Lecture 08

Hinzugefügt: 2026-05-10

[00:00:03][music] [music] Hello everyone, welcome back to the MOO's course on advanced analytical technique. This is week two and the third lecture. This is the third lecture of the second week of this MOS course.

[00:00:33]In the previous few lectures, we have discussed about the atomic absorption spectroscopy and its principle working instrumentation as well as the possibility of interferences and how to avoid those interferences. Today we'll start with another technique which is very versatile and widely used in industries as well as research centers.

[00:00:58]This is called as atomic emission spectroscopy. In contrast to the atomic absorption spectroscopy, this technique is based on the emission spectrum. This atomic emission spectroscopy is a multi-anal mult is a multi-element analysis technique which can be used even for certain elements for which atomic absorption fails to provide reliable data.

[00:01:25]For example, in case of atomic absorption, we cover the alkali metals, alkaline earth metals and transition metals. And the scope of AAS is limited to 50 to 55 elements of the periodic table only. While the atomic emission spectroscopy also known as AES or optical emission spectroscopy, OEES.

[00:01:54]And this technique can be used for the analysis of around 75 elements of the periodic table.

[00:02:04]In addition to the metals as well as metaloids certain metales, this technique can be extended to leninites as well as few actctinites analysis as well. Just like atomic absorption spectroscopy, the data provided by atomic emission is very reliable and robust data. But the major difference between the atomic absorption and atomic emission spectroscopy. This atomic emission spectroscopy does not require a light source. As I I have explained you in the previous lectures in the very first lectures where I was explaining you the difference between an absorption spectrum and an emission spectrum. There I told you that for absorption spectrum a light radiation from a light source is needed that passes through the sample where interaction will occur and electrons when and atoms absorb energy so that their electrons are promoted to the next higher level. This absorption creates an absorption spectrum. While in case of emission spectrum, I told you that in case of emission spectrum, we do not eat a light source. We heat the sample provide thermal energy either by fl heating in a flame directly or by giving an electric discharge and then those atoms absorb energy and their electrons are promoted to the next higher level. when those electron relax and comes back to the ground state they emit light radiations and an emission spectrum is generated. So in atomic emission spectroscopy we do not need a light source which reduces the cost of the light source and which makes the equipment even more um versatile and robust. But since we do not need a light source in this case. So we excite the sample by thermal energy and therefore uh the thermal energy required in atomic emission spectroscopy is quite high as compared to the requirement of thermal energy or the requirement of heat in atomic absorption spectroscopy. So high thermal energy is needed. Height and heat is required in this technique because here the role of the automizer is not only to convert molecules into individual atoms but to excite those atoms as well. So here the automizer serves the purpose of automization.

[00:04:40]Here the automizer serves the purpose of automization and excitation.

[00:04:49]In atomic absorption automizer only automizes the sample convert the molecules or the compounds into individual atom while the excitation is being done by the light radiations coming from the hyoathode tube and since in this case we do not require a light source. So the same job of excitation which is being done by holocath tube in the atomic absorption here it is done by the atom by the automizer itself. So we need extra heat for excitation of these samples to put those vaporized sample in excited state not in the ground state.

[00:05:25]As I just mentioned that this technique can be used to analyze around 75 elements. So you can see in the periodic table we can these all these blue color elements can be analyzed. We have alkali metals, alkaline earth metals, we have transition metals, certain metaloids and then we have almost all we have all lenides and to actites that can be analyzed with this technique. Someone may ask a question sir why atomic absorption cannot be used for the analysis of lenthnites or these elements as well as two actinites. While this atomic emission can be used, the answer to this question is that since atomic absorption uses low thermal energy only, it has only flame automizers or graphite furnace automizers that can provide temperature of up to 3,000°C only. So the metals that can easily vaporized and dissociated in this particular temperature range can be analyzed by atomic absorption spectroscopy. While metals like these few requires excessively high temperature which is not possible with normal flame or a graphite furnace. Such metals can only be analyzed by providing very high temperature which is only poss which is only possible by using certain other modes of heating like electric arc or an electric spark or maybe a plasma. Plasma is a very good source of energy for automization as well excitation of the sample. In the later slides, I'll explain you more about plasma, what it is and how a plasma is formed and how much temperature it can achieve.

[00:07:20]So as I said atomic emission spectroscopy is another interesting and very useful analytical technique that is used for the qualitative as well as quantitative analysis of metals ranging from alkaline and alkaline earth metals to transition metals certain metalides lenise lenthnides as well as actinides.

[00:07:46]This technique is also known as optical emission spectroscopy.

[00:07:51]It works by measuring the intensity of the light emitted by the excited atoms when they return back from the excited energy level to the ground state.

[00:08:02]They when they come back to the ground state they emit light of a specific wavelength which is a characteristic of a particular type of element. And from that characteristic wavelength we can identify the type of elements present in the sample. And in and in this way we can do the analysis or we can analyze the sample and identify that what kind of elements are present.

