Combined Assessment/Nakuru Heavy Metals
1. Knowledge
What is an x-ray?
Define the relationship between wavelength and energy.
Is anodic stripping voltammetry a good technique to analyze Pb in soil at ppb levels?
Below is a typical anodic stripping voltammogram. Label the x- and y-axes and give appropriate
units for each.
What region of the electromagnetic spectrum is used for atomic absorption measurements?
Draw a diagram illustrating the process of x-ray fluorescence.
2. Comprehension
Explain the difference between atomic absorption and atomic emission.
In x-ray fluorescence, is the emitted photon of shorter or longer wavelength than the incident
photon?
Give the steps involved in a normal ASV determination of M+2, and explain what is occurring in
each step?
Describe why a stirred solution is important in an ASV analysis.
Explain the process whereby isolated atoms of the metal of interest are formed in atomic
spectroscopy.
3. Application
For the XRF spectrum shown below, explain how element specific information can be obtained.
What specific types of information are available from the spectrum?
Inductively coupled plasma (ICP) instruments operate at higher temperatures than flame atomic
absorption. Explain what advantage this gives to ICP measurements.
Would it be possible to detect Au+3 in industrial effluent by ASV in the presence of Pb+2, Cu+2,
and Zn+2? Sketch the voltammogram expected and label each peak.
The XRF calibration graph below was prepared for lead spiked into a sample of silica sand.
Using this graph, calculate the level of lead in a Lake Nakuru sediment sample producing an
intensity of 53 cps.
Determination of Lead by XRF
Intensity (CPS)
500
y = 0.4223x + 0.4354
400
300
200
100
0
0
500
1000
1500
mg/g
4. Analysis
Because matrix effects can be significant in atomic spectroscopy, measurements are
sometimes performed using the method of standard additions. Explain in your own words how a
standard additions experiment is performed and how it can eliminate problems arising from
matrix effects.
Smelters can create heavy metal contamination in the surrounding areas with levels varying
based on wind direction and flow. What challenges might you encounter in an XRF survey for
lead in the areas around the smelter. Design a plan for field measurements to evaluate the
dispersion of contaminants and selection of soil samples for further lab-based analysis.
5. Synthesis
Devise an atomic spectroscopy method that would be suitable for the analysis of As in river
sediment. Include aspects of sampling, sample size, sample prep, instrument calibration, and
analysis using a target concentration in the sediment of 10 (+/- 1) ppb As.
You are developing a method to determine the metal M2+ in waste water samples via ASV. Your
detection limits are not sufficiently low. Describe three methods you might try in order to improve
your detection limits. Include your reasoning for each method.
6. Evaluation
Considering the following requirements: ease of use, capability of multielement measurements,
matrix, and limits-of-detection compare the relative merits of XRF, ASV and atomic
spectroscopy for metal analyses in the field and in the laboratory.
You are working in a dental research lab and wish to measure calcium levels in the saliva of
human subjects after rinsing with a calcium-containing remineralizing mouthwash. The expected
concentrations are in the low ppm level. You will need to run about 10 samples per day. The two
methods available to you are flame atomic absorption spectrophotometry and anodic stripping
voltammetry. Which method will you choose? Compare and contrast these two methods for this
application and justify your choice.