Method for detecting concentration of trace TOC (Total Organic Carbon) by utilizing spectrophotometer

Abstract

The invention discloses a method for detecting the concentration of trace TOC by using a spectrophotometer, and belongs to the technical field of detection of trace organic carbon in a water body. The method specifically comprises the following steps: preparing humic acid standard solutions with different concentrations; detecting absorbance values and TOC concentrations of the humic acid standard solutions with different concentrations at 254nm, 260nm and 280nm; by taking the TOC concentrations of the humic acid standard solutions with different concentrations as a horizontal coordinate and respectively taking the absorbance values of the humic acid standard solutions with different concentrations at 254nm, 260nm and 280nm as vertical coordinates, fitting to obtain an equation of the absorbance values and the TOC concentrations under three different wavelengths; and detecting absorbance values of a to-be-detected water sample at 254 nm, 260 nm and 280 nm, substituting the absorbance values into the obtained three equations to obtain three numerical values, and calculating the average number of the three numerical values, namely the TOC concentration in the to-be-detected water sample. The method is suitable for detecting trace humic acid with the humic acid mass concentration smaller than 50 mg / L, the TOC concentration smaller than 20 mg C / L and the molecular weight smaller than 3500 Da.

Description

Technical Field

[0001] The present invention relates to a method for detecting trace TOC concentration by using a spectrophotometer, and belongs to the technical field of detecting trace organic carbon in water bodies. Background Art

[0002] Humic acid (HA) is widely distributed in nature. Its skeleton structure is distributed with polar functional groups such as phenolic hydroxyl groups, carboxyl groups, alcoholic hydroxyl groups and amino groups. These active sites can form stable complex systems with heavy metal ions in water bodies through mechanisms such as complexation, chelation and surface adsorption. In addition, humic acid is a precursor for the formation of disinfection by-products during the chlorination treatment of water sources. In view of its potential threat to water quality safety, establishing an efficient humic acid monitoring system has become an important link in ensuring drinking water safety. In recent years, the development of accurate and convenient detection methods has attracted much attention in the field of environmental analysis.

[0003] The existing humic acid TOC concentration detection technologies include total organic carbon analyzers. However, for trace TOC concentrations with small molecular weights, it is impossible to accurately detect them with total organic carbon analyzers. Therefore, based on a conventional ultraviolet spectrophotometer and a total organic carbon analyzer in the laboratory, the present invention constructs a method for detecting trace TOC concentration with small molecular weights. Summary of the Invention

[0004] In order to solve the deficiencies existing in the detection process of total organic carbon analyzers, the present invention provides a method for detecting trace TOC concentration by using a spectrophotometer, and the specific steps are as follows:

[0005] (1) Prepare humic acid standard solutions with different concentrations.

[0006] (2) Detect the absorbance values of humic acid standard solutions with different concentrations at 254 nm, 260 nm and 280 nm.

[0007] (3) Detect the TOC concentration of humic acid standard solutions with different concentrations.

[0008] (4) Taking the TOC concentration of humic acid standard solutions with different concentrations as the abscissa and the absorbance values of humic acid standard solutions with different concentrations at 254 nm, 260 nm and 280 nm as the ordinate, fit to obtain equations of absorbance values and TOC concentration at 3 different wavelengths.

[0009] (5) Detect the absorbance values of the water sample to be tested at 254 nm, 260 nm and 280 nm, substitute them into the 3 equations obtained in step (4), obtain 3 values, and calculate the average of the 3 values, which is the TOC concentration in the water sample to be tested.

[0010] Preferably, the concentration of the humic acid standard solution in step (1) is 10 - 50 mg / L.

[0011] Preferably, the equations of the absorbance values at three different wavelengths obtained in step (4) and the TOC concentration are as shown in I to III:

[0012] T 1＝ 0.11705×X A254 +0.13225 (I);

[0013] T2 = 0.11155×X A260 +0.13288 (II);

[0014] T3 = 0.09418×X A280 +0.12148 (III);

[0015] Where X A254 is the absorbance value at A254nm, and X A260 is the absorbance value at A260nm, and X A280 is the absorbance value at A280nm.

