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Spiked Recovery Test Method For Interferences
The Spiked Recovery Test Method (also referred to as Standard Additions Method) is used to verify that
LEADQuick
™
gives accurate results for your water sample. If you find an unexplained difference in results
when compared to another test method it is reasonable that you should resolve the issue. A good way
to resolve the issue is by a technique commonly used by analytical chemists every day. This technique
is often referred to as Spiked Recovery Test Method for Interferences. The following information explains
how to perform the test method. You will need a Lead standard solution. The idea behind this method is
as follows:
1. Add a known amount or concentration of Lead standard solution to the questionable sample. This is
now the “spiked sample.” It is recommended to add a Standard Solution amount that is at least equivalent
to three times the minimum detectable limit of the test (10 µg/L or ppb).
2. Test the spiked and un-spiked (original) samples using the same reagents, instrument and technique
or test method. The spiked sample should show an increase equal to the amount of standard added. The
value obtained is called the
Recovery. Ideally the % recovery is 100%. Results are acceptable if the %
recovery is +/- 10%. The formula for calculating percent Recovery is below.
3. If the percent recovery is not in the acceptable range there may be interferences. You can consider
diluting the sample with lead free water past the point of interference, within the detection limit of the test
kit. You can also consider calculating the actual lead in the sample (see below) as long as the %Recovery
is above a reasonable level such as 40%.The percent recovery formula is as follows:
% Recovery = 100(cs-cu)
K
Where:
cs = concentration found through testing of the spiked sample
cu = concentration found through testing of the un-spiked sample
(NOTE: result should be adjusted for the dilution of the spike volume if volume change is more then 5%)
K = concentration of the spike added to the sample
Example 1: An un-spiked questionable sample measures 10 ppb Lead. A separate 10mL portion of the
questionable sample was spiked by adding 20 µL of a 10 ppm Lead standard solution. This is the equiva-
lent of adding 20 ppb Lead to the water sample. The spiked solution was measured by the same method
as the original sample. The Spiked result was 28 ppb (Cs)
cs = 28 ppb
cu = 10 ppb
k = 20 ppb
% Recovery = 100(28-10) = 90% (Recovery result acceptable)
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Example 2: In another water sample using a similar spiked method as in Example 1 the results were
cs = 30 ppb
cu = 18 ppb
k = 20 ppb
% Recovery = 100(30-18) = 60% (Recovery result unacceptable)
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Calculating the Lead: In this example the percent recovery value is low and suggests that, with this
test, the water sample gives lead results that are 60% of the actual concentration. To calculate the
concentration of lead in this sample divide the expected recovery (100%) by the observed recovery
(60%) to get the interference correction factor (100% / 60% = 1.67). Multiply the interference correction
factor by the un-spiked sample result (cu) for the actual concentration of lead in the sample (1.67 x 18
ppb = 30 ppb).
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