Ir Spectroscopy Working Principle

[Doris Joo]

Learnabout the basics of atomic absorption analysis and design. The FAQ addresses such questions as what is atomic absorption spectroscopy, how does it work and why use it. It also describes the key features of the instruments, and other important factors that can impact successful elemental analysis by AAS.

If you wish to view a commercial atomic absorption instrument, or are interested in using AAS in your laboratory, then please visit our product pages where you can view the range of flame and graphite furnace instruments, as well as accessories, software and consumable items.

Atomic absorption spectroscopy (AAS or AA spectroscopy) is one of the earliest elemental analysis techniques to be commercially developed. So, what is atomic absorption spectroscopy? How does atomic absorption spectroscopy work?

Flame atomic absorption spectroscopy (Flame AAS or FAAS) was developed in 1952 and first commercially released as an analytical technique in the 1960s. Since then, the technique has remained popular for its reliability and simplicity. AAS is an analytical technique used to determine how much of certain elements are in a sample. It uses the principle that atoms (and ions) can absorb light at a specific, unique wavelength. When this specific wavelength of light is provided, the energy (light) is absorbed by the atom. Electrons in the atom move from the ground state to an excited state. The amount of light absorbed is measured and the concentration of the element in the sample can be calculated.

An electron is excited from the ground state to higher energy level by absorbing energy (light) at a specific wavelength. In atomic absorption spectroscopy, the wavelength of absorbed light is determined by the type of atom (which element it is) and the energy levels the electrons are moving to. How much light is absorbed is determined by the concentration of the element in the sample.

In AAS, a solution containing the analyte is introduced into a flame. The flame converts samples into free ground state atoms that can be excited. A lamp emitting light at a wavelength specific to the atoms is passed through the flame, and as the light energy is absorbed, the electrons in the atoms are elevated to an excited state.

The Beer-Lambert Law describes the relationship between light absorption and concentration of the element. According to the law, the amount of light absorbed is proportional to the number of atoms excited from the ground state in the flame.

The following diagram shows the energy levels in a lead (Pb) atom. Energy from the heat of the flame causes atoms to freely dissociate. The amount of energy required for the electrons to move between energy levels corresponds to specific wavelengths of light. As the diagram shows, moving an electron from the ground state of a Pb atom to the first energy level (E1) requires energy equivalent to light at 283.3 nm.

It requires more energy to move an electron from the ground state to the second energy level (which is further away from the nucleus). For AAS analysis, the wavelength of the ground state to the E1 level is frequently of most interest, as it is the most intense. A strong absorbance band gives the best (lowest) detection limits. In samples where the concentration of the element is higher, an alternate wavelength can be used.

For Pb analysis, a thin beam of light is passed through the flame containing the analyte. The beam contains light at 283.3 nm. The light is absorbed by Pb atoms as excitation of electrons from the Pb ground state to the first energy level occurs. The amount of absorption allows a calculation of the concentration of Pb in the sample to be determined. Only free, ground state Pb atoms in the flame will absorb at 283.3 nm. Electrons moving between other energy levels in the Pb atoms will absorb light at different wavelengths.

The liquid sample is transported via capillary tubing into the nebulizer. The pneumatic nebulizer makes use of the Venturi effect, the principle that fluid flows at a higher velocity through a narrower tube, to accelerate the solution stream. The fluid then impacts a glass bead to create a fine spray of droplets, known as an aerosol. Larger droplets drain to waste, while the fine aerosol is passed up into the spray chamber. The mixing paddles assist to further remove large droplets to maintain a homogenous flow of fine droplets into the spray chamber and burner. Mixing paddles also assist in minimizing burner blockage and ensuring thorough mixing of the oxidant/acetylene gases with the sample droplets.

As the design requires the drain trap to be partially filled with liquid, a float is included to ensure that the liquid level is always maintained. The spray chamber bung is a safety device. It will safely release upon any abnormal build-up of gases in the sample introduction system.

The atomic absorption spectrometer (AAS) burner provides a steady state of ground state atoms. In flame AAS, the burner converts the aerosol/gas mixture created by the spray chamber and nebulizer, into free, ground state atoms. There are two common gas mixtures that are burnt to fuel the flame. They are air-acetylene and nitrous oxide-acetylene. Producing a flame around 2300 C, air-acetylene is suitable for most elements. At around 2700 C, the nitrous oxide-acetylene flame creates a more reducing environment, suitable for elements that are prone to form oxides.

