Seawater total nitrogen in-situ analysis device and seawater total nitrogen concentration detection method

Abstract

The invention discloses a seawater total nitrogen in-situ analysis device and a seawater total nitrogen concentration detection method, and the seawater total nitrogen in-situ analysis device comprises a fluid distribution module, a digestion reduction module, a micro-flow light detection module, a control module and a mechanical shell, the seawater total nitrogen in-situ analysis device can be directly immersed in seawater to realize on-site in-situ measurement of total nitrogen. The seawater total nitrogen detection method comprises the following steps: mixing seawater with an alkaline digestion agent, and carrying out digestion reaction on total nitrogen in the seawater and the alkaline digestion agent at high temperature to generate nitrate; nitrate and an acidic metal reducing agent are heated for a reduction reaction, and nitrite is generated; the method comprises the following steps: carrying out chromogenic reaction on nitrite and a color developing agent to generate a diazo compound which shows rose red at 525-540nm, and measuring the absorbance x of total nitrogen in seawater at 525-540nm; the total nitrogen concentration y is calculated and output through the temperature correction model, the seawater total nitrogen analysis model under temperature correction is y = (k1t + c1) x + (k2t + c2), k1, c1, k2 and c2 are all compensation coefficients, and t is the environment temperature. According to the seawater total nitrogen in-situ analysis device and the seawater total nitrogen concentration detection method, seawater in-situ measurement can be realized, and the seawater total nitrogen detection accuracy and sensitivity are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of marine ecological environment monitoring, and in particular to an in-situ analysis device for total nitrogen in seawater and a method for detecting total nitrogen concentration in seawater. Background Art

[0002] Total nitrogen in seawater plays an important role in marine aquatic ecosystems. Changes in total nitrogen content in water bodies are important indicators of water quality deterioration and can be used to assess the ecological status of water bodies. Human agricultural production and wastewater discharge may lead to an increase in nitrogen-based compounds in seawater, causing water pollution and marine ecological disasters such as red tides and green tides. Therefore, monitoring total nitrogen in the ocean, especially in coastal waters, is of vital importance for disaster prevention and mitigation and scientific research.

[0003] The method for detecting total nitrogen in seawater usually adopts the potassium persulfate oxidation method in Part 4 of the Marine Monitoring Specifications: Seawater Analysis (GB17378.4-2007), that is, the total nitrogen in seawater is digested at 120°C by potassium persulfate to generate nitrate, and then the nitrate is reduced to nitrite using a cadmium column, and finally reacted with sulfonamide and α-naphthylethylenediamine to generate a diazo compound. However, the biological toxicity of the cadmium column is relatively high, and it will be worn out with use. This method is basically only used as a laboratory manual detection method or for laboratory flow analyzers. It has not been promoted on seawater total nitrogen monitoring equipment and the analysis results have the disadvantage of lagging.

[0004] In addition to the total nitrogen reduction method used in the above-mentioned laboratories, most of the seawater total nitrogen monitoring equipment on the market currently uses ultraviolet reduction methods. This method uses DTPA as a reducing agent and achieves the purpose of reduction through 254nm ultraviolet light irradiation. However, due to the complex seawater substrate, the reduction efficiency of this method is low, only about 40%, which is difficult to meet the detection of low concentrations of total nitrogen in seawater. It may also lead to low accuracy of measurement results and false negative data. At the same time, due to the diverse forms of total nitrogen in seawater, changes in ambient temperature have a great impact on its chemical reaction dynamics and the light intensity measurement of optical detection elements, resulting in deviations in measurement data. Summary of the invention

[0005] The present invention provides a seawater total nitrogen in-situ analysis device and a seawater total nitrogen concentration detection method, which can realize on-site in-situ measurement of seawater, ensure the acquisition of real-time data, and improve the accuracy and sensitivity of seawater total nitrogen detection.

