The factors that Lambert-Beer's law does not hold

The factors that Lambert - Beer's law does not hold

The quantitative test law is commonly known as Lambert-Beer's law. The condition is that the test object is a uniform dilute solution, gas, etc., without scattering caused by solute, solvent and suspension; the incident light is monochromatic parallel light. Then the reason that the Lambert-Beer law is not established is basically due to the above reasons.

1. The Lambert-Beer law caused by high concentration is not established.

When the concentration of the absorbing material is very high, the absorption point is very close, which affects each other's charge distribution, and the absorption capacity of the absorption point changes to a specific wavelength of light, which causes the law to be unsatisfactory. In some cases, when the concentration is large, it causes a change in the refractive index, and the law does not hold. Also, when the concentration of the solution is high, the particle distance is very close, which causes an overlap of radiation and absorption, thereby reducing the overall absorption capacity and making the law unsatisfactory.

2. Non-monochromatic light causes the Lambert-Beer law to be unsuccessful

In the current manufacture of spectrophotometers at home and abroad, it is impossible to produce a photometer capable of separating monochromatic light. The light split by the monochromator is not purely sharp light, but has a spectrum of a certain width. After the light passes through the sample, the light of a non-specific wavelength is absorbed very little, enters the detector, and the optical signal is converted into an electrical signal to obtain data. If the spectral bandwidth of the instrument is too large, the law does not hold.

3. The excessive stray light causes the Lambert-Beer law to be unsuccessful.

When testing low-concentration samples, the effect of stray light on the results is not seen, but when testing high-concentration samples, the effect of stray light on the test results is great. For example, when testing a sample with a certain concentration, the transmittance is 3%. If the stray light of the instrument is: 0.5%, this will undoubtedly have a great influence on the data results. Therefore, the stray light over the assembly makes the law unfounded.

4. Suspension and turbidity make Lambert-Beer law not true

Because the particle will keep moving in the test container, because the particle size of the suspension or turbid liquid is relatively large, it will directly affect the instability of the test result. When the particle moves to the position of the light, the transmittance will become smaller. The absorbance becomes large and an accurate value cannot be read, which naturally makes the law not hold.

5, non-parallel light makes the law not established

At present, the light sources used in domestic and foreign instruments are divergent, and need to be focused by lens or concave mirror. Most of the instruments are not parallel when passing through the sample cell, which will cause complicated changes of light, making the readings inaccurate, and the law is not established. .

There are many other factors that make the law untenable, such as: colloidal solution, luminosity accuracy, and the effects of scattering. Here, only a brief introduction to the law caused by the above main factors is provided, for reference only.

Shanghai Yuan Analysis Instrument Co., Ltd.

Xie Yusheng

2010, 2, 25