Analysis Methods - Overall Methodology for Organics
So last class we were discussing the analysis, we started discussing analysis of organic chemicals in water, we looked at the process where we have a water sample A we remove the A here by extraction and then we have another process which is A plus solvent and then we concentrate to make this a smaller volume, and this goes into the analytical instrument and then we get concentration data on that So we discussed one method of this and we also looked at some standards methods for that are there listed in regulatory agencies. The analysis of different types of components in water, so here one method that we use for extraction is what is called liquid- liquid extraction and this usually involves the adding some amount of solvent to the water sample and then shaking it to extraction by whatever means you can bring the solvent to water in contact so that there is sufficient exchange of air between the water and the solvent. The idea is to use the solvent that has very good capacity to hold the solid that you are interested in so that’s a matter of experience and there a lot of people who have collected this data, so the standard methods that you are looking at will recommend a particular solvent for the analysis or several solvents if they are applicable. For the extraction of one class of chemicals A from the water and we looked at some of the solvents, there is no point in me giving you a list of solvents by and large the chlorinated organic solvents are very strong for any of this process but they also have a problem, they themselves are listed a hazardous chemicals so you have to worry about their disposal so people always look at alternatives but if extraction is the concern then people still use these chemicals they are not banned you just have to take care of their disposal properly if you are using it. So one option because this is a concern, using a liquid solvent, there are several problems in using liquid- liquid extraction one is the waste management, second is safety itself, whilst doing the concentration, what we mean is when evaporating the solvent , the solvent is coming out and its potentially realized into the environment the analysts themselves can get exposed to the solvent okay so we have to be careful when doing that. So in the standard methods if you are doing liquid- liquid extraction you will also see the safety methods that you are to follow when handling these things which includes the concentration to be done inside what is called as .. I will not discuss some of the methods in detail you will read. So there are two issues here, waste management and safety and third is the amount of handling can lead to a lot of errors, so you have to manually taking samples, taking out the solvent, concentrating it, taking the instrument all this can lead to additional errors in sample loss in the process so in order to circumvent all of this there is another method that people use now which is called as solid phase extraction. This doesn’t remove the problem completely this means that instead of using a solvent to extract we are using this A plus water, we are sending this entire thing into a solid, the water comes out the air is retained on the solid column so essentially what we are doing is we are doing another mass transfer process called as absorption, we are absorbing all air from water onto the solid phase which is usually a packed column and all the air is retained in the solid phase wo what we do is say if you are processing 1 liter of water a large quantity, you send it through the solid phase and what ever quantity of water is there gets retained on the solid column and the water goes through there is no solvent involved here so safety issues are lesser, then you take this solid phase which contains air, you have to take it out bring it to the instrument again which means that I have to remove the air from here and then go to the instrument for analysis, there are very few instruments that people have especially at the organics that we are interested in that an analysis of A on the solid phase is not possible, the method doesn’t exist as of now. So you first have to remove it out of the solid phase, so you have to extract it some how so this process is called as desorption. So for water usually the instrument that we are going to be using is typically take a liquid or some solvent so one way is you send a solvent now this solvent interaction with the chemical with refence to water is very different to the interaction to the solid so we are doing the portion in this case the liquid extraction we are looking at the partition constant of A between the solvent and water in a liquid- liquid extraction in the case of this adsorption we are talking about the solvent and the solid phase. So this option you have to select the solvent, now the organic is there on the SP column it must be, its properties are very similar to that of a solvent, which means that if you are using another solvent to take it ut of its place it must be very strong so the portioning of the constant from the chemical has to be very good. So there are recommendations for the selection of solvents for a particular class of chemicals from a particular solid. So there is adsorption ad desorption this is mass transfer and the equilibrium of a chemical between two phases in these kind of systems so what we are effectively doing is for example you have 1 liter of sample, t goes into a small cartridge which contains a few milligrams 100 mg or few grams of a solid and then there is, then you take a small amount of