Method for solving abnormities in absorbance measurement of low-absorption liquid

Abstract

The invention relates to a low-absorption liquid absorbance measurement method, in particular to a method for solving absorbance measurement abnormity of low-absorption liquid, which changes the conventional reference sample, enables the optical loss of the reference sample and the optical loss of a sample to be measured to be consistent, adds the same liquid to be measured into cuvettes made of the same material, and constructs an absorption difference by using cuvettes with different optical paths, so that the absorbance measurement abnormity of the low-absorption liquid is solved. The obtained absorbance data is the absorbance of the liquid under the optical path size difference, and the real and accurate absorbance of the sample to be measured is finally obtained by integrally multiplying the data by the reciprocal of the ratio of the optical path difference to the optical path to be measured. The method is low in cost, convenient to operate and high in measurement precision, and is applied to the field of liquid absorbance testing.

Description

Technical Field

[0001] The present invention relates to a method for measuring the absorbance of a low-absorbing liquid, belonging to the field of optical measurement. Background Art

[0002] In optical measurement, absorbance measurement is an important means to analyze the absorption ability of a liquid to lasers of different wavelengths. In the absorbance measurement scenario, there are significant differences between low-absorbing, low-concentration liquids and high-absorbing, high-concentration liquids. For high-absorbing, high-concentration liquids, because their signals are strong, a little error in measurement has a relatively small impact on the final result. However, for low-absorbing, low-concentration liquids, due to their weak absorption signals, extremely small measurement errors in the measurement process may cause large deviations in the results, thereby affecting subsequent experimental analysis and calculation, etc. Therefore, compared with high-absorbing, high-concentration liquids, low-absorbing, low-concentration liquids have a higher demand for the accuracy of absorption data when performing absorbance measurement.

[0003] A cuvette is a commonly used instrument in the process of liquid absorbance measurement. During measurement, the cuvette containing the liquid to be measured has four interfaces, namely air-cuvette, cuvette-liquid, liquid-cuvette, and cuvette-air. When light propagates to these interfaces, reflection phenomena will occur. Due to the reflection effect, the energy of the transmitted light will decrease, which will inevitably affect the accuracy of absorbance measurement. In addition, a reference sample needs to be used for calibration during the measurement process. However, the light loss caused by the reflection of the interfaces of the reference sample and the sample to be measured is different. This difference causes deviations in the finally measured data and interferes with the accurate determination of the true absorbance of the liquid. Especially when measuring low-absorbing and low-concentration liquids, this energy loss and measurement interference caused by reflection are particularly obvious, seriously affecting the accuracy of the data. Summary of the Invention

[0004] The present invention aims to solve the technical problem of poor data accuracy when measuring the absorbance of low-absorbing and low-concentration liquids, and provides a method for solving the abnormality of low-absorbing liquid absorbance measurement. The method of the present invention is to add the same liquid to be measured into cuvettes of the same material, and use cuvettes with different optical paths to construct an absorption difference; at the same time, multiply the obtained data by the reciprocal of the ratio of the optical path difference to the optical path to be measured to obtain the actual absorbance of the liquid to be measured, eliminating the data abnormality in the absorbance measurement of low-absorbing liquids.

[0005] The method for solving the abnormality of low-absorbing liquid absorbance measurement of the present invention is carried out according to the following steps:

[0006] 1. Take cuvettes of the same material, the same wall thickness, and different optical paths, denoted as cuvette I and cuvette II; where the optical path of cuvette I is h1, and the optical path of cuvette II is h2; h2 > h1;

[0007] II. Add the liquid to be measured into cuvette I and perform a baseline scan using this as a reference.

[0008] III. Then add the liquid to be measured into cuvette II and measure the absorbance value A1 of the liquid.

[0009] IV. Calculate the true absorbance A of the liquid to be measured, where A = A1h2 / (h2 - h1), and complete the absorbance measurement of the liquid to be measured.

[0010] Furthermore, the liquid to be measured in step II is a liquid with low absorption capacity.

[0011] Furthermore, in step I, h2 = (2 - 10)h1.

[0012] Furthermore, the cuvette is a quartz cuvette or a glass cuvette.

