Tap water is measured for arsenic and found to have a mean signal of 0.0068. Next, a small amount of Arsenic is spiked

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Tap water is measured for arsenic and found to have a mean
signal of 0.0068.
Next, a small amount of Arsenic is spiked into the tap water
such that the new value is 0.500 ppb higher than before. The
mean signal from this new low-level concentration is 0.0394,
with a standard deviation of 0.00490.
What is the concentration detection limit?
(Hint: estimate slope as ,
where is
the difference between the low level spike and the blank signals;
and is
the concentration difference after spiking with the analyte.