[00:08:29]Here in this technique both excitation as well as deexitation occurs during the analysis. Excitation occur not by light but by the absorption of heat radiation provided by the automizer whose job is to automize the sample as well as excite the sample. So resulting into excitation and then which causes electrons to move to the higher energy levels from the ground state and when these electrons return back the the process called as deexitation relax and come back or we can also say relaxation. When they relax and come back to the ground state, they emit light of a specific wavelength. Based on the light emitted by these relaxation phenomena, we can identify the structure of the compound, the composition of the sample as well as its surrounding environment or its surrounding elements. What other elements or groups are present in the sample?

[00:09:40]again.

[00:09:42]And this technique is used for both qualitative as well as quantitative analysis.

[00:09:57]Just like atomic absorption, atomic emission can also be used for qualitative as well as quantitative analysis. Once again, I would like to apprise you that qualitative means to know the nature of the compound. its structure and its type and behavior.

[00:10:12]While quantitative analysis means to determine its amount or concentration in the given matrix.

[00:10:21]The basics principle of atomic emission spectroscopy. The principle of emission spectroscopy. The intensity of the light or the amount of light emitted by the sample is directly proportional to the concentration of excited atoms in vaporized form of that element. If the intensity of the emitted light is high then that means that those particular atoms are higher in number. The concentration of those atoms is higher in excited state in excited vaporized state and this is only possible when their concentration in ground state is higher.

[00:11:04]So high intensity high intensity of the emitted light means means high concentration of the sample.

[00:11:24]On the other hand, if we have light with low intensity, it means the concentration of that particular element is low in the sample matrix.

[00:11:39]So the principle of atomic emission spectroscopy based upon this phenomena that the intensity of the light is directly proportional to the concentration of vaporized atoms in excited state and it is directly proportional and from the intensity it automatically measures the concentration or the quantity of those target analyte elements.

[00:12:08]Since the atoms or molecules when absorb energy they promote their electrons to higher energy levels and we all know that these higher energy levels are not very stable. They are unstable states.

[00:12:19]So electrons immediately return back to lower energy level by releasing photons by releasing or emitting light. So this is the phenomena of excitation. This is represented in this schematic diagram where the electrons are pro getting energy where the atom are getting energy and electrons are promoted to a higher level and when these electrons return because it is an unstable state electrons decay back to the ground state and when this process of deactivation or deexitation or even relaxation occur there is of course there's a release of light which is the basis of emission spectrum.

[00:13:00]This light which is being emitted by the deactivation process by the deexitation process then detected by a photo sensitive detector which measures the intensity of the light which counts the total number of photons and based on the total number of photons it automatically calculates the amount of atoms or molecules that are present in the excited state and that are coming back from excited state to the lower energy state because when they come from higher energy state to lower energy state then they emit light. If the light intensity is more then that means then atoms that are coming from excited state to the ground state is more and that ultimately determines their concentration. So by measuring the amount of light we can determine the concentration and by measuring the wavelength of light we can identify its structure. As I said before in case of as as well since the spectrum consists of intensity on the yaxis while wavelength or lambda on the x-axis. This x-axis data is used for qualitative analysis.

[00:14:21]While the y-axis data means the peak height or peak area is used for quantitative analysis.

[00:14:37]Quantitative analysis and for quantitative analysis we need a calibration curve. While for qualitative analysis we only need a standard which through which we can compare the data the sample data as well as the standard data and if the peaks are matching we can easily identify that this peak belongs to that particular compound. But for quantitative or quantification or quantitative analysis we need to prepare a calibration curve.

[00:15:09]Now since in the discussion so far on atomic emission spectroscopy we realize that since no light source is used in atomic emission spectroscopy which means that the excitation will not occur by absorption of light. Then what are the other modes of excitation? The other mode of excitation left is the heat or the thermal energy. So since we are not using light radiations, we are using thermal energy or heat to excite the samples. Now as we are already using heat for automization of the sample in atomic absorption spectroscopy and then that automized sample is excited with the help of light radiation. But here the whole job of automization as well as excitation is being done by the thermal energy is being done by the thermal energy which is provided by the automizer. And therefore the automizers that are used in atomic emission spectroscopy they are of very u high effectivity. They are of very high uh power so that they can provide so that they can achieve very high temperatures that are not required in atomic absorption spectroscopy. And there are three types of automizers or excitation sources that are being used here. Excitation means to first of all they automize the sample converting the sample in the solvent to they first of all remove the solvent dissolvation followed by vaporization followed by vaporization then dissoc association of these molecules into individual atoms.

