Comparison of some methods for the determination of total sulfur in plant tissues

Abstract

There are several methods for determining total sulfur (S) in plant materials but few studies comparing several methods. In this study, we compared six methods, viz, dry ashing‐inductively coupled plasma emission spectrometry (ICP), oxygen flask combustion ‐ ICP, oxygen flask combustion ‐ ion chromatography (IC), automated combustion (LECO CNS‐2000), and two variants of micro‐ wave‐acid digestion (HNO₃/HClO₄ or HNO₃/H₂O₂/HCl) followed by ICP analysis. Six tissue types (citrus leaf, canola seed, com leaf, pine needle, red clover shoot, and wheat flour) were decomposed by the above methods in two batches (days) of three subsamples each. Average tissue S content as determined by the methods was: automated combustion = HNO₃/H₂O₂/HCl ‐ICP > oxygen flask combustion = dry ashing > HNO₃/HClO₄‐ICP. There was a small but significant method x tissue interaction, with some methods giving higher S values for certain tissue types and vice versa. Oxygen flask combustion followed by ICP or ion chromatographic analysis gave similar S values for all tissue types indicating complete oxidation of plant S to sulfate. Mean coefficients of variation ranged from 3.1 to 5. 5%. Both the HNO₃/HClO₄ and the HNO₃/H₂O₂/HCl microwave digestion‐ICP methods were suitable to somewhat different degrees for multi‐elemental analysis, but the tested dry ashing‐ICP method was not suitable without modification.