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Tips and Tricks for the Analysis of Polar Pesticides – Q&A With an IC-MS Expert

Team TFS
Team TFS

In a previous  blog post  titled “IC-MS: The Solution to the Problem Analysis of Polar Pesticides!” I outlined the advantages of Ion chromatography-mass spectrometry (IC-MS) for the analysis of polar pesticides in complex food matrices.  I also indicated our commitment to continue to develop the IC-MS/MS approach, so, where are we now?

Where are we now with Polar Pesticides Analysis?

Since posting the blog, Thermo Fisher has published an Application Note, “Multi-residue analysis of polar anionic pesticides in food samples using a compact ion chromatograph... and launched a validated Anionic pesticides analytical workflow based on IC-MS/MS.  More laboratories are now using IC-MS/MS, many successfully, so it is puzzling why some laboratories still face challenges with this method. Could it be unfamiliarity with the care and maintenance of IC-MS systems?  Hence, I reached out to my colleague Wai-Chi Man, Product Marketing Manager Ion Chromatography and Sample Preparation, to give us her top tips for success with the IC-MS polar pesticides application.  Here are Wai-Chi’s answers to my questions.

 Q: What top tips do you have for someone who is familiar with LC-MS, but new to IC-MS?

A: The quality of water is of utmost importance. Always use 18.2MΩ water for eluent generation and the suppressor feed, otherwise, impurities in the water can affect the baseline. The background conductivity at the start, before connecting to MS, should be <1µS.  LC eluents normally contain organic solvents which help to prevent microbial contamination. For IC we are using water, so it is recommended to replace, not simply top-up, the water in the reservoir, to avoid unnecessary challenges with microbial contamination.

HPLC connections and plumbing are usually with stainless steel fittings and tubings which are less impacted by overtightening. IC connections on the other hand are usually pursued with PEEK polymeric fittings and PEEK tubing and overtightening these fittings can constrict the tubing causing increases in backpressure. It is recommended to finger tighten these fittings with a quarter turn with a wrench. This procedure ensures that the tubings do not add unnecessarily to the backpressure of the system.

Always monitor and record the back pressure of your system so that you can quickly identify any changes that may cause a problem. When installing a suppressor, you should always measure the backpressure before installing a mass spectrometer (MS), usually downstream of the conductivity detector. After connection, the pressure should not change by more than 100 psi.

When connecting the MS (perhaps via a PEEK tee) it is important to use tubing with the correct dimensions. The safest option is to use pre-cut IC viper tubing for IC-MS.  If this is not possible then do not use excessive lengths or PEEKtubing with an internal diameter less than  0.01”. It is also important to check that sample components are soluble in the aqueous environment during chromatography and in the solvent environment post chromatography. Samples that are not soluble will tend to precipitate and may cause issues with backpressure and leakage.

Q: In your opinion which is the best column-suppressor combination for polar pesticides in food and why?

A: There are several different ion exchange columns that could theoretically be used for polar pesticides. For multi-residue anionic pesticides analysis, the Thermo Scientific™ Dionex™ IonPac™ AG19-4μm Guard column, 2 × 50 mm, coupled to a Thermo Scientific™ Dionex™ IonPac™ AS19-4μm Analytical column, 2 × 250 mm, and with KOH gradient elution is the most appropriate choice as described in the application note (AN 73204) mentioned previously.

Q: Can you provide any advice for maintaining optimum performance and longevity of the suppressors?

A: It is essential to restore the regenerant flow to rehydrate the suppressor before powering the suppressor device to avoid burning out the suppressor. The suppressor should always be rehydrated after a period of inactivity.

It is also important to remember sample co-extractives may gradually contaminate the suppressor. I recommend that you use a flow-through On-Guard II RP cartridges to remove co-extractives as described in application note AN 73204.  These SPE cartridges have proved very successful for leek and wheat matrices with minimal, if any, losses of analytes, but have not yet been tested on samples of animal origin.

The use of the recommended cartridge clean-up option will also help to minimize contamination of the column. Dionex ion-exchange columns are very robust to matrix, but inevitably become contaminated with continuous injection of hundreds of extracts of complex food samples.  It is good practice to clean the columns regularly to avoid any build-up of contamination in the first instance. A simple but effective procedure is to connect the column outlet directly to waste and use a KOH eluent concentration not exceeding 200mM.  The clean-up is more effective if using a low flow for a longer period. When cleaning the column always use an offline PEEK pump, not a stainless steel pump, as the harsh pH of the cleaning solution could strip out metal ions in the flow path and contaminate the column. Always disconnect the guard column from the analytical column and clean separately and NEVER backflush ion -exchange columns. Or, you reverse the order to analytical then guard to avoid contamination from the guard column fouling the analytical column. If in doubt refer to the column manual.

Thank you, Wai-Chi, for your time and for sharing your knowledge and expertise. It seems the implementation of simple maintenance protocols for IC-MS can enable the robust multi-residue analysis of polar pesticides at low mg/kg concentrations in foods. Furthermore, much of your advice should be equally applicable to a wide range of other IC-MS food safety and environmental applications’.

For further information, you may also wish to read an LCGC E-book entitled IC-MS: A Solution for the Analysis of Polar Pesticides.