[0006] The present invention proposes an in-situ analysis device for total nitrogen in seawater, comprising: a digestion and reduction module, the digestion and reduction module comprising a reaction device, a temperature sensor and a high-pressure solenoid valve; the outer wall of the reaction device is wound with a heating wire, the heating wire is dense at the bottom and sparse at the top; when the digestion reaction and the reduction reaction are performed, the heating wire plays a role in heat exchange, making the reaction temperature more uniform; the temperature sensor is arranged on the outer wall of the reaction device, the temperature sensor and the surface of the reaction device are coated with thermal conductive silicone grease, and the temperature sensor is used to monitor the temperature change of the reaction device and the in-situ analysis of total nitrogen in seawater The ambient temperature inside the device; there are two high-pressure solenoid valves, which are respectively located above and below the reaction device; when the digestion reaction and the reduction reaction are carried out, the high-pressure solenoid valve is closed to form a closed space for the reaction device to ensure the preservation of the reaction pressure at high temperature; the lower high-pressure solenoid valve is connected to the valve port of the multi-channel valve injection integrated pump through a reagent tube and then to the lower port of the reaction device, the upper port of the reaction device is connected to the upper high-pressure solenoid valve through a reagent tube, and then the high-pressure solenoid valve is connected to the atmosphere through the pipeline through-hole of the upper end cover of the mechanical housing.

[0007] Preferably, it further comprises a mechanical housing, wherein the mechanical housing comprises an upper end cover, a lower end cover and a housing shell; The upper end cover is connected with a reagent tube, which is used to extract seawater, air and discharge waste liquid; the upper end cover is provided with a watertight head and a watertight cable, which are used to power the analysis device; the mechanical shell is made of UPVC and is subjected to a watertight test to play a waterproof and anti-corrosion role.

[0008] Preferably, it also includes a fluid distribution module, which includes a multi-channel valve injection pump; the multi-channel valve injection pump includes a multi-channel valve and a syringe integrated therewith, and is connected to the atmosphere, samples, digesters, reducing agents, color developers and waste liquids through reagent tubes; the multi-channel valve injection pump is controlled by a circuit to select the valve port, and the syringe is used to quantitatively extract and discharge different liquids and to extract air to vortex mix the sample reaction liquid.

[0009] Preferably, it also includes a microfluidic detection module, which is used to detect absorbance, and includes a micro flow cuvette, an LED and a photodiode; the volume of the micro flow cuvette is 30-80 μL, the lower port is connected to the multi-channel valve port through a reagent tube, and the upper port is connected to the waste liquid; the LED and the photodiode are respectively located at both ends of the flow cuvette.

[0010] Preferably, it further comprises a control module, which is used to control the actions of each module, receive the absorbance electrical signal fed back by the detection module, and calculate the total nitrogen concentration in the seawater to be tested through an embedded temperature correction model.

[0011] The present invention also provides a method for detecting the total nitrogen concentration in seawater, the method comprising: A digestion reaction and a reduction reaction, wherein the digestion reaction and the reduction reaction are both carried out in the digestion reduction module as claimed in claim 1; The seawater is mixed with a digester, and the total nitrogen in the seawater reacts with the alkaline digester at high temperature to generate nitrate; The nitrate is heated with an acidic metal reducing agent to undergo a reduction reaction to generate nitrite; Nitrite reacts with the color developer to generate a diazo compound, which appears rose red at 525-540nm. The absorbance x of total nitrogen in seawater at 525-540nm is measured; The volume ratio of seawater: digester: reducing agent: color developer is 15: (1-2.1): (1-3): (1.5-2.5); The total nitrogen concentration y is calculated and outputted using the temperature correction model. The temperature correction model is: y=(k1t+c1)x+(k2t+c2), where k1, c1, k2, c2 are compensation coefficients; Among them, the method for obtaining the temperature correction model is: Step 1, when the water temperature is t1, use the seawater total nitrogen in-situ analysis device to measure the absorbance of total nitrogen standard solutions with concentrations of 100 μg / L, 200 μg / L, 500 μg / L, 800 μg / L and 1000 μg / L, respectively, and use 35‰ NaCl as a blank solution to calculate the concentration-absorbance standard curve of the standard solution: y=a1x+b1; y is the concentration, the unit is μg / L; x is the absorbance; a and b are regression coefficients, wherein a1 is the slope; b1 is the intercept.

[0012] Step 2: Change the water sample temperature to t2, t3, ... t n , and repeat step 1 to obtain the standard curve of total nitrogen under various temperature conditions, and obtain the univariate linear regression equation group at different temperatures:

[0013] In the above system of equations: a2, a3...a n is the slope; b2, b3…b n is the intercept.

