1.6. Purge and Trap Technique for VOCs in Water

There is another application of this kind of system when you are doing analysis of VOCs in water. 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 the sample, the sample is constantly going out, the chance of losses are quite high so people don’t do liquid-liquid extraction for VOCs.

The Purge and Trap technique is specifically designed for analyzing Volatile Organic Compounds (VOCs) in water. An example of VOC is benzene, which has a high Henry's constant—a measure of how easily it enters the air from water. Because of its volatility, when researchers take a water sample, benzene can easily escape into the air, leading to inaccurate results if traditional liquid-liquid extraction methods are used.

Imagine trying to catch bubbles rising in a glass of soda. If you wait too long, those bubbles float away, and you lose your chance to measure how much carbonation is in your drink. Similarly, when extracting VOCs from water, you need to act quickly to prevent them from escaping.

They use another technique called purge and trap. This is a sample processing accessory. What this does is it takes the water sample directly and it purges it. You purge with nitrogen, nitrogen gas is released. When nitrogen gas is flushing out all the volatiles in the system you are stripping the water.

In the purge and trap system, a sample of water is treated with nitrogen gas. This means that nitrogen is bubbled through the water, which helps to carry the VOCs out of the water and into a gas phase. The nitrogen does not chemically react with the VOCs; it simply helps to separate them from the liquid, making it easier to measure.

Think of using a straw to drink soda. When you suck on the straw, you remove the liquid and leave the bubbles behind. Similarly, nitrogen works like the straw in the purge and trap technique, removing the gases from the water sample.

Now at this point this becomes like a vapor sample, air sample. What do you do? You put a small thermal desorption tube here, we are doing the adsorption here, it goes in and sits here this is trap, this is the purge, we trap it until all of it is purged.

Once the water is flushed with nitrogen, the volatile compounds become part of a vapor sample. These vapors are then passed into a thermal desorption tube where they are trapped for later analysis. This tube acts like a sponge, holding onto the vaporized VOCs until the analytical instrument is ready to process them.

Imagine a sponge soaking up spilled juice. Once the sponge is full, it doesn’t let any more juice in until it’s squeezed out. In the same way, the thermal desorption tube holds onto the gaseous VOCs until it is time to analyze them.

Once you trap it, trapping cycle is done then you do what we did in the thermal desorption, you do thermal desorption after that and then it goes to a GC or GCMS or whatever through a sampling loop again, it has to go through a sample loop to do this.

After trapping the VOCs, the next step involves 'thermal desorption,' where the trapped compounds are heated, causing them to be released as a gas. This gas is then introduced into a Gas Chromatograph (GC) or a Gas Chromatograph-Mass Spectrometer (GCMS) for detailed analysis. The 'sampling loop' helps ensure accurate measurement by controlling the volume of gas analyzed.

Think of a popcorn kernel that’s heated up in a pot. When it pops, it releases steam and flavor into the air. Similarly, thermal desorption releases the VOCs for analysis, ensuring that researchers can accurately detect what they were able to trap.

--