[0013] The present invention changes the conventional reference sample for comparison, making the light losses suffered by the reference sample and the sample to be measured the same. Using cuvettes of the same material and adding the same liquid to be measured, an absorption difference is constructed by using cuvettes with different optical paths. The obtained absorbance data is the absorbance of the liquid under the optical path size difference. Multiply the data as a whole by the reciprocal of the ratio of the optical path difference to the measured optical path, and finally obtain the true and accurate absorbance of the sample to be measured.

[0014] The principle of the present invention is as follows:

[0015] When measuring the absorbance of a liquid, it is first necessary to perform a baseline scan on the reference sample, aiming to provide a reference line for the instrument so that the absorbance values measured subsequently can be compared and calculated on an accurate basis. Using different reference samples has different effects on the measured liquid absorbance data.

[0016] According to the Fresnel reflectivity formula, when the incidence is perpendicular, the reflectivity R is determined by the refractive index n1 of the incident medium and the refractive index n2 of the transmission medium. The formula is:

[0017]

[0018] Obviously, the greater the difference between n1 and n2, the higher the reflectivity R and the stronger the reflected light. And regardless of whether the light travels from n1 to n2 or in the reverse direction, the reflectivity is the same. Therefore, when a light beam directly irradiates an empty glass cuvette, the reflectivities of the two types of interfaces from air (n = 1) to glass (n = 1.5) and from glass to air are both 4%. If the cuvette is filled with water (n = 1.333), the reflectivity of the glass-to-water interface will drop to ~3.5‰. If it is filled with cyclohexane (n = 1.426), the reflectivity will further drop to .

[0019] For the test of liquid absorption, an ideal reference sample should be another liquid with no absorption and a refractive index close to that of the liquid to be measured. Such strict conditions make it difficult to accurately measure the absorbance of liquids with low absorption capacity.

[0020] In experiments, air or an empty cuvette is usually directly used as a reference.

[0021] When air is used as a reference sample, the reflection situations during the tests of air quartz cuvettes and solid quartz cuvettes are as Figure 1 shown. Since each layer of the air quartz cuvette has two interfaces, it will undergo four reflections, reducing the transmitted light energy. Compared with the solid quartz cuvette, there are two more reflections, and the measured absorbance curve is as Figure 2 shown. As can be seen from Figure 2 this, the measured absorbance data will show a falsely high situation.

[0022] When using a blank cuvette as a reference, the reflection situations during the tests of air quartz cuvettes and solid quartz cuvettes are as Figure 3 shown. Since the air inside the air cuvette will produce two reflections of approximately 4% with the inner surface of the (glass) cuvette, and among the four reflections (four interfaces: air - cuvette, cuvette - liquid, liquid - cuvette, cuvette - air) of the cuvette filled with the liquid to be measured, due to the refractive index of the liquid being significantly higher than that of air, the reflection ratio of the inner surface is much smaller. This difference will greatly mislead the measurement results and even result in a situation of negative absorbance, as Figure 4 shown.

[0023] Another commonly used material for quartz cuvettes is fused quartz, whose refractive index is between 1.444 and 1.458, close to that of glass cuvettes. Therefore, the above problems are still significant.

[0024] Taking cyclohexane as the liquid to be measured as an example, considering that the refractive index of the liquid to be measured is close to that of the quartz cuvette, a solid quartz cuvette can be selected as a reference to ensure that the reflection situations of the sample and the reference are close, as Figure 5 shown, and the obtained data is as Figure 6 shown. However, the method of using a solid cuvette as a reference sample has limitations. It is only applicable to liquids with the same refractive index as the material of this solid cuvette.

[0025] The present invention is not limited by the requirement of refractive index matching in the above methods. By adding the same liquid to be measured to cuvettes of the same material in the reference sample and the sample to be measured, and using cuvettes with different optical paths to construct an absorption difference, making the optical path of the reference sample smaller than that of the sample to be measured, the obtained absorbance data is the absorbance of the liquid under the optical path size difference. Multiplying the data as a whole by the reciprocal of the ratio of the optical path difference to the optical path to be measured, the true and accurate absorbance of the sample to be measured can be finally obtained.

