Calibration method of ultraviolet spectrophotometer:
We know that the most important physicochemical quantity of spectrophotometry is absorbance. In order to obtain accurate research results, it is very important to accurately measure the absorbance of the sample solution. Generally, the unreliability of the analysis results is related to accidental errors and systematic errors. Accidental error affects the precision of measurement, which can be reduced by statistical processing of a sufficient number of measurements; system error affects the accuracy of measurement results, which can be used to compare the accurate measurement results of a substance under substantially the same experimental conditions to unify the system errors stand up. The system errors of the spectrophotometer (wavelength correction, slow astigmatism of the spectrophotometer, linear response of the amplifier, dark current and optical path of the cuvette) and operating errors (temperature changes, instrument readings, operator changes, substances used The purity, weighing and concentration, pH) of the measured absorbance can be checked and corrected. With regard to operating errors, in most cases, the generation of such errors is controlled or reduced by strictly following the operating procedures for measurement, instrument zero adjustment, and accurate weighing. The systematic errors of the instrument can be overcome by regular calibration of the spectrophotometer. If a highly accurate measurement is required, it must be corrected every day.
Correction content:
1. The wavelength accuracy test is expressed by the error between the wavelength value displayed by the instrument and the actual wavelength value of the monochromatic light, which should be within ± 1.0nm. The 486.02nm and 656.10nm spectral lines of the deuterium lamp in the instrument can be used for correction.
2. Absorption accuracy test
3. Stray light test
4. Wavelength reproducibility test
5. Resolution test
Correction method of absorbance:
For the calibration of absorbance, a solution with a certain concentration of a very pure substance is usually used as the standard, and the absorbance coefficient of this solution has been checked by different laboratories. Substances, at the same time, the solution is required to be stable, and the absorption change in a considerable wavelength range complies with Beer-Lambert's law. Commonly used solutions of copper sulfate, ammonium cobalt sulfate, and sodium or potassium nitrate. Potassium chromate solution is the most commonly used standard solution. This solution is suitable for both the ultraviolet and visible regions.
Wavelength or wave number correction method:
Solutions with narrow absorption bands, filters or vapors can be used to correct the required light wave range. If a high degree of precision is required, it can be corrected by the radiation emitted by the discharge bulb. Some spectrometers are already equipped with a lamp for calibration. Benzene vapor is also useful for correcting a certain range of wavelengths. A small drop of benzene can be placed in a one-centimeter thick absorption cup to measure its absorption wavelength. In the far ultraviolet region, oxygen absorption bands can be used for correction. With various rare earth metal filters, the wavelength can be corrected quickly, but the accuracy is not as high as the above method. Commonly used filters containing holmium, neodymium and praseodymium ions.
Correction method of stray light
A small amount of stray light will often cause a large measurement error. Its correction can use a solution that can completely absorb monochromatic light at a certain wavelength and absorb very weakly at other wavelengths. The approximate value of stray light can be inferred from the light transmission conditions exhibited by this solution. The pseudo-absorption band caused by stray light can also be checked by Beer-Lambert's law, but the error of checking the pseudo-absorption band by this law is large. The approximate percentage of stray light can be obtained from the transmittance shown outside the cut-off range. If the stray light contained is greater than 0.1%, try to reduce it, or correct the measured absorbance. The error caused by stray light is proportional to the stray radiation, so the correction value is easily calculated from the near-correct curve of the compound. In addition, an appropriate filter can also be used, which completely transmits light in the measurement wavelength range, but absorbs light waves outside this range, thereby eliminating stray light.
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