Method for measuring chlorine ion in a special medical food

Abstract

This invention belongs to the field of food testing technology, and particularly relates to a method for determining chloride ions in special medical foods, including the following steps: (1) Sampling: Weigh 1.0g of sample into a graduated plastic tube; (2) Standard curve formulation: Optimize the standard working curve, and according to the optimized standard working curve, measure the standard working solution from low concentration to high concentration using a spectrophotometer, and plot the standard curve with absorbance as the ordinate and chloride ion concentration as the abscissa; (3) Sample pretreatment: Weigh the sample into a special medical food pretreatment device, add water and nitric acid solution, mix thoroughly, seal and soak for extraction, separate solid and liquid, make up the volume of the separated liquid, and let it stand to obtain the test solution; (4) Determination: Measure a certain amount of the test solution into a 100mL volumetric flask, add nitric acid solution and silver nitrate, let it stand, and then measure it at 400nm using an ultraviolet spectrophotometer.

Description

Technical Field

[0001] This invention belongs to the field of food testing technology, and in particular relates to a method for determining chloride ions in special medical foods. Background Technology

[0002] Chlorine is one of the important macroelements in the human body, usually present as chloride ions (Cl-). - Chloride exists in the form of electrolytes and acids, working alongside potassium and sodium ions to maintain the body's acid-base balance. An imbalance in the chloride ion content of the blood can lead to electrolyte and acid-base imbalances, and in severe cases, even hyperchloremic acidosis. In GB25596-2010, the "National Food Safety Standard for Infant Formula for Special Medical Purposes," chloride is listed as an essential nutrient. Compared to ordinary infant formula, formulas for special medical purposes are powdered or liquid formulations designed to meet the nutritional needs of infants with specific disorders, diseases, or medical conditions. Therefore, controlling the quality of these products is crucial.

[0003] GB 5009.44-2016, "Determination of Chloride in Food," specifies three methods for determining chlorine: potentiometric titration, indirect precipitation titration, and direct titration. However, foods for special medical purposes contain milk proteins, amino acids, short peptides, and other complex matrices. Using the direct or indirect precipitation methods specified in the standard cannot completely precipitate proteins and fats, easily leading to solution turbidity and affecting the titration. Potentiometric titration, a new method in GB 5009.44-2016, is highly applicable, but requires a potentiometric titrator, which is expensive and unsuitable for primary laboratories. Furthermore, it requires large amounts of acetone, which is inconvenient for personnel and the environment. Therefore, establishing a spectrophotometric method for determining chlorine in foods for special medical purposes is crucial. This method is rapid, accurate, low-cost, and widely applicable, and plays a vital role in identifying potential risks in these foods, meeting regulatory requirements, and ensuring the high-quality development of foods for special medical purposes. Summary of the Invention

[0004] To address the problems of the prior art, this application provides a method for determining chloride ions in special medical foods.

[0005] This application is achieved through the following scheme:

[0006] A method for determining chloride ions in a special medical food includes the following steps:

[0007] (5) Sampling: Weigh 1.0g of the sample into a graduated plastic tube;

[0008] (6) Development of standard curve: The standard working curve is optimized. Based on the optimized standard working curve, the standard working solution is measured by spectrophotometer from low concentration to high concentration. The standard curve is plotted with absorbance as the ordinate and chloride ion concentration as the abscissa.

[0009] (7) Sample pretreatment: Weigh the sample into the special medical purpose formula food pretreatment equipment, add water and nitric acid solution, mix thoroughly, soak and extract in a sealed container, separate solid and liquid, make up the volume of the separated liquid, and let it stand to obtain the test solution.

[0010] (8) Measurement: Take a certain amount of the test solution and put it into a 100mL volumetric flask. Add nitric acid solution and silver nitrate. After standing, measure it at 400nm using an ultraviolet spectrophotometer.

[0011] The pretreatment equipment includes a base, a volume balancer on top of the base, a sedimentator and a fixed container on top of the volume balancer, the bottom of the sedimentator being connected to the volume balancer via a sediment discharge pipe, an elastic membrane on the top surface of the volume balancer, the fixed container being connected to the volume balancer via the elastic membrane, a piston adapted to the sedimentator structure on top of the sedimentator, the sedimentator being connected to the fixed container via a sedimentator discharge pipe, the volume balancer being filled with a variable-shape, constant-volume filler, a filter device on the sedimentator discharge pipe, and a sedimentator discharge valve on the sedimentator discharge pipe.

