I am trying to measure creatinine in fluids by using an enzymatic (creatinine) test. I try to determine the concentration by the difference in absorbance before and after 5 minutes after a reagent is added. Have a look:
My problem is that the measurements ‘jump’ all over the place, but seem centered at the right value. My absorbance is on the order of 0.0010 (the photometer provides 4 decimal places resolution) for my sample material for a concentration of say 60 umol/l. That means a change in the least significant digit in the absorbance gives an error of minimum 10%. I think this could be causing my fluctuating results, because simply rotating the cuvette changes the absorbance by at least 0.0001. I also see that the temperature in the room change 2 degrees during the day. Perhaps it could influence the speed of the reaction.
What is the right way to troubleshoot a situation like this to find out why my measurements are unstable?