What a spectrophotometer is
A spectrophotometer measures how much light a sample absorbs at a chosen wavelength. Because many substances absorb light in proportion to their concentration, the instrument turns an absorbance reading into a quantitative result - which is why it appears in water and wastewater testing, in food and beverage analysis, in pharmaceutical quality control and in any laboratory that follows a colorimetric method.
The visible range is the common case: a coloured complex develops in the sample and the instrument measures how strongly it absorbs. The ultraviolet range extends this to substances that are colourless but still absorb below the visible band, and it needs a different lamp and a different cuvette material. The two bands are frequently wanted in one laboratory, which is why the wavelength range is the first thing to establish.
What separates instruments is not the display but how the light is produced, separated and detected: the wavelength range, the spectral bandwidth, whether one beam or two, the lamp type, and the cuvette format the work requires. This page covers those, and the boundary against the neighbouring analytical instruments that answer different questions on the same bench.
How a spectrophotometer measures
The instrument passes light through a monochromator - a grating or a filter - to isolate a narrow band at the chosen wavelength, sends it through the sample, and measures what arrives at the detector. Absorbance is calculated from the ratio between the light transmitted by the sample and the light transmitted by a reference, which is why a blank measurement is part of every method rather than an optional extra.
The relationship between absorbance and concentration is linear only within limits, and only in a well-behaved system. Above a certain absorbance the linearity breaks down, matrix effects in the sample interfere, and the reading stops being a dependable measure of concentration. A method therefore defines not only the wavelength but the path length, the reagent and the range over which the calibration was established.
Bandwidth and stray light set what the instrument can do at the edges of its range. A narrow bandwidth resolves a sharp absorption peak that a wide one would flatten and under-read; stray light limits the highest absorbance that can be trusted, because at high absorbance the small proportion of stray light becomes a large proportion of the light reaching the detector. Both are specification figures rather than operating adjustments.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Wavelength range | Visible only, or ultraviolet and visible together | Which substances and which standard methods the instrument can run, and which lamp and cuvette material are needed. |
| Spectral bandwidth | The width of the band of light that reaches the sample | Whether a sharp absorption peak is resolved correctly. A peak measured with too wide a band reads low, and the calibration absorbs that error until the instrument is changed. |
| Optical arrangement | Single beam, or double beam with a reference channel | Whether the reference is measured at the same time as the sample. A double beam compensates for drift in the lamp and the detector, which matters over a long run of samples. |
| Light source | The lamp or lamps fitted, and the range each covers | Which end of the wavelength range is available. The ultraviolet band needs a source of its own, and the crossover point between sources is a specification detail. |
| Detector | The type of detector, and the range it responds to | The sensitivity at the extremes of the wavelength range, and the noise level of the reading. |
| Sample format | Standard cuvettes, microvolume cells, flow-through cells, and test-tube or vial holders | Whether the method can be run as written, and how much sample it consumes. A microvolume format is only worth having if the samples are genuinely small. |
| Software and method storage | Built-in methods, stored calibration curves, and whether results can be exported | Whether routine work is a stored method or a manual sequence, and whether results have to be documented. |
| Wavelength accuracy and verification | How wavelength accuracy is set, and the reference filters or standards supplied | Whether the instrument can be shown to read at the wavelength the method requires, which an accredited laboratory has to demonstrate. |
How to choose a spectrophotometer
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| What is being measured | Colour, a concentration by a standard method, or a scan across a range each place different demands | Name the substance and the method, or describe what information is wanted from the reading. |
| Wavelength band needed | It decides the lamp, the cuvette material, and often the price band of the instrument | State the wavelengths the method calls for, and whether any fall below the visible range. |
| Bandwidth required by the method | A method written around a peak needs a bandwidth that can resolve it | Quote the method, including the wavelength and bandwidth where it states them. |
| Concentration range | It sets the path length and the dilution the method will need | Give the expected minimum and maximum concentration. |
| Sample volume available | Some methods and some samples are volume-limited | State how much sample there is per determination. |
| Number of samples and throughput | It decides whether a single-beam instrument is adequate or a double beam saves time | Give the batch size and how often the work is run. |
| Documentation requirement | Accredited work usually requires records and instrument verification | State whether results must be documented, and whether verification standards have to be supplied. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a spectrophotometer usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a spectrophotometer |
| Colorimeters | Use a filter to isolate a band rather than a monochromator to select a wavelength, so they are simpler and suited to a method that defines one colour reaction. They cannot scan a spectrum or resolve a narrow peak, which makes them a fixed-routine instrument rather than one for method development. |
| Atomic absorption spectrometers | Measure the absorption of light by atoms in a flame or a furnace to determine elemental concentration, often at very low levels. They answer which elements are present and in what quantity, where a UV-Vis instrument usually answers how much of a compound is present. |
| Gas chromatographs | Separate a mixture into its components before measuring them, which a spectrophotometer cannot do. Where a sample contains several substances that absorb at the same wavelength, that separation is what makes the measurement possible at all. |
| Liquid chromatographs | The same separation principle applied in the liquid phase, and often coupled to a spectrophotometric detector. The distinction is that the chromatography provides the separation; the detector reports only what passes through it. |
| TOC analyzers | Measure total organic carbon by oxidising the sample and detecting the resulting carbon dioxide. They report an aggregate figure for organic loading, where a spectrophotometric method measures a specific substance or reaction. |
| Mass spectrometers | Identify compounds by their mass-to-charge ratio, giving structural and confirmatory information. They are a different order of instrument in cost and in operation, and they are used where identification, rather than routine quantification, is the requirement. |
The boundary worth settling first is that between a colorimeter and a spectrophotometer. Where the method names one wavelength for one colour reaction, a colorimeter may be sufficient; where a method has to be established or transferred, the wavelength selectivity of a spectrophotometer is what makes it possible.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The substance or parameter to be measured, and the method to be followed
- Wavelength or wavelength range the method requires
- Bandwidth specified by the method, where it states one
- Expected concentration range
- Sample volume available per determination
- Whether the work is a fixed routine method or a range of methods
- Batch size, and how often the work is run
- Whether results must be documented or exported
- Quantity, destination port and required delivery date
