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The LC-MS: How We Test for Pesticides

One of the many tests we perform at APRC is on the LC-MS (Liquid Chromatography - Mass Spectrometry) for pesticides. Our board member, Dr. Dosoky, has developed a method to detect pesticides in essential oils.

When testing for pesticides, the instrument first pumps two liquids through the tubing of the LC. The computer is then used to initiate the injection of the sample.

The auto-sampler in Tower 2 pulls 1µl (microliter) from the sample vial and mixes it with the mixed solution of A and B. The solution pulls the sample through the tubing to Tower 1, where the oven with the columns are. There are 6 columns in the oven, but only one is used for testing pesticides. Each column has a different matrix, connectivity, length, and width to allow for different kinds of testing. For pesticides, we use the biphenyl column for testing pesticides. This column effectively separates the pesticides for detection.

After the column, the sample is moved to the “source” on the Mass Spectra (MS). The source moves the sample through the capillary, a needle with a hair’s thickness opening. The sample moves down and is sprayed out. Due to hot temperatures, the sample is changed into a gas and hits the interface, a metal block that is 350 degrees Celsius (662 Fahrenheit), which is perpendicular to the capillary. The sample then moves through different chambers in the MS.

Q1 (Quad 1) separates a specific, predetermined molecule from the rest of the sample. In this case, the molecule will be the pesticide, if it is present in the sample. In Q2 (the collision cell), the isolated molecules are hit with magnetic and electric energy to fragment the parent molecule into daughter molecules. The daughter molecules are detected in Q3. The mass of the parent molecules is measured and compared to the daughter molecules.

The mass of a specific parent molecule and the mass of the daughter molecule are specific, predetermined numbers we input into the computer. For example, if a pesticide parent molecule has a mass of 142, the daughter molecule will always have a mass of 93. If the daughter molecule is recorded with that mass, that means the pesticide is present in the oil sample. If the sample is run and the daughter molecule mass is not the predetermined number, it will not be recorded on the computer. This means the parent molecule is not the pesticide, so that pesticide is not present in the sample.

You can watch our video on the LC-MS on our YouTube channel here.



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