Describe atomic absorption spectrophotometry

1. AtomicAbsorption
Spectrophotometry (AAS)
By: Zeravan Ali

2. Contents:
 Introduction.
 History.
 Principle.
 Instrumentation.
• Hollow Cathode Lamp.
• Electrodeless Discharge Lamp.
 Optical Considerations.
 AAS Detectors.
Applications of AAS.
Advantages of AAS.
Limitations of AAS.
References.

3.  Atomic absorption spectroscopy has become one of the most frequently
used tools in analytical chemistry.
 Atomic absorption methods measure the amount of energy (in the form of
photons of light, and thus a change in the wavelength) absorbed by the
sample. Specifically, a detector measures the wavelengths of light
transmitted by the sample (the "after" wavelengths), and compares them to
the wavelengths, which originally passed through the sample (the "before"
wavelengths).
 Atomic absorption spectrometry (AAS) is an easy, high-throughput, and
inexpensive technology used primarily to analyze compounds in solution.
As such, AAS is used in food and beverage, water, clinical, and
pharmaceutical analysis. It is also used in mining operations, such as to
determine the percentage of precious metal in rocks.
Introduction:

4. Introduction:
 Atomic absorption is so sensitive that it can measure down to parts per
billion of a grams (µg dm–3) in a sample. The technique makes use of the
wavelengths of light specifically absorbed by an element. They correspond
to the energies needed to promote electrons from one energy level to
another, higher, energy level.
 The process of atomic absorption is illustrated in Figure 1.
Figure 1: Atomic Absorption Process.

5.  The phenomenon of atomic absorption (AA) was first observed in 1802
with the discovery of the Fraunhofer lines in the sun's spectrum. It was not
until 1953 that Australian physicist Sir Alan Walsh demonstrated that
atomic absorption could be used as a quantitative analytical tool.
 In 1955 the modern era of atomic absorption spectroscopy began with the
work of Walsh and Alkemade and Milatz. The time since 1955 can be
divided into seven year periods. The first was an induction period (1955–
1962) when AA received attention from only a very few people. This was
followed by a growth period (1962–1969) when most of what we see today
was developed, and then by a period of relative stability (1969–1976) when
AA contributed greatly to other fields.
 We are now in a period of great change, which started in about 1976, due to
the impact of computer technology on individual laboratory instruments.
History:

6.  The absorption spectrum of an element in its gaseous atomic state consists of a series
of well defined and extremely narrow lines arising by the electronic transitions of outer
most electrons. In case of metals, most of these transitions belongs to visible and UV
regions.
 Most obvious wavelengths at which absorption or emission is observed are associated
with the transitions where minimal energy change occurs e.g., 3s-3p transition in Na
atom gives rise the emission of yellow light and this is referred as D-line transition.
 For the analysis, the sample (element) is converted into atomic vapour and then the
absorption, either in visible and UV region (at a selected wavelength), which is specific
for a given element is measured.
 For converting element into its gaseous atomic state initial step in the whole procedure
of estimation involves spraying a solution of the sample into the flame. This process is
accomplished by drawing the solution of the sample as a fine mist into a suitable flame.
In terms of Functionality, the flame serves as analogous to cell or cuvette containing
the solution in the conventional spectroscopy.
Principle:

7.  Spectrophotometric measurements are based on Beer’s Law (sometimes referred to as
the Beer-Lambert Law). When a monochromatic light beam (light with a particular
wavelength) is passed through a cell containing a specimen in a solution, part of the
light is absorbed and the rest is passed through the cell and reaches the detector.
 In spectrophotometry, transmittance is often measured as absorption (A) because there
is a linear relationship between absorbance and concentration of the analyte in the
solution
Lambert-beer’s Law
Principle:
Density C
l
I0 I
I = I0 e-k .l .C
Abs = -logI/I0 = k .l. C
k : proportional constant
l : path length
C : density (concentration)

8.  The major difference in the instrumentation of AAS and flame spectrophotometry is the
presence of a radiation source (a particular resonance wavelength cannot be isolated
from the continuous source using a prism or diffraction gratings). So, for this purpose, a
hollow cathode lamp is used (Figure 2).
Instrumentation:
Figure 2: Instrumentation of atomic absorption Spectrophotometer.

