Question

A researcher was attemping to quantify the amount of DDT (dichlorodiphenyltrichloroethane) in spinach with gas chromotography using a chloroform internal standard. To begin, the researcher examined a sample containing 5.05 mg/L DDT standard and 2.10 mg/L chloroform as the internal standard, producing peak areas of 4997 and 9697, respectively. Then, the researcher collected 10.52 g of spinach, homogenized the sample, and extracted the DDT using an established method (assume 100% extraction), producing a 2.48 mL volume of unknown sample. The researcher then prepared a sample that contained 1.00 mL of the unknown sample and 1.25 mL of 10.29 mg/L chloroform, which was diluted to a final volume of 25.00 mL. The sample was analzed using GCMS, producing peak areas of 6277 and 14795 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.

Answer 1

Analysis of DDT in spinach sample

concentration of CHCl3 standard = 2.10 mg/L

concentration of known solution of DDT = 5.05 mg/L

peak area for CHCl3 = 9697

peak area for DDT = 4997

So,

response factor (F) = (area/concentration)for DDT/(area/concentration)for CHCl3

feeding values from above,

F = (9697/2.10)/(4997/5.05) = 4.7

Analysis of spinach sample

concentration of standard CHCl3 in diluted spinach sample = 10.29 mg/L x 1.25 ml/25 ml = 0.5145 mg/L

peak area for unknown = 6277

peak area for standard CHCl3 = 14795

So,

4.7 = (6277/concentration of DDT in unknown)/(14795/0.5145)

concentration of DDT in unknown (25 ml dilute) = 0.0464 mg/L

concentration of DDT in 1 ml unknown = 25 x 0.0464 = 1.16 mg

concentration of DDT in 2.48 ml unknown = 1.16 x 2.48 = 2.877 mg

So,

DDT concentration in unknown spinach sample = 2.877 mg/10.52 g = 0.2735