LIMITATIONS OF CHEMICAL TESTING

Because of the limitations imposed by minimum detection level of a chemical analyte, it is impossible to ever determine with any degree of certainty if there are zero atoms of any analyte in a sample.  When you see in a test report of  some analyte, such as lead, listed as < A

Where A is a very low number, A is the MDL of lead, and a result listed thusly, can only reliably mean that the actual concentration of lead is less than A, not that it is A.  There indeed may be no lead in the sample at all, but the required test cannot accurately determine that. 

How difficult is it to determine what the concentration of an analyte is?  Imagine having 20 decks of cards, all with the same pattern on the back, but slightly different colors. Pick groups of cards that are approximately the same amount as  a full deck and sort them out by the colors of the backs.  At the end when they have all been sorted go back and determine how many of the selected packs have wrong cards in them.

Or imagine you have a huge pack of 1000000 hearts and one diamond.  If you had the patience to sit down and try to find the diamond in this pack of hearts multiple times, you would probably miss it at least once.  Assume that you ran an MDL study as outlined in the previous blog, and you determined that the MDL for finding the diamond was 2.   That means the result for testing one million hearts for diamonds would get reported as <2, whether the actual number was 2,1, or none. 

I hope after these two blog entries my readers, all two of you, have a better understanding of the limitations of chemical testing.