System for detecting TMA in seawater by adopting simple HS-GC-NPD

Abstract

The utility model relates to a system for detecting TMA in seawater by adopting simple HS-GC-NPD, which belongs to the field of environmental monitoring and comprises an Agilent 7890A gas chromatograph system with the model of G3440A, a high-purity hydrogen generator with the model of HG-1803A and an air pump with the model of AG-1602. A nitrogen and phosphorus detector (NPD) is connected and mounted on the Agilent 7890A gas chromatograph with the model number of G3440A; according to the system, ammonia water is added into a sample, so that the adsorption of the chromatographic column to organic amine is effectively reduced, and the detection of the sample with relatively low concentration is realized. And on the basis, the extraction volume is reduced from 0.85 L to 0.085 L, so that the repeatability of the sample is improved. The above process further improves the accuracy and reproducibility of sample analysis.

Description

Technical Field

[0001] The invention belongs to the field of environmental monitoring, and specifically relates to a system for detecting trimethylamine (TMA) in seawater by using a headspace-gas chromatograph-nitrogen-phosphorus detector (HS-GC-NPD). Background Art

[0002] Establishment of TMA detection method in seawater: Since the 1980s, domestic and foreign scholars have begun to establish detection methods for organic amines in seawater. However, due to the low concentration of organic amines in seawater, the lack of sensitivity of detection methods, and the difficulty in promoting established methods, there are only more than 20 studies on the content variation characteristics of organic amines in seawater. Cree et al (Cree CHL, Airs R, Archer SD, et al. Measurement of methylamines in seawater using solid phase microextraction and gas chromatography [J]. Limnology and Oceanography: Methods, 2018, 16 (7): 411-420.) established a method for detecting organic amines in seawater using solid phase microextraction-gas chromatography-nitrogen phosphorus detector. This method has the advantages of high sensitivity and the ability to simultaneously detect TMA, DMA, and MMA. However, this method also has certain shortcomings: (1) It requires gravity filtration of 850ml seawater samples, which is a time-consuming process; (2) When enriching 850ml seawater samples, it is difficult to maintain a constant temperature and uniform state for a long time, resulting in poor repeatability of sample test results; (3) Based on this method, further verification experiments were carried out and found that when using solid phase microextraction needles to extract samples, the peak shape of the first few needles was good, but the repeatability of the same sample deteriorated with the increase in the number of uses. In addition, the unit price of solid phase microextraction needles is about 1,000 yuan, and each solid phase microextraction needle is used about 10 times, which increases the detection cost. Therefore, it is particularly important to develop a detection method with good repeatability and low cost. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a system for detecting TMA in seawater by combining a simple headspace device, a gas chromatograph, and a nitrogen-phosphorus detector (HS-GC-NPD). The method adopts a 100 mL crimp-top headspace bottle, a 1 mL gas-tight needle (SGE injection needle), and an Agilent 7890A gas chromatograph (G3440A). The NPD detector is connected to the G3440A gas chromatograph and connected to an HG-1803A high-purity hydrogen generator and an AG-1602 air generator. By combining the simple headspace bottle, the gas chromatograph, and the nitrogen-phosphorus detector, the volume of solid-phase microextraction is optimized to 85 mL, the repeatability and accuracy of sample analysis are improved, and the pretreatment cost is low.

[0004] The present invention is achieved through the following technical solutions:

[0005] A system for detecting TMA in seawater using HS-GC-NPD, the system comprising an Agilent 7890A gas chromatograph system (model G3440A), a high-purity hydrogen generator (model HG-1803A), and an air pump (model AG-1602); an NPD detector is connected to the Agilent 7890A gas chromatograph system (model G3440A);

[0006] Flow path of the detection sample: An Agilent 7890A gas chromatograph system with model G3440A was equipped with a split inlet, a nitrogen-phosphorus detector (NPD), and an Agilent CP7448 chromatographic column (265°C, 60m×320μm). The split inlet was connected to one end of the Agilent CP7448 chromatographic column (265°C, 60m×320μm), and the other end of the Agilent CP7448 chromatographic column (265°C, 60m×320μm) was connected to the NPD.

