A method for obtaining the optimum absorption wavelength of near ultraviolet end visible light with visible spectrophotometer

Abstract

The application discloses a method for obtaining the best absorption wavelength of near ultraviolet end visible light by using a visible spectrophotometer, and belongs to the technical field of analytical chemistry detection. In view of the problem that the background of a near ultraviolet region (400nm to 435nm) light source is high, and a characteristic absorption peak is easy to be submerged in a background signal and difficult to be identified, the method comprises the following steps: selecting a wavelength step less than 5nm to scan a target waveband, obtaining the absorbance data of an air blank and a colored solution to be detected, and deducting the air blank from an instrument without an automatic zero function; adopting a quadratic function approximation method, taking three continuous wavelength points as a wavelength segment group, constructing a quadratic function y=ax 2 +bx+c in sequence, and recording the quadratic term coefficient a corresponding to each wavelength segment group; continuously comparing the quadratic term coefficients a, if the a value of a middle wavelength segment group is significantly smaller than that of adjacent segment groups, it is determined that the segment has a characteristic absorption peak, and the best absorption wavelength is determined by selecting a waveband where the a value is the minimum value. The application effectively strips the high light source background interference by using the curve curvature change, does not need to modify the instrument, is simple to operate, is strong in universality, and can realize accurate positioning of a weak spectrum peak on an ordinary spectrophotometer.

Description

Technical Field

[0001] This invention belongs to the field of analytical chemistry and optical detection technology, specifically relating to a method for spectral measurement and data processing. More specifically, this invention relates to a method for accurately extracting and determining the optimal visible light absorption wavelength of an analyte under high background interference in the near-ultraviolet region (particularly the 400 nm to 435 nm band) using a visible spectrophotometer, through mathematical modeling and data processing. This invention can also be used in the search for other characteristic spectral features with high background spectral characteristics. Background Technology

[0002] In visible spectrophotometry applications of analytical chemistry, wavelength selection is a crucial step in determining the sensitivity and accuracy of the method. Typically, the analyte has a specific maximum absorption wavelength in the visible light region, and quantitative analysis is performed using the absorbance at that wavelength. However, in practical applications, near-ultraviolet (e.g., 400 nm–435 nm) visible light absorption measurements are often involved. This band lies at the edge of the emission spectrum of a visible spectrophotometer's light source.

[0003] Currently, the limitation of existing technology lies in the following: In the near-ultraviolet region of a visible spectrophotometer, the luminous intensity of the instrument's light source (such as a tungsten lamp or halogen tungsten lamp) is low in this band, or it is limited by the instrument's optical path system (such as grating diffraction efficiency and detector sensitivity), resulting in an extremely high "light source background" in this region. During actual measurements, the absorbance signal of the analyte in this band contains a large proportion of the light source background signal, causing the useful signal to be submerged in the extremely high background signal. On the wavelength-absorbance scan, it only appears as a monotonically decreasing curve, while the original characteristic absorption peaks, due to their minute changes and being submerged by the background, are completely indistinguishable to the naked eye. With conventional visible spectrophotometers, operators often cannot determine whether the absorbance fluctuations at this point are due to random measurement errors of the instrument or genuine characteristic absorption peaks of the analyte.

[0004] Due to the lack of effective signal processing methods, current techniques for finding the optimal absorption wavelength in the near-ultraviolet region often rely on blind trial and error or reference to literature, lacking rigorous data support and universally applicable discrimination rules. This significantly limits the application scope of visible spectrophotometry in the near-ultraviolet region. Therefore, to address these technical problems, there is an urgent need to develop a data processing method that can effectively remove background interference from high-light sources and accurately locate characteristic absorption peaks in the near-ultraviolet region using a visible spectrophotometer. Summary of the Invention

