Lecture 12

Added: 2026-05-12

[00:00:20]Welcome to the third lecture of week three of MOO's course on advanced analytical technique. In the second lecture of the third week, we have discussed about the different components of inductively coupled plasma. We have we have witnessed different types of nebilizers. We have also discussed about a new type of nebulizer called as the frit nebulizers which utilizes a glass membrane and those frit nebilizers have a counterurren flow means gas is coming from the top while the liquid uh sample is coming from the bottom. Also we have briefly discussed about the pneumatic nebilizers as well as ultrasonic nebilizers. Uh in pneumatic nebilizers we have one of them is called as the coentric nebilizer where in a single tube we have aerosol. We have where in a single tube we have sample as well as argan gas and they are mixed and then they are entered and then they entered into the spray chamber. While in case of the uh cross flow we have one tube which allows sample to pass through it while at 90° we have another tube in a cross flow pattern which allows argan gas and both are being mixed into into the spray chamber to convert the aerosol. Then after this nebilizer we have discussed about the plasma source the ICP torch and then we moved on to monochrometers.

[00:01:42]Now we'll discuss about the two common types of spectromemeters that are being used in atomic emission spectroscopy. These two spectrometers are called as sequential spectrometers and simultaneous spectrometers. We can from their name we can easily perceive that sequential spectrometers. They basically works upon the principle of sequence.

[00:02:06]Means these spectrometers are used to first of all we allow one light radiation to pass through it.

[00:02:17]Uh the light which is coming from the excitation of the sample. We allow one particular wavelength to pass through it to and finally falls on the detector.

[00:02:27]Then we change the sample. Then we again go for another light radiation to identify another element. So this means that we can identify different element.

[00:02:41]For example, if we have several if we have multiple elements present in a system or in a matrix, we have to do the analysis repeatedly. Then only we can be able to identify or determine all these elements. Of course, it is a tedious task which requires long times. Although the machine is inexpensive because all basic equipments are used like we have one single photo multiplier tube as a detector, we have one monochrometer.

[00:03:13]Similarly the uh since we are not doing the multipen analysis at a time we may not need a very high excitation source and therefore although it requires a skilled operator but very highly trained person is not needed for these sequential spectrometers. On the other hand we have simultaneous spectrometers. These spectrometers have the beauty that we can go and perform analysis of multiple elements at a time. We can detect multiple elements simultaneously.

[00:03:49]Therefore, of course, there therefore these simultaneous spectrometers are very quick in response. We can just by doing one or two run, we can identify numerous samples. We can go up to 30 40 or 50 elements in a sample by these simultaneous spectrometer just in few minutes. Uh but of course since they require multiple element analysis so they use uh various type of monochrometer or I should say instead of a monochromter they have a polychromter which allows different type of light radiation to pass through it. And then of course they have multiple photo multiplier tubes or some other sort of uh detector that can detect all these elements at a time. I'm not going to tell you the name of that detector which is being used that we'll discuss during this presentation in the later slides of this particular topic.

[00:04:49]As I said sequential spectrometers they are basically used for single elemental analysis as a at a time and for the second element we have to perform the analysis again. So we have movable grating to select relevant wavelength.

[00:05:03]We move the grating select particular wavelength with the help of the entrance and exit slit.

[00:05:11]And since they are used to detect single element at a time, they uses a single photo multiplier tube to detect. For the second element, we again draw put the sample which which is being drawn with the help of peristoaltic pump into the nebulizer and it again gets converted into aerosol which is then injected into the spray chamber and finally reaches to the excitation source. Upon excitation is it emits light and the light radiation that is being emitted again it passes through monorometer. We allow specific wavelength to pass through it which falls on the detector so that the second element can be detected. In this way we do the analysis.

[00:05:52]Sometimes these monometer uses normal grating but sometimes we prefer holographic grating which has large number of grooves which can narrow down the light radiation to a greater extent.

