Lecture 06

Added: 2026-05-10

[00:00:03][music] [music] Welcome back to the MOO's course on advanced analytical techniques. Myself Dr. from Mohammed Zan Khan working as assistant professor in the department of industrial chemistry Alig Muslim University Alig today we are going to start the first lecture of the second week of this MOO's course in the previous week we have completed the types of automizers which are being used in the atomic absorption spectroscopy we have discussed about the flame automizers and the graphite furnace based automizers they are working they are um design as well as their limitations and strength. Now we'll in today's lecture we I'll talk about the light source which is being used in the atomic absorption spectroscopy as well as other components like monochrometer and the detectors. So let us begin with the light source in the atomic absorption spectroscopy.

[00:01:21]The light source which is commonly used is the holo cathode lamp.

[00:01:25]This holo cathode lamp consists of a cylindrical tube containing two electrodes.

[00:01:36]One we have cathode that is made up of the metal which is being analyzed while anode is made up of tungsten and these two electrodes are sealed in a glass tube. So the light source which is commonly used in atomic absorption spectroscopy is the holo cathode lamp.

[00:01:59]This lamp has two electrodes that are sealed in a cylindrical glass or quartz chamber. And these two electrodes. The anode is made of tungsten. While the cathode which is placed in the middle is made up of the element of interest. Means cathode is made of the target analyte metal itself. For example, if we are analyzing calcium, the cathode is made up of calcium. While if we are analyzing some other metal, it is made up of that particular metal because in that case only it will produce the wavelength which is needed to absorb the atoms of that analyte resulting into an absorption spectrum. If we are using some other metal in the holocath tube, the wavelength that are the light the wavelength of the light radiation that is being generated will not exactly matches with the energy gap between the ground level and excited state level of the target analyte resulting into low absorption or there will be no absorption resulting into and there will be no spectrum. Therefore for analyzing a particular sample the holocath tube is selected specifically based on the target analyte which is present in the form of cathode while anode is made of tensen. Then in order to avoid the oxidation of those metals we fill this cylindrical tube with argan gas or neon gas. Organ gas as compared to neon its availability is quite good and the rate is low and the price is also low. So we mostly most of the time we go for argan gas and in order to generate a light radiation of that particular wavelength we pass electric current through these electrodes. The magnitude of the electric current is around 10 miamp.

[00:04:12]And when this current is passed, the cathode starts emitting light radiation that are ultimately absorbed by the atoms of individ that are ultimately absorbed by the individual atoms present in vaporized form in the automizer.

[00:04:32]For example, if you are using copper, then the line that are being emitted are having the wavelength. For example, if we are for example, if we are working on copper, the excited metal emits some characteristic spectral lines for copper which is around 322 324.7 nanometers.

[00:04:56]Similarly, if you are working on selenium and cathode is based on selium selenium metal, the wavelength that is of the the wavelength that is being covered by the light radiation is around 196 nanometer.

[00:05:10]In case the pure metal is not available or the pure metal is quite costly, we may also go for an alloy of the same element. Sometimes the pure metals are quite soft as well. So we mix them up with some other alloying element in order to improve their strength or to reduce their price and then we can use that alloy in the cathode instead of the pure metal. So there is a flexibility that if the pure metal is not available or is not feasible to be used in the cathode directly we can use its alloy.

[00:05:44]So this is the design of the holo cathode tube. We have this cylindrical chamber which is made of quartz or even Pyrex glass because they have high uh optical properties and they are thermally stable as well. And this tube contains anode. And then we have this U-shaped holo cathode which is made up of the metal that is being analyzed.

[00:06:12]And uh if that pure metal is not available, this cathode is made up of the alloy of that target analyte metal.

[00:06:19]And when the current is passed around 10 m, this cathode start producing light radiation that has a specific wavelength needed for absorption of those elements.

