This is the forth 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 is linked here: Elusion Issues and Moisture Control

Troubleshooting linearity and reproducibility issues in your systems can be difficult. Problems with linearity and reproducibility can make calibrating your systems frustrating, and if internal standards are varying, it can cause all other target compound to look unstable.

Hopefully, following previous tips from this series and the tips below will help make troubleshooting these issues easier. Starting with troubleshooting the Mass Spectrometer (MS), issues with linearity and reproducibility could mean that source maintenance needs to be performed, there could be vacuum issues occurring, or the multiplier could be going bad and needs to be replaced.

If you start to see response from your internal standards increase as target compound concentration increases, you should clean the MS source. This can be validated by preparing three increasing target concentrations with internal standards in 1 milliliter (mL) vials and then performing a direct injection of 1 microliter (µL) into the Gas Chromatograph (GC). If the internal standards area counts are still increasing, you have isolated the active site to be either the MS source and/or the GC inlet liner. If the internal standards area counts remain consistent, that means the active site is not within the GC-MS and the problem could be in the analytical trap of the Purge and Trap (P&T) or the sample tubing of the autosampler.

In the GC, the cause of linearity and reproducibility issues could be a dirty inlet liner, a failure with the Electronic Pneumatic Controller (EPC), a bad column, or a method that isn’t optimized enough. When re-evaluating your method, be sure to optimize your oven temperature program.

 In the P&T, linearity and reproducibility issues could be caused by a failing trap. This can be true if your brominated compounds are displaying low recovery or the heavy, later eluting compounds are not recovering well. If the internal standards are varying, this could be causing reproducibility issues and there could be an active site. It is also possible that the drain valve could be leaking. There could also be excess water in the system.

Making sure there is enough bake-time and you are baking at the correct temperature can ensure that the system is cleaned up of that excess water from sample to sample. It’s also possible that temperatures are not reaching set-points; possibly there is a faulty heater somewhere. Too much carryover could also cause linearity and especially reproducibility issues. Another common and frustrating reason why there could be linearity and reproducibility issues could be sample preparation errors.

In the autosampler, if you are seeing those inconsistencies with the internal standards, you can handspike vials with internal standard to test if there is a leak in the internal standard vessel to the point it is not consistently dosing the same amount of internal standard to each sample. You could also do a manual injection right into the sparger as well. Linearity and reproducibility issues in the autosampler could also be caused by it not pulling the same amount of sample each run or not transferring the sample correctly. There could be improper rinsing between samples.

One comparison you could try if you have an Atomx XYZ would be to compare a soil and a water method sample. These method types have different sample pathways, so if you see an issue in one, that is where your problem could be. You can check pressure in the internal standard vessels in the tools menu if you have a Tekmar product. You want that to be between 6-8 psi. If the pressure of the vessels isn’t reached, this could cause issues with the amount of internal standard getting purged with the sample.

Please continue to follow this series as we will cover the TekLink troubleshooting tools of leak check and benchmark test.

• 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 is linked here, Troubleshooting Part 3: Elution issues and moisture control