Ultraviolet detector is the most commonly used detector for high performance liquid chromatography. There are many manufacturers and models on the market. Manufacturers usually use the noise of the detector, baseline drift and other technical parameters as the sensitivity performance evaluation index of the product. Considering that the UV absorption response of the sample is mainly related to the sample, it has little relationship with the instrument. It is generally considered that this can basically reflect the sensitivity of the instrument.
Based on the experimental results, Luchuang Analytical Instruments Co., Ltd. proved that the response values ​​of the detectors of the same sample actually have different program differences, indicating that only the noise and baseline drift indicators can not accurately display the performance difference of the instrument in terms of sensitivity, so The signal-to-noise ratio is proposed to evaluate the sensitivity performance of the instrument.
1. Purpose and basis of sensitivity evaluation index setting
High performance liquid chromatography and its UV detector are mainly used for the analysis and testing of chemical samples. Its evaluation indicators are necessarily related to its use. The requirements of an analytical method for detectors are primarily in terms of sensitivity, selectivity, accuracy and accuracy. Sensitivity is a very important indicator in chromatographic analysis. For sensitivity, the signal-to-noise ratio, the lowest detection limit, or the lowest limit of quantitation are mainly used in the analysis of analytical test methods. Among them, the signal-to-noise ratio is the core of the evaluation, because the minimum detection limit and the minimum quantitative limit are usually defined by the signal-to-noise ratio of 2 to 3 and 10.
However, current manufacturers usually use detector noise, baseline drift and other technical parameters as the sensitivity performance evaluation index of the product. This is actually based on the assumption that the UV absorption response of the sample is primarily related to the sample and has little to do with the instrument. According to Lambert Beer's law, for the same length of the test cell, the same concentration of sample solution should have the same absorbance. Thus, for the same concentration of sample solution, the detector response should be the same, so the above noise, baseline drift parameters should be used to evaluate the sensitivity performance of the detector.
However, in the experiment, it was found that there was a significant difference in the response of the same sample solution in different detectors. Table 1 shows that for the same sample solution, the response values ​​of different manufacturers and models of detectors may differ by more than three times. At this time, since chromatographic analysis mainly uses a relatively indirect measurement method, its use is not actually affected much. It is difficult to simply identify problems with the instrument. However, because the signal-to-noise ratio has changed, the noise and baseline drift parameters cannot be used to evaluate the sensitivity performance of the detector.
At this time, directly using the signal-to-noise ratio to evaluate the sensitivity performance of the liquid chromatograph detector is a reliable and accurate method.
2. Signal to noise ratio indicator evaluation method
To evaluate the signal-to-noise ratio, it is necessary to determine the response value. It is necessary to select the sample. It is recommended to absorb the sample near 254 nm such as naphthalene and L-phenylalanine. The former is often used for column performance evaluation; the latter is non-toxic and may exist for the instrument. The polarization phenomenon is sensitive. Of course, even if a different evaluation sample is used, it can be corrected and compared with a spectrophotometer as long as the concentration and solvent are fixed.
According to the existing signal-to-noise ratio measurement method, after selecting a specific concentration of the sample, the detector is balanced at the selected wavelength (the solution in the detector is the solvent for dissolving the evaluation sample), and after the baseline is stabilized, it will be prepared. The sample solution is directly perfused into the flow path of the liquid chromatography instrument and filled therein, taking care not to infuse the bubbles, and after the signal is stabilized, the sample solution is flushed out with the solvent. The amplitude of the fluctuation above and below the baseline is measured as the noise value (N), and the difference between the baseline median value and the median value of the sample signal is measured as the signal value (S). Then the signal to noise ratio is the ratio of the two (SN).
3. Conclusion
Evaluation of the detector using the signal-to-noise ratio (SN) indicator allows a more accurate and comprehensive evaluation of the sensitivity performance of various detectors. The evaluation can refer to the existing signal-to-noise ratio measurement method. It is recommended to use 254nm as the detection wavelength to select a simple and easy-to-obtain sample. The concentration value should be selected so that the signal-to-noise ratio SN is within 10 to facilitate measurement. After formulating the specific concentration of the evaluation solution, the response value S of the relative solvent at this time is measured by the perfusion method, and the value of the signal-to-noise ratio SN is measured and calculated as the signal-to-noise ratio evaluation of the liquid chromatograph detector for the specific evaluation sample solution. The value of the indicator.
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