[00:06:34]And in this cylindrical chamber, we fill this chamber up with the noble gas or an inert gas like argan and neon which reduces the possibility of any reaction or oxidation of the metal present in the electrodes. As I said earlier, for example, if we are going to analyze selenium metal, if we have a sample that contains selenium metal and we are going to quantify it, then we use the cathode that is made specifically of selenium or an alloy of selenium so that it may generate light radiation with wavelength necessary for absorption of selenium metal present in the sample. If for example you are working as a scientist or you are a professor in a university and your department is going to procure that atomic absorption spectroscopy machine then the vendor will ask you or the company will ask you that what type of metals you are going to analyze because while selecting you have to tell them what holocath tube you need. For example, if you're going to analyze only alkal or alkaline and met alkal or alkaline earth metal like calcium, sodium, potassium, then they will provide you those tubes, those lamps that can cover their wavelengths.

[00:07:49]Similarly, if you are going for transition metals, then you have to tell them what kind of transition metals you are um exploring so that they may so that they will provide you a specific light source for those elements.

[00:08:06]The so this is the schematic diagram of the holo cathode tube and this is the actual diagram. This is a quartz window or the cost chamber which is of cylindrical shape and then we have this is this is our anode. Then we have a cathode placed over here which is based on uh which is made of the target metal itself target metal or its alloy.

[00:08:42]So it is made of target metal or its alloy and from here we pass current which ultimately results into the light radiation of specific wavelengths. And this chamber is filled with these buffer gases or the noble gases or the or you can say inert gases that avoids any reaction.

[00:09:04]Then after the light source one of the most important component is the monochromator as its name is reflecting monochromator. Mono means one chroma. This chromator comes from the word chroma which means color. So this monochrometer is used to select a particular color wavelength or I should say it is used to select a specific region of the wavelength or I should say it is used to cover the specific region of the electromagnetic radiation for the analysis. Now this monochrometer assembly consists of two parts. On one side it has a prism or a grating which is used to disperse light to distribute the light into its individual components or I should say into its individual colors. This pris you have you all have seen a pris which convert an ordinary light into a specific colors depending upon the level of scattering.

[00:10:17]The color which is scatters more they are towards this side while the colors that is scattered very least while the colors that is scattered least will be on the lower side. So based upon their scattering they this prism can divide or distribute an ordinary white light into individual colors. Now depending upon our requirement we can select any color. So if for example we have a prism over here and the light is coming in this direction it is being distributed or dispersed into different color and suppose we want red color. We want this particular color then we put a slit over here. so that all other light radiations will be blocked and only this particular light radiation is allowed for onward transmission resulting into the absorption.

[00:11:14]For example, if we do not want this but we want this particular radiation, then we use a slit over here and we use a slit over here so that it can allow this particular radiation only while these radiations are being restricted to enter into the automizer. So in this way a monochromter is used which is a combination of slits. There are two slits used entrance slits and exit slit and a prism or a grating. The purpose of as I said before the purpose of slit is to the as I said before the purpose of prism or grating is to distribute or disperse the light into individual components while slits are used to select a specific wavelength while cutting off the other wavelengths. So it restrict as it is mentioned here it restricts any scattered light from passing through the sample or the detector. It only allows specific light radiation which are needed by the user who is operating the machine. It allows only those radiations to pass through it. Rest all the radiations are being stopped from entering into the sample and thereby finally falling on the detector. So a monochrometer selects a few wavelengths only while it excludes the other wavelengths that are not needed because if all the light radiation passes through the sample and ultimately all the light radiation falls on the detector and our since our detector is photosensitive it is highly sensitive to the light radiations. It will give you number of peaks which will create large number of lines in our spectrum ultimately causing complications for us to analyze the spectrum. Therefore the role of a monochromator is very important. It selects only the desired wavelength while it excludes the other radiations. Let us see the schematic diagram of a monochrometer. Here you can see it is a rectangular box where we have two slits. This is called an entrance slit while this is the exit slit. And light from the light source have holo cathode lamp enters into this entrance slit. It first of all falls on the culminating it first of all falls on the cumating mirror. Its role is to just reflect the light radiation towards the grating. If the grating is not available, we can also use a prism over here. Otherwise the mostly all the commercially available uh atomic absorption spectrophotometers most of them uses grating instead of a prism. So this light is being reflected towards the grating which convert this light into individual components. These individual light radiations further falls on another mirror which is called as the focusing mirror and th this focusing mirror pushes them towards the exit slit and exit slit is played in such a manner that it was that it will allow only the desired light radiation to pass through it and falls to pass through it and enters into the sample tube and finally falls on the detector.

[00:14:33]while this exit slit restrict or is stopped the other undesired wavelength.

