Analytical Chemistry - Atomic Absorption

ATOMIC ABSORPTION

Atomic Absorption Spectrometry (AAS)	Sampling Techniques for Atomic Absorption Spectrometry
Light sources for atomic absorption	Quantitative Analysis by Atomic Absorption
Advantages and Disadvantages of Atomic Absorption Spectrometry and Graphite Furnace

 

Atomic absorption spectrometry (AAS) is an analytical technique that measures the concentration of an element by measuring the amount of light (intensity of light) that is absorbed - at a characteristic wavelength - when it passes through a cloud of atoms of this element. As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way. Modern atomic absorption spectrometry was introduced in 1955 as a result of the independent work  of A. Walsh and C.T.J Alkemade. Commercial atomic absorption spectrophotometers were introduced by the early 1960’s, and the importance of atomic absorption as an analytical technique was soon evident.

 

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References

1. (a) A. Walsh, Anal. Chem., 63, 933A–941A, 1991 (b) S.R. Koirtyohann, Anal. Chem., 63, 1024A–1031A, 1991 (c) W. Slavin, Anal. Chem., 63, 1033A–1038A, 1991.

2. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000

3. D.A. Skoog, F.J.  Holler, T.A.  Nieman,  “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.

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Advantages and Disadvantages of Atomic Absorption Spectrometry and Graphite Furnace

Atomic Absorption Spectrometry (AAS) is an analytical technique that measures the concentration of an element by measuring the amount of light (intensity of light) that is absorbed - at a characteristic wavelength - when it passes through a cloud of atoms of this element.

As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way.

The main advantages of AAS  are given below:

High sample throughput
Easy to use
High precision
Inexpensive technique

The main disadvantages of AAS  are as follows:

only solutions can be analyzed
less sensitivity compared to graphite furnace
relatively large sample quantities are required (1-3 ml)
problems with refractory elements

 

Graphite furnace is by far the most advanced and widely used high sensitivity sampling technique for atomic absorption .

The main advantages of graphite furnace (GFAAS) over AAS can be summarized as follows:

Slurries and solid samples can be analyzed in addition to samples in solution
It shows greater sensitivity than AAS
Smaller quantities of sample are required (normally 5-60 μL)
The atomization process is more efficient comparing to AAS

The main disadvantages of  graphite furnace are given :

It is an expensive technique
Low sample throughput
It requires experienced operators.

 

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Relevant Posts

Atomic Absorption Spectrometry (AAS)

Sampling Techniques for Atomic Absorption Spectrometry

 

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References

1. (a) A. Walsh, Anal. Chem., 63, 933A–941A, 1991 (b) S.R. Koirtyohann, Anal. Chem., 63, 1024A–1031A, 1991 (c) W. Slavin, Anal. Chem., 63, 1033A–1038A, 1991.
2. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
3. D.A. Skoog, F.J.  Holler, T.A.  Nieman,  “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.

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Sampling Techniques for Atomic Absorption Spectrometry

 

Two systems are commonly used to produce atoms from the sample in atomic absorption spectrometry:

Aspiration
&
electrothermal atomization (graphite furnace).

 Aspiration involves sucking a solution of the sample into a flame. Figure I.1 shows a typical burner and spray chamber. Ethylene/air or ethylene /dinitrogen oxide are used. A flexible capillary tube connects the solution to the nebulizer. At the tip of the capillary, the solution is “nebulized” that means is broken into small drops. The larger drops fall out and drain off while smaller ones vaporize in the flame. Only 1% of the sample is nebulized.

 

Electrothermal atomization (graphite furnace, GFAAS) is where a drop of sample is placed into a graphite tube that is then heated electrically. This is by far the most advanced and widely used high sensitivity sampling technique for atomic absorption known also as graphite furnace. In this technique, a tube of graphite is located in the sample compartment of the atomic absorption spectrometer, with the light path passing through it. A small volume of the sample solution quantitatively placed into the tube through a sample injection hole located in the center of the tube wall. The tube is heated through a programmed temperature sequence until finally the analyte present in the sample is dissociated into atoms and atomic absorption occurs.

There are several advantages of the graphite furnace technique comparing to aspiration:

The detection limits for the graphite furnace  fall in the ppb range (ng/l) for most elements. The sample is atomized in a very short period of time, concentrating the available atoms in the heated cell and resulting in the observed increased sensitivity.
The graphite furnace is much more automated than the other techniques. Even though heating programs can be very sophisticated, the entire process is automated once the sample has been introduced and the furnace program initiated. Automatic samplers are used to facilitate the process that can be completed unattended.
Interference problems have been minimized with the development of improved instrumentation
The graphite furnace can determine most elements measurable by AA in a wide variety of matrices.