[0007] The present invention also provides a method for detecting using the above system, wherein the method comprises the following steps: using a simple headspace sampling device to process a sample, filtering a 100 ml seawater sample through a microfiltration membrane and then pouring it into 1.0 ml of 37% by volume HCl to inactivate the sample, while protonating the organic amine; taking 85 ml of the water sample and adding it to a 100 ml headspace bottle, and adding 32 g of NaCl, 12 g of K2SO4, and 4.25 ml of 10 mmol·L -1 NaOH and 0.85 ml of ammonia water were stirred with a magnetic stirrer at a constant temperature of 60 °C for 2.5 h at a speed of 1500 rpm. A 1 mL gas-tight needle (SGE injection needle) was inserted into the top of the headspace bottle above the liquid level. 1 mL of gas was taken and quickly injected into the rear injection port of the gas chromatograph to start sampling.

[0008] As one of the preferred embodiments, the gas chromatograph conditions are: front detector temperature 300 ° C, hydrogen flow rate 3 mL min -1 , air flow rate 60mL min -1 , carrier gas plus tail gas flow rate 12ml·min -1 , column temperature 260 °C, rear injection port temperature 200 °C, rubidium bead voltage controlled at 0.5-1 V, output maintained at 20 pA, and potassium hydroxide was used as the electrolyte.

[0009] The beneficial effects of the present invention compared with the prior art are as follows:

[0010] The method of the present invention uses an NPD installed on an Agilent 7890A gas chromatograph system (the Agilent 7890A gas chromatograph system is equipped with a split inlet, which is connected to one end of an Agilent CP7448 chromatographic column (265°C, 60m×320μm). The other end of the CP7448 chromatographic column (265°C, 60m×320μm) is connected to the NPD, as well as the HG-1803A high-purity hydrogen generator and AG-1602 air generator connected to the Agilent7890A gas chromatograph system. The existing technology mainly uses an extraction fiber head to adsorb volatile organic amines in 850ml of seawater samples. This method takes a long time to gravity filter 850ml of sample, and due to the large sample volume, the sample repeatability is poor. In addition, the extraction fiber head used is expensive. This project effectively reduces the adsorption of organic amines by the chromatographic column by adding ammonia water to the sample, thereby realizing the detection of lower concentration samples. On this basis, the extraction volume is reduced from 850ml to 85mL, which can reduce the extraction volume of the sample and improve the repeatability of the sample. In addition, a 1mL gas-tight needle is used to directly extract and inject the sample, which is relatively low in cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is the result of analyzing organic amines using a G3440A gas chromatograph-nitrogen-phosphorus detector combined with headspace sampling technology;

[0012] Figure 2 This is the optimal condition diagram obtained by analyzing TMA using G3440A gas chromatograph-nitrogen-phosphorus detector combined with headspace sampling technology;

[0013] Figure 3 is the standard and correlation coefficient of TMA;

[0014] Figure 4 Schematic diagram of the instrument setup for the solid phase microextraction experiment: 1-heater, 2-1L beaker, 3-SGE syringe, 4-100mL crimp-top headspace vial; DETAILED DESCRIPTION

[0015] The technical solution of the present invention is further explained below through examples, but the protection scope of the present invention is not limited in any form by the examples.

[0016] Example 1

[0017] A system for detecting TMA in seawater using a headspace sampling-gas chromatograph-nitrogen-phosphorus detector (HS-GC-NPD) is disclosed. The system comprises an Agilent 7890A gas chromatograph system (model G3440A), a high-purity hydrogen generator (model HG-1803A), and an air pump (model AG-1602). The gas chromatograph (model G3440A) is equipped with a nitrogen-phosphorus detector, which is used in conjunction with the high-purity hydrogen generator (model HG-1803A) and the air pump (model AG-1602). A headspace device ( Figure 4 ) for sample processing.

[0018] Flow path of the detection sample: an Agilent 7890A gas chromatograph system with model G3440A is equipped with an inlet, a nitrogen-phosphorus detector and a chromatographic column. The inlet is connected to one end of the chromatographic column, and the other end of the chromatographic column is connected to the nitrogen-phosphorus detector.

[0019] A nitrogen-phosphorus detector was installed on an Agilent 7890A gas chromatograph system, connected to an HG-1803A high-purity hydrogen generator and an AG-1602 air generator, and combined with solid-phase microextraction technology. The separation of organic amines (TMA) in seawater was effective, thereby improving data accuracy, and the detection method could be completed within 21 minutes.

