Cuvettes Guide
Cuvettes Guide
Cuvette Terminology
Cuvette cells are used to hold liquid samples for spectroscopic measurements. Some simple spectrometers use round cuvettes for quick measurements, but square cuvettes are required for accurate concentration determinations. All Wilson Analytical systems are designed for square cuvettes.
Cuvettes are referred to by the distance between the inside faces of opposing cuvette walls, representing pathlength (usually in mm), that the spectrometer light travels through the sample during the measurement. 1, 5, and 10 mm are common, although shorter and longer pathlengths are used for special purposes. A standard 10 mm cuvette has exactly 10 mm between the inside walls of the cuvette, giving a sample pathlength of 10 mm. This precise value is used as the sample pathlength in the Beer's Law calculation to determine solution concentrations.
The outside dimensions of cuvettes can vary depending on the cell material and construction type, but a "10 mm cuvette" is 10 mm internally.
Cuvette Types
In addition to pathlength, there are two common types of cells used for optical spectroscopy depending on the type of measurement being made:
Absorbance
Absorbance or transmission determinations shine spectrometer light directly through the cuvette (and sample) to measure the amount of light absorbed or transmitted through the sample. In this measurement configuration, the opposite-side cuvette walls must be optically "clear" to let light pass unimpeded through the sample. The other two sides of the cuvette are often "frosted" to allow handling of the cell without dirtying the optical surfaces of the other two "measurement" sides of the cuvette. These are known as "two sides clear" or absorbance cuvettes.
Fluorescence
Fluorescence measurements are typically made by optically "exciting" the same solution at a 90-degree angle to the spectrometer system. This requires cuvettes with optically clear walls 90-degrees to each other, and this is typically accomplished by making all four walls on the cuvette optically clear with no "frosted" sides for handling the cuvette. These are known as "four sides clear" or fluorescence cuvettes.
Cuvette Materials
In addition to pathlength, there are two common types of cells used for optical spectroscopy depending on the type of measurement being made:
Disposable Cells
Disposable cells have a major advantage in that being single-use new from the box, they are always "clean" if stored properly. They are also inexpensive and are almost unbreakable with normal handling and use. Their low-end UV transmission cutoff depends on the cuvette material used, as does their organic solvent resistance. All plastic cells are fine to use with water but are only useful with a select set of organic solvents. See below for solvent compatibility and spectral range details.
- PMMA (poly(methyl methacrylate))
- 300 nm - 900 nm measurement range.
- Poor with most solvents, but good for several hours with Hexane.
- UV Plastic
- 200 nm - 900 nm measurement range.
- Poor with most solvents, but good with Acetic Acid, Acetone, Acetonitrile, Butanone, Dimethyl Sulfoxide, Ethanol, Formaldehyde, Hydrochloric Acid, Isopropanol, Perchloric Acid, Phenol, Sodium Hydroxide, Sulfuric Acid, and Trichloroacetic Acid.
- PS (polystyrene)
- 340 nm - 900 nm measurement range.
- Poor with most solvents, but good with Sodium Hydroxide.
Reusable Cells
Reusable cells are expensive to buy, are easily broken (especially when being cleaned), and must be thoroughly washed between each use. Cleaning can be difficult if the materials being measured stick to the cell walls (such as oilfield corrosion inhibitors), or if the determinations are being performed out of the laboratory. They do have two major advantages over disposable cuvettes in that they are resistant to almost all organic solvents and they transmit much further down into the UV. This is especially true for quartz, which is transparent down to the UV cutoff of water at 190 nm, while most glass cells cutoff at around 300 nm in the UV.