[0012] Furthermore, the filler is water or bubble wrap.

[0013] Furthermore, the inner wall of the volume balancer is provided with an elastic inner membrane, the filler is filled in the elastic inner membrane, and the precipitate discharge pipe is not connected to the elastic inner membrane.

[0014] Furthermore, the sum of the volume change of the volume balancer and the internal volume of the fixed container is 100 ml.

[0015] Furthermore, the ratio of the volume change of the volume balancer to the internal volume of the fixed container is 1-6:94-99.

[0016] Furthermore, the container is equipped with graduated damping plates.

[0017] Furthermore, the fixed container is equipped with a fixed container drain pipe, and the fixed container drain pipe is equipped with a filter membrane. The precipitator is wider at the top and narrower at the bottom. The precipitate discharge pipe is equipped with a pressure pipe, and the other end of the pressure pipe is equipped with a pressure device and a pressure valve. A filter screen is provided at the connection between the pressure pipe and the precipitate discharge pipe. The precipitator discharge pipe is located above the pressure pipe. The precipitate discharge pipe is equipped with an exhaust pipe, and the exhaust pipe is equipped with an exhaust valve. The fixed container is equipped with a fixed container cover that matches the fixed container. The fixed container cover is equipped with a controller and a switch. The wall of the fixed container is equipped with a scale. The fixed container is equipped with a liquid level sensor, which is electrically connected to the controller. The controller is electrically connected to the switch. The outer wall of the fixed container is equipped with an alarm, which is connected to the controller.

[0018] Furthermore, the slope of the optimized standard working curve is 0.0546, and the correlation coefficient R0 is [missing information]. 2 =0.9992; The precipitator is equipped with two precipitator discharge pipes, denoted as precipitator discharge pipe one and precipitator discharge pipe two. The precipitator discharge pipe one is located at 1 / 2 of the precipitator volume, and the precipitator discharge pipe two is located at 3 / 10 of the precipitator volume.

[0019] Furthermore, in step (3) sample pretreatment: weigh the sample into the special medical purpose formula food pretreatment equipment, add 3mL of water and 2mL of nitric acid solution, mix thoroughly, and soak and extract in a sealed container for 15min. Drain the test liquid from the precipitator outlet tube into the fixed container, add a fixed volume solution to the precipitator, wash the precipitate 7-11 times, drain the washing liquid into the fixed container through the precipitator outlet tube, and drain the precipitate through the precipitate discharge tube. At this time, add water to make up to 100mL, mix well, let stand, and after standing, drain the test liquid from the fixed container with filter membrane outlet tube. The nitric acid solution in step (3) is made by mixing 1 volume of concentrated nitric acid with 5 volumes of deionized water.

[0020] Furthermore, in step (4), the following steps are performed: 5 mL of the test solution is placed in a 100 mL volumetric flask, 1 mL of nitric acid solution and 2 mL of silver nitrate (15 g / L) are added, and the solution is kept at a constant temperature of 35 °C in the dark for 15 min. The solution is then measured at 400 nm using a 1 cm cuvette and an ultraviolet spectrophotometer. The nitric acid solution in step (4) is prepared by mixing 1 volume of concentrated nitric acid with 2 volumes of deionized water.

[0021] Beneficial effects:

[0022] (1) This invention discloses a method for determining chloride ions in special medical foods. According to the requirements of GB / T27407-2010 "Laboratory Quality Control Standard for Physicochemical Testing of Food" and GB / T27417-2017 "Guidelines for Conformity Assessment and Validation of Chemical Analysis Methods", the recovery rate is 96.06-104.71% under different concentration levels of spikes, which meets the requirements of the method recovery rate range; the precision is 0.79-3.49%, which meets the precision requirements, indicating that the method is feasible and applicable to the determination of chloride ions in special medical foods.