[0016] Principle of the present invention:

[0017] The detection method of humic acid concentration in water of the present invention is based on the principle that: A 254 is the absorbance of organic matter in water under ultraviolet light at a wavelength of 254nm, which can characterize the content of aromatic compounds (such as humic acid and fulvic acid), corresponding to the π→π* transition of the benzene ring, and is often used as a surrogate indicator for dissolved organic carbon (DOC) in water quality monitoring; A 260 is the absorbance of organic matter in water under ultraviolet light at a wavelength of 260nm. The aromatic ring structure (such as benzene ring and quinone group) in humic acid has absorption near 260nm, which can be used to evaluate its content. Compared with A 254 , A 260 is more sensitive to the presence of low molecular weight humic substances; A 280 is the absorbance of organic matter in water under ultraviolet light at a wavelength of 280nm, which can reflect the content of proteins or amino acid substances (such as tyrosine and tryptophan), corresponding to the conjugated double bond and heteroatom transition;

[0018] Technical effects of the present invention

[0019] (1) The present invention can solve the problems of fluctuations and errors in the measurement of TOC for trace small molecule humic acid in water samples.

[0020] (2) This method is applicable to the detection of trace humic acid with a humic acid mass concentration less than 50mg / L, a TOC concentration less than 20mg C / L, and a molecular weight < 3500Da.

[0021] (3) The method of the present invention is simple, convenient to operate, time-consuming, and suitable for rapid detection by non-professionals. Description of the Drawings

[0022] Figure 1 Standard curve of TOC value - UV value of humic acid standard solution obtained from the example. Detailed Implementation Manner

[0023] The following examples are used to further illustrate the present invention, but should not be construed as limiting the present invention. Any other changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art.

[0024] Example 1

[0025] Fitting of the absorbance value and the TOC concentration equation, and the specific steps are as follows:

[0026] (1) Prepare a humic acid standard solution: Accurately weigh 100 mg of humic acid with a small molecular weight (molecular weight < 3500 Da), add it to 20 mL of ultrapure water, shake well until the humic acid is completely dissolved, then add deionized water to make the total volume reach 100 mL. After filtering through a 0.45 μm filter membrane, a standard humic acid solution is prepared. Dilute the standard humic acid solution respectively to obtain humic acid solutions with concentrations of 10, 20, 30, 40, and 50 mg / L.

[0027] (2) Take 2 mL of the humic acid solutions with concentrations of 10, 20, 30, 40, and 50 mg / L respectively, and measure their A 254 nm, A 260 nm, and A 280 nm values with a UV spectrophotometer; then take 20 mL of the humic acid solutions with concentrations of 10, 20, 30, 40, and 50 mg / L, and measure their TOC concentrations with a total organic carbon analyzer.

[0028] (3) Using the TOC concentrations of the humic acid standard solutions with different concentrations as the abscissa, and the absorbance values of the humic acid standard solutions with different concentrations at 254 nm, 260 nm, and 280 nm as the ordinate, as Figure 1 shown, three equations shown in Formulas Ⅰ - Ⅲ are obtained by fitting:

[0029] T1 = 0.11705 × A 254 + 0.13225 (Ⅰ)

[0030] T2 = 0.11155 × A 260 + 0.13288 (Ⅱ)

[0031] T3 = 0.09418 × A 280 + 0.12148 (Ⅲ)

[0032] where A 254 is the ultraviolet absorbance value at 254 nm, A 260 is the ultraviolet absorbance value at 260 nm, A 280 is the ultraviolet absorbance value at 280 nm.

[0033] Example 2

[0034] Apply the equation obtained in Example 1 to the detection of TOC concentration in water samples. The specific steps include:

[0035] (1) Prepare water sample A and water sample B: Prepare humic acid solutions with mass concentrations of 10 mg / L and 20 mg / L respectively, with a molecular weight less than 3500 Da. After adsorbing with 200 mg of birnessite for 24 h (the purpose of this step is to reduce the TOC concentration), the supernatant is separated. The supernatant is filtered through a 0.45 μm water-based filter membrane to obtain water sample A and water sample B. At this concentration, birnessite can adsorb >70% of humic acid.

