1.8. Analysis Techniques for Air Pollutants

So, while coming back to our thermal desorption what is done is the following. So, you take your sample in a tube, you attach it to a pump in the usual way and then this tube is brought to what is called now called as an automated thermal desorption unit. It is an accessory to a GC, it is kept before a GC.

Thermal desorption is a technique used to analyze air samples by heating them. A sample is placed in a tube and connected to a pump that transfers the sample to an automated thermal desorption unit, which is an accessory for a Gas Chromatograph (GC). This unit heats the sample, causing the pollutants to evaporate and become detectable in the GC.

Think of it like cooking food. When you heat food, the smells (aromas) get released into the air. Similarly, thermal desorption heats the sample to release the pollutants so they can be analyzed.

When you heat it to a 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 flash desorption.

Flash desorption involves rapidly heating the sample to cause all the pollutants to vaporize quickly. This prevents the pollutants from re-adsorbing into the sample tube and allows for a more concentrated and faster input into the GC.

Imagine popping popcorn in the microwave; if the heat is applied quickly, the kernels pop instantly. If they heat slowly, some might become chewy instead of crunchy. Similarly, quick heat ensures a clear, concentrated sample for analysis.

So it is usually the carrier gas in the GC and it goes into the GC, how can it go into the GC? GC has what is called a sample loop. The sample loop is the holding place for the sample, you are sending in a certain concentration.

The carrier gas, which is typically argon, helium, or nitrogen, helps transport the vaporized sample into the GC. The sample loop serves as a holding area that ensures a specific volume of the sample is injected into the GC for analysis.

Think of the sample loop as a water reservoir. Just like a reservoir holds enough water to release into a system as needed, the sample loop holds the exact amount of sample for the GC to analyze at a controlled rate.

Suppose you want to analyze benzene, the Henry's constant of benzene in water is very high. So, when you are doing all that extraction, the moment you take sample, sample is constantly going out, the chance of losses are quite high so people don’t do liquid-liquid extraction for VOCs.

For volatile organic compounds (VOCs), a method called 'purge and trap' is used instead of traditional extraction methods. In this method, a nitrogen purge gas is used to strip volatile compounds from a water sample without losing them, allowing for better analysis of substances like benzene.

Consider trying to catch bubbles in a fish tank; if you just try to scoop them up with a net (like liquid-liquid extraction), many will escape. Instead, using a vacuum cleaner to suck them up (like purge and trap) ensures you capture most bubbles effectively.

So, this is something called as passive sampler, this is a new thing, and not a standard method, this is still under development. So, this is an equilibrium based sampler.

Passive samplers collect air samples without needing a pump; they sit in the environment and equilibrate with the air. This method is cost-effective and useful for long-term monitoring of air pollutants, as it can gather data without frequent intervention.

Think of passive sampling like leaving a sponge in a bucket of water. Eventually, the sponge will absorb water to match the water level in the bucket without needing manual help — that's how the passive sampler collects air quality data over time.

We are looking at analysis of different species, let's just summarize that PM and vapor. The organic analysis will have something called OC/EC (organic carbon/elemental carbon).

In analyzing air pollutants, it's important to differentiate between organic carbon (OC) produced from various sources and elemental carbon (EC) usually found in soot. Techniques involve burning samples to measure these forms of carbon and their ratios, which provide insights into pollution sources.

It's like examining the smoke from a fire; some smoke comes from burning wood (organic carbon) and some from the soot (elemental carbon). By analyzing both, we can understand what kind of fire we’re dealing with and better manage air quality.

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