[00:06:05]So these spectrometers are designed to take measurements turn-wise or in sequential order means one by one. First analysis is done then we go for the second elemental analysis. However, due to their uh time-taking protocols, uh when these sequential spectrometers were developed, there were uh high demand from the industries as well as from research institution to reduce the time and uh to enhance the possibility of detection of multiple element. So just after the development of these sequential spectrometers there has been some new developments in which uh slooh scan spectrometers have been developed.

[00:06:52]These slooh scan spectrometers are an advanced version of these sequential spectrometers where we have a little bit quicker analysis time which ultimately uh shortens the whole protocol and and now with the development of the SL scan we can perform analysis of four five or six element in a short span of time. In this particular figure, uh I'm showing you a sequential spectrometer, but uh there is no point of showing the nebulizer and all those uh components like the nebulizer, paristaltic pump or spray chamber because uh our point of discussion here is when the light radiation has been created and in case of ICP or in case of AES the light radiation is created after the excitation source. So this is our excitation source. When the sample is excited, light radiations now are coming. They passes through a prism and then again we uses mirrors to direct it towards these gratings. These are holographic gratings. Holo we have a normal grating like this. In holographic grating we have uh some thickness as well. We have this type of um plates which have some thickness. And then the on these plates we have these grooves created. When these light radiation falls here they are distributed into wavelength of they are distributed into different wavelengths.

[00:08:22]Then they again passes through a refracting mirror and finally they passes through the exit slit and they are detector and they are detected on a detector. Sometimes we use dual detector for even more reliable analysis. For the second sample, for this for the second element, we again carry out the same procedure.

[00:08:46]The sample is being injected into the automizing source from the spray chamber. And then in this automization or and excitation source means the plasma to the sample is excited again.

[00:08:57]We have some light radiation. This time, this time we move the grating as well as entrance slit to select some other wavelength corresponding to another element. And finally when that radiation passes through the exit slit and falls upon the detector, we again uh we we are we are able to analyze or determine the second element. So in this way we can perform repeated analysis for multiple elements. Now in order to complete this sequential analysis the monochromter grating or even the holographic grating is rotated in a controlled manner. We rotate it in a controlled manner and select different wavelength so that next time we have a different element which is being detected. Then third time we rotate it and allow the third wavelength to pass through it and falls on the detector to detect the third element. In this way we have to keep uh moving the monochrometer grating so as to select different wavelengths and light radiations that ultimately helps us to identify different elements.

[00:10:03]And then in order to reduce the tedious protocol as I said scan spectrometers have been developed which allows very easy and controlled movement of these monochrometer so that we can perform the analysis even in a short period of time.

[00:10:21]These silhouan spectrometers they reduces the longer analysis time of sequential spectrometers.

[00:10:30]So we can reduce the longer time longer analysis time of sequential spectrometers by using these sill scan by using these slan spectrometers. These sill scan these sill scan spectrometers have su scan detectors that have a combination of monochrometer and slid to select a specific wavelength. We we already have a combination of monochrom and slid. We in the first case we select one combination of monochrome metroate which allows a particular light addition. Then we move the knob just like uh we have a knob over there just like we have our volume knob in our uh systems. So we move that knob we select a different combination of monochrometer and sit and this way we change we keep we and in this way we keeps on changing the combinations of this monochrometer and slit and we can very easily go to the next combination thereby selecting different wavelengths that ultimately helps us to ele to analyze more samples in a short period of time. So because in the pre because in the previous case in this one we have to every time change the position of these slits and in order to select a different wavelength. But in case of silly scan which a combination is already fixed by just by rotating the knob we can select different combination of slit and monochrometer means different combination of grating and slit that allows a different uh light radiation to pass through it and a new element has been detected. Then again we rotate the knob to adjust or to fix another combination of slit and grating.

[00:12:21]And then the third radiation is selected while the first two are being excluded.