[0014] Step 3: Establish slopes a1, a2, ... a n and intercepts b1, b2, ... b n With different temperatures t1, t2, ... t nA linear function is obtained, and the temperature fitting coefficients k and c are obtained, where the fitting coefficients of the slope a are k1 and c1, and the fitting coefficients of the intercept b are k2 and c2; a= k1t+c1, b= k2t+c2; Step 4: Substitute the linear function obtained in step 3 into the univariate linear regression equation of the total nitrogen standard solution in step 2 to establish a correction absorbance formula for total nitrogen in seawater at any temperature, and obtain a temperature correction model: y=(k1t+c1)x+(k2t+c2); The actual temperature t and absorbance x of seawater are detected and brought into the temperature correction model to obtain the concentration of total nitrogen in the actual seawater.

[0015] Preferably, the digester comprises potassium persulfate, sodium hydroxide and water, the concentration of potassium persulfate is 8-12 g / L, and the concentration of sodium hydroxide is 7-12 g / L; the temperature of the digestion reaction is 100° C.-135° C., and the reaction time is 10-40 min.

[0016] Preferably, the acidic metal reducing agent includes a vanadium chloride solution and a hydrochloric acid or sulfuric acid solution, the concentration of the vanadium chloride is 15-25 g / L, and the mass fraction of the hydrochloric acid or sulfuric acid is 5-20%; the temperature of the reduction reaction is 45°C-75°C, and the reaction time is 1-10 min.

[0017] Preferably, the developer comprises a sulfonamide solution and an α-naphthylethylenediamine solution mixed in a volume ratio of (1-3):1.

[0018] Preferably, the sulfonamide solution comprises sulfonamide and hydrochloric acid solution, the concentration of sulfonamide is 15-25 g / L, the mass fraction of hydrochloric acid solution is 10%-20%; the concentration of α-naphthylethylenediamine solution is 2-4 g / L.

[0019] Compared with the prior art, the advantages and positive effects of the present invention include: (1) The toxicity, concentration and dosage of the reducing agent vanadium chloride used in the method for detecting total nitrogen in seawater described in the present invention are all low, thus avoiding the use and replacement of highly toxic cadmium columns, reducing the maintenance frequency and increasing the consistency and accuracy of the measurement.

[0020] (2) The heating reduction efficiency of vanadium chloride described in the present invention is more than twice that of DTPA ultraviolet reduction efficiency, which can ensure the sensitivity and accuracy of measurement.

[0021] (3) The in-situ analysis device for total nitrogen in seawater described in the present invention utilizes an integrated flow injection pump to select samples and reagents and to extract and mix them quantitatively. All reactions described in the present invention are carried out in a digestion and reduction module. The design is compact and centralized, the volume is small, and the mechanical casing is waterproof and corrosion-resistant, making it suitable for in-situ analysis of seawater.

[0022] (4) The temperature correction method for the wet chemical method seawater total nitrogen in-situ analyzer of the present invention takes into account the interference of ambient temperature changes on total nitrogen analysis. When detecting the total nitrogen concentration in seawater, it is only necessary to determine the current water temperature t while obtaining the sample absorbance and bring it into the seawater total nitrogen analysis model under temperature correction to obtain the current total nitrogen concentration in seawater, thereby improving the accuracy of total nitrogen detection and ensuring the accuracy and reliability of the seawater total nitrogen in-situ analyzer.

[0023] Other features and advantages of the present invention will become more apparent after reading the detailed description of the embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is the univariate linear regression equation of the method for detecting total nitrogen concentration in seawater based on vanadium chloride reduction proposed in the present invention (without temperature correction); Figure 2 This is a comparison chart of the reduction efficiency of total nitrogen concentration in seawater measured by the vanadium chloride reduction method and the DTPA reduction method; Figure 3 It is a partial schematic diagram of the process of obtaining the total nitrogen concentration of seawater by the in-situ analysis device for total nitrogen in seawater proposed by the present invention; Figure 4 It is a schematic diagram of the internal structure of the in-situ analysis device for total nitrogen in seawater proposed by the present invention; Figure 5 It is a schematic diagram of the structure of the mechanical housing of the in-situ analysis device for total nitrogen in seawater proposed by the present invention; Figure 6 The in-situ analysis device for total nitrogen in seawater proposed by the present invention measures the concentration of total nitrogen in seawater and compares the data with the manual method in the laboratory. DETAILED DESCRIPTION

[0025] The technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Example 1 Figure 4 , Figure 5 As shown, the in-situ analysis device for total nitrogen in seawater of the present invention comprises a mechanical housing 10 , a fluid distribution module 20 , a digestion and reduction module 30 and a microfluidic detection module 40 .

