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5940022ChemAnal.fm
Because it may not be feasible to determine the cause of the interference,
diluting the sample past the point of interference is often the most
economical and efficient means of getting the correct result. If it is not
possible to dilute out an interference without diluting out the parameter
to be measured, use a different method, such as a different chemistry or
an ion-selective electrode to measure the parameter.
Calculating percent recovery:
1. Measure the unknown sample concentration.
2. Calculate the theoretical concentration of the spiked sample using the
following formula:
Theoretical Concentration
=
Where:
C
= measured concentration of the unknown sample
u
V
= volume of the unknown sample
u
C
= concentration of the standard
s
V
= volume of the standard
s
3. Measure the spiked sample concentration.
4. Divide the spiked sample concentration by the theoretical concentration and
multiply by 100.
For example:
A sample was tested for manganese and the result was 4.5 mg/L. A separate
97-mL portion of the same sample was spiked with 3 mL of a 100 mg/L standard
solution of manganese. This spiked solution was tested again for manganese
using the same method. The result was 7.1 mg/L.
The theoretical concentration of the spiked sample is:
(
×
)
(
4.5 mg/L
97 mL
+
100 mg/L
------------------------------------------------------------------------------------------------------------
97 mL
+
3 mL
The percent spike recovery is:
7.1 mg/L
×
---------------------- -
100
=
96%
7.4 mg/L
USEPA Calculation
The USEPA requires a more stringent calculation for percent recovery. This
formula calculates the percent recovery only for the standard added to the
spiked sample and yields a lower value than the above calculation. A complete
explanation for the USEPA formula is in USEPA Publication SW-846. The USEPA
percent recovery formula is:
(
)
100 X s X u
–
%R
=
----------------------------------- -
K
Where:
X
= measured value of the spiked sample
s
X
= measured value for the unspiked sample, adjusted for the dilution of the spike volume
u
K = known value of the spike in the sample
(
×
)
(
×
)
C
V
+
C
V
u
u
s
s
------------------------------------------------------- -
V
+
V
u
s
×
)
3 mL
=
7.4 mg/L
Chemical Analysis
Chemical Analysis
Page 39
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