Collection, preparation, pretreatment and storage of test samples

Collection of examples
Sampling: extract a representative portion of samples from a large number of analyte species. As the material is analyzed, this process is called sampling .

1. Importance of correct sampling:
Sampling is an important part of food analysis. For the same type of food products or raw materials, due to the different origins, ripening periods, processing and storage conditions of the species, their ingredients and contents may also vary widely. For the same analyte, the composition and content of different components can also vary widely. From a large, uneven composition, the species of the tested substances with inconsistent components should be collected to represent the analytical samples of all tested substances. Scientific sampling techniques should be are used to prevent components from escaping and contamination, and representative samples should be collected in a balanced manner, otherwise the test results will be worthless, even if the post-testing and other links are very accurate and accurate.=\"box-sizing: border-box; color: rgb(51, 51, 51); font-family: \"Helvetica Neue\", Helvetica, Tahoma, Arial, sans-serif; font-size: 16px; - space: normal ; background-color: rgb(255, 255, 255);\"/>
2. Principles of correct sampling:
①The collected samples should be uniform and representative, which can reflect the composition, quality and sanitary status of all inspected foods.

② During the sampling process, try to maintain the original physical and chemical indicators to avoid the escape of components or the ingress of impurities.

③Sampling Steps

Sampling→Original sample→Average sample

Sampling: A small amount of material collected from different parts of the bulk material to be analyzed.

Original example: Many examples are merged and called the original example.

Average sample: The original sample is technically edited and then a portion of it is taken for analysis and inspection.

3. Sampling method:
① randomly selectedSampling: Randomly selects a few samples from a large number of materials.

② representative samples: use the system The sampling method is used to sample, that is, to know the law that the sample changes with space and time, and to sampling according to this law, so that the collected sample can represent the composition and quality of the corresponding component.For example: stratified sampling, sampling along with each link of the production process, regular sampling of food displayed on the shelves at different times, etc.

Random sampling can prevent artificial tendency, but in some cases, such as difficult to mix food (such as viscous liquid, vegetable) sampling, it is not enough to just random sampling to be used, it must be combined with representative sampling, and samples are taken from representative parts. Therefore, sampling is usually a combination of random sampling and representative sampling. The specific sampling method depends on the nature of the object of analysis.

Smooth solid material (such as grain, powdered food)

a. With full packaging (bags, drums, boxes, etc.)

Determine the number of samples → sample from different parts of the stack.

b. Unpacked bulk samples are averaged using the quartering method

thick semi-solid material (such as cream, animal fat, jam, etc.)

Mix well→take out test sample→shrink to required sample Number of averaged samples.

Liquid materials (such as vegetable oil, fresh milk, etc.)

Mix well → tiered sampling (siphon) → reduce to required number of samples Average sample composition of non-uniform solid foods (meat, fish, fruits, vegetables, etc.).

Small packaged foods (canned, bagged or canned milk powder, bottled beverages, etc.)

This kind of food is usually accompanied by shift or lot number Packed together for sampling.

4. Number of examples:
Samples should be used in triplicate for inspection, re-inspection and backup. The quantity of each sample is generally not less than 0.5 kg.

5. Sampling Precautions:
① All sampling tools are clean and no substances are allowed to influence the measurement results. Samples for microbiological examination must be carried out strictly according to aseptic procedures. Try the original microbial status and physical and chemical indicators.

②Samples with extremely unsystematic sensory properties should not bemixed together and packed separately and their properties noted.

③ After the sample is collected, it should be quickly sent to the test room for analysis to avoid changes. Labels should be firmly attached to the utensils containing samples, indicating sample name, sampling location, sampling date, sample lot number, sampling method, sampling quantity, analysis items and sampling personnel, packaging conditions, hygiene conditionslocal conditions, transport and storage conditions and appearance.

Sample Preparation

1. Sample prep:

①Sample preparation: Separation, pulverization, mixing and shrinking of the collected samples etc. process.

② Purpose: to ensure that the sample is highly uniform.

Note: Prevent the escape of volatile components and avoid changes in sample composition and physical and chemical properties.