[0027] The fluid distribution module 20 includes a standard solution 21, a seawater sample 22, a digester 23, a reducing agent 24, a color developer 25, a waste liquid 26 and a multi-channel valve injection integrated pump 27. During the operation of the device, different valve ports of the multi-channel injection integrated pump 27 quantitatively extract the standard solution 21, the seawater sample 22, the digester 23, the reducing agent 24, the color developer 25 and the first atmosphere 1 through the PTFE pipeline, and discharge the reacted solution into the waste liquid 26 through the PTFE pipeline.

[0028] The digestion module 30 includes a reaction device 31 , a heating wire 32 , an upper high-pressure solenoid valve 331 , a lower high-pressure solenoid valve 332 and a temperature sensor.

[0029] The reaction device 31 is made of quartz, and its outer wall is wound with heating wires 32. The heating wires 32 are arranged densely at the bottom and sparsely at the top, which can play a role in heat exchange during the digestion reaction and the reduction reaction, making the reaction temperature more uniform; The temperature sensor is arranged on the outer wall of the reaction device 31. The temperature sensor and the surface of the reaction device 31 are coated with thermal conductive silicone grease. The temperature sensor is used to monitor the temperature change of the reaction device 31. There are two high-pressure solenoid valves, namely an upper high-pressure solenoid valve 331 and a lower high-pressure solenoid valve 332. The upper high-pressure solenoid valve 331 and the lower high-pressure solenoid valve 332 are respectively located above and below the reaction device 31. When the digestion reaction and the reduction reaction are carried out, the upper high-pressure solenoid valve 331 and the lower high-pressure solenoid valve 332 are closed, so that the reaction device 31 forms a closed space to ensure the preservation of the reaction pressure at high temperature.

[0030] The lower high-pressure solenoid valve 332 is connected to the valve port of the multi-channel valve injection integrated pump 27 through a reagent tube and then connected to the lower port of the reaction device 31. The upper port of the reaction device 31 is connected to the upper high-pressure solenoid valve 331 through a reagent tube, and then the upper high-pressure solenoid valve 331 is connected to the second atmosphere 2 through the pipeline through hole of the upper end cover of the mechanical housing.

[0031] The microfluidic detection module 40 is used to detect the absorbance of the sample. The detection module 40 includes a micro flow cuvette 43, an LED 41 and a photodiode 42. The lower port of the flow cuvette 43 is connected to the valve port of the multi-channel valve injection integrated pump 27 through a reagent tube, and the upper port is connected to the waste liquid; the LED 41 and the photodiode 42 are respectively located at both ends of the flow cuvette 43, and the upper end of the flow cuvette 43 is connected to the PTFE pipeline to access the waste liquid 26.

[0032] like Figure 5As shown, the in-situ analysis device for total nitrogen in seawater of the present invention further includes a mechanical housing 10, which includes an upper end cover 11, a lower end cover 12 and a housing shell 13. The upper end cover 11 is provided with a first through hole 14 for installing a reagent tube, and the reagent tube is used to extract seawater, air and discharge waste liquid; the upper end cover 11 is provided with a threaded hole 15, and the threaded hole 15 can be installed with a handle, which can facilitate the transfer and carrying of the in-situ analysis device for total nitrogen in seawater; the upper end cover 11 is provided with a second through hole 16 for installing a watertight head and a watertight cable to power the analysis device.

[0033] Embodiment 2 The measurement process of the in-situ analysis device for total nitrogen in seawater of the present invention comprises the following steps: Clean the reaction device and circulation pool: open the upper high-pressure solenoid valve 331 and the lower high-pressure solenoid valve 332, and the multi-channel injection integrated pump 27 draws liquid from the standard solution 21 and discharges it into the reaction device 31. After staying for 2 seconds, the multi-channel injection integrated pump 27 draws liquid from the reaction device 31 and discharges it into the waste liquid 26 through the detection module 40, completing a cleaning action. This cleaning action is performed twice in total.

