Atomic emission spectroscopy (AES) is an analytical technique used for the quantification of metal atoms by measuring the intensity of light emitted by the atoms in excited states. When an excited atom returns to the ground level, it emits radiation in a discrete wavelength. Atomic emission spectroscopy involves both excitation (absorption of radiation) and de-excitation (emission of radiation) of electrons.
Atomic emission spectroscopy, also known as ICP spectroscopy stands for inductively coupled plasma. It is named altogether as inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductive coupled plasma optical emission spectrometry (ICP-OES). It involves the excitation and de-excitation processes for electrons by absorption of radiation. When an electron emits electromagnetic radiation while coming back from an excited to de-excited state, the EMR is measured and analyzed. It is called OES due to the optical property of radiation during the de-excitation process.
Development of inductively coupled plasma atomic emission spectroscopy (ICP-AES)
This technique was developed by Sir Norman Lockyer from the United Kingdom, although it was Henrick Lundegardn who pioneered it. This was one of the best techniques used at that time for quantitative analysis. Atomic emission spectroscopy is a useful technique for determining the inorganic constituents in different samples and much more.
Working Principle of atomic emission spectroscopy
AES technique is based on the excitation of electrons to higher energy levels by absorption of a specific wavelength when heated at high temperatures. When the excited species leave the high-temperature region, they return to the ground states by emission of radiation in the form of discrete wavelength packets. These emissions pass through a monochromator or filter before detectors. The excited levels have a very short lifetime (∼10-8 sec).
Instrumentation of AES
A cup-shaped graphite electrode acts as a sample cell in atomic emission spectroscopy. The sample is introduced into plasma with the help of nebulizers.
Various types of nebulizers are used for carrying samples to the plasma flame section. The most commonly used nebulizers are concentric glass nebulizers and cross-flow nebulizers.
The sample is forced into a mixing chamber at a flow rate of 1 mL /min by a peristaltic pump and nebulized by the steam of argon flowing at about 1L /min. Argon gas flowing at very high pressure is used for changing liquid to an aerosol spray.
Atomizers consist of flames and burners. Atomizers are used for the conversion of solid or liquid samples to free gaseous atoms.
Flame atomizers require oxidant or flame gases.
The different types of burners used for atomizing purposes are
- Total consumption burner
- Laminar flow burner
- Meker burner
Electrothermal atomizers are also used for the atomization of samples. It is also known as a graphite furnace atomizer. Electrically heated graphite tubes are used instead of flame for atomizing purposes in electrothermal atomizers.
ICP (Inductively Coupled Plasma) source
ICP source is extremely hot and produces a maximum temperature of up to 6500 °K. This temperature is enough for the ionization of samples. ICP source comprises three concentric silica quartz tubes each of which is open at the top. The argon stream carries the sample in the form of aerosol and passes through the central tube. The excitation is provided by the radiofrequency range radiations (∼ 27 MHz).
Prism and diffraction grating acts as monochromators. These are used to choose the specific radiation type, emitted by the analyte and the removal of all other unwanted radiations. For this reason, it is sometimes called ‘the wavelength selector’. Diffraction gratings give better results and resolution than prisms.
Photomultiplier tubes, photoemissive cells, or array detectors act as detectors in atomic emission spectroscopy. They are used to convert optical signals into electrical current which is then amplified by the amplifier.
Signals from detectors are fed into the amplifiers which amplify signals several times making them workable or comparable.
Computers are used as readout devices in atomic emission spectroscopy. Computers analyze the data in the form of spectra and plot the calibration curve using the atomic emission ranges library.
Types of spectrophotometers
There are two types of spectrophotometers used in ICP-AES.
- Sequential spectrophotometer
- Direct spectrophotometers (simultaneous multielement spectrophotometers)
Sequential spectrometers are less expensive and more flexible than the latter. They include a single photomultiplier tube and movable gratings to select wavelength in sequential orders. Some sequential spectrophotometers use two to three photomultiplier tubes to reduce the analyzing time.
- Sequential instruments have a longer analysis time.
- A greater sample volume is required for spectroscopy to complete.
- They have poor accuracy and precision.
