Chem 434 Instrumental Analysis
Test 1
Answer any 6 questions
1. Describe four of the five techniques used to build instruments so they have low noise levels in
their signal.
Here are 5.
Grounding and Shielding - Minimize lengths of all wires, put electronics in a shielded
container, make sure electronics and the box they are contained in are well grounded.
Difference and Instrumentation Amplifiers - build first stage of electronic amplification
stages with difference amplifier. Build all other stages with instrument amplifiers. (We didn’t
worry about how these were actually built from components)
Analog filtering - Use low pass an high pass filters in circuit design as appropriate.
Modulation - chop signal and source signals, at a given frequency that has low noise
levels.
Chopping Amplifiers/Lock-in amplifiers. Once signal has been chopped, signals only at
the frequency of the chopped signal can be amplified. Can also use lock in amplifiers tht will
only amplify signals at a set frequency.
2. In Chapter 6 we used several terms to discuss how light interacted with matter. Define these
terms:
Coherent Radiation - light that contains a single frequency and where all the wave are in
phase eith each other
Refraction of light - The change of the direction of a light beam as it moves between
mediums of different refractive indicies
Reflection of light - Light that is bounces back when a light beam moves between
mediums of different refractive indicies
Scattering of radiation - When a quantum of light interacts with matter briefly and is then
released with the same energy, but if a new direction.
Line Spectra - A spectrum that contains sharp lines, usually due to atomic absorption.
Band Spectra - A spectrum that contains band instead of lines, usually due to molecular
interactions
Continuum Spectra - A spectrum that contains a continuum of energy reflecting the
temperature of the source.
3. Diagram and discuss 4 different kinds of photon transducers.
Here are 5.
Photovoltaic cells - Barrier layer cells - Cheap, rugged, not very sensitive, used in the
visible region
Vacuum Tubes- Photoemissive metal sealed in a vacuum tube. When light hits electrode,
electrons are released and move toward anode to generate a current.
Photomultiplier tube - uses a photoemissive element as above for first stage, but then, at
each dynode the electrons knock loose more electrons to amplify the resulting current. Very fast
response
Silicon Diode Transducers - A transistor that passes current in the presence of light
Multichannel Transducers - an array of silicon diode transducers, or other charge transfer
devices that contain 1000's of detectors that can be sampled simultaneouse\ly.
4. In theory an atomic line spectra should be infinitely sharp because it involves a single unique
quantum transition. Discuss at least 3 different factors that tend to ‘broaden’ atomic spectra.
Here are 4.
Uncertainty effects - Frequency and time are a pair of variables that are linked together in
Hiesenbergs uncertainty principle. Thus the time between the absorption of light and it’s reemission is unknown, and this makes the exact frequency at which the light is emitted vary
slightly. Net effect is a linewidth of .0001 Å
Doppler broadening. In a flame, the atoms that are emitting light are moving in all
directions. This introduces Doppler broadening into the emission frequency that gives linewidths
of about .01Å.
Pressure Broadening -The emitting atoms collide with each other and with other atoms
and molecules in the flame to change their energy, and this introduces broadening in the.1-.01Å
range.
The book also mentions Electric field and Magnetic field effects that we did not go into in
class.
5. Discuss the ‘anatomy’ of a flame. Where is it hotter, colder, where is the region of primary
combustion, the interzonal region, the region of secondary combustion, where do atoms tend to
become oxides, where are they more reduced, etc.
Make a diagram of a flame, similar to figure 9-2 of text. At the base of the flame we have
the primary combustion region. This is the light blue flame observed in a Bunsen burner. This is
probably the coldest part fo the flame, and the atoms tend to be more reduced in nature. In the
middle of the flame is the interzonal region. This is the hottest part of the flame and has lots of
free atoms. Finally toward the tip of the flame is the secondary combustion zone, the flame is
now a bit cooler, and one finds that more oxides are forming here.
6. Discuss the various ‘interferences’ that can occur in Atomic Spectroscopy.
Spectral interference can occur when the sample contains two elements that emit or
absorb light at the same frequency. This is actually fairly rare, because most spectra do not
overlap. Spectral interference can also occur when particles form in the flame to scatter light or
when other chemicals in the matrix you are working with interfere with the flame.
Chemical Interferences are reactions that occur in the flame that change the amoun of free
atoms observed in the flame. Three common forms of Chemical interference are: Formation of
low volatility compounds like phosphates or sulfates binding to Ca; Dissociation Equilibria,
where the presence of oxygen allows metal oxide and hydroxides to occur in the flame; and
Ionization Equilibria, where the flame has enough energy to ionize the atom. This typically can
happen wth the high atomic mass alkali metals.
7. Discuss at least three different sources used in UV/Vis spectrophotometers.
Here are four.
D2 and H2 lamps - contain either D2 or H2 at low pressure As a 40V potential is arced
through this gas, the molecules are excited and undergo photodissociation with the release of
light energy. This yields a continuum of light between 165 and 375 nm
Xenon lamps. Contains Xenon at a high pressure. Give a continum between 200 and
1000 nm.
Tungsten Filaments - Basically a standard light bulb. Give light between 350 and 2500
nm so only good in the visible and near IR
Halogen lamps - Similar to the above, but bulb includes so I2. This added halogen reacts
with any Tungsten that sublimes off the filament to make it re-aneal to the filament. This allows
one to heat the filament to a higher T to make the lamp brighter and to give it more output in the
UV range.
8. Discuss the four major transitions observed in organic molecules; what are the transitions,
what kind of organic structures give rise to such a transition, at what wavelengths are the
transitions observed, what are their typical å values, etc.
ó 6ó* transitions can arise from any single bond. Very high energy so they occur in the
vacuum ultraviolet range and are not normally observed.
n6ó* transitions observed in molecules with lone pairs of electrons. Fairly high energy
so typically between 150 and 250 nm. Low to medium å values.
n6ð* transitions , very low absorbance values (å ~ 10-100) , typically in 200-700 nm
range. Undergo large ( up to 30 nm) hypsochromic shifts (Blue shifts) in polar solvent
ð6ð* transitions, high absorptivities (å 1000-10000) also in 200-700 nm range. Undergo
small (~5nm) bathchromic (red) shifts in polar solvents.