[0034] Inject sample and perform digestion reaction: Figure 3 As shown, the multi-channel valve injection integrated pump 27 draws the seawater to be tested from the standard solution 21 and discharges it into the reaction device 31, then draws the detoxifying agent 23 and discharges it into the reaction device 31, the multi-channel injection integrated pump 27 draws the air of the first atmosphere 1 and discharges it into the reaction device 31 for vortex mixing, then closes the upper high-pressure solenoid valve 331 and the lower high-pressure solenoid valve 332, and performs digestion heating through the heating wire 32, and the digestion temperature is monitored by the temperature sensor to be 130°C.

[0035] Reduction reaction: After the digestion reaction has been carried out for 15 minutes, the upper high-pressure solenoid valve 331 and the lower high-pressure solenoid valve 332 are opened, and the multi-channel valve injection integrated pump 27 extracts the reducing agent 24 and discharges it into the reaction device 31. Then, the upper high-pressure solenoid valve 331 and the lower high-pressure solenoid valve 332 are closed, and digestion heating is carried out through the heating wire 32. The reduction temperature is monitored by the temperature sensor to be 65°C.

[0036] Sample detection: After the reduction reaction has been carried out for 5 minutes, the multi-channel valve injection integrated pump 27 extracts the standard solution 21 and discharges it into the flow cuvette 43, the LED 41 is turned on, the photodiode 42 receives the light intensity of the detection light path, and calculates the blank absorbance A0 of the standard solution; the upper high-pressure solenoid valve 331 and the lower high-pressure solenoid valve 332 are opened, the multi-channel valve injection integrated pump 27 extracts the solution in the reaction device 31 and discharges it into the flow cuvette 43, the photodiode 42 receives the light intensity of the detection light path, and calculates the absorbance A1 of the solution after the reaction; the difference between A1 and A0 is the absorbance x of total nitrogen in seawater.

[0037] Clean and empty the reaction device and circulation pool: after the multi-channel valve injection integrated pump 27 draws liquid from the standard solution 21 and discharges it into the reaction device 31, it stays for 2 seconds, and then the multi-channel valve injection integrated pump 27 draws liquid in the reaction device 31, empties the reaction device 31, and discharges it into the waste liquid 26 through the flow cuvette 43; the multi-channel valve injection integrated pump 27 draws air from the first atmosphere 1 and empties it through the flow cuvette 43.

[0038] Calculate the total nitrogen concentration: Substitute the absorbance x and temperature t into the temperature-corrected univariate linear regression equation y=(-2.146t+657.77)x+(0.3977t-42.412), and the total nitrogen concentration y of seawater is obtained.

[0039] The volume ratio of seawater to be tested: digester: reducing agent: color developer is 15:1.5:1:1.

[0040] The digester was prepared by dissolving 10 g potassium persulfate and 9 g sodium hydroxide in 1 L of water.

[0041] The reducing agent is prepared by dissolving 20 g of vanadium chloride in 500 mL of ultrapure water, which is the standby solution 1; 78 mL of concentrated hydrochloric acid is dissolved in 400 mL of ultrapure water, which is the standby solution 2; the standby solution 1 and the standby solution 2 are mixed and fixed to 1 L to obtain a reducing solution.

[0042] The color developer is prepared by mixing a sulfonamide solution and an α-naphthylethylenediamine solution in a certain proportion. The sulfonamide solution is prepared by dissolving 4 g of sulfonamide in 140 mL of a hydrochloric acid aqueous solution and then settling the volume to 200 mL, wherein the hydrochloric acid aqueous solution contains 30 mL of concentrated hydrochloric acid; the α-naphthylethylenediamine solution is prepared by dissolving 3 g of α-naphthylethylenediamine in 1 L of ultrapure water. Further preferably, the sulfonamide solution and the α-naphthylethylenediamine solution are mixed in a volume ratio of 1:1 to obtain the color developer.

