Determination method for chloride ions in oil material

Abstract

The invention relates to a method for determining chloride ions in oil, which comprises the following steps: preparing a chloride ion standard substance working solution with gradient concentration, injecting the standard series solution into an ion chromatograph for determination to obtain a mass chromatogram of the standard solution, and drawing a standard working curve; taking a sample, fully covering the sample with strong base, carbonizing, firing, ashing, dissolving, extracting, centrifuging, purifying, and taking a clear liquid; detecting the clear liquid according to the same instrument parameters to obtain a sample chromatogram, performing qualitative analysis on a target object according to retention time, and calculating the concentration of a to-be-detected object according to the peak area of the chromatogram and a linear regression equation.

Description

Technical Field

[0001] The invention relates to the technical field of oil identification, and in particular to a method for determining chloride ions in oil. Background Art

[0002] Traditional cooking oils provide the human body with rich fatty acids and also give food a better taste, so they are favored by consumers.

[0003] However, the high temperature during the refining process of edible oil will cause the oil to produce harmful substances, such as 3-MCPD. After 3-MCPD enters the human body, it will be hydrolyzed into 3-MCPD (3-MCPD) under the action of intestinal lipase. This is a more toxic substance with genotoxicity, nephrotoxicity and even carcinogenicity. The International Agency for Research on Cancer (IARC) classifies it as a Class 2B carcinogen. In order to deal with the toxic effects of 3-MCPD in vegetable oil on the human body, many countries and regions in the world have evaluated its limits. For example, the European Commission recommends setting the limit requirement for 3-MCPD in some edible oils to 1.25 mg / kg, and the limit requirement in infant food and formula food to 0.75 mg / kg. Similarly, the United Nations Expert Committee on Food Additives (JECFA) has also established a limit standard for 3-MCPD, with a daily allowable intake (TDI) of no more than 2 μg / kg (based on body weight).

[0004] Chloride ions are an active non-metallic element with a wide range of sources, such as hypochlorite used for tap water disinfection. They diffuse into groundwater and are absorbed by oil crops. Studies have shown that the chlorine content in oil is positively correlated with the formation of 3-MCPA esters, and is one of the precursors of 3-MCPA esters in oil processing. Therefore, the detection and control of the chloride ion content in oil is crucial to the formation of 3-MCPA esters.

[0005] At present, the detection method of chloride ions in oil shows incomplete matrix coverage, and it is difficult to meet the detection of chloride ions in different types of oil. Therefore, it is necessary to develop a sensitive, accurate, reliable and applicable method for detecting chloride ions in different types of oil. Summary of the invention

[0006] In view of the above problems, a method for determining chloride ions in oil with simple pretreatment operation, good purification effect and high sensitivity is provided to effectively solve the problems existing in the prior art.

[0007] The specific technical solutions are as follows:

[0008] A method for determining chloride ions in oil, comprising:

[0009] Prepare a chloride ion standard working solution with gradient concentration, inject the standard series solution into an ion chromatograph for measurement, obtain a mass chromatogram of the standard solution, and draw a standard working curve;

[0010] Take a sample, fully cover it with a strong base, dissolve and extract it after carbonization, burning and ashing, centrifuge and purify it, and take the clear liquid;

[0011] The clear liquid was tested according to the same instrument parameters to obtain a sample chromatogram, and the target substance was qualitatively analyzed by retention time. The concentration of the analyte was calculated by the peak area of ​​the chromatogram and the linear regression equation.

[0012] Furthermore, the concentrations of the chloride ion standard working solutions are 0.00 mg / L, 0.10 mg / L, 0.50 mg / L, 1.00 mg / L, 2.00 mg / L, and 5.00 mg / L, respectively.

[0013] Furthermore, a standard curve is drawn with the concentration of the standard solution as the abscissa and the peak area as the ordinate to obtain a linear regression equation.

[0014] Furthermore, the ion chromatography conditions are: chromatographic column Dionex AS19 (250 mm×4 mm, 7.5 μm); mobile phase is NaOH solution; injection volume is 9-10 μL; column temperature is 25-35° C.; flow rate: 0.8-1.2 mL / min; gradient elution.

[0015] Further, gradient elution was as follows: 6 mmol / L NaOH (0-15 min), 40 mmol / L NaOH (15-23 min), 6 mmol / L NaOH (23-40 min), with a total running time of 40 min.

[0016] Furthermore, the strong base is one or more of sodium hydroxide or potassium hydroxide.

[0017] Furthermore, the carbonization burning method is: take a sample, crush it, sieve it, and evenly cover the surface of the sample with a strong base, heat it until the strong base melts, and continue heating until the sample is burned without smoke.

