Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is an analytical technique for analyzing the chemical properties of elements in materials and has become a well-established analytical method in the 1990s. It can accurately determine qualitatively and quantitatively almost all elements in the periodic table, including refractory elements, and has a wide analytical range. It is capable of simultaneous analysis of multiple elements and provides effective information on elemental composition. It can also be used for isotopic analysis of the elements to be measured, and is widely used for isotope dilution quantification and stable isotope run completion. It can determine very low concentrations of the elements to be measured, and the lower limit of quantification reaches 1~10ng/L. The response sensitivity is higher than most other modern analytical instruments.
The sample introduction system is an important part of ICP-MS and has a great impact on the analytical performance. The high sensitivity of ICP-MS and the wide range of trace elements analyzed by most analytical protocols require special attention to contamination loss during sample preparation.
Contamination or loss of the elements to be measured during sample preparation, such as sampling, storage, sample preparation, decomposition, separation, and pre-precipitation, is the most important systematic error. The lower the concentration of the element to be measured, e.g. down to mg-kg-1 or even ng-kg-1, the more pronounced the effect of contamination and loss.
The primary sources of contamination include the material of the utensils and tools used, the reagents and the experimental air environment. The material of utensils and tools used in the experiment is extremely important in trace analysis. Contaminants may be leached from the vessels or tools or desorbed from their surfaces into the sample solution.
The following categories of materials are recommended for use in trace analysis.
1, silica oxide glass, especially artificial quartz, high purity, is one of the best materials in trace analysis. The main advantages are: not easy to be contaminated, the surface is smooth and poorly adsorbed, can withstand high temperature of 1200℃, and can resist most inorganic acids, except HF and H3PO4.
2, Polyphenylene fluoride, PTFE, PFA and TFM, these materials are preferred in the ablation process. Due to the non-polarized surface, the adsorption of polar ions is minimal, so the contamination and loss caused by adsorption or desorption is minimal. The maximum temperature resistance is 250°C. The advantage is that these materials are resistant to almost all inorganic acids and can therefore be used for HF digestion of samples. PFA has the same excellent physical and mechanical properties, electrical properties, chemical stability, non-stick properties, flame retardancy and atmospheric aging resistance as PTFE. However, the melting temperature of PTFE is 327℃, but the resin is in the molten state only above 380℃, and the melt viscosity is high, plus its solvent resistance is extremely strong, so it can neither melt processing, nor dissolution processing. It can not be used melt extrusion, injection molding and other conventional thermoplastic molding process, can only be used similar to the powder metallurgy method of sintering molding. The melting point of PFA is lower than PTFE, but the long-term use temperature is the same as PTFE, and the mechanical strength at high temperature is better than PTFE, and the outstanding feature of PFA is good thermoplasticity, no processing difficulties like PTFE, so it has the name of fusible PTFE, PFA can be processed by molding, extrusion, injection molding. Here, PFA is recommended to be used as the sample pre-treatment vessel.
The same container made of PFA, due to different production and processing methods, will lead to long or short service life of the product; different grades of raw materials selected will also lead to different precipitation of impurities during long-term storage of reagents; due to the good processing performance of PFA, it is recommended to choose When using PFA containers, it is also necessary to check whether they are made of recycled materials. Once PFA products made of recycled materials are accidentally purchased and used, the consequences of contamination to the sample will be immeasurable.
Another major source of contamination is impurities adsorbed on the surface. Usually need to be fully cleaned before using the container. For example, acid vapor (usually HNO3) is used to clean quartz vessels, and borosilicate glass, PTFE, PFA, TFM, etc. through an acid counter-current cleaner.
Acid Counterflow Cleaner
Another example is the cleaning of polymer material containers:
① remove polymer surface residues;
② rinse with deionized water;
③ placed in the PFA large capacity cleaning tank, filled with 1:4HCl, soak, placed at least 1 week;
④ rinse with deionized water;
⑤ placed in the PFA large capacity cleaning tank, filled with 1:4HNO3, soak, placed at least 1 week;
⑥ rinse with deionized water;
⑦ dry. PTFE container cleaning process is similar to the polymer material containers, only the HCl, HNO3 concentration used is 1:1, and kept at least 4h at 80 ~ 90 ℃.
The proper vessel and the correct cleaning method will minimize the interference of contamination to the ICP-MS and obtain the desired analytical results.