[0023] (2) The pretreatment equipment provided by the present invention has a simple structure and is suitable for use in any basic laboratory. The present invention uses few reagents, is green and environmentally friendly, and can still achieve precipitation of special medical foods with the least amount of nitric acid. It can also determine the analyte without adding a stabilizer. It is easy to operate in batches, meets regulatory requirements, and is also a new exploration in the field of food chemical testing.

[0024] (3) The matrix of special medical foods is complex. Extraction with a nitric acid solution (1 volume of concentrated nitric acid mixed with 11 volumes of deionized water) still results in a turbid solution. Adding 3 mL of a nitric acid solution (1 volume of concentrated nitric acid mixed with 5 volumes of deionized water) for precipitation leads to insufficient filtrate and even adsorption by the filter paper, resulting in an overestimation of the precipitate. To ensure the correct results, the volume of the precipitant (nitric acid) needs to be increased by 100 mL; otherwise, incomplete precipitation and poor filtration will occur. However, this wastes nitric acid, which is also a precursor to toxic substances. To avoid this, a pretreatment device specifically for special medical foods is used. This invention utilizes a piston to discharge the precipitate from the precipitate discharge pipe into a volume balancer. The volume balancer is filled with a variable-shape, constant-volume material. When the precipitate enters the volume balancer, pressure causes the elastic membrane to bulge at the bottom of the container, balancing the volume change. The final size of the bulge matches the size of the precipitate. Finally, the volume is adjusted. When the volume of water, the volume of the test solution discharged from the precipitator outlet pipe, and the volume of the bulge reach 100 mL, the scale stop plate automatically floats out to prevent over-adjustment. This invention achieves precipitation with the smallest volume of nitric acid and a final volume of 100 mL, ensuring that the error is minimized. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some preferred embodiments of this application, and not all embodiments. For those skilled in the art, other embodiments and drawings can be obtained based on these embodiments and drawings without creative effort, and all of them fall within the protection scope of this application.

[0026] Figure 1 Absorption curves for wavelengths of 200–600 nm;

[0027] Figure 2 Linear range graph;

[0028] Figure 3 This is a graph showing the effect of reaction temperature.

[0029] Figure 4 This is a schematic diagram of the pretreatment equipment structure;

[0030] Figure 5 This is a schematic diagram of the connection structure between the pressurization device and the sediment discharge port in an embodiment of this application;

[0031] In the diagram: 1. Base, 2. Sedimenter, 3. Volume balancer, 4. Fixed container, 5. Piston, 6. Sediment discharge pipe, 7. Sedimenter drain pipe, 8. Packing material, 9. Elastic membrane, 10. Restriction plate, 11. Fixed container drain pipe, 12. Elastic inner membrane, 13. Pressurizing device, 14. Filtering device, 15. Pressurizing valve, 16. Pressurizing pipe, 17. Exhaust pipe, 18. Exhaust valve, 19. Fixed container cover, 20. Controller, 21. Switch, 22. Liquid level sensor, 23. Alarm, 24. Sediment discharge valve, 25. Sedimenter drain valve, 26. Filter screen. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below. It should be understood that the terms "upper," "lower," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. The foregoing definitions are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the structure referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0033] Example 1

[0034] Optimization of the method and procedure for determining chloride ions in special medical foods

[0035] (1) Selection of measurement wavelength

[0036] Some literature uses 300nm and 330nm as the maximum absorption wavelengths, but silver ions have significant absorption in the 200nm-300nm range, leading to inaccurate results. This invention performs a wavelength scan from 200 to 600nm, finding a maximum absorption peak around 310nm, but with an absorbance as high as 3.5 Å. This is the absorbance of silver ion plasma, see [reference needed]. Figure 1 Furthermore, absorbance measurements were taken at 310 nm at chloride ion concentrations ranging from 0 to 10 μg / mL. The absorbance remained essentially the same, indicating that besides silver ions, other substances also contribute to the interference at 310 nm. The maximum absorption peak around 310 nm is not the optimal absorption wavelength. This is likely due to the silver chloride suspension scattering the light, and is not a characteristic peak. As the wavelength increases, absorbance decreases. To avoid scattering and interference from other substances, the inflection point of 350 nm was bypassed, and 400 nm was chosen as the experimental measurement wavelength.