solvent, maybe 10ml of solvent and dissolve it all the air now comes to a this solvent so you are essentially doing the same thing that you did, what you did in this zone is equivalent to what we are doing in the SP except that you are not liquid-liquid extraction we are using the solid phase extraction so I have a Iarge showing that, this is w sold phase, extraction, so essentially what we do is we have a cartridge, then allows material to be sent through and then the analyte is received then it is stored here and then we elude it there is a standard method for that, so yesterday we talked about solid waste standard methods. So you will get in the method very specific instructions as to what is the flow rate you need to use, the time rate, purity of solvents and all that, how much of solvent to use, mass of SP to use, and same in liquid liquid extraction for a given volume of water, all those instructions are given, you can modify it whatever way you want, as long as you have sufficient justification for doing it.So the next step is the concentration step sometimes when you extract you extract large volumes of solvent, this is sometimes possible that our only extract 20 or 30 m l or even larger. So in order to reduce this volume, solid volume is going from this to this which means that the solvent is evaporating you already seeing the volume of the solid assuming that all the air that is n the system already is retained in the second set also that the concentration is higher in this case and two ways of doing it, one is if you have a very large amount of solvent you can use what is called as a rotary evaporator, this is essentially a system where you have, the sample is kept inside a container and this is kept in a heating for a particular temperature and then the vapor that comes out is condensed and collected but here we are also applying this evaporation and condensation to collect this thing, we use vacuum. So at a particular temperature by using a vacuum we are, its equivalent to boiling but we are not boiling we reduce the pressure so evaporation occurs at a lower temperature at room temperature you have a certain amount of vacuum you can apply on certain solvents so there is a listing for that so at 35 degrees this is the vacuum you apply , different solvents based on the vacuum will evaporates this is rotated which is why its called a rotary vacuum. So beyond certain volume you can’t reduce this so if you have 100 ml solvent you can reduce it to say 5ml or so beyond that the factor volume is not felt by this thing it will go up and come straight away you can’t reduce it all the way but it’s a very, people use for various reason s but in environmental science we do it for concentration, the advantage of this thing is you can also recover the solvent, now how pure the solvent is it’s a different question you may have to check it because several things will go out but it is like distillation which means there is evaporation on the basis of the volatility of that thing, so we are influencing the volatility by applying the vacuum okay this is the more sophisticated way for doing large volumes for small volumes let us say this is 10ml or 5ml in the case of SP the volume you extract is 10 ml or something like that but you still want to reduce it, you cant use a rotary evaporator its too much so we use a nitrogen blowdown, its very simple and straight forward a sample is placed here and you have nitrogen flow on the surface s it is essentially evaporating just like that, so its very precise you will have a needle that is suppling nitrogen from a nitrogen cylinder storage and it evaporates so you can keep on bringing this needle as the level keeps going down so this will go down, its nitrogen because its inert that’s the reason why we use nitrogen it will react and also wont because of the problems in the system, there are commercially available but there is no specific, but essentially it is evaporation.So for quality control issues during evaporation you will lose some value of your interest depending on the solubility of that if you are doing naphthalene analysis it has some volatility also, one of the main problems we have is what we call as co-evaporation solvent. The reason we choose a particular solvent for extraction is that if that if it has high volatility its easy for us to concentrate, if you take a solvent that is not easy to evaporate you cant evaporate it easily you have to expend a lot of energy to evaporate and in the process you may lose a lot of the chemical itself so you always have to worry about quality control, the recovery of the analyte of the main chemical is critical here, sometimes you will have a lot of loses. So losses can occur by way of incomplete extraction during the extraction process and also can be loss during this process so what you recover in the instrument is less than what is in the original sample for these reasons. So these are the wo ways in which we do nitrogen blowdown. If you are doing solid samples, we talk about the extraction, if you are extracting solid samples, these are more rigorous therefore you can’t do liquili-liqiud extraction you can add a solvent and mix it but one of the problems is solids don’t, lets take soil, soil and sediment they contain moisture it won’t mix very well, so the solvent must be a sufficient mixture where it can allow water, that is one way of doing it sometimes people use a solvent as a mixture for example you have a mixture of hexane and acetone all of them are good as organic solvents but one of them is good in water, the other one mixes well with water hexene