An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 259

weighed. While the principles used in a method range from the general to the specific,
they are all derived from natural laws. This means that an analysis done in one location
will give the same reproducible results when done the same way in a different location.
Analytical methods consist of a series of intermediate steps that, when combined, make
up the analytical procedure. Therefore, the analytical method could be considered as a set
of steps that are determined by the process of measurement, which is in turn based on
natural scientific laws. It can include some of the intermediate steps of sample
preparation and evaluation and represents the overall concept for the acquisition of data
from the sample. The analytical procedure consists of (a) instructions for the initial
sampling event, (b) the sample preparation with directions or indications for required
equipment and chemicals, and (c) the analytical instrumentation and instrument settings
used. The method also gives the required analytical calibration, the area or scope of
application, and possible selectivity and/or interferences [4].
On this basis two types of quantitative analysis can be defined: the classic wet
chemical methods and instrumental methods. Many of the classic analytical methods still
in use today are simple yet reliable “wet” chemistry procedures, including gravimetry
(analysis by weight) and volumetric or titrimetry methods. Instrumental methods are
based on the chemical nature of the compounds and elements being analyzed for and are
separated into organic and inorganic methods. Organic methods are those analytical
procedures for compounds that are mostly made of carbon and hydrogen (e.g.,
pesticides), and generally man-made chemical compounds. Inorganic methods are for
non-carbon-containing compounds and such natural elements as the metals.
10.7.1. Gravimetry
If the analytical procedure used to quantify a substance is based on weight, it belongs to
the field of gravimetry. This involves precipitating a compound out of the sample or an
extract of the sample, drying, and weighing. It is preferred over other methods for
microanalysis using sample amounts in the gram range. Its advantages over the
instrumental methods to be discussed later include the absence of calibration, thus
making it an absolute method. It is a high-precision method and requires limited
apparatus. Its disadvantage in a production environment is the large expenditure of time
and effort required to perform an analysis. A general procedure for a gravimetric method
is as follows: (a) taking of a subsample and weighing, (b) dissolution of the sample, (c)
subsequent precipitation of the component of interest according to a prescribed
procedure, (d) filtering and washing of the precipitate, (e) drying, and (f) weighing.
A second gravimetric method is called thermal analysis (TA), which is a general term
for methods whereby a physical or chemical change in the properties of a substance or
mixture is measured as a function of time or temperature. The sample is subjected to a
controlled increase in temperature and its change in mass is measured over the course of
a preset temperature/ time program. A mass change occurs when a volatile component is
released from the sample (water vapor, carbon dioxide, organic contaminants, or other
similar substances). In addition to decomposition, oxidation can occur, depending on the
atmosphere surrounding the sample. Weight measurements are made using a special
thermal balance and the resultant data are graphed in terms of temperature versus time