Page 280 - Modern Analytical Chemistry

1400-CH08 9/9/99 2:18 PM Page 263

Chapter 8 Gravimetric Methods of Analysis 263

8D.1 Theory and Practice
Two approaches have been used to separate the analyte from its matrix in particu-
late gravimetry. The most common approach is filtration, in which solid particu-
lates are separated from their gas, liquid, or solid matrix. A second approach uses a
liquid-phase or solid-phase extraction.

Filtration Liquid samples are filtered by pulling the liquid through an appropriate
filtering medium, either by gravity or by applying suction from a vacuum pump or
aspirator. The choice of filtering medium is dictated primarily by the size of the
solid particles and the sample’s matrix. Filters are constructed from a variety of ma-
terials, including cellulose fibers, glass fibers, cellulose nitrate, and polytetrafluo-
roethylene (PTFE). Particle retention depends on the size of the filter’s pores. Cellu-
lose fiber filters, commonly referred to as filter paper, range in pore size from 30 mm
to 2–3 mm. Glass fiber filters, constructed from chemically inert borosilicate glass,
range in pore size from 2.5 mm to 0.3 mm. Membrane filters, which are made from a
variety of materials, including cellulose nitrate and PTFE, are available with pore
sizes from 5.0 mm to 0.1 mm.
Solid aerosol particulates in gas samples are filtered using either a single or
multiple stage. In a single-stage system the gas is passed through a single filter, re-
taining particles larger than the filter’s pore size. When sampling a gas line, the filter A A A
is placed directly in line. Atmospheric gases are sampled with a high-volume sam-
pler that uses a vacuum pump to pull air through the filter at a rate of approxi-
3
mately 75 m /h. In either case, the filtering medium used for liquid samples also can (a)
be used for gas samples. In a multiple-stage system, a series of filtering units is used
to separate the particles by size. A A A
Solid samples are separated by particle size using one or more sieves. By select-
ing several sieves of different mesh size, particulates with a narrow size range can be
isolated from the solid matrix. Sieves are available in a variety of mesh sizes, ranging (b)
from approximately 25 mm to 40 mm.
A A A
Extraction Filtering limits particulate gravimetry to solid particulate analytes that L L L
are easily separated from their matrix. Particulate gravimetry can be extended to the
analysis of gas-phase analytes, solutes, and poorly filterable solids if the analyte can
be extracted from its matrix with a suitable solvent. After extraction, the solvent can
be evaporated and the mass of the extracted analyte determined. Alternatively, the (c)
analyte can be determined indirectly by measuring the change in a sample’s mass
after extracting the analyte. Solid-phase extractions, such as those described in A A A
Chapter 7, also may be used.
More recently, methods for particulate gravimetry have been developed in
which the analyte is separated by adsorption onto a metal surface, by absorption
into a thin polymer or chemical film coated on a solid support, or by chemically
binding to a suitable receptor covalently bound to a solid support (Figure 8.10). Ad-
sorption, absorption, and binding occur at the interface between the metal surface, (d)
the thin film, or the receptor, and the solution containing the analyte. Conse- Figure 8.10
quently, the amount of analyte extracted is minuscule, and the resulting change in Four possible mechanisms for solid-state
mass is too small to detect with a conventional balance. This problem is overcome extraction: (a) adsorption onto a solid
substrate; (b) absorption into a thin polymer
by using a quartz crystal microbalance as a support.
or chemical film coated on a solid substrate;
The measurement of mass using a quartz crystal microbalance is based on the (c) metal–ligand complexation in which the
10
piezoelectric effect. When a piezoelectric material, such as a quartz crystal, experi- ligand is covalently bound to the solid
substrate; and (d) antibody–antigen binding
ences a mechanical stress, it generates an electrical potential whose magnitude is
in which the receptor is covalently bound to
proportional to the applied stress. Conversely, when an alternating electrical field is the solid substrate.