[0037] (2) Selection of extraction method

[0038] To precipitate proteins and fats, previous standards and literature typically employed the addition of zinc acetate and potassium ferrocyanide. However, the matrix of special medical purpose food formulations is complex, and this method often results in turbidity, and CN- ions react with silver ions in the system to form a precipitate. This invention optimizes the extraction method to make it simpler, more convenient, and cost-effective.

[0039] To avoid introducing other interfering ions and to ensure the simplicity and speed of the method, different proportions of nitric acid were selected. It was found that using 1+11 nitric acid did not produce significant precipitation, while 1+5 or 1+2 nitric acid could achieve precipitation. Therefore, 1+5 nitric acid was ultimately chosen.

[0040] The nitric acid solution described in this application as n+m refers to a nitric acid solution composed of n volumes of concentrated nitric acid and m volumes of deionized water. Similarly, the nitric acid solution as 1+2 refers to a nitric acid solution composed of 1 volume of concentrated nitric acid and 2 volumes of deionized water.

[0041] (3) Determination of sample size

[0042] Foods for special medical purposes (FSMP) have relatively high chloride ion content. Weighing too few samples leads to significant errors, and the system's limitations mean that increasing the sample size increases filtration difficulty. Considering that currently available FSMP products typically contain 300–800 mg / 100g of chloride ions, GB29922-2013, the "National Food Safety Standard for Foods for Special Medical Purposes (General Rules)," stipulates that chloride ions are a mandatory test item in FSMP products, with a limit of 52 mg / 100 KJ. Based on the energy level of most FSMP products being 2000 KJ / 100g, the limit for FSMP products is approximately 1040 mg / 100g. Considering that at a dilution factor of 200, the chloride ion content in the sample is low, even below 300 mg / 100g, the chloride ion content in the test solution is below 0.375 μg / mL, or even lower, leading to a larger error in the results; when the chloride ion content in the sample is high, approaching the maximum limit of 1040 mg / 100g, the chloride ion content in the test solution reaches 26 μg / mL, resulting in resource waste. Furthermore, due to system limitations, increasing the sample weight increases the difficulty of filtration. Therefore, considering all factors, a sample weight of 1g is selected.

[0043] Table 1 Selection of Sample Size Conditions

[0044] Chloride ion content in the sample (mg / 100g) Sample weight (g) Dilution factor Chloride ion content in the test solution (μg / mL) 300 0.25 200 0.375 300 1 200 1.5 300 5 200 7.5 1040 0.25 200 1.3 1040 1 200 5.2 1040 5 200 26

[0045] (4) Determining the linear range

[0046] By optimizing the sample weight and considering the limit levels in foods for special medical purposes, the concentration ranges of chloride ion standard solutions were determined to be 0 μg / mL, 0.8 μg / mL, 1.6 μg / mL, 2.4 μg / mL, 3.2 μg / mL, 8 μg / mL, and 10 μg / mL. Under the finally determined experimental conditions, experiments were conducted, and a good linear relationship was observed. Details are as follows. Figure 2 .

[0047] (5) Selection of reaction temperature

[0048] The stability of silver chloride suspension has been studied. Some literature uses the addition of stabilizers for optimization, while this invention studies stability by selecting the reaction temperature. To date, no literature has optimized the reaction temperature conditions for silver chloride. However, since silver chloride itself is a suspension, ensuring that the generated silver chloride is uniformly dispersed in the solution is the primary problem to be solved in this experiment. To ensure that the silver chloride suspension does not become turbid and to increase sensitivity, the temperature conditions were optimized. It was found that different temperatures in the reaction system have a significant impact on the absorbance of the system. As the temperature increases, the absorbance gradually increases, and the absorbance increases significantly after 35°C. Figure 3As shown. To ensure high sensitivity and accuracy, and considering that excessively high temperatures would have a certain impact on the spectrophotometer during colorimetric analysis, the reaction temperature was determined to be 35℃.

