The first supplement of the United States Pharmacopoeia (USP40 S1) has come into effect on 2017.08.01. Among them, the general rule USP<232> “Elemental Impurities – Limits” on metal elemental impurities is in harmony with ICH Q3D. Increase the types of elements investigated from 15 to 24, and divide them into four categories: 1, 2A, 2B, and 3 according to the toxicity of elements and the possibility of appearing in nature. General Chapter USP <233> “Elemental Impurities – Analytical Procedures” section, it mainly introduces the selection requirements of element determination methods, the processing methods of different forms of samples, the introduction of ICP-MS and ICP-OES two test methods, the verification requirements of limit determination methods, the verification requirements of quantitative determination methods, etc.
Sample preparation is very important when analyzing trace metal elements using inductively coupled plasma spectroscopy (ICP). Usually, the key issue of sample preparation is how to choose the most suitable method to obtain the sample test solution. The ideal state is to dissolve or dilute the sample directly with water or dilute acid, which can save time and effort, and can get the result faster. However, for most samples analyzed by ICP, this simple method is not applicable.
Sample preparation method
The United States Pharmacopoeia General Chapter USP <233> “Elemental Impurities – Analytical Procedures” proposes 4 methods for sample solution preparation. These solutions can be analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS). Liquid samples can be tested directly, and solid or liquid samples can be tested in aqueous solution, organic solution or digestion solution. It is very important to ensure that the blank solution and standard solution are prepared in the same matrix as the sample (such as the same solvent, the same concentration of acid, the same stabilizer, etc.).
- Liquid sample direct test: A small number of liquid samples can be analyzed directly without dilution or digestion. This method is suitable for samples with low viscosity that can be injected directly, a typical example is water.
- Direct analysis of aqueous solution: For water-soluble samples, dilute acid can be used to dissolve the sample and analyze directly. It is very important for the test to ensure that the sample is completely dissolved without precipitation or turbidity.
- Direct analysis of organic solutions: For fat-soluble samples, organic solvents can be used for preparation. When analyzing organic solutions by ICP-OES or ICP-MS, a cooled spray chamber, special torches, and possibly a separate oxygen connection are required. The ICP parameters used to analyze aqueous solutions and organic solutions may be very different, so when analyzing organic solutions, ICP parameters need to be optimized.
- Indirect sample solution preparation: For samples that require concentrated acid for digestion, a microwave digestion device can be used for preparation. This method can reduce the loss of volatile elements, but in order to ensure complete digestion, multiple times of acid digestion are required. When using microwave digestion to prepare samples, analysts should take into account the high price of the digestion vessel and the contamination of PTFE materials. Extra care should be taken when using hydrofluoric acid, and in addition to safety considerations, special microwave digestion vessels, spray chambers, and rectangular tubes should be considered.
In addition to the sample preparation methods recorded in the United States Pharmacopoeia, there are other sample preparation methods. For example, the US Environmental Protection Agency 3050B uses the reflux digestion method of dilute acid solution to prepare sample solutions.
Other considerations
All glassware used for trace element analysis must be absolutely clean before use. First of all, it needs to be soaked in high-grade pure dilute acid solution, and then cleaned with purified water; For aqueous solution, the cleaned glassware can be used directly; Sample solutions prepared by other methods need to be used after the glassware is dry. Standard solutions can be prepared by diluting commercial standard solutions. When preparing mixed standard solutions, care should be taken that all elements in the mixed standard can coexist without precipitation. In order to eliminate interference, be sure to use the same matrix as the sample solution to prepare the standard solution and blank solution.
In addition, during the analysis of mercury element, in order to prevent the mercury element from being reduced to elemental mercury and ensure that the mercury element exists in the form of mercury ions, gold trichloride is usually added as a stabilizer.
