Monitoring methods for Air - Vapor - Part 3
So coming back to our thermal desorption, what is done is the following so you sample in a tube the usual way you attach it to a sample and then bring the sample, this tube I brought to an automated thermal desorption unit it is an accessory to a GC ITS KEPT BEFORE A GC it doesn’t have to be a GC it can be anything. Its an accessory before any instrument. The processing accessory so this thermal desorption tube is paced in this in a heated chamber or jacket or what ever and it needs to go to an analytical instrument so let us say it needs to go to a GC for example. So you heat this tube to very high temperature, what will happen is it will all come out, the desorption will happen.
But it can’t stay there it has to be taken out quickly, so what we do is we do flush desorption, flash desorption means its very quick the temperature shoots up very fast and everything comes off in one shot. If you are doing slow temperature increases It will come the adsorption, but you want all of it to come off at once, why do you want all of it to come off once because when you are sending gas here to sweep everything out you want the volume of the sample to be as small as possible.
Because now you are going to inject t into a GC and you don’t, you will recall the GC our injection is very quick we are injecting a pulse there so we want it to be as close as possible to a pulse. Otherwise separation will have a problem, sample injection your sample chromatography retention time will change. So its very difficult to manage it to an extent possible they try to do this as quickly as possible, it’s a very interesting system a lot of chemical engineering concepts go into this, and how quickly you can convert this into aa pulse sample.
There is usually a sweep gas, essentially that’s what it does it sweeps it whatever is desorbed it just sweeps it and flushes it out its usually the carrier gas that is used in GC you don’t want anything else that’s mixing, and it goes into the GC, the GC has what is called as a sample loop is the holding place for the sample. You would like the sample loop to hold al the sample that you are bringing so this is important because what you measure in the GC depends on what you are injecting and what you are desorbing has to be in a finite well defined volume so that you know what the sample volume is.
So the sample loop will allow you to keep this in place, and calibration now has to be done on this basis when you are doing that you have to calibrate like this which means that I have to inject a known concentration of gas. But you are now eliminating one processing step, you are automating everything your hope is that things are, this is especially good for what is called as volatile organic compound or VOCS. When you are doing analysis of VOC USING WATER, the Henry’s constant between benzene and water is very high, the moment you take that ample, the sample is constantly going out so one of the things people do is if you are extracting again there is chance of losses so people don’t do liquid-liquid extraction for VOCs they use another technique called as purge and trap you will see this in the sample you are using, this is a processing accessory.What this does is it takes the water sample directly and it purges it, with nitrogen, nitrogen gas is released in the system, you are stripping the water this is the water sample. So the accessories which are very useful in analyzing this so all of these are directed towards only one thing all the accessories, we are trying to increase the confidence in our analysis, quality control and quality analysis is the focus in most of these methodologies that developed for these kind of things.So we will go over quickly this is another type of samples, there is something called as passive samplers, this is a new thing, this is not a standard method its still under development so this is an equilibrium based sampler which means that all he other things that we have done is we take a sample of air, we know what’s the volume of air and we extract what is there and we get concentration mass volume here in some cases if you want to take ambient air samples in the way we described what is called as active sampling, this is where I am actively pulling a sample of air but the problem is that sometimes I need a pump, sometimes I need somebody there. So what people have come up with is what is called a passive sampler, you put hundreds of these passive samplers and it comes to equilibrium with ambient air or water. Its an equilibrium based method we are using a partition constant and some subtract or solid. This is a long time average its useful for processes which are, this is a longtime process.
There are various types of passive sampling devices this is used in indoor pollution a lot, there use it find out passive smoking in restaurants and bars. We have a little bit of balance material remaining in the analysis of different species, analysis of chemical composition in PM and vapor, in organic we have OC/EC OC is the analyzer what they do here, this is similar to GC for one section of this as I mentioned earlier the OC/EC instrument itself works in terms of the OC component is all organic compounds which have some structure, the EC compound is what is called as elemental carbon, so this portion I can combust it and what ever comes out you can measure CO2 using infra red. You get carbon dioxide or carbon monoxide, this is brought back and you get methane.
So people use this data OC/EC the ratio of organic data the ratio, to look at where the source could be, so if the elemental carbon is high the source is very likely to be from combustion, diesel combustion and all that which forms soot elemental carbon some specific combustion sources specifically form soot so you know that if the OC/EC ratio is different possibly the source contributing to the pollution in that area is coming from this source, so based on profiles from various sources you can get information regarding what is the source contribution which source is contributing to air pollution in that given area this is one instrument which is a very specialized instrument so here because this filter paper is directly put into the instrument and can withstand temperature up to 1000 degrees this filter we use quartz filter paper, we had mentioned in the previous class that we use quartz Teflon, nylon different materials, the quartz s specifically used for organic carbon elemental carbon analysis .