I would like to figure out our labs method detection limit for phosphorus (sm 4500 P B, E ascorbic acid).  We use a Spec 20 to read the absorbance.  My question is what value do I use for my lower instrumentation limit?  My understanding is that the concentrations are only linear from 0.20 to 0.70.  I obviously can't use 0.20 as my lower limit since 5X this limit would give me 1.0 mg/l.

Thanks,

Don

Thanks James,

Do I develop a standard curve around this low detection limit of 0.02 mg/L-P?

Don

What you need to do is develop a curve ranging from about 0.005 mg/L to 1.5 mg/L.  Then you read each of these points and graph your responses.  You will need to visually inspect the resultant curve and find the range where your instrument, procedure, analyst, etc. produce a linear response.  You should try to shoot for having a detection limit of whatever the end user will require.  If you are a commercial lab, you need to know the detection limits required by your clients.  If you are doing this as a lab in a plant, you need to know the detection limit on your permits.  Once you have established your curve you can determine your MDL.  Do your seven replicates at the previously determined detection limit and apply the necessary statistical analysis.  Your final MDL should be less than your detection limit.  The 5 times you are referring to in your original post is a 5 times less than.  I believe the actual requirement is less than half the detection limit but greater than one tenth the detection limit.  If your detection limit is 0.2 mg/L, (Somewhat high in my opinion.  You really ought to be able to get down to 0.05 mg/L) than your MDL should end up 0.02-0.1.  If your calculated MDL is consistently lower than one tenth your detection limit, that tells you that your detection limit is too high.

David