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The Most Useful Pesticide Standard Ever…monitoring GC inlet “dirtiness”

15 Jun 2015

I have spent most of my time over the past several years testing pesticides and there is one standard that I simply can’t live without…QuEChERS Performance Standards Kit.

It is based on a really interesting article* on analyte protectants with a few compounds added.

The standard has 40 compounds housed in three vials for long term stability. The smart folks in our Reference Standards department made each vial 300 ppm in acetonitrile/ acetic acid (99.9:0.1). This makes it simple to blend equal volumes of the three vials to get a mix at 100 ppm.

  • It has compounds from different pesticide classes
  • Covers a range of polarities and volatilities (so good for GC and LC)
  • Has “good” or easy pesticides and problem pesticides

The mix is a great method development tool and we use it to evaluate both sample preparation and instrument performance.  Certain compounds make excellent probes for certain aspects of instrumental performance and I will show you one here. I monitor deltamethrin to determine the "dirtiness" of my inlet. From our experience, we know that deltamethrin forms an isomer as nonvolatile material builds up in the GC inlet especially in the liner. It can also happen with high inlet temperatures too.

I am always monitoring the formation of this deltamethrin isomer to keep an eye on my liner performance and can quickly replace my liner when the breakdown is bad enough. In order to monitor the breakdown isomer, I have to make sure to add it to my mass spec method. I have been using GC-MS/MS lately and I simply use the same MS/MS transitions that I use for deltamethrin. The breakdown isomer will elute just before deltamethrin on “5” type columns.

In this example, I am running on an Rxi-5ms column using efficiency optimized flow and optimal heating rate.

You can see that tracking the deltamethrin isomer formation and the decrease in deltamethrin signal is a good indication of inlet performance. As more orange samples are injected, more nonvolatile material deposits on the liner causing a decrease in deltamethrin signal. After some predetermined point...maybe loss of 20% of the signal...I replace the inlet liner.

Stay tuned for more ways I use The Most Useful Pesticide Standard Ever...



 *Combination of Analyte Protectants To Overcome Matrix Effects in Routine GC Analysis of Pesticides in Food Matrixes,  Katerina Mastovska, Steven J. Lehotay, and Michelangelo Anastassiades, Anal. Chem. 2005, 77, 8129-8137.