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Several laboratory sample pre-treatment methods

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Several laboratory sample pre-treatment methods

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Common Disgestion Methods

A) wet dissipation method

1. Nitric acid digestion method (for clearer aqueous samples)

2. Nitric acid-perchloric acid digestion method (digestion of samples containing difficult to oxidize organic substances)

3. nitric acid – sulfuric acid digestion method (nitric acid: sulfuric acid = 5: 2, often adding a small amount of hydrogen peroxide)

4. sulfuric acid – phosphoric acid digestion method (conducive to the determination of the elimination of Fe3+ plasma interference)

5. Sulfuric acid – potassium permanganate decomposition method (commonly used in the determination of mercury samples in aqueous solution)


6. Nitric acid – hydrogen peroxide ablation method: some people use this method to ablate biological products to determine nitrogen, phosphorus, potassium, boron, arsenic, fluorine and other elements

7. Multiple digestion method: requires the use of more than three acids or oxidants digestion system.



B) dry ashing method (pyrolysis method)

1. The ashing method does not use or use a small amount of chemical reagents to decompose the sample, and can handle larger samples, so it is beneficial to improve the accuracy of the determination of trace elements;

2. The ashing temperature is generally 450-550°C, which is not suitable for processing samples for the determination of volatile components, and the ashing time is also longer;

3. According to the different types of samples and the properties of the components to be tested, choose crucibles of different materials and ashing temperatures. Commonly used crucibles are quartz, platinum, silver, nickel, iron, porcelain, polytetrafluoroethylene and other properties. The principle is that the crucible does not react with the sample and is stable at the processing temperature;

4. Usually no other reagents are added to ashing biological samples, but in order to promote decomposition and inhibit the volatilization loss of certain elements, an appropriate amount of auxiliary ashing agent is often added. After the sample is completely ashed, it is dissolved in dilute nitric acid or hydrochloric acid for analysis and determination.


Extraction and enrichment

A) Extraction method
1. Vibration extraction method (vegetables, fruits, grains)
2. Tissue crushing and extraction (extracting organic pollutants from animal and plant tissues)
3. Soxhlet extraction (commonly used to extract pesticides, petroleum, phenylhydrazine pyrene and other organic pollutants in biological and soil samples)


B) Volatilization and evaporation concentration
The volatile separation method is to use some components with high volatility or convert the components to be measured into volatile substances, and then use inert gas to take them out to achieve the purpose of separation. Evaporation and concentration refers to heating the water sample on an electric heating plate or in a water bath to slowly evaporate the water, so as to reduce the volume of the water sample and concentrate the components to be measured.


C) Distillation
The components of water samples have different boiling points to separate them from each other; when measuring volatile phenols, cyanides, and fluorides in water samples, pre-distillation and separation in acidic media are required; distillation has the functions of digestion, enrichment and separation Three functions.


D) Ion exchange method
The use of ion exchange agent and the ions in solution exchange reaction for separation. Ion exchange agent can be divided into inorganic ion exchange agent and organic ion exchange agent (ion exchange resin);

A phenomenon in which an insoluble compound in solution is carried out together with certain trace components that coexist in the process of forming a precipitation. The principle of coprecipitation is based on surface adsorption, formation of mixed crystals, interaction of heteronuclear colloidal substances and encapsulation.

 

E) co-precipitation method

  1. Co-precipitation separation using adsorption:
    Commonly used to contain Fe (OH) 3, Al (OH) 3, Mn (OH) 2 and sulfide, etc.
    2. Separation by co-precipitation using the generation of mixed crystals;
    3. co-precipitation separation using organic co-precipitating agents.



F) adsorption method
The use of porous solid adsorbent will be adsorbed on the surface of one or several components of the water sample, has achieved the purpose of separation. Commonly used adsorbents are activated carbon, alumina, molecular sieve, large mesh resin, etc.. The adsorbed enriched in the adsorbent surface pollution components, available organic solvents or heating desorption out for determination.


G) Chromatography
Chromatography is divided into column chromatography, thin-layer chromatography, paper chromatography, etc. The adsorbent is divided into inorganic adsorbent and organic adsorbent.


H) sulfonation method and saponification method
Sulfonation method:
The use of fat, waxes and other interfering substances in the extract can be sulfonated with concentrated sulfuric acid reaction, generating a very polar sulfonic acid-based compounds, with the separation of sulfuric acid layer, and the purpose of separating the pesticides from the extract. Sulfonation method uses fats and oils, etc. can occur with strong alkali saponification reaction to generate fatty acid salts and will be separated.


