First can I just point out that there are different reasons for spiking the sample before digestions versus spiking after digestion - so it may also depend
a little on what your goal is.
Spiking the sample before digestion is a test you can use to check for potential loss of analyte or contamination during digestion - as well as checking the effect of the sample matrix on analyte recoveries. This is commonly referred to as a Matrix Spike or Laboratory Fortified Sample Matrix.
The post digestion spike will only check the effect of the sample matrix on analyte recoveries. So this type of spiking will help confirm if there are interferences present which may require correction techniques or potentially, use of alternate masses for quantification.
If you refer to the US EPA requirements (using Method 200.8 as an example), the recommendation is that the pre-digest spike should have a concentration ranging from 40-100 μg/L for each analyte, except selenium and mercury (in the sample).
A post digest spike would normally be at a concentration that is 20-100 times the MDL.
Hope that helps you,
Thanks that does explain a little of where the trainer was coming from. We are following the USP's specifications and have historically done pre digestion spiking and this is most likely how we will continue at least for validation processes. Do you have any opinion on my spiking and dilution question?
For a pre-digest spike, the logic you outlined in your original message makes sense. If the sample prep. involves a 100x dilution, you need to allow for this when deciding what concentration to add to the sample.
Ideally, you want the spike to be a similar concentration to the expected analyte concentration in your sample. If you add a much larger spike
concentration, this may mask the matrix effects. And using a spike that is an order of magnitude different (smaller) to the sample analyte concentration may mean you do not see or recover the spike added concentration. So your suggestion to spike at the spec (around 30 ppb) is reasonable.
As noted in the earlier reply, the US EPA methodology does also recommend a spiking concentration of 40-100 μg/L for each analyte, except selenium and mercury. That is also a good guide (a little lower than your planned spike concentration of the spec of 30ppb).
In terms of the maths, you will need to account for the 100 fold dilution applied during preparation to both the spike and the sample - since you will calculate the recovery with respect to the original spike added concentration (before dilution - or your spec. of 30 ppb).
Hope that helps you,