Development of 9 Kinds of Plant Matrix Reference Materials for Elemental Analysis and Quantitative Characteristics

With the increasing concern for food safety, especially for the elemental composition and efficacy of plant-derived foods and health products, research on the nutrient analysis of related foods is continuously carried out. At present, due to the wide variety of agricultural and health products, and the different environments in which the plant matrix grows, the elements absorbed by plants during the growth process vary greatly, making it difficult for many foods to meet the requirements of the calibration and testing system simply by using pure standards. It is necessary to combine the matrix reference materials for calibration and develop new plant-based reference materials.

To improve the situation that there are few existing plant matrix reference materials, by developing 9 kinds of plant matrix reference materials for elemental analysis. Except for the reference materials for elemental analysis in notoginseng powder, all of them were developed for the first time, which supplemented the types of plant matrix elemental analysis reference materials and were mainly used in food nutrition and safety analysis and testing technology research of plant samples.

In the early stage of development, the selection of candidate reference materials with the representative matrix were investigated and studied. The samples were pretreated, dried, crushed, sieved, and examined by a laser particle size analyzer before being packed in 45mL HDPE plastic bottles and inactivated by ⁶⁰Co irradiation, which can be stored for a certain period of time. The samples were randomly selected from 30 bottles during the whole packing process, and the homogeneity test was performed by inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-optical emission spectrometry (ICP-OES), atomic fluorescence spectrometry (AFS) and volumetric method (VOL). The long-term stability of the reference materials under the specified storage conditions and the transport stability at 60℃ were investigated and the results were evaluated by trend analysis, which showed that the samples were stable under the specified conditions. The operation guideline of reference materials characterization analysis was developed, and 11 laboratories worked together to analyze 59 elements: Ag, Al, As, B, Ba, Be, Bi, Br, Ca, Cd, Ce, Cl, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Gd, Ge, Hg, Ho, I, K, La, Li, Lu, Mg, Mn, Mo, N, Na, Nd, Ni, P, Pb, Pr, Rb, S, Sb, Sc, Se, Si, Sm, Sn, Sr, Tb, Th, Ti, Tl, Tm, U, V, Y, Yb, Zn.

The data of the developed series of reference materials were statistically judged by Grubbs and Dixon and the suspicious values were rejected, with a rejection rate of less than 3%. Shapiro-Wilk was used to test the normality of the fixed value data, in which the element Ti in red bean, Bi, I and Th in needle mushroom, Ti in tremella, and I in jasmine had a skewed distribution of the original measurement data, and no certified value was taken only as the median value for reference, and the arithmetic mean of the customized data was used as the certified value for the remaining elements. The certified values and uncertainties of the nine reference materials were above 40, among which, 53 certified values and uncertainties were given for hawthorn and notoginseng. The uncertainty of reference materials consists of homogeneity uncertainty, stability uncertainty and characterization uncertainty. This series of reference materials was developed under the concept of metrological traceability, and a variety of quality control measures were taken to ensure the accuracy and reliability of analytical results and was applied as national level reference materials in 2021. The names and number are as follows: chemical composition of red bean (GBW10228), onion (GBW10232), rape (GBW10233), needle mushroom (GBW10247), tremella (GBW10248), root starch (GBW10249), hawthorn (GBW10250), notoginseng (GBW10251), and jasmine (GBW10234).

The existing plant matrix elemental analysis reference materials are mostly found in rice, wheat, tea and other major agricultural and sideline food, and the certified value elements are mainly macronutrients and heavy metals. The present series of reference materials cover agricultural and health products with a large amount of daily sampling, except for notoginseng, which is developed for the first time, supplementing the variety of relevant quality control samples in the food testing field, with 59 certified value elements, which can effectively reflect the composition of elements in relevant products.

In this study, more regionally representative candidates were selected, focusing on the beneficial nutritional elements K, Ca, Na, Mg, P, Zn, Fe and harmful metal elements As, Hg, Cd, Pb, whose contents can reflect the distribution characteristics of the elements in different samples and origins, such as red beans, which is a health food, and the candidates were picked from Yi'an County in Songnen Plain (one of the important commodity grain bases in China). It contains high contents of beneficial nutrients such as K (1.392g/100g), Ca (0.104g/100g), Mg (0.165g/100g), P (0.457g/100g), Zn (24μg/g) and Fe (71μg/g), and low contents of Pb (0.033μg/g), Cd (5.4ng/g) and other harmful metal elements, which are far below the national limit standards, reflecting the fertile black soil of Songnen Plain and the effectiveness of China's land management. For elements of Cd and Pb, red beans (5.4ng/g) and (0.033μg/g), onions (17ng/g) and (0.322μg/g), needle mushroom (33ng/g) and (0.126μg/g), rape (170ng/g) and (0.9μg/g), notoginseng (311ng/g) and (0.7μg/g), jasmine (420ng/g) and (1.65μg/g) form a gradient range, which is more applicable for food testing quality control. It can be found that the content of heavy metals in health products such as notoginseng and jasmine are higher than that in vegetable products such as rape and needle mushroom, which has a certain guidance for selecting suitable quality control samples in analytical testing. This is a guideline for the selection of suitable quality control samples.

Due to the progress of the current testing methods and instruments, the precision of the data for the determination of rare earths and other elements with very low contents in some samples was good, and standard values were given, such as Eu (9.4±1.7ng/g), Ge (7.8±2.3ng/g), Lu (1.7±0.5ng/g), Tb (5.3±0.9ng/g), Tm (1.8±0.6ng/g) in rape, while the previous reference materials of similar vegetable matrixes did not reach such definite levels.

The development of the series of plant matrix reference materials is supplemental and improves the existing plant matrix reference materials, with obvious distribution characteristics of elements and wide applicability of the gradient range. This series of reference substances covers lots of elements with certified values, which is suitable for the quantity transfer and quality control of biological samples analysis and provides basis for the research on the correlation between the medicinal value and elements of health products.