What are the sample pretreatment methods?

As the cornerstone of the experimental analysis process, the importance of sample pretreatment is self-evident. It is directly related to the accuracy, reliability and repeatability of analysis results. This process is like laying an accurate channel for the analytical instrument, ensuring that the substance to be measured can be displayed in the "field of view" of the analytical equipment in the most appropriate state. Diverse sample pretreatment methods are like a carefully crafted process, aiming to eliminate or reduce sample matrix interference, enrich target analytes, adjust samples to an appropriate detection range, and protect analytical instruments from potential damage. Some common sample pretreatment methods are introduced below:

1. Digestion Digestion is the conversion of organic or inorganic substances in samples into measurable forms through chemical or physical means. 1. Wet digestion method (1) Nitric acid digestion method (for clearer aqueous solution samples) (2) Nitric acid-perchloric acid digestion method (digestion of samples containing difficult-to-oxidize organic matter) (3) Nitric acid-sulfuric acid digestion method (nitric acid: sulfuric acid =5:2, often add a small amount of hydrogen peroxide) (4) Sulfuric acid-phosphoric acid digestion method (favorable for eliminating F during measurement e3+ plasma interference) (5) Sulfuric acid-potassium permanganate digestion method (commonly used to measure aqueous solution samples of mercury) (6) Nitric acid-hydrogen peroxide digestion method: Some people use this method to digest biological products to determine nitrogen, phosphorus, potassium, Boron, arsenic, fluorine and other elements (7) Multi-element digestion method: A three-element or higher acid or oxidant digestion system is required. 2. Dry ashing method: The sample reacts with oxygen in the air at high temperature. After dehydration, carbonization, decomposition and oxidation, the organic matter is completely decomposed and volatilized, and the residue is dissolved with acid for analysis. 3. Microwave digestion method: The thermal effect of microwave is used to quickly react the sample with acid in a closed container to achieve the purpose of digestion. This method has the advantages of high efficiency, energy saving, time saving and low blank space.

2. Extraction and enrichment Extraction and enrichment are to separate the target components in the sample from the matrix and increase their concentration to facilitate detection. 1. Oscillation extraction method: suitable for the extraction of vegetables, fruits, grain and other samples. 2. Tissue crushing and extraction: Extract organic pollutants from animal and plant tissues. 3. Soxhlet extraction: commonly used to extract organic pollutants such as pesticides, petroleum, and phenylhydrazine pyrene from biological and soil samples. 4. Volatile separation method: The purpose of separation is achieved by utilizing the high volatility of certain components or converting the components to be measured into volatile substances, and then taking them out with inert gas. 5. Evaporation concentration method: refers to heating the water sample on an electric hot plate or in a water bath to slowly evaporate the water, thereby reducing the volume of the water sample and concentrating the components to be measured. 6. Ultrasonic extraction: Based on the physical properties of ultrasonic waves, rapid mechanical vibration waves reduce the force between the target extract and the sample matrix to achieve solid-liquid extraction and separation.

3. Purification Purification is the process of removing impurities from samples that interfere with analysis. 1. Adsorption method: Use porous solid adsorbents to adsorb one or several components in the water sample to the surface to achieve the purpose of separation. Commonly used adsorbents include activated carbon, alumina, molecular sieves, large mesh resin, etc. The pollutant components that are adsorbed and enriched on the surface of the adsorbent can be desorbed using organic solvents or heating for measurement. 2. Chemical method: chemically reacting impurities or substances to be measured to change their solubility, thereby separating them from the original system. 3. Chromatography: Separation is achieved by utilizing the different physical and chemical properties of each component in the mixture and the different moving speeds on the support.

4. Concentration Concentration is the process of reducing the sample volume and increasing the concentration of the analyte. 1. Normal pressure concentration: suitable for components with relatively low volatility and boiling point. 2. Concentration under reduced pressure: reduce the boiling point of the substance by vacuuming, allowing the solvent to evaporate at low temperature. 3. Freeze drying: reduce pressure and vacuum while freezing to sublimate the solvent. 4. Nitrogen purging and concentration: Use inert gas to purge the heated sample liquid to quickly concentrate the sample.

5. Separation Separation is the process of separating different components in a sample. 1. Filtration: intercept suspended solids and other impurities in the sample through the surface or filter layer of the filter medium. 2. Centrifugal separation: Utilize the density and other differences between different substances to separate using a centrifugal force field. 3. Distillation: Separation by utilizing the different boiling points of the components in a mixed liquid or liquid-solid system. 4. Low-temperature freezing method: Separate each other based on the principle that the solubility of different substances in the same solvent varies with temperature.

There are various sample pretreatment methods, and the specific method chosen depends on factors such as the nature of the sample, the purpose of analysis, and laboratory conditions. In actual operations, pre-processing steps should be flexibly selected and optimized according to specific circumstances to ensure the accuracy and reliability of analysis results.