Method for determining iron tartrate content in edible salt by ion chromatography

Abstract

This invention discloses a method for determining the ferric tartrate content in edible salt, belonging to the field of food analysis and detection technology. The method includes: dissolving the edible salt sample in water, adding excess sodium hydroxide to dissociate the ferric tartrate and release tartrate ions; then purifying the sample by passing it through two tandem Ag columns to remove chloride ion interference; determining the tartrate ions using ion chromatography, quantifying the sample using L-tartaric acid as a standard, and calculating the ferric tartrate content by molecular weight conversion. This method is simple to operate, highly sensitive, and accurate, with a detection limit of 17 mg / kg and a quantitation limit of 50 mg / kg. The recoveries in refined salt (iodized / non-iodized) and low-sodium salt matrices range from 86.1% to 107.9%, with a relative standard deviation of less than 3%. It is suitable for the determination of ferric tartrate in edible salt and its salt substitutes.

Description

Technical Field

[0001] This invention belongs to the field of food analysis and testing technology, specifically relating to a method for determining the ferric tartrate content in edible salt using ion chromatography. Background Technology

[0002] Salt, with sodium chloride as its main component, is one of the most common condiments in people's lives. It plays a role in increasing the saltiness and umami flavor of food and stimulating appetite during cooking. It is also widely used in the food industry to improve the flavor, sensory properties, water retention, and extend the shelf life of food. GB2721-2015, the National Food Safety Standard for Edible Salt, defines edible salt as: salt with sodium chloride as its main component, used for consumption, including table salt and salt substitutes (such as low-sodium salt).

[0003] Ferric tartrate is an important compound with the chemical name D,L-meta-2,3-dihydroxysuccinic acid ferric(III), molecular formula Fe(OH)2C4H4O6Na, and relative molecular mass 260.92. This compound is readily soluble in water, and its solubility is affected by temperature and pH. In the food industry, it can be used as an anti-caking agent in salt and salt substitute products to improve their flowability and prevent clumping during moisture and storage.

[0004] GB1886.381-2024, "National Food Safety Standard for Food Additives: Ferric Tartrate," clearly specifies the quality specifications for ferric tartrate. This standard defines ferric tartrate as a food additive prepared by complexation of L-tartaric acid, sodium hydroxide, and ferric chloride. GB2760-2024, "National Food Safety Standard for the Use of Food Additives," stipulates that ferric tartrate is permitted for use in salt and salt substitutes, with a maximum usage of 0.106 g / kg (based on ferric tartrate content).

[0005] Abroad, (EC) No 1333 / 2008, the EU Food Additives Regulation, stipulates that ferric tartrate is also used as an anti-caking agent in the edible salt industry, and the limit for ferric tartrate in edible salt is 0.106 g / kg (calculated as ferric tartrate content) [4-5]. However, there are currently no detection methods or standards for determining ferric tartrate in edible salt, which to some extent restricts the implementation of relevant food safety supervision work.

[0006] In existing technologies, there are few methods for detecting ferric tartrate, mainly because the high concentration of chloride ions in edible salt can severely interfere with chromatographic analysis systems, and ferric tartrate exhibits poor stability during sample pretreatment. Therefore, there is an urgent need to establish a simple, sensitive, and accurate method for determining the ferric tartrate content in edible salt. Summary of the Invention

[0007] The purpose of this invention is to establish a simple and highly sensitive ion chromatography method for determining the content of ferric tartrate in edible salt, providing technical support for the supervision of imported and exported food safety, filling the gap in domestic detection methods for ferric tartrate in edible salt, and providing a basis for the formulation of relevant detection standards.

[0008] The technical solution adopted in this invention is as follows: A method for determining the ferric tartrate content in edible salt includes the following steps: (1) Sample pretreatment: Dissolve the edible salt sample in water, add excess sodium hydroxide solution to dissociate ferric tartrate and release tartrate ions to obtain the solution to be purified; (2) Purification: The liquid to be purified obtained in step (1) is sequentially purified by passing it through an Ag column to remove chloride ions and collect the effluent; (3) Ion chromatography determination: The effluent obtained in step (2) was measured using an ion chromatograph. The content of tartrate ions was determined using L-tartaric acid as a standard substance. (4) Calculation of results: Based on the content of tartrate ions, the content of ferric tartrate in edible salt is calculated by molecular weight conversion.

[0009] Further, in step (1), the amount of sodium hydroxide solution added is 2 mL of 1 mol / L sodium hydroxide solution per 2.5 g sample.

[0010] Furthermore, the Ag column mentioned in step (2) is an OnGuard II Ag column, and the two Ag columns are used in series. The collected effluent is the subsequent effluent after discarding the first 10 mL.