[0005] The purpose of this invention is to address the technical shortcomings of existing visible spectrophotometers in the near-ultraviolet region by providing a method for obtaining the optimal absorption wavelength of visible light in the near-ultraviolet range using a visible spectrophotometer. In the near-ultraviolet region (wavelengths 400 nm to 435 nm) of a visible spectrophotometer, the background light is extremely high and exhibits a monotonically rapid decline due to limitations in the instrument's light source intensity, grating diffraction efficiency, and detector sensitivity. The characteristic absorption peaks of the analyte solution in this band show very weak changes and are easily obscured by the high background signal. Operators cannot visually distinguish the true characteristic absorption peaks from the random errors in instrument measurement using conventional wavelength-absorbance scans, making it difficult to accurately select the optimal analytical wavelength. This invention aims to effectively eliminate background light interference and accurately extract and locate the characteristic absorption peaks in the high-background spectrum of the near-ultraviolet region through data modeling and mathematical processing.

[0006] To achieve the above objectives, this invention provides a method for obtaining the optimal absorption wavelength of near-ultraviolet visible light using a visible spectrophotometer, which employs a quadratic function approximation method as its core data processing principle. Specifically, it includes the following technical steps: Step S1, Spectral data acquisition: On a visible spectrophotometer, set the target scanning wavelength range (preferably 400 nm to 435 nm) and select a wavelength step size of less than 5 nm. Scan from one end of the target wavelength range to the other end to obtain the raw wavelength-absorbance scanning data of the air blank solution (light source background) and the colored solution to be tested.

[0007] Step S2, Background subtraction processing: If the visible spectrophotometer used does not have an automatic zeroing function, the absorbance value of the colored solution to be tested at each wavelength point is subtracted from the absorbance value of the air blank solution at the same wavelength to obtain the new absorbance data after deducting the light source background. If the visible spectrophotometer used has an automatic zeroing function (such as the 723C spectrophotometer, which resets the absorbance to zero every time the wavelength is adjusted), then the absorbance value of the colored solution to be tested read directly from the instrument is used as the target absorbance data required for subsequent calculations.

[0008] Step S3, quadratic curve fitting: Within the target wavelength band after background subtraction, following the order of wavelength from low to high, three consecutive wavelength points are used as a wavelength segment group (i.e., x1, x2, x3 and their corresponding absorbances y1, y2, y3). The quadratic function equation y=ax is then constructed by sliding the wavelengths sequentially to each wavelength segment. 2 +bx+c, a≠0; and record the quadratic coefficient a value corresponding to each wavelength segment group in order from low to high wavelength.

[0009] Step S4, Characteristic absorption peak identification: The quadratic coefficient 'a' values ​​of each adjacent wavelength group are compared sequentially. According to the mathematical properties of quadratic functions, if a characteristic absorption peak appears in a high-light-source background (where overall absorbance decreases), it will change the curvature of the curve in that region, resulting in a significant change in the quadratic coefficient 'a' value of that wavelength group. The specific peak-finding rule is as follows: If, during the sequential comparison process, it is found that the quadratic coefficient 'a' of a certain intermediate wavelength group is significantly smaller than the 'a' value of its adjacent preceding wavelength group and significantly smaller than the 'a' value of its subsequent wavelength group (i.e., a 'trough' characteristic appears), then it is determined that the intermediate wavelength group contains a characteristic absorption peak.

[0010] Step S5, Determining the optimal wavelength: In step S4, among all the possible wavelength ranges containing the characteristic absorption peaks, the wavelength range where the quadratic coefficient a shows the most significant "trough" change or the absolute value is the smallest is selected. The position corresponding to this wavelength range is the optimal visible light absorption wavelength of the colored solution in the near-ultraviolet region.

[0011] Compared with the prior art, the present invention has the following outstanding substantive features and significant progress: 1. Effective signal extraction: This invention creatively utilizes the quadratic function approximation method. By calculating the change in local curvature (quadratic coefficient a), it successfully transforms the weak feature absorption signal that was originally buried in high background noise into a quantifiable and comparable mathematical index, thus completely solving the problem that the true spectral peaks cannot be distinguished by the naked eye in traditional methods.

