An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 264

in the test solution except the component of interest. These blanks are used to zero the
instrument before readings are made. The second requirement is a standard or calibration
curve. These are prepared by making a series of solutions containing different known
amounts of the component of interest. These are then measured and a graph of the
amount of component present versus the instrument response is prepared. Using this
graph the concentration of analyte in an unknown can be determined. Statistically this
standard curve should have a regression coefficient r 2 of 0.99 or 99%. This shows that
the graph is nearly a straight line and that there is a good relationship between the actual
amount of material present and the instrument reading [7].
10.7.3.5. Mass Spectroscopy
Mass spectroscopy is different from other Spectroscopy techniques in that it measures the
mass of molecular ions or fragments produced by ionizing the sample. The fragments
produced from a compound are unique, and can thus be used for its identification. A
sample of a compound is introduced into the mass spectrometer, and the ions formed are
separated in the analyzer using either a magnetic or an electrical field. The separated ions
and fragments are recorded and their unique fragmentation pattern is used for
identification. When this same fragmentation pattern is obtained from an unknown in a
mass spectrum, it shows that this compound is present. Because mass spectroscopy is
best used to determine the composition of a pure single molecule, it is often used after
chromatographic separation of a mixture into it component parts [8, 9].
10.7.4. Separation Methods
Initial physical and chemical separation may be necessary as adjuncts to final separation
and determination methods. Two reasons for separation are:
1. If the selectivity of the determination method is insufficient and interfering impurities
must be separated out and the sample concentrated before analysis.
2. If as many substances as possible are to be qualified and quantified in a single
procedure. In this case, the sample must be separated into its respective components
(e.g., aqueous/nonaqueous, solid/liquid).
Separation methods can be initially divided into two groups, depending on whether or not
they act on the basis of analyte transformation. Transformation includes the classic
gravimetry, electrogravimetry, which is when a substance is transformed with the aid of
an electric current, and volatization of a substance in an aqueous phase.
Separation without substance transformation can be accomplished by partitioning on
the basis of partial charges, masses, sizes, or shapes, and different vapor pressures of the
compounds of interest. Separations caused by partitioning between two immiscible
phases are also very important. Depending on the types of components and solvents
involved one differentiates adsorption or desorption processes (solid-liquid or solid-gas),
liquid-liquid partitioning (usually as liquid extraction), ion exchange (between a solid ion
exchange matrix with a substitutable charge group and a liquid phase), and liquid—solid
extractions.