[0049] Example 2

[0050] Pretreatment equipment for determining chloride ions in special medical foods, such as... Figure 4 , Figure 5 ;

[0051] The pretreatment equipment includes a base 1, a volume balancer 3 on the top of the base 1, a sedimentation tank 2 and a fixed container 4 on the top of the volume balancer 3, the bottom of the sedimentation tank 2 being connected to the volume balancer 3 via a sediment discharge pipe 6, an elastic membrane 9 on the top surface of the volume balancer 3, and the fixed container 4 being connected to the volume balancer 3 via the elastic membrane 9. A piston 5 adapted to the sedimentation tank structure is provided on the top of the sedimentation tank 2, and the sedimentation tank 2 is connected to the fixed container 4 via a sedimentation tank discharge pipe 7. The volume balancer 3 is filled with a filler 8 whose shape is variable but whose volume is constant. A filter device 14 is provided on the sedimentation tank discharge pipe 7, and a sedimentation tank discharge valve 25 is provided on the sedimentation tank discharge pipe 7.

[0052] At least one embodiment is implemented through the following scheme: Figure 4 As shown, the fixed container 4 and the volume balancer 3 are connected by an elastic membrane 9. That is to say, the elastic membrane 9 is both a part of the fixed container 4 and a part of the volume balancer 3.

[0053] The filtration device 14 used in this application is an existing device with filtration function, such as a filter membrane, ultrafiltration membrane, nanofiltration membrane, filter membrane module, etc.

[0054] At least one embodiment is achieved by the following scheme: the bottom of the fixed container 4 is an elastic bottom, the elastic bottom of the fixed container 4 is in contact with the elastic membrane 9, and the elastic bottom of the fixed container 4 matches the size of the elastic membrane 9.

[0055] Furthermore, the filler 8 is water or bubble wrap.

[0056] At least one embodiment is implemented through the following scheme: Figure 5 As shown, the inner wall of the volume balancer 3 is provided with an elastic inner membrane 12, the filler 8 is filled in the elastic inner membrane 12, and the precipitate discharge pipe 6 is not connected to the elastic inner membrane 12; when the elastic inner membrane 12 is used, the volume balancer 3 has an opening at the top, the fixed container 4 is set at the opening, the bottom of the fixed container 4 is set as an elastic bottom, and the size of the bottom of the fixed container 4 is larger than the size of the opening;

[0057] Furthermore, the sum of the volume change of the volume balancer 3 and the internal volume of the fixed container 4 is 100 ml.

[0058] Furthermore, the ratio of the volume change of the volume balancer 3 to the internal volume of the fixed container 4 is 1-6:99-94.

[0059] At least one embodiment is implemented by the following scheme: the fixed container 4 is provided with a graduated blocking plate 10.

[0060] Furthermore, the fixed container 4 is provided with a fixed container drain pipe 11, and the fixed container drain pipe 11 is provided with a filter membrane.

[0061] At least one embodiment is implemented by the following scheme: the precipitator 2 is wider at the top and narrower at the bottom (or it can be designed to be the same size at the top and bottom), the precipitate discharge pipe 6 is provided with a pressurizing pipe 16, the other end of the pressurizing pipe 16 is provided with a pressurizing device 13, the pressurizing pipe 16 is provided with a pressurizing valve 15, a filter screen 26 is provided at the connection between the pressurizing pipe 16 and the precipitate discharge pipe 6, and the precipitate discharge pipe 6 is provided with a precipitate discharge valve 24.

[0062] At least one embodiment is implemented by the following scheme: the precipitator 2 is provided with two precipitator discharge pipes 7, referred to as precipitator discharge pipe one and precipitator discharge pipe two. The precipitator discharge pipe one is set at 1 / 2 of the volume of the precipitator 2, and the precipitator discharge pipe two is set at 3 / 10 of the volume of the precipitator 2. By providing two precipitator discharge pipes 7, it is prevented that the precipitator discharge pipes 7 are set too high, so that the liquid below the precipitator 2 cannot be transported to the fixed container 4 through the precipitator discharge pipes 7. If the precipitator discharge pipes 7 are set too low, the liquid will be transported to the fixed container 4 slowly due to the effect of the precipitate, and the time will be too long. By setting the two precipitator discharge pipes 7 at 1 / 2 and 3 / 10 of the volume of the precipitator 2 respectively, not only can the liquid in the upper layer reach the fixed container 4 quickly, but it can also prevent some liquid from not being transported to the fixed container 4 through the precipitator discharge pipes 7.

