Following the contamination of the natural raw material Botox by Fonterra, another New Zealand dairy company had problems with its products. On August 19th, local time in New Zealand, China’s AQSIQ, New Zealand Ministry of Primary Industries (MPI) and New Zealand dairy company Westland announced that Westland’s 390 kg of lactoferrin detected excessive nitrate levels and all of the problem products were exported to China. .
The nitrite in food mainly refers to sodium nitrite. Nitrite can oxidize normal oxygen-carrying methemoglobin in the blood into methemoglobin, thus losing oxygen carrying capacity and causing tissue hypoxia. There is literature that the semi-lethal dose is 22 mg/kg. According to the requirements of GB 2762-2012 National Food Safety Standards for Contaminants in Foods, nitrite (calculated as NaNO2) in infant-based milk-based foods shall not exceed 2.0 mg/kg. RephiLe introduced the nitrite detection program in milk powder according to the “GB 5009.33-2010 National Food Safety Standard for Determination of Nitrite and Nitrate in Foodâ€.
First, the experimental principle
The milk powder sample is precipitated protein, fat is removed, extracted and purified, and the potassium hydroxide solution is used as a eluent, separated by an anion exchange column, and detected by a conductivity detector. Characterize by retention time and quantify by external standard method.
Second, instruments and reagents
ICS-2000 ion chromatograph (Dionex), AS11-HC anion column (Dionex), PURIST UV ultrapure water system (RephiLe).
RephiQuik Nylon needle filter (RephiLe). All chromatographic reagents are chromatographically pure.
Third, the test method
1. Sample Pretreatment The sample was placed in a covered container capable of accommodating 2 times the sample volume, and the sample was thoroughly mixed by repeatedly shaking and inverting the container until the sample was homogenized.
2. Extract 2.5 g (accurate to 0.01 g) of the sample, place it in a 100 ml volumetric flask, add 80 ml of ultrapure water, shake well, sonicate for 30 min, add 2 ml of 3 % acetic acid solution, and place at 4 °C. After 20 min, take it out to room temperature and dilute to the mark with water. The solution was filtered through a filter paper, and the supernatant was taken for use. Take about 15 ml of the above supernatant, pass the 0.22 μm aqueous filter needle filter, C18 column, discard the first 3 ml (if the chloride ion is greater than 100 mg/L, you need to pass the needle filter, C18 column, Ag column in turn). With the Na column, discard the previous 7 ml) and collect the back eluent for testing.
3. Reference chromatographic conditions
3.1 Column: AS11-HC anion column.
3.2 Eluent: Potassium hydroxide solution, concentration 5 mmol / L ~ 50 mmol / L; elution gradient of 5 mmol / L 33min, 50 mmol / L 5min, 5 mmol / L 5 min; flow rate 1.3 ml / min .
3.3 Suppressor: ASRS 300.
3.4 Detector: Conductivity detector with a detection cell temperature of 30 °C.
3.5 Injection volume: 50 μL.
4, determination
4.1 The standard curve is taken from the nitrite standard solution and diluted with water to prepare a series of standard solutions containing nitrite ions at concentrations of 0.00 mg/L, 0.02 mg/L, 0.04 mg/L, 0.06 mg/L, and 0.08 mg/L. Standard solutions of 0.10 mg/L, 0.15 mg/L, and 0.20 mg/L were injected sequentially from low to high concentrations. A chromatogram of each of the above concentration standard solutions was obtained. Taking the concentration of nitrite ions (mg/L) as the abscissa and the peak height (μS) or peak area as the ordinate, draw a standard curve or calculate a linear regression equation.
4.2 Sample measurement Take 50 μL of blank and sample solution separately, and inject them into the ion chromatograph under the same working conditions, and record the chromatogram. The peak height and the peak height (μS) or peak area of ​​the sample were measured separately according to the retention time.
5. Expression of analytical results The content of nitrite (in terms of NO2-) in the sample is calculated according to formula (1):
In the formula:
X - the content of nitrite ions in the sample, in mg/kg;
C——the concentration of nitrite ions in the sample solution for measurement, in mg/L;
C0——the concentration of nitrite ions in the reagent blank, in mg/L;
V——the volume of the sample solution, in ml;
F——the dilution factor of the sample solution;
M——sample sample amount, in g.
Description: The nitrite ion content measured in the sample is multiplied by a conversion factor of 1.5 to obtain the content of nitrite (calculated as sodium nitrite);
The arithmetic mean of the results of two independent measurements obtained under repetitive conditions indicates that the result retains two significant figures.
6. Precision The absolute difference between two independent determinations obtained under repetitive conditions shall not exceed 10% of the arithmetic mean.
The detection limit of nitrite in this method is 0.2 mg/kg.
In ion chromatography, various contaminants, particulate impurities, ions, organic matter and bacteria in water have a very large impact on the service life and test results of the instrument. First, particulate matter can clog the pump and tubing, causing damage to the equipment or column. Second, the ionic impurities in the water can interfere with normal analysis to a large extent, which increases the difficulty of the experimenter; the presence of organic matter will lower the column. Lifetime and interference with measurement and analysis; while bacteria continuously release ions and small molecules of organic matter, on the other hand, bacteria can also be regarded as particulate matter, causing damage to the instrument.
In order to achieve satisfactory experimental results and ensure the accuracy of the analysis, RephiLe especially recommends that the research staff use the PURIST UV ultrapure water system, which is a product that is especially suitable for providing ultrapure water for instrumental analysis. Easy to use. The PURIST UV ultra-pure water meter uses an ultra-large capacity four-tube purification column and an internal ultra-pure water timed circulation system. The dual-wavelength UV lamp can effectively remove ionic impurities from water and control the organic matter and microbial content in the ultrapure water system. The water production resistivity can reach 18.2 MΩ•cm. The terminal uses a 0.22 μm filter to effectively remove particles and bacteria. At the same time, RephiLe can also provide customers with a comprehensive range of pure water, ultra-pure water use solutions - Direct-Pure EDI + PURIST UV combined pure water treatment system. The combination kit can provide both secondary pure water and ultrapure water to meet the water needs of users in different situations in instrumental analysis. Direct-Pure EDI secondary pure water system, using Siemens's EDI module, the water quality is stable and reliable, and its water production as the water inlet of PURIST UV can maximize the effect of PURIST, ensuring users have no worries about water use. In addition, RephiLe can also provide Direct-Pure UP ultra-pure water machine. The pure water meter uses tap water as the inlet water, and prepares reverse osmosis RO water and high-quality ultra-pure water, one machine and two waters, giving customers more select.
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