Briefly describe the methods and standards for elemental speciation analysis in food

　　The form of an element refers to a specific form in which an element exists with different isotopic compositions, different electronic configurations or valence states, and different molecular structures. Element forms are divided into physical forms and chemical forms. Physical form refers to the physical state of an element in a sample, such as dissolved, colloid, and granular states; chemical form refers to the existence of an element in the form of certain ions or molecules, including the valence state, bound state, aggregated state, and structure of the element. In a general sense, element form refers to chemical form. Element form is different from element valence state. The same valence state of the same element may have multiple forms. For example, arsenic with a valence of five can be divided into inorganic and multiple organic forms.

　　Elements exist in different forms in food, and the effects of elements on the human body are closely related to their forms. The form mentioned here refers to the performance or distribution of the element in different types of compounds. For example, chromium, trivalent chromium is a component of the human glucose tolerance factor. Many diabetes cases are related to the lack of trivalent chromium in the human body, while hexavalent chromium is a relatively strong carcinogen. The toxicity of arsenic in different forms also varies greatly. For example, arsenosugar and arsenobetaine in the form of organic arsenic are almost non-toxic, while inorganic arsenic compounds are very toxic. Therefore, for some elements, it is not enough to only know the total amount of an element in food. While we understand the total amount, we also hope to understand the form composition of an element in food.

　　Measuring the morphology of elements can be achieved through the following methods:

　　Spectrophotometry: There are specific requirements for the form of elements when developing colors. This characteristic can be used to perform morphological analysis. A typical example is the measurement of hexavalent chromium in water. This method usually has large interference and low sensitivity, and has a certain range of application in simple matrices.

　　Atomic fluorescence spectrometry (AFS): Since the generation of hydrides has certain requirements on the form of elements, this feature can be used for form analysis. For example, organic arsenic almost never generates arsenic hydride with borohydride, and hydride-atomic fluorescence spectrometry cannot directly detect organic arsenic, while inorganic arsenic can react with borohydride and be detected. This feature can be used to measure different forms of certain elements. The characteristic of this method is high sensitivity. The disadvantage is that it has strong specificity and can only analyze certain forms of a limited number of elements, so it is not widely used.

　　Chromatography: Use a chromatographic column to separate different forms, and then measure them with a detector such as a spectrophotometer or conductivity. For example, ion chromatography is a commonly used method. Due to the pre-separation treatment, this method has less interference than spectrophotometry and better sensitivity.

　　Pre-separation method: The sample is pre-separated according to the characteristics of different forms of elements, such as organic extraction, ion adsorption and exchange, to separate a specific form from other forms and collect them, and then use some spectral analysis methods to measure. This method has a relatively high sensitivity, but the pre-treatment is relatively complicated and is also easily interfered.

　　Chromatography-spectrometry (mass spectrometry): This method uses online chromatographic separation, and after separation, each component directly enters the spectrometer for measurement. It combines the advantages of chromatography and spectroscopy, and has the advantages of good separation effect, high sensitivity, and wide application. The disadvantage is that the equipment is relatively expensive, the interface technology from chromatography to spectroscopy needs to be solved, and the pre-treatment method also needs to be strengthened. There are literature reports on different chromatography and spectroscopy coupling techniques, mainly focusing on the coupling of chromatography and plasma mass spectrometer (ICP-MS). The following coupling methods are currently common.

　　1. Liquid chromatography-ICP-MS

　　Liquid chromatography (HPLC)-ICP-MS coupling technology is suitable for the analysis of non-volatile compounds in food samples. Since the flow rate of liquid chromatography is consistent with the injection speed of ICP-MS, the connection is very simple and convenient, and its coupling interface is very simple. In addition, due to the characteristics of liquid chromatography, it has the advantages of small injection volume, fast analysis speed, and good separation effect. Therefore, the coupling technology of HPLC and ICP-MS has been increasingly used in the field of arsenic, selenium, tin, mercury and other elemental morphological analysis in various foods, and related research is also the most. When using this technology, pay attention to whether the composition of the liquid mobile phase meets the injection solution requirements of ICP-MS. If the proportion of organic phase is too high, auxiliary oxidation technology is required.

　　2. Ion chromatography-ICP-MS

　　As an effective separation and detection technology, ion chromatography (IC) has been widely used in the determination of metal and non-metal ions. It has become an effective tool for solving the analysis of ultra-trace ion forms in complex organisms. It is also one of the hot spots in the research of ICP-MS related combined technologies, and has more and more applications in the field of food analysis. Its combined method is also very simple, just like liquid chromatography. At present, relevant literature focuses on the detection and research of chromium, arsenic, antimony, bromine, iodine and other forms. Similarly, when using this technology, attention should be paid to the matching of ion chromatography mobile phase and ICP-MS injection requirements, and the soluble solid content of the mobile phase should not be too high.

