2.2 Components of a Flame Atomic Absorption/Emission Spectrometer System
2.2.6 Instrumental Noise in the Source Lamp and Flame
Now that the source lamps and flames have been introduced, it is time to discuss sources of noise in AAS measurements; some of these also apply to AES measurements. FAAS and FAES instruments usually start with a pure source of light, and it is desirable to end with that same wavelength in as pure of a form as possible. Noise results when this process breaks down and the intensity of the wavelength of interest decreases or impure radiation reaches the detector. Decreases in the quality of light occur as the radiation passes through air and at the interfaces of surfaces (Section 1.3). In addition, there are three common causes of line broadening: natural, Doppler, and pressure broadening.
Natural broadening and the Uncertainty Effect: Natural broadening of pure spectral lines occurs due to the finite amount of time an atom spends in its excited electronic state. As the absolute time of the two states (ground and excited) approaches infinity, the width of the line resulting from a transition approaches zero; this is a direct result of the Heisenberg Uncertainty Principle for time and energy.
For example, the time required for absorption of a photon by an atomic species is approximately 10-15 seconds, while the lifetime of the excited state is about 10-9 seconds. This excited state transition is sufficiently short enough that the uncertainty in energy is greater than 10-25 J. For UV and visible wavelengths in the system discussed here, the line broadening from this uncertainty in energy affects the wavelength by 10-5 to 10-6 nm, and is considered negligible compared to other forms of line broadening.
Doppler broadening: The Doppler shift of a wavelength is an important observation in physics. This broadening is caused when an object is moving with respect to a detector while it simultaneously is emitting a wave such as a photon or sound. The observed wavelength will be slightly different when the emitter is moving towards or away from the detector. Everyone who has listened to a train whistle at a railroad crossing has observed this principle; as the train approaches a stationary observer the sound frequencies are compressed and a slightly higher frequency is heard. In contrast, as the train passes the observer the frequency is broadened and a lower frequency pitch results. For the instruments discussed in this text, the Doppler effect is observed only in a hollow cathode lamp. If an excited atom is moving toward the sample cell and detector, a slightly shorter wavelength will be observed while an atom moving away from the detector will emit a longer wavelength. This is also referred to as thermal motion. Even though atomic speeds are significantly less than the speed of light (1000 m/s), this effect can result in spectral broadening since the wavelength of interest may now overlay with another wavelength present in the sample or flame. The net result is an increase in noise and possibly an overlap with another absorbing or emitting atomic species. For the conditions in common FAAS flames, the width of a spectral line is about two orders of magnitude greater than the breadth present in the natural occurring line due to natural broadening. This is calculated for individual wavelengths by
where ν is the frequency of interest, R is the ideal gas law constant, T is temperature and M is the atomic mass of the element. For typical operating conditions, the broadening caused by the Doppler Effect is about 10-4 nm. This effect accounts for most of the line width broadening in flame-based instruments.
Pressure broadening: Pressure broadening, also known as collisional or Lorentzian broadening, results from collisions between the gaseous atom of interest and any other atom. Collisions result in radiation-less relaxations by distributing electronic energy into vibrational and rotational energy that lengthens the wavelength of the line as compared to its central frequency (unaffected frequency or wavelength in a vacuum). Pressure broadening can occur in the lamp and the flame in an AAS instrument. In a source lamp, such as a hollow cathode lamp, most collisions are between gaseous sputtered metal atoms and the noble gas. The pressure of Ar or Ne in the source lamp is very low to decrease the frequency of these collisions; most of these collisions result from other gaseous sputtered metal atoms present in the source lamp. The net result is a line broadening of approximately 10-6 nm and is much less significant that the Doppler Effect (approximately 10-4 nm). The observed two order of magnitude difference illustrates the lack of importance of pressure broadening in hollow cathode lamps. However, in high-pressure background source lamps, such as deuterium, Hg and Xe lamps, collisions are more common and the resulting broadening is capitalized upon to emit a broad range of wavelengths in the UV and visible regions. In the flame, the reaction cell used in most AAS units, collisions occur between the analyte of interest, fuel and oxidant molecules and other ions.
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