Elemental fingerprint analysis of barley (Hordeum vulgare) using inductively coupled plasma mass spectrometry, isotope-ratio mass spectrometry, and multivariate statistics

Abstract

Inductively coupled plasma mass spectrometry (ICP–MS) and isotope-ratio mass spectrometry (IR-MS) have been used to examine the multi-elemental composition and ¹⁵N/¹⁴N and ¹³C/¹²C isotope ratios of three spring barley (Hordeum vulgare) genotypes (Orthega, Barke, and Bartok) grown in three typical Danish agricultural soils (North Jutland, West Jutland, and East Zealand) differing in soil fertility. The aim of the study was to examine whether it was possible to generate a unique elemental fingerprint of individual barley genotypes irrespective of the elemental imprint plants had received from soils differing in fertility and agricultural practice. Multivariate statistics were used to analyze the elemental fingerprints of the barley genotypes at different times during a full growing season from early tillering to full maturity of the barley grains. Initially, 36 elements were analyzed in the plant samples but this number was subsequently reduced to 15 elements: B, Ba, C, Ca, Cu, Fe, K, Mg, Mn, N, Na, P, S, Sr, and Zn. These elements exceeded the limit of detection (LOD) for all genotypes, soil types, and plant growth stages and for these elements the accuracy was better than 90% compared with apple leaf certified reference material (CRM). Principal component analysis (PCA) separated multi-elemental data in accordance with soil type when plants of similar physiological age were compared, whereas this separation disappeared if plants of all ages were compared simultaneously. Isotope ratios (δ¹⁵N) of plants also proved to be a highly accurate property for classification of samples according to soil type. In contrast, the differences in δ¹³C were too small to enable such classification. The differences in δ¹⁵N among soils were so pronounced that separation of samples according to the physiological age of plants became redundant. However, δ¹⁵N and the multi-elemental analysis revealed no differences between the three barley genotypes, indicating that the influence of soil chemistry and possibly also climate and agricultural practice was too large to allow an unique elemental fingerprint for the genotypes. This finding was substantiated by analyzing the multi-elemental composition of grain from two additional genotypes (Otira and Barthos) grown at the north and east locations, respectively. PCA showed not only that the elemental fingerprints of these two genotypes were similar to those of the others, but also that the soil in which the plant had been growing could be accurately predicted on the basis of the PCA scores from the genotypes Orthega, Barke, and Bartok. Similar conclusions could be drawn using δ¹⁵N data.