I) Cryogenic freezing method
Based on the principle that the solubility of different substances in the same solvent varies with temperature to separate from each other.
Principle: substances in different solvent phases with different partition coefficients, and to achieve the separation and enrichment of components.


J) Extraction method
Types of conventional liquid-liquid extraction
Extraction of organic substances:
Separation of organic substances in the aqueous phase easily extracted by organic solvents;
Extraction of inorganic substances:
First add a reagent, so that it combines with the ionic components in the aqueous phase, generating uncharged, easily soluble in organic solvents, the reagent and the organic phase, the aqueous phase together to form an extraction system. According to the type of extractables generated, it can be divided into chelate extraction system, ionic conjugate extraction system, ternary complex extraction system and synergistic extraction system, etc..


Solid Phase Extraction (SPE)

Device: offline and online SPE

Off-line SPE.
1. SPE and analysis are performed independently, and SPE only provides suitable specimens for later analysis.
2. The solvent flow rate should not be too high in order to make sufficient contact between the sample solution and the filler.
3. It can be done by the automatic instrument. The automatic SPE instrument consists of a column holder, plunger pump, reservoir, tubing and specimen processor.

On-line SPE:
Also known as on-line purification and enrichment technique, mainly used for HLPC analysis.

Column pretreatment

Purpose:
1. to remove possible impurities in the packing material;
2. to solventize the packing material and improve the reproducibility of solid phase extraction;


Addition of samples:
1. the concentration of the sample solvent should not be too high in order to prevent the loss of analytes;
2. when extracting by reversed-phase mechanism, use water or buffer as solvent, of which the amount of organic solvent should not exceed 10% (V/V);
3. To overcome the loss of analytes during sample addition, weak solvents can be used to dilute the specimen, reduce the volume of the specimen, increase the amount of packing in the SPE column and select a sorbent with strong retention of analytes.


SPE method establishment:
Elution and collection of analytes (another case where impurities are retained while analytes pass through the column)

1. for reversed-phase extraction columns, the cleaning solvent is water or buffer containing an appropriate concentration of organic solvent;
2. To determine the best cleaning solvent concentration and volume, add the sample on the SPE column, with 5 to 10 times the volume of the SPE column bed solvent cleaning, sequentially collect and analyze the effluent to obtain the elution profile of the cleaning solvent on the analytes. Increasing the intensity of the cleaning solvent in turn, according to the elution profile of the analytes under different different strengths, to determine the appropriate strength and volume of cleaning solvent;
3. elution and collection purpose: the analyte will be completely eluted and collected in the smallest volume of the grade fraction, while making stronger than the analyte retention of impurities as much as possible on the SPE column;
4. To increase the analyte concentration or to adjust the solvent properties for later analysis, the collected analyte cascade can be blown dry with nitrogen and then dissolved in a small volume of solvent.


Applications of SPE:
Environmental analysis, pharmaceutical analysis, clinical analysis, food and beverage analysis (four fields)

1. Environmental samples such as surface water have very low analyte concentrations and must be enriched for analytes prior to analysis.
2. The composition of biological fluids is complex and contains a large amount of proteins, so it is necessary to pretreat the specimen to remove proteins before analysis.


Solid phase extraction theory
Equilibrium theory:
The adsorption equilibrium is established between the solid-liquid or solid-gas phase during the adsorption process.
(In a certain time, the equilibrium is not completely reached due to the slow mass transfer process.)
The selectivity of the extraction depends mainly on the properties of the coating material. A suitable SPE coating is selected according to the principle that the analyte is easily extracted by a solid phase similar to its polarity.


Polydimethylsiloxane-divinylbenzene (PDMS-DVB) for aromatics and volatile compounds.

Coating Materials
The most commonly used substances for solid-phase coatings are polymethylsiloxane (PDMS) and polyacrylate (PA), both of which can be used in gas and liquid chromatography. The former is mostly used for non-polar compounds such as volatile compounds, polycyclic aromatic hydrocarbons and aromatic hydrocarbons, while the latter is mostly applied to polar compounds such as triazines and phenolics. The solid-phase layer can be coated on quartz fibers in non-bonded, bonded or partially cross-linked form. The addition of some polymers to the coating can increase the surface area of the coating and improve the efficiency of SPME.