[0020] Method for detecting TMA in seawater using the system:

[0021] The specific steps are as follows:

[0022] Before using the system, replace the electrolyte in the HG-1803A high-purity hydrogen generator electrolytic cell storage barrel. Specific replacement method: Add 100g KOH (super pure) and 1000mL distilled water or secondary deionized water into a measuring cylinder, stir evenly, and pour into the storage barrel after cooling. Soak for 20 minutes before use;

[0023] The HG-1803A high-purity hydrogen generator, AG1602 air generator and gas chromatograph need to be self-checked before being connected:

[0024] HG-1803A high-purity hydrogen generator self-test: Tighten the drain valve, turn on the instrument switch, the red light will light up and the green light for electrolytic hydrogen production will light up; bubbles will be generated in the electrolyte in the electrolytic cell; the hydrogen flow rate will show 300ml / min, and the pressure indicator needle will rise to 0.4Mpa, and then the hydrogen flow rate will drop to 0, proving that the instrument is well sealed and the system is working properly. Turn off the power and it can be used online with the gas chromatograph;

[0025] AG-1602 air generator self-test: Tighten the exhaust valve, check whether the air outlet nut on the back is tightened, and check whether the two covers on the filter are tightened. Turn on the instrument switch, the red light on the switch will light up, and the pressure gauge pointer will begin to rise to about 0.4MPa. Turn off the instrument switch and observe the pressure gauge. After ten minutes, the pointer remains at 0.4MPa, indicating that the air circuit system has good sealing performance and the instrument is normal and can be used online with the gas chromatograph;

[0026] Turn on the workstation, turn on the carrier gas switch, start running the gas chromatograph, and let the instrument equilibrate for at least 10 hours;

[0027] The specific operating steps are: turn on the nitrogen, turn on the gas chromatograph switch, tighten the hydrogen generator and air generator exhaust valves to the right, turn on the hydrogen generator, the red button of the air generator, turn on the workstation, select the working method "trimethylamine detection", click the Front det button on the instrument control panel, adjust the detector temperature, and slowly increase it in stages according to 90℃, 120℃, 150℃, 200℃ and 300℃ to prevent the rubidium bead from getting damp and generating overcurrent. Run for more than 10 hours (under normal circumstances, the rubidium bead can maintain a stable base current signal output after 72 hours of operation), keep the instrument baseline stable, and start sampling.

[0028] Sample collection: 100 mL of seawater sample was placed in a high-density polyethylene bottle and immediately gravity filtered through a 47 mm GF / F membrane (pore size 0.7 μm). The filtered solution was poured into another 100 mL high-density polyethylene bottle containing 1.0 mL of 37% HCl to inactivate the solution and protonate the organic amines to prevent their volatilization. The sample was then refrigerated and transported back to the laboratory for analysis.

[0029] Experimental conditions exploration stage: using Figure 4 The experimental device shown in the figure selects NaCl: 30, 31, 32, 34, 35g for repeated experiments, and it is found that 32g is the most suitable ( Figure 2 A); select 10mmol L -1 NaOH: 3ml, 4ml, 4.25ml, 4.5ml, 5mlNaOH: Repeat the experiment and find that 4.25ml is the most suitable ( Figure 2 B in the experiment); select K2SO4: 10, 11, 12, 13, 14g for the experiment, and find that the best result is obtained when K2SO4 is 12g ( Figure 2 C in the experiment); select 40, 50, 60, 70, 80℃ and repeat the experiment several times to get the result ( Figure 2D) With the continuous increase of heating temperature, the peak area of ​​TMA increases more and more. However, starting from 70℃, it is affected by water vapor, and the obtained TMA peak area is unstable and has poor repeatability. Therefore, 60℃ is selected as the optimal heating temperature. NH3·H2O: 0.6, 0.8, 0.85, 0.9, and 1mL are selected for the experiment. It is found that the TMA peak area increases significantly at 0.85mL ( Figure 2 E), 0.85mL was selected as the best; the optimal heating time was tested in the following order: 1h, 2h, 2.5h, 3h, 4h, and the results showed that ( Figure 2 F) For TMA, there was no significant difference between heating times of 2.5 h and 3 h, so 2.5 h was selected as the optimal heating time. The optimal conditions for TMA analysis using a G3440A gas chromatograph coupled with a nitrogen-phosphorus detector (NPD) coupled with solid-phase microextraction (SPME) were as follows: 32 g of NaCl, 4.25 mL of NaOH, 0.85 mL of NH₃·H₂O, and 12 g of K₂SO₄ were added to a 100 mL crimp-top headspace vial at 60°C (the optimal temperature is set at 60°C; above 60°C, the temperature is significantly affected by water vapor, resulting in unstable experimental results), 2.5 h of heating time, and 85 mL of seawater sample. A magnetic stirrer was added, and the vial was sealed with a 20 mm silver aluminum cap.