[00:14:40]So in this way this monochrometer assembly is used as a light filter that filters a specific wavelength while excludes the other.

[00:14:51]After the monochrometer the light enters into the detector which is one of the most important component.

[00:15:00]It is a photosensitive device that is very sensitive towards light radiation.

[00:15:06]It can detect the intensity of the light very easily because it has a photosensitive cathode placed inside it along with an anode.

[00:15:20]So it is used to study the transmitted light to identify or to calculate the intensity of the transmitted light and from the intensity of and from the intensity difference of the incident light as well as the transmitted light. We can easily calculate the intensity of the absorbed light. For example, if the transmitted light intensity is very high is it is if it is almost equal to the incident light then that means the absorption did not occur or if the absorption occur it occurs to a very lower degree. While if the transmitted light has lower intensity then it means that absorption occur to a higher degree. There has been some very good interaction with the sample molecules or there has been very good interaction with the sample atoms the target analyte atoms and the light radiations resulting into good absorption and therefore the intensity of the transmitted light is low.

[00:16:24]So this detector there are different types of detectors used in different techniques in atomic absorption spectroscopy. The detector which is used is called as the photo multiplier tube.

[00:16:35]The the detector is called as PMT or the photo multiplier tube. The detector which is used in atomic absorption spectoscopy is the photo multiplier tube or in short we can say it is PMT detector which is a photosensitive equipment used to detect the intensity of the transmitted light.

[00:16:56]What it does is that it captures the transmitted light photons and then convert it into electrical signal. So what we see in the display is the electrical signal which is being obtained by converting the photons which which are present in the transmitted light with the help of a circuit that converts the light radiation or the intensity of light radiation or the total number of photons that are falling on the photoactive cathode into an electrical signal. Now the design of this detector includes two electrodes. One is called a photocathode which is a photosensitive material. Photosensitive means when electron when photons falls on this object it emits electron. I hope you know about photoelectric effect.

[00:17:51]Photoelectric effect says that light when falls on a surface of the metal it ejects electron depending upon the intensity of the light we can have more number of electrons or if the light intensity is low we can have lower number of electrons ejected from the metal and the electrons that are ejected depends upon the work function of the metal. Similarly, the photosensitive cathode which is used has lower work function. So that when light ray falls or when the light photons falls on it, it results into number of electrons ejected from it and then these electrons are accelerated towards the anode. Here a node is not a photosensitive material because we only need photosensitive material in the cathode because cathode is facing the transmitted light when the because only the transmitted light is falling on the cathode. So we need a photosensitive material to be present in the cathode. When transmitted light falls upon this cathode, it ejects electron. Then in between these two electrodes mean the photocathode as well as the anode we have these diodes. These specifically designed metallic electrodes that serves as a diodes. The purpose of these diodes is to eject secondary electrons. The electrons that are being ejected from the photosensitive cathode they are called as primary electrons or I can say 1 degree. One degree is generally used as a synonym for in chemical sciences we prefer we use one degree for primary. So these when the light radiation falls the trans when the transmitted light radiation falls on the photosensitive cathode primary electron the electrons detect the electrons ejected are called as primary electrons.

[00:19:41]Then these electrons since the signal is weak the number of electrons ejected are not very high.

[00:19:50]Therefore in order to make that signal readable we place diodes. These dian nodes will eject extra electrons. When these primary electrons collide with these dodes they will move like this. And in this way more electrons are rejected. And finally when the these electrons reaches anode the total number of electrons is quite high as compared to the primary electrons ejected from this photosensitive cathode and we have good number of electrons that are reaching to anode resulting into an amplified signal. Otherwise if we are not using these dodes the signal would would be very weak and it is very difficult to analyze that signal in without using dot the signal would be very small and we it will be difficult for us to analyze whether it is an impurity signal or it is the analyte signal because the concentration or the signal size or the peak size because the concentration of the peak size would almost be the same. But if we are using diodes which eject secondary electrons thereby enhancing the signal or amplify the total number of electrons and the current would ultimately increase and a bigger peak is obtained so that we can easily identify the target analyte and if some background noise is there or if some impurities are present in lower amount they definitely have a very small peak. So we can easily distinguish between the peak responsible for an impurity or a background noise and the peak that is uh responsible for the target and a light. So in this way a photo multiplied tube is used as a photo detector to detect the light intensity of the transmitted light and it converts it into an electrical signal and the display device shows us a large peak or a small peak depending upon the current that is being generated which is which depends upon the total number of secondary electrons reaches to the anode of the photo multiplier tube.