9. 1. Light Sources
An atom absorbs light at discrete wavelengths. In order to measure this narrow light
absorption with maximum sensitivity, it is necessary to use a line source, which emits the
specific wavelengths which can be absorbed by the atom. Narrow line sources not only
provide high sensitivity, but also make atomic absorption a very specific analytical
technique with few spectral interferences. The two most common line sources used in
atomic absorption are the ‘‘hollow cathode lamp’’ and the ‘‘electrodeless discharge
lamp.’’
a. Hollow Cathode Lamp (HCL):
It contains a tungsten anode and cathode is a hollow cylindrical tube which is lined by the
element to be determined. These are sealed in the glass tube filled with an inert gas like
neon or argon at a low pressure. At the end of the cylinder is a window, made up of quartz
or pyrex, transparent to the emitted radiation. Each element in question will thus emit
monochromatic radiation characteristic of the emission spectrum of that particular element
involved. So, each element has its own unique lamp which must be used for the analysis.
Instrumentation:

10.  HCLs are available for most metallic elements. A schematic diagram of a HCL is shown
in Figure 3.
Instrumentation:
Figure 3: Hollow Cathode Lamp.

11. b. Electrodeless Discharge Lamp (EDL):
 Electrodeless discharge lamps are used less frequently than the HCLs except for analytes
such as arsenic and selenium. These lamps may be excited using either microwave energy
(although these tend to be less stable) or radiofrequency energy. The radiofrequency-
excited lamps are less intense than the microwave-excited ones, but are still 5–100 times
more intense than a standard HCL.
 In the electrodeless discharge lamp, a bulb contains the element of interest (or one of its
salts) in an argon atmosphere. The radiofrequency energy ionizes the argon and this, in
turn, excites the analyte element, causing it to produce its characteristic spectrum. For
analytes such as arsenic and selenium, these lamps
give a better signal-to-noise ratio than HCLs
and have a longer useful lifetime
Instrumentation:
Figure 4: Electrodeless Discharge Lamp.

12. 2. Nebulizer
 It creates a fine spray of the sample for the introduction in the flame. The aerosol and
the fuel and oxidant are mixed thoroughly for the introduction into the flame.
 The standard nebulizer, which provides best performance with respect to minimizing
chemical interferences, is recommended for general-purpose applications. A High
Sensitivity Nebulizer is available for applications that require maximum sensitivity and
the lowest flame detection limits. The High Sensitivity Nebulizer utilizes an integral
ceramic impact bead to enhance atomization efficiency.
3. Atomizer:
 The elements which needs to be analysed needs to be in the atomic state. Here comes
the role of atomizer. It breaks down the molecules into the atoms by exposing the
analyte to high temperatures in a flame.
Instrumentation:

13.  There are two basic atom cells (a means of converting the sample, usually a liquid, into
free atoms) used in atomic absorption spectroscopy:
a. The Flame:
For flame systems, the sample is introduced via a nebulizer and spray chamber assembly.
Sample is drawn up the intake capillary by the Venturi effect. The liquid sample column
comes into contact with the fast moving flame gases and is shattered into small droplets. In
many systems it is further smashed into still smaller droplets by an impact bead. The flame
gases then carry the aerosol (the nebular) through a spray chamber containing a series of
baffles or flow spoilers. These act as a droplet size filter, passing the larger droplets to
waste (85–90% of the sample) and allowing the finer droplets to be transported to the
flame.
Instrumentation:

14. Instrumentation:
Figure 5: The flame process.

15. b. Electrothermal atomization:
 In electrothermal atomization the sample is introduced into the tube, which is then
heated in a series of steps at increasing temperature. The sample is dried at a
temperature just above the boiling point of the solvent (but not so hot as to cause
frothing and spitting of the sample); ashed (charred at an intermediate temperature to
remove as much of the concomitant matrix and potential interferences as possible
without losing any analyte); and then atomized at a high temperature. During the
atomization stage, the atoms leave the graphite surface and enter the light beam, where
they absorb the incident radiation. The ashing and atomizing temperatures used will
depend on the analyte of interest.
Instrumentation:
Figure 6: Electrothermal atomization.

