Chemistry 421/821 – First Exam Spring 2009 page 1
Name
1) (15 points)
a. (5 points) Describe the primary difference between a prism and grating monochromator.
A prism is based on the refraction of light and the fact that different wavelength’s of light
have different values of refraction index in a medium. A grating monochromator is based on
the diffraction of light (constructive and deconstructive interference).
b. (5 points) Explain how a narrow bandwidth can be both an advantage and disadvantage in
UV/vis spectroscopy.
A narrow bandwidth means that a smaller range of wavelengths are transmitted to the
sample. Thus, the observed absorbance is more closely correlated to the desired wavelength.
This is important since molar absorptivity is wavelength dependent. You are only getting the
wavelength you want. Conversely, a lower bandwidth means the intensity of light reaching
the sample has also decreased. This may be problematic since the sensitivity of measuring an
absorbance is directly dependent on the intensity or power of the source wavelength.
c. (5 points) Explain how the additive property of Beer’s law may lead to errors (negative deviation
from the expected absorbance)
This is related to the bandwidth question above. The observed total absorbance is the sum of
each absorbance at each individual wavelength transmitted to the sample (bandwidth):
AT =A1 +A2 + A3 + … = 3bc + 3bc + 3bc + …
But, the expected absorbance is based on a single wavelength, the center of the bandwidth. If
the molar absorptivity is significantly different at each wavelength, the observed absorption
at each wavelength will differ. Thus:
A1 ≠ A2 ≠ A3, so AT is not proportional to A1
8
7
6
A =  bc
5
4
3
Negative Deviation
2
1
0
2 10-4
4 10-4
6 10-4
Concentration (M)
Similar effect if the analyte is involved in a chemical equilibrium, where different species
exhibit different molar absorptivity at the observed wavelength.
8 10-4
1 10-3
Chemistry 421/821 – First Exam Spring 2009 page 2
Name
2) (15 points) Given the following calibration curve:
a. (5 points) Which analyte has the lowest sensitivity? Explain.
C, lowest slope
b. (5 points) What are the selectivity coefficients?
kb,a = mb/ma = 3.14/11.714 = 0.27
kc,a = mc/ma = 0.987/11.714 = 0.084
kc,b = mc/mb = 0.987/3.14 = 0.31
c. (5 points) Which analyte has the lowest limit of detection? Explain.
A, highest slope: cm = (Sm-Sbl)/slope
Chemistry 421/821 – First Exam Spring 2009 page 3
Name
3) (9 points)
a. (3 points) Which compound is likely to have an observable UV/vis spectra?
b. (3 points) The product of a chemical reaction resulted in a red-shift in the UV spectra, which
compound is the likely product?
c. (3 points) Which compound is likely to fluoresce?
4) (10 points) Please identify which of the situations described below is a source of systematic or random
error:
a. (2 points) Glass cuvette measuring an absorbance at 300 nm.
Systematic (glass absorbs below 350 nm)
b. (2 points) Power fluctuations that affect the stability of the light source.
Random, (random, unpredictable change light power/intensity)
d. (2 points) Rapid decay in the intensity of a deuterium lamp in a Spectronic 20.
Systematic (intensity of light would change by a fixed amount between samples)
e. (2 points) Thermal electron emission in a phototube.
Random (random increase in current)
f. (2 points) Presence of a contaminant that absorbs over the same frequency range as the
analyte.
Systematic (contaminant absorbs a constant amount of light based on concentration)
Chemistry 421/821 – First Exam Spring 2009 page 4
Name
5) (15 points)
a. (5 points) What is the difference between internal conversion and external conversion in
fluorescence?
Internal conversion: crossing of e- to lower electronic state, particularly if vibrational levels
overlap. Efficient and main reason most compounds do not fluoresce. External collesion:
deactivation or loss of energy through collision with solvent.
b. (5 points) What is the difference between singlet and triplet states in fluorescence?
Singlet state: electron spins are paired no net magnetic field.
Triplet state: electron spins unpaired, net magnetic field.
Singlet state is higher energy than triplet state.
c. (5 points) Identify one mechanism by which fluorescence can be quenched.
Increase oxygen concentration or collision with solvent (increase viscosity or temperature)
Chemistry 421/821 – First Exam Spring 2009 page 5
Name
6) (25 points) An echellette grating that contains 2800 blazes/mm is used in a UV/vis spectrometer
where the incident angle of the polychromatic beam is 63.26o.
a. (5 points) At what angle from the grating would you be able to observe light with a wavelength of
600nm?
n = d(sin  + sin )
d = 1 mm/2800 blazes x 106nm/mm = 357.1 nm/blaze
600 nm = 357.1 nm(sin r + sin 63.26)
sin r = 600 nm/357.1 nm - sin 63.26
sin r = 0.787
r = sin-1 0.787 = 51.9o
b. (5 points) What is the energy of a photon of this light?
E=hhc: E(600 nm) = (6.63x10-34J.s x 3.00 x 108 m/s)/600 x10-9 m = 3.32 x10-19 J
b. (5 points) If an analyte exhibited a percent transmission of 25% at 600nm, what is its absorbance?
A = -log(%T/100)
A = -log(25%/100)
A = -log(0.25) = 0.60
c. (10 points) If the light strikes a glass cuvette at an angle of 10o, what are the angles of reflection
and refraction (assume index of refraction for air = 1.0003 and glass = 1.52)?
angle of reflection = 10O (other side of normal)
angle of refraction (2) = sin-1(1sin1 /2) = sin-1(1.0003*sin(10)/1.52) sin-1 (0.5699) = 6.6O
Chemistry 421/821 – First Exam Spring 2009 page 6
Name
7) (11 points)
a. (6 points) If an absorption filter with a bandwidth of 100 nm and a maximum transmission at 260
nm is used in a UV/vis spectrometer, what is the approximate range of wavelengths available for
measuring an analytes absorption spectrum?
Conservatively: 210-310 nm or liberal: 160-360nm
b. (5 points) Consider two light sources that both generate light with a wavelength of exactly 550 nm
that are both pointed at an object in a darken room. If the distance between the light sources and
the object are 1.75 m and 1.2 m, respectively, would the object be illuminated?
Yes, distance difference (0.550m) is integer multiple of wavelength