Atomic
Absorption
Introduction
Atomic
absorption spectrophotometers make it relatively easy to obtain analytical results
which are consistently accurate and precise. Nonetheless, occasional operational
problems can be encountered, and the causes may not be immediately obvious,
especially to the less experienced operator. Most of these problems,
however, can be solved by the operator or analyst without the need for a
service call. This guide is designed to help in diagnosing such problems.
Set-Up
Your spectrophotometer
cannot be expected to provide maximum analytical performance unless it is first
set up correctly. Complete details of the setting up procedure are given in the
instrument operation manual, but the following specific points are worth
mentioning here: • Ensure that the burner is completely clear of the optical path
before attempting to align the hollow cathode lamp and adjust the
monochromator.
• Ensure that the
hollow cathode lamp is operating correctly at the recommended lamp current.
• Set the monochromator
EXACTLY to the wavelength peak approaching from the low wavelength side. Note
that some of the elements have complex spectra. Be particularly careful with
the monochromator adjustment for these elements, otherwise the instrument will
be set on the wrong wavelength.
• Select the
recommended spectral band width.
• Align the hollow
cathode lamp so that the maximum available light energy is directed along the
optical path. Before igniting the flame, move the burner close to the position required
for the final AA measurement.
Use
the following procedure:
1.
Zero the instrument to read 0.000 Abs.
2.
With the aid
of a business card or paper, use the horizontal control to align the centre of
the burner slot with respect to the light beam. Carefully move the burner
vertically until it just commences to block the light beam. This position can
be detected with the aid of the digital or peak meter readout. Again use the
horizontal control, and ensure that the centre of the burner slot is aligned
with the light beam. If this is not so, repeat the procedure. 3. The instrument
should still read zero absorbance, for example the burner must not block the
light path. This position is the best burner location from which to commence an
analytical measurement. Further “fine tuning” can only be carried out with the
flame on and aspiration
of a standard solution.
Optimization
(Flame On)
Light
the flame according to the operation manual, and aspirate a standard solution
of the analyte element which will give an absorbance of between 0.1 and 0.8.
Now carry out
the
following sequence:
1.
Carefully adjust the burner position rotationally and horizontally (not
vertically) to achieve the maximum
absorbance.
2.
Downward vertical movement of the burner may be necessary, especially with
nitrous oxide acetylene elements. Do not raise the burner so that the light
path is blocked.
3.
Adjust the flame composition to give the desired absorbance.
4.
Always adjust the position of the glass bead to give the maximum absorbance
consistent with minimum noise. The optimum bead position can be readily
established using a standard copper solution (5 mg/L), air-acetylene flame,
wavelength 324.8 nm, 0.5 nm spectral bandwidth, lamp current 4 mA. It is
recommended that the glass bead be positioned close to the nebulizer venturi
before placing the whole assembly into the spray chamber. Then move the glass
bead away from the venturi with the screwdriver adjustment (anticlockwise).
This procedure eliminates the possibility of fouling the glass bead on the
venturi
and fracturing the glass. This optimum position will be generally suitable for
all elements and most applications. For a few analyses, however, the noise level
may be unacceptably high and it will be necessary to readjust the glass bead
for the specific application.
Fault Finding
Difficulties
encountered in the operation of a flame AA can usually be categorized into four
topics which are covered in this section.
Low
Absorbance
Use
the following sequence to check possible causes:
1.
The nebulizer may be partially blocked and absorbance may
therefore be lower than expected. Nebulizer blockage can occur if the solution
being aspirated has a high dissolved solids content or if particulate matter is
suspended in solution. Seawater analysis and
some clinical samples are good examples of these problems. You should not attempt
to aspirate continuously, solutions which have a dissolved solid content of
more than about 1—2% w/v. If the old style nebulizers are being used, clean the
capillary with the cleaning wire provided. Aspirate distilled water through the
nebulizer for approximately 15 minutes to ensure that the capillary and venturi
are thoroughly cleaned. If the new (demountable) nebulizers are being used, the
nebulizer can be disassembled and cleaned as described in the operation manual.