[0026] The beneficial effects of the present invention are as follows:

[0027] 1. The method of the present invention does not add hardware to the existing test system. It only needs to change the size of the cuvette of the reference sample used as a control. Therefore, the present invention has low cost and convenient operation.

[0028] 2. This present invention is applicable to the measurement of any liquid, so the present invention has universality.

[0029] 3. By using references and samples of the same material but different thicknesses to construct exactly the same reflection loss, the relevant test errors caused by reflection are eliminated, thus improving the measurement accuracy. Description of the Drawings

[0030] Figure 1 Schematic diagram of light loss generated by using air as a reference, an air cuvette and a solid cuvette as samples to be measured;

[0031] Figure 2 Absorbance data measured with air as a reference, an air cuvette and a solid cuvette as samples to be measured;

[0032] Figure 3 Schematic diagram of light loss generated by using air and a cuvette as a reference, and a liquid and a cuvette as samples to be measured;

[0033] Figure 4 Absorbance data of cyclohexane measured with air and a quartz cuvette as a reference, and cyclohexane and a quartz cuvette as samples to be measured;

[0034] Figure 5 Schematic diagram of light loss generated by using a solid quartz cuvette as a reference, and a liquid and a quartz cuvette as samples to be measured;

[0035] Figure 6 Absorbance of cyclohexane measured with a solid quartz cuvette as a reference, and cyclohexane and a quartz cuvette as samples to be measured;

[0036] Figure 7 Schematic diagram of light loss generated in Example 1 with a 1 - mm quartz cuvette as a reference, and a liquid and a 10 - mm quartz cuvette as samples to be measured;

[0037] Figure 8 Absorbance graph of cyclohexane measured in Example 1 with a 1 - mm quartz cuvette as a reference, and cyclohexane and a 10 - mm quartz cuvette as samples to be measured;

[0038] Figure 9 Absorbance curve of 1 mol / L ErCl3 aqueous solution measured by the conventional method and absorbance curve of 0.05 mol / L ErCl3 measured by the method of the present invention; Detailed Embodiments

[0039] The beneficial effects of the present invention are verified by the following embodiments.

[0040] Embodiment 1: The method for solving the abnormal measurement of the absorbance of low-absorbing liquid is carried out according to the following steps:

[0041] I. Take quartz cuvettes with the same material, the same wall thickness, and different optical paths, denoted as cuvette I and cuvette II; among them, the optical path of cuvette I is h1 = 1 mm, and the optical path of cuvette II is h2 = 10 mm;

[0042] II. Add the liquid cyclohexane to be measured into cuvette I, and use this as a reference for baseline scanning;

[0043] III. Then add the liquid to be measured into cuvette II, and measure the absorbance value A1 of the liquid;

[0044] IV. Calculate the true absorbance A of the liquid to be measured, A = 10 / (10 - 1)A1, and complete the absorbance measurement of the liquid to be measured.

[0045] In this Embodiment 1, the liquid to be measured is cyclohexane, and quartz cuvettes with optical paths of 1 mm and 10 mm are selected. This is because the signal of the liquid to be measured itself is low, and the larger the optical path difference, the more obvious the data signal. The quartz cuvette with cyclohexane and an optical path of 1 mm is used as a reference, and the cyclohexane of the sample to be measured is added to the cuvette with an optical path of 10 mm. The light losses of the two are the same. Ignoring the influence of the cuvette walls with different thicknesses, the difference in absorption between the two is only caused by the 9 mm liquid difference. Taking the 1 mm quartz cuvette as a reference, the schematic diagram of the light loss generated by the liquid and the 10 mm cuvette as the sample to be measured is as Figure 7 shown.

[0046] According to the Beer-Lambert law: A = εcl, where ε is the molar absorption coefficient, c is the molar concentration, and l is the optical path. Obviously, for the same absorbent substance with the same concentration, its absorbance is proportional to the optical path. Therefore, the measured result can be multiplied by a coefficient of 10 / 9 to obtain the absorbance of the 10 mm liquid.

