Chromatography Forum

How can I know, my sample have how much concentration of pesticides?
I do OC pesticide analysis by GC-ECD.
I do calibration curve (0.01,0.02,0.05,0.25 and 0.5 ppm levels).
I did not use internal standards.
For my sample,
I weigh 5 g of rice sample and do sample preparation by QuEChERS step by step.
I take final extract 4 ml and dry under flow of nitrogen.
And I redissolve with acetonitrile 500 uL and inject to GC 1 uL.
eg. The Totalchrom software show the adjusted amount,
0.014
so, how many concentration in my sample?
How can I calculate?
Please kindly help me.

I would strongly recommending use of internal standards or at least a couple surrogate standards to monitor losses due to Quechers, evaporation etc.

However, you have a calibration curve/plot which should be plotted Area vs. Concentration (µg/mL or PPM). You do a linear regression analysis and get an equation for a line y=mx+b or Area = Slope × Conc. + Y-intercept. Solve Equation for Conc. and input the Area of your sample result. You get Concentration back (µg/mL).

IMPORTANT NOTE:
This is the concentration in the evaporated sample at 500 µL (0.5 mL). Since you are probably more concerned with how much is in the Rice (sample of 5 g) you need to calculate the total mass of analyte in your 0.5 mL sample. You take the concentration you had calculated before (in µg/mL) and multiply by the final volume of 0.5 mL. You get "total µg extracted" as a result. You now take this value and divide it by the dry weight (if applicable) of your sample (sample weight × [1-fraction of water]) in grams. You should now get a value in units of µg/g which is also in PPM.

I usually prefer to stay away from the unit labels such as PPM or PPB as sometimes they can be rather ambiguous. It is a lot easier to see how to calculate a solution when you have all of the units right in front of you.

Hope this helps.

Thank you for your kind suggestions.