[0063] The precipitator discharge pipe 7 is located above the pressurization pipe 16; the precipitate discharge pipe 6 is provided with an exhaust pipe 17, and the exhaust pipe 17 is provided with an exhaust valve 18; the fixed container 4 is provided with a fixed container cover 19 that matches the fixed container 4, the fixed container cover 19 is provided with a controller 20 and a switch 21, the wall of the fixed container 4 is provided with a scale, the fixed container 4 is provided with a liquid level sensor 22, the liquid level sensor 22 is electrically connected to the controller 20, the controller 20 is electrically connected to the switch 21, the outer wall of the fixed container 4 is provided with an alarm 23, and the alarm 23 is connected to the controller 20.

[0064] After sedimentation, open the sedimentation drain valve 25 on the sedimentation drain pipe 7 to place the liquid into the fixed container, then close the sedimentation drain valve 25, add a fixed volume solution to the sedimentation tank 2 to wash the liquid remaining in the sediment, open the sedimentation drain valve 25 to transfer the washing solution to the fixed container 4, and repeat the washing 7-11 times. Then close the sedimentation drain valve 25 and open the sediment discharge valve 24 on the sediment discharge pipe 6. The sediment is transferred to the volume balancer 3 through the sediment discharge pipe 6. The piston 5 pushes the residue on the inner wall of the sedimentation tank 2 downward to the sediment discharge pipe 6. Close the sediment discharge valve 24 and open the pressure valve 15. The pressure device 13 pressurizes the sediment in the sediment discharge pipe 6 and pushes it into the volume balancer 3. Close the pressure valve 15 and open the exhaust valve 18 to make the pressure in the pretreatment equipment the same as the atmospheric pressure. Close the exhaust valve 18.

[0065] When in use, when the liquid level in the container 4 reaches the set scale, the liquid level sensor 22 transmits a signal to the controller 20. The controller 20 controls the alarm 23 to remind the R&D personnel to stop adding the volumetric solution into the container 4. At the same time, the controller 20 controls the start of the switch to close the container lid 19.

[0066] Preferably, the base 1 is a vortex oscillation base, which is a device for mixing;

[0067] The precipitator 2 is a device for precipitating proteins and fats.

[0068] The volume balancer 3: a device for balancing volume;

[0069] The fixed container 4 has a fixed volume device with a built-in graduated stop plate;

[0070] The piston 5 is used to discharge the precipitate from the precipitator;

[0071] The precipitate discharge pipe 6 is used to release precipitate.

[0072] The precipitator discharge pipe 7 is used for releasing the test liquid;

[0073] The filler 8: a substance used to balance the volume of the precipitate;

[0074] The elastic membrane 9: a pressure transmitter;

[0075] The graduated retaining plate 10: prevents overfilling.

[0076] The fixed container drain pipe 11 with filter membrane: releases the filtered supernatant.

[0077] Example 3

[0078] Methods for determining chloride ions in foods for special medical use

[0079] 1. Reagents and materials

[0080] Reagents: Silver nitrate (AgNO3), nitric acid (HNO3), standards;

[0081] Standard sodium chloride (NaCl) with a purity ≥99.8%. Or a chlorine standard solution that has been nationally certified and granted a standard substance certificate.

[0082] Reagent preparation

[0083] (1) Nitric acid solution (1+2): Measure 100 mL of concentrated nitric acid, add 200 mL of deionized water, and mix well;

[0084] (2)(2) Silver nitrate solution (15g / L): Weigh 1.75g ​​of silver nitrate and dissolve it in 100mL of water. Store the solution in a sealed brown bottle.

[0085] Preparation of standard solutions

[0086] (1) Chlorine standard stock solution (1000 μg / mL).

[0087] (2) Chlorine standard intermediate solution (10.0 μg / mL): Accurately transfer 2.00 mL of chlorine standard stock solution (1000 μg / mL) to a 200 mL brown volumetric flask, add water to make up to volume, and mix well.