[00:12:26]The when this third radiation falls on the detector the photo multiply tube the third element is detected. Although in this case also we have to rotate the knob but rotating this knob is very easy to select different combinations and therefore the time for analysis of multiple elements is quite reduced as compared to the sequential spectrophotometers.

[00:12:48]Then we have the simultaneous spectrometers. So in case of sequential spectrometers we have a normal sequential spectrometers and then we have a silu scan spectrum photometers.

[00:12:59]Although in both the cases we have to u set different combinations. But in case of su scan the selection of new combinations of slit and grating is quite easy. Just by rotating the knob we can select different different combinations very easily and very quickly as well. So that we can identify element after element. We can identify different element one after the other in a short period of time. While in case of the old version of these spectrometers which are called which are called as the sequential spectrometers, we have to every time adjust the grating as well as every time adjust the so as to select different wavelength and different light radiations in order to determine or detect all the elements present in the sample. Now u this slide shows the simultaneous spectrometers that are basically known for simultaneous analysis of multiple elements at a time. Now since we are doing simultaneous analysis of multiple elements means for example if we have a ele sample from river water which is close to an industry and there are some news that that industry is polluting that river and we have to analyze the sample to identify all the heavy metals present in that particular sample which is coming from a particular liver or for example if we have a blood sample of a person who is having some type of poisoning. So in in that case as well further in case of drinks like juices or soft drinks in order to identify uh the possibility of different elements we may use these simultaneous spectrometers.

[00:14:51]And since we are going for the detection of multiple element that to at a particular point we need number of monochrometers or I should use the word polychrometers and we also need need a multiple detectors. So there are two possibilities. Either we can use num either we can use multiple PMTs means the multiple photo multiplier tubes which you know that they are photosensitive. They have a photocathode and photo to a node placed on the either side. Photocathode once captures the photons that are coming from into the that are present into the light radiation they convert it into an electrical signal. Means electrons are ejected by photoelectric effect that photo that photocathode is light sensitive and those electrons of course they are not very accelerated and the number of electrons are also low. So they first of all falls on the diodes which create secondary electrons and these accelerated secondary electrons they falls on the cathode creating current and a peak is obtained.

[00:16:01]So this is how a PMT works. just I just I wanted to do a recap to make you realize what uh PMT is. Uh the other type of uh detectors that are even more useful and even more authentic and reliable are called as the solid state array detectors.

[00:16:22]I told you in the beginning of this lecture that I'm not going to tell you the name. I'll tell you the name once that particular slide comes in. So now the second type of detector that are used for multiple analysis are called as the solid state array detectors. These solid state array detectors they basically work on charge coupled devices. Charge coupled devices.

[00:16:48]There are multiple types of solid state detectors. We may have solid state detectors that works on charge couple devices, CCT.

[00:16:59]And we also have solid state array detectors that works on charge injection devices. But uh and both these detectors can be used for the analysis of 50 or 60 elements at a time. In their spectrum we can have hundreds of spectral lines and these detectors can be able to identify so much lines because one element can have more than one spectral lines present in the sample. Uh because sometimes the excitation occurs from E not to E1. Sometimes it happens to E2 while in some other case it happens to E3. So when in a so when in AES the electrons are coming back from E3 to E not the light radiation emitted of course are having more energy or shorter wavelength. Then from when from E2 it is coming they have some different wavelength and when from E1 they are coming they have some different wavelength. It is up to us that we can deselect or exclude any two of these wavelengths. But if you want we can have all these wavelengths in the form of a spectral lines in the spectrum. So therefore I'm saying that with charge couple devices or charge injection devices based solid state area detectors we can go up to 50 60 elements detection at a time and uh you can uh calculate that if for one sample we can have three or four lines then how many lines may be present in the spectrum. Of course that creates some complications but still they have the capability to go up to so many spectral lines and those can be detected as well.

