1.3. Sampling Methods

This is for PM. What about vapor? So there is a bunch of things we talked about for sampling of PM using impactors and all that. How do we do vapor phase analysis? Vapor phase, but in the atmosphere we have everything vapor phase and PM, we only want vapor phase, the first thing we need to do is cut off the particles. So, you need a filter so, vapor phase at the downside of the filter paper usually. And so, we use an air stream we have a pre filter here we take out all the particulate matter here and then we have we use an absorbent typically.

Vapor phase analysis is a method used to examine gases in the atmosphere while filtering out particulate matter (PM). The first step involves using a filter to separate the vapor from solid particles. The vapor is collected on the underside of the filter paper. An air stream is used to push air through the pre-filter, which captures PM, allowing only vapor to go through, subsequently using an absorbent to collect the vapor for analysis.

Think of this process like a coffee filter. When making a cup of coffee, the filter allows liquid coffee to drip through while capturing coffee grounds. Similarly, the air filter separates vapor from solid particles to ensure only the vapor goes through for further analysis.

Sampling, one method of sampling is what is called as grab sampling. This is what you do in water also, in water, you take samples you take you grab a sample in a container. I can also grab a sample of air in a container. What is the problem? So you have to talk about again, sampling all the way to the instrument. What prevents me from doing it if I can do it, I will do it. But if I cannot do it, what is preventing me from doing this? What is my constraint?

Grab sampling is a method where a sample of air or liquid is taken directly in a container for immediate analysis. This method can be practical for liquids, like water, where a sample can be easily collected. However, capturing a sample of air is more challenging because air is invisible. The potential issue arises if the concentration of the vapor of interest is low; in this case, grab sampling may not provide a detectable amount for analysis.

Imagine trying to collect a sample of invisible perfume in the air. If you're close enough to the perfume bottle (the source), you can easily capture its scent. However, if you're far away, you might not detect any odor at all, similar to how grab sampling works with air concentrations.

Grab sampling essentially means it takes some volume of sample and I analyze it as is, so if I am doing grab sampling, I have to define a volume there is a certain volume, I take gas or vapor in that volume and I directly go and take a sample and inject it into a GC or HPLC or something and get some signal or I have a sensor that will directly, if I can put a sensor directly I am I do noteven have to grab something.

During grab sampling, a specific volume of the sample is drawn from the environment and analyzed directly. The sample is injected into analytical instruments like Gas Chromatography (GC) or High-Performance Liquid Chromatography (HPLC) for analysis. However, this assumes the concentration is high enough for detection. If not, sampling may yield insufficient or incorrect data, leading to misinterpretation.

Consider a chef who needs a taste of soup to assess its seasoning. If the soup is flavorful (high concentration), a small spoonful will suffice for tasting. However, if the soup is bland (low concentration), that small spoonful might not provide enough flavor to make an assessment, similar to how grab sampling might fail with low concentrations.

The biggest problem in the air and you can’t see it; you can’t see the sample is there. That is one of the main problems in air, ok, this is this is this is ah. So, there are ways to work around that. So how do you sample air? How do you grab some? How do you grab a piece of air? Vacuum pump.

Sampling air presents unique challenges due to its invisible nature. To collect a sample, methods like using a vacuum pump are employed. The vacuum creates a pressure difference, allowing ambient air to be drawn into a container where the sample can be collected for analysis. This method ensures that a defined volume of air is captured, which is crucial for accurate measurements.

Think of a vacuum cleaner. It uses suction power to draw in dirt and debris from the floor into its canister. Similarly, a vacuum pump acts to draw air into a sample container, making it possible to collect air samples for analysis.

One possible container for a grab sample? Need a container wait what containers, will you use. So you can use a cylinder for all gas storage everything we use a cylinder, so we use a small cylinder, a small cylinder which has a valve well here is a cylinder. And, we also have a, some kind of a pressure gauge which shows that there is gas inside.

Air samples are typically collected in specialized containers, such as cylinders, which have valves to control the flow of air in and out. Additionally, pressure gauges may be present to indicate whether the container is holding air, ensuring that the sample collection process is accurate and secure. This setup helps prevent contamination and loss of the sample during retrieval.

Imagine a balloon filled with air. When you want to seal it, you tie a knot to prevent air from escaping. In a similar way, sample containers are equipped with valves that 'seal' the air inside, allowing for safe transport and preventing contaminants from entering.

If you did not have enough concentration, you have to accumulate that you cannot do grab sampling the concentration is not enough. So you have to accumulate volume just like we accumulate mass so we use an absorbent in this case for trace analysis where grab sampling would not work.

When the concentration of the vapor is too low for effective grab sampling, absorption techniques are utilized. In this method, air is pulled through an absorbent material that traps the vapor components of interest. This allows for the accumulation of a measurable amount of the target vapor over time, which can later be extracted for analysis.

Think of a sponge soaking up water. If you dip a dry sponge into water (the air sample), it can absorb a large amount of moisture. Similarly, absorbent materials can trap low concentrations of vapors over a period of time, allowing for further analysis.

There are a lot of absorbents that are available in the market which are generic absorbents in which means that you did not know what is an atmosphere it is open, it will absorb a wide array of compounds and then you can trap it here.

Various absorbents are available in the market, and they can capture a wide range of chemical compounds from the air. This versatility is useful when the specific composition of the atmosphere is unknown, allowing for comprehensive sampling and analysis of various vapors present without prior knowledge of their identities.

Imagine going to a buffet with a variety of foods. You grab a plate and take small portions from each dish without knowing their exact ingredients. Just as the plate captures a mix of flavors, generic absorbents can capture a mixture of vapors, making them useful in atmospheric studies.

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