[0088] (3) Chlorine standard working solution: Accurately transfer 0.80 mL, 1.60 mL, 2.40 mL, 3.20 mL, 8.00 mL, and 10.00 mL of fluorine standard intermediate solution (10.0 μg / mL) into 25 mL volumetric flasks, then add 2 mL of nitric acid (1+2) to each flask, add water to the mark, mix well, and let stand in the dark at a constant temperature of 35 °C for 15 min. Prepare fresh before use.

[0089] Instruments and equipment

[0090] (1) Ultraviolet-visible spectrophotometer.

[0091] (2) Balance: sensitivity is 1 mg.

[0092] (3) Constant temperature water bath.

[0093] 2. Measurement Method

[0094] (1) Sampling: Weigh 1.0g of the sample into a graduated plastic tube.

[0095] (2) Development of the standard curve: Based on the optimized standard working curve, the standard working solutions were measured using a spectrophotometer from low to high concentrations. The standard curve was plotted with absorbance as the ordinate and chloride ion concentration as the abscissa.

[0096] (3) Sample pretreatment: Weigh the sample into the special medical purpose formula food pretreatment equipment, add 3mL of water and 2mL of nitric acid solution, mix thoroughly, and soak and extract in a sealed container for 15min. Drain the test solution from the precipitator outlet tube 7 into the fixed container 4, add a fixed volume solution to the precipitator 4, wash the precipitate 7-11 times, and drain the washing solution from the precipitator outlet tube 7 into the fixed container 4. After the precipitate is discharged through the precipitate discharge tube 6, add water to make up to 100mL, mix well, and let stand. After standing, drain the test solution from the fixed container outlet tube 11 with filter membrane. The nitric acid solution in step (3) is made by mixing 1 volume of concentrated nitric acid with 5 volumes of deionized water.

[0097] (4) Determination: Take 5 mL of the test solution and put it into a 100 mL volumetric flask. Add 1 mL of nitric acid solution and 2 mL of silver nitrate (15 g / L). Keep it in the dark at a constant temperature of 35 °C for 15 min. Measure it at 400 nm using a 1 cm cuvette and an ultraviolet spectrophotometer. The nitric acid solution in step (4) is made by mixing 1 volume of concentrated nitric acid with 2 volumes of deionized water.

[0098] 3. Verification of indicators for special medical foods

[0099] Experiments were conducted on the recovery rates of spiked samples with low chlorine content at 1 / 2 limit level and limit level. The results are shown in Table 2.

[0100] Table 2. Results of Confirmation of Methods for Special Medical Foods

[0101]

[0102] As shown in the table, according to GB / T27417-2017 "Guideline for Conformity Assessment and Validation of Chemical Analysis Methods", the recoveries at different concentration levels ranged from 96.06% to 104.71%, meeting the requirements for the method's recovery range; the precision ranged from 0.79% to 3.49%, meeting the precision requirements. This indicates that the method is feasible and applicable to the determination of chloride ions in foods for special medical purposes.

[0103] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions claimed by the present invention.

Claims

1. A method for determining chloride ions in a special medical food, characterized in that, Includes the following steps: (1) Sampling: Weigh 1.0g of the sample into a graduated plastic tube; (2) Formulation of standard curve: The standard working curve is optimized. Based on the optimized standard working curve, the standard working solution is measured by spectrophotometer from low concentration to high concentration. The standard curve is plotted with absorbance as the vertical axis and chloride ion concentration as the horizontal axis. (3) Sample pretreatment: Weigh the sample into the special medical purpose formula food pretreatment equipment, add water and nitric acid solution, mix thoroughly, soak and extract in a sealed container, separate the solid and liquid to obtain the precipitate, make up the volume of the separated liquid, and let it stand to obtain the test solution. (4) Measurement: Take a certain amount of the test solution into a 100mL volumetric flask, add nitric acid solution and silver nitrate, let it stand, and then measure it at 400nm using an ultraviolet spectrophotometer. The pretreatment equipment includes a base, a volume balancer on the top of the base, a sedimentator and a fixed container on the top of the volume balancer, the bottom of the sedimentator being connected to the volume balancer via a sediment discharge pipe, an elastic membrane on the top surface of the volume balancer, the fixed container being connected to the volume balancer via the elastic membrane, the bottom of the fixed container being an elastic bottom that contacts the elastic membrane, and the size of the elastic bottom of the fixed container matching that of the elastic membrane, a piston adapted to the structure of the sedimentator being provided on the top of the sedimentator, the sedimentator being connected to the fixed container via a sedimentator discharge pipe, the volume balancer being filled with a filler that has a variable shape but a constant volume, a filter device being provided on the sedimentator discharge pipe, and a sedimentator discharge valve being provided on the sedimentator discharge pipe; The sample pretreatment in step (3) is as follows: the precipitate is discharged through the precipitate discharge tube. When the precipitate enters the volume balancer, due to the pressure, the elastic membrane will bulge at the bottom of the fixed container to balance the volume change. Finally, the size of the bulge is consistent with the size of the precipitate. Finally, water is added to make up the volume. When the volume of water, the test liquid discharged from the precipitator discharge tube and the bulge volume reach 100mL, mix well and let stand. After standing, the test liquid is discharged from the fixed container with filter membrane discharge tube. The nitric acid solution in step (3) is prepared by mixing 1 volume of concentrated nitric acid with 5 volumes of deionized water.

2. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The filler is water or bubble wrap.

3. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The inner wall of the volume balancer is provided with an elastic inner membrane, the filler is filled in the elastic inner membrane, and the precipitate discharge pipe is not connected to the elastic inner membrane.

4. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The sum of the volume change of the volume balancer and the internal volume of the fixed container is 100 ml.

5. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The ratio of the volume change of the volume balancer to the internal volume of the fixed container is 1-6:94-99.

6. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The container is equipped with graduated damping plates.

7. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The fixed container is equipped with a fixed container drain pipe, and a filter membrane is installed on the fixed container drain pipe. The precipitator is wider at the top and narrower at the bottom. A pressure pipe is installed on the precipitate discharge pipe, and a pressure device is installed at the other end of the pressure pipe. A pressure valve is installed on the pressure pipe, and a filter screen is installed at the connection between the pressure pipe and the precipitate discharge pipe. The precipitator discharge pipe is located above the pressure pipe. An exhaust pipe is installed on the precipitate discharge pipe, and an exhaust valve is installed on the exhaust pipe. The fixed container is equipped with a fixed container cover that matches the fixed container. The fixed container cover is equipped with a controller and a switch. The fixed container wall is equipped with graduations. A liquid level sensor is installed inside the fixed container. The liquid level sensor is electrically connected to the controller. The controller is electrically connected to the switch. An alarm is installed on the outer wall of the fixed container, and the alarm is connected to the controller.

8. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The slope of the optimized standard working curve is 0.0546, and the correlation coefficient R0 is [missing value]. 2 =0.9992; The precipitator is equipped with two precipitator discharge pipes, denoted as precipitator discharge pipe one and precipitator discharge pipe two. The precipitator discharge pipe one is located at 1 / 2 of the precipitator volume, and the precipitator discharge pipe two is located at 3 / 10 of the precipitator volume.

9. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, Step (3) Sample pretreatment: Weigh the sample into the special medical purpose formula food pretreatment equipment, add 3mL of water and 2mL of nitric acid solution, mix thoroughly, and soak and extract in a sealed container for 15min. Drain the test liquid from the precipitator discharge tube into the fixed container, add a fixed volume solution to the precipitator, wash the precipitate 7-11 times, and drain the washing liquid into the fixed container through the precipitator discharge tube. After the precipitate is discharged through the precipitate discharge tube, add water to make up to 100mL, mix well, and let stand. After standing, drain the test liquid from the fixed container with filter membrane through the discharge tube.

10. The method for determining chloride ions in a special medical food as described in claim 1, characterized in that, The determination in step (4) is as follows: Take 5 mL of the test solution and put it into a 100 mL volumetric flask. Add 1 mL of nitric acid solution and 2 mL of 15 g / L silver nitrate solution. Let it stand in the dark at a constant temperature of 35 °C for 15 min. Measure it at 400 nm using a 1 cm cuvette and an ultraviolet spectrophotometer. The nitric acid solution in step (4) is made by mixing 1 volume of concentrated nitric acid with 2 volumes of deionized water.

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