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Module 1: Monitoring Methods

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    Monitoring methods for Air - Vapor - Part 1
    THIS is 4 PM what about vapor, there is a bunch of things we talked about in sampling of PM HOW do we do vapor phase analysis but in the atmosphere we have everything vapor and air. The first thing we need to do is cut off the particles so we need a filter, vapor phase is at the down side of the filter paper, and so we use an air try, then we use an adsorbent typically before we go to the adsorbent vapor phase there are two kinds of analysis that is done suppose what is the simplest method when you want to sample why are we sampling in a filter paper for PM, what is the basic reason, the sampling material in a filter paper, over a period of time the reason we are doing that is, preferably I would like to take this vapor sample as it is and measure it.
    One method of sampling is what is called GRAB sampling. This is what you do in water also, in water you grab a sample in a container I can also grab a sample of air in a container, what’s the problem? We talk about sampling again all the way into the instrument what prevents me from doing it? What is my constraint? So if I am doing sampling I have to define a volume, thee is a volume I take gas or vapor in that sample and I directly inject in into a GC and get a signal.
    Grab sample is done when concentration is high enough for direct analysis no processing required. Which are the possible candidates for this? Which sources are candidate for this? Cn you think of anything that we can do grab sampling? We can do grab sampling on anything but which one do you think we will have high concentrations? If your emission source is here if you want to measure I can use grab sampling it may work but if I am standing far away and I am trying to do grab sampling chances are it may not work, so this is a question of source sampling versus ambient sampling.
    Ambient sampling is general atmosphere I want to know what is the average concentration of some compound there grab sampling may not work if I do grab sampling I might get nothing so people can make a wrong inference that there is nothing there. So how will you do grab sampling with air? It’s the biggest problem because you cant see the sample is there, how do you grab a piece of air?
    So your container itself so what is one possible container for a grab sample? You need a container but what container will you use? So you can use a cylinder, for all gas storage we use a cylinder a small cylinder which has a valve, you have a valve here and there is a cylinder and we also have some kind of a pressure gauge which shows that there is a gas inside.
    So what they do is they evaporate, they pull the vacuum in this thing and then they close the valve so the contents of the container are in vacuum, so if you want a sample I open the valve and the air from inside will come in and you will let it equilibrate and then close the valve. So as long as there is no leak in the valve here your sample is secure and you can see it by, you need a very sensitive gauge.
    The only way you can see it is if you pressurize it more than the atmospheric so it will go up and if you see a leak it will start going down but in the atmospheric pressure usually the gauges are very sensitive but anywhere this is one way of doing it and from here there ways in which you can take a sample into a syringe or a GC.
    There is another way which is called a canister and there is another instrument called a tedlar bag, this is like a plastic bag so here same way you can see it has an air sample. The canisters are usually stainless steel so they wont, but plastic bags are nice because you can see if there is gas there or no gas I will show you pictures of it tomorrow. If you don’t have enough sampling you have to accumulate you cant do grab sampling so you use trace samples. You choose a perfect adsorbent.In the process of sampling there is a chance of a lot of losses that can happen especially if you are looking a volatile compounds, if you are looking at compounds like benzene and all those things they are fairly volatile so in the process of extraction they will come up with, you may have a significant amount of losses and many of these things there are ways in which here as well as in water analysis, there is extra extended technique that people use not to do manual extraction processing, you have automated extraction and processing and how to do that, we will just look at those 2 cases and this in tomorrow’s class.