 

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Relevant Posts

Atomic Absorption Spectrometry (AAS)

Light sources for atomic absorption

 

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References

1. (a) A. Walsh, Anal. Chem., 63, 933A–941A, 1991 (b) S.R. Koirtyohann, Anal. Chem., 63, 1024A–1031A, 1991 (c) W. Slavin, Anal. Chem., 63, 1033A–1038A, 1991.
2. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
3. D.A. Skoog, F.J.  Holler, T.A.  Nieman,  “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.

 

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Light sources for atomic absorption

It is well known that an atom in its ground state absorbs light at discrete wavelengths (E = h*c/λ). 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.

The two most common line sources used in atomic absorption are the “hollow cathode lamp, HCL” and the “electrodeless discharge lamp, EDL”.

 

The Hollow Cathode Lamp HCL

The common source of light is a “hollow cathode lamp”. This contains a tungsten anode and a cylindrical hollow cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas – e.g. argon or neon – at a pressure between 1Nm-2 – 5Nm-2. 

When an electrical potential is applied between the anode and cathode, some of the inert gas atoms are ionized. These gaseous ions bombard the cathode and eject metal atoms from the cathode in a process called sputtering (Fig. I.1). Some sputtered atoms are in excited states and emit radiation characteristic of the metal as they fall back to the ground state. The shape of the cathode concentrates the radiation into a beam which passes through a quartz window, and the shape of the lamp is such that most of the sputtered atoms are redeposited on the cathode.

Hollow cathode lamps have a finite lifetime. With extended use, the sputtering process removes some of the metal atoms from the cathode and these are deposited elsewhere.

 

 

The Electrodeless Discharge Lamp  (EDL)

For most elements, the hollow cathode lamp is a completely satisfactory source for atomic absorption. In a few cases, however  such as the more volatile elements, the quality of the analysis is impaired by limitations of the hollow cathode lamp.

In cases like the above the electrodeless discharge lamp (EDL) is used. A small amount of the metal or salt of the element for which the source is to be used is sealed inside a quartz bulb. The bulb is placed inside a small, self-contained RF generator. When power is applied to the RF generator, an RF field is created. The energy will vaporize and excite the atoms, inside the bulb, causing them to emit their characteristic spectrum. Electrodeless discharge lamps are typically much more intense and in some cases, more sensitive than comparable hollow cathode lamps. They therefore offer a better precision and lower detection limits.

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Relevant Posts

Atomic Absorption Spectrometry (AAS)

Quantitative Analysis by Atomic Absorption

 

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References

1. (a) A. Walsh, Anal. Chem., 63, 933A–941A, 1991 (b) S.R. Koirtyohann, Anal. Chem., 63, 1024A–1031A, 1991 (c) W. Slavin, Anal. Chem., 63, 1033A–1038A, 1991.
2. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
3. D.A. Skoog, F.J.  Holler, T.A.  Nieman,  “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.

 

Quantitative Analysis by Atomic Absorption

 

The capability of an atom to absorb very specific wavelengths of light is utilized in atomic absorption spectrometry.

Light of a specific wavelength and of initial intensity I_o is focused on the flame cell containing ground state atoms. The initial light intensity is decreased by an amount determined by the atom concentration in the flame cell. The light is then directed to the detector where the reduced intensity, I_t, is measured. The amount of light absorbed is determined by comparing I_t  to I_o and according to Beer’s law:

Atomic Absorption Spectrometry (AAS)

 

Atomic absorption spectrometry (AAS) is an analytical technique that measures the concentration of an element by measuring the amount of light (intensity of light) that is absorbed - at a characteristic wavelength - when it passes through a cloud of atoms of this element.

As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way.

 

History of Atomic Absorption Spectrometry

The phenomenon of light absorption had already been investigated at the beginning of the 18th century. It was observed that the original radiation intensity is resolved into three components: into reflected, transmitted and absorbed radiation.

The two fundamental laws governing the fraction of incident radiation absorbed on passing through a sample are the Lambert’s law (1768) and the Beer’s law (1852).

The first law, was stated by Bouguer in 1729 and restated by Lambert in 1768 is called the Lambert’s law and predicts the effect of thickness of a sample medium upon the fraction of radiation which is absorbed. Lambert reached the intuitively reasonable conclusion that each unit length of material through which radiation passes absorbs the same fraction of radiation. If for example a monochromatic beam of intensity Io passes through a thickness d of absorbing material the transmitted beam intensity It will be reduced and will be equal with:

I_t  =  I_o * e^-k.d             (1)

The proportionality constant k is called absorption coefficient and depends on the wavelength and the temperature of a given substance.