[0012] 2. High versatility and low cost: This invention does not require hardware modification of existing visible spectrophotometers. It can be implemented on various ordinary laboratory visible spectrophotometers by relying only on conventional wavelength scanning functions and basic mathematical fitting, and has extremely high value for promotion and application.

[0013] 3. Objective and accurate judgment results: Verification through examples shows that the method of the present invention is effective in various colorimetric systems (such as silicic molybdenum yellow solution, EDTA-Fe...). 3+ The method can accurately and consistently locate the optimal absorption wavelength in the near-ultraviolet region on solutions and different types of spectrophotometers (such as devices with and without automatic zeroing function), providing reliable technical support for establishing a new method for near-ultraviolet-visible spectrophotometric analysis with high sensitivity and high accuracy. Attached Figure Description

[0014] Figure 1 This is the original wavelength-absorbance fitting curve of the silicic molybdenum yellow solution in the 400-440nm band on the 723C spectrophotometer in Embodiment 1 of the present invention.

[0015] Figure 2 This is a comparison chart of the continuous distribution of the quadratic function coefficient 'a' values ​​of the silicic molybdenum yellow solution in each wavelength band on the 723C spectrophotometer in Embodiment 1 of the present invention.

[0016] Figure 3 It is the EDTA-Fe on the 723C spectrophotometer in Embodiment 1 of the present invention. 3+ The original wavelength-absorbance fitting curve of the solution in the 400-440nm band.

[0017] Figure 4 It is the EDTA-Fe on the 723C spectrophotometer in Embodiment 1 of the present invention. 3+ A comparison of the continuous distribution of the coefficients 'a' of the quadratic function in the solution across different wavelength bands.

[0018] Figure 5 This is a graph showing the original wavelength-absorbance fitting curves of air blank and silicomolybdenum yellow solution in the 395-445nm band on a 7230G spectrophotometer in Embodiment 2 of the present invention.

[0019] Figure 6 This is the fitting curve of the silicic molybdenum yellow solution after subtracting the air blank on the 7230G spectrophotometer in Embodiment 2 of the present invention.

[0020] Figure 7 This is a comparison chart of the continuous distribution of the quadratic function coefficients 'a' values ​​in each wavelength band after subtracting the blank from the silicomolybdenum yellow solution on the 7230G spectrophotometer in Embodiment 2 of the present invention.

[0021] Figure 8 This refers to the air blank and EDTA-Fe on the 7230G spectrophotometer in Embodiment 2 of the present invention. 3+ The original wavelength-absorbance fitting curve of the solution in the 400-440nm band.

[0022] Figure 9 It is the EDTA-Fe on the 7230G spectrophotometer in Embodiment 2 of the present invention. 3+ Fitted curve of the solution after subtracting the air blank.

[0023] Figure 10 It is the EDTA-Fe on the 7230G spectrophotometer in Embodiment 2 of the present invention. 3+ A comparison of the continuous distribution of the coefficients 'a' of the quadratic function across different wavelength bands after subtracting the blank from the solution. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. These specific embodiments are intended to provide detailed illustrative examples of the invention and are not intended to limit the scope of the invention. Those skilled in the art, after reading this specification, can make appropriate changes and adjustments to the embodiments as needed without departing from the spirit and scope of the invention.

[0025] Example 1 This embodiment uses a 723C visible spectrophotometer with an "automatic zeroing" function. This model of instrument automatically resets the absorbance to zero each time the wavelength is adjusted. Therefore, its measurement values ​​have automatically eliminated the background factor of the light source at a specific wavelength. Thus, in this embodiment, there is no need to perform additional air blank subtraction, and the absorbance data of the test solution can be obtained directly.

[0026] 1. Preparation of experimental reagent rain solution Silicate molybdenum yellow solution: Take 5 mL of 0.4 g / L SiO2 standard solution into a 100 mL volumetric flask, add 10 mL of 100 mL H2SO4 (3+97) solution and 5 mL of ammonium molybdate (50 g / L) solution, and make up to volume and mix well.

