Page 129 - Modern Analytical Chemistry

1400-CH05 9/8/99 3:59 PM Page 112

112 Modern Analytical Chemistry

EXAMPLE .4
5
A third spectrophotometric method for the quantitative determination of the
concentration of Pb 2+ in blood yields an S samp of 0.193 for a 1.00-mL sample of
blood that has been diluted to 5.00 mL. A second 1.00-mL sample is spiked
with 1.00 mL of a 1560-ppb Pb 2+ standard and diluted to 5.00 mL, yielding an
S spike of 0.419. Determine the concentration of Pb 2+ in the original sample of
blood.
SOLUTION
The concentration of Pb 2+ in the original sample of blood can be determined
by making appropriate substitutions into equation 5.7 and solving for C A . Note
that all volumes must be in the same units, thus V s is converted from 1.00 mL to
–3
1.00 ´10 mL.
.
0 193 0 419
.
=
3 –
æ 100 mL ö æ 100 mL ö æ 100 ´ 10 mL ö
.
.
.
C A ç ÷ C A ç ÷ + 1560 ppb ç ÷
.
.
è 500 mL ø è 500 mL ø è 5.00 mL ø
.
.
0 193 0 419
=
. 0 200C A + 0 312 ppb
.
.

0 200C A

.
.
.
0 0386C A + 0 0602 ppb = 0 0838C A

.
.
0 0452C A = 0 0602 ppb
C A =133. ppb
A
2+
Thus, the concentration of Pb in the original sample of blood is 1.33 ppb.
It also is possible to make a standard addition directly to the sample after mea-
suring S samp (Figure 5.6). In this case, the final volume after the standard addition is
V o + V s and equations 5.5–5.7 become
S samp = kC A
æ V o V s ö
S spike = k C A + C S ÷ 5.8
ç
è V o + V s V o + V s ø
Add V of C S
S
V o V o
Figure 5.6
Illustration showing an alternative form of
the method of standard additions. In this Total concentration Total concentration
case a sample containing the analyte is of analyte of analyte
spiked with a known volume of a standard V V
solution of analyte without further diluting C A C A o + C S S
o
o
either the sample or the spiked sample. V + V S V + V S