Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 163

(a) (b)

Glass sample container
connected to a
water-cooled reflux
condenser

Sample tube

Heating block

Protective glass sheath

Sample
+ Acid

Microwave irradiation

Sample + Acid Sample well

Figure 7.3 Schematic diagram of two common acid digestion apparatus: (a) heat block acid
digestion system, (b) microwave-assisted acid digestion system. (Dean, JR, 2003, Methods for
Enviromental Trace Analysis, # John Wiley & Sons Limited. Reproduced with Permission)

7.2.2 Principles of Acid Digestion
and Selection of Acid

Acid digestion is often conducted on a hotplate under a ventilation hood especially
designed for minimal exposure to metals, while allowing corrosive acids to be
washed from the hood after the digestion is completed. The purpose of acid
digestion is to dissolve metals from sample matrix so that metals can be in a
measurable form. Various acids are used in acid digestion, which could be confusing
in differentiating one protocol from the other in the standard methods (EPA 300
series methods for water and wastewaters and 3000 series method in SW-846 for
liquid and solid wastes, Section 5.2.1). In selecting which single or combined acid(s)
to use, one should first consider what instrument is available for metal analysis. In
general, HCl is not preferred if graphite furnace atomic absorption spectroscopy
(GFAA, see Chapter 9) is the method of analysis because of the interference from
chloride. The second factor relates to the characteristics of the sample matrix. The
general guidelines on acid selection are as follows: (a) For clean samples or easily
oxidized materials, the use of HNO 3 alone is adequate, (b) For readily oxidizable
organic matter, the HNO 3 -HCl or HNO 3 -H 2 SO 4 digestion is adequate, (c) For
difficult to-oxidize organic matter, HNO 3 -HClO 4 is needed, and (d) If matrix
contains silicates, then HNO 3 -HClO 4 -HF digestion is necessary. No acids other
than HF will liberate the metal of interest from the silica matrix.
HNO 3 is an acid of preferred choice either alone or in combination with other
acid(s). The only exception to use HNO 3 is when samples contain highly
concentrated alcohols and aromatic rings that can form explosive compounds
(e.g., nitro glycerine and TNT). There are several reasons why HNO 3 is preferred
over other acids. First, HNO 3 is acting as both an acid and an oxidizing agent in