Title of Experiment
Determination of Metals with ICP Atomic Emission Spectrometry
Objectives of Experiment
To determine dissolved analytes in aqueous samples after suitable filtration and acid preservation.
Introduction
Inductively coupled plasma-atomic emission spectrometry (ICP-AES) is one of the most intensive and technique which is widely used in the determination of trace elements. This method uses high-frequency inductively-coupled plasma as the light source, and is convenient for the analysis of aqueous sample solutions. The spontaneous emission of photons from atoms and ions that have been excited in a RF discharge is the base of this technique. The sample solution which is directly injected to the instrument is then converted to an aerosol and directed into the central channel of the plasma. The aerosol is quickly vaporized due to high temperature at the inductively coupled plasma (ICP). Analyte elements are discharged as free atoms in the gaseous state. Further collisional excitation within the plasma transmits additional energy to the atoms, raising them to excited states. Adequate amount of energy is available to convert the atoms to ions and to promote the ions to excited states. The emission of a photon is used to relax the atomic and ionic excited state species to the ground state. The wavelength of the photons can be used to determine the elements from which they came from. The total number of photons is directly proportional to the concentration of the originating element in the sample. This method has become highly recommended due to its speed and accuracy. It is widely used as the official analysis method according to JIS and ISO standards and the Japanese Water Supply Act. This instrument supports an extensive range of analysis from trace levels to high concentrations. It also supports analysis for research and development, automated analysis for pro p ro d uc ti o n c o nt ro l,
an d
wa t e r -q ua li t y
mo n it or in g an al ys i s fo r en v ir on m en ta l
management. 1
Reagents and Apparatus
ICP stock solution containing 4 solutions with several concentrations (in ppm) of Cu and Mg respectively, 4 unknown concentrations of samples for each magnesium and copper, blank solution, centrifuge tubes, ICP-AES Spectrometer
Diagram
Nitrogen gas flow
Argon gas flow Purifies air flow
Inductively coupled plasma-atomic emission spectrometry (ICP-AES) machine
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Results and Calculations
Figure 1.1 Graph of intensity versus concentration of magnesium
Intensity versus Concentration of Magnesium 3500000 3000000 2500000
y t i s n e t n I
2000000 1500000 1000000 500000 0 0 -500000
1
2
3
4
5
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Concentration (mg/L)
The equation obtained from calibration curve for Mg: = 642399 − 42524 Sample calculation for unknown concentration of Magnesium (UM1):
= 642399 − 42524 1312493.7 = 642399 − 42524 = 2.109 /
Table 1.1: The table of concentration of each unknown of magnesium sample Unknown
Intensity
Concentration (mg/L)
UM1
1312493.7
2.109
UM2
2530633.3
4.006
UM3
4148926.9
6.525
UM4
5275925.7
8.279
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Figure 1.2 Graph of intensity versus concentration o f copper
Intensity versus Concentration of Copper 4500000 4000000 3500000 y3000000 t i 2500000 s n2000000 e t 1500000 n I 1000000
500000 0 -500000 0
1
2
3
4
5
6
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Concentration (mg/L) The equation obtained from calibration curve for Cu: = 644854 − 563 Sample calculation for unknown concentration of Copper (UC1):
= 644854 − 563 3217990.8 = 644854 − 563 = 4.991 /
Table 1.2: The table of concentration of each unknown of copper sample Unknown
Intensity
Concentration (mg/L)
UC1
3217990.8
4.991
UC2
1937753.7
3.006
UC3
4612372.0
7.153
UC4
5989315.3
9.289
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Discussion
In this experiment, induced coupled plasma atomic emission spectrometry, (ICP-AES), were used to determine the concentration of magnesium and copper in a sample. This inductively coupled plasma (ICP) machine provides fast and precise methods for observing up to 50 elements simultaneously for minor and trace levels. It also has a superior detection capability, especially for the rare-earth elements. When excited atoms from a sample are produced, they emit electromagnetic radiation at wavelength characteristic of a particular element. On the other hand, a spectrometer separates and settles these lines and measures their strength.
The ICP-AES consists of two parts, the ICP and the optical spectrometer. The ICP torch contains 3 concentric quartz glass tubes. Argon gas is typically used to create the plasma. At the point when the sample is brought into the spectrometer, it atomized into a fog like cloud. This fog is conveyed into the argon plasma with a stream of argon gas. Argon gas flows into the concentric channels of the ICP light. The radio frequency load coil is connected to its generator. As power is switched on, oscillating electric and magnetic fields are set up at the end of light. As the argon flows through the ICP light, electrons are peeled off, causing the argon atoms to form argon ions.
Based on the ICP discharge, the ions formed are normally positive ions whereas elements which form negative ions are hard to be determined via ICP-AES. These particles collide with other argon atoms to produce further ionization, framing argon plasma and causing temperature raise. The ionized argon produces temperatures near 6000-10000K, which thermally energizes the external shell electrons of the components in the sample. In some designs, nitrogen or dry compressed air is used to cut the plasma at a specific spot. A compressed nitrogen is utilized to expel the plasma tail from the optical way in order to limit obstructions and extend the dynamic range. The shear gas configuration offers an upkeep free and lower-cost way to deal with expelling the cooler plasma zone.