doesn’t mix well, so the water will go on and all that but its not important, what will help is that the solvent and the matrix can mix well, you need good mass transfer, second is there will also add a drying agent so you will, if there is moisture we add a drying agent or something like we have here as sodium sulphate. This will absorb he moisture and their sample will become more, and it will be easier for us to do mass transfer, so when you have soil or sediment essentially what we do is that we take some sample solid and then we add the solvent, then we have to bring these two together and pull off the air that is sitting, so we pull off the chemical that is sitting with the organic matter or such phase or it is sitting as pure phase so we have to pull that out into this and it’s a solid matrix, so by doing all this moisture reduction and drying agent it becomes particles that are easily mixed well with a solvent there for extraction efficiencies are higher. But in some cases one way of doing this is to just mixing it, mixing itself is not sometimes enough so the methods of extraction one older method is what is called Soxhlet extraction, it’s a very old method and its used to extract all kinds of solid matrixes, any solid big chunk, this is a rotary evaporator that I showed you . This is what is called a a Soxhlet apparatus, what we do is that there is this small thing that you see here, on the right hand side, this is a sample holder like a cup. One of the other things in extraction is that the extraction like liquid extraction offers you have to separate the liquid and the extract. So these apparatus a solvent is paced here, it is kept close to its boiling point and it starts boiling it goes up here you see this condensation this thing goes up, condenses and drops down into the thimble so its dropping down close to its boiling point. So it’s a hot extraction and it drops down, very close to the solid and it fills up, when it reaches this level there is a small system here, it creates an effect and the entire liquid in this chamber is now transferred back into the main reservoir, so this happens in cycles of about 30 minutes, when it fills up everything empties. So its like one cycle of extraction you are extracting at that temperature with a certain volume of solvent for 45 minutes and do another one. So if you look at the reflux of solvent and it is done for 4 to 24 hours depending on how hard the matrix is. Its very time consuming and expensive in the sense of energy. At the end of the process the extract is here, this is the extract. In this method you need a large amount of extract, in this kind of situation we have to use a rotary evaporator or some such. The other instrument to use is called as Kuderna - Danish apparatus.This is a very old technique its to prevent/ reduce the loss analyte because one the problems if you rapidly evaporate the concentration this solvent will evaporate very fast because its at boiling point the rate of evaporation is very fast that it will take other things with. This is seen as a way of reducing it a little bit but this is a very time consuming method and it will not entirely reduce the volume. But the idea is once you get it inside this tube you can take this tube and do and reduce it to what small volume e you want. So far for solids the other option that we have is a more recent method of using ultra sonic extraction, this takes a sample and add what ever moisture reducing solvent and the sulphate in it mix it well and put it in ultra sonic bath. In both these methods once you get an extract in solid phase it will contain a lot of things if you are extracting from soil the extract will look yellow or dark brown which means it contains a lot of things. So we were discussing this the other day we will call this as an interference, but if you don’t want the interference, so it contains the extract, to remove the interferences you have to filter the sample, which is called as the clean up before we go any further, one of the main interferences is the solid itself, so one way of doing. Possible interferences one is solid, chemical groups. So there are cleanup methods to remove whatever type of interferences essentially it looks like this so we have adsorption, these are all standard methods the numbers you see here are solid based. And these are the type of material that is used for the cleanup for example you have silica gel cleaner alumina cleaner. So what happens here is like a filter, your sample is added on the top and allowed to go through this column. It retains what ever you don’t want.You have to elute the sample and this term of elution is very specific, it’s a term that is used in chromatography and we will come to chromatography in the next topic so we are doing what is called as column chromatography here in some sense but if it is just a filter we are using that’s is filtration but in the case of silica gel its also what is called as liquid chromatography it’s the oldest form of chromatography which means that you should have a mobile phase you see that there is a mobile phase moving through when you add this here and we will discuss chromatography after this you will understand more, so basically you must have ac continuous flow of this going through. So there are various kinds of cleanup procedures you for cleaning up sulfur, for cleaning up acids all kind of things, so depending on what your sample is and what you are expecting to be has a significant interference will move many of these things. We will come back to this any questions on this so far? In terms of what we are doing?