Lecture 11

Added: 2026-05-09

[00:00:03][music] [music] Hello everyone. Welcome back to the third week of the MOO's course on advanced analytical technique. Today I'm going to start the second lecture of this third week where I'm going to discuss about the instrumentation of the inductively coupled plasma technique.

[00:00:37]Uh in the previous lecture we have discussed about this particular figure where we have shown uh different components of the ICP in the form of a flow sheet and since the very first component is the nebilizer which is basically based on the different types of pumps which we use.

[00:00:57]So the very first uh component is the nebilizer which is used as a sample delivery system. So let us discuss about this nebilizer. The nebulizer basically consists of a paristaltic pump that automatically draws the sample depending upon the previously uh adjusted flow rate. It sucks the sample from the sample container or the beaker containing this sample with the help of an inlet pipe and then it draws the sample and it injects into the nebulizer. Now when the sample enters the nebilizer, we also have argan gas coming into this nebulizer which converts the sample which converts the sample into an aerosol and I hope you remember about the nebilizers that we have discussed in the atomic emission spectroscopy. We have three commonly used nebilizers. the ultrasonic nebilizers, the nebilizers which works on the principle of ventury called as jet or pneumatic nebilizers. And then we have a third type of nebilizer called as the electrothermal nebilizer which first of all vaporizes the sample and then mix it with the uh gas to convert it into an aerosol. So any of these nebilizers can be used and then when we and then we have one more uh nebilizer called as the frit nebilizers that we'll discuss today.

[00:02:21]They uses a glass membrane to convert sample into small droplets that can be further converted into aerosol with the help of organ gas. So the paristaltic pump sucks the sample and pushed it into the nebulizer where it is converted into fine aerosol with the help of our uh inert gas organ with the help of our inert gas and then it is being sprayed into the automizer or into the excite excitation source.

[00:02:51]So in the spray chamber the large droplets which are still not converted into very small droplets they are settled down while small droplets that have been evenly converted into aerosol they are basically sprayed into the excitation source. Any excess liquid uh any excess solvent is drained out from this nebulizer.

[00:03:17]So this is uh a schematic diagram showing this is our sample. Then we have this paristaltic pump. Then we have this paris paristaltic pump which pushes the sample into the nebilizer. And in this nebulizer the excess sample is drained out from towards the bottom. And then we have argan gas which converts the sample into aerosol. from this argan gas s based aerosol is then injected into the plasma torch with the help of a straight with the help of a spray chambers. This is spray chamber. This spray chamber is sprayed the aerosol into the innermost tube of the quartz base plasma torch where the sample is heated at very high temperature so that it immediately automizes as well as excitation as well as excited so that where where the sample immediately uh where the sample is immediately automized as well as excited to create an emission spectrum. So this is uh schematic diagram. Then finally the emitted light radiations falls on the detector. And finally we can have sometimes sensor is mentioned because a detector is basically a sensor and then we can have it on the computer. As I've already discussed that nebulizer is the component which basically used to convert the sample into an aerosol. Uh that can be done either by using air or oxygen gas. But in order to avoid the possibility of oxidation of the sample, oxidation of the elements that are present in the sample in case of inductively coupled plasma because the temperature is very high. The chances of oxidation are quite high. Therefore, we avoid using oxygen or air. Instead, we use an inert gas argan to create this aerosol.

[00:05:03]The size of the particles ranges from 1 mm to 2 mm or even lesser. Then the samples are injected into the plasma torch in its innermost tube with the help of a spray chamber. Then there are common type of nebilizers. These are pneumatic type. They are also known as jet nebilizers. And then there is another type which we have not discussed yet the frit type neilizers. Uh in addition to these we have ultrasonic nebilizers as well as the electrothermal nebilizers.

[00:05:35]The pneumatic nebilizers we have already discussed they works on the ventury principle or I should say the broli principle in which uh you know that if liquid is flowing or if gas uh in which if a fluid is flowing through a pipe and if the diameter of the pipe is reduced the velocity increases while the pressure falls on the basis of this concept these nebulizers create a negative pressure or very low pressure outside uh the tube so that the high pressure inside the tube pushes the aerosol with very high force and it immediately enters into the excitation source.

[00:06:12]Then there are two types of nebulizer co-entric and cross flow. The difference between coentric and cross flow is that in this particular case we have the sample as well as argan gas they are entered into this particular pipe and they are at very high pressure. They have been and they are being injected into this spray chamber at very high pressure so that they are converted into aerosol. Here they are basically here they directly entered into this pipe.