[00:18:49]So as of now the most advanced atomic emission spectro meters that are based on inductively coupled plasma or other type of plasma like the direct current or the microwave plasma. They used the solid state array detectors. But of course since these SSAs are very sensitive devices they are highly costly. So the instrument that have these SSA for multi-element analysis simultaneously of course they are very very expensive.

[00:19:20]They are very very expensive.

[00:19:26]Now let us move on to discuss about the polychrometers. So there are two types of simultaneous spectrometers from this discussion. those which utilizes polychrometers and those which utilizes solid state array detectors. Now what are polychrometers? Polychromators have a fixed defraction grating means they have a defraction grating that is being fixed but they have multiple exit slit. For example, we we have a light radiation of u wide wavelength is coming. We have an exit slit that allows this light radiation. Now we have a mirror over here. And then when this mirror reflects this radiation backward, we have a grating placed over here. Sometimes we use a shell grating as well.

[00:20:24]This is a particular type of grating which is used for uh dispersing light radiation into individual very narrow wavelengths. So when this light ray falls on the grating it is converted into different light radiations. Now in case of monochromat we have only one exit slate but now in this particular case we have multiple exit slate that allows more than one light radiation to pass through it. Of course it does not mean that we cannot stop any light radiation. We can identify some we can exclude some light radiations but still we can allow at a time four five or even more number of light radiations to pass through it with the help of these multiple exit slit. They are multiple exit slit.

[00:21:19]Now the question is that now the question is how these multiple light radiations that are passing through these multiple exit slits are going to be detected and the answer is multiple PMTs installed in this particular case of spectrometers in this particular case of simultaneous spectrometers. So when these multiple lines are passing through, so when these light editions are passing through multiple slits, we have multiple detectors. We have multiple detectors placed over here in front of these poly in front of this polychrometer so that they can detect each photo tube detects a particular wavelength confirming the presence of a given element in the sample and we can go up to 60 elements. Can you believe this? 60 elements can be detected at a time with the help of these polychrometers based simultaneous spectrometers.

[00:22:21]So this is uh a design of uh an ICP which is based on a polychrometer. We have a plasma source as I said earlier since we are focusing on monochromes and polychrometers which are used for the selection of particular light radiation or selection of multiple light radiations. there is no point of showing nebulizer as well as the spray chamber in this particular uh diagram. So this is the plasma torch which is also shown um in very small size just to give more space for these component like the polychrometers. Then the light radiation that are coming they are passed through an aperture and then finally we have mirror that reflect this light radiation. Then we have concave defraction grating or sometime we have a shell grating.

[00:23:15]Now in case of this polychrometer we have multiple exit slits. These multiple exit slits they are placed on a circle.

[00:23:22]This circle is called as rolland circle.

[00:23:26]This circle can be moved very easily and by moving the circle we can put different slits in front of this light radiation so that we can pass multiple light radiations in a single run. And once this light radiation passes through the exit slit, we have different types of PMT devices.

[00:23:48]The photo multiplier tube. The position of the photo multiplier tubes can be moved up and down, left and right. So as to capture a particular wavelength and in this way the analysis can be done. These photo multiplier tube are connected to our output device. They created they collect they capture photons converted in converted them they capture photons converting them into an electrical signal which is then amplified and finally presented in the form of peaks on the spectrum. So this shows how a polychrometer works. So a polychromter works by taking a roll and circle on which we have multiple exit slits we have a grating either the concave defraction grating or shell grating. This grating convert light radiation into individual components.

[00:24:44]And then with the help of this rolling and with and then with the help of this roland circle we moves these multiple exit slits in order to select different wavelengths. And finally we have and finally we have these photo multiplier tubes which can be moved up and down. We have multiple photo multiplier tubes here and these photo multiplier tubes then capture those radiations coming from the different multiple exit slits and convert them into electrical signals. So in this way polychromter works. So basically a polychromter means we are having multiple monochrometers and this works when we have a grating and multiple exit slits. We don't need multiple entrance lit multiple and only multiple exit slits are needed which allows multiple light radiations to pass through it. And in order to have a combination that can easily be And in order to have a combination that is easily movable that can be easily uh moved to select different wavelength we have both these devices placed on the Roland circle.

