Environmental Engineering Reference
In-Depth Information
methods used in environmental analysis. The method numbers of both EPA and
APHA are also given so the reader can refer to the references for details.
6.2.2 Common Laboratory Techniques
The basic laboratory techniques introduced here are not only related to the wet
chemical methods shown in Table 6.3, but also the basics to all phases of
environmental lab work. Several key points or even pitfalls for each of these
techniques are briefly introduced. Readers can locate these materials in traditional
analytical chemistry topics. Several sample preparation techniques, such as
extraction and digestion, will be further described in Chapter 7.
Filtration
Filtration is used to remove (collect) materials from a liquid or air matrix in which
the materials are suspended. The filter media can be filter papers of various types or
under gravity filtration or vacuum (suction) filtration using a water aspirator or a
vacuum pump. Filtration is used for the analysis of suspended solids in water, or the
analysis of dissolved phase contaminant after the removal of suspended solids. In air
sampling, the filter medium is connected with a sampling pump to collect atmospheric
particulates. For quantitative analysis of solids, use ashless quantitative filter papers
such as Whatman and Schleicher & Schuell. To remove (collect) small particles with
pore sizes of 0.025 to 14
m
m, use only membrane filter such as Millipore and Gelman.
The 0.22
m is themost common in removing bacteria or collecting colloidal particles.
The membrane filters have a thin polymeric structure with extraordinarily fine pores.
When vacuum filtration is used, use hardened-grade papers and thick wall flasks.
The hardened-grade papers have great wet strength to sustain vacuum.
m
Centrifugation
Acentrifuge is used toseparate solidsor liquidparticles of different densities or sizesby
rotating them in a centrifuge tube. If colloidal particles or macromolecules (protein,
nucleic acids) are to be separated, an ultracentrifuge is used at a higher speed. The low
speed centrifuge can run up to 5, 000 revolutions per minute (rpm) and the high speed
ultracentrifuge can run up to 100,000 rpm. Another form of common centrifuge is the
microcentrifuge used with 0.5 or 1.5 mL disposable plastic tubes, which can generate
10,000 to13,000 rpm. Followingare thecommonprecautions: (a)Alwaysusebalanced
pairs of centrifuge tubes. Balancing is extremely important to avoid excessive heating
and vibrations or even damaging the centrifuge. Turn off immediately if vibrations or
noise occur. (b)Never fill centrifuge tubes above themaximumthat is recommendedby
the manufacturer. Use only centrifuge tubes with protective rubber cushions. The reg-
ular test tubeswill not sustain centrifugal force andwill break. (c) Fully secure the rotor
and rotor seal before operation. Rotors on high-speed centrifuge or ultracentrifuge
units are subject to powerful mechanical stress that can result in rotor failure. In the
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