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Atomic Absorption Spectrometry _AAS_ and Emission Spectrometry _ES__1_

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					Atomic Absorption Spectrometry (AAS) and Emission Spectrometry (ES)

Homework due next time
Page WMDS, pg 256-258: 4, 5,8,11

Details of AAS
 Hollow Cathode Lamps
As mentioned above the light source for AAS is the hollow cathode lamp. A diagram of a HCL is shown below.




As the name suggests the lamp consists of 2 parts (anode and cathode) and is filled with an inert gas (generally Ar or Ne). The
HCL is connected to a regulated low voltage low current DC power supply. The flowing electrical current causes excitation of the
element present on the cathode and generates the characteristic line spectra of that element. The shape and configuration of the
HCL helps to focus the radiation into the optical path of the instrument. Provision is generally made in the instrument to further
enhance the focus of the radiation. Although there are multiple element hollow cathode lamps, generally one uses the specific
HCL for the element being analyzed. Each specific HCL has specified current settings, so be sure to adjust the DC power supply
to be within the operational parameters of that lamp. The reason for using the line source of the element for the radiation source is
its very narrow and well-defined lines. The bandwidth of a selected line may approach 0.005nm, something that most
monochromators could not accomplish with sources that produce a continuum of radiation
.
 The Atomizer
The atomizer may be a flame or an electro thermal apparatus known as the graphite furnace. We have already looked at the burner;
pictured below is the graphite furnace.




 Again, its function is to atomize the sample, not cause the excitation of the atoms to higher electronic states. The length of the
burner (or plasma region of the graphite furnace) is the approximate path length of the cell. The path length is generally not
known exactly, but its value remains a constant during standardization and analysis. For the burner a variety of fuels and oxidants
may be used as shown in your textbook, table 9.3, page 231 selected according to the required temperature. The burner can
generally be adjusted vertically to select the oxidizing (Oxidant-rich) or reducing (Fuel-rich) portion of the flame. This is a
parameter the analyst may change to obtain the best sensitivity for a given element. The most common fuel and oxidant is
acetylene and air.

Background Correction
           It is not possible to easily construct a double beam instrument as was done in uv and visible spectrometry to correct for
source lamp fluctuations and background effects because of the difficulty in setting up a duplicate blank burner and flame. What is
done is to use a continuous source simultaneously with the HCL source. Either the hydrogen or deuterium lamp is most common.
Its radiation passes through the sample along with the resonance radiation from the HCL and then the electronic system sorts out
the two sources and determines their ratio. The sorting out of the two different radiations is accomplished by optical choppers. The
first transparency shows the block diagram of the AA spectrometer with background correction. The principle here is that any
factors such as background absorption or scattering that attenuate the desired signal is equally attenuating the hydrogen/deuterium
radiation, and only the resonance line of the element being measured is absorbed in the sample.

Sensitivity
          The sensitivity of the AAS methods are dependent on many parameters which most all be controlled for accurate analysis.
These include the following:
      The temperature of the burner
      The selection of the resonance line for that element
      That portion of the flame that the light path traverses
      The flow rate of the sample or standard into the nebulizer.
All of these factors are taken into consideration (or kept constant) in a determination. The sensitivities of AAS for several elements
are shown in the following
table.
          Limits of detection
           g/mL (ppm)                Elements
                   10-3              Ag, Ca, Cd, Mg

                  10-2              Au, Be, Co, Cr, Cu, Fe,
                                     K, Li, Mn, Na, Ni, Pb,
                                     Rb, Sr, Zn

                  10-1              Al, As, Ba, Bi, Cs, Ga, Ge, In, Mo, Pt, Sb, Se, Si, Sn, Ti, TI, V

