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 x 4.6 mm i.d., 1.8 mum) 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.

Keywords: liquid chromatography-tandem mass spectrometry; pesticides; phenoxyacid herbicides; post-column reagent addition; water analysis.

Figure 1.

Percentage response of degradation products relative to response when no post-column reagent system is used as a function of molarity of ammonia in methanol. Degradation Products: DBHBA measured at 295.0 > 250.3; DCP at 160.9 > 124.7; CMP at 140.9 > 105.2; and TCP at 194.6 > 158.9. Percentage response is response of post-column system/response no post-column system multiplied by 100. LC-ESI⁻-SRM response shown is average peak height of three replicates. Sample injection 5 μL degradation product at 750 μg L⁻¹ with post-column flow of 50 μL min⁻¹ and column flow of 150 μL min⁻¹.

Figure 2.

Selected Reaction Monitoring (SRM) chromatograms of phenoxyacid herbicides and degradation products. Shown are quantitative SRMs with SRM transition labelled for each chromatogram. Note SRM at 140.9 > 105.2 and 160.9 > 124.7 are also the qualitative SRM transition for some analytes. Peak numbering for phenoxyacid herbicides are as follows: 1. dicamba; 2. bromoxynil; 3. MCPA; 4. 2,4-D; 5. mecoprop; 6. dichlorprop; 7. 2,4-DB; 8. MCPB; 9. 2,4,5-TP; and degradation products: 10. DBHBA; 11. CMP; 12. DCP; 13. TCP. See Table 1 for retention times. Standard 75 ng L⁻¹.

Figure 3.

Selected Monitoring Chromatograms (SRMs) for Sample WP216 containing 2,4-D and its degradation product DCP, and mecoprop and its degradation product CMP. Shown are both the quantitative and qualitative SRM transitions (SRM1 and SRM2) for analytes with expanded x-scale scale for the degradation products DCP and CMP. Quantitative and Qualitative SRM Transitions: 2,4-D are 219.0 > 174.9 and 174.9 > 144.9; DCP are 160.9 > 124.7 and 163.0 > 124.7; mecoprop are 213.0 > 140.9 and 140.9 > 105.2; and CMP are 140.9 > 105.2 and 142.8 > 105.2 (see Table 1). Analytes identified: 4. 2,4-D; 5. mecoprop; 11. CMP; and 12. DCP. Sample WP216 diluted at 1:2 with addition of IS for 5 μL injection.