Introduction to Atomic Spectroscopy Atomic Absorption Spectroscopy. Lecture 12. Introduction of Solid Samples. A variety of techniques were used to introduce solid samples into atomizers. These include: 1. Direct Sample Insertion
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A variety of techniques were used to introduce solid samples into atomizers. These include:
1. Direct Sample Insertion
Samples are first powdered and placed in a boat-like holder (from graphite or tantalum) which is placed in a flame or an electrothermal atomizer.
2. If the sample is conductive and is of a shape that can be directly used as an electrode (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. Otherwise, powdered solid samples are mixed with fine graphite and made into a paste. Upon drying, this solid composite can be used as an electrode. The discharge caused by arcs and sparks interacts with the surface of the solid sample creating a plume of very fine particulates and atoms that are swept into the atomizer by a flow of an inert gas. This process of sample introduction is called ablation
Sufficient energy from a focused intense laser will interact with the surface of samples (in a similar manner like arcs and sparks) resulting in ablation. The formed plume of vapor and fine particulates are swept into the atomizer by the flow of an inert gas. Laser ablation is becoming increasingly used since it is applicable to conductive and nonconductive samples
A low pressure envelope (1 to 10 torr argon) with two electrodes with the conductive solid sample is the cathode, as in the figure below. The technique is used for sample introduction and atomization as well. The electrodes are kept at a 250 to 1000 V DC. This high potential is sufficient to cause ionization of argon which will be accelerated to the cathode where the sample is introduced. Collision of the fast moving energetic argon ions with the sample (cathode) causes atomization by a process called sputtering.
We will cover two main techniques of atomic absorption spectroscopy (AAS), depending of the type atomizer. Two atomization techniques are usually used in AAS:
Flames are regarded as continuous atomizers since samples are continuously introduced and a constant or continuous signal is obtained. Samples in solution form are nebulized by one of the described nebulization techniques discussed previously. The most common nebulization technique is the pneumatic nebulization. Nebulized solutions are carried into a flame where atomization takes place.
a. Nebulized samples are sprayed into a flame as a spray of very fine droplets
b. Droplets will lose their solvent content due to very high flame temperatures in a process called desolvation and will thus be converted into a solid aerosol.
c. The solid aerosol is volatilized to form gaseous molecules
d. Gaseous molecules will then be atomized and neutral atoms are obtained which can be excited by absorption of enough energy. If energy is not enough for atomization, gaseous molecules will not be atomized and we may see molecular absorption or emission
e. Atoms in the gaseous state can absorb energy and are excited. If energy is too much, we may observe ionization.
The different processes occurring in flames are complicated and are not closely controlled and predicted. Therefore, it can be fairly stated that the atomization process in flames may be one of the important parameters limiting the precision of the method. It is therefore justified that we have a closer look at flames and their characteristics and the different variables contributing to their performance.
Flames can be classified into several types depending on fuel/oxidant used. For example, the following table summarizes the features of most familiar flames.
Therefore, it can be clearly seen that significant variations in flame temperatures can be obtained by changing the composition of fuel and oxidant.
On the other hand, flames are only stable at certain flow rates and thus the flow rate of the gas is very important where at low flow rates (less than the maximum burning velocity) the flame propagates into the burner body causing flashback and, in some cases, an explosion. As the flow rate is increased, the flame starts to rise above the burner body. Best flames are obtained when the flow rate of the gas is equal to the maximum burning velocity. At this equity ratio the flame is most stable. At higher ratios, flames will reach a point where they will no longer form and blow off the burner.
Three well characterized regions can be identified in a conventional flame. A lower region, close to the burner tip, with blue luminescence. This region is called the primary combustion zone which is characterized by existence of some non atomized species and presence of fuel species (C2 and CH, etc.) that emit in the blue region of the electromagnetic spectrum. The second well defined region is called the interzonal region just above the primary combustion zone. The interzonal region is rich in free atoms and is the region of choice for performing atomic spectroscopy.
It also contains the regions of highest temperatures. The third region in the flame is the outer region which is called the secondary combustion region. It is characterized by reformation of molecules as the temperature at the edges is much lower than the core. These regions can be schematically represented by the following schematic:
Flame Absorption Profiles third region in the flame is the outer region which is called the secondary combustion region. It is characterized by reformation of molecules as the temperature at the edges is much lower than the core. These regions can be schematically represented by the following schematic: We have seen that there are different temperature profiles in a flame and temperature changes as the distance from the burner tip is change
Since the temperature of a flame depends on the position from its tip, it is necessary to concentrate our work on one spot in a flame and preferably adjust the height of the flame to get best signal. In fact, not all elements require a specific height above burner tip but rather each element has its own requirements which largely reflect some of its properties. For example, one can use higher distances from the tip so that higher temperatures are achieved to analyze for silver. This is possible since silver will not be easily oxidized.
However, best results for the analysis of chromium occur at lower heights (fuel rich flames) since at higher heights oxygen from atmosphere will force chromium to convert to the oxide which will not be atomized at flame temperatures. A third situation can be observed for magnesium where increasing the height above tip will increase the signal due to increased atomization at higher temperatures. However, at higher distances the oxide starts to form leading to a decrease in signal.
There are several types of flame atomizers available. The simplest is a turbulent flow burner that is very similar to conventional Bunsen burner. This type of burner suffers from fluctuations in temperature since there is no good mechanism for homogeneous mixing of fuel and oxidant. The drop size of nebulized sample is also inhomogeneous which adds to fluctuations in signal. The path length of radiation through the flame is small which suggests a lower sensitivity of the technique.