                  1              Hg
Table from "Instrumental Methods of Chemical Analysis", GW Ewing

Interferences
          Due to the complex chemical reactions that occur in the flame, significant interferences may arise in the analysis. The
major such difficulties are the incomplete dissociation of the sample into its free atomic state or the formation of refractory
compounds (oxides, especially for Ti, Al, and V). The later problem may be overcome by the use of the hotter acetylene-nitrous
oxide flame.
          In addition to the above sometimes the element being analyzed reacts with other components of the sample mixture under
the conditions of the flame. These interferences are often eliminated by the addition of a substance that suppresses the reaction. For
example in the analysis of Sr when Al or Si is present, the Sr reacts with the Al or Si. The addition of a small amount of La which
preferentially reacts with the Al or Si allows determination of the Sr.
          Another problem sometimes encountered is that the temperature required for atomization of the sample also is sufficient
to ionize a significant fraction of the analyte. For example in the determination of Ca ionization is suppressed by the addition of
the more easily oxidized Na.
          Finally, another interference may arise from differences in the flow properties such as the viscosity of the solution
containing the analyte. This is accounted for by matching the flow properties of the sample and the solutions used to make the
calibration curve.

The Calibration Curve and Evaluation of the Unknown Data
         The establishment of the standard calibration curve in AAS is identical to the use of that method in other absorption
spectroscopy methods. Solutions of known concentrations in solutions similar in flow properties to the sample solution are used to
construct a plot of absorption vs concentration. This is expected to be linear over at least 1 power of ten, though a nonlinear
response may be used if the response is well known.

         In addition to the use of a calibration curve, the methods of standard addition and internal standards are also used. In
standard addition the sample is measured and then the sample is spiked with a known amount of the analyte and re-measured. The
increase in the response (A here) is related to the amount of analyte added. The slope/point method may be used to evaluate the
concentration in the original sample, or more commonly, the response curve is extrapolated to the zero value of the y-axis (A); the
intercept (in quadrant II) is the concentration of the sample in the same units as the x-axis (concentration). The use of the standard
addition method is valuable in removing or minimizing the matrix effects of the sample, which are all of the parameters that affect
the flow and atomization of the sample such as viscosity, ionic strength, etc.
          In the internal standard method the sample is again spiked, but now with an element other than the analyte. A calibration
curve is prepared by plotting the ratio of the A of the analyte : A of the standard vs the concentration of the analyte. The sample is
then treated in exactly the same way as was the standards used to establish the calibration curve. The use of the internal standard
method is especially useful whenever a multi-step pre-analysis step may have caused sample losses or some nonspecific matrix
interference is expected. The reason for the use of the ratio is so that in the cases of loss for whatever reason, the ratio of
absorbances is the same although the absorbances themselves may have changed.

Non-flame methods
          Although less common, electro thermal atomizers generally known as the graphite furnace may also be used in AAS. As
the name suggests this device consists of a graphite tube that is mounted inside a larger cylinder electrically insulated. The graphite
tube is connected to a low voltage, high current power supply and rapidly heats to ~ 3000 K. This then atomizes the sample. The
sample is confined in the space defined by the tube and it is this space through which the light travels. Unlike the flame method of
atomization, the sample is not swept away by combusting gases. One important advantage of this method of atomization is that by
controlling the current, one may also accomplish such other functions such as drying, evaporating, and ashing prior to the
atomization. Most GF units have programmable power supplies that allow variable times for each of these operations.
          In addition to an increased sensitivity for certain elements (up to 1000 times) the GF allows the atomization to be done in
a more controlled manner and permits the analysis of some samples in a solid form without the necessity of solution pre treatment.
Generally the precision for the electro thermal methods of atomization is poorer than the flame methods. GF units are also most
expensive, both initially and to maintain. Most instruments are now constructed so an electro thermal unit can replace the burner
assembly.
          A few elements (As, Se, Sb, Bi, Ge, Sn, Te, and Pb) chemically react to form their volatile hydrides which then can be
swept with an inert gas into either the flame or a quartz cylinder in the light path for measurement. Hg is sufficiently volatile that
its measurement is accomplished by reaction of the sample with SnCl 2 which reduces all forms of Hg to the elemental form.

Advantages and Disadvantages of AAS
    + generally rapid analysis without difficult pre treatment of the sample
    + analysis of all metals at low and very low concentration levels.
    + amenable to auto sampling and computerization
    + requires same quantities (sometimes as little as microliters)
    - applicable only for metallic elements, as one must be able to make a HCL source.
    - generally requires a separate hollow cathode lamp source for each element to be
       analyzed

				
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