Chem 360 October Quiz
Please answer questions in the spaces provided
[2] 1. Distinguish between sensitivity and LOD.
Sensitivity refers to slope
Limit of detection is the smallest amount that will give a signal that is definitely bigger
than the blank
[2] 2. Distinguish between accuracy and precision.
Accuracy is whether the mean is close to the true mean
Precision is how reproducible the results are
[2] 3. What is meant by a 95 % confidence limit?
There is a 95/100 probability that the actual answer lies somewhere in the given range
[3] 4. What is solid phase extraction?
Solution is poured through a cartridge (column) packed with a powdered solid which is
coated with a material that will selectively adsorb the material of interest but not
interferents. The substance can then be eluted in a smaller volume
What is it used for?
Cleanup and preconcentration
[3] 5. You are extracting acetic acid from water into hexane.
- Is it more effective to do one extraction with 300 mL, or three extractions each with 100
mL? Three
-Would the extraction be more effective if the pH was 3 or 8?
3
Why? At low pH the acetic acid will be less ionic and have greater affinity for the
hexane
[2]6.
What can you do to facilitate the extraction of a metal ion from water into an organic
solvent?
Add a ligand so as to form a neutral complex, or add a ligand and then a hydrophobic
counterion to form an ion pair
[2]7.
What does LASER stand for?
Light amplification by stimulated emission of radiation
[2]8.
What are the distinguishing features of Laser light as opposed to ‘ordinary’ light?
Collimated
In phase
Monochromatic
High intensity
[1]9.
Why would using a laser as the excitation source, rather than a tungsten or deuterium
lamp, greatly increase fluorescence?
More power will excite more species which will the fluoresce more
(F is proportional to the power of the exciting light)
[5]10. What is the function of a photodiode array?
To detect all wavelengths of light effectively simultaneously
Describe the principal of operation of a single photodiode, and include a diagram to
illustrate your explanation.
p-n junction with a voltage applied to give a reverse bias so that a depletion layer is
formed at the junction and no current can flow
when light falls on the depletion layer, current carriers are formed and current can flow
the amount of current depends on the intensity of the light
[4]11. Why does one use a wavelength selector in conjunction with a photomultiplier tube?
A PMT cannot distinguish one wavelength of light from another
The selector is used to allow one wavelength at a time to fall on the PMT so the absorbance of
each wavelength can be monitored
Do you need a wavelength selector with a photodiode array? Explain
No – you only require a grating for dispersion
PDA consists of a row of detectors, each positioned to detect a particular wavelength of
light
[6]12. Interference filters and gratings have similarities in the way they operate.
a)
State the function of each.
I filter – to select (transmit) a particular bandwidth of light
Grating – to disperse light
b)
Explain the general principle of operation that applies to both.
The device is designed so that beams of light which enter and exit parallel travel
different distances. Constructive interference occurs if the extra distance traveled = nλ
The constructive interference means that wavelength of light will be transmitted
(filter)– or come off at that particular angle (grating)
[4]13. What is Beers law.
A = abc
What causes deviations from Beer’s law?
High concentrations – energy levels are perturbed, refractive index changes
Scattered light
Changing equilibria
Non-monochromatic light
[6]14. Using absorption spectroscopy, describe various ways that can be used to do
multicomponent analysis of a solution, and state what determines which option you
would use.
[3]14. By definition, how does one distinguish between fluorescence and phosphorescence.
[2]15. What are some other differences between fluorescence and phosphorescence?
[5]16. Explain the basic design of an ion-selective electrode.
What determines the lower limit for the LOD of an ion selective electrode?