Hi,
I have a doubt about using an internal standard when running a calibration curve.
I am using SPME for my sample prep/extraction. When building my calibration curve, do I need to do the same for my internal standard, meaning do I make a calibration curve for the internal standard as well ?
What about a surrogate? I am getting these terms confused and since SPME the extraction and analysis process is all the same step I am not sure if I don't need a surrogate, or if I need to build a calibration curve for the surrogate and not the internal standard.
Any input would be great! Thanks!
Anamari
This would be similar to purge and trap when using internal standards and surrogates.
You don't actually build a calibration curve for an internal standard. The internal standard is spiked into each sample/calibration/blank at the exact same concentration. The calibration curve is then generated using the internal standard concentration as a constant, with the internal standard response as a variable which corrects for variations in extraction/analysis efficiency and applied to the response of the target analytes while the concentration of the target analyte is held constant.
RF=(AsxCis)/(AisxCs)
As=Peak area of analyte
Ais=Peak area of internal standard
Cs=Concentration of analyte
Cis=Concentration of internal standard
This generates a response factor which is then used to calculate the concentration of analyte in samples using the response of the internal standard, concentration of internal standard and response of analyte.
Calculate analyte and surrogate concentrations.
Cs=(AsxQisx1000)/(AisxRFxV)
where:
Cs = concentration of analyte or surrogate in μg/L in the water sample.
As = integrated abundance of the quantitation ion of the analyte in the sample.
Ais = integrated abundance of the quantitation ion of the internal standard in the sample.
Qis = total quantity (in micrograms) of internal standard added to the water sample.
V = original water sample volume in mL.
RF = mean response factor of analyte from the initial calibration.
Surrogates are normally used to evaluate extraction efficiency by comparing the known amount added versus the amount calculated from the analysis. This is more representative for actual extraction where the internal standard is not extracted unlike in a volatiles analysis where the internal standard is part of the extraction. It can still tell you a little about your extraction/analysis performance since you calculate the concentration instead of holding it as a constant. It will show if some conpounds are extracted better than others (eg the internal standard).
Internal standard works well for correcting variance in a method, so try it if you can.