To reduce
risk of blocking, nebulize the sample alternately with the blank solution at
intervals of approximately 15 seconds.
2.
With the flame OFF, check that the nebulizer is
fitted correctly by using a soap solution or a commercially available solution
such as “Snoop” to check for gas leaks. Check around the nebulizer and bung for
leaks. If leakages do exist, either tighten the appropriate clamps or remove
the nebulizer from the bung and refit, following the installation instructions
in the operation manual.
3.
The plastic capillary tubing occasionally becomes
blocked with dust or fluff from the surrounding atmosphere. Blockage will occur
where the plastic capillary meets the nebulizer and the sample uptake rate will
be severely reduced. Should this blockage occur, cut the end off the plastic capillary
tube containing the dust particles and replace the tube on to the nebulizer.
4.
The sample uptake rate can be measured by using a 10 mL calibrated cylinder, aspirating distilled
water for 60 seconds and calculating the uptake rate. For a fixed nebulizer
this should be between 4–6 mL per minute. When using an adjustable nebulizer
the uptake control can vary the aspiration rate over the range of 0 to 10 mL
per minute. If the nebulizer does not meet these standards then parts may have
worn out. With the new style nebulizers, disassemble and replace the worn parts
as described in the
operation
manual. The old style nebulizers are not demountable and need to be replaced.
5.
Careful adjustment of the glass bead is vital to ensure that the best
absorbance is achievable. Once set, however, the same position will effectively
apply to all elements.
6.
Use the procedure outlined in Initial Set-Up and Optimization (Flame On) to
achieve the burner position
which
will give the maximum absorbance. If the burner is too low or rotated about the
optical path, low
absorbance
will result.
7. Check the flame stoichiometry. The chemical composition of
a flame can have a marked effect on the decomposition process within the flame
and can thus have a considerable effect on the characteristic concentration. It
is therefore necessary to optimize the fuel-to-oxidant ratio for each element
being analyzed. Elements such as barium, molybdenum and silicon require a
reducing environment
(a fuel-rich flame) to obtain maximum response. Other elements such
as cadmium, copper and nickel require an oxidizing environment (that is, a
fuel-lean flame).When adjusting flame conditions it is best to keep the oxidant
flow constant and vary the fuel flow to obtain the maximum absorbance signal
while aspirating the standard solution. If necessary, change the oxidant flow
rate and again vary the fuel flow to obtain the desired absorbance.
8.
Check the stability of solutions. When very
dilute solutions of an element are being analyzed there is a high probability
that the metal concentration of the solution will change over a short period of
time. It is good practice to prepare fresh standards and solutions each day
prior to each analysis. Standards should be prepared from stock solutions
(typically 1000 mg/L) which contain about 1% v/v of an appropriate acid. Since
many standards for flame AA can be of very low concentration (possibly less than
1 mg/L) it will be necessary to add extra acid to ensure that the pH remains
low (between pH 1 and 2).
9.
The sample must be in liquid form for analysis by flame atomic absorption. If
there are particulates in solution which contain the analyte, these
particulates may settle out and give an absorbance much lower than the true value
would be. Small particles (less than 10 ,am) can be analyzed by flame AA but
large ones may not be completely atomized. It is necessary to frequently shake
solutions containing particulate matter to ensure complete dispersion
throughout the liquid. Ideally solutions for analysis should be free from suspended
matter.
10.
Check the ionization suppression. Atomic absorption spectroscopy requires that
the atoms be in their ground state. At high temperatures ionization may occur,
reducing the number of ground state atoms and therefore reducing the absorbance.
This especially occurs in the nitrous oxideacetylene flame, and some of the
more easily ionized elements include calcium, strontium and barium.
In
order to prevent ionisation an “ionization suppressant” is added. An ionization
suppressant is a high concentration of another more easily ionized element. The
elements used normally are the alkaline elements (potassium or cesium). These
elements have a lower ionization potential than the elements previously listed
and therefore minimize their ionization by supplying an excess of electrons in
the flame. Typically 5000 mg/L of potassium is added to the sample (and
standards) to suppress ionization.
11.
A fault in the hollow cathode lamp itself is rarely a cause of low absorbance.
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