Research On Collision/Reaction Cell Inductively Coupled Plasma Mass Spectrometry For Analysis Of Food Samples

Trace elements are the specific physiologic element or activator forming human cells or body fluids, to maintain the body’s special physiological function of enzymes, it is essential in the life activities and physiological processes in the human body, and it plays an obviously nutritional role in the growth of the human organism, the process of metabolism, the biochemical reaction of physiology, and energy conversion. So it shoulders the important mission. The lack of trace elements will cause the biochemical disorders of human body system, lead to physiological dysfunction, and produce diseases and pathological problems. Rich trace elements in food are the main source of trace elements of human body. Because different trace elements play different roles in the life activity of human body, their biological effects are greatly different. The trace elements of heavy metal in food brought by food production, processing, delivery, storage and sales have been a latent threat to human health, which is a serious problem of food safety. Therefore, inspecting trace elements of food is an important step in the work of food quality control. For this, the contents and the distributions of the trace elements in foods must be accurately analyzed.With its simple spectrum, extremely low detection limit, wide linearity range, fast analyzing speed and offering the ratios of isotopes, inductively coupled plasma mass spectrometry (ICP-MS) has been widely applied in food analysis. But as a result of the diversity of food species, the complexity of matrix and the spectral overlap interference, caused by the added solvents in the course of processing pre-sample, are still problems faced in ICP-MS analysis. However, the greatest difference of collision/reaction cell (CRC)-ICP-MS from the common quadrupole ICP-MS is to generate the differences of mass and electric charges among the analyzed ions, interfering ions and other fragment ions by putting a collision/reaction cell between the ion lens and the quadrupole mass analyzer when the sample is ionized by ICP and is taken with a sampling cone and an skimmer cone and then has a collision reaction/reaction with the collision/reaction gas in the sealed CRC after focusing with ion lens. Finally the corrections of interferences are achieved. The paper systematically expounds the theoretical foundation of CRC-ICP-MS method. And the analysis and application of CRC-ICP-MS method in food were explored in the following aspects:(1) This paper describes a simple method for the determination of Li, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Hg, Tl and Pb in nuts by using ICP-MS with octopole reaction system(ORS). Under the boost control, the sample was directly determined in nitric acid and hydrogen peroxide medium after its microwave digestion. The enhancement effect arising in the plasma of5%methanol improved the sensitivity of ICP-MS measurement of the elements ionized difficultly and the working parameters of the instrument were optimized. In this work, matrix effects were corrected in the mixed standard solution of Sc, In and Bi, and an octopole reaction system (ORS) and shield torch system (STS) was used to eliminate the interference of polyatomic ions. The applicability of the proposed method was validated by the analysis of rice standard reference material (GBW10045). The result showed that the detection limits of the24elements are in the range of0.002～0.290μg/L. This method was sensitive, precise and applicable for the analysis of the trace elements in the five kinds of nuts such as Amercan hazelnut, almond, pistachio, and Brazilian cashew and pinenut. And the result of the analysis suggested that there were rich micro-nutrients in the nuts and more Selenium in cashew and pistachio. According to GB/T22165-2008(Chinese fried snack standard of nuts), all nuts are very low in heavy metal element, so with high edible value. The technique can be applied to the quality and safety evaluations of nuts.