Direct spectrophotometer (simultaneous multielement spectrophotometer)
Direct spectrometers are faster, more precise, and more accurate than sequential ones. All elements ( up to Z=60 ) are determined simultaneously by increasing the analytical speed. Radiations from the plasma enter through single slits and are dispersed by a concave reflection grating. Then these wavelengths reach a series of exit slits which isolate specific wavelengths for specific elements.
Calibration of ICP-AES
All instruments must be calibrated before analysis for better results. For calibration, there must be three to four standard solutions containing a known concentration of metals to be analyzed and of the same concentration as sample solutions. These solutions are called primary standards.
Different high purity universal calibration standard solutions are available at a modest cost for the calibration of instruments because it is very tedious to prepare a large number of calibration solutions.
Callibirating solution is generally prepared as a sample salt dissolved in water or dilute acid.
The calibration curve is plotted between concentration and emission.
Calibration of ICP-AES is usually preferred by osmium salts rather than osmium metal itself because the heat energy will oxidize osmium to OsO4. This way Os metal is lost and the calibration process stops or gets disturbed.
Quantification process for ICP-AES/OES
Results are calculated by plotting a graph between concentration and emission.
For the quantification purpose, the instrument must be calibrated and must be free from background interferences as background interferences affect the detection limit. The correction factor is also applied for the removal of interferences.
Five to ten successive determinations are performed for the quantification and their standard deviation is calculated. Many elements are ionized because of very high plasma temperature. Therefore ionic lines are analyzed. Most of the elements even show a detection limit of 10 ppb.
To overcome the spectral overlaps and background shifts, modern instruments are equipped with PCs and monitors to allow the acquisition and display of spectral regions in the vicinity of analytical lines.
After calibration and verification of samples, they are analyzed to generate a calibration curve and results are generally printed out. There are two types of spikes in AES:
1. Pre-digestion spikes
For verification of sample preparation and analysis process, predigestion processes are used.
2. Post-digestion spikes
Post digestion spikes entirely focus on matrix effects.
Applications of Atomic Emission spectroscopy
AES is used for the following purposes:
- Metallic deficiency in living organisms can be diagnosed.
- Clinical diagnosis of different metals i.e Na, K, Ca, Mg, Fe, Zn, etc in body fluids like blood, urine, etc.
- Trace metal and rare earth elemental detections.
- The analysis of agricultural substances like fertilizers, composts, etc.
- Different metals like Fe, Ni, Mn, etc can be analyzed in motor lubricants.
- Pharmaceutical tablets can be analyzed for hazardous substances.
- Determination of alkali and alkaline earth metals.
- Igneous and metamorphic rocks can be analyzed for mineral compositions which can then be used for the dating process.
- Detection of metallic ions and minerals in water i.e. sea and freshwater have such differences.
- The analysis of trace and other elements, including multi-elemental analysis in stainless steel, frozen lava, emerging islands, etc.
1. Australian Journal of forensic sciences published an article;
It said that the bone fragments of different species like a cat, coyote, deer, fisher, fur seal, mouflon, pig, raccoon, and rat were analyzed by AE Spectroscopy. It showed significantly different levels of elements like aluminum, iron, potassium, magnesium, and sodium in different feeder types. Feeder types are the modes of nutrition, an organism adapts to. Carnivores had significantly high levels of these elements than both herbivores and omnivores. This spectroscopic evidence can prove to be a great distinguishing parameter of fossilized bones, on the basis of the mode of nutrition.
2. The human brain has also been studied using AES and mass spectrometry coupling to analyze the trace elements in the brain and their functions.
3. The chemical components of electronic cigarettes have been analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). This study focuses on the carcinogens, toxicants, and impurities which contaminate these products.
4. International Journal of Pharmacy and Life Sciences published an article on;
This study was focused on the applications of ICP-AES in the pharmaceutical analysis of impurities and quality maintenance of drugs.
What is meant by atomic emission spectroscopy?
Atomic emission spectroscopy is an analytical technique used to find the concentration of metal atoms by measuring the intensity of light emitted in their excited states.
What is the principle of atomic emission spectroscopy?
When a metal atom is directed to a high temperature, it absorbs a specific wavelength and is promoted to a higher energy state. When it returns to the ground state, it emits absorbed radiations in the form of discrete wavelengths which are then used for quantification.
What are the types of emission spectrum?
