1. Our gas chromatogram with electron capture detector (GC-ECD) can detect very

Question

1.     Our gas chromatogram with electron capture detector (GC-ECD) can detect very low concentrations of nitro containing compounds (like trinitrotoluene, TNT).  We can easily detect concentrations in the low ppb range.  Suppose we wanted to prepare a solution that is 12.00 ppb TNT for analysis using hexane for a solvent (density = 0.6458 g/mL).  The solution is prepared by first making a 250.00 mL stock solution; a 1.000 mL aliquot of the stock is diluted into a 250.00 mL volumetric, then 1.000 mL of this third solution is diluted to 250.00 mL to make the 12.00 ppb TNT solution.  What mass is required for the initial stock solution?

Explanation / Answer

consider ,

stock solution = solution 1

1st dilution = solution 2

2nd dilution = final solution = solution 3

so,

For solution 3

mass of solvent in 250 mL solution 3 = density*volume =  0.6458*250 = 161.45 grams.

[solution 3] is 12 ppb that is 12*10^-9 grams per 1 grams of solvent

so,

mass of TNT in sol. 3= 12*10^-9*161.45 = 0.0000019374 grams

For solution 2

1 mL of sol. 2 is diluted to form sol. 3

so,

Mass of TNT in 1 mL of sol. 2 = mass of TNT in sol. 3 = 0.0000019374 grams

so,

mass of TNT in 250 mL sol. 2 = 250*0.0000019374 = 0.00048435 grams

For solution 1

1 mL of sol. 1 is diluted to form sol. 2

so,

Mass of TNT in 1 mL of sol. 1 = mass of TNT in sol. 2 = 0.00048435 grams

so,

mass of TNT in 250 mL sol. 1 = 250*0.00048435 = 0.1210875 grams

so,

required mass of TNT for the initial stock solution = 0.1210875 grams = 0.1211 grams

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