[0011] Further, the ion chromatography conditions described in step (3) are as follows: the chromatographic column is an anion analysis column IonPac AS19 (4 mm × 250 mm) or an equivalent column, the eluent is potassium hydroxide solution with a concentration of 20 mmol / L, the flow rate is 1.0 mL / min, the injection volume is 25 μL, the column temperature is 30 ℃, and the suppressor current is 50 mA.

[0012] Furthermore, the edible salt includes one or more of refined salt (iodized or non-iodized), low-sodium salt, sun-dried salt, and crushed washing salt.

[0013] The beneficial effects of this invention are as follows: (1) Simple pretreatment: By adding excess sodium hydroxide, ferric tartrate is fully dissociated. The operation is simple and the dissociation efficiency can reach more than 95%.

[0014] (2) Good purification effect: The purification of two Ag columns in series can effectively remove the interference of high concentration of chloride ions in edible salt and protect the ion chromatography system.

[0015] (3) High sensitivity: The method detection limit is 17 mg / kg and the quantitation limit is 50 mg / kg, which meets the limit requirements of GB 2760-2024.

[0016] (4) Good accuracy and precision: the spiked recovery rate is 86.1%~107.9%, and the relative standard deviation is less than 3%.

[0017] (5) High specificity: Common additives in table salt (such as potassium iodate, potassium ferrocyanide, etc.) do not interfere with the determination. Attached Figure Description

[0018] Figure 1-1 , Figure 1-2 , Figure 1-3 The figures show a comparison of the purification effects of the method of the present invention without using an Ag column, using one Ag column, and using two Ag columns in series.

[0019] Figure 2 This is an ion chromatogram of the L-tartaric acid standard solution (1.0 mg / L) used in the method of this invention.

[0020] Figure 3 This is a standard curve of L-tartaric acid in the method of the present invention.

[0021] Figure 4 This is a superimposed chromatogram of refined salt (with added potassium ferrocyanide) and 0.4 mg / L L-tartaric acid standard in the method of this invention.

[0022] Figure 5 This is a superimposed chromatogram of low-sodium salt (without iodine) and 0.4 mg / L L-tartaric acid standard in the method of this invention.

[0023] Figure 6 This is a superimposed chromatogram of refined salt (with added potassium iodate and potassium ferrocyanide) and 0.4 mg / L L-tartaric acid standard in the method of this invention. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0025] Example 1: Selection of Purification Method Because table salt contains a large amount of chloride ions, there is a risk of contaminating the ion chromatography system, requiring effective purification methods to reduce this risk. In this example, an Ag column was selected for purification testing. The results are shown in Figure 1 and Table 1. When using a single Ag column, a large amount of chloride ions still existed in the sample solution; however, when two Ag columns were used in series, the chloride ions in the sample solution could be effectively removed. Elution test results showed that when two Ag columns were used in series, the recovery rate of the last 10 mL of sample solution after eluting 20 mL met the requirements (98.7%~102.0%).

[0026] Table 1 Elution curves of two Ag columns in series

[0027] Example 2 Sample Pretreatment Based on the purification method selected in Example 1, this example provides a sample pretreatment step, the specific steps of which are as follows: Accurately weigh 2.5 g (accurate to 0.001 g) of edible salt sample (refined salt, low-sodium salt, etc.) into a 50 mL centrifuge tube, add 30 mL of water, sonicate for 10 min, add 2 mL of 1 mol / L sodium hydroxide solution, vortex to mix, let stand for 5 min to allow ferric tartrate to fully dissociate, and dilute to 50 mL with water and shake well. Filter an appropriate amount of the solution through a 0.22 μm filter membrane. Pass the filtrate sequentially through two tandem OnGuard II Ag columns (1.0 mL each). Discard the first 10 mL of eluent and collect the subsequent eluent for ion chromatography analysis.

[0028] Example 3 Selection of Ion Chromatography Conditions 3.1 Selection of chromatographic column Based on the chemical properties of tartaric acid and referring to GB 5009.157-2016 "National Food Safety Standard - Determination of Organic Acids in Food" and relevant literature on the detection of organic acids by ion chromatography, this example compares the separation effects of three anion exchange analytical columns: DIONEX IonPac AS19 4 mm × 250 mm (guard column IonPac AG19 4 mm × 50 mm), DIONEX lonPac AS11 4 mm × 250 mm (guard column lonPac AG11 4 mm × 50 mm), and Shenghan DZ-A-1 4 mm × 250 mm (guard column DZ-G-1 4 mm × 50 mm). Results Figure 2 The results show that, under the same elution conditions, all three anion exchange columns exhibit good retention of tartaric acid with symmetrical peak shapes, high sensitivity, and good specificity.