[0047] In this Embodiment 1, taking the 1 mm quartz cuvette as a reference, the absorbance diagram of cyclohexane measured with cyclohexane in the 10 mm cuvette as the sample to be measured is as Figure 8 shown. It can be seen that the measured results do not show the phenomena of being falsely high or negative. And the curves of repeated tests coincide, indicating that the stability of this method is good.

[0048] To verify the accuracy of the testing in this embodiment, the absorbance values per unit concentration of the same liquid at different concentrations were used for comparison. Since the influence of light loss on the absorbance of the high-absorbance liquid can be neglected, the absorbance data per unit concentration of the high-absorbance signal liquid tested by the conventional method was used as the standard, and compared with the absorbance per unit concentration of the diluted liquid tested by the method of the present invention. Taking the ErCl3 aqueous solution as an example, the specific method is as follows:

[0049] I. Take quartz cuvettes with the same wall thickness and the same optical path, denoted as cuvette I and cuvette II; the optical paths of cuvette I and cuvette II are both 10 mm.

[0050] II. Add water to cuvette I and perform a baseline scan using this as a reference.

[0051] III. Then add the ErCl3 aqueous solution with a concentration of 1 mol / L as the liquid to be measured into cuvette II, and test the absorbance curve of the liquid, as shown in Figure 9 a of.

[0052] IV. Take quartz cuvettes with the same wall thickness and different optical paths, denoted as cuvette III and cuvette IV; the optical path of cuvette III is h3 = 5 mm, and the optical path of cuvette IV is h4 = 10 mm.

[0053] V. Add the ErCl3 aqueous solution with a concentration of 0.05 mol / L into cuvette III and perform a baseline scan using this as a reference.

[0054] VI. Then add the ErCl3 aqueous solution with a concentration of 0.05 mol / L as the liquid to be measured into cuvette IV, and test the absorbance curve of the liquid, as shown in Figure 9 b of.

[0055] From Figure 9 it can be seen that the absorbance curve of the high-concentration ErCl3 aqueous solution measured by the conventional method has 6 absorption peaks in the visible region, while the absorbance curve of the low-concentration ErCl3 aqueous solution measured by the method of the present invention has 6 corresponding absorption peaks in the visible region. Comparing the absorbance per unit concentration of the 6 peaks, as shown in Table 1, it can be seen that the absorbance value per unit concentration measured by the method of the present invention is almost the same as the test result of the conventional method. This shows that the method of the present invention has high accuracy.

[0056] Table 1 Comparison between the conventional method and the method of the present invention

[0057]

[0058] The present invention changes the reference sample in the traditional measurement method. By using cuvettes made of the same material but with different optical path dimensions, the light losses suffered by the reference sample and the sample to be measured are made consistent, fundamentally solving the data anomaly of the liquid absorbance. It has the advantages of not adding accessories and simple operation.

Claims

1. A method for solving abnormal absorbance measurement of low-absorbing liquids, characterized in that, The method is carried out according to the following steps:

1. Take colorimetric cuvettes with the same material, the same wall thickness, and different optical paths, denoted as colorimetric cuvette I and colorimetric cuvette II; where the optical path of colorimetric cuvette I is h1, and the optical path of colorimetric cuvette II is h2; h2 > h1; 2. Add the liquid to be measured into colorimetric cuvette I, and use this as a reference for baseline scanning; 3. Then add the liquid to be measured into colorimetric cuvette II, and measure the absorbance value A1 of the liquid; 4. Calculate the true absorbance A of the liquid to be measured, A = A1h2 / (h2 - h1), and complete the absorbance measurement of the liquid to be measured.

2. A method for solving abnormal absorbance measurement of low-absorbing liquid according to claim 1, characterized in that, The liquid to be measured described in step 2 is a liquid with low absorption capacity.

3. A method for solving the abnormal measurement of the absorbance of a low-absorbing liquid according to claim 1 or 2, characterized in that, In step 1, h2 = (2 - 10)h1.

4. A method for solving the abnormal measurement of the absorbance of a low-absorbing liquid according to claim 1 or 2, characterized in that, The colorimetric cuvette is a quartz colorimetric cuvette or a glass colorimetric cuvette.

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