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?

 

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

 

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.

 

 

 

 

 

 

 

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 µg/L

0.39 µg/L

0.52 µg/L

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.

 

 

5. General

Answer each of the following as either true (T) or false (F).

 

               a.                        Atomic absorption spectrophotometry relies on line sources of light

 

               b.                        Diode array spectrophotometers use filters instead of monochromators for wavelength selection

 

               c.                        One important type of sample preparation in gas chromatography is derivatization, which results in chemical changes in the analytes

 

               d.                        Stray light will result in abnormally high absorbance readings with double-beam instruments

 

               e.                        High levels of sample absorbance can interfere with TOC analysis

 

               f.                         TOX analyzers can measure organic chlorine, organic bromine, organic iodine, but not organic fluorine

 

               g.                        Most gas chromatographic detectors use light absorption for detection of compounds leaving the GC column.

 

               h.                        Flame Atomic absorption spectrophotometry is enhanced by use of an L’vov platform

 

               i.                         Chromatographic separations generally improve as particle size, or film thickness decreases

 

               j.                         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?

 

 

 

 

 

 

 

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m/e

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