Pesticide Analysis at Pace
Application chemist Kirk Jensen tells us how a low-pressure gas chromatography kit and short collision cell technology helped him measure 244 pesticides in 11 minutes
Lauren Robertson |
Speed is king in the world of pesticide analysis; as many lab managers know, being able to process more samples in a shorter time frame – while maintaining data quality – can be invaluable in high-throughput applications. Current gas chromatography-tandem mass spectrometry (GC-MS/MS) methods are certainly sensitive, but they require longer analysis times to effectively separate complex mixtures. Feeling the need for speed, Kirk Jensen, Robert “Chip” Cody, and John Dane from JEOL decided to test an approach using a low-pressure GC (LPGC) kit (Restek) with the enhanced selected reaction monitoring (SRM) switching speed of the short collision cell in a GC-triple quadrupole MS system (JMS-TQ4000GC, JEOL) (1).
The result? Three transitions for each of 244 pesticides were measured in a standard mixture in just 11 minutes. We spoke to Kirk Jensen to find out a little more about his work.
Meet Kirk Jensen
After graduating with a Bachelor’s in chemistry from the University of Northern Colorado, I worked in the pharmaceutical industry for a while. I soon decided I wanted more of a challenge, so I went back to grad school at Colorado School of Mines where I studied under Kent Voorhees – a big name in the MS sphere. I worked on a variety of projects; from combustion of biofuels to detecting Bacillus anthracis with phage amplification. I then took up a position as an invited researcher at Osaka University in Japan where I was looking at non-invasive ways of measuring stress markers in saliva. In my third year there, I was promoted to assistant professor and I was studying time-of-flight MS – specifically a closed loop kind of technology that used a figure of eight (or “infinite”) flight path.
Eventually, I moved back to the US and took up a position as an applications chemist at JEOL. Now, my job primarily involves helping customers with their application ideas, and providing them with guidance on how to make their ideas a reality. My secondary role is to find applications for our own systems. So I work on projects with our mass spectrometers and I try to disseminate this information as best I can.
What prompted this study?
My colleague, Chip Cody, discovered that Restek were offering a preassembled low-pressure gas chromatography (LPGC) kit. Now, this is not new technology – but the implementation of it is new. The kit uses a restrictor column connected to the analytical column to calculate pressure correctly. In theory, you could just buy two different columns and set this up yourself, but it can be very time consuming and the connections don’t always work. The LPGC kit makes this entire process a lot simpler.
The kit was of interest because we were looking for ways to increase throughput in pesticide testing labs – any way we can figure out how to do GC in less time is a boon for the industry. The idea with LPGC is to find a way to move more things through the column faster with similar separation efficiency. With a wider column, you should be able to push more things through. A wide-bore column combined with the MS vacuum reduces the pressure within the column, decreases carrier gas viscosity and increases optimum linear velocity. The whole idea is to increase your optimal linear velocity while minimizing the plate height to maximize efficiency. MS plays a key role in this process because the vacuum helps evacuate the column, meaning more shift in the optimal linear velocity. We were inspired to use the LPGC kit with our own triple quadrupole MS to see if we could push more pesticides through faster, but separate them in a similar fashion.
Why is the triple quad MS so pertinent to this type of research?
When analyzing pesticides in particular, but also for other compounds like PFAS, it’s inevitable that some ions are going to co-elute. With triple quad MS, or tandem MS, you take a single fragment ion and you break it apart further (with a collision gas for example) and measure that mass spectrum. This means that even if two pesticides are coming out at the same time, you should be able to measure one or two qualifying ions as well which enable you to distinguish between the different pesticides. The other reason a triple quad is so important to cannabis analysis is due to the complexity of the matrix. You get a lot of different substances that co-elute with the pesticides, meaning a lot of interference ions and a messy chromatogram. With a triple quad, the ion you fragment is very specific and you can see the individual peaks quite clearly – it doesn’t mean you won’t ever get an overlap, but you’re increasing your ability to be able to tell the two things apart.
Could you tell us about the short collision cell technology used?
The short collision cell incorporates two different patented technologies that allow it to perform differently compared with other systems on the market. The first technology accumulates all the ions in a small volume, and then produces a single ejection. So rather than the ions coming in a stream and being fragmented, it traps the ions for a short duration (<1 ms) while it’s fragmenting them, and then it pushes the whole packet out. The longer accumulation time increases sensitivity and decreases the number of interference ions – while the pesticide is trapped there, all the other ions are being sent off into the vacuum and lost. The second technology relates to highly specific timing. While the pesticide is trapped in the cell, nothing is hitting the detector in theory – so we simply shut it off! Because of this, we are able to significantly lower the noise and, in turn, increase sensitivity.
These two technologies working together allow us to do a couple of things. The most important for LPGC is that it enables us to switch between ions faster – the maximum switching speed is 1000 SRMs per second (the highest currently on the market). The JEOL JMS-TQ4000GC also has high pumping capacity and a good vacuum, meaning more penetration into the column, better linear velocity, and therefore higher efficiency.
What was the biggest challenge you faced?
The number one challenge was that all these different transitions have to be developed for every single pesticide. So anytime there’s a pesticide that isn’t in our library, we have to figure it out. We have tools built into our software to do this, but the real obstacle came when pesticides had very similar structures because they fragment the same and have similar ions. For these pesticides, instead of using the most intense transition, I often had to pick a more selective transition – something that was unique to one pesticide over the other. Sometimes these would be very low intensity but highly specific ions – and that’s when we must rely on the sensitivity of the triple quad to pick them up.
You used a standard mixture in this work; do you have any plans to test your approach with real-world samples?
Yes! The next thing I’d like to explore is how this works for real-world cannabis samples. Can I still measure all these pesticides with different matrices? Cannabis testing labs have different jurisdictional pesticides they are looking for – and they want to know if our approach can measure a specific list of pesticides in that particular matrix. They don’t need to know about all 244 pesticides.
Clearly, pesticides are not limited to cannabis, so I’d love to see how those doing routine testing of vegetables or other food products could benefit from our work. The USDA and EPA are concerned about many more pesticides than the cannabis industry – these are the scientists who will actually care about the 244 pesticides we’ve managed to measure in our work! And that brings me to another hope for the future – I’d love to expand the pesticide capability on this instrument. I am almost certain there is scope to get more than 244 pesticides in 11 minutes, and the LPGC is so quick it’s possible to develop an SRM profile in less than a day. I can’t wait to explore the potential!
- K Jensen, J Dane and R Cody, Rapid Commun Mass Spectrom, 36, e9258 (2022). DOI: 10.1002/rcm.9258