[00:25:50]This is again uh another type of a polychromatic device where we have a poly chromator used but in this case we do not use a concave defraction grating like we have in this particular case but in this case we use an isial grating. This is an initial grating where we have this is in the like this. We have this shell grating like this which so this is our shell grating which is like this. We have grooves on this plate which convert light radiation into individual component. But there is a peculiar but there is a peculiarity in this particular design.

[00:26:41]I hope you have identified in this diagram that we are using a grating as well as a prism. Now when the we are using both these uh in combination means we are using a grating as well as a prism it means we are further narrowing down the wavelength of the light radiation. For example, if we have a light radiation that is covering different wavelengths ranging from 300 to 400 say for example. So when it it enters first of all the light rate that is coming it is reflected with the help of this source mirror that allows it to enter through the entrance lit and then we have a culminating mirror which again reflect this light radiation to the shell grating. Now when it falls in the shell grating, a shell grating convert this light radiation into individual radiations. For example, we have uh from 100 it converts its different 100 wavelengths to different individual light radiations. Then we have a prisma that further reduces or narrow down the wavelengths of these radiation. And then we have an aperture plate placed over here which has different holes. Now through these holes different light radiation passes through the sample and falls on the multiple PMT devices.

[00:28:07]Multiple PMT devices which detects these multiple devices which detects multiple elements simultaneously. So here in this case we can go up easily up to 60 elements. So in this in case of this particular combination we have to move this roland circle or rotate this roland circle to uh select different uh wavelength coming from the multiple exit slit so that uh multiple elements can be detected. But in this particular case when we are using an ael grating and a prism which narrow downs the light radiation to even a smaller uh interval or to even a smaller level and then it passes it it is they are being reflected from the plane mirror and finally there's a plate aperture where we have different holes which allows multiple light radiations to pass through it simultaneously and then we have multiple PMT devices. So here we do not need to move anything or move a device or a circle like a roll and circle. We can very easily we can very easily make these multiple light radiations to focus on the uh PMT device. Of course that are also multiple PMT devices. So we can we can very easily uh have these light radiation focusing on the PMT devices and multiple elements can be detected in a single run simultaneously.

[00:29:27]So we can say that polychrometers makes our work very faster and the data is also and the result is also quite reliable. However the cost goes up because we are using polychrometers which have multiple slits. Sometimes we are using a shell grating which has a grating and and a prism together and also the most important thing we since we are using polychrometer we need multiple detectors which makes the instrument very expensive. So if we are having a budget limitation or a small industry which cannot uh go for this particular um equipment. So they may satisfy themsel by using a sleeve scan spectrophotometer or just a sequential dete uh or just a sequential spectrometer that can be used for having single analysis at a time. But for a multiple analysis system, we need uh these polychrometers or the sequential spectrometers where we have polychrometers that are placed on a rolling circle or those polychrometers that uses grating and a prism which creates which uh narrow downs the light to a greater extent that passes through a plate aperture and finally detected and finally finally detected based on different types of and finally detect and finally detected based on multiple PMTs. So that at a time we can have multiple element analysis although this since we are going for large number of equipments uh since we are going although uh but since we are going for large number of samples at a time so we may have more spectral lines which may complicate the spectrum. Then there comes the last topic of today's discussion. The solid state array detectors. In case of these solid state array detectors, as I said there are two types of solid state array detectors.