[0027] EDTA-Fe 3+ Solution: Take 0.5 g / L of Fe 3+ Add 10 mL of the standard solution to a 100 mL volumetric flask, add 5 mL of EDTA (50 g / L) solution, and dilute to volume and mix well.

[0028] 2. Determination and Data Processing of Silicate Molybdenum Yellow Solution Turn on the 723C spectrophotometer and allow it to warm up and stabilize. Set the wavelength scanning range to 400 nm to 440 nm, and select a step size of approximately 2 nm to 3 nm. Record the absorbance values ​​of the silicic molybdenum yellow solution at 17 points from low to high wavelength: 400 nm, 403 nm, 405 nm, 408 nm, 410 nm, 412 nm, 415 nm, 418 nm, 420 nm, 422 nm, 425 nm, 428 nm, 430 nm, 432 nm, 435 nm, 438 nm, and 440 nm (see Table 1 for specific data). Plot a fitted curve with wavelength λ on the x-axis and absorbance λ on the y-axis (e.g., ...). Figure 1 As shown in the figure. It can be clearly seen from the fitted curve that the absorbance decreases monotonically with increasing wavelength, which is a high background environment, and no characteristic absorption peaks can be observed with the naked eye.

[0029] Table 1. Absorbance distribution of molybdenum yellow on 723C spectrophotometer as a function of wavelength Subsequently, data processing is performed according to the quadratic function approximation method proposed in this invention: starting from the lowest wavelength end, three consecutive wavelength points are used as a wavelength segment group. For example, the first wavelength segment group is 400nm, 403nm, and 405nm; the second wavelength segment group is 403nm, 405nm, and 408nm; and so on, sliding sequentially. Each wavelength point group is denoted as (x1, y1), (x2, y2), and (x3, y3), and substituted into the quadratic function equation y = ax 2 Solving for +bx+c (a≠0) yields the quadratic coefficient 'a' for this wavelength group. Calculate the 'a' values ​​for all wavelength groups (numerical values ​​are shown in Table 2).

[0030] Table 2. Distribution of the quadratic function coefficient 'a' of the three molybdenum yellow spots on the 723C spectrophotometer as a function of wavelength (×1000) Compare all the quadratic function coefficients 'a' calculated in Table 2 sequentially according to wavelength, and plot a line graph of coefficient 'a' varying with wavelength range (e.g., ...). Figure 2 (As shown). Through Figure 2 It can be visually observed that a significant "concave" transition occurs near 410 nm. Based on this, it is determined that there is a characteristic absorption peak in the wavelength range of 408 nm-412 nm, and the optimal absorption wavelength of this solution is 410 nm.

[0031] 3. EDTA-Fe 3+ Determination and Data Processing of Solutions Replace with EDTA-Fe 3+ The absorbance values ​​at the above 17 wavelength points were measured using the same scanning parameters (see Table 3 for specific data), and a fitting curve was plotted (e.g., ...). Figure 3 (As shown). Its absorbance also shows a monotonically decreasing trend, making it impossible to directly observe the spectral peaks.

[0032] Table 3. Absorbance distribution of EDTA-Fe as a function of wavelength on a 723C spectrophotometer. Following the same method as with the molybdenum yellow solution, a quadratic function was fitted using three consecutive wavelength points as wavelength group, and the coefficients a of the quadratic term of each wavelength group were calculated (the values ​​are shown in Table 4).

[0033] Table 4. Distribution of the quadratic function coefficient 'a' of EDTA-Fe at three points on the 723C spectrophotometer as a function of wavelength (×1000) Plot the values ​​of 'a' in Table 4 as a line graph (e.g.) Figure 4 (As shown). From Figure 4The analysis revealed that the quadratic coefficient 'a' showed significant low values ​​or abrupt changes near 410 nm, 420 nm, and 428 nm. However, within the range of 418 nm to 425 nm, the change in 'a' value exhibited a clear stationary characteristic (i.e., the 'a' value experienced a fluctuation of rising, falling, and rising again in this range). Considering the continuity of the spectral peaks and the clustering of the minimum values ​​of 'a', the optimal absorption wavelength of the solution in the near-ultraviolet region was determined to be 420 nm.

