An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 268

separated compounds exit a gas chromatographic column they are introduced into a mass
spectrometer. This method thus not only separates a complex mixture into its component
parts but also identifies the components, making it an extremely powerful analytical tool.
The output from a GC/MS is sent to a computer, where it is recorded and displayed.
The computer also contains a library of the unique fragments of a wide range of
compounds. It can be used to search this library for compounds with the same
fragmentation patterns as the unknown component from the chromatographic column and
thus identify it. In one analytical procedure taking little time, a very complex mixture can
be separated and its component parts identified.
There are two very important limitations to this method that must be kept in mind at all
times. It is possible that two different compounds will come out of a chromatographic
column at the same time. It is also possible that these two compounds will have similar
fragmentation patterns. When this happens the computer output can misidentify the
compound exiting the chromatographic column. This is a particular problem when the
GC and MS are set up for a particular analysis and, without changing the instrument
parameters, it is used for a very different analysis [9, 16].
10.7.7. Electroanalytical Analysis
Electroanalytical methods use an electric current (i.e., amperage and voltage) or a change
in potential to produce an analytical result. These methods are often based on the change
in a material that occurs upon the application of an electrical potential. They are
commonly used as the indication method in such classic wet chemistry methods as
titration processes. Electroanalytical methods use ion and electron movement and phase
interfaces of electrodes and ions to obtain information about the type and concentration
of organic or inorganic material present in a solution.
The two main electroanalytical forms are transformation of electrical energy into
chemical energy (i.e., electrolysis with current flow across a cell) and the transformation
of chemical energy into electrical energy using a galvanic cell. As an example,
electrolysis takes place when current is applied to a copper rod immersed in a copper
solution and is connected with a second half cell made up of a platinum rod immersed in
an acidic solution. Copper ions will take up electrons and be deposited as metal on the
cathode. A galvanic cell (wet cell) is made when of two half cells, each consisting of a
metal electrode immersed in its cationic solution, are connected to each other. The
measured potential of this cell is the difference between the individual potentials of the
electrodes in their solutions. If one cell is copper ions in a copper sulfate solution and the
other a zinc rod in a zinc sulfate solution, the connection between the solutions and rods
makes a galvanic cell. The reduction of copper ions to metallic copper at the copper rod
and the oxidation of the zinc metal to zinc ions at the zinc rod takes place. This combined
reaction generates a current.
Testing methods employing the principles of electroanalytical chemistry are mainly
limited to the two forms known as potentiometry and conductometry.
In potentiometry, the characterization of the amounts and types of the materials present
are determined by measuring the potential differences between a measuring or indicator
electrode and a reference electrode using a constant potential. This is the principle used to