[00:06:40]They meet here and they start forming the aerosol. While in this particular case, While in this particular case where we have a counterurren where we have a cross flow, this is called as co-cententric flow because both the argan gas as well as the sample they are being injected in injected into the same tube. While in this case argan gas is injected directly from this point while the sample comes at 90° called as cross flow and then they meet into the spray chamber and form aerosol. This is the only difference between these two. Otherwise the principle is same. Then the ultrasonic nabilizer we have a po electric crystal which receive power from a direct current source and start vibrating at high frequency creating an impact which causes the droplets to break down into small droplets and then we have argan gas coming which convert these small droplets to aerosol which is then injected into the plasma source. Any excess liquid is drained out from the bottom.

[00:07:46]This is uh third type of nebilizers which we have not discussed previously when we were discussing the types of nebilizers in atomic emission spectroscopy. These nebulizers are basically these nebilizer basically works on the principle of frit membrane.

[00:08:04]Frit membranes are membranes which are frit membranes are glass-based membranes. They are glass membranes and they are quite porous as well. So through these pores when the sample is passed the sample is converted from large droplets.

[00:08:24]The sample forms very small droplets.

[00:08:27]When it is passed through this frit membrane and then it is mixed with the argan gas. Here we inject the sample from the bottom and the sample is pushes through this membrane to form these large droplets of the sample. They forms very small droplets and then we have an we have argan gas coming from the top.

[00:08:48]So we can say it is a type of countercurren flow. Earlier we have seen a coentric flow in case of pneumatic neilizers. We we have also seen uh a cross flow. Now we can see the flows from the opposite direction. It is called as countercurren flow where gas is coming from the top while sample is coming from the bottom. They both mix with each other forming a very super fine aeros and then this aerosol is passed into the plasma chamber.

[00:09:18]This advantage of fit nubilizer is that they produce a very fine and uniform aerosol and it is very ideal for efficient transport of sample into the plasma or even for flame direct aspiration. We can also use these flip nebilizers.

[00:09:34]Then comes the IC source. The inductively coupled plasma source. Uh we have already discussed it in detail. U just uh to give you uh an idea in the nutshell. It is basically a plasma source. In this case is called as a plasma torch which consists of three coentric tubes and in all these tubes we passes organ gas. In the innermost tube, the aerosol coming from the spray chamber which is connected to the nebulizer. This aerosol enters into the innermost tube. This aerosol contains this aerosol contains argan gas plus sample.

[00:10:14]This one auxiliary argan gas at the rate of 10 to 20 L per minute is injected to create the plasma and in the outermost tube argan gas is used as a coolant to control the temperature and to protect this tube as well as the copper coils placed here for passing radio frequency are being protected with the help of this coolant argan gas. uh here we have uh as I said co copper coils which are connected to the radio frequency source radio frequency is passed which has an oscillating magnetic field which creates very high temperature. Basically that uh the mechanism is like that oxillating magnetic field creates electric current.

[00:10:57]As you all know that magnetic field creates electric field while electric field create a magnet while electric field creates a magnetic field. These two forces are not different but they are one forces and therefore they are called as electromagnetic forces.

[00:11:10]Earlier it was believed that they are only uh there are five forces uh fundamental forces in nature. The gravity, the weak nuclear force, the strong nuclear force, electric force and magnetic force. Later it was realized that electric and magnetic force they are not different but they are same thing because when electric force is created it automatically creates magnetic field and the vice versa is also true. So they they have been combined and now there are only four forces. The electromagnetic force, the weak nuclear force, strong nuclear force and the gravity. So this oscillating magnetic field create eddyurren. Those eddyurrens creates very high temperature and that high temperature causes this auxiliary plasma gas and that very high temperature causes this auxiliary argan gas to ionize forming a ions along with electrons. Although in the plasma there are some unionized AR gas molecules as well and the rate of the coolant organ is 13 to 20 L per minute while in case of auxiliary argan it is around 10 to 20 L per minute. So we can say that in case of inductively coupled plasma the requirement of argan gas is quite high which is maintained by I hope you know the answer the diwar flask which is used to store all the liquids which have very low liquid temperature means argan gas which is having a temperature of - 185 for nitrogen - 196 and for helium it is around 260 uh°C they can be used and that diar flask consist of two concentric uh tubes uh cylindrical tubes made of stainless steel with a vacuum created in between to avoid heat losses uh through conduction or convection to avoid heat passage of heat due to conduction and convection. And finally this uh outermost cylindrical stainless steel uh container is coated with silver to avoid to avoid any absorption of heat to reflect all the heat. So Diwir flask is used and we have a plasma created over here. The plasma is very bright. It emits a continuum of bright light. But as we moves away from the plasma, as we moves away from this torch, the brightness of the plasma decreases. And at around 3 cm from this tube where the plasma is created, the temperature uh the brightness goes completely the brightness is completely lost. Al although we have very high temperature over there. So in this particular transparent region we ionize. So in this particular transparent region we excite the sample in the transparent region.

[00:14:01]Let us move forward.

