CE 772                                                                                                  18 Dec 1997

 

FINAL EXAM

 

 

Open Book, Open Notes

 

 

Answer five of the following eight questions:

   (each is worth 20%)

 

 

1. Sample Preparation

 

A 1-liter aqueous sample is being extracted in a sequential batch mode with diethyl ether.  Pollutant "Y" has an ether/water dimensionless partition coefficient of 95.

 

a. After one extraction with 25 mL of ether, how much of the original amount of pollutant "Y" remains in the water phase?

b. If it is desired that 99% of pollutant "Y" be extracted into the ether phase, how many sequential extractions using 25 mL of the organic phase must be made?

 

2. Chromatographic Theory

 

An extract of drinking water shows two closely-eluting GC (ECD) peaks which are believed to be methyl trichloroacetate and methyl dichloroacetate.  The column used is a 2 m x 4 mm glass column packed with 3% SP-1000 on 100/200 mesh Supelcoport.  The stationary phase weight is 2.54 g, and it has a density of 0.699 g/mL.  The inlet column pressure was 22 psig, the outlet flow rate was 60 mL/min, and the column temperature was isothermal at 65oC.

 

Retention Time (min)

Peak Width (min)

Identity

0.40

 

Unretained solvent

8.85

0.63

methyl trichloroacetate?

9.55

0.71

methyl dichloroacetate?

 

            a. Calculate k' for each of the suspected esters

            b. Calculate and average number of theoretical plates

c. Describe how you would determine the true identity of the two suspected esters.  Your only have access to a GC with an FID and an ECD.

 

3. GC/MS

 

Interpret the mass spectrum summarized on the table below and the attached figure.  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.

 

 

m/e

% Abundance

m/e

% Abundance

   28

  0.9

   70

  5.1

   29

  0.6

   71

  2.1

   30

 15.1

   72

100

   31

  0.2

   73

  3.7

   42

 16.2

   86

  1.5

   43

 14.1

  100

  2.1

   44

 39.2

  112

  1.1

   45

  1.0

  113

  0.6

   55

  2.1

  114

 25.8

   56

  3.1

  115

  2.1

   57

  4.4

  127

  0.8

   58

  5.3

  128

  0.5

 

 

  129

 18.1

 

 

  130

  1.7

 

 

 

 

4. General

 

You have a surface water sample that you suspect to be contaminated with pentabromobenzoic acid (C6Br5COOH).  This aromatic compound is considered to be very toxic, so you are interested in trace quantities.  One of your concerns is interference from naturally-occurring organic matter which is present at concentrations that are 3 or more orders of magnitude greater than the analyte.  Propose how you would analyze such a sample for pentabromobenzoic acid.  Explain the reasons for your choice of methods and procedures.  Consider the following points:

 

            a. extraction and/or concentration

            b. sample cleanup

            c. derivatization

            d. separation and quantification

            e. identification

 

 

 

5. Error Analysis

 

A series of replicate measurements were made for pollutant "X" in identical aliquots of a sludge sample.  The measurements are as follows:

 

33.5 µg

35.9 µg

29.1 µg

22.6 µg

35.0 µg

38.2 µg

 

a.      What is the mean and relative standard deviation for these measurements?

b.     Assuming these measurements were all made on aliquots of precisely 5 g each, what is the mean concentration in ppm, and the associated relative standard deviation?

c.      Repeat "b", but this time assume that the standard error on the weight measurements for each aliquot was 0.5 g

 

 

6. UV-Vis Spectrophotometry

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

 

            a.                   Double-beam spectrophotometers exhibit a greater light throughput than single-beam instruments

 

            b.                   Diode array spectrophotometers are commonly reverse optics (geometry) instruments

 

            c.                   Normal optics (geometry) spectrophotometers are more susceptible to problems with sample photodegredation

 

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

 

            e.                   Absorbance is affected by sample pH

 

            f.                   Transmission of polychromatic radiation through a monochromator leads to positive deviations from Beer's Law

 

            g.                   Double dispersing spectrophotometers are less subject to stray light.

 

 

 

7. TOC

 

Propose a method for the analysis of Total Organic Carbon (TOC) in solid samples (soils/sediments).  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?

 

 

 

8. General

 

Bromate (BrO3)is a disinfection byproduct found in some drinking waters containing small amounts of naturally-occurring bromide.  Since it is a known animal carcinogen, it will probably be regulated (at the high ppb range) in the next few years by U.S. EPA.   However, at the present, one obstacle to the regulation of this chemical is the lack of an accepted method for its analysis at these low levels.  In particular, classical sample concentration techniques give losses, because bromate is a strong general oxidant (i.e., it is subject to chemical reduction).  Discuss the issue of bromate analysis and propose an outline for a sensitive (detection limit ~100 ppb) analytical method.  Be as specific as you can.  Discuss important analyte properties that determine how it can be analyzed at low concentrations.

 

 

9. GC/MS

 

Interpret the attached mass spectrum.  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.