[00:22:02]So this is uh the text of what I have just explained. These photogenerated electrons means the electrons that are being ejected from the cathode once it receives incident photons of the transmitted light they are accelerated with the voltage and fall on the first diode to generate secondary electrons.

[00:22:21]These secondary electrons are generated.

[00:22:23]Then these secondary electrons falls on the second diode to further enhance to further reject secondary electrons.

[00:22:30]Ultimately when it reaches to the last dode then total number of electron is very high which creates a bigger signal which creates an amplified signal based on the high value of the current which makes our analysis easy.

[00:22:49]This amplified signal is then sent to the display device or the computer.

[00:22:56]This is a sample spectra. Uh this is a sample spectra of three elements, three metals, tin, iron and cadmium that are being analyzed with a graphite, furnace base, atomic absorption spectrophotometer.

[00:23:11]And here you can see that these three elements the green represents iron while red represents tin and then we have calc and and then we have cadmium. These it can easily be converted into its individual atoms and the absorption also occurs at comparatively.

[00:23:32]So the green represents iron while red represents tin and blue is for cadmium.

[00:23:38]So we can see different um we can see highly resolved peak separate peaks for each elements and sharp peaks are obtained and depending upon the size of these peaks we can say that the amount of cadmium is highest in the sample while tin is lowest in that particular sample and this sample we have obtained with the help of a graphite furnacebased atomic absorption specttophotometer.

[00:24:08]Which means that the sample containing iron, tin and cadmium is taken in liquid form in solution form and then a pipe inlet pipe of and the inlet pipe of the nebulizer is immersed into that solution which sucks in the sample then mix it with fuel and oxygen converted into aerosol and it pushed it by creating a lower pressure or negative pressure at the end of the nebilizing tube. It pushed it at high pressure into the graphite furnace where the sample immediately automized in three steps basically dissolvation, vaporization and dissociation or automization so that compound is that so that the molecules gets converted into individual atoms in vaporized state. Then through this graphite furnace there are optical windows through which the light from the holo cathode tube is coming when the light interacts with these vaporized individual atoms of these metals like iron, tin and cadmium they absorb certain radiations the transmitted light which passes through the other window of the graphite furnace falls first of all passes through the monochromator where we select particular wavelength otherwise there will be too much lines or too much of excess excessive peaks in the spectrum making our analysis quite complicated. So we select some wavelength and we exclude the other wavelengths and then after passing through the exit slit of the monochrometer the transmitted light finally falls on the detector which detect which is a photosensitive equipment has a photosensitive cathode which detected which detects the signal by ejecting primary electrons. Then those primary electrons falls subsequently upon the dodes creating more number of secondary electrons ultimately increasing the ultimately amplifying the signal giving creating more current and large peaks and separate individual and large peaks of these elements are shown in the display device which are being shown here. So all these peaks correspond to three different elements that are quite away from each other. So there is no overlap and also the size of the peaks tells us about the concentration of the individual elements present in the sample.

[00:26:33]Now comes the calibration curve.

[00:26:35]Calibration curve is an important plot which is being used to determine the concentration of an unknown sample.

[00:26:47]Without a calibration curve, we can not go for a quantitative analysis. As I told you in the beginning that atomic absorption spectroscopy can be used for qualitative as well as quantitative analysis. So it can be used for qualitative [cough and clears throat] sorry and quantitative analysis. For qualitative analysis, we generally for qualitative analysis, we focus on the x-axis of the spectrum means the wavelength based on the wavelength at which absorption occur. We gets an idea about the type of element. But based on the data given on the y-axis means the data which is showing the current value or the peak height or the peak area gives us the amount or the concentration of this that element in the given sample matrix. So for quantitative analysis we cannot do it directly. We can do the qualitative analysis by comparing the spectrum with the standard spectrum. And if the peaks are matching up more than 90% or if the peaks are m matching up 100% or even more than 90% we can say that those peaks are we can say that those peaks belongs to the compound which is shown in this standard. But for quantitative analysis we cannot do it directly. We have to first of all prepare a calibration curve. And a calibration curve is a curve that is based on known concentrations and it is used to determine an unknown concentration.

