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CEE 772 |
16 November 2005 |
Mid-term Exam
Open Book, Open Notes
Answer any two of the following six questions:
(each is worth 50%)
1. Sample Preparation
A 1-liter aqueous sample is being extracted in a
sequential batch mode with methyl-tertiary-butyl ether (MtBE). Pollutant "Y" has an MtBE/water dimensionless partition coefficient of 25.
a.
After one extraction with 50 mL of MtBE, how much of the original amount of pollutant
"Y" remains in the water phase?
b.
What is the overall % of pollutant "Y" that would be extracted into
the MtBE phase if 5 sequential extractions using 50 mL of the organic phase were made?
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KD |
25 |
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V-water |
1000 |
mL |
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V-solvent |
50 |
mL |
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One extraction |
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fe = |
0.555556 |
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fw =1-fe= |
0.444444 |
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Two sequential
extractions |
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f2e= |
0.802469 |
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f2w =1-f2e= |
0.197531 |
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Three sequential
extractions |
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f3e= |
0.912209 |
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f3w =1-f3e= |
0.087791 |
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Four sequential
extractions |
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f4e= |
0.960982 |
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f4w =1-f4e= |
0.039018 |
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Five sequential
extractions |
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f5e= |
0.982658 |
or |
98.27% |
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f5w =1-f5e= |
0.017342 |
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2. GC/MS #1
Interpret the mass spectrum depicted in the
figure below. Indicate your rationale
for the determination of empirical formulae for each of the major fragment
ions, as well as for the molecular ion.
Propose a structure for the parent molecule.
No one elected to solve this problem
3. GC/MS #2
Interpret the mass spectrum depicted in the
figure below. Indicate your rationale
for the determination of empirical formulae for each of the major fragment
ions, as well as for the molecular ion.
Propose a structure for the parent molecule.
No one elected to solve this problem
4. Error Analysis
A series of replicate measurements were made for
dichloracetamide in identical aliquots of a drinking
water sample. The measurements are as
follows:
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0.35 µg/L |
0.39 µg/L |
0.52 µg/L |
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0.44 µg/L |
0.40 µg/L |
0.41 µg/L |
a. What is the mean and relative standard deviation
for these measurements?
b. If these measurement represent concentrations
near the method detection limit (MDL), what would you calculated the MDL to be?
c. Calculate a 95% confidence interval for the mean
of these measurements.
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DW sample |
0.35 |
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0.44 |
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0.39 |
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0.4 |
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0.52 |
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0.41 |
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4a. |
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Mean = |
0.418333 |
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SD = |
0.057764 |
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RSD = |
0.138081 |
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4b |
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t = |
3.365 |
alpha = 1% |
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df = 5 |
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MDL = |
0.19 |
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4c |
sx= |
0.023582 |
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t = |
2.571 |
alpha = 2.5% |
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df = 5 |
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t*sx
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0.060629 |
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95% CI = |
0.357704 |
to |
0.478963 |
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5. General
Answer each of the following as either true (T)
or false (F).
a. T
Atomic absorption spectrophotometry relies on
line sources of light
b. F
Diode array spectrophotometers use filters instead of monochromators for wavelength selection
c. T
One important type of sample preparation in gas chromatography is derivatization, which results in chemical changes in the analytes
d. F
Stray light will result in abnormally high absorbance readings with
double-beam instruments
e. F
High levels of sample absorbance can interfere with TOC analysis
f. T
TOX analyzers can measure organic chlorine, organic bromine, organic
iodine, but not organic fluorine
g. F
Most gas chromatographic detectors use light absorption for detection of
compounds leaving the GC column.
h. T
Flame Atomic absorption spectrophotometry is
enhanced by use of an L’vov platform
i. T Chromatographic separations generally improve
as particle size, or film thickness decreases
j. T
Sample preparation in atomic absorption spectrophotometry
frequently involves addition of matrix modifiers, which can help avoid interferents.
6. TOC
Explain how you would measure TOC in
gasoline-contaminated groundwater.
Consider matrix problems, interferents, analyte recovery and any other pertinent issues. Be as specific as you can. How could one best verify the accuracy of
this method?
In this
problem, I wanted you to think and comment about several issues:
- The fate of
volatile organic compounds during the pre-purge step to remove inorganic
carbon
- Use of different
types of standards for assessment of analyte
recovery
- Other types of
TOC protocols that might be more appropriate
Of course,
the volatile gasoline constituents would be partly lost during pre-purge. This would create a negative bias on the
measurements. Aside from the usual
calibration standards (KHP), you cold run standards of some pure hydrocarbons
in water of the type that you know are found in gasoline. You could also run a gasoline mixture
dissolved in water to assess recovery. Each
has some interesting and very practical experimental difficulties, but they
could be done. An alterative approach
would be to use the two channel method for TOC. Here you measure total carbon without purging
by means of direct injection into a high temperature combustion furnace. Then the inorganic carbon is determined
separately by injection into a low temp furnace and measurement of the off gas
by NDIR. A second alternative would be
to measure purgeable organic carbon directly. Some TOC analyzers are also equipped to do
this. Basically, the sample is purged
and the off-gas is sent directly through the combustion furnace and NDIR
instead of just venting to the air.