[0043] In the present invention, the total nitrogen in seawater needs to be reduced to nitrite after digestion before it can be measured. Therefore, by directly comparing the nitrite standard containing the same concentration of N element and the nitrite generated after nitrate reduction, the reduction efficiency of vanadium chloride and DTPA reduction methods can be known. Figure 2 It can be seen that the reduction efficiency of the vanadium chloride of the present invention is about twice that of the DTPA method. The vanadium chloride of the present invention has high reduction efficiency and can improve the accuracy and sensitivity of total nitrogen detection in seawater.

[0044] Example 3 The measurement accuracy of the in-situ analysis device for total nitrogen in seawater of the present invention before and after temperature correction is compared. Accuracy is represented by relative error, and the formula is: relative error (%) = (measured value - true value) / true value × 100. The smaller the relative error, the higher the accuracy of the analyzer measurement.

[0045] The seawater total nitrogen in-situ analysis device was placed in a high and low temperature test chamber, and the temperature of the high and low temperature test chamber was adjusted to 15.5℃ and 31.2℃, respectively, to analyze the measurement accuracy of the device after temperature correction. A standard solution with a total nitrogen concentration of 500μg / L was selected as the sample to be tested. The sample to be tested was tested, and the absorbance corresponding to 15.5℃ and 31.2℃ was 0.8125 and 0.8876, respectively, which was substituted into the univariate linear regression equation before temperature correction (such as Figure 1 As shown in the figure, the concentration of the sample to be tested at 15.5°C is 479.65μg / L, with a relative error of -4.07%; the concentration of the sample to be tested at 31.2°C is 527.17μg / L, with a relative error of 5.43%.

[0046] The in-situ analysis device for total nitrogen in seawater was placed in a high and low temperature test chamber, and the temperature was set to 10℃-40℃. A standard curve for total nitrogen in seawater was established every 5℃, so as to obtain the slope a and intercept b of the standard curve corresponding to each temperature t. The linear function of temperature t and slope a was established as a=-2.146t+675.77, and the linear function of temperature t and intercept b was established as b=0.3977t-42.412, that is, the fitting coefficients of slope a were k1=-2.146 and c1=675.77, and the fitting coefficients of intercept b were k2=0.3977 and c2=-42.412. The linear function of temperature t and slope a and the linear function of temperature t and intercept b were substituted into the standard curve of total nitrogen in seawater y=ax+b, and the corrected univariate linear regression equation was obtained as y=(-2.146t+675.77)x+(0.3977t-42.412).

[0047] Where y is the sample concentration in μg / L; x is the absorbance; t is the temperature in °C; Substituting the temperature set in the high and low temperature test chamber and the absorbance measured at the corresponding temperature into the equation, the concentration of the sample to be tested at 15.5°C was 485.79μg / L, with a relative error of -2.84%; the concentration of the sample to be tested at 31.2°C was 510.38μg / L, with a relative error of 2.08%. It can be seen that the accuracy of the calibrated seawater total nitrogen in-situ analysis device has been improved, and the measurement results of the samples to be tested are more accurate.

[0048] At present, the method for detecting total nitrogen in seawater is usually measured by the laboratory manual method shown in Part 4 of the Marine Monitoring Specifications: Seawater Analysis (GB17378.4-2007). The measurement data of the in-situ analysis device for total nitrogen in seawater after applying the vanadium chloride reduction method and temperature correction are compared with the laboratory manual method. The results show that the two are consistent with each other, which can replace the manual method and reduce manpower and material resources.

[0049] The above-mentioned in-situ analysis device for total nitrogen in seawater was installed in the shaft of the pontoon, and the measurement steps of the in-situ analysis device for total nitrogen in seawater were embedded in the device control module to determine the total nitrogen in seawater. The equipment was placed at the pontoon dock for a two-week analysis test. The total nitrogen in seawater was tested once every 4 hours, and a total of 84 groups of samples were tested; during the test, manual sampling was performed every day and sent back to the laboratory for method comparison tests. A total of 14 comparisons were performed, and 42 groups of data were measured. Three parallel samples were taken for each group of data and compared with the results of the analysis device. The measurement time during the comparison is shown in Table 1. Table 1 shows that the sampling time of the manual method must be consistent with the analysis time of the instrument to ensure the comparability of the samples collected by the two; at the same time, the manual method measurement takes into account the time, energy and subsequent time to return to the laboratory for processing, and only selects daytime for sample collection.