[0029] 3.2 Selection of rinsing fluid conditions and selection of suppressor current The selection of eluent and the optimization of eluent conditions are crucial for ensuring separation efficiency and improving analytical efficiency in ion chromatography. Currently, sodium hydroxide solution and sodium carbonate-sodium bicarbonate buffer solutions are commonly used eluent systems, as both can generate low-conductivity compounds through suppression reactions via a suppressor. This example compares the elution effects of isocratic elution and gradient elution with online sodium hydroxide solution on target ions. The results show that when the sodium hydroxide concentration is 20 mmol / L and the flow rate is 1.0 mL / min, the target compound can be effectively separated from chloride ions in the salt matrix. The suppressor current for ion chromatography elution typically corresponds to the current value recommended for the eluent concentration; 50 mA was selected as the suppressor current value.

[0030] In summary, the ion chromatography conditions were determined as follows: This embodiment uses a Thermo Fisher ICS-5000+ ion chromatograph equipped with a conductivity detector and an automated KOH eluent generator. The chromatographic column was an IonPac AS19 (4 mm × 250 mm) and a guard column was an IonPac AG19 (4 mm × 50 mm). The eluent was potassium hydroxide solution at a concentration of 20 mmol / L, with a flow rate of 1.0 mL / min. The suppressor was an ASRS 300 anion suppressor with a current of 50 mA. The injection volume was 25 μL, and the column temperature was 30 °C.

[0031] Example 4: Standard Curve and Quantitative Analysis Accurately weigh 0.1000 g of L-tartaric acid standard, dissolve it in water, and dilute to 100 mL to prepare a 1000 mg / L standard stock solution. Before use, dilute with water to prepare a series of working standard solutions of 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.5 mg / L. Plot a standard curve with peak area on the ordinate and concentration on the abscissa. The linear equation is y = 0.309x - 0.08, and the correlation coefficient is r = 0.9986.

[0032] The ferric tartrate content in the sample is calculated according to the following formula (1): 1.74……………(1) In the formula: X —The content of ferric tartrate in the sample, in milligrams per kilogram (mg / kg); ρ —The mass concentration of tartaric acid in the sample solution obtained from the standard curve, in milligrams per liter (mg / L); ρ 0—The mass concentration of tartaric acid in the blank sample solution obtained from the standard curve, in milligrams per liter (mg / L); V —Volume at constant volume, in milliliters (mL); m —The mass of the sample, in grams (g); 1,000 – Unit conversion factor.

[0033] 1.74 — Conversion factor from tartaric acid to ferric tartrate.

[0034] The calculation result should be rounded to three significant figures.

[0035] Where: X is the content of ferric tartrate (mg / kg); ρ is the mass concentration of tartaric acid in the sample solution (mg / L); ρ0 is the mass concentration of tartaric acid in the blank sample solution (mg / L); V is the volume of the final volume (mL); m is the mass of the sample (g); 1.74 is the coefficient for converting tartaric acid to ferric tartrate.

[0036] Example 4: Method Validation 4.1 Determination of linear range and detection limit L-tartaric acid exhibits good linearity in the concentration range of 0.1–1.5 mg / L (0.1 mg / L, 0.2 mg / L, 0.4 mg / L, 0.6 mg / L, 0.8 mg / L, 1.0 mg / L, 1.2 mg / L, 1.5 mg / L), with a linear equation of y = 0.309x - 0.08 and a correlation coefficient of 0.9986. (See...) Figure 4 GB 2760-2024, the National Food Safety Standard for the Use of Food Additives, stipulates that the maximum permitted usage of ferric tartrate in salt and salt substitutes (calculated as ferric tartrate content) is 0.106 g / kg. GB5009.295-2023, the National Food Safety Standard for the Validation of Chemical Analysis Methods, stipulates that for target analytes with finite values, the limit of quantitation (LOQ) should be 0.5 times or less of the limit value; the signal-to-noise ratio (SNR) of the analyte at the limit of detection (LOD) level should not be less than 3, and at the limit of quantitation (LOQ) level, the SNR should not be less than 10. Because edible salt contains a large amount of chloride ions, posing a risk of contaminating the ion chromatography system, and considering factors such as instrument differences and column wear in practical applications, to enhance the applicability of the method, the LOD of this method is determined to be 17 mg / kg and the LOQ to be 50 mg / kg (corresponding to tartaric acid concentrations of approximately 10 mg / kg and 30 mg / kg, respectively) when the sample weight is 2.5 g. Spiking tests (with L-tartaric acid added) were conducted using refined salt (iodized), refined salt (non-iodized), and low-sodium salt as the matrix. Six parallel determinations were performed to obtain the signal-to-noise ratio of the target analyte. The results are shown in [Figure number missing]. Figure 3 And Table 2.