The graphite tube (A) is around 20 mm long. A hole in the center of the tube allows sample to be placed inside. The open ends of the tube allow light to pass through the atomized sample. (B) Cutaway of the graphite tube showing the platform design, where the sample is deposited.

A hollow cathode lamp (HCL) is the most common light source used with an AA spectrometer. The light source is a critical component. It is the absorbance of the light from the lamp as it passes through the atomized sample, that is measured.

A lamp containing a single element is most often used. Multi-element lamps can be produced, but typically are less sensitive than single element lamps. The combination of elements in multi-element lamps is restricted to avoid any spectral interferences and chemical incompatibility.

The hollow cathode lamp emits many narrow emission lines. A monochromator is used to isolate a single resonance emission line. This happens after the light passes through the sample within the atomization source, e.g. the flame.

The information collected by the instrument is fed to the controlling computer. Specialized instrument control software calculates the concentration of each element in the sample, using the calibration that was performed before the sample analysis. Statistical analysis of results, saving of instrument settings as analytical methods and the generation of reports on the analysis, are all done by the instrument software.

Modern AA spectrometers contain a network of sensors and use sophisticated algorithms to monitor and control their operating conditions. They notify the user when there is a problem with the system and ensure safety of operation.

A single-beam AA spectrometer analysis compares two light intensities. The first is the intensity of light as it passes through the flame containing none of the element of interest. This is compared to the light intensity when a sample containing the element is aspirated into the flame. The comparison allows the measurement of the total absorbance of the element-containing solution.

Double-beam AA spectrometers use the deuterium (D2) background correction technique. D2 background correction is frequently used with flame AA spectrometer instrumentation. It uses a D2 lamp to measure the non-atomic absorption and is effective between 190 to 425 nm. To perform deuterium background correction, the continuum signal from the D2 and the HCL resonance light sources are rapidly alternated. This can be achieved using a chopper mirror (a mirrored wheel that has sections cut out of it). The signal from the D2 lamp is subtracted from the HCL signal. This gives a background-corrected signal. It should be noted that the D2 lamp emits spectral components at the resonance frequency. However, the absorbance of this light by analyte elements is assumed to be negligible, as light at the target wavelength makes only a small contribution to the total radiation of the lamp.

Another type of background correction technique uses the Zeeman technique. The Zeeman technique is commonly used with a graphite furnace AA spectrometer (GF-AAS). The Zeeman technique does not require the use of an extra lamp but uses the application of a magnetic field to alternately polarize light. In simple terms, by activating a magnetic field around the HCL light beam, a background measurement can be made. When the magnetic field is off, the analytical measurements can be made.

Despite being one of the first elemental analytical techniques on the market, Atomic Absorption Spectroscopy (AAS) is still in widespread use across many industries. This is largely because the benefits of AAS are simplicity, reliability, and low cost while still delivering precise, accurate results.

This makes flame atomic absorption a very useful technique for most laboratories that are preparing their labs for elemental analysis, as well as established laboratories that have a particular need for a low-cost technique for the analysis of a handful of elements.

However, there are some limitations of flame AAS that mean alternative techniques can be more favorable. Conventional FAAS systems can be slow, requiring the measurement of the same sample multiple times, once for each element. This can be time-consuming and is less than ideal when only a small amount of sample is available. To avoid this issue, choose a fast sequential system that allows the rapid determination of all elements from a single aspiration. Additionally, the sensitivity offered by FAAS cannot match other plasma-based spectroscopy techniques as the flame temperature is much lower than that of a plasma. This makes flame AAS unsuitable for applications requiring low detection limits. Finally, FAAS should not be operated unattended due to the use of acetylene (a flammable gas), which also contributes to operational costs, but there are other techniques that overcome these issues. For example, Microwave Plasma - Atomic Emission Spectroscopy (MP-AES) runs on nitrogen, which is an inert gas that can even be extracted from air using a nitrogen generator. This offers simultaneous analysis, delivers lower detection limits, and provides a safer and cost-effective alternative to flame AAS.

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