Lambert’s law (1) is an exact law and applies to any homogeneous, nonscattering medium, regardless of whether it is a gas, liquid, solid or solution.

Lambert’s law underwent a thorough examination by Beer and he proposed a more useful law for chemistry:

A = log I_o / I_t = α * c * d       (2)

It states that the absorbance A is proportional to the concentration c of the absorbing substance and to the thickness d of the absorbing layer. The constant α is a new proportionality constant.

 

The Atomic Absorption Process

If light of just the right wavelength strikes on a free, ground state atom, the atom absorbs the light as it enters in an excited state. This process is called atomic absorption and is illustrated in Fig. I.1:

 

 

The capability of an atom to absorb very specific wavelengths of light is utilized in atomic absorption spectrometry.

Light of a specific wavelength and of initial intensity Io is focused on the flame cell containing ground state atoms. The initial light intensity is decreased by an amount determined by the atom concentration in the flame cell. The light is then directed to the detector where the reduced intensity, It, is measured. The amount of light absorbed is determined by comparing It  to Io and according to Beer’s law:

A = log I_o / I_t = α * c * d       (2)

Absorbance A is the most convenient term for characterizing light absorption in absorption spectrophotometry, as this quantity follows a linear relationship with concentration.

 

Atomic Absorption Spectrophotometer

Modern atomic absorption spectrometry was introduced in 1955 as a result of the independent work  of A. Walsh and C.T.J Alkemade. Commercial atomic absorption spectrophotometers were introduced by the early 1960’s, and the importance of atomic absorption as an analytical technique was soon evident. Perkin-Elmer has been the undisputed global leader in AAS for over 50 years.

There are five basic components of an atomic absorption instrument: The light source, an “absorption cell”, a monochromator, a detector and a screen.

The light source that emits light of a specific frequency for the element under investigation. The light source is usually a hollow cathode lamp or an electrodeless  discharge lamp. The light source is electronically modulated or mechanically chopped to differentiate between the light from the source and the emission from the sample cell.

An “absorption cell” in which atoms of the sample are produced (flame, graphite furnace, FIAS cell, FIMS cell, MHS cell). When flame atomization is used the sample is first converted into a fine mist consisting of small droplets of solution. This is accomplished by using a nebulizer assemply. Subsequently, thermal energy volatizes the particles producing a vapor consisting of molecular species, ionic species and free atoms. Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Air-acetylene (temperature: 2100-2400 C) and nitrous oxide – acetylene flames (temperature: 2600-2800  C) are used most frequently.

 

 

A monochromator for light dispersion. The monochromator is used to select the specific wavelength of light – a spectral line – which is absorbed by the sample, and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others. The light selected by the monochromator is directed onto a detector that is a photomultiplier tube. This produces an electrical signal proportional to the light intensity.

A detector, which measures the light intensity and amplifies the signal.

A screen that shows the reading after it has been processed by the instrument.

 

Uses of Atomic Absorption Spectrometry

Atomic absorption spectrometry has many uses in different areas of chemistry such as:

Clinical analysis: Analyzing metals in biological fluids and tissues such as whole blood, plasma, urine, saliva, brain tissue, liver, muscle tissue, semen, umbilical cord.
Pharmaceuticals: In some pharmaceutical manufacturing processes, minute quantities of a catalyst that remain in the final drug product, active pharmaceutical ingredient.
Water analysis: Analyzing water for its metal content
Food Analysis: The technique is increasingly being used in the food and beverage industries to ensure compliance with stringent global legislation. Combining flame, furnace, and vapor techniques, atomic absorption spectrophotometry facilitates the analysis of a large number of toxic trace elements across a wide analytical range, from parts per million (ppm) down to subparts per billion (ppb).
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 precious metals 

 

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Relevant Posts

Sampling techniques for atomic absorption spectrometry

Light sources for atomic absorption

Quantitative analysis by atomic absorption

Advantages and disadvantages atomic absorption and graphite furnace

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References

1. (a) A. Walsh, Anal. Chem., 63, 933A–941A, 1991 (b) S.R. Koirtyohann, Anal. Chem., 63, 1024A–1031A, 1991 (c) W. Slavin, Anal. Chem., 63, 1033A–1038A, 1991.
2. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
3. D.A. Skoog, F.J.  Holler, T.A.  Nieman,  “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.

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