Atomic Emission Spectroscopy

Atomic Emission Spectroscopy (AES) is a chemical analysis method in which the quantity of an element is determined by measuring the intensity of light emitted from a flame, plasma, arc or spark. The element can be recognized by the wavelength of the emitted light.^[1][2] There are three common methods that follow this phenomenon: flame emission spectroscopy, inductively coupled plasma atomic emission spectroscopy and arc atomic emission spectroscopy.^[1] 

Basic principle

In atomic emission spectroscopy, energy is applied to a molecule in form of light or heat. This energy then excites the electrons in the molecule to a higher energy level. The excited state is, however, less stable than the ground state of an atom, this results in decay of the excited state. When decaying, the electron returns from the higher energy level to a lower one and energy is released. The energy is released as a quantum of electromagnetic radiation, better known as a photon.^[3][2] This phenomenon is demonstrated in Figure 1.^[2]

Since the energy is quantified by the higher and lower energy levels which are specific to each element, the wavelength of the emitted photon is also quantified. Therefore, the element can be detected from the wavelength of the radiation and the concentration of said element can be determined from the intensity of emitted radiation.^[3]

Figure 1. Excitation and decay of an electron, and emission of photon, depicted in Bohr atomic model. (Figure: Eero Kuusisto, adapted from^[2])

The dependence of energy level difference and wavelength is presented in equation (1). Where, ΔE is energy level difference, E₁ and E₂ are the energy levels, h is Planck constant, c is the speed of light and γ is the wavelength.^[3]

The AES instrument system typically consists of flame or plasma, monochromator (or polychromator) and detector. As previously already mentioned plasma is used to excite the atoms of the sample to higher energy level. The purpose of monochromator is to limit the radiation emitted from the sample to detector in small wavelength bands. This is done to determine the intensity of different wave lengths throughout the spectrum. Lastly the detector detects the photons of wavelengths limited by the monochromator that hit its surface. To detect low concentrations, where only few photons are emitted, photon multiplier can be used.^[1][2]

Inductively coupled plasma atomic emission spectroscopy

When it comes to AES methods, the plasma spectroscopies are mainstay of modern day chemistry.^[4][5] The main difference between AES instruments is the method of excitation. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) uses a high-frequency inductively-coupled plasma as light source.^[6] The wavelengths used for AES range from upper part of vacuum ultraviolet (160 nm) to visible light (800 nm). The lower end of spectrum causes limitations for instrumentation as borosilicate glass absorbs light below 310 nm and oxygen in air absorbs below 200 nm. This can be partly bypassed using quartz glass and filling optical paths with argon.^[7]

When examining a typical ICP-AES instrument, its structure can be divided into two main parts: the ICP torch (Figure 2) and optical spectrometer. The spectrometers used in different AES methods are very similar and the main difference between the methods comes from the different paths used for excitation of sample material.^[5] In the torch typically three coils are used to control the plasma (ionized argon), which can have temperatures close to 7000 K. The hot plasma is used to excite the sample into excited state and the emissions are measured.

Figure 2. An Argon plasma torch of ICP-AES (License: Open Government Licence). ^[8]

The method has become highly regarded in past few years as the number of analyzed samples and elements has increased.^[6] In fact as many as 50 elements can be measured simultaneously for minor- or trace levels.^[5]

ICP-AES can be used in many kinds of situations to determine the chemical composition of material. For example, Takada et al. used ICP-AES in their study of superconducting CoO₂ layers to determine the molecular composition of synthesized materials. In the study, the composition of Na_0.35CoO₂·1.3H₂O was determined for a successful superconductor (T_c of 5 K) from the Na and Co content of sample.^[9]

Flame emission spectroscopy

In flame emission spectroscopy (FES), the studied material is brought into the flame as a gas or in solution which is first nebulized (converted into a fine aerosol). The heat of the flame then evaporates the solvent and breaks the chemical bonding between the atom (or ions) of the compound to generate free atoms. In addition, the thermal energy also excites the atoms to excited states.^[10][11] Similarly to other AES methods, the emitted photons of transition from higher energy state to a lower or ground energy state are studied. The radiation is passed through monochromator to isolate a specific wavelength band which is directed to the detector.

In the schematic (Figure 3), the typical process of flame emission spectroscopy measurement can be seen. First the sample solution, which in this example is metal salt, is nebulized, this is typically done using high velocity gas jet. The gas used is commonly the oxidant of the burning flame. In following steps the solvent is evaporated to afford solid aerosol, which the is sublimated to gas. In the gas molecular excitations and emission can be observed, until the sample is dissociated into its elements. In some cases side reactions for the metal occur. Alkaline metals such as K, Cs or Sr ionize easily, which creates inaccuracy in measurements of their concentration. For other metals, this phenomenon can be exploited, by adding excessive concentrations before mentioned easily ionized elements, the ionization in sample and calibration solutions can be suppressed. The addition of suppressants is evermore vital in analyzes that require hotter acetylene/nitrous oxide flames. The final step shows the similarity of atom absorption spectrometry (AAS) and flame emission spectrometry (FES), where the difference lays in whether the excitation or emission is measured.^[10]

Figure 3. Flame emission spectroscopy process schematic of salt MX. (Figure: Eero Kuusisto, adapted from ^[10]).

The FES is commonly used in determination of trace metals, especially in liquid samples. For FES offers an inexpensive and sensitive method for detecting common metals, this includes alkali, alkaline earth and many transition metals (ex. Fe, Mn, Cu and Zn) as well.^[10][11] Also FES detectors for P and S are also available for use in gas chromatography.^[10] The FES has been largely displaced be plasma spectroscopies, due to their higher atomization ratios. However, in regards of alkali and alkaline earth metals FES could be advantageous for less ionization will occur in lower temperatures of flame compared to plasma.^[4]

References