.

Suited to
Molecular identification, aqueous samples, through-container measurement.
Limitations
Fluorescent samples; anything where the laser damages the material.
06

Laser-induced breakdown spectroscopy

Reads the emission from a laser-generated microplasma.

Elemental rather than molecular. A focused pulse ablates a microscopic amount of material and the cooling plasma emits atomic lines. It is fast, requires no preparation and works at stand-off distance, which is why it is used on conveyors and at mine faces.

Suited to
Elemental composition of solids, at speed, in harsh environments.
Limitations
Matrix-matched calibration; heterogeneous materials need many shots.

Next step

Discuss the right method for your sample

An Evolve Sensing engineer can review the sample and the measurement goal and recommend a method before any instrument is selected.

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