[0018] Furthermore, the ashing method is: cooling the smokeless sample to room temperature, then transferring it into a muffle furnace, ashing it at 450-550° C. for 1.5-2.5 h, and taking it out.

[0019] Furthermore, the purification method is: the supernatant obtained by centrifugation is passed through an H column, a Na column, and an aqueous phase filter membrane in sequence to obtain the supernatant.

[0020] Furthermore, the water phase filter membrane is a 0.22 μm water phase filter membrane.

[0021] The matrix raw materials of oil crops are mainly oily raw materials, including oils, hydrocarbons and higher fatty acids. The matrix raw materials will interfere with the determination of analytes. The present invention adopts a strong base to adsorb chloride ions for the first time to avoid chlorine volatilization; then uses an H column to remove hydroxide in the system to ensure that the pH value of the solution is controlled at a neutral level; then uses a Na column to remove the detached metal and the metal ions in the sample solution to remove the interference of the matrix raw materials on the analytes. Under the above-mentioned specific pretreatment process, the final determination result has the advantage of high accuracy.

[0022] The present invention adopts an anion chromatographic column with high throughput and uses a low-concentration sodium hydroxide solution as a mobile phase, which is beneficial to improving the separation efficiency of chloride ions. High-concentration sodium hydroxide is then used for elution to remove impurities in the sample solution, thereby improving the stability of the chromatographic column.

[0023] The detection limit of the present invention is 0.2 mg / kg, and the quantitative limit is 0.5 mg / kg, which fully meets the requirements for the determination of chloride ions in oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A chromatogram of 1.00 mg / L chloride ion provided in the present invention;

[0025] Figure 2 A chromatogram of chloride ions in a blank solution provided in the present invention;

[0026] Figure 3 A chromatogram of chloride ions in the sample provided in the present invention;

[0027] Figure 4 The chromatogram is of chloride ions after the sample provided in the present invention is spiked. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the 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.

[0029] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0030] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0031] An embodiment of the present invention provides a method for determining chloride ions in oil, comprising:

[0032] Prepare chloride ion standard working solutions with concentrations of 0.00 mg / L, 0.10 mg / L, 0.50 mg / L, 1.00 mg / L, 2.00 mg / L and 5.00 mg / L, respectively, inject the standard series solutions into an ion chromatograph for determination, obtain a mass-chromatogram of the standard solution, plot a standard curve with the concentration of the standard solution as the abscissa and the peak area as the ordinate, and obtain a linear regression equation;

[0033] Weigh 0.5-1.0g (accurate to 0.001g) of crushed and sieved (particle size 100-600μm) oil into a 50mL nickel crucible, add granular sodium hydroxide at a mass ratio of 2:1 (to ensure that sodium ions adsorb chloride ions in the oil to form ionic bonds during the subsequent carbonization process to increase the boiling point of chlorine and avoid chlorine volatilization), evenly cover the surface of the oil, heat on an electric furnace until the sodium hydroxide is molten, wait for the sample to burn until smokeless and then cool it, move the sample into a muffle furnace, incinerate at 500℃ for 2h, take it out, cool it to room temperature, add pure water to dissolve the sample (clean the sample 5 times), and move it into a 50mL container. The sample was placed in a volumetric flask, and finally water was added to make the volume 50 mL. The sample was centrifuged at 8000 r for 5 min, and the supernatant was taken and passed through an H column (to remove the hydroxide in the supernatant and ensure that the pH value of the solution is controlled at neutral), a Na column (to remove the metal shed by the hydrogen column and adsorb the metal ions in the sample solution), and a 0.22 μm aqueous filter membrane. The sample solution was finally transferred to a polypropylene vial, and 10 μL of the filtrate was injected into the ion chromatograph system. The clear solution was tested according to the same instrument parameters to obtain a sample chromatogram. The target substance was qualitatively analyzed by retention time, and the concentration of the analyte was calculated by the peak area of ​​the chromatogram and the linear regression equation.

[0034] The ion chromatography conditions are as follows: chromatographic column Dionex AS19 (250 mm × 4 mm, 7.5 μm); mobile phase is NaOH solution; injection volume is 10 μL; column temperature is 30°C; flow rate is 1.0 mL / min; gradient elution. The gradient elution is as follows: 6 mmol / L NaOH (0-15 min), 40 mmol / L NaOH (15-23 min), 6 mmol / L NaOH (23-40 min), and the total running time is 40 minutes.

