Chapter 5 - Analytical Sciences Digital Library

164 Analytical Chemistry 2.0
S
= k C
samp A A
V
V
o
std
S k C C
spike
=
Af A
+ V V
std p
+ V + V
5.10
o
std
o
std
S
samp
C
A
=
C
A
V
o
V
o
+ V
std
S
spike
+ C
std
V
o
V
std
+ V
std
5.11
Example 5.4
A fourth spectrophotometric method for the quantitative analysis of Pb 2+
in blood yields an S samp of 0.712 for a 5.00 mL sample of blood. After spiking
the blood sample with 5.00 mL of a 1560-ppb Pb 2+ external standard,
an S spike of 1.546 is measured. What is the concentration of Pb 2+ in the
original sample of blood.
V o + V std = 5.00 mL + 5.00×10 –3 mL
= 5.005 mL
So l u t i o n
To determine the concentration of Pb 2+ in the original sample of blood, we
make appropriate substitutions into equation 5.11 and solve for C A .
0. 712 1.
546
=
3
C
A 500 . mL
500 . × 10
− mL
C
+ 1560
ppb
A
5.
005 mL
5.
005 mL
0. 712 1.
546
=
C 0. 9990C
+ 1.
558 ppb
A A
0.7113C A + 1.109 ppb = 1.546C A
C A = 1.33 ppb
The concentration of Pb 2+ in the original sample of blood is 1.33 ppb.
Mu l t i p l e St a n d a r d Ad d i t i o ns
We can adapt the single-point standard addition into a multiple-point
standard addition by preparing a series of samples containing increasing
amounts of the external standard. Figure 5.7 shows two ways to plot a
standard addition calibration curve based on equation 5.8. In Figure 5.7a
we plot S spike against the volume of the spikes, V std . If k A is constant, then
the calibration curve is a straight-line. It is easy to show that the x-intercept
is equivalent to –C A V o /C std .