1. polydimethylsiloxane-divinylbenzene (PDMS-DVB) for aromatics and volatile compounds;
2. polyethylene glycol-divinylbenzene (CW-DVB) for polar compounds such as alcohols;
3. polyethylene glycol-template resin (CW-TPR) for ionized surfactants;
4. quartz fibers coated with graphitic carbon black for the analysis of trace contaminants in water and air;
5. carbon nanotubes and titanium dioxide nanotubes.


Establishment of the method
1. Maintain the consistency of sampling conditions.
2. The factors affecting sampling are sampling time, temperature, fiber depth, etc.
3. Maintain a linear relationship between the response value and the initial concentration of the analyte, the concentration of the specimen should not be too high and the volume of the specimen should not be too small, so that the extraction is within the linear range of the sorption isotherm.
4. Adding electrolytes to the specimen can increase the ionic strength of the solution, thus reducing the solubility of the analytes and improving the extraction efficiency; changing the pH of the specimen has a greater effect on the extraction rate of acid and alkaline substances.
Note: The role of salt addition in microextraction is sometimes different from that of conventional liquid-liquid extraction and requires optimization of experimental conditions.
5. Stirring can shorten the extraction time.


Microwave extraction

Short extraction time, good selectivity, high recovery, low reagent dosage, low pollution, water can be used as extractant, automatic control of sample preparation conditions; less application objects, currently applied to soil, sediment in polycyclic aromatic hydrocarbons, pesticide residues, organometallic compounds, active ingredients in plants, harmful substances, extraction of metals in minerals, drugs in blood and extraction of pesticide residues in biological samples research.


High efficiency
1. the direct action of microwaves with the separated substances;
2. microwave extraction using polar solvents is more beneficial than using non-polar solvents;
3. The application of closed containers so that microwave extraction can be carried out at a much higher temperature than the boiling point of the solvent, significantly improving the efficiency of microwave extraction


Microwave extraction equipment and its method (the main components are specially manufactured microwave heating devices, extraction vessels and pressure and temperature control devices equipped according to different requirements)

Multi-cavity 2450MHz:

Multiple samples can be prepared at one time, the extraction conditions are easy to control, and the extraction is fast.


Single-mode focused 2450MHz:

There is no need to control pressure and temperature, the sample preparation volume is large, only one sample can be prepared at a time, and the extraction time is longer.


Conventional microwave extraction method:

The polar solvent or a mixture of polar solvent and non-polar solvent is mixed with the extracted sample, loaded into the microwave sample preparation vessel, and heated in the microwave sample preparation system under the airtight condition. According to the requirements of the extracted components, control the extraction pressure or temperature and time; at the end of heating, the sample is filtered and the filtrate is measured directly, or after corresponding treatment. In general, the heating time of microwave extraction is about 5-10 min. the total volume of extraction solvent and sample does not exceed 1/3 of the volume of the sample making cup.


Supercritical fluid extraction


Supercritical Fluid (SCF)


A fluid whose temperature and pressure are higher than the critical point has the following characteristics:

1. whose diffusion coefficient is smaller than that of a gas, but an order of magnitude higher than that of a liquid;
2. its viscosity is close to that of a gas
3. its density is similar to that of a liquid, and small changes in pressure can lead to significant changes in its density
4. changes in pressure or temperature can lead to phase changes.


Fundamental

In the supercritical state, the supercritical fluid is put in contact with the material to be separated, so that it selectively extracts the components of polarity size, boiling point height and relative molecular mass size in turn, and the density and dielectric constant of the supercritical fluid increase with the increase of the pressure of the confined system, and the polarity increases, and the components of different polarities can be extracted in steps by using the programmed pressure boost.


The dissolving power of supercritical CO2

1. Lipophilic and low boiling point components can be extracted at low pressure (104kPa);
2. The more polar groups the compound has, the harder it is to extract;
3. The higher the relative molecular mass of the compound, the harder it is to extract.


Modifier

CO2 is a non-polar solvent, and generally polar solvents have to be added to improve its solubility in CO2, so it is called a modifier. The more commonly used ones are methanol, acetone, ethanol, ethyl acetate, etc.

Modifiers have a limited role in changing the solubility of supercritical fluids, while also weakening the capture of the extraction system, leading to an increase in co-extracts, which is likely to interfere with the analytical determination. The amount of modifier should be small, generally do not exceed 5%.

Application of Supercritical Fluid Extraction Technology

It has great advantages in the extraction of natural substances, and can be used in conjunction with GC, IR, MS, LC, etc. to become an efficient analysis method.


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