In this experiment, the manipulated variable is the concentration of samples and the responding variable is the intensity of light emitted. From the result of the experiment, a graph of intensity of emitted light versus concentration is drawn for both magnesium and copper samples. 5
Let y-axis to be the intensity and x-axis is the concentration. By using the three known concentration of sample, a best fit graph can be obtained. The straight line graph show the relationship of concentration of sample and intensity of light emitted to be proportional to each other. From the graph, an equation is obtained for magnesium sample that is = 642399 −
42524 and another equation = 644854 − 563 for copper sample. By using the equation obtained, the 4 unknown concentrations of samples for each magnesium and copper samples can be calculated. The concentration of each unknown Cu and Mg is recorded in Table 1.1 and Table 1.2. Nevertheless, the concentration for the unknown sample for both Mg and Cu through manual calculation is slightly different from the concentration obtained from the software. But the concentration obtained through calculation is still acceptable as it falls in the range of standard deviation.
There are some differences between ICP-AES and AAS.
AES stands for 'Atomic
Emission Spectroscopy' whereas AAS stands for 'Atomic Absorption Spectroscopy'. Both of these are spectro-analytical methods used in Chemistry to quantify the amount of a chemical species; in other words, to measure the concentration of a specific chemical species. AAS and AES differ in their operating principle where AAS employs the method of absorption of light by the atoms and, in AES, the light emitted by the atoms is what is taken into consideration. In AAS, a monochromatic light source is used to provide energy for the excitation of electrons. In the case of AES, it is a flame that is often used. Another aspect that differentiates AES from AAS is their operating principle. In AAS, there is a separate chamber for atomization of the sample in AAS. When monochromatic light is bombarded through the sample the atoms absorb energy, and the extent of absorption is recorded. However, in AES, atomization takes place step by step upon the introduction of the sample to the flame. The sample which gets atomized in the flame then absorbs the energy through the electrons which get excited. Later this energy is released upon the relaxation of the atoms and is measured by the instrument as the emitted energy.
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In aspect of detection limits, ICP-OES spectrometer has shown impressive detection limits in the sub-ppb region for some elements using a high-resolution monochromator with radially viewed plasma. Other spectrometers have been able to get improvements using an axially viewed ICP, although this view has problematic matrix interferences. Flame AAS has generally poorer detection limits than ICP-OES except for the alkalis metals for example sodium and potassium. This is because AAS technique relies on ground state atoms and the absorption of light that passes through the flame containing the atoms. But for ICP-OES requires different, much higher temperatures for emission to take place for most of the elements. Because of the high temperatures present in inductively coupled argon plasma not only atomic, but also ionic emission takes place. This is an advantage since many metals have sensitive ionic emission lines. This is the reason why ICP-OES, compared to the AAS technique, provides higher detection sensitivity.
In aspect of analysis element, ICP-OES systems can analysis few elements in one time but AAS only able to purely sequential which means element by element analysis. Although AAS are fast, require only about 3 sec for one replicate measurement versus 30 sec which is ICPOES typically used. This means that when more than 10 elements need to be analyzed an ICP is faster than an AAS.
In aspect of precision, the precision for ICP-OES up to 0.3%-1% and the precision for AAS is up to 0.1% -1%. Sample preparation is critical to the success of the method. The sample is dissolved in a solvent, typically a dilute acid, but deionized water or an organic solvent may also be used. Typically dilute nitric acid or hydrochloric acid is used to eliminate interference, but other acids may be used if necessary. If the sample is not soluble or the solution clogs the nebulizer, an alternate sample preparation method is required. Acid digestion may also be used; however precaution must be taken to prevent loss of elements during the heating process. Precautions must be taken when making mixed standard preparations to ensure all of the elements are compatible in solution and no precipitation occurs. It is important to prepare standards and 7
blanks in the same matrix as that of the sample to eliminate interference. During the experiment, student should wear protective equipment such as laboratory coat and avoid staying near the ICP-AES Spectrometer as it is operating at high temperature.
The calibration curve that we obtained in this experiment is when the concentration for magnesium and copper increase the intensity we get from ICP for magnesium and copper will also increase. This is due to the higher the concentration will having a high number of molecules to absorb the light energy.
In overall by comparing between ICP-OES and AAS, the result obtain by ICP-OES is more superior than AAS. This is because the detection limit for ICP-OES is better than AAS and ICP-OES having high detection sensitivity than AAS.
Conclusion
References
Lajunen, L. H. J. and Peramaki, P. (2005). Spectrochemical analysis by atomic absorption and emission. (1st ed.). Cambridge: Royal Society of Chemistry.
Velez, G. (2009). Inductively coupled plasma: the future of heavy metals testing. Retrieved from http://www.sgs.co.ao/~/media/Global/Documents/Technical%20Documents/sgs- regulatoryheavy%20metals-en-09.pdf
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