[0037] Table 2. Determination of method detection limit and quantitation limit (n=6)

[0038] 4.2 Accuracy and Precision Spiking tests were conducted at three levels (LOQ, 2x LOQ, and 5x LOQ) using refined salt (iodized), refined salt (non-iodized), and low-sodium salt as matrices (corresponding to tartaric acid concentrations of approximately 30, 60, and 150 mg / kg, respectively). Each spiking level was measured in parallel six times, and the recovery rate and precision were calculated. The results are shown in Table 3. The results indicate that the recoveries at the three matrix spiking levels (LOQ, 2x LOQ, and 5x LOQ) ranged from 86.1% to 107.9%, and the relative standard deviations ranged from 0.45% to 2.71%, meeting the technical requirements of GB 5009.295-2023 "General Rules for Validation of Chemical Analysis Methods in National Food Safety Standards".

[0039] Table 3 Recovery rates and precision of different matrices (n=6)

[0040] 4.3 Method Specificity In addition to sodium chloride, the main component of table salt, fortifying agents (such as potassium iodide and potassium iodate) and anti-caking agents (such as potassium ferrocyanide) are added. Three commercially available samples were selected for testing in the laboratory—refined salt (with added potassium ferrocyanide), low-sodium salt (with added potassium ferrocyanide), and refined salt (with added potassium iodate and potassium ferrocyanide). Their spectra were superimposed with the spectra of 0.4 mg / L L-tartaric acid standard (approximately equivalent to 70 mg / kg of ferric tartrate in the table salt sample). No interference with the target analytes was found in any of the samples. The results are shown in [Figure number missing]. Figures 4-6 This indicates that the method has good specificity.

[0041] 4.4 Collaborative Verification In this embodiment, in accordance with the method validation requirements, three technical verification institutions (Chongqing Customs Technical Center, Xi'an Customs Technical Center, and Hefei Customs Technical Center) were selected to conduct collaborative validation. Table 4 shows the validation results, indicating that the linear correlation coefficients of the three laboratories were r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, r14 ...3, r 2 =0.9986、r 2 =0.999 and r 2 =0.9998, the average recovery rate ranged from 94.6% to 105.8%, and the relative standard deviation was from 0.54% to 4.4%. The method was verified to be simple to operate, accurate and reliable, and the correlation coefficient of the standard curve, the method precision and the recovery rate met the requirements of GB 5009.295-2023 standard.

[0042] Table 4. Collaborative laboratory validation data in representative sample matrices (n=6)

[0043] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should be protected by the present invention.

Claims

1. A method for determining the ferric tartrate content in edible salt by ion chromatography, characterized in that, Includes the following steps: (1) Sample pretreatment: Dissolve the edible salt sample in water, add excess sodium hydroxide solution to dissociate ferric tartrate and release tartrate ions to obtain the solution to be purified; (2) Purification: The liquid to be purified obtained in step (1) is sequentially purified by passing it through an Ag column to remove chloride ions and collect the effluent; (3) Ion chromatography determination: The effluent obtained in step (2) was measured using an ion chromatograph. The content of tartrate ions was determined using L-tartaric acid as a standard substance. (4) Calculation of results: Based on the content of tartrate ions, the content of ferric tartrate in edible salt is calculated by molecular weight conversion.

2. The method according to claim 1, characterized in that, The amount of sodium hydroxide solution added in step (1) is 2 mL of 1 mol / L sodium hydroxide solution for every 2.5 g of sample.

3. The method according to claim 1, characterized in that, The Ag column mentioned in step (2) is two Ag columns connected in series.

4. The method according to claim 1, characterized in that, The Ag column mentioned in step (2) is an OnGuard II Ag column.

5. The method according to claim 1, characterized in that, In step (2), the effluent collected is obtained by discarding the first 10 mL of effluent and collecting the subsequent effluent for ion chromatography determination.

6. The method according to claim 1, characterized in that, The ion chromatography conditions described in step (3) are as follows: the chromatographic column is an IonPac AS19 anion analyzer column or an equivalent column, the eluent is potassium hydroxide solution with a concentration of 20 mmol / L, the flow rate is 1.0 mL / min, the injection volume is 25 μL, the column temperature is 30 ℃, and the suppressor current is 50 mA.

7. The method according to claim 1, characterized in that, The edible salt includes one or more of refined salt, low-sodium salt, sun-dried salt, and crushed washing salt.

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