Atomic Absorption Spectrophotometry (AAS) Analysis
by
Christian Bogdal
(Laboratory of Joseph W. Stucki,
Before
starting your first AAS analysis, you should watch the video tape provided by
Professor Stucki. It gives a detailed demonstration of
the proper use, procedures, and settings for the Varian Model SpectrAA210
spectrometer for AAS analysis.
Preparations
Prior
to AAS analysis, preparation of sample solutions is required, followed proper
protocols depending on the type of analysis being performed (absorption,
emission, element(s), etc.).
The
Varian instruction booklet entitled Analytical
Methods lists all the chemical elements one can identify with AAS. It is
recommended to go through this book and consider the element one wants to
analyze.
Table
1 provides some indications concerning two elements commonly analyzed in Professor
Stuckis group.
|
Table
1: Sample preparation for AAS analysis |
||
|
Element |
Sample
preparation |
Interferences |
|
Na |
To analyze Na it is
recommended to use Cs+ as an ionization suppressor. A final
concentration of 0.1% (1mgCs+/ml) is a suitable value and is
obtained by adding CsCl to the samples as well as
to the standards. |
Na is an element which is
present in numerous stuff, e.g. towels, skin Glassware
too frees a lot of Na. Hence it is recommended to
be very careful with Na analysis and follow this advice: -prepare solutions in plastic containers, do not
use glass. -wear disposable gloves, a clean lab-coat |
|
Fe |
If the samples contain Fe
in a precipitated form, dissolution is required. Add HCl
to the solutions in order to obtain a pH<2. The acidified solutions have
to remain 3 days before analysis. |
(See
Loveland
(1988) for more information about Fe analysis) |
Preparation
of Standards
The
establishment of a calibration curve is obtained by standards which have to be
prepared in the same way as the samples. Thus, the standards must also be
treated as described in Table 1.
Starting
the AAS Analysis
-Turn on the Varian software and the spectrometer
-Align the burner
-Prepare the software
settings (The video tape contains detailed information about the software
programming)
-The light has to be switched
on at least 20 minutes before the analysis, in order to let the light beam
stabilize
-Open the valve of the hood above the spectrometer
-The acetylene flame has to
be turned on some 10 minutes prior to the analysis, to heat the burner up to a
constant temperature
-Use fresh distilled water and always leave the sucking
tube in it
-Measure the sucking flow by
using a chronometer and a graduated cylinder. A flow rate of 5 ml / minute is
an accurate value. The flow rate can be modified by adapting the position of
the black screw which fastens the sucking tube.
Procedure
of AAS Analysis
-Between the readings of any two
samples, the sipper tube must remain in a solution. It is recommended to use
the background matrix of your samples as a rinsing solution, e.g., if you want
to analyze Na in a MgCl2 matrix, you should
rinse the system with a blank MgCl2 solution of the same molarity.
-Start by setting the
calibration curve with at least 5 standards. The concentration range depends on
the specific element. Refer to the Varian instruction booklet entitled Analytical Methods for the linear range
of the absorbance vs. concentration relationship.
-After the last standard, a
long rinsing time of 1 minute at least is required, as the last standard is the
most concentrated and precedes immediately your first sample.
-Between each analyzed
solution a rinsing time of 30 seconds with the rinsing solution is suggested,
to clear out all influences from the previous sample. After sipping the sample
solution one should wait for 10 seconds before requesting the software to read
the absorbance. This allows the system to be filled up with the sample
solution.
-After 10 samples it is
recommended to analyze one of the standards used to establish the calibration
curve. The comparison of the obtained absorbances
gives an indication about the stability of the analysis.
-Between samples, the reading
of a spy sample allows you to get an idea of the accuracy of your analysis. A
spy sample is a sample which is prepared from a certified reference solution in
the same way as the standards. However, it should be prepared independently
from your standards, which means that it should not derive from a standard. The
discrepancy between the expected concentration and the measured concentration
(according to the AAS absorbance) of the spy sample is a good way to evaluate
your results.
-At the end of the analysis
the sipping tube should remain for 5 minutes in fresh distilled water, to rinse
the system completely.
After the
AAS analysis
-Turn off the flame, the spectrometer and the
software
-Close the valve of the fume hood
-Let the burner cool down
-Immerse the burner in nitric acid (HNO3 5
% v/v) for 12 hours
-Rinse the burner abundantly
with distilled water and let it dry before installing it in the spectrometer