This gets even stranger...

Another HI 736 user indicated he was able to get far more consistent results by allowing the C2 sample to sit for 7 minutes prior to testing. While I still don't understand the mechanism, in addition to my regular testing, I followed up with a timed test conducted after allowing the cuvette to sit still for 7 minutes. The results were...

5/03/12 7:25AM 7 min - 04 ppb. Immediate follow up tests - 3, 1, 2, 6, 1
5/03/12 7:00AM Initial - 12 ppb. Immediate Follow up tests - 1, 0, 1, 3, 1
5/02/12 7:00AM Initial - 12 ppb. Immediate follow up tests - 3, 0, 0, 0, 0 (fresh GFO)
4/30/12 7:00AM Initial - 25 ppb. Immediate follow up tests - 3, 12, 10, 4
4/27/12 7:00AM Initial - 17 ppb. Immediate follow-up tests - 4, 2, 7, 0, 6
4/26/12 7:00AM Initial - 16 ppb. Immediate follow up tests - 0, 4, 6, 5, 4
4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3
4/24/12 7:00AM Initial - 29 ppb. Immediate follow up tests - 11, 9, 8, 12
4/23/12 7:00AM Initial - 43 ppb. Immediate follow up tests - 5, 8, 9, 13, 8

So my 7 minute results were much like those reported, although I'm still puzzled as to why. My C2 sample was about 25 minutes old when I conducted the 7 minute test - I wonder if the reagent in solution might be breaking down and altering the results? But even that does not explain the variance between the initial and follow up tests. Today's tests were also made using a new box of reagents (lot HO13). Assuming the reagent is completely dissolved (which I believe it is) the results are hard to explain.

I would love to trust the follow up and 7 minute values as they provide more favorable results - It sure would be nice if Hanna provided some type of input.