Spectroil MC-W User

JC,

I stopped played with these toys approx. 20 yrs ago, but as far as I remember, not all elements’ concentrations when rerun diluted will mathematically correctly agree with the straight run concentrations; especially not if strong interferences are present that cause false-positive or false-negative results.

It is possible that Cu may have caused false-positive reading on Na. However, if you additionally made 1/2 and 1/10 dilutions and run them, you would have more info to better assess the degree of the interferences and report correct results. Additional runs of spiked straight and diluted samples with Na and Cu would help you reveal if there was any interference with Na result or how much it affected the results.

Also, it would be good to know what concentrations of Cu and Na you used for calibration, and what the detection limits and max acceptable reporting limit for those two elements are, after which you need to make dilutions.

I don’t think it is a good idea to dilute your sample with any oil as long as it does not contain Cu. Dilutions should be made with solvents or oils standardized for this purpose.

I wish you noted other trace metals in this sample and how their straight run results agreed with results of the 1/5 dilution.

Another aspect to consider - I assume your spectrometer is using a rotating disk electrode - the oil film produced on the rotating disk surface is strongly affected by the sample's composition,viscosity, other properties. So if the oil you are diluting with doesn't match the composition and viscosity of the used oil, then the determined concentration x the dilution factor won't equal the undiluted sample.
Do you know the linearity range for Cu on this spectrometer? If you are outside the linear range, further discussions are not really useful.

Not sure how similar the MC-W is the the Spec M or 100 - but : have you tried changing the crossover values for the copper channels i.e. using either the CU low (0-250ppm) or hi (250-900ppm) channel exclusively? And seeing which one gets more or less interference.