[00:14:03]So again it is written what which have already explained the torch surrounded by highend energy induction coil through copper wires through copper coils and radio frequencies passed which creates eddyurrens which causes heat excessive heat ultimately converting or argan gas into a plasma and everything is being done in quartz tube coentric quartz tube. The sample is introduced in the organ stream in the form of uh mostly liquid aerosol or vapor but most of the time we prefer liquid aerosol and it enters uh from the innermost tube and at a distance of 3 cm where the zone is transparent we do the um analysis by creating an emission spectrum.

[00:14:46]So this is all about these are three concentric tube. This is the innermost tube. Then we have this opening for passing auxiliary gas which creates a plasma. And this is for coolant gas inlet which is again organ liquid organ is used to control the temperature and protect these uh the and protect the whole assembly. So we carry out the analysis where we have a transparent region. Here we have very bright continuum. So we do not uh analyze or excite the sample here. We analyze the sample at a distance of around 3 cm. so that we can have no interference or minimum interference.

[00:15:27]Uh this plasma has different temperature ranges. Even we have 10,000° Kelvin temperature range. That can also be possible. It is at the center of this plasma. But as we move away from the center, the temperature decreases at immediately at the center when the distance is zero. We have 10,000 Kelvin at 1 15 mm means 1.5 cm the temperature is reduced to 7,000 but is still it is very high temperature and at around 2.5 to 3 cm or 25 to 30 mm the temperature is higher than 6,000 Kelvin but this particular region is transparent region and therefore we carry out the analysis and excite our sample to create emission spectrum in this transparent region only just to avoid any background radiation otherwise we would be having large number of otherwise we would be having excessive of background radiation which makes the analysis highly complicated then again we have monochromes as I said earlier monochrometers are very useful device they are used to select particular wavelengths and they are also uh helpful to exclude the other non desired wavelengths although in case of atomic absorption spectroscopy we use the monochrometer before the light path interact with the sample. Atomic absorption where the light radiation is where the light radiation is coming from this light source. Light radiation is coming. So this is a light source where the light radiation is coming and then we have an automizer which is connected to a nebulizer. So this is our nebilizer. We have automizer over here. Here the sample is injected from the nebulizer and converted into aerosol where it interacts with light. So before this uh automizer we use a monochrometer over here so that we can select a particular wavelength of light in which we want to make the study. While in case of atomic emission spectroscopy because no light source is used initially the light radiations is created by the sample itself. Therefore we use first of all we have a nebulizer which is connected to an automizer or the excitation source. Now here in this particular case this is our excitation source of atomic emission spectroscopy or more specifically I should say the inductively coupled plasma. Uh for example if this is our plasma torch and sample upon heating and excitation and the sample upon heating and excitation produces emission spectrum. So light is coming from here.

[00:18:03]So we use a monochrometer after the excitation source. This is the monochrometer. So always remember in atomic absorption spectroscopy we use monochrom before the automizer is placed. Just after the light source we use monochrometer. We select light and then the selected light. Monochromatic light enters into the automizer for absorption. While in case of atomic emission spectroscopy we do not use that. We we use monochromter after the excitation or automization source. Again monochrometer as you all know consist of a grating or a prism. So we can use prism or grating can be used to disperse light distribute light into different wavelengths small they basically it monometers basically narrow down the wavelength.

[00:18:52]For example, if light radiations is coming from the wavelength from 200 to 400, then morometer can be used to convert this uh light radiation with a wavelength of 200 nanometers. From 200 to 400, it covers a region of 200. It converts it into say 50 nanometer. And if we want to have more specific wavelength, we may use Brisman grating together. So that we can have first of all a 200 nanome wide light radiation is converted into 50 nanometer and then uh prisma grating is used again which converts this 50 nanometer again into 10 nanome. So we can bring down the uh wide gap of this electromagnetic radiation of 200 to and make it very narrow. We bring it down to 10 nanometer.

[00:19:42]So for this purpose monochrometers are used.

[00:19:46]So monochrom these monochrometers also these monochrometers are very useful because they reduces spectral interferences. If we are not using monochrometer there are chances of having spe there are chances of having spectral interferences which may spoil the analysis. Therefore, monoproters are always very useful because they are they basically act as a filtering device which filter some of the desired spectral lines only while cut off the other and both prism or grating can be used.

[00:20:21]Most of the cases we prefer grating. In some cases where we need to narrow down the light up to a greater extent, we use grating as well as prism both. Sometimes holographic grating is also used to create uh light radiation into individual components.

[00:20:42]With this we have completed uh the components of inductively coupled plasma uh like the nebulizer, the excitation source and monochrometer. In the next lecture we'll discuss about the two common types of spectrophotometers that are basically preferred for atomic emission spectroscopy. One in one case uh the first are called a sequential or sel scan spectrometers while in second case we have simultaneous spectrometer that are used for the detection of multiple element at a time. So we'll discuss that in our ne next lecture.

[00:21:16]Till then thank you very much.

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