1. Introduction to Atomic Spectroscopy
Lecture 11

2. Effect of Temperature on Atomic Absorption and Emission
The question here is which technique would be affected more as a result of fluctuations in temperature? The answer to this important question is rather simple. Atomic emission is the technique that will be severely affected by fluctuations in temperature since signal is dependent on the number of atoms in the excited state. This number is significantly affected by fluctuations in temperature as seen from the example above. However, in the case of atomic absorption, the signal depends on the number of atoms in ground state that will absorb energy.

3. very high as related to the number of excited atoms:
Nj/No = 1.72x10-4 or
172 excited atoms for each 106 atoms in ground state
This suggests a very high population of the ground state even at high temperatures. Therefore, atomic absorption will not be affected to any significant extent by fluctuations in temperature, if compared to atomic emission spectroscopy.

4. However, there are some indirect effects of temperature on atomic absorption spectroscopy. These effects can be summarized as:
Better sensitivities are obtained at higher temperatures since higher temperatures can increase the number of vaporized atoms at any time.
Higher temperatures will increase the velocities of gaseous atoms, thus causing line broadening as a result of the Doppler and collisional effects.
High temperatures increase the number of ionized analyte and thus decrease the number of atoms available for absorption.

5. Band and Continuum Spectra Associated with Atomic Spectra
When the atomization temperature is insufficient to cause atomization of all species in the sample matrix, the existent molecular entities, at the temperature of the analysis, impose very important problems on the results of atomic absorption and emission spectroscopy. The background band spectrum should be removed for reasonable determination of analytes. Otherwise, the sensitivity of the instrument will be significantly decreased.

7. As the signal for the blank is considered zero and thus the instrument is made to read zero, when the analyte is to be determined, it got to have an absorbance greater than the highest point on the continuum and the instrument will assume that the absorbance related to analyte is just the value exceeding the background blank value. This will severely limit the sensitivity of the technique.

8. Putting this conclusion in other words we may say that if the analyte signal is less than the background blank, the instrument will read it as zero. Therefore, it is very important to correct for the background or simply eliminate it through use of very high temperatures that will practically atomize all species in the matrix. We will come to background correction methods in the next chapter.

9. Atomization Methods
It is essential, as we have seen from previous discussion, that all sample components (including analytes, additives, etc.) should be atomized. The atoms in the gaseous state absorb or emit radiation and can thus be determined. Many ionization methods are available which will be detailed in the next two chapters. Generally, atomization methods can be summarized below:

11. Sample Introduction Methods
The method of choice for a specific sample will mainly depend on whether the sample is in solution or solid form. The method for sample introduction in atomic spectroscopy affects the precision, accuracy and detection limit of the analytical procedure.

12. Introduction of Solution Samples
1. Pneumatic Nebulizers
Samples in solution are usually easily introduced into the atomizer by a simple nebulization, aspiration, process. Nebulization converts the solution into an aerosol of very fine droplets using a jet of compressed gas. The flow of gas carries the aerosol droplets to the atomization chamber or region. Several versions of nebulizers are available and few are shown in the figure below:

13. Concentric Tube Nebulizer

14. Cross Flow Nebulizer

15. Babington Nebulizer

16. Fritted Disc Nebulizer

17. 2. Ultrasonic Nebulizers
In this case samples are pumped onto the surface of a piezoelectric crystal that vibrates in the kHz to MHz range. Such vibrations convert samples into homogeneous aerosols that can be driven into atomizers. Ultrasonic nebulization is preferred over pneumatic nebulization since finer droplets and more homogeneous aerosols are usually achieved. However, most instruments use pneumatic nebulization.

19. 3. Electrothermal Vaporization
An accurately measured quantity of sample (few mL) is introduced into an electrically heated cylindrical chamber through which an inert gas flows. Usually, the cylinder is made of pyrolytic carbon but tungsten cylinders are now available. The signal produced by instruments which use electrothermal vaporization (ETV) is a discrete signal for each sample injection. Electrothemal vaporizers are called discrete atomizers to differentiate them from nebulizers which are called continuous atomizers

21. 4. Hydride Generation Techniques
Samples that contain arsenic, antimony, tin, selenium, bismuth, and lead can be vaporized by converting them to volatile hydrides by addition of sodium borohydride. Volatile hydrides are then swept into the atomizer by a stream of an inert gas.