Samples for microbiological testing should be based on microbiological according to the requirements of aseptic surgical procedures.

③Method: (varies by product type).

Liquid, slurry or suspension liquid: generally the sample Shake and stir well.

Immiscible liquids: make incompatible Separate the dissolved components, then taste them separately.

Solid samples : cut, pulverise, mash, grind and other methods to make the sample uniform.

canned food: canned fruit before pureeing fruit core must be removed; canned meat and poultry must be removed beforehand.

bones: canned fish with herbs Separated and mashed .

Preprocessing samples
General principles:

1. Eliminate distractions.

2. Keep measured components intact.

3. Concentrate the measured components to obtain reliable analysis results.

(1) Organic Destruction Method:

Application: Mainly used for the determination of inorganic elements in food.
Organic matter: carbohydrates; composed of C.H.O.N, halogen, sulfur, phosphorus.

Properties: flammable; pyrolysis; weak polarity; many intermolecular reactions and side reactions.

Inorganic: non-carbohydrate.

Principle: use high temperature or high temperature to strengthen the oxidation conditions so that organics decompose and escape in gaseous state, while the measured comonponents remain.

Depending on the specific operating conditions, it can be divided into two categories: dry method and wet method.

1. Dryverassen

(1) Method of action: carbonization (can be added to fixative e.g.: alkaline or acidic substances) → ashing until the residue is white or light gray (ashing oven 550°C).

(2) Functions:

①The empty value is low ;

② can enrich the measured components, lower detection limit;

③ The organic matter is completely decomposed and the workers have to monitor it regularly ；

④It\'s taking a long time
⑤Evanescent elements are easy to lose;

⑥Measurement results and recovery rate are low.

(3) Measures to improve the recovery rate:

According to the properties of the measured components, take the appropriate ashing temperature. Add an ashing agent to prevent volatilization loss and crucible absorption of measured components. For example, by magnesium chloride or magnesiumnitrate, phosphorus and sulfur can be converted into magnesium phosphate and sulfur magnesium. Prevent their loss; add hydrogen. Sodium oxide or calcium hydroxide can convert the halogen into non-volatile sodium iodide or calcium fluoride; adding magnesium chloride and magnesium nitrate can convert arsenic into non-volatile magnesium pyrethate; adding sulfuric acid can make some volatile Lead chloride, chlorine/cadmium chloride, etc. are converted to hard volatile sulfates .

Newly developed low-temperature ashing technology in recent years, this methodthe to put the sample into a low temperature ashing furnace, first pump the air to 0-133.3 Pa, then inject oxygen continuously, 0.3-0.8 O2 every minute, the oxygen is activated by radio frequency irradiation, and the sample can be completely incinerated at a temperature of <150℃.

New sample digestion technology: high pressure digestion method in sealed tank. This method is to add the correct amount of sample and oxidant to a polytetrafluorene container and place it in a sealed tank for 120 hours.places. -150 ℃ oven for several hours and then cooled to room temperature.

2. Wet Digestion

(1) Control method: sample → oxidizerdel amplify → heat Cook .

(2) Commonly used strong oxidizing agents: concentrated nitric acid, concentrated sulfuric acid, perchloric acid, potassium peroxide, hydrogen peroxide.

(3) Functions:

①Decomposition is fast and time is short.

②Reduce volatilization loss, container occlusion little.

③ often produce a lot of noxious gas.

④In the initial stages of digestion, a large amount of foam is likely to overflow, so the operator should take care of it at any time.

⑤The amount of reagent is large and the blank value is large.
(4) Just Wet Digestion Method:

① nitric acid-perchloric acid-sulfuric acid method

Weigh 5~10 g of the crushed sample in 250~500 in a Kjeldahl flask, moisten it with a little water, add a few glass beads, add 10-15 ml of a 3:1 nitric acid-perchloric acid mixturein, leave it for a while, heat it slowly over a low heat and let it cool down after the reaction is stable Add 5~10ml of concentrated sulfuric acid to the wall of the bottle, keep heating until the liquid in the bottle starts to turn brown , then add the nitric acid-perchloric acid mixture (3:1) drop by drop until the organic matter is completely broken down. Increase the firepower to white smoke, the solution should be clear, colorless or slightly yellow. Pay attention to explosion-proof during use. After cooling, transfer to a volumetric flask to make up the volume.