[00:17:13]And finally, one more process called as excitation in which the same excitation source provides extra energy causing the electrons to be promoted to the higher energy state and therefore these excitation sources have excessive energy. There are three types of excitation sources here. The flame excitation source of course the energy of flame in this case a little bit higher but still we cannot analyze all the metals especially those metals which require high temperature by this technique. So for those metals we either go for furnace excitation or the best way because plasma excitation achieves very very high temperature and the thermal energy that it provides is very high that can immediately perform automization as well as excitation of almost all the metals and therefore it increases the scope of this atomic emission spectroscopic technique as well as makes it worse. acetile the instrumentation of AES is almost similar to the AAS except the very known fact that we do not need a light source here.

[00:18:29]So in case of atomic absorption we have nebulizer we have we have first of first of all we have a nebulizer then we have automizers then in atomic absorption spectroscopy we have a nebulizer we also have a automizers then we Have a light source followed by a monochromator for selecting specific wavelengths detector and amplifier.

[00:19:19]But in this case, we do not need a light source. So I'm striking I strike out this light source. We only have a nebulizer over here. We have an automizer or it is also called as excitation source because it is also exciting this sample by promoting electrons.

[00:19:45]We have monochrometer followed by detector and amplifier. So in terms of components it has no light source. So one component is lesser as compared to atomic absorption spectroscopy.

[00:20:03]This is the flow diagram of atomic emission spectroscopy which is quite similar to atomic absorption spectroscopy with the fact that it does not have a light source. Again as you have you can recall the slide where I have explained you the instrumentation of atomic absorption spectroscopy. We have a sample holder in the form of a standard flask or a conical flask where we put the inlet pipe of the nebulizer. This nebulizer has pump which sucks in the sample and mix this sample with fuel and oxidant. The common fuel for flame automizers are either hydrogen gas or acetylene gas CH bond CH while the common oxidants are oxygen or maybe air or nitrous oxide they are used. So this nebulizer So this nebulizer sucks in the sample at a controlled flow rate and then mix it with fuel as well as oxidant. The excessive sample is drained out from this pipe. From this pipe it can be drained out while the sample is uniformly mixed with fuel and oxidant to convert it into an aerosol. So the sample is here it is in the form of solution but before injecting into the flame or into the excitation source or automizer it is converted into an aerosol just like we did in atomic absorption spectroscopy and once this aerosol is formed it is in the form of a spray in which liquid droplets from this solution are dispersed in water. As you all know that aerosol is a collidal system.

[00:22:00]Aerosol is a collideral system where liquid is dispersed in gas. So the liquid from the solution containing our target analyte molecules is mixed with gas is dispersed in gas and then at a controlled flow rate it enters into the automizer or the excitation source where excitation is provided either by flame which is called as direct excitation or by some electric discharge.

[00:22:41]And the third and best way is to use a plasma.

[00:22:50]These three either of these sources are used as excitation source. And when the high temperature high thermal energy is provided by this excitation source it immediately converts the liquid the aerosol containing liquid droplets in which the sample is present in the solvent and the whole system is dispersed in a gas. This high in temperature provided by the excitation source immediately causes drying up of the sample by evaporation of the solvent and then it dissociates the molecule or the compound into individual atoms followed by their vaporization. Finally the extra amount of energy that is being provided by the u excitation source is utilized for excitation of the sample atoms. And when these sample atoms are excited, their electrons are promoted as you all know to the higher level. And when these electrons return back from the higher level back to the ground state, they emit photons.

[00:24:04]Then these photons enters into the monochrometer by an entrance slit. Now you are aware of all these technical words. What is a monochromator? I have already explained you in the uh previous lecture of atomic absor absorption spectroscopy. So you all uh are aware about the monochromator and how a monochromator is formed by using an entrance slit and an exit slit along with few mirror like a columnating mirror and a focusing mirror and the most important thing is the prism or a grating which is used to disperse light into individual uh regions. So this this light that is being emitted by the relaxation or the deactivation process enters through an the entrance lit and they falls on the grating to be dispersed into individual radiations and falls on the focusing mirror which reflects it towards the photosensitive detector. And again as you all know that since in this technique also we are measuring the light photons. So we again use the same detector the photo multiplier tube which consists of a photosensitive anode which creates electron when it receives photons and these electrons are called as primary electrons. These electrons then falls on these diodes to create more secondary electrons which ultimately reaches to the anodic side and an amplified current signal is obtained which is displayed in the readout device or the computer. So this is a flowheet or a working diagram of an atomic emission spectroscopy and and in this way we can perform the analysis of elements of interest present in various metrics like water, it may be tab water, it may be waste water or the water from a river. Of course, after filtering the river water because it must not contains any suspended impurities as it will block these pumps or the pipes which are connecting these nebulizer with the automizer. So we avoid uh such suspended material and in addition to water sample the same technique can be used for analysis of blood, urine, serum or any other sample like juice sample in industries. With this uh I hope that you have got you have got the you have got a brief idea about the working of atomic emission spectrophotometer. In the next few lectures we'll discuss about its components and how plasma is being generated which is very important for you to learn for uh competitive for competition purposes as well as for job interviews. Till then thank you very much. [music]