(2) The sample of potato was achieved by microwave digestion in a digestion system of the mixed acid (HNO3and H2O2). And the contents of16inorganic elements (Na, Mg, Al, P, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se and Pb) in the sample were determined by ICP-MS. The pretreatment of sample was studied in detail. With ORS, different collision/reaction modes were selected, the gas flow of collision/reaction was changed, and the sensitivity of the method was ensured. Consequently, the spectral interferences caused by the overlaps of polyatomic molecules were effectively decreased. Rh as internal standard element was used to compensate matrix effect and signal drift. Under the optimal conditions, the detection limits of the16inorganic elements are in the range of0.005～0.236μg/L. The recovery was92.20%～109.65%by adding standard recovery experiment, and the relative standard deviation (RSD) was less than3.18%for all the elements. This method turned out to be precise and reliable. Four potatoes from different places of origin were analyzed in the way, and the result showed that the contents of inorganic elements in potato are greatly regional. A potato contains rich potassium which is an essential and major nutrient of human body for regulating fluid balance and maintaining muscles, nerve functions and metabolism, and even can be of cancer prevention and resistance. Potato can be safely eaten because of its rich phosphorus, magnesium and calcium, and very few heavy metal elements which are far lower than the Chinese standard of the limited food quantity. This method provides scientific rationale for determining inorganic elements of potato.(3) An analysis method of microwave digestion and ICP-MS with ORS were established for the determination of10heavy metal elements including Cr, Co, Ni, Cu, As, Cd, Sn, Sb, Hg and Pb in sweetener. The effect of long-time spectral analysis on the signal intensities of the determined elements was observed. Ultrapure helium and hydrogen was respectively inlet in the octopole/reaction cell system. The use of ORS can eliminate the interference of polyatomic ions dramatically. Sc, Y, In and Bi as internal standard elements were used to compensate matrix effect and signal drift. The optimum conditions for the determination was tested and discussed. Under the optimal conditions, the detection limits of the10elements was in the range of0.003～0.038μg/L, the recovery of the samples was in the range of93.0%～106.6%and the relative standard deviation(RSD)≤3.4%, which showed that the method was very precise. In the way four common non-nutritious sweeteners including xylitol, aspartame, saccharin sodium, and sodium cyclamate were analyzed. It turned out that the contents of the heavy metal elements in the four sweeteners are all less than10μg/g. The technique was simple, quick and precise, and it could be completely applied to the quality control and safety evaluation of food additive and sweetener.(4) An analytical method for simultaneous determination of18trace elements in honey including Na, Mg, Al, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Mo, Cd, Ba, Hg and Pb by ORS-ICP-MS has been established. And the background equivalent concentrations (BEC) of the determined elements were respectively studied under the modes of normal, hydrogen reaction, and helium collision. It showed that under the modes of hydrogen reaction and helium collision the BECs easily interfering with elements greatly declined by one to three orders of magnitude, and the interference of polyatomic ions was eliminated effectively. The method was sensitive, accurate and reproducible with the detection limits in the range of1.05～73.45ng/L, good linear relations whose relevant coefficients are all more than0.9995, the RSDless than2.73%, and the recovery in the range of92.2%～106.8%. In this way four kinds of honey bought in the market were analyzed including osmanthus honey, medlar honey, jujube honey and acacia honey. The study proved that the contents of18trace elements in different kinds of honey were of great difference. In honey there are very few heavy metal elements but sufficient trace elements which are essential in human body. So honey is rather nutritious and safe to drink.(5)An analytical method was examined to determine14incidental elements including Be, Mg, Al, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Hg and Pb in sodium bicarbonate-a food bulking agent respectively by adopting sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) and ORS-ICP-MS. The sample was dissolved by HNO3in a closed-vessel microwave system, and then the above14trace elements in the test solution were determined directly by SF-ICP-MS and ORS-ICP-MS. This technique covered in detail the digestion method of the sample, the selection of the isotopes of the determined elements and the differences between SF-ICP-MS and ORS-ICP-MS. SF-ICP-MS was applied to the low or middle resolution mode to eliminate the interference of polyatomic ions with the determined elements, while ORS-ICP-Ms was applied to helium collision mode and hydrogen reaction mode to eliminate the interference of polyatomic ions with the determined elements. Ge, In and Bi as internal standard elements apparently improved the stability and precision of instrumental analysis. As a result, the determinations in the two analytical methods were almost identical, and the detection limits of ORS-ICP-MS were lower than ones of SF-ICP-MS.