HOW TO CHOOSE APPROPRATE CONCENTRION FOR INTERNAL STANDARD?

[Cathy001]

what factors we need consider? such as calibration range? How?

Thanks very much.

[Peter Apps]

Hi Cathy

All other things being equal, and without knowing anything alse about what you are analysing; the concentration of the internal standard needs to be close to the concentration of the analyte.

Peter

[Cathy001]

[quote="Peter Apps"]Hi Cathy

All other things being equal, and without knowing anything alse about what you are analysing; the concentration of the internal standard needs to be close to the concentration of the analyte.

Peter[/quote]

Hi Peter

Thank you for your replying.
I want know the general rules for the ISTD concentration choosing. I am using GC/MS to test semi-volatile compounds. I need to change method from External standard to internal standard menthod. I was told that choosing the concentration close to calibration Level 1 to level 2(For example 5 levels of 2000, 1000, 500, 200, 100ppb, level 1=2000ppb).
Is there any response ratio range considerarion?

Thanks again. Have a good day.

[Peter Apps]

Hi Cathy

If you are planning to calculate the concentration of the analyte by comparing its peak area to the peak area from an internal standard of known concentration the response factors for analyte and standard have to be the same. Unless you use isotope labelled standards, with MS this requirement is not likely to be met.

If you are going to calibrate by running standards with a known concentration of analyte and a fixed concentration of standard, plotting peak area ratio against caoncentration of analyte the response factors are not important. I would select an internal standard concentration that gave a 1:1 peak area ratio for the analyte concentration in the middle of the calibration range.

Peter