[00:31:24]One which uses charge coupled devices and the other which uses charge injection devices. We basically we mostly prefer solid state array detectors that uses charge coupled devices. Now these charge coupled devices are basically light sensitive materials. light sensitive materials or you can say they are sensors that works on the principle of photoelectric effect. For example, a small one single light sensitive instrument is like this. When the light falls on it, it basically captures light photons and they work on the concept of photoelectric effect. When the light photons falls upon these light sensitive uh plates which are connected to capacitors, electrons are created. Means electrons are ejected. As you all know that photoelectric effect means when light photons falls on a metal surface electrons are ejected. When electrons are ejected they leave behind holes. So electron hole pairs are created. These electron holes pairs create an electrical signal which is then stored into the capacitor which is connected with these devices. Therefore they are called as charge coupled devices because this charge pairs are created. They are being stored in the capacitor and an electric current or electric signal is generated and the this is only a one device. There are multiple devices that are connected uh to form a array. Array means a series of those uh sensitive elements connected with each other that can be used for each single element is used for a single wavelength means each these sensitive element can be used for one elemental analysis.

[00:33:14]So they are very s highly they are highly sensitive. They have very fast response time. Excellent wavelength resolution means with this we can have different wavelengths that are very easily resolved very clearly resolved from each other and therefore these solid state array detectors are useful for instrument like ICPE inductively plasma optical emission spectroscopy they are also used sometimes in U visible spectrometers and in ramen systems these machines these S solid state area detectors are also useful in so and these solid State area detectors are also useful in medical imaging. They are also used to study about the different types of cells and they are also useful in uh remote sensing. So they have uh multiple applications in different areas. So the sol so a a solid state error detectors they are always used with polychromters means either they are connected to rolland circle which have combination of multiple exit slits and uh concave defraction grating or they are used with a shell grating where we have a shell grating and a prism that create that narrow that narrow downs the light to a greater extent and then we have a plate aperture which allows multiple light radiation to pass through the small holes. to pass through those small apertures of that plate and then those multiple light radiations then falls on the detector. So these solid state array detectors are always used with polychrometers and therefore they are part of simultaneous spectrometers.

[00:34:56]So this is a type of solid state array detectors where we have different light sensitive elements placed or light sensor. This light sensor is shown here.

[00:35:06]These light sensitive elements sometimes called as pixel or large or light sensitive elements when they receive light photons. H new they create electrons that are being ejected and then we have a holes remaining. This electron hole pair create charge carriers and these charge carriers are connected with because in in between these we have uh capacitors that source this charge and ultimately the charge is released and an electrical signal is generated and that electrical signal is then amplified and finally a signal or a peak is obtained in then in the computer or the readout Guys, here you can see we have a plasma torch.

[00:35:55]The sample is excited. The excited light radiations are coming and they are first of all focused with the help of these mirrors to pass through the entrance lit. And then we have so then there's a parabolic culminator which focus this light radiation to the aial grating which is having these types of grooves which converts this light radiation into uh smaller light radiation into light radiations of smaller wavelength or I should say into individual wavelengths.

[00:36:25]Then they again passed through a visible prism and then finally they focused upon a detector which is based on solid state detector and it has the capability to detect multiple elements at a time. So this is the flowheet of a solid state array based detector which uses a polychrometer either in the form of a rolland circle which has a concave defraction grating and multiple exit slit or it has an echel grating with a prism in combination and a plate aperture that allows multiple light radiation to pass through it. either of these two polychrometers are used and they are coupled with solid state array detectors so that we can detect multiple elements at a time. So with this we have discussed the different components of ICP inductively coupled plasma right from the beginning that is the peristatic pump the nebulizer spray chamber its torch for creating the inductively coupled plasma then the monochrometer the type of spectrometers that uses monochrometers like our sequential spectrometers and the slooh scan spectrometers uh slightly reduces the time of analysis and then we have sequential spectrometers that works on polychromator or the poly chromatic devices that you either use multiple PMTs, multiple photo multiplier tubes for detection or they used solid state array detectors which is a combination of light sensitive material based on charge coupled devices CCD. So with this we have completed the instrumentation of inductively coupled plasma AES where we there and that probably be the last lecture of this week where we'll discuss about the different types of spectral and non-spectral interferences possible in case of ICP. Till then take care.

[00:38:20]Thank you very much.