[0030] Working curve drawing: using TMA standard solution ρ = 1004 μg mL -1 The concentration of trimethylamine was 0.2 μg L -1 , 0.50 μg L -1 , 1.00 μg L -1 , 5.00 μg L -1 、10.00μg L -1 , 20.00μgL -1 , 50.00 μg L -1 、100.00μg L -1 , 200.00 μg L -1 TMA standard solution; weigh 32g sodium chloride and 12g potassium sulfate into 8 100mL headspace bottles, and use a pipette to add 0.85mL ammonia water and 4.25mL sodium hydroxide (concentration of 10mol L -1 ), pipette 85.0 ml of the prepared TMA solution of various concentrations into a headspace vial, immediately seal the vial, and gently shake to mix. After solid-phase microextraction, analyze according to the instrument's reference conditions. A standard series of concentrations was injected and analyzed, and a calibration curve was plotted using peak area versus concentration. For trimethylamine, Y = 3.92X - 2.63, with a correlation coefficient of r = 0.999; Figure 3 This is the standard curve of TMA.

[0031] Sample treatment stage: Take 85ml water sample and add it to 100ml headspace bottle, add 32g NaCl and 12g K2SO4 respectively, and then quickly add 4.25ml NaOH (10mmol L -1 ) and 0.85ml ammonia water. Figure 4 As shown, the mixture was stirred at 60°C for 2.5 hours using a magnetic stirrer and rotor 4 at 1500 rpm. The headspace vial was placed on heater 1 and heated in water bath 2 to maintain constant temperature. A 1 mL SGE syringe 3 was inserted into the top of the headspace vial (above the liquid level but not touching it), and 1 mL of gas was rapidly injected into the gas chromatograph to begin sampling.

[0032] The gas chromatograph conditions are: front detector temperature 300°C, hydrogen flow rate 3 mL·min -1 , air flow rate 60mL·min -1 , carrier gas plus tail gas flow rate 12ml min -1 The column temperature was 260°C, the back injection port temperature was 200°C, the bead voltage was controlled at 0.5-1 V, and the output was maintained at 20 pA. Potassium hydroxide was used as the electrolyte.

[0033] The effect of using G3440A gas chromatograph combined with nitrogen-phosphorus detector to separate trimethylamine TMA by solid phase microextraction technology is shown in the figure. Figure 1 ;TMA( Figure 1 ) has a good separation effect and the detection can be completed in 21 minutes. The correlation coefficient of organic amines can reach above 0.999 ( Figure 3 The detection limit of TMA is 0.1 μg L -1 The limit of quantification was 0.5 μg·L -1 , repeatability is ±15%.

Claims

1. A system for detecting TMA in seawater using a simple HS-GC-NPD method, characterized in that: The system includes a sample processing system, which includes a 100 ml headspace bottle, a magnetic stirrer, and a 1 ml gas-tight needle; the rotor of the magnetic stirrer is placed in the 100 ml headspace bottle, the 100 ml headspace bottle containing the sample is placed on a heater, and the 1 ml gas-tight needle is inserted into the upper part of the sample in the 100 ml headspace bottle; Agilent 7890A gas chromatograph system model G3440A, high-purity hydrogen generator model HG-1803A, and air pump model AG-1602; the Agilent 7890A gas chromatograph model G3440A is connected to an NPD detector; A high-purity hydrogen generator (model HG-1803A) and an air pump (model AG-1602) were both connected to an Agilent 7890A gas chromatograph system (model G3440A). The Agilent 7890A gas chromatograph system (model G3440A) was equipped with a split inlet, a nitrogen-phosphorus detector, and an Agilent CP7448 chromatographic column. The split inlet was connected to one end of the Agilent CP7448 chromatographic column, and the other end of the Agilent CP7448 chromatographic column was connected to the NPD detector.

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