A new method for correlation analysis of compositional (environmental) data - a worked example.

Most data in environmental sciences and geochemistry are compositional. Already the unit used to report the data (e.g., µg/l, mg/kg, wt%) implies that the analytical results for each element are not free to vary independently of the other measured variables. This is often neglected in statistical analysis, where a simple log-transformation of the single variables is insufficient to put the data into an acceptable geometry. This is also important for bivariate data analysis and for correlation analysis, for which the data need to be appropriately log-ratio transformed. A new approach based on the isometric log-ratio (ilr) transformation, leading to so-called symmetric coordinates, is presented here. Summarizing the correlations in a heat-map gives a powerful tool for bivariate data analysis. Here an application of the new method using a data set from a regional geochemical mapping project based on soil O and C horizon samples is demonstrated. Differences to 'classical' correlation analysis based on log-transformed data are highlighted. The fact that some expected strong positive correlations appear and remain unchanged even following a log-ratio transformation has probably led to the misconception that the special nature of compositional data can be ignored when working with trace elements. The example dataset is employed to demonstrate that using 'classical' correlation analysis and plotting XY diagrams, scatterplots, based on the original or simply log-transformed data can easily lead to severe misinterpretations of the relationships between elements.

Trace elements in Canadian field peas: a grain safety assurance perspective.

Canada is a significant exporter of field peas and, thus, issues of food safety are important in all grain and food products within the international marketplace. Environmental contaminants, such as trace elements, may be present in all foods and, as a result, international standards have been established for a number of toxic trace elements, such as cadmium, lead, mercury and arsenic, in raw food commodities and food products. The Canadian Grain Commission has undertaken a baseline study of various trace elements in Canadian peas to ensure compliance with international food safety legislation. Mean total cadmium content was found to be 0.023 mg kg(-1); arsenic and lead mean values were below the method limit of quantification of 0.050 mg kg(-1) and the total mean mercury level was below the quantification level of 0.002 mg kg(-1). All measured values in the study were below the maximum residue levels (MRLs) established by the FAO and WHO in the Codex Alimentarius. The mean total selenium content was 0.331 mg kg(-1), with 56% of the measured values exceeding the MRL established by the People's Republic of China (PRC) of 0.3 mg kg(-1). No Codex MRL has been established for selenium as it is regarded as an essential trace element for human health and the PRC is currently reviewing its MRL for selenium in light of this fact. For those parts of the world where selenium-deficiency is of nutritional concern, the higher level of selenium in Canadian peas and their products may be of nutritional benefit.

Trace elements in western Canadian hard red spring wheat (Triticum aestivum L.): levels and quality assurance.

A monitoring program was conducted for trace elements in Western Canadian Hard Red Spring wheat (Triticum aestivum L.). Samples were selected from harvest survey samples submitted by producers from crop districts in Manitoba, Saskatchewan, and Alberta for 1996, 1997, and 1998 crops. The analytical quality control measures used in these surveys are described along with the results for Cd, Cu, Fe, Mn, Se, and Zn. Accuracy and precision for the analyses fell within the acceptable control limits. Year-to-year variations in grain chemistry were small for Cd, Mn, Se, and Zn, but Cu and Fe contents showed 12 and 9% decreases, respectively, over the 3 years. The overall variability for the plant-essential trace elements-Cu, Fe, Mn, and Zn-was low compared with that for Cd and Se. The spatial variation in crop chemistry across the Canadian Prairie wheat-producing region was greater than the year-to-year variations. Soil properties were major factors in controlling Cd and Se levels in grain.

Primary hyperparathyroidism in the primary care setting.

A series of 36 patients with surgically proved primary hyperparathyroidism is reported. From this group a clinical profile consisting of obesity, anxiety, and/or depression in a mildly hypertensive, middleaged female was derived. Women constituted 92 percent of the patients. The serum calcium was confirmed again as the single most valuable test in the evaluation of this disorder.