my analyte conc. in standard is not stable why ?

[Ronaldo]

Hi,

I used to prepare my analyte in water only ( its solubility in water is 90 mg/L) and using methanol water as mobile phase . Actually to prepare this analyte in water i need to wait many days to stabilise then i filter it then doing dilution to make stock solution 40 ppm . From this stock , the 1 ppm was prepared succesfully .The question is

in the first week , second week also i measure it by hplc and it gives me 1ppm but i found this 1ppm in water is untrustable after 2 monthes because i measure in the two weeks was ok and rsd is less than 1 .But now after two months the variation in its measurement is very high ( rsd 50 or more )

i didnt see any degredation product into the chromatogram even after one hour later , therfore we can tell that the analyte is breakdown .I

i filter 1ppm again and again but i still have not constant concentration

Could you please help

[Don_Hilton]

you have not given us much information. Something is changing in the time that your solution stabilizes. It could be the analyte. The fact that you do not see degredation products does not mean that changin there are none.

If you can tell use somethign about the analyte and your analysis, perhaps that information will help to give a more focused answer.

[danko]

In addition to Don’s highly relevant comments, I would focus the attention on the assumed analyte concentration. I wonder how one can assume that the standard solution contains 40 or 1 or whatever ppm, when filtering solids/ non dissolved compound.

Best Regards