This is the third in our Troubleshooting series for VOC Analysis.
- Part one is linked here, VOC Analysis Troubleshooting Part 1: How to Begin Troubleshooting
- Part two is linked here, Troubleshooting Part 2: No or low analyte response and carryover
Part three will discuss elusion issues and moisture control for VOC analysis.
If you’re having issues with the elution of your compounds of interest, first check the tune on your Mass Spectrometer (MS) to ensure everything looks normal and is passing recommended requirements. If the MS is working properly, you should perform a direct injection in the Gas Chromatograph (GC) inlet. If there are any missing peaks, you could be dealing with a faulty electronic pneumatic controller (EPC) or a bad column. If the problem lies within the early elution groups, there could be a leak in the GC inlet or column.
To determine if the Purge and Trap (P&T) is causing the elution issues, you should run a leak check. A leak in the trap could be causing the loss of compounds, especially early eluting compounds of interest. If a top-of-trap injection leads to peaks still missing, the sample tubing or transfer line might need to be replaced. In the P&T method, you might not be efficiently managing the moisture by having too long of a dry purge or desorb time and pushing too much water to the GC. Also, having the temperature of the trap set too high could push the compounds of interest to the vent, leaving them not recovered at all.
Very rarely, you could have issues with your middle elution groups. In this case, your GC column could need to be replaced, or you could have the wrong column for the method you’re trying to run. On the P&T side, the flow might be set too low to push everything off the trap and to the GC-MS. There might also be a leak in the Mass Flow Controller (MFC), or you might need to replace the trap.
For issues with the late elution groups of your compounds of interest, the MS source might need to be cleaned or the carrier gas flow rate could be set too low. On the P&T side, you could be dealing with poor purge efficiency, restrictive tubing, dirty glassware, a faulty heater, a plugged vent valve. Or you might need a new trap.
Water vapor accumulating in systems can minimize sensitivity and cause target compounds to co-elude, shift in retention time, and produce poor peak shape, even in compounds outside of the water peak. If you’re seeing increased moisture in your chromatogram, you could be having vacuum issues on the MS. On the GC, your split ratio might be set too low, allowing too much water to get onto the column. A long water peak may be a sign of a bad column or a sign that it’s time to replace your column.
For the P&T, you want to make sure you’re using the correct trap for the method. Also, be sure to double check your sample purge temperature. Purging your sample at a higher temperature can drive target compounds out of solution better, but it also increases the amount of moisture sent to the trap. Other method optimization studies you can perform include dry purge, desorb, and bake settings. Testing one setting at a time can determine your overall system sweet spot and run your methods as efficiently as possible.
Read the blog series or watch our webinar on moisture control for a more detailed description of how water vapor disrupts the volatiles analysis, and tips to optimize methods for moisture control.
- Part one is linked here, VOC Analysis Troubleshooting Part 1: How to Begin Troubleshooting
- Part two is linked here, Troubleshooting Part 2: No or low analyte response and carryover
Continue to follow this troubleshooting series. We’ll cover linearity and reproducibility issues next, followed by a run-through of our TekLink troubleshooting tools of leak check and benchmark test.
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