[00:28:44]How this curve is made? For making this curve we prepare different solutions that different standard solution. Standard solution means a solution whose concentration is known to us and whose concentration is constant. It's not changing. So we prepare few solutions of the analyte. For example, if we are working on the if we are analyzing copper 2+ so we prepare five different solutions of copper in one sample. We take blank only with no copper. We name it as 0 ppm copper. Then we have sample number two. For example, if we prepare it 10 ppm, then the third sample is 20 ppm. Then 30, then 40. We may take six samples as well. So the so the last sample is 50 ppm of copper only prepared from copper nitrate or copper chloride or any [snorts] salt of copper having cubric ion present because we are going to analyze CO2 plus and then these standard solutions because their concentration is known. We one by one put these samples into the nebulizers and a spectrum is generated based on the peak height. We plot a graph between the peak height as well as the concentration. As you can see either the absorbance or the peak height. In case of UV visible spectroscopy we use absorbance.

[00:30:17]Otherwise we plot peak height versus concentrations.

[00:30:21]We can put our concentration 0 ppm because this is the blank where we do not have any copper ion. Then we have 10 ppm. And then we have 20, 30, 40 and 50 ppm. And we put these values draw a curve. Now on the basis of this curve, we can identify an unknown concentration. For an unknown sample, we just put the sample. A spectrum is generated. We take the peak height, put the peak height in this graph and calculate the corresponding concentration value. And in this way we can use this calibration curve to identify the unknown sample. So we can say that a calibration curve is a vital component that is being that must be prepared to analyze the sample quantitatively.

[00:31:13]So if we want to determine the amount of a element in the given sample or its concentration, we need a calibration curve which is based on some known values, some known concentration of that element that are being run through the same atomic absorption spectrophotometer to obtain it their spectrum. And from their peak height as well as the concentration we plot a graph based on known values of concentration and then this graph is used to determine an unknown concentration in a real sample.

[00:31:47]Do you know the difference between difference between a real sample and a synthetic sample. A synthetic sample is the sample that we prepare in lab. As I said in the previous slide that we prepare five to six solutions of known concentration. So they are being prepared by the user only by us only.

[00:32:02]They are synthetic sample. They are imitated sample and the real sample is the sample for example if we have copper contamination in river or the copper contamination in your tap water. The tap water you which you brings in for analysis in the laboratory that is the real sample or the river water that you brings in is your real sample that is being analyzed with the help of atomic absorption spectroscopy.

[00:32:29]So with this we have covered all the major components of atomic absorption spectrum photometers starting from the nebulizer. Then we have atom automizers the job of the nebilizer in a nutshell.

[00:32:44]So with this we have discussed about all the major component of atomic absorption spectrophotometer right from the beginning nebulizer. Then we have automizers.

[00:32:56]Then finally we have then light source, monochrometers and detectors. The job in a nutshell I once again u telling you that a nebulizer's jobs is to the job of the nebilizer is to convert the sample into aerosol and that so that that aerosol can be easily converted into individual vaporized atoms. That means it can be easily automized. Then the job of the then then the function of the automizer is to convert that aerosol sample means the sample in liquid form which is mixed with gas. The liquid dispersed in gas is then burnt or is heated at high temperature causing dissolvation.

[00:33:40]Vaporization and dissociation converting the sample into individual atoms. And finally there's a light source which produces light of a specific wavelength because it contains cathode of the same element which we are going to analyze. That specific wavelength passes through these individual vaporized atoms causing absorption to occur and the transmitted light then passes through a monochrometer which allows only selected wavelength while while excludes the other and then that light radiation transmitted light falls on a photosensitive detector called as the photo multiplier tube which has a photosensitive cathode which produces primary electrons and then these primary electrons falls on the diodes producing secondary electrons more number of secondary electrons thereby amplifying the signal and a spectrum is obtained in the display device or the computer. So this is all about the different components of atomic absorption spectrophotometer. In the next lecture we'll discuss about the types of interferences, the type of um contamination that are possible which affects the spectral analysis with this technique. Thank you very much. [music]