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?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

KD

25

 

 

 

 

 

 

V-water

1000

mL

 

 

 

 

 

V-solvent

50

mL

 

 

 

 

 

 

 

 

One extraction

 

 

 

fe =

0.555556

 

 

fw =1-fe=

0.444444

 

 

 

 

 

Two sequential extractions

 

 

f2e=

0.802469

 

 

f2w =1-f2e=

0.197531

 

 

 

 

 

Three sequential extractions

 

 

 

 

 

 

f3e=

0.912209

 

 

 

 

 

 

f3w =1-f3e=

0.087791

 

 

 

 

 

 

 

 

 

 

 

 

 

Four sequential extractions

 

 

 

 

 

 

f4e=

0.960982

 

 

 

 

 

 

f4w =1-f4e=

0.039018

 

 

 

 

 

 

 

 

 

 

 

 

 

Five sequential extractions

 

 

 

 

 

 

f5e=

0.982658

or

98.27%

 

 

 

 

f5w =1-f5e=

0.017342

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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.

 

Mass Spectrum

 

 

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:

0.35 g/L

0.39 g/L

0.52 g/L

0.44 g/L

0.40 g/L

0.41 g/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.

 

 

 

 

 

 

 

 

 

 

 

 

DW sample

0.35

 

 

 

 

 

 

 

0.44

 

 

 

 

 

 

 

0.39

 

 

 

 

 

 

 

0.4

 

 

 

 

 

 

 

0.52

 

 

 

 

 

 

 

0.41

 

4a.

 

 

 

 

Mean =

0.418333

 

 

SD =

0.057764

 

 

RSD =

0.138081

 

 

 

 

 

 

 

 

 

4b

 

 

 

 

 

 

 

 

t =

3.365

alpha = 1%

 

 

 

 

 

 

df = 5

 

 

 

 

 

MDL =

0.19

 

 

 

 

 

 

 

 

 

4c

sx=

0.023582

 

 

 

 

 

 

 

 

 

 

t =

2.571

alpha = 2.5%

 

 

 

 

 

 

df = 5

 

 

 

 

 

 

 

 

 

 

 

 

 

t*sx =

0.060629

 

 

 

95% CI =

0.357704

to

0.478963

 

 

 

 

 

 

 

 

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 Lvov 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:

 

  1. The fate of volatile organic compounds during the pre-purge step to remove inorganic carbon
  2. Use of different types of standards for assessment of analyte recovery
  3. 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.