[0034] Example 2 This embodiment uses a 7230G visible spectrophotometer that does not have an "automatic zeroing" function. In order to eliminate the influence of the high light source background in the near-ultraviolet region, this embodiment must perform an "air blank" subtraction operation.

[0035] 1. Preparation of experimental reagents and solutions Silicomolybdenum yellow solution and EDTA-Fe 3+ The solution preparation method is exactly the same as in Example 1.

[0036] 2. Determination, blank subtraction, and data processing of silicomolybdenum yellow solution. Step 1: Background Scan and Solution Scan: Fill the cuvette with pure water (air blank) and perform a blank scan in the range of 395 nm to 445 nm, recording the absorbance values ​​at wavelength intervals of 5 nm (see Table 5, fitting curves are shown in Table 5). Figure 5 (Left side). Subsequently, the sample was loaded with molybdenum yellow solution and scanned under the same conditions (see Table 6, fitting curves are shown in Table 6). Figure 5 (Right side). A comparison reveals that both the air blank and the sample scan images exhibit extremely high light source backgrounds and are remarkably similar in shape.

[0037] Table 5. Absorbance of air as a function of wavelength on a 7230G spectrophotometer Table 6. Absorbance distribution of molybdenum silicoylate as a function of wavelength on a 7230G spectrophotometer Step 2: Background subtraction: Subtract the absorbance of the air blank at the same wavelength from the absorbance values ​​of the molybdenum yellow solution at each wavelength to obtain the "new absorbance" after subtracting the light source background (see Table 7 for specific data, and the fitting curve is shown in...). Figure 6 Although the curve after removing the background still shows a decreasing trend, the curvature of the curve has changed.

[0038] Table 7. Absorbance distribution of molybdenum silicate yellow after air blank on 7230G spectrophotometer as a function of wavelength. Step 3: Quadratic function approximation method processing: For the new absorbance data after subtracting the blank, the same "continuous three-point" quadratic function fitting method as in Example 1 is used to calculate the quadratic coefficient a of each wavelength group (see Table 8 for specific data).

[0039] Table 8. Distribution of the quadratic coefficient 'a' of molybdenum silicate yellow after air subtraction on the 7230G spectrophotometer as a function of wavelength (×1000) Step 4: Peak Value Determination: Plot the values ​​of 'a' in Table 8 sequentially (e.g., ...). Figure 7 (As shown). From Figure 7 It can be clearly observed that in the wavelength range of 400nm-410nm, the coefficient 'a' of the quadratic term gradually decreases, forming a significant local minimum near 410nm. Combined with... Figure 7 Based on the distribution characteristics of the α-value, the optimal absorption wavelength of the solution was determined to be 410 nm. This result is completely consistent with the result obtained in Example 1 using a 723C spectrophotometer.

[0040] 3. EDTA-Fe 3+ Solution determination, blank subtraction and data processing Step 1: Background Scan and Solution Scan: The absorbance of the air blank was measured in steps of 2-3 nm within the wavelength range of 400 nm to 440 nm (see Table 9, fitted curves are shown in Table 9). Figure 8 (left side), and EDTA-Fe 3+ The absorbance of the solution (see Table 10, and the fitted curve is shown in the figure) Figure 8 (Right side).

[0041] Table 9. Absorbance of air as a function of wavelength on a 7230G spectrophotometer Table 10 Absorbance distribution of EDTA-Fe on 7230G spectrophotometer as a function of wavelength Step 2: Background subtraction: Subtract the blank absorbance from the solution absorbance point by point to obtain the new absorbance (see Table 11 for specific data, and see the fitted curve for the curve). Figure 9 ).

[0042] Table 11. Absorbance distribution of EDTA-Fe after air blank subtraction on a 7230G spectrophotometer as a function of wavelength. Step 3: Quadratic function approximation method: Using three consecutive points as a group, perform quadratic function fitting on the new absorbance data to calculate the quadratic coefficient 'a' of each wavelength group (see Table 12 for specific data).