② salpeteric acid-sulphuric acid method
Put the squashed sample 250~500ml Add 20ml concentrated nitric acid to a Kjeldahl bottle (the sample size can be weighed as 10~20g), mix gently, first melt the sample over low heat, then add 10ml concentrated sulfuric acid, gradually increase the heat, keep a slightly boiling state, and concentrated nitric acid was added dropwise until the solution became transparent and no longer turned black. When the solution darkens, immediately add nitric acid, otherwise it will become incompleteearth. After the solution stops turning black, continue to heat for several minutes until thick white smoke comes out and the digestive juice should be clear and transparent at this point. After the digestion solution has cooled, carefully dilute it with water, transfer it to a volumetric flask and at the same time wash the Kjeldahl flask with water, transfer the washing solution to the volumetric flask, make up to the mark and mix well to To test.

③Comparison of dry and wet methods

(2) Solvent extraction method:

Principle: Use the difference in solubility of each component of the sample in a given solvent to partially or completely separate the components.

Applies to: Determination of vitamins, heavy metals, pesticides and aflatoxins.

Category: Solvent stratification method, soaking method, salting out method.

(1) Solvent stratification method (solvent extraction method)

1. Principle: Using the difference in the partition coefficient of a given component in two immiscible solventsparts, it is transferred from one solvent to another to separate it from other components.

Partition coefficient: at a certain temperature component B dissolves into immiscible In the case of two liquid phases , when the two phases reach equilibrium, this component will be divided into the two phases in a certain ratio.

KB=content of component B in E/content of component B in R

2. Extractor selection:

not miscible with original solvent;

for the measured components Has the greatest solubility and the least solubility for impurities;

The degree of difficulty and whether the two solvents are separated should be considered There will be problems such as bubbles;

3. Instruments: separatory funnel, continuous liquid aspiration.

4. Extraction method:
Separation funnel: generally 4-5 Full separation can in principle only be achieved after one extraction.

Continuous liquid extractor: inside flask A The solvent is heated and the generated steam rises to the condenser C through tube B and is cooled. After condensed and liquefied be, it falls into the central tube and descends down the central tube, and becomes small droplets from the lower end, making the extracted liquid layer D. At this time, extraction. After the extract is refluxed into flask A, the solvent becomes reevaporated so that the repeated extraction can extract all tested components in flask A.

(2) soaking method (leaching method)

1. Principle: use the difference in solubility of the solid mixture in the extractant to extract objects are separated.

2. Extractor option :

① selected extraction The agent can dissolve a large amount of extracts without destroying the properties of the extracts, for example, the selection of sugar extracts ethanol in water.

② According to the polarity of the extract force to choose the extraction medium. For weak polar components (such as organochlorine pesticides) can be extracted with low polar solvents (such as petroleum / petroleum ether); for strong memory components (such as aflatoxin) can be extracted with highly polar solvents (such as methanol and water mixture) extraction

The boiling pointof the solvent should be between 45-80 Between ℃, the boiling point is too low to be volatile, and the boiling point is too high to be difficult to concentrate, and it is unfavorable to the extracted components with poor thermal stability.

The solvent must be stable and not interact with the sample.

3. Instrument: common Soxter-type extractor.

(3) Salting out method:

Salting out: I want to add a certain substance to the solution to make the solute dissolve in the original Solvent solubility is greatly reduced, resulting in precipitation from solution .

Note: ① The added substance does not destroy the precipitated substance .

②Select the appropriate separation method (such as filtration, centrifugation, evaporation, etc., which should be determined according to the properties of the solution, solvent, precipitated substance and the experimental requirements).
③ Pay attention to the requirements of pH value, temperature and other conditions.

(4) Sulphonation and Saponification:

This is a method commonly used when handling oil or fat containing samples and is often used for sample purification in pesticide analysis.

