For Test 2 - U. of M. WWW server

CHEM 4590 - Practice questions for Test 2 –
i) Quantitative IR spectroscopy
1.Describe how 2-D images are obtained by either FTIR microscopy.
2.What is measured by diffuse reflectance FTIR and why would a Fourier Transform instrument be
better than a dispersive spectrometer to perform in vivo measurements?
3. IR spectra of glycerol and urea are shown below. Using either technique, describe the development of
a quantitative method for the analysis of homogeneous mixtures of these two compounds. Your
description should include i) why you decided to use IR or raman; ii) choice of wavelengths; iii)
measurement of “blanks” for each compound; iv) calibration curves; v) determination of proportions of
each compound in an unknown mixture.
4. The fourier transform infrared (FTIR) spectrum shown below was obtained for pure vanillin
prepared as a carbon tetrachloride solution. Vanillin is the main compound found in vanilla bean
extracts.
If you were to develop a quantitative FTIR method to determine the amount of vanillin in diverse
extracts, describe how you would conduct the following steps:




Selection of analytical wavelength
Calculation of absorbance (use a drawing if necessary)
Calibration curve
Determination of unknown
5. a) What is the function of the Michelson interferometer in an infrared (IR) fourier transform (FT)
spectrometer?
b) Describe two main advantages of using FT over dispersive instruments.
c) How do you define time domain vs. frequency domain spectroscopy?
6. What is attenuated reflectance IR spectroscopy and how does it differ from transmission IR
spectroscopy?
7. Aspartame is a methyl ester of the didpeptide aspartyl-phenylalanine. It is a well known low calorie
sweetener. It is crystallized as aspartame hemihydrate, which has a stable conformation at room
temperature. If heated, however, aspartame hemihydrate undergoes dehydration followed by cyclization.
The pathway of water evaporation, dehydration and intramolecular
cyclization of aspartame hemihydrate.
With FTIR spectroscopy, it is possible to monitor the gradual changes occurring as aspartame
hemihydrate is heated. The portions of FTIR spectra shown below show these changes (50, 63, 75, 110,
153, 165 and 174oC).
Individual spectra of aspartame hemihydrate at seven temperatures
Knowing that a sample of aspartame hemihydrate has spent several weeks in the heat at temperatures
reaching nearly 200oC , describe how you would best determine how much of the original compound is
left or is gone relative to the starting amount, in %.
8. The pharmaceutical compound (2R,3S)-2-({(1R)-1-[3,5 bis(trifluoromethyl)phenyl]ethyl}oxy)-3-(4fluorophenyl) morpholine hydrochloride (denoted here as FMH), has been found to crystallize in at least
two polymorphic forms.
FMH
Each form has a distinct infrared (IR) spectrum, as shown below. The band at 1009 cm-1 varies with
concentration of Form I; the band at 1058 cm-1 is present for both forms but does not vary with
concentration; it can therefore be used as a correction factor for variations in sample preparation.
Discuss the elaboration of a quantitative method for the determination of Form I in mixtures of I and II.
In your answer include:
a) analytical wavelength selection;
b) making a calibration curve for Form I, i.e. Absorbance vs. % of I in mixture;
c) determination of unknown percentage of II;
d) use of the signal at 1058 cm-1 as correction factor.
ii) SNIF NMR
1. The alcoholic content of Caribbean rum from fermenting sugar cane can be increased illegally by
adding another source of sugar prior to fermentation, e.g. corn syrup from Canada.
In the event of this happening, how would you expect this to affect the deuterium (2H) NMR
spectrum? Your answer will consist of comparing hypothetical spectra (including R and C ratios)
obtained for rum if:
i) The traditional local Caribbean recipe was followed;
ii) Canadian corn syrup was added to sugar cane and fermentation was done in Canada;
iii) Canadian corn syrup was added to sugar cane but fermentation was done in a Caribbean country.
2. The deuterium NMR spectrum shown on the left was measured on alcohol found in Bacardi rum
from Cuba. The other two spectra were measured on unknown alcohols.
a) How can the relative R ratios obtained for unknowns 1 and 2 give information about the
geographic origin of these samples? Do they come from warmer or colder climates than the Cuban
climate ?
b) How is ratio C complementary to R?
c) Why does it take at least two hours to obtain each of these spectra ?
Bacardi rum (Cuba)
Benzene ref.
-CH2DHOD/-OD
-CHD-
Unknown alcohol 1
Unknown alcohol 2
iii) SEM-AFM
1.
a) Give a summary description of the apparatus used in scanning electron microscopy (SEM). You
may draw a diagram if needed.
b) How does one typically prepare a biological sample for dry SEM?
c) Name three detection techniques that could be used on the same instrument.
2. Give comparative features of atomic force microscopy (AFM) and scanning electron microscopy
(SEM) in terms of:

Sample preparation requirements

How images are obtained

Fields of application
3.
a) Atomic force microscopy is a microscopy/imaging technique which does not require
conductivity in samples. Explain why this technique can work for non-conducting samples and
conductive samples.
b) Describe how (no chemical reactions needed) and why one would chemically modify a
silicon tip surface in AFM.
c) There are three main rastering modes in AFM: direct contact, no contact and contact with
tapping. Suggest one type of application for each mode.
4.
a) Explain how you would prepare a biological sample (e.g. skin cells) to obtain a 3-D image by
scanning electron microscopy (SEM).
b) SEM can use secondary electron emission for the generation of an image. Where do these
secondary electrons originate from?
c) Why is SEM more useful to biologists than EPXMA (electron probe X-ray microanalysis)? What
type of information can the latter provide?
5. In a forensic-related case you as the expert are asked to perform qualitative and quantitative
analysis of arsenic in a sample of human hair. The techniques available in your laboratory are AFM,
SEM and EPXMA. Select your technique(s)* of choice and elaborate on sample preparation
procedures and nature of results expected.
*Select one or two technique(s).
6. A biopolymer can be made into a thin film and studied by surface techniques such as scanning
electron microscopy (SEM) and atomic force microscopy (AFM). If there are three main questions
to be answered about the biopolymer properties, namely:
a) presence of heavy metal contaminants;
b) film topography;
c) affinity of the biopolymer for a specific protein (e.g. insulin),
Which surface technique(s) would you use in each case, how would you prepare the film sample and
what type of information would you obtain?