Sample preparation is the critical step of any analytical analysis to obtain high-quality analytical results and valid conclusions. It involves steps from simple dilution to partial or total digestion. Wet digestion with oxidizing acids is the most common sample preparation procedure. Many of the sample preparation methods currently in use were actually developed during the 19^th century.

Wet digestion is a process of oxidizing decomposition of organic samples by liquid oxidizing reagents such as HNO₃, H₂SO₄, HClO₄ or mixture of these acids.

Sample preparation by wet digestion before AAS analysis

In the early 1800s, Berzelius developed test tubes, separatory funnels and platinum crucibles; in 1831 he first made use of the conversion of SiO₂ to SiF₄ by reaction with HF for analytical purposes. In 1834, Henry and Zeise developed methods for the gravimetric determination of sulfur as sulfate in organic samples. Their method was for the sample to be digested with fuming nitric acid or aqua regia and fused with potassium hydroxide or potassium nitrate.

The first published wet digestion reagent was chloric acid from HCl + KClO₃, as described in 1938 by Duflos as well as by Remigius Fresenius and Babo in 1844. The classical wet digestion reagent HNO₃ + H₂SO₄ (the most important and most versatile of the so-called wet-oxidation mixtures (Henryk)) was investigated by Danger and Flandin in 1841, for the destruction of organic matter.

Hydrogen peroxide is a very popular oxidizing agent as it is converted to water and oxygen during the oxidation of biological material. No acid corrosion of the digestion vessel PTFE (polytetrafluorethylene) walls, no formation of insoluble salts with an acid anion and no change of the sample matrix by an acid are additional advantages. Because of its strong oxidation power, only small amounts of H₂O₂ need to be used, so that concentrated sample solutions can be obtained (Henryk).

Campbell and Plank, (1992), Organic Matter Destruction, Wet Ashing, Plank   Analysis Reference Procedures for the Southern Region, US, pg. 7-9. Retrieved February 28, 2007 from http://www.cropsoil.uga.edu/~oplank/sera368.pdf

Douglas A. Skoog, (2004), Decomposing and Dissolving the Sample, Fundamentals of Analytical Chemistry 8th Edition, Chapter 36, pg. 1042-1049.

Jones, J. B., Jr., and V. W. Case. (1990), Sampling, handling, and analyzing plant tissue samples, Soil testing and plant analysis, SSSA, Inc., Madison, WI