Liquid Chromatography with Post-Column Reagent Addition of Ammonia in Methanol Coupled to Negative Ion Electrospray Ionization Tandem Mass Spectrometry for Determination of Phenoxyacid Herbicides and their Degradation Products in Surface Water

Abstract

A new liquid chromatography (LC)-negative ion electrospray ionization (ESI−)–tandem mass spectrometry (MS/MS) method with post-column addition of ammonia in methanol has been developed for the analysis of acid herbicides: 2,4-dichlorophenoxy acetic acid, 4-chloro-o-tolyloxyacetic acid, 2-(2-methyl-4-chlorophenoxy)butyric acid, mecoprop, dichlorprop, 4-(2,4-dichlorophenoxy) butyric acid, 2,4,5-trichlorophenoxy propionic acid, dicamba and bromoxynil, along with their degradation products: 4-chloro-2-methylphenol, 2,4-dichlorophenol, 2,4,5-trichlorophenol and 3,5-dibromo-4-hydroxybenzoic acid. The samples were extracted from the surface water matrix using solid-phase extraction (SPE) with a polymeric sorbent and analyzed with LC ESI− with selected reaction monitoring (SRM) using a three-point confirmation approach. Chromatography was performed on a Zorbax Eclipse XDB-C18 (50 × 4.6 mm i.d., 1.8 μm) with a gradient elution using water-methanol with 2 mM ammonium acetate mobile phase at a flow rate of 0.15 mL/min. Ammonia in methanol (0.8 M) was added post-column at a flow rate of 0.05 mL/min to enhance ionization of the degradation products in the MS source. One SRM transition was used for quantitative analysis while the second SRM along with the ratio of SRM1/SRM2 within the relative standard deviation determined by standards for each individual pesticide and retention time match were used for confirmation. The standard deviation of ratio of SRM1/SRM2 obtained from standards run on the day of analysis for different phenoxyacid herbicides ranged from 3.9 to 18.5%. Limits of detection (LOD) were between 1 and 15 ng L−1 and method detection limits (MDL) with strict criteria requiring <25% deviation of peak area from best-fit line for both SRM1 and SRM2 ranged from 5 to 10 ng L−1 for acid ingredients (except dicamba at 30 ng L−1) and from 2 to 30 ng L−1 for degradation products. The SPE-LC-ESI− MS/MS method permitted low nanogram-per-liter determination of pesticides and degradation products for surface water samples.

Chemical Analysis of Pesticides Using GC/MS, GC/MS/MS, and LC/MS/MS

“…With ESI-the phenoxy acid herbicides MCPA, mecoprop, MCPB have a common confirmation SRM transition of 140.9 > 105.2 which is also the quantitative SRM for degradation product, chloromethylphenol. In addition, 2,4-D, dichlorprop, and 2,4-DB also have the sample confirmation SRM of 160.9 > 124.7 which is also the quantitative SRM for dichlorophenol (Raina and Etter, 2010). Separated of these phenyoxyacid herbicides can be achieved using a short C18 column with methanol gradient and mobile phase containing 2 mM ammonium acetate (Raina and Etter, 2010).…”

“…As a general rule the choice of positive or negative mode depends upon polarity and acidity of analytes and sample matrix impacts. In general, ESI-is more sensitive for phenoxyacid herbicides and their transformation products (Raina and Etter, 2010;Koppen et al, 1998;Dijkman et al, 2001) and chloroacetanilide transformation products; ESI+ for sulfonylureas, phenylureas, N-methylcarbamates, organophosphorus pesticides (Cessna et al, 2006;Degenhardt et al, 2010;Hernandez et al, 2006;Steen et al, 1999;Raina and Sun, 2008); APCI+ for triazines (Dagnac et al, 2005;Jeannot et al, 2000); and APCI+ or ESI+ for chloroacetanilides (Dagnac et al, 2005;Ferrer et al, 2007;Banerjee et al, 2009 …”

“…On-column derivatization with trimethylphenylammonium hydroxide and trimethylsulfonium hydroxide has been used to give thermally stable products for a variety of carbamates including carbaryl, methiocarb, chlorpropham, propham, and promecarb that can be analyzed by GC-EI/MS (Zhang and Lee, 2006). In more recent years there has been a shift to LC/MS/MS methods (see section 3.2) for both phenoxyacid herbicides and carbamates as these methods do not require the derivatization step and can provide an ability to simultaneous analyze transformation products and often a wider range of pesticides within the same chemical class (Raina and Etter, 2010;Charlton et al, 2009;Chung and Chan, 2010). To achieve the necessary MDLs required for environmental or food analysis the majority of GC/MS pesticide analysis methods are in SIM mode with either single quadrupole or iontrap systems with ion-traps providing similar or slightly higher MDLs than the more popular quadrupole systems.…”

Phenoxyacid Herbicides in Stormwater Retention Ponds: Urban Inputs

“…A detailed description of the sample filtration, solid phase extraction (SPE), and liquid chromatography-tandem mass spectrometry (LC/MS/MS) analysis used for quantifying the acid herbicides and their transformation products can be found in Raina and Etter [28]. Filters used for water sample filtration included Whatman 934-AD, Whatman 41 ashless and Whatman 0.45 µm nylon membrane filters (Canadawide Scientific, Ottawa, ON, Canada).…”

“…The filtration and SPE procedures were completed within 24 hours of sample collection and dried SPE cartridges were stored at -4˚C until analysis. Herbicides were eluted with 8 mL of 60/40 v/v% methanol/ethylacetate and dried to ~0.95 mL followed by addition of a dilution standard 13 ) was used for the LC/MS/MS analysis [28]. LC/MS/MS analyses were performed at the Trace Analysis Facility at the University of Regina using a Waters LC system consisting of a 1525 µ binary pump, column heater, and a Quattro Premier tandem mass spectrometer (Waters-Micromass, Milford, MA, USA) with electrospray ionization operated in negative ion mode.…”