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Purge and Trap Water Management: Using Desorption Time to Your Advantage

Posted by Teledyne Tekmar on Wed, Jun 13, 2012 @ 04:21 PM

In gas phase analysis, water can be a big challenge. Like the analytes of interest, it is volatile. Generally in gas chromatography, water can be the enemy of just about every component of the GC, from the injector to the column to the detector. And when using a purge and trap concentrator for the sample preparation, the matrix is water, so dealing with it is unavoidable. Our task, then, is to try to decrease the amount of water and minimize the effects of water on the GC and the resulting chromatography.

This chromatogram may look familiar, especially to those running EPA Method 524.2.

EPA Method 524.2 Water

Figure 1: EPA Method 524.2 Chromatogram

So what is the noise in the window of about 1 to 4½ minutes? That’s water. Even though the mass spectrometer, per most methods, is set to scan only as low as 35 m/z, the effect of water (18 m/z) can definitely be seen in this rise in the baseline. Just because we aren’t looking for it, doesn’t mean it’s not there and we are immune to its effects.

In an effort to monitor this water effect, we set up a SIM window for 20 m/z which helps to mirror the chromatography, but avoids saturating the detector since it is a very minor isotope of water (Figure 2). Scanning down to 18 m/z to quantify the water is not a good idea, and even setting up a SIM window would saturate the detector with the extreme amount of water.

P + T SIM Window

Figure 2: 20 m/z SIM Window

So, now that we can see the water, how can we work to reduce it? There are a few tools an analyst has in regards to water management when using P&T. One parameter that can be overlooked, sometimes due to method restrictions, is desorption time. This is the amount of time used to transfer analytes off of the analytical trap to the GC for separation and quantification. This plays a big role in the amount of water transferred to the GC, as the compounds of interest are removed from the trap well before the end of the 4 minute desorption time prescribed in 524.3. Any extra time is simply sending more water to the GC.

Analytical traps used in P&T analysis are a fixed volume and can hold a set amount of volatile compounds. These traps are selective and have a preference for hydrocarbons, but they will pick up water if there are none available. So what happens in a purge and trap situation where the sample you introduce is normally 99%+ water in a gas phase and the compounds of interest are in the part-per-billion range? All the extra spaces fill up with water, the solvent which is readily available.

The good news is that all of the compounds of interest are removed from the trap in about the first 30 seconds of the desorption step. By using shorter desorption times, we effectively “shut the door” on the excess water, leaving it to be removed during the bake cycle. Given sufficient flow, this does not negatively impact the compounds of interest, and in most cases, actually improves the resulting chromatography. An example of this study can be found in Figure 3, showing a drastic decrease in the amount of water seen using a 30 second versus 4 minute desorption cycle.

P+T Decrease in Water

Figure 3:  Drastic Decrease in the Amount of Water

With EPA methods 524.3 and 524.4, flexibility with purge and trap parameters is increasing, allowing for quicker cycle times, better water management, and cost savings, especially with the scarcity of helium. Running a shorter desorption time keeps that extra water out of the mass spectrometer and can help to lengthen the time between those dreaded source cleanings. Tekmar has multiple application notes for both of these methods (see links below), and our customer support team is always available to answer any questions about getting them set up in your lab.

Access to our Applications Library: http://www.teledynetekmar.com/applications/index.asp

Related Application Notes:

Using Nitrogen Purge Gas for USEPA Drinking Water Methods

Evaluating USEPA Method 524.3 Utilizing Newly Permissible Method Modifications to Purge and Trap Techniques

Tags: Purge and Trap Concentrator, chromatography, US EPA Method 524.3, SIM, US EPA Method 524.2, gas phase analysis

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