Using a GCMS analysis drug money extract in a 50 mL methanol

Using a GC/MS analysis, drug money extract in a 5.0 mL methanol containing cocaine gave a signal of 45644 area counts. After 200 microliters of 500.0 ppm cocaine were added to the 5.0 mL methanolic extract, this spiked solution sample gave a signal a 57055 area count. (i) Find the concentration of cocaine in the original extract in ppm. (ii) Is this technique external standard, standard addition, or internal standard method?

Solution

Gas Chromatography/ Mass Spectrometry (GC/MS) analysis of 5ml or 5000 microliters gave a signal of 45644 area counts say having cocaine of x ppm.

Adding 200 microliters of 500.0 ppm cocaine to x ppm initial concentration the new concentration becomes 5000+200=5200 microliters gave a signal a 57055 area count.

So xppm in 5000 microliters=45644 area counts
or x ppm in 5200 microliters=5000/5200*45644=43888.5 area counts
x+500.0 ppm in 5200 microliters =57055 area count.
or 500ppm=57055 area count-43888.5 area counts=13166.5 area counts
or 1 area count=500/13166.5ppm

or 45644 area counts=500/13166.5*45644 =1733.34 particles per million concentration of cocaine

So 1733.34 particles per million concentration of cocaine in new concentration

i) In original extract it was 1733.34*5200/5000=1802.67 ppm

ii) This technique is internal standard method.