A researcher was attempting to quantify the amount of DDT (dichlorodiphenyltrich

Question

A researcher was attempting to quantify the amount of DDT (dichlorodiphenyltrichloroethane) in spinach with gas using a chloroform internal standard. To begin. the researcher examined a sample containing 8.55 mgfi DDT standard and 1.30 mg/L chloroform as the internal standard, producing peak areas of 4835 and 9615, respectively Then, the researcher collected 12.00 g of spinach, homogenized the sample, and extracted the DOT using an established method (assume 100% extraction), producing a 2.18 ml volume of unknown sample. The researcher then prepared a sample that contained 1.00 mL of the unknown sample and 1.75 mL of 11.41 mg/L chloroform, which was diluted to a final volume of 25.00 mL. The sample was analzed using GCMs. producing peak areas of 6991 and 11237 for the unknown and chloroform, respectively. Calculate the DDT concentration in the spinach sample. Express the final answer as milligrams DDT per gram of spinach.

Explanation / Answer

Since the internal std is used in GC, first we can calculate the Relative Response Factor for DDT std samples

RRF (DDT)= Response Factor of DDT/ Response factor of IS = (Peak Area DDT / Conc. DDT) / (Peak Area IS / Conc. IS)

= (4835/8.55) / (9615/1.3) = 0.0765

Now we know peak area of unknown samples. Then RRF= (Peak area of unknown/ conc. of unknown) / (Peak Area IS / Conc. IS)

From this conc. of unknown = (Peak area of unknown x Conc. IS)/ (Peak Area IS x RRF)

= (6991 x 0.0199 mg/mL)/ (11237 x 0.0765)

= 0.162 mg/ml , in 2.8 mL= 0.4536mg

ie, 0.4536 mg in 12 g spinach sample, then 0.0378 mg DDT/ g of spinach

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