[0050] Table 1 Frequency of inspection of instrument technical indicators

[0051] The principle of the total nitrogen detection method of the present invention is as follows: the total nitrogen in the seawater to be tested reacts with a digester at high temperature to generate nitrate, the nitrate reacts with a reducing solution by heating to generate nitrite, the nitrite reacts with a color developer to generate a diazo compound, which displays rose red at 540 nm. Different concentrations of total nitrogen finally generate diazo compounds with different color depths, and the total nitrogen content in the seawater is determined by measuring the absorbance of the product at 540 nm.

[0052] The collected data are shown in Table 2. Indication error = laboratory measurement average value − instrument measurement value. Relative error (%) = indication error / laboratory measurement average value × 100. The comparison results are shown in Figure 6 As shown, Figure 6 It is a comparison curve diagram drawn according to Table 2.

[0053] Table 2 shows the measurement results obtained by the in-situ analysis device for total nitrogen in seawater and the laboratory analysis method, as well as the relative error between the two. The smaller the error, the better the consistency between the two. Generally speaking, the relative error within 10% is the best. Some individual values ​​differ greatly and are abnormal values ​​that need to be removed.

[0054] Table 2 Collection data and analysis results

[0055] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for a person skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to replace some of the technical features therein by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A seawater total nitrogen in-situ analysis device, characterized in that: include: A digestion and reduction module, comprising a reaction device, a temperature sensor and a high-pressure solenoid valve; The outer wall of the reaction device is wound with heating wires, and the heating wires are arranged densely at the bottom and sparsely at the top; when the digestion reaction and the reduction reaction are carried out, the heating wires play a role in heat exchange, making the reaction temperature more uniform; The temperature sensor is arranged on the outer wall of the reaction device, and the surface of the temperature sensor and the reaction device are coated with thermal conductive silicone grease, and the temperature sensor is used to monitor the temperature change of the reaction device; There are two high-pressure solenoid valves, which are respectively located above and below the reaction device; when the digestion reaction and the reduction reaction are performed, the high-pressure solenoid valve is closed, so that the reaction device forms a closed space to ensure that the reaction pressure is maintained at high temperature; The high-pressure solenoid valve at the bottom is connected to the valve port of the multi-channel valve injection integrated pump through a reagent tube and then connected to the lower port of the reaction device; the upper port of the reaction device is connected to the high-pressure solenoid valve at the top through a reagent tube, and then the high-pressure solenoid valve at the top is connected to the atmosphere through the pipeline through hole of the upper end cover of the mechanical housing.

2. The seawater total nitrogen in-situ analysis device according to claim 1, characterized in that: Also included is a mechanical housing, the mechanical housing including an upper end cover, a lower end cover and a housing shell; The upper end cover is connected with a reagent tube, which is used to extract seawater, air and discharge waste liquid; The upper end cover is provided with a watertight head and a watertight cable for supplying power to the analysis device; the mechanical housing is made of UPVC and is subjected to a watertight test to play a waterproof and anti-corrosion role.

3. The in-situ analysis device for total nitrogen in seawater according to claim 1, characterized in that: Also included is a fluid distribution module, the fluid distribution module including a multi-channel valve injection integrated pump; The multi-channel valve injection integrated pump comprises a multi-channel valve and an integrated syringe, and is connected to the atmosphere, sample, digester, reducing agent, developer and waste liquid through a reagent tube; The multi-channel valve injection integrated pump is controlled by a circuit to select a valve port, and the syringe is used to quantitatively extract and discharge different liquids and to extract air to vortex mix the sample reaction liquid.

4. The in-situ analysis device for total nitrogen in seawater according to claim 1, characterized in that: It also includes a microfluidic detection module, which is used to detect absorbance and includes a micro-flow cuvette, an LED and a photodiode; The volume of the micro-flow cuvette is 30-80 μL, the lower port is connected to the multi-channel valve port through a reagent tube, and the upper port is connected to the waste liquid; The LED and the photodiode are respectively located at two ends of the flow cuvette.

