Polar ionic pesticides are widely used but notoriously difficult to analyze with generic multi-residue methods. A recent SelectScience webinar addresses new advances that enable the sensitive and specific detection of polar pesticides such as glyphosate, AMPA, glufosinate, chlormequat, diquat, mepiquat, paraquat and propamocarb.
Contaminants such as glyphosate, one of the world’s most widely used pesticides, are commonly excluded from pesticide monitoring programs, as their analysis has been limited to complex detection methods to overcome unwanted ionic interactions. In this webinar, Dr. Jonathan Beck, who focuses on IC-MS/MS and LC-MS/MS solutions for environmental food and safety laboratories worldwide, discusses the Quick Polar Pesticides (QuPPe) method combined with IC-MS/MS for successful detection of polar pesticides. Joined by Dr. John Madden, an analytical chemist with more than 20 years' experience in the field of ion chromatography, the webinar also covers the advantages of IC-MS/MS and the separation of both anionic and cationic pesticides.
This webinar is the second part of a 5-part series of webinars we are running in partnership with Thermo Fisher Scientific, which explores ways to optimize your mass spectrometry small molecule workflows so as to improve lab productivity, selectivity and sensitivity, and to meet current and future testing challenges.
At the end of the webinar, Beck and Madden answered live questions from our listeners, and the highlights of this Q&A session can be read below:
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Q: How does the eluent generator produce ions?
JM: The eluent generator produces ions by electrolysis of water to produce hydroxide and hydronium ions. The counter ions are stored in a reservoir which sits on top of the eluent generator assembly itself, so the EGC does tend to look like a bottle turned on its head when you look at it. The actual ions used for ion exchange chromatography are literally generated from water itself.
Typically for 2mm operation, which is what we would be using for an IC-MS application, these cartridges last anywhere from about one to four years depending on the application and, of course, how much they are used. For a pretty heavily used instrument you should get a year out of it. After about two years, we do recommend you test the eluent generator to make sure it’s working properly. But up to about four years.
Q: How robust is the peak pump compared to a traditional LC pump?
JM: A peak pump comes with advantages and disadvantages as you’d expect. Its biggest advantage is of course its chemical inertness. In that respect, it is far more robust than a traditional LC pump. It can handle extremes of acids and bases, large concentrations of salt, deionized water and most solvents without any problem. On the other side of the equation, it cannot handle the pressures of a traditional LC pump, which is why all our columns are designed to run somewhere in the order of 2,000 psi to 4,000 psi, which is pretty much the operating limit of the pump. The pumps will go up to 6,000 psi, but we do not recommend running them much over 5,000, which gives you an idea of the limitations of the pump.
Q: Why do you need a conductivity detector if you have a mass spectrometer?
JM: The conductivity detector is there to make sure the matrix ions are not moved into the mass spectrometer. So, the matrix that the sample is in typically contains lots of potassium and magnesium for food, sodium and calcium for water matrices. These should not go into the mass spectrometer, they cause a lot of problems, so the conductivity detector is there to detect them. When they are found, a matrix divert valve is activated which sweeps those ions away to waste, away from the mass spectrometer. It also acts as a back-up in case the suppressor fails, and eluent starts flowing onto the mass spectrometer; again, it will detect that condition and activate the bypass valve accordingly.
Q: How do you perform method development for IC?
JB: Typically, we would optimize the mass spectrometer by infusing the same way we would in an LC-MS experiment. Since the IC eluent is converted to essentially water after it passes through the suppressor, we can infuse the compounds by teeing them into that IC flow and do our optimization as we would typically do on an LC-MS type application — really no difference there. In terms of method development with the IC, in terms of the chromatography it’s just like optimizing an HPLC method. You would experiment with either an isocratic separation or use a gradient separation. In the presentation today, both methods were gradient elution.
I like to explain to new IC users coming from an LC background that in ion chromatography, in the case of an anionic system, as you increase the current or concentration of potassium hydroxide, it is analogous to increasing the percentage of organic solvent in the mobile phase of a HPLC reverse phase experiment. So, it works the same way: as you increase that strength of the eluent, the compounds that are retained more would come off the column. We are just using ions instead of organic solvent to take the compounds off the column.
Q: Have you tried to analyze glyphosate in milk? It is reported to be found in breast milk by ELISA and milk has calcium that may bind with it.
JB: Yeah, we actually have. We had a collaboration with an organization called Fera in the UK in the city of York. They did analysis of glyphosate and other polar pesticides in infant formula. If you want that has been published as a peer reviewed journal search for my name or the name of author Stuart Adams. But yes, it did work, glyphosate was detected at the low ppb high ppt level in infant formula.
Q. Your method used RFIC systems with eluent generators, is it possible to manually prepare eluents?
JM: Very quickly, for doing anion separations, the answer is no. You can’t use manually prepared eluents. The reason is carbon dioxide from the air tends to be absorbed by sodium or potassium hydroxide, forming sodium or potassium carbonate. The suppressor turns this into carbonic acid and the carbonic acid acts as an ion suppressant in the ion source reducing sensitivity. So, we don’t recommend that.
For cationic systems, yes, you can use manually prepared eluent; it actually makes surprisingly little difference. We recommend an eluent generator primarily for ease-of-use and reproducibility. It eliminates lots of lab to lab, day to day, week to week irreproducibility. So, it is not required but is recommended.
Q. Can high-resolution instrumentation like the Orbitrap (LC-MS) be used instead of a triple quadrupole?
JB: Absolutely, we can do that. We have quite a few customers especially in the European Union, who use high-resolution instrumentation for the detection. It comes with all the same advantages of using a high-resolution instrument for LC.