ADVANCED INSTRUMENTAL TECHNIQUES
Written Practical Test - Content
On the same day as the final theory test, you will be required to do a short “practical” test
(written only) dealing with the practical aspects of the instruments that you have used
throughout the year.
The questions will cover sample preparation and how the instruments are used, but not
how/why they work. Some of the questions will be applied, rather than just memory-type.
It is RESTRICTED OPEN BOOK – you can you use these notes, but nothing else.
Example questions
What steps must be performed before operating the electrothermal AAS?
A technician does a QC test on bitter lemon by fluorimetry and measures a quinine intensity
of 456, which is around the normal value. Because he is in a rush, he simply reports that
the batch is OK. A month later, there is a complaint about that batch. On re-analysis, it is
found to be 10 times higher in quinine than it should be. What else should have been done in
the first analysis which would have picked up this problem?
The following is what the test will be based on.
Graphite Furnace AAS
Setting up the instrument for a run requires a number of steps extra to the usual flame AAS
(optimising lamp etc):
•
rinse the autosampler tip
•
check that the autosampler tip delivers into the tube
•
run a tube clean
If a sample has a high absorbance (> 2) due to an unexpectedly high concentration, a number
of tube cleans should be run, until a blank gives a low absorbance.
Ultrapure acids and water must be used to avoid contamination.
All glassware must be rinsed in dilute ultrapure nitric acid to remove traces of adsorbed
metals from the glass and made up in dilute ultrapure nitric acid to minimise the adsorption
of the analyte from solution onto the glass.
When developing a new temperature program, listen for fizzing at the end of the drying stage,
and look for lots of smoke at the end of the ashing stage. If these occur, more time is
required. The temperatures should be as high as possible anyway to reduce the times.
AIT Written Practical Information
Fluorimetry
To determine the correct filters, run a UV/VIS absorption spectrum and find the filter closest
to the absorption maximum: this is used as the excitation filter. Add 50 nm to the absorption
maximum to identify the most likely emission filter.
Make sure that the gain/span settings are such that all calibration standards and samples are
on the same gain scale.
Always dilute samples roughly 1:1 in the sample cell to confirm that the intensity decreases,
meaning that self-quenching is not occurring.
X-ray fluorescence
Granular samples (eg rocks) should be ground into powder to avoid heterogeneity problems.
Solid samples (eg a piece of metal) should be polished unless you want to analyse the top
surface (eg for corrosion).
Always make sure that the pair of Kα and Kβ (or L) peaks exist in the correct intensity ratio to
definitely identify the element. If one is missing or they are in the wrong ratio, it is not the
element you thought it was.
The tube voltage should be at least twice that of the highest energy element line of interest, eg
if you are interested in copper and zinc, the highest energy line is the Zn Kβ at 9.6, so a tube
voltage of 20 kV minimum should be used.
High tube currents cause peak broadening without significant improvement in intensity. If
in doubt, choose the Auto option which determines the optimum current.
HPLC
Solvents should be freshly prepared as they don’t seem to be sufficiently stable for storage
beyond a week or so. They must be Millepore-filtered to remove particulates, as must all
solutions – standards and samples. A syringe filter is used as the final solution cleaning
device.
The mobile phase should be run through the system for about 30 minutes before use. Check
for a flat baseline and adjust the zero as required.
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AIT Written Practical Information
Gas chromatography
With any GC, simple or sophisticated, turn on the carrier gas first before turning on the
instrument. This guarantees that the oven will not switch on without gas flowing through the
column.
To determine the best temperature program, run a series of isothermal injections across the
temperature range (eg 50, 100, 150, 200) and use these as the basis for determining when to
increase the temperature.
Generally ramp the temperature up as fast as possible from one step to the next.
If you have two peaks close together, once the first one is half-eluted, you can adjust the
temperature because the second one must be already separated from it.
The solvent delay on the MS detector is intended to save wear on the filament in the
ionisation chamber. To determine the appropriate time for the delay, run the sample with no
delay once (or using a different detector) and identify when the solvent peak is through.
ICP emission spectroscopy
Sensitivity can improved by:
•
a different wavelength
•
increased RF power to the plasma
•
increased nebuliser flow
Filtration of samples is necessary because of the ease of blocking the nebuliser and the
difficulty in unblocking it.
Levels of organic solvents or viscous samples can cause changes in sample flow rates into the
nebuliser which will lead to changes in intensity for other than concentration reasons. An
internal standard is the best response.
Polarography/voltammetry
Samples should be:
•
treated with a neutral electrolyte to increase conductivity and reduce diffusion rate
variations
•
purged with nitrogen to remove dissolved oxygen
Scan ranges should start and finish at least 100 mV past the analyte half-wave potential (eg
Pb is -0.4 V so a suitable scan range would be -0.6 to -0.2 V).
In ASV, the reduction potential to plate the metal onto the mercury drop should be 100 mV
past the highest reduction potential (eg Zn is –1.0 V, set the voltage to –1.1V).
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