[0035] The linear regression equation, linear range, LOD and LOQ of the chloride ion content in the oil measured by the above method are shown in the following table:

[0036]

[0037] The recovery and precision (n=6) of the spiked chloride ion content in oil measured by the above method are shown in the following table:

[0038]

[0039] The present invention adopts the matrix matching external standard method to identify and quantify the analyte of the blank sample spiked at four concentrations (0.5 (LOQ), 1.0, 2.0, 3.0 mg / kg of chloride ion), and the results are all within the linear range of 0.00-5.00 mg / L, and the correlation linear coefficient (r) is 0.99994. The calculation of LOD and LOQ is based on 3 times and 10 times of the signal-to-noise ratio (S / N), respectively. The LOD and LOQ of the target analyte are 0.2 mg / kg and 0.5 mg / kg, respectively. In order to obtain an accurate limit of quantification, the chloride ion added at the limit of quantification concentration is tested six times repeatedly, and the recovery rate of the limit of quantification is 85.1-91.5%, and the RSD is 3.29%-4.57%.

[0040] In order to verify the repeatability of the method, the recovery was calculated by analyzing the spiked chloride ions at three concentration levels (1.5 mg / kg, 3.0 mg / kg and 15.0 mg / kg), and six repeated experiments were performed. The results showed that the recovery of the target was between 88.1-93.5%, and the RSD was between 2.91% and 4.57%. In the present invention, the inter-day and intra-day repeatability were evaluated by adding a blank sample of 1.5 mg / kg, and the inter-day recovery and precision were determined by repeatedly analyzing six oil samples within six days, and the intra-day recovery and precision were determined by analyzing six spiked samples every day. The results showed that the recoveries of the inter-day and intra-day tests were 87.3-92.2% and 88.5-92.6%, respectively, and the inter-day and intra-day analysis precisions were 3.29% and 3.67%, respectively. The above explanation shows that the method provided by the present invention fully meets the monitoring requirements of chloride ions in oil.

[0041] The above description is only a preferred embodiment of the present invention, and does not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the description and illustrations of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for determining chloride ions in oil, characterized in that: include: Prepare a chloride ion standard working solution with gradient concentration, inject the standard series solution into an ion chromatograph for measurement, obtain the mass-chromatogram of the standard solution, and draw a standard working curve; Take a sample, fully cover it with a strong base, dissolve and extract it after carbonization, burning and ashing, centrifuge and purify it, and take the clear liquid; The clear liquid was tested according to the same instrument parameters to obtain a sample chromatogram, and the target substance was qualitatively analyzed by retention time. The concentration of the analyte was calculated by the peak area of ​​the chromatogram and the linear regression equation.

2. The method for determining chloride ions in oil according to claim 1, characterized in that: The concentrations of the chloride ion standard working solutions are 0.00 mg / L, 0.10 mg / L, 0.50 mg / L, 1.00 mg / L, 2.00 mg / L, and 5.00 mg / L, respectively.

3. The method for determining chloride ions in oil according to claim 1, characterized in that: The standard curve is drawn with the concentration of the standard solution as the abscissa and the peak area as the ordinate to obtain the linear regression equation.

4. The method for determining chloride ions in oil according to claim 1, characterized in that: The ion chromatography conditions were as follows: chromatographic column Dionex AS19; mobile phase NaOH solution; injection volume 9-10 μL; column temperature 25-35° C.; flow rate: 0.8-1.2 mL / min; gradient elution.

5. The method for determining chloride ions in oil according to claim 1, characterized in that: Gradient elution was as follows: 6 mmol / L NaOH (0-15 min), 40 mmol / L NaOH (15-23 min), 6 mmol / L NaOH (23-40 min), with a total run time of 40 min.

6. The method for determining chloride ions in oil according to claim 1, characterized in that: The strong base is one or more of sodium hydroxide or potassium hydroxide.

7. The method for determining chloride ions in oil according to claim 1 or 6, characterized in that: The carbonization burning method is: take a sample, crush it, sieve it, and evenly cover the surface of the sample with a strong base, heat it until the strong base melts, and continue heating until the sample is burned without smoke.

8. The method for determining chloride ions in oil according to claim 6, characterized in that: The ashing method is as follows: cool the sample burned to smokeless to room temperature, move it into a muffle furnace, ash it at 450-550°C for 1.5-2.5h, and take it out.

9. The method for determining chloride ions in oil according to claim 1, characterized in that: The purification method is: the supernatant obtained by centrifugation is passed through an H column, a Na column, and an aqueous phase filter membrane in sequence, and then the supernatant is taken.

10. The method for determining chloride ions in oil according to claim 9, characterized in that: The water phase filter membrane is 0.22 μm water phase filter membrane.

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