Abstract
Selenium (Se) has multiple biochemical effects ranging from a nutritional deficiency at low levels to toxicity at high levels. Cases of Se-deficiency are more reported than cases of Se toxicosis. South Africa is one of the countries with uneven distribution of Se in soil, primarily dominated by low Se content. In some areas with high Se content in soil, animals grazing in the regions are still Se deficient. It is presumed that there are chemical forms of Se that do not biotransform into crops and livestock. Farmers (livestock) are greatly affected by Se deficiency, evident through weak calves, stillbirths, and perinatal mortality. However, it is challenging to develop significant supplementation efforts without accurate measurement techniques for Se and its chemical forms. This study focuses on developing analytical methods for quantifying total Se and Se species in South African soils, maize plants, and maize flour.
Test samples of each matrix were sampled simultaneously in KwaZulu-Natal, Free State, and Eastern Cape Province. Se species were separated employing a Dionex ICS-5000+ high-pressure ion chromatograph (IC) and all concentrations of Se were quantified using an Agilent 8900 inductively coupled plasma triple quadrupole mass spectrometer (ICP-QqQ). The methods were validated using matrix certified reference materials (CRMs), namely NMIJ CRM 7303-a (trace elements in lake sediments), NIST SRM 1646-a (estuarine sediments), NIST SRM 1570-a (spinach leaves), NIST SRM 1571 (orchard leaves), NCS ZC 73010 (maize flour), NIST SRM 1568b (maize flour), and NIST RM 8437 (hard red spring wheat flour).
Microwave digestion methods for determining total Se concentration in the respective matrices were developed. All methods fulfilled the performance requirements of the AOAC International. An acceptable accuracy was achieved with percentage recoveries between 57 to 102%, and πΈπscores within Β±1 limits. Repeatability (π
ππ·π,%) and intermediate precision (π
ππ·π
,%) ranged from 4 to 12%, and 9 to 15%, respectively. The limits of detection (LODs) ranged from 0.0036 to 0.024 ng g-1, and limits of quantification (LOQs) from 0.00179 to 0.082 ng g-1. The analytical characteristics of the methods were comparable with previously reported methods, confirming reliability. The measurement uncertainty was evaluated following the Guide to the Expression of Uncertainty in Measurement (GUM). Relative expanded uncertainty at a 95% level of confidence (π=2) was less than 24% for the final measurement results. Methods for separation and extraction of four (4) Se species, selenite (Se(IV)), selenate (Se(VI)), selenomethionine (SeMet), and methylselenocysteine (MeSeCys) were developed. An optimal separation method was achieved using a Dionex IonPac AS7 anion-exchange column and IonPac AG7 guard column. Separation of species was obtained with a 40 mM HNO3 mobile phase using an isocratic elution. The retention times (π‘π) for the Se species were, Se(IV), π‘π=2.3; MeSeCys, π‘π=4.3; Se(VI), π‘π=5.4; and SeMet, π‘π=7.4 min. Extraction in soil were conducted using 5% hydrofluoric acid (HF) in a hot water ultrasonic bath, at a temperature of 80 ΒΊC for 1 hour. In maize plants and maize flour, extractions were carried out employing microwave assisted extraction, using hot water for 2 hours and a stirring rate of 60%. An acceptable accuracy was achieved with percentage recoveries between 39 to 42%, Repeatability (π
ππ·π,%) and intermediate precision (π
ππ·π
,%) ranged from 4 to 10%, and 11 to 20%, respectively. The LODs for Se species ranged from 0.189 to 2.62 ng g-1, and LOQs from 0.276 to 6.12 ng g-1. The Relative expanded uncertainty at a 95 % level of confidence (π=2) was less than 4% for the final measurement results.
Total Se concentrations in soil were between 123 to 4349 ng g-1, 2.5% of the tested samples were Se-deficient, 25% Se-marginal, 50% Se-sufficient, 20% Se-rich and 2.5% Se-excessive. The detected Se species in soil were Se(IV) and Se(VI), accounting for 1-46% of the total Se. The pH of the soils ranged between 4.89-8.38 with alkaline soil showing higher accumulation of Se in corresponding maize plants and maize flour. Selenium concentrations in maize plant organs were between 1 to 739 ng g-1, with Se(IV), as the detected species, accounting for 5-88% of the total Se.
The results also showed that the Se concentrations in maize plant organs did not meet the recommended Se concentration of 100 ΞΌg kg1 in animal feed to avoid Se deficiency in cattle. The concentration of Se in maize flour was between 1 to 571 ng g-1. The Free State and Eastern Cape had Se deficient maize flour which did not meet the daily requirement of 40-400 ΞΌg day-1 from the World Health Organisation (WHO). KwaZulu-Natal maize flour was found to be Se adequate, with the exception of a few samples that were below and above the recommended daily limit indicating that potential Se toxicosis or deficiency can occur if necessary, interventions are not taken. Selenium species were not detected in maize flour.