For obvious reasons, pesticide levels in food need to be carefully controlled. Traditionally, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography (LC)-MS have been two of the most popular techniques for identifying and quantifying pesticides in food, but due to the often complex chemical nature of food products, low levels of pesticides can be challenging to analyze with these single quadrupole instruments. Interference from the sample matrix (which can distort chromatographic peak shapes or peak areas) often makes it difficult to perform reliable measurements of the chromatographic data of analytes. This matrix-interference problem has largely been solved by the introduction of MS/MS in GC-MS/MS and LC-MS/MS systems. Using two quadrupoles in tandem with a quadrupole collision cell in-between (hence the name “triple quadrupole”), adds a very high degree of specificity by eliminating the effects of matrices. Therefore, the benefit of triple quadrupole mass spectrometric detection is most pronounced for samples with very complex matrices. In this study, a QuEChERS method, followed by thermal desorption and GC-MS/MS with an Evolution-upgraded Agilent GC-MS, were used to analyze 89 types of pesticides in one GC run. The samples were red currant pulp and mushroom, both spiked with 5 µg/kg of various pesticides, including aldrin, dieldrin, propaconazol, chlorthal-dimethyl, pentachloraniline, pirimiphos-methyl, tebuconazol and others.

The objective of this study was to obtain reliable chromatographic data in a single run for 89 different pesticides in a complex food matrix at a trace concentration of 5 µg/kg using a QuEChERS sample preparation, followed by Thermal Desorption and GC-MS/MS separation and detection. Analyte peaks should have a well-defined, approximately Gaussian shape.

Materials and Methods
Sample Preparation
A QuEChERS method was used to prepare extracts for injection. The solvent used was acetonitrile.

Thermal Desorption: TDAS
Autosampler: CTC Combi Pal with Chromtech GmbH’s Thermal Desorption Autosampler (TDAS)option. Ten µL QuEChERS extract injected into unpacked glass tube at -20 °C. Desorption was for 4 min at 330 °C. Flow through the tube was 20 mL/min.

GC-MS/MS
Gas chromatograph: Agilent Technologies 6890N
Column: Agilent HP-5 ms, 30-m length, 250 µm i.d., 0.25 µm film thickness
Carrier gas: Helium, 0.7mL/min constant flow
GC Inlet: Split mode, 300° C, Split Ratio 30:1
Oven program: Initial 40 °C, ramp 35 °C to 100° C; ramp 8 °C/min to 320 °C. Hold 10 min. Total time 41.21 min.

Triple quadrupole mass spectrometer: CHROMTECH EVOLUTION Triple Quadrupole-upgraded Agilent 5975 mass spectrometric detector with diffusion pump. (Turbo pump more common and recommended).
Source: 230° C, EI mode, 70 eV
Quadrupole 1 temperature: 150 °C
Collision cell: 90-degree curved with argon collision gas at 14.5 psi.
EMVolts: 1,400 V over Autotune (2,900 V Abs)

Results
Below are a series of chromatographic data obtained from the red currant and mushroom samples spiked with 5 µg/kg (5 ppb) of 89 different pesticides. Note the absence, or at least the extremely low amounts, of matrix. For each pesticide, two chromatograms are shown—red currant (top) and mushroom (bottom) (Graphs).

*Note that the ion chromatograms shown use the product ions as labels for the respective transitions when using Agilent Chemstation or MassHunter Data Analysis for Evolution Triple Quad-generated data. When more than one transition has the same product ion, the labels used are simply xxx.1, xxx.2, etc. Therefore, there is no risk of interference.

Figure 1: Chlorthal-dimethyl (5 ppb) in Red Currant (top) and Mushroom (bottom)

Figure 2: Pentachloraniline (5 ppb) in Red Currant (top) and Mushroom (bottom)

Figure 3: Aldrin, Penconazole and Dieldrin (5 ppb) in Red Currant (top) and Mushroom (bottom)

Figure 4: Pirimiphos-methyl (5 ppb) in Red Currant (top) and Mushroom (bottom)

Figure 5: DDT and DDE (5 ppb) in Red Currant (top) and Mushroom (bottom)

Figure 6: o,p-DDD, p,p-DDT and o,p-DDT (5 ppb) in Red Currant (top) and Mushroom (bottom)

Figure 7: Tebuconazol (5 ppb) in Red Currant (top) and Mushroom (bottom)

Figure 8: Terbufenpyrad (5 ppb) in Red Currant (top) and Mushroom (bottom)

Conclusion
Complex food samples usually present significant challenges for accurately determining trace levels of pesticides using conventional single quadrupole MS analysis. MS/MS eliminates matrix interference almost completely, resulting in far superior signal:noise ratios and, therefore, in far lower detection limits. A further benefit is a higher degree of accuracy and confidence in the data obtained.

Extremely low concentrations (5 µg/kg) of a wide variety of pesticides are easily determined in complex matrices such as red currant and mushroom using an Evolution Triple Quadrupole GC-MS/MS instrument. ♦

For more information on CHROMSYS’ analytical solutions for the food and flavor industry, please visit www.chromsys.com, e-mail info@chromsys.com or call 1-877-865-7712.