Water vapor entering the gas chromatograph (GC) inlet, column, and, ultimately, the mass spectrometer (MS) can minimize the sensitivity of our early eluting compounds, including our gases. It can also cause poor peak shape, compounds to co-elude, and shift in retention time, even in compounds outside of the water peak. So, once the water is introduced into our system, it starts to wreak havoc on all our compounds. Some say water doesn’t affect column phase or lifespan, but some have seen an effect, especially if you’re running a lot of samples; water can shorten the column life of your GC column and can cause vacuum issues if you’re using a MS system, which could lead to shorter filament lifetime and having to clean the source more often.
Throughout this blog series, we’ve discussed ways to minimize the amount of water vapor introduced to the GC-MS, using Teledyne Tekmar’s moisture control fitting and application control. We can continue to control moisture not only through this hardware designed for the Purge and Trap, but also through method parameters we can adjust and using smart method parameters on the GC.
By using shorter, narrow bore GC columns, we are able to adjust our split ratio. If our split ratio is set too low, it will allow too much water vapor onto the column. A higher split ratio means more of that desorbing gas is going off to vent before going into the inlet and onto the column. This leads to less water being transferred onto the column, but also less of our target analytes because they are going off to vent as well. Therefore, if we have our split ratio set too high, we will start to lose analytical sensitivity. Analyzing a mid-level standard at different split ratios can help you assess what split will work best for your setup. You want to make sure the water peak is minimized as much as possible, while still getting the response needed for your analysis. A long water peak may be a sign of the need to replace a bad column.
On the MS, seeing excess water in your analysis could be a sign of vacuum issues. If you don’t have adequate vacuuming, then the water that is getting into the MS is causing more problems than it otherwise would. Less water getting to the GC means less water is getting to the MS, which can extend filament life and allows for less source cleaning by the analyst, which is a huge time-saver because source cleaning is a long process.
In short, using Teledyne Tekmar’s innovative moisture control system, choosing the correct trap for your target analytes, purging your sample at the optimized temperature, adding dry purge time as needed, shortening your desorb time if the method allows, and adding in adequate bake time will not only remove water vapor from the sample going to the GC-MS, but also remove moisture in the system from sample to sample, leading to improved and sensitive volatile organic compound analysis.
This is a four part series discussing moisture control. For the final part of this series, we will explore how to optimize the method parameters on the GC-MS side of the analysis for moisture control.
- Part 1: Why is water vapor bad for VOC Analysis?
- Part 2: Ways to optimize moisture control in the Purge and Trap
- Part 3: Ways to optimize moisture control in the Purge and Trap - desorb and bake settings
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