You are measuring mercury (Hg) in peanut butter. You prepare a stock solution of
ID: 936383 • Letter: Y
Question
You are measuring mercury (Hg) in peanut butter. You prepare a stock solution of 10 ppb Hg in ddH2O and then prepare a standard curve as follows:
Std Solution
mL Hg Stock
mL ddH2O
A
5
0
B
4
1
C
3
2
D
2
3
E
1
4
F
0
5
You perform wet ashing on a 5.29 g sample of peanut butter. During ashing, the solution is boiled down to a final ashed volume of 25 mL. The ashed solution is then diluted to 50 mL with ddH2O. You also prepare a reagent blank by performing wet ashing by the same procedure with no sample and diluting to 50 mL with ddH2O. The purpose of the reagent blank is to account for minerals in the reagents that do not come from the sample, so the minerals in the sample can be more accurately quantified. You measure the diluted sample, reagent blank, and standard curve samples by AAS (253.7 nm). The following data are obtained:
Sample
A253.7 nm
Std A
1.682
Std B
1.147
Std C
0.875
Std D
0.64
Std E
0.223
Std F
0.061
Sample
0.389
Reagent Blank
0.167
Prepare a blanked standard curve, and use the reagent blank to determine the true Hg content in the sample.
What is the slope of the blanked standard curve?
Std Solution
mL Hg Stock
mL ddH2O
A
5
0
B
4
1
C
3
2
D
2
3
E
1
4
F
0
5
You perform wet ashing on a 5.29 g sample of peanut butter. During ashing, the solution is boiled down to a final ashed volume of 25 mL. The ashed solution is then diluted to 50 mL with ddH2O. You also prepare a reagent blank by performing wet ashing by the same procedure with no sample and diluting to 50 mL with ddH2O. The purpose of the reagent blank is to account for minerals in the reagents that do not come from the sample, so the minerals in the sample can be more accurately quantified. You measure the diluted sample, reagent blank, and standard curve samples by AAS (253.7 nm). The following data are obtained:
Sample
A253.7 nm
Std A
1.682
Std B
1.147
Std C
0.875
Std D
0.64
Std E
0.223
Std F
0.061
Sample
0.389
Reagent Blank
0.167
Prepare a blanked standard curve, and use the reagent blank to determine the true Hg content in the sample.
What is the slope of the blanked standard curve?
Explanation / Answer
Stock solution is 10 ppb
ddH2O = double distiled water
SLOPE is 0.159 absorbance unit/ppm
Std Solution
mL from stock sol.
mL dH2O
Conc. of std.sol, ppm
Absorbance,A
A
5
0
10
1.683
B
4
1
8
1.147
C
3
2
6
0.875
D
2
3
4
0.640
E
1
4
2
0.223
F
0
5
0
0.061
Sample
0.389
Reagent Blank
0.167
Conc. of std.sol, C ,ppm
Absorbance, A
10
1.683
8
1.147
6
0.875
4
0.640
2
0.223
0
0.061
Put these values on a graph or in an Excel table. Read or calculate SLOPE and INTERCEPT (these are Statistical functions in Excel).
SLOPE
0.159
INTERCEPT
-0.022
SLOPE is 0.159 absorbance unit/ppm
The calibration curve is :
A = -0.022 + 0.159 C
Correct the sample absorbance for the reagent blank : 0.389
Std Solution
mL from stock sol.
mL dH2O
Conc. of std.sol, ppm
Absorbance,A
A
5
0
10
1.683
B
4
1
8
1.147
C
3
2
6
0.875
D
2
3
4
0.640
E
1
4
2
0.223
F
0
5
0
0.061
Sample
0.389
Reagent Blank
0.167
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