16. Photometers
 The portion of an atomic absorption spectrometer’s optical system which conveys the
light from the source to the monochromator is referred to as the photometer. Three types
of photometers are typically used in atomic absorption instruments: single-beam,
double-beam and what might be called compensated single-beam or pseudo double-
beam.
1. Single-Beam Photometers:
 The instrument diagrammed in Figure 7 represents a fully functional ‘‘single-beam’’
atomic absorption spectrometer. It is called ‘‘single-beam’’ because all measurements
are based on the varying intensity of a single beam of light in a single optical path.
Optical Considerations:

17. Single-Beam Photometers:
Figure 7: Single-beam A.A Spectrometer.

18. 2. Double-Beam Photometers:
 An alternate photometer configuration, known as ‘‘double-beam’’ (Figure 8) uses
additional optics to divide the light from the lamp into a ‘‘sample beam’’ (directed
through the sample cell) and a ‘‘reference beam’’ (directed around the sample cell). In
the double-beam system, the reference beam serves as a monitor of lamp intensity and
the response characteristics of common electronic circuitry. Therefore, the observed
absorbance, determined from a ratio of sample beam and reference beam readings, is
more free of effects due to drifting lamp intensities and other electronic anomalies
which similarly affect both sample and reference beams.
Optical Considerations:

19. Double-Beam Photometers:
Figure 8: Double-beam A.A Spectrometer.

20. 3. Alternative Photometer Designs:
 There are several alternative system designs which provide advantages
similar to those of double-beam optical systems and the light throughput
characteristic of single-beam systems. Such systems can be described as
compensated single-beam or pseudo double-beam systems. One such
design uses two mechanically-adjusted mirrors to alternately direct the
entire output of the source through either the sample path (during sample
measurements) or through a reference path.
Optical Considerations:

21.  The photomultiplier tube is almost universally used as the detector type in AAS figure 9.
 Upon exiting the sample cell the beam of monochromatic light eventually strikes the
spectrophotometer’s detector.
 The role of the detector is to convert a light signal into an electrical signal. The
characteristics of an ideal detector are fast response times and a linear response over a
wide range of wavelengths with low noise and high sensitivity.
AAS Detectors:
Figure 9: photomultiplier tube.

22. Clinical analysis: Analyzing metals in biological fluids such as blood and urine.
Environmental analysis: Monitoring our environment – eg finding out the levels of
various elements in rivers, seawater, drinking water, air, petrol and drinks such as wine,
beer and fruit drinks.
Pharmaceuticals: In some pharmaceutical manufacturing processes, minute quantities of
a catalyst used in the process (usually a metal) are sometimes present in the final product.
By using AAS the amount of catalyst present can be determined.
Industry: Many raw materials are examined and AAS is widely used to check that the
major elements are present and that toxic impurities are lower than specified – eg in
concrete, where calcium is a major constituent, the lead level should be low because it is
toxic.
Mining: By using AAS the amount of metals such as gold in rocks can be determined to
see whether it is worth mining the rocks to extract the gold.
Applications of AAS:

23. • Minute amount of an element can be determined even in the
presence of high concentration of other elements.
• The technique is more sensitive than emission flame photometry.
• It is highly specific for a given element.
• It is rapid and requires only small amount of material.
Advantages of AAS:

24. • There is requirement of separate lamp for each of the element to be
determined.
• Elements such as Al, Ti, W, Mo, V, Si cannot be detected. This is due to the
fact that these elements give rise oxides of the atoms in the flame.
• The use of this technique is limited to metals only. With non-metals
difficulties arise from the strong absorption of light by the oxygen in the
light path and from the flame gases themselves.
• If aqueous solutions are used the prominent anion can affects the signal to a
noticeable degree.
Limitations of AAS:

25. 1. Sneddon, J., Atomic absorption spectrometry: Atomic Absorption Spectrometry. Theory, Design and
Applications, edited by SJ Haswell, Elsevier Science Publishers, Amsterdam, 1992, Dfl. 345.00,(529
pages), ISBN: 0-444-88217-0, 1992, Elsevier.
2. Paul, V., et al., Atomic Absorption Spectroscopy (AAS) for Elemental Analysis of Plant Samples.
Manual of ICAR Sponsored Training Programme for Technical Staff of ICAR Institutes on
“Physiological Techniques to Analyze the Impact of Climate Change on Crop Plants”, 2017: p. 84.
3. Sommer, L., Analytical absorption spectrophotometry in the visible and ultraviolet: the principles2012:
Elsevier.
4. Koirtyohann, S., A history of atomic absorption spectroscopy. Spectrochimica Acta Part B: Atomic
Spectroscopy, 1980. 35(11-12): p. 663-670.
References: