Internal standard

[Juan Pedro]

Hi everybody!

I am trying to measure OC pesticides with CG-ECD and want to use PCDBF as internal standard. in human biological samples.

When I do the calibration, it is impossible to use "clean" samples, so I have to use a "contaminated" human sample. How can I substract the effect of the OC burden when I extrapolate the results with the equation?

Thanks a lot!

[Don_Hilton]

Obtain your regression equation. You can calculate the value at which the calibration curve (assuming you use a linear fit) crosses the X axis (wiht concentration on the X axis and response on the Y axis). Solve your regression equation for a response value of 0. The concentration you get, which will be a negative number, is the concentration of the analye (or an interference, but measured as the analyte) in the matrix. Look up "Standard addition methods" for more details.

[Juan Pedro]

Thank you very much for your answer!

So, how do I compensate for this concentration of the blank samples? I mean that if I use this regression ecuation for calculating the concentration of the samples, I would be committing an error, because it was made with the concentrations that were in the matrix + the standards that I added.

Thank you!

[Don_Hilton]

Since you know the concentration of each material you added and the concentration of the analyte in the blank, you can add the two and recompute the calibration. It works like this: You add standards so the theoretical concentrations for your curve are 0.1, 0.5, 1, 5, etc. After you compute the background level of analyte, you find that the the concentration of analyte is 0.3 in the matirix. So, you know that your concentrations for builllding a curve were really (0.1 + 0.3). (0.5+0.3), (1+0.3), (5+0.3), etc or 0.4, 0.8 1.3, 5.3, etc.

The slope of the lnie remains the same, but it now passes through the origin of the plot.

There are a number assumptions made in doing this. These include that youre response forthe compound is linear and that the signal you are calling analyte in the matrix is, indeed, the analyte.

[fsistere]

Hi
If you want do the standard adition with a only one point you can do this:
Sample1: Add the Internal standard
Sample 2: Add standard of OC at kown concentration and Internal standard (same concentration than 1)

Analize sample 1 and 2 and measure the areas.


Cs = Cadd * ((A1/Ais1)/((A2/Ais2)-(A1/Ais1))) where:

Cs .- OC Concentration in your sample
Cadd.- OC concentration added in sample 2
A1 and A2 .- OC area in sample 1 and 2
Ais1 and Ais2.- Internal standard area in sample 1 and 2.

Is better do the analysis like said Don_Hilton but you have to work more.
Like this you work less but you quantify only with one point . Is important add a OC concentration similar than you are waiting for in the sample.

[Juan Pedro]

Thanks a lot for your responses, they have been very very useful for me!!!!

[mun]

I would like to know,
Can we validate a method by using standard addition method?
(especially the recovery calculation, i spike with standard in 3 different concentration, get a four point linear curve, and from the curve calculate concentration of spiked and non-spiked sample, calculate the recovery of each spiked sample as the percentage of the ratio of calulated concentration from curve to the summation of the calculated concentration of sample from curve and concentration which i spike)
If the concentration of analyte in sample is too high, how the standard addition method can be used, as i don't think it is a good idea to spike sample with high concentration standard?

Thank you.