The fluorescent compound chrysene is determined by molecular fluorescence, using
ID: 482149 • Letter: T
Question
The fluorescent compound chrysene is determined by molecular fluorescence, using the method of standard additions (MSA).
B. A 0.891 g sample of a solid sample is extracted with organic solvents and is diluted to 100. mL volume. The fluorescence intensity of chrysene in the sample is measured, and then, known quantities of chrysene are added, and the fluorescence intensity is re-measured. The following results are obtained:
Solution Chrysene Intensity (arbitrary units)
Blank 16
Sample 427
Sample + 1.00 mg chrysene 694
Sample + 2.00 mg chrysene 964
Sample + 3.00 mg chrysene 1229
Calculate the concentration of chysene in the solid sample; you may assume that there is no change in volume of the 100. mL sample extract upon addition of the 1.00, 2.00 or 3.00 mg chrysene.
Explanation / Answer
Ans.
I. Blank = 16 arbitrary units (au)
II. Sample = 427 au
III. Sample + 1.0 mg C = 694 au ; [C = chrysene]
Increase in intensity due to addition of 1 mg chrysene = Intensity III- II
= 694 au – 427 au = 267 au
IV. Sample + 2.0 mg C = 964 au
Increase in intensity due to addition of 1 mg chrysene = Intensity IV- III
= 964 au – 694 au = 270 au
V. Sample + 3.0 mg C = 1229 au
Increase in intensity due to addition of 1 mg chrysene = Intensity V- IV
= 1229 au – 964 au = 265 au
Now,
Calculated average intensity for 1 mg/100 mL chrysene = Average of (III, IV and V)
= [(267 + 270 + 265) / 3] au
= 267.33 au
Corrected average intensity for 1 mg/100 mL chrysene =
Calculated average intensity – intensity of blank
= 267.33 au – 16 au = 251.33 au
Therefore, an intensity of 251.33 au is equivalent to the concentration of 1.0 mg / 100.0 mL chrysene. This is taken as the standard because it is the value of standard spiked solution.
Corrected intensity sample = Experimental average intensity – intensity of blank
= 427 au – 16 au = 411 au
[Chrysene] in mg/100 mL =
(intensity of sample / intensity of standard) x concentration of standard
= (411 au/ 251.33 au) x (1 mg/ 100 mL)
= 1.635 mg/ 100 mL
Thus, [Chrysene] in the given sample = 1.635 mg/100 mL
##. Since the 100.0 mL solution contains chrysene extracted from 0.891 g solid sample, so amount of chrysene in solid sample is equal to amount of chrysene in 100.0 mL sample.
So, amount of chrysene in 0.891 g solid sample = 1.0 mg
[Chrysene] in solid sample as % (wt / wt) = (Mass of chrysene / mass of solid sample) x 100
= (0.001 g/ 0.891 g) x 100 ; [1 mg = 0.001 g]
= 0.1122 %