Page 222 - Modern Analytical Chemistry

1400-CH07 9/8/99 4:03 PM Page 205

Chapter 7 Obtaining and Preparing Samples for Analysis 205

é ( . )( )185 1 ù
E =(.0 9891 - )1 + ê ´.0 032 ú
7
ë () û

0
=-.0109 ) +( .0085 )
(
0
2
4
=-.0024, or - . %
0
0
Note that a negative determinate error introduced by failing to recover all
the analyte is partially offset by a positive determinate error due to a failure
to remove all the interferent.
(c) To determine the maximum allowed recovery for Zn, we make appropriate
substitutions into equation 7.17
( é 185 1) ù
.
)(
0 0010 = ( 1 000 - 1) + ê ´ R Zn ú
.
.
ë 7 () û
and solve for R Zn , obtaining a recovery of 0.0038, or 0.38%. Thus, at least
99.62% of the Zn must be removed by the separation.

7 F Classifying Separation Techniques

An analyte and an interferent can be separated if there is a significant difference in
at least one of their chemical or physical properties. Table 7.4 provides a partial list
of several separation techniques, classified by the chemical or physical property that
is exploited.

7 F.1 Separations Based on Size

The simplest physical property that can be exploited in a
7 4
separation is size. The separation is accomplished using a Table . Classification of Separation
porous medium through which only the analyte or inter- Techniques
ferent can pass. Filtration, in which gravity, suction, or
Basis of Separation Separation Technique
pressure is used to pass a sample through a porous filter is
the most commonly encountered separation technique size filtration
based on size. dialysis
Particulate interferents can be separated from dis- size-exclusion chromatography
solved analytes by filtration, using a filter whose pore mass and density centrifugation
size retains the interferent. This separation technique is complex formation masking
important in the analysis of many natural waters, for change in physical state distillation
sublimation
which the presence of suspended solids may interfere in
recrystallization
the analysis. Filtration also can be used to isolate analytes
change in chemical state precipitation
present as solid particulates from dissolved ions in the
ion exchange
sample matrix. For example, this is a necessary step in electrodeposition
gravimetry, in which the analyte is isolated as a precipi- volatilization
tate. A more detailed description of the types of available partitioning between phases extraction
filters is found in the discussion of precipitation chromatography
gravimetry and particulate gravimetry in Chapter 8.