Environmental Engineering Reference
In-Depth Information
Chemicals are sometimes categorized by one of their properties or by their analytical
procedures (the so-called analytical definition). Examples include those loosely
defined compound categories based on the following:
1. Density: Heavy metals are elements of generally higher atomic weight with
specific gravities greater than five and especially those toxic to organisms
such as Cu, Zn, Pb, Cd, Ni, Hg, Cr and the metalloid (As)
2. Volatility: Volatile organic compounds (VOCs) are group of compounds that
vaporize at a relatively low (room) temperature. VOCs have boiling points
below 200 C. Examples of VOCs include trichloroethane, trichloethylene,
and BTEXs. Semi-volatile organic compounds (SVOCs) evaporate slowly at
normal room temperature and are operationally defined as a group of
organic compounds that are solvent-extractable and can be determined by
chromatography including phenols, phthalates, PAHs, and PCBs. Volatility
as defined by Henry's law constant will be introduced in Chapter 7.
3. Extractability: Extraction with methylene chloride initially under basic
conditions to isolate the base/neutral fraction (B/N) and then acid condition
to obtain the acidic fraction (mostly phenolic compounds).
2.1.3 Analytical Precision, Accuracy, and Recovery
Accuracy is the degree of agreement of a measured value with the true or expected
value. Accuracy is measured and expressed as % recovery and calculated according to:
% Recovery ¼ Analytical value100
=
True value
ð2
:
The true value (concentration) is rarely known for environmental samples. Thus,
accuracy is typically determined by spiking a sample with a known quantity of a
standard:
Spiked sample value Sample value
Spiked Value
% Recovery on spike ¼
100
ð2
:
An accurate test method should achieve a percentage recovery close to 100%. To
determine percentage recovery, the amount of spiking chemical should be close to
(0.52 times) the analytical concentration. In no case should the amount spiked
exceed three times the concentration of the analyte.
Precision is the degree of mutual agreement among individual measurements
ðx 1 ;
x n Þ as the result of repeated applications under the same condition.
Precision measures the variation among measurements and may be expressed in
different terms. The first term is standard deviation (s), which is defined as follows
for a finite set of analytical data (generally n
x 2 ; ...
<
30):
s
ð x i x Þ
2
s ¼
ð2
:
n1
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