5. The in-situ analysis device for total nitrogen in seawater according to claim 4, characterized in that: It also includes a control module, which is used to control the actions of each module, receive the absorbance electrical signal fed back by the microfluidic detection module, and calculate the total nitrogen concentration in the seawater to be tested through an embedded temperature correction model.

6. A method for detecting total nitrogen concentration in seawater, characterized in that: The method comprises: A digestion reaction and a reduction reaction, wherein the digestion reaction and the reduction reaction are both carried out in the digestion reduction module as claimed in claim 1; The seawater is mixed with a digester, and the total nitrogen in the seawater reacts with the alkaline digester at high temperature to generate nitrate; The nitrate is heated with an acidic reducing agent to undergo a reduction reaction to generate nitrite; Nitrite reacts with the color developer to generate a diazo compound, which appears rose red at 525-540nm. The absorbance x of total nitrogen in seawater at 525-540nm is measured; The volume ratio of seawater: digester: reducing agent: color developer is 15: (1-2.1): (1-3): (1.5-2.5); The total nitrogen concentration y is calculated and outputted using the temperature correction model. The temperature correction model is: y=(k1t+c1)x+(k2t+c2), where k1, c1, k2, c2 are compensation coefficients; Among them, the method for obtaining the temperature correction model is: Step 1, when the water temperature is t1, using the seawater total nitrogen in-situ analysis device, measuring the absorbance of total nitrogen standard solutions with concentrations of 100 μg / L, 200 μg / L, 500 μg / L, 800 μg / L and 1000 μg / L, respectively, using 35‰ NaCl as a blank solution, and calculating the concentration-absorbance standard curve of the standard solution: y=a1x+b1; y is the concentration, the unit is μg / L; x is the absorbance; a and b are regression coefficients, wherein a1 is the slope; b1 is the intercept; Step 2: Change the water sample temperature to t2, t3, ... t n , and repeat step 1 to obtain the standard curve of total nitrogen under various temperature conditions, and obtain the univariate linear regression equation group at different temperatures: ； In the above equations: a2, a3...a n is the slope; b2, b3…b n is the intercept; Step 3: Establish slopes a1, a2, ... a n and intercepts b1, b2, ... b n With different temperatures t1, t2, ... t n A linear function is obtained, and the temperature fitting coefficients k and c are obtained, where the fitting coefficients of the slope a are k1 and c1, and the fitting coefficients of the intercept b are k2 and c2; a= k1t+c1, b= k2t+c2; Step 4: Substitute the linear function obtained in step 3 into the univariate linear regression equation of the total nitrogen standard solution in step 2 to establish a correction absorbance formula for total nitrogen in seawater at any temperature, and obtain a temperature correction model: y=(k1t+c1)x+(k2t+c2); The actual temperature t and absorbance x of seawater are detected and brought into the temperature correction model to obtain the concentration of total nitrogen in the actual seawater.

7. The method for detecting total nitrogen concentration in seawater according to claim 1, characterized in that: The digester comprises potassium persulfate, sodium hydroxide and water, the concentration of potassium persulfate is 8-12 g / L, and the concentration of sodium hydroxide is 7-12 g / L; The temperature of the digestion reaction is 100°C-135°C, and the reaction time is 10-40 minutes.

8. The method for detecting total nitrogen concentration in seawater according to claim 1, characterized in that: The reducing agent comprises a vanadium chloride solution and an acidic solution, wherein the acidic solution is a hydrochloric acid solution or a sulfuric acid solution; The concentration of vanadium chloride is 15-25g / L, and the mass fraction of hydrochloric acid solution or sulfuric acid solution is 5-20%; The temperature of the reduction reaction is 45°C-75°C, and the reaction time is 1-10 min.

9. The method for detecting total nitrogen concentration in seawater according to claim 1, characterized in that: The color developer comprises a sulfonamide solution and an α-naphthylethylenediamine solution mixed in a volume ratio of (1-3):

1.

10. The method for detecting total nitrogen concentration in seawater according to claim 9, characterized in that: The sulfonamide solution includes sulfonamide and hydrochloric acid solution, the concentration of sulfonamide is 15-25g / L, and the mass fraction of hydrochloric acid solution is 10%-20%; The concentration of the α-naphthylethylenediamine solution is 2-4 g / L.

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