　　3. Gas chromatography-ICP-MS

　　Gas chromatography (GC) is suitable for the separation of volatile or moderately volatile organometallic compounds. The derivatization step before separation not only complicates the separation and analysis process, but also increases the possibility of loss or contamination of the form to be measured. In addition, the connection between the gas phase and ICP-MS requires a dedicated interface. Therefore, the application of GC and ICP-MS in the morphological analysis of elements has certain limitations. At present, GC-ICP-MS technology is limited to the analysis of forms such as alkyl lead, alkyl tin and alkyl mercury.

　　4. Capillary electrophoresis-ICP-MS

　　Compared with gas chromatography and liquid chromatography, capillary electrophoresis (CE) has the characteristics of high separation efficiency, low sample consumption, fast separation time, etc. It has a wide range of applications and can separate various compounds from simple ions, non-ionic compounds to biomacromolecules. However, during the separation process, the original form of the analyte in the sample may change due to the adjustment of electrolytes or pH values. The composition of the sample is also an important factor affecting CE separation. Since the interface between CE and ICP-MS is not as mature as HPLC, the application of CE-ICP-MS coupling technology is restricted to a certain extent. However, there are still many related studies, mainly focusing on the analysis of the forms of elements such as arsenic, selenium, and mercury in food.

　　5. Liquid chromatography-AFS

　　Since China's AFS technology is ahead of the world, the research has developed rapidly in China. Since AFS has high detection sensitivity for certain elements, such as As, Se, Hg, etc., and these elements are also the elements of greatest concern in morphological analysis, AFS is very useful in elemental morphological analysis. As mentioned earlier, AFS alone can perform some specific morphological analysis, but to achieve better separation and detection, it needs to be combined with chromatography. Now it is mainly combined with liquid chromatography, and many HPLC-AFS instruments have been launched. The advantage of this technology is that it has the advantages of liquid phase separation, can also take advantage of the high sensitivity and element specificity of AFS, and the overall price of the instrument is not high. Its disadvantage is that the detected elements are limited by AFS, and the stability of the AFS detection state is also difficult to guarantee.

　　Standards for elemental speciation analysis in food:

　　1. Arsenic speciation analysis standards

　　According to GB 2762-2012 "Limits of Contaminants in Food", the limit standards for inorganic arsenic in food are stipulated, so there are also relevant detection methods:

　　GB/T 5009.11-2003 Determination of total arsenic and inorganic arsenic in food: Inorganic arsenic is detected by atomic fluorescence method, and the pretreatment is different from that of total arsenic.

　　GB/T 23372-2009 Determination of inorganic arsenic in food by liquid chromatography-inductively coupled plasma mass spectrometry: This standard adopts HPLC-ICP-MS combined technology, with strong separation and detection capabilities.

　　There is an industry standard for the detection of organoarsenic pesticides: SN/T 2316-2009 Detection of Arsanilic Acid, Nitrophenyl Arsenic Acid and Roxarsone Residues in Animal Origin Food for Import and Export Ion Chromatography-Inductively Coupled Plasma Mass Spectrometry

　　2. Mercury speciation analysis standards

　　According to GB 2762-2012 "Limits of Contaminants in Food", the limit standards for organic mercury (measured in methylmercury) in food are stipulated, so there are also relevant detection methods:

　　GB/T 5009.15-2003 Determination of total mercury and organic mercury in food: Organic mercury is determined by gas chromatography and pre-separation-cold atomic photometry.

　　The detection methods for inorganic arsenic and organic mercury both have defects. The revised new method (draft) uses liquid chromatography-atomic fluorescence coupling, but it also has problems and has not yet been promulgated as an updated method.

　　3. Morphological analysis standard of bromate

　　Since bromate is a Class 2B carcinogen, it is no longer allowed to be used as an additive. There are two standards for the speciation analysis of bromate in food, both using ion chromatography:

　　GB/T 20188-2006 Determination of bromate in wheat flour - Ion chromatography

　　SN/T 3138-2012 Determination of bromate in exported noodle products - Post-column derivatization ion chromatography

　　There are also limit standards and detection methods for bromate in water, and the relevant water detection standards also use ion chromatography.

　　4. Chromium morphology analysis standards

　　There is an industry standard for the detection method of hexavalent chromium:

　　SN/T 2210-2008 Determination of hexavalent chromium in health foods Ion chromatography-inductively coupled plasma mass spectrometry

　　There is also a corresponding standard test method for hexavalent chromium in water, using the classic colorimetric method.