Stability of th calibration curve , what is the limit ?

[Ronaldo]

Hi ,

please help me here may god help you anywhere

i am using RP-HPLC isocratic method ( 65 % methanol water , flow : 1ml/min , injection volum always 20 microL)

six months ago , I prepared my two analytes on the concentration range of ( 0.001 - 100 ppm ) in methanol .My next job was to measure the concentration of my analyte on water ( its solubility on water is 90 mg/L and in organic solvents is 35000 mg/l).

my Questions are :

1- Is it acceptable to measure my analyte concentration on water by refering to the external calibration curve of methanol ( no difference between the peak area of my analyte in methanol or water nearly because if the peak area of 1 ppm in methanol is 16 000 in water will be 15 989 ).

I am asking this because my friend say (no you must measure your standard methanol every day because the condition of previous calibration is different form this time you measure your analyte in water !)
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My second quesion is very vital to me

It is suggested to use internal standard ( my second analyte ) and i am alreadly prepared my external calibration curve for the two analytes .How can i measure the concentration of my analyte based on internal standard ( but do i use the data of external standard to make the ratio againest my analyte concentration or what )

if there is book or papers help me here would be very gratefull

Thanks

[Don_Hilton]

You need to recalibrate a method to ensure that any changes in the instruments are accounted for. If you do not recalibrate your instrument daily, you need to run check standards to ensure the method does not need to be recalibrated. As far as the difference in condition of your measuring your calibratin and the standards - you have not given us much information to address that.


For information on internal standard calibrations see if this helps: http://www.wfu.edu/chemistry/courses/jo ... ration.ppt The example uses a different analytical technique, but once you have integrated peaks - the mathematics works the same way. I would expect a more complete discussion in an elementary analytical chemistry text or a text on instrumental methods of analysis. Mine are apparently in boxes somewhere - but it has been a few years since I took those classes...