Question

A researcher was attempting to quantify the amount of DDT (dichlorodiphenyltrichloroethane) in spinach with gas chromatography using a chloroform internal standard. To begin, the researcher examined a sample containing 6.00 mg/L DDT standard and 2.50 mg/L chloroform as the internal standard, producing peak areas of 5041 and 9537, respectively. Then, the researcher collected 10.85 g of spinach, homogenized the sample, and extracted the DDT using an established method (assume 100% extraction), producing a 3.32 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 10.45 mg/L chloroform, which was diluted to a final volume of 25.00 mL. The sample was analyzed using GCMS, producing peak areas of 7671 and 11805 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

Let’s calculate Relative Response Factor (RRF) F for DDT with respect to Chloroform from known data,

Formula,

F = (A_DDT/M_DDT) / (A_CHCl3/M_CHCl3) …….. …………… (1)

We have,

M_DDT = 6 mg/L, M_CHCl3 = 2.50 mg/L

A_DDT = 5041, A_CHCl3 = 9537

Let us put all these values in above eq. (1)

F = (5041/6)/(9537/2.50)

F = 0.2202

Now for Unknown DDT concentration,

A_DDT = 7671, A_CHCl3 = 11805

M_CHCl3 = 10.45 mg/L in 25 mL     M_DDT =?

F = 0.2203

Let us put all these values in eq. (1) again and calculate M_DDT=?

0.2202 = (7671/M_DDT) / (11805/10.45)

O.2202 = (7671/M_DDT) /(1129.6)

0.2202x1129.6 = (7671/M_DDT)

248.75 = (7671/M_DDT)

M_DDT = 7671/248.75

M_DDT = 30.84 mg/L in 25 mL

This DDT sample was made from 1mL sample of spinach extract.

Hence in 1 mL unknown sample of DDT concentration of spinach = 30.84/25 = 1.234 g

Hence in 3.32 mL = 3.32 x 1.234 = 4.1 mg.

This 3.32 mL extract was obtained from 10.85 g of spinach,,

Hence we can say ,

10.85 g spinach = 4.1 mg of DDT

Hence concentration of DDT in spinach (in mg DDT/g of spinach) = 4.1/10.85 = 0.377 mg

Ans: 0.377 mg DDT per gram of spinach.

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