The types of emission spectrum are:
- Band spectrum
- Line spectrum
- Continuous spectrum
What is ICP AES?
ICP-AES stands for Inductively Coupled Plasma Atomic Emission Spectroscopy.
- At very high temperatures (65000 °K – 150000 °K). Argon stream carries samples through the central tube in the form of an aerosol.
- Plasma sources can produce greater excitation as compared to low temperatures which are then used to analyze elements.
- It can produce spectra for a large number of elements so it is suitable for simultaneous elemental analysis.
What is DCP AES?
DCP stands for Direct Current Plasma Atomic Absorption Spectroscopy. It consists of a high voltage discharge between two electrodes. The sample is nebulized at a flow rate of 1 mL /min using an argon carrier gas. Here, argon is ionized by high voltage discharge and able to sustain the current of ∼20 A.
Why ICP AES is better than AAS?
ICP-AES is better than AAS because
- ICP AES is less expensive than AAS.
- It can be used for direct solid analysis.
- AAS requires a light source for every element whereas ICP does not require a light source.
- ICP AES can be used for multi-elemental analysis at a time whereas AAS can only be used for one element.
Why ICP AES is better than DCP AES?
ICP AES is better than DCP AES because
- ICP has a superior detection limit with greater accuracy.
- In DCP-AES, the graphite electrodes need to be replaced after a few hours of use.
- ICP has a very high-temperature resistance than DCP-AES.
What are the advantages of a direct reader spectrometer?
- Direct readers are very faster and can be used for up to 60 elements at a time.
- Direct readers are more accurate and more precise.
- Background corrections are easily handled in direct reader spectrophotometers.
What are the disadvantages of a direct reader spectrometer?
- They are very expensive
- The instrumentation of direct reader spectrometers is very complex.
- It requires a large number of detectors for multi-elemental analysis.
What are the advantages of a sequential spectrometer?
- Sequential spectrometers are very cheap
- Instrumentation is very simple.
- Easy to use.
What are the disadvantages of a sequential spectrometer?
- Longer analysis time (one element at one time).
- Poor accuracy and precision.
- Reproducibility of wavelength selection.
What are the types of atomic emission spectroscopy?
Types of atomic emission spectroscopy are:
- Flame emission spectroscopy
- Plasma emission spectroscopy
- Inductively coupled plasma (ICP-AES)
- Direct current plasma (DCP-AES)
How many types of spectroscopy are there?
- UV Visible spectroscopy
- Atomic absorption spectroscopy
- Atomic emission spectroscopy (ICP-AES)
- Infrared spectroscopy
- Raman spectroscopy
- Nuclear magnetic resonance (NMR) spectroscopy
- X-ray Fluorescence spectroscopy
- Mass spectrometry, etc.
Different types of nebulizers used for sampling in ICP-AES?
Types of nebulizers are:
- Concentric glass nebulizers
- Crossflow nebulizers
- Hildebrand grid nebulizers
- Babington nebulizers
- Ultrasonic nebulizers
What is a plasma torch?
A plasma torch is used as an ICP source in atomic emission spectroscopy which provides energy for the excitation process. Torch is secured within an electromagnetic field arising from conducting helical coil that transforms power (27- 40 MHz) from a radiofrequency generator. Three argon streams are supplied to the torch. The largest flow is called plasma or cooling gas flowing through the upper arm at 15-20 L /min. The second flow is auxiliary gas flowing through the lower arm at 1 L /min. The third flow is sample gas passing through the nebulizer at 0.8 L /min. This whole instrumentation makes a plasma torch.
What are the ten uses of atomic emission spectroscopy?
Atomic emission spectroscopy (ICP-AES/PES) is used in:
- The detection of metal deficiency in soils and plants.
- Clinical diagnosis of different metals i.e Na, K, Ca, Mg, etc in body fluids.
- The determination of trace metals in solution.
- The analysis of agricultural materials.
- Lubrication oil can be analyzed for different metals i.e Fe, Ni, Mn, etc.
- Analysis of multicomponent pharmaceutical tablets.
- The determination of alkali and alkaline earth metals.
- The determination of the mineral composition of igneous and metamorphic rocks
- Detection of metallic ions and minerals in the water.
- The analysis of trace elements in stainless steel, etc.