[0043] Table 12. Distribution of the quadratic coefficient 'a' of molybdenum silicate yellow under three-point air subtraction on the 7230G spectrophotometer as a function of wavelength (×1000) Step 4: Peak Value Determination: Plot a line graph of the a value as a function of wavelength (e.g., ...). Figure 10 (As shown). From Figure 10 It was found that the coefficient 'a' of the quadratic term changed abruptly at 410 nm, 420 nm, and 428 nm. However, considering the continuity of the curve and the overall curvature change in the 418 nm-425 nm wavelength range, it was determined that 420 nm was the optimal absorption wavelength of the solution in the near-ultraviolet region.

[0044] In summary, Examples 1 and 2 employed two different models of visible spectrophotometers with different optical detection mechanisms. Experiments on two completely different chemical colorimetric systems accurately and consistently determined the optimal absorption wavelength (410 nm for both silicon molybdenum yellow and EDTA-Fe) under weak near-ultraviolet signals. 3+ (All are 420nm). This fully demonstrates that the quadratic function approximation method proposed in this invention has extremely strong anti-interference ability and excellent universality, and can effectively solve the technical problem that existing technologies cannot identify characteristic absorption peaks in the high background environment of the near ultraviolet region.

Claims

1. A method for obtaining the optimal absorption wavelength of near-ultraviolet visible light using a visible spectrophotometer, characterized in that, Includes the following steps: S1: Set the scanning wavelength range to the target band in the near ultraviolet region, select a wavelength scanning step size of less than 5nm, scan from one end of the target band to the other end, and obtain the wavelength-absorbance scanning data of the air blank solution and the colored solution to be tested respectively. S2: If the visible spectrophotometer does not have an automatic zeroing function, the absorbance of the air blank solution at the corresponding wavelength point is subtracted from the absorbance at each wavelength point in the colored solution to be tested to obtain the target absorbance data of the solution to be tested; if the visible spectrophotometer has an automatic zeroing function, the absorbance of the colored solution to be tested directly measured by the instrument is used as the target absorbance data. S3: Within the target wavelength range, following the order of wavelength from low to high, take three consecutive wavelength points as a wavelength segment group, and successively slide and construct the quadratic function equation y=ax for each wavelength segment group. 2 +bx+c, a≠0; and record the coefficient a of the quadratic term for each group; S4: Compare the quadratic coefficients a of each adjacent wavelength group in turn. If the quadratic coefficients a of the middle wavelength group are significantly smaller than those of the preceding wavelength group and significantly smaller than those of the following wavelength group, then the middle wavelength group is determined to be the wavelength group where the characteristic absorption peak may exist. S5: From all the wavelength groups that are determined to be characteristic absorption peaks, select the wavelength group with the smallest corresponding quadratic coefficient a. The wavelength contained in this wavelength group is the optimal absorption wavelength position.

2. The method according to claim 1, characterized in that, In step S1, the target wavelength in the near-ultraviolet region is 400nm to 435nm.

3. The method according to claim 1, characterized in that, In step S1, the wavelength scanning step size is set between 1 nm and 3 nm.

4. The method according to claim 1, characterized in that, In step S3, the construction of the quadratic function equation is to take the wavelength value as the independent variable x and the corresponding target absorbance data as the dependent variable y, and solve the quadratic coefficient a by substituting the three coordinates into a system of simultaneous equations.

5. The method according to claim 1, characterized in that, When the method is applied, it also includes a data filtering preprocessing step: before step S3, the target absorbance data is smoothed to reduce the interference of instrument random noise on the calculation of the quadratic coefficient a.

6. The method according to claim 1, characterized in that, The target wavelength band includes the characteristic absorption peak of yellow visible light in the near-ultraviolet region, and the colored solution is a potassium thiocyanate colorimetric system solution, a silicomolybdenum yellow colorimetric system solution, or an EDTA-Fe system solution. 3+ One of the complex solutions.

Images