I recently read a publication that appeared in the Journal of Agricultural and Food Chemistry (Yang et al, published ahead of print on January 25, 2015), titled, Effect of Sample Dilution on Matrix Effects in Pesticide Analysis of Several Matrices by Liquid-Chromatography−High-Resolution Mass Spectrometry, (link to article abstract). This in-depth evaluation of the effect of sample dilution on matrix effects in pesticides residue analysis is an excellent read and was coauthored by Paul Yang from Ontario Ministry of the Environment and Climate Change, Jon W. Wong, Kai Zhang, and Alexander J. Krynitsky at the U.S. Food and Drug Administration, Jian Wang at the Canadian Food Inspection Agency, and our very own James S. Chang and Maciej Bromirski.
Matrix effects, most often seen as suppression or enhancement of the analyte signal in the electrospray ionization source, are commonly observed in liquid-chromatography electrospray ionization mass spectrometry (LC-ESI-MS). Approaches to reduce matrix effects in food samples include sample clean-up techniques such as QuEChERS and solid-phase extraction (SPE), sample dilution, chromatographic separation of analytes from interfering matrix components, use of matrix-matched standards, and other approaches. Each of these solutions has its disadvantages. Sample dilution, also referred to as dilute-and-shoot, is one of the most attractive solutions because the technique is simple and rapid, doesn’t require its own method development, and introduces less chance of error and variation compared to some of the other techniques.
The growing availability of High-Resolution Accurate Mass (HRAM) mass spectrometry has allowed for the closer investigation of the contribution of matrix components to pesticide residue analysis assays. The teams at the Canadian and U.S. labs used one of our benchtop LC-MS systems (Thermo Scientific Exactive Plus Orbitrap HRAM mass spectrometer) coupled to one of our HPLC systems (Thermo Scientific UltiMate 3000 HPLC system to evaluate 381 pesticide standards at 3 dilutions in 5 matrices: avocado, spinach, hazelnut, orange, and honey. Dilutions of 1X, 1/10X, and 1/100X were interrogated and matrix effects were measured via principal component analysis and through slope ratios of the calibration curves.
For all matrices except honey, suppression decreased with increasing sample dilution. In avocado, the matrix in which suppression was most prevalent, over 70% of pesticides were affected by suppression at 1X dilution whereas less than 15% were suppressed at 1/100X dilution. Effects did vary somewhat depending on the LC column used. Interestingly, suppression increased with increasing dilutions of the honey matrix.
Ion enhancement increased with increasing dilution, but as expected the overall number of pesticides affected by enhancement was low in all matrices compared to the number affected by suppression. In avocado, for example, at most 44 of the pesticides were affected by enhancement (1/100X dilution). Ion enhancement affected early-eluting, hydrophilic pesticides the most. As per the results, the authors concluded that for all but the honey matrix, dilution was an effective approach to minimize matrix effects.
Like what you are learning?
By the way, Paul Yang and Jon Wong have collaborated with us on several other projects. Take a look at the following:
- Application Note by Paul Yang: Routine, Targeted and Non-Targeted Analysis of Environmental Contaminants of Emerging Concern – Development and Validation of a UHPLC Orbitrap MS Method (downloadable pdf)
- On-demand webinar by Jon Wong: Development and Applications of Liquid Chromatography-High Resolution Mass Spectrometry for the Analysis of Chemical Contaminants in Foods
- Poster presentation by Paul Yang and Jon Wong: A Turn-key System for Automated Detection of Organic Contaminants in Food Matrices and Economic Adulteration (downloadable pdf)
What is the biggest challenge in your pesticides analysis? Is it matrix effects or something else? Give us your feedback in the comments box below.