Appropriate pretreatment methods can not only ensure the accuracy of your test results, but also reduce the damage caused by the analysis to the instrument. Take ICP-MS as an example, because its tolerance to the matrix is worse than that of atomic absorption and atomic fluorescence, and the sample liquid with high matrix is easy to cause interference, moreover, the loss of the instrument itself is more serious. Next, let me give a brief introduction to some pretreatment methods of inorganic analysis.
1. Dilution method
Many people may say that this is also called pre-processing?
Yes, this is a relatively time-saving pretreatment, but if you say it is easy, you are indeed still in the early stages of inorganic analysis. Some samples do not need complex pretreatment process, such as serum, tissue fluid and other liquid samples, when determining the content of metal elements, water, dilute acid solution, surfactant (such as Triton X-100) or The aqueous solution of organic solvents (such as n-butanol and ethyl acetate) is simply diluted and measured.
2. Acid extraction
The components to be tested are directly extracted from the sample with an acid solution, without completely decomposing and destroying the organic matter, only the components to be tested are quantitatively transferred to the solution, so the amount of reagents used is relatively small, the treatment process is simple, the treatment conditions are mild, and the blank value is low. There is little possibility of loss or contamination of the components to be measured. However, attention should be paid to whether matrix interference and extraction efficiency can meet the analysis requirements.
3. Mineralization method
The method widely used in inorganic analysis is generally called digestion method. It can be divided into dry method and wet method. This method essentially eliminates the organic matter in the sample, hence the name mineralization. Because most samples exist in the form of organic matter, the purpose of digestion is to destroy and decompose the organic components of the sample itself, so that the tested inorganic ions are separated. Wet digestion refers to the use of oxidative strong acid or mixed acid to destroy and decompose organic matter under heating conditions, which is suitable for most samples.
Commonly used acids include nitric acid, hydrochloric acid, perchloric acid and their mixtures. The commonly used mixed acid ratio is nitric acid + perchloric acid = 4+1 or 5+1. Sulfuric acid and hydrofluoric acid are used in special industries. For the determination of arsenic in scallops, sulfuric acid is generally added to increase the digestion temperature to release organic arsenic, otherwise the results will be low if atomic fluorescence measurement is used. If use ICP-MS to detect, try to avoid introducing chlorine and sulfur elements, because polyatomic ion interference will be introduced. The dry digester is much simpler and rougher. The sample is directly carbonized at high temperature, then placed in a muffle furnace at a high temperature of 550°C for ashing, and then the residue is dissolved by careful calculation. After filtration, it can be analyzed on the machine.
4. Pressurized decomposition method
Strictly speaking, this is also a kind of mineralization method. In fact, the sample is placed in a closed container, and the pressure is used to increase the boiling point of the acid to accelerate the destruction of organic matter in the sample.
Compared with wet digestion under normal pressure, the advantage of pressurized decomposition method is that it improves the utilization rate of acid, effectively prevents the volatilization loss of volatile elements, reduces the blank value and reduces the possibility of pollution; the processing temperature Relatively low, generally no more than 150 ℃, saving energy. However, the amount of sample processing is small, usually less than 0.5g.
For the low content of the analyte, in order to achieve the purpose of its detection, it may be necessary to combine the digestion solution to make it easier to be detected.
5. Microwave digestion
Strictly speaking, this is also a kind of mineralization method. The microwave digestion method has a fast speed of processing samples, generally 3min-4min can completely decompose the sample, the consumption of reagents is small, the blank value is low, the digestion vessel is airtight, there is no volatilization loss of materials and reagents, it has little environmental pollution, saves energy, and is easy to automate. And the sample preparation precision is high. The disadvantage is that the same sampling volume is small, generally no more than 0.5g. But now there is a super microwave, which is said to be able to take a sample of 1g or more.
6. Enzymatic decomposition
The method of sample processing by using enzymes to decompose proteins is called enzymatic decomposition, and this type of method is especially suitable for biological samples. Its advantage is that the action conditions are mild, so it can effectively prevent the volatilization loss of the analyte; another feature is that it can maintain the original valence state of the metal ion, so it can be used for morphological analysis; for example, trypsin, papain and Bacillus subtilis The mixed solution of protease acts on the meat to hydrolyze the protein at pH 7.8 and 37°C, and the Cr(VI) and Cr(III) in the sample can be determined.
7. UV radiation digestion
This is a method of digesting samples by ultraviolet radiation. There are automated instruments. For example, when digesting blood samples, take 0.05ml of sample, put it in a quartz tube with a cover, add 0.1ml~0.2ml perchloric acid-hydrogen peroxide or perchloric acid-nitric acid (1+1) mixed acid into each tube, mix well, put it into the machine, turn on the ultraviolet radiation device, until the sample is colorless. This method has the advantages of less sample required, low blank value, automatic operation, etc., and is suitable for the pretreatment of trace metal analysis in blood samples. If you use atomic fluorescence and liquid phase online for speciation analysis of metals, you may be familiar with it, because this digestion step is integrated into the entire analysis process.
After talking about the technical points of ICP-MS pre-processing, let’s talk about why we choose ICP-MS/MS compared to ICP-MS?
1. In ICP-MS, ICP plays the role of ion source. ICP uses high-frequency radio frequency signal with strong power on the inductance coil to form high-temperature plasma inside the coil, and through the promotion of gas, the balance of plasma is ensured. And continuous ionization, the sample to be analyzed is sent into the atomizer by the peristaltic pump to form an aerosol, which is brought into the central area of the plasma torch by the carrier gas, where evaporation, decomposition, excitation and ionization occur. The high-temperature plasma causes most elements in the sample to ionize an electron to form monovalent positive ions;
2. The ions in the plasma are effectively transmitted to the mass spectrometer through the ICP-MS interface;
3. Mass spectrometry is a mass screening and analyzer. It detects the intensity of an ion by selecting ions with different mass-to-nucleus ratios (m/z), and then analyzes and calculates the intensity of an element.
To sum it up is
atomization
Converts atomized atoms mostly into ions
Ions are separated according to their mass-to-charge ratio
Count the number of various ions
As a rapid multi-element analysis technique for the analysis of toxic elements and nutritional elements in food, ICP-MS is increasingly used in daily food analysis, but some food analysis applications require higher sensitivity for specific elements;
Other complex sample matrices may cause spectral interference (such as the determination of low-concentration inorganic As and Se, the determination of Hg, S and Si), but ICP-MS/MS can solve this problem well.
ICP-MS/MS can remove most polyatomic interferences by adding O2 reaction gas. For example, O2 can convert As+ into AsO+, so that the interference of ArCl+ and REE2+ (rare earth elements) on As+ analysis can be removed.
The interference of WO+ on Hg+ analysis can be removed by converting WO+ to WOO+, which can reduce the detection limit of related elements to sub-ppt level.
The methionine and cysteine residues of most proteins contain elemental sulfur, which is difficult to determine by ICP-MS because of its high ionization energy, resulting in very low sensitivity, and the presence of polyatomic ions increases all Mass spectral overlap of sulfur isotopes, especially in organic matrices.
ICP-MS/MS can well avoid such problems, and quantitatively analyze sulfur by measuring 32S and 34S isotopes, so as to quickly and accurately perform absolute quantitative analysis on proteins and antibodies.