(1) Sulphonation method

Applies to acid-stable organochlorine pesticides Example: DDT, 666.

(2) Saponification

1. Saponification: alkaline hydrolysis of esters.

2. Application: Alkaline stable pesticides.

(5) Precipitation separation method
The precipitation separation method is to use the precipitation reaction to separate methods. Add a suitable precipitating agent to the sample to precipitate the measured component, and separate the precipitate from the mother liquor by filtration or centrifugation to achieve the purpose of separation

Example: When measuring sodium saccharin content in coldbeverages, basic copper sulfate can be added to the reagent to precipitate the interfering impurities such as protein, while the sodium saccharin remains in the test solution. After removing the precipitate by filtration, the filtrate was taken for analysis.

(6) masking method:

In this method, the masking agent interacts with the interfering components in the sample solution to create the interference. The composition is changed to a state that does not interfere with the determination, and it is masked. Using this method, the interference can be eliminated without separating the interfering components. The analysis steps are simplified. It is often used in defining metallic elements.

Exampled: when using the dithizone colorimetric method to determine lead, the determination conditions Under the ambient (PH=9), Cu2+, Cd2+, etc. will interfere with the determination, you can add potassium cyanate and citric acid to mask and eliminate their interference .

(7) Concentration:

Normal pressure concentration method: used for concentration of non- volatile sample purification solution.

Decompression concentration method: K-D concentrator.

(eight) distillation:
Principle: Use the difference in volatility of each component in the liquid mixture to separate.

Features: It has dual effects of separation and purification, and the equipment and operation are more complicated.

Classification: normal pressure, reduced pressure, steam distillation.

(1) Atmospheric Distillation
  is suitable for substances that do not decompose after heating or have a low boiling point.

Heating: Boiling point<90℃, heating with water bath, boiling point>90℃, heating with oil bath, sand bath, salt bath or asbestos bath
condensation: boiling point <150℃ with cold water Boiling point > 150℃ with air condenser

Note: 1. If a heating bath is used, the temperature T of the heating bath should exceed the boiling point T1, and T—T1<30°C.

2. Use short neck distillation flasks for high boiling point substances.

3. Must understand the properties of the distilled substance.

（2）Decompression Distillation

Suitable for substances that break down easily when heated or have a high boiling point

1. Applicable: Substances that are readily degradable at boiling point and have a high boiling point
2. Principle: at a given temperature, The vapor pressure of the two-component mixed liquid is equal to the sum of the vapor pressures of each liquid alone, so the mixedThe boiling point of the liquid is lower than the respective boiling points of the two liquids.In steam distillation, if water vapor passes through a two-component liquid mixture and the component in the mixture is non-volatile, thenl the vapor pressure required for the other component to boil and vaporize will be reduced due to the existence of the partial pressure of water vapor. Therefore, distillation can be performed at lower temperatures. When the material is distilled, the water vapor brings out volatiles, and after condensation, it is divided into an immiscible water phase and a volatile liquid phase.

（4）fractionation

1. Applies: Mixed liquids that are miscible and have similar boiling points.

2. Principle: The liquid mixture is processed multiple times in one device Partial evaporation and partial condensation separates a liquid mixture into its components.

(5) Sweeping co-distillation method

1. Application: vegetables, organochlorine and organophosphorus pesticides in fruits, edible oils and dairy products.

2. Principle: food extract → injected into the Steller tube → converted to steam → nitrogen stream blown into the condenser tube → passed through the microchromatographic column into the collector.

Preserve examples
Prepared samples should be sealed and clean. Store in a container in a dark place. Specimens prone to spoilage should be stored in a refrigerator at 0-5°C, but the storage time should not be too long. The analytical components susceptible to photodecomposition are samples of analytical items and should be stored away from light. Under special circumstances, an appropriate amount of preservatives that do not affect the analysis results can be added to the sample, or the sample can be stored in a freeze dryer for sublimation drying. of food, avoid moisture, air drying and spoilage to ensure that the appearance and composition do not change.

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