A cuvette is a container that holds the liquid sample during the measurement process by the spectrophotometer. The most used cuvette are plastic cuvettes, optical glass cuvettes, and quartz cuvettes. Similarly, the path length of these cuvettes ranges from about 5 mm to 100 mm.
1. On the basis of the material used
- Plastic Cuvette
- Optical Glass Cuvette
- Quartz Cuvette, etc
2. On the basis of size
The assumption here is that only 80% of the cuvette can be filled with the sample. If it occupies more than 80%, then the liquid might spill.
Based on the volume of the sample that the cuvette can hold, they are mainly of 4 types.
- Macro Cuvette:- It has a storage capacity of more than 3.5 ml of sample (7 ml, 14 ml, 17.5 ml, 35 ml, etc.).
- Standard Cuvette:- It has a storage capacity of 3.5 ml of sample.
- Semi-Micro Cuvette:- It has a storage capacity of around 0.35 ml to 1. 7 ml of sample (1.4 ml, 1 ml, 0.7 ml, 0.35 ml, etc. ).
- Sub-Micro Volume Cuvette:- It has a storage capacity of around 20 microliters to 0.35 ml of sample.
3. On the basis of design
- Absorption Cuvette:- The cuvette, which depends upon the principle of absorption such as photometers, will have two polished sides. The remaining two walls are clear. This is because, when the light enters into the sample there is a chance that the light undergoes diffraction. In that case, it will scatter around and pass through the sidewalls and cause a disturbance to the photodetector. This can fluctuate the measurement of the sample. So a polished surface can prevent the light from escaping.
- Fluorescence Cuvette:- It can have 3-4 clear walls.
The PTFE cover or the round Teflon stopper is mostly in use.
Cuvette Path Length
The path length is the length that the light travels within the sample. In other words, it is the displacement between the internal front and back windows of the cuvette. The standard path length is 10 mm. But you may also find it, with a path length of 1 mm, 5 mm, 20 mm, 40 mm, 50 mm, 100 mm, etc. in the market.
- In terms of price, plastic is the cheapest one. After that comes the optical glass followed by the quartz. The price of quartz cuvettes maybe around 2.5 to 3.5 times that of optical glass cuvettes.
- Plastic is a disposable product. So, it can either be used for one time or more. On the other hand, quartz and glass cuvettes are both reusable in nature. So, if handled properly, they can be reused for more than 6 months.
- For applications such as photometers, quartz is better than plastic and optical glass. This is because the photometer (filter photometer, the spectrophotometer, or fluorimeter) both depend upon the principle of absorbance of light by the sample present in the cuvettes. In such cases, any cuvette which does not absorb waves is preferred. If the applications (e.g U.V. Kinetics in biochemistry analyzer) use ultraviolet rays, then quartz is better. This is because the rate of absorption of UV rays by plastic and optical glass is more than that of quartz. But, if the applications consist of only the use of visible light or IR waves, then plastic or optical cuvettes can also be used.
- The Mohs scale value for optical glass (5) is less than that of quartz (7). A higher Mohs scale implies that the material is less likely to be scratched. Thus, you can say that quartz is more scratch-proof than glass. Also, if the cuvette gets scratched, it will affect the absorbance pattern of the light.
- Quartz is more fragile than optical glass and plastic. So, you need to take extra care while handling the quartz material.
How to identify between quartz and optical glass
Optical glass is a good electrical insulator. On the other hand, quartz glass acts as a good conductor of electricity. Similarly, quartz has got a higher tolerance for heat and pressure than normal optical glasses.
Take an iron nail. Then try to scratch the given substance with it. If you can easily scratch the surface, then that can be a normal optical glass. However, if you are unable to scratch the surface, then that can be quartz glass.
It is really important to clean the cuvette after every test. This is because the residue present in the cuvette from the previous test can cause cross-contamination of the sample. It will also hinder the absorbance of light by the sample.
To reduce this type of cross-contamination, it is better to use multiple cuvettes for different types of tests. You may rinse the cuvettes with ethanol or distilled water a few times. But, make sure that the inner surface is completely dried out prior to its use for the measurement. Else, it will cause the dilution of the sample which may give you the fault reading.
Alternatively, you can rinse the cuvette a few times with the same sample which will be used for the measurement. In this case, you may not need to dry out the inner surface of the cuvette.
Always dry out the outer surface of the cuvette before taking a measurement. Else, some of the light will be absorbed by the outer wet surface. Similarly, fingerprints can also cause scattering of some of the incident rays. Finally, do not fill up the cuvette by more than 80% of the sample while taking the measurement.