[0036] (2) Detect the absorbance values of water sample A and water sample B at A 254 nm, A 260 nm, A 280 nm values. The absorbance values are shown in Table 1. Substitute the absorbance value at A 254 nm into Equation (Ⅰ) to obtain the T1 value, substitute the absorbance value at A 260 nm into Equation (Ⅱ) to obtain the T1 value, and substitute the absorbance value at A 280 nm into Equation (Ⅲ) to obtain the T3 value.

[0037] (3) Calculate the average values of the T1 value, T2 value and T3 value to obtain the final TOC concentrations in water sample A and water sample B. The results are shown in Table 1. The TOC concentration in water sample A is 0.14 mg C / L, and the TOC concentration in water sample B is 1.23 mg C / L.

[0038] Then test the TOC concentrations of water sample A and water sample B with a total organic carbon analyzer. The results are shown in Table 1.

[0039] Table 1

[0040]

[0041] Calculate the adsorption rate of birnessite on water samples A and B. The calculation formula for the adsorption rate D n (%)

[0042]

[0043] where T x are the TOC concentrations of the water samples before adsorption by birnessite (detected by a TOC analyzer), where x is A or B, TA = 1.51, T B = 4.189;

[0044] D n is the actual adsorption rate after adsorption obtained by calculation or detection, where n = 1, 2, 3, 4.

[0045] The adsorption rate D1 (the adsorption rate of birnessite adsorbed on water sample A calculated when the TOC concentration detected in Table 1 is T1) > 90%, D3 (the adsorption rate of birnessite adsorbed on water sample B calculated when the TOC concentration detected in Table 1 is T3) > 70%, D2 (the adsorption rate of birnessite adsorbed on water sample A calculated when the TOC concentration detected in Table 1 is T2) and D4 (the adsorption rate of birnessite adsorbed on water sample B calculated when the TOC concentration detected in Table 1 is T4) < 70%. It is known that birnessite can adsorb > 70%. Therefore, the adsorption rates calculated by substituting T2 and T4 into the adsorption rate calculation formula do not conform to the adsorption law, indicating that it is incorrect to measure the TOC concentration with a total organic carbon analyzer. Therefore, this method is more accurate than the existing methods for determining trace small molecular weight humic acid. This method is applicable to the detection of trace humic acid with a humic acid mass concentration less than 50 mg / L, a TOC concentration less than 20 mg C / L, and a molecular weight < 3500 Da.

Claims

1. A method for detecting trace TOC concentration using a spectrophotometer, characterized in that: The specific steps are as follows: (1) Prepare humic acid standard solutions with different concentrations; (2) Detect the absorbance values of humic acid standard solutions with different concentrations at 254 nm, 260 nm, and 280 nm; (3) Detect the TOC concentrations of humic acid standard solutions with different concentrations; (4) Using the TOC concentrations of humic acid standard solutions with different concentrations as the abscissa and the absorbance values of humic acid standard solutions with different concentrations at 254 nm, 260 nm, and 280 nm as the ordinate, fit the equations of the absorbance values and TOC concentrations at 3 different wavelengths; (5) Detect the absorbance values of the water sample to be measured at 254 nm, 260 nm, and 280 nm, substitute them into the 3 equations obtained in step (4), obtain 3 values, and calculate the average of the 3 values, which is the TOC concentration in the water sample to be measured.

2. The method for detecting the trace TOC concentration using a spectrophotometer according to claim 1, wherein: In step (1), the concentration of the humic acid standard solution is 10 - 50 mg / L.

3. The method for detecting trace TOC concentration using a spectrophotometer according to claim 1, characterized in that: The equations of the absorbance values and TOC concentrations at 3 different wavelengths obtained in step (4) are as shown in Ⅰ - Ⅲ: T 1＝ 0.11705×X A254 +0.13225(Ⅰ); T2 = 0.11155×X A260 + 0.13288 (Ⅱ); T3 = 0.09418×X A280 + 0.12148(Ⅲ); where X A254 is the absorbance value at 254 nm, and X A260 is the absorbance value at 260 nm, and X A280 is the absorbance value at 280 nm.

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