A New Approach for the Detection of Acidic Pesticides in Water by MS

By: Jens Boertz, Rudolf Köhling, Chengdong Xu, Zachary S. Breitbach, Daniel W. Armstrong, Analytix 2014, Issue 4

Increasing the Sensitivity in Pesticide Analysis by Paired-Ion ESI Detection

In previous publications, di- and tri-cationic ion-pair reagents were used to detect inorganic compounds, such as chlorides and sulfates [1-2]. A recent publication by C. Xu et al [3] demonstrates the versatility of such ion-pair reagents, which are added post-column to the mobile phase and increase the ESI-MS sensitivity extraordinarily by forming positively charged adducts of reagent and anionic analytes (Figure 1).


Figure 1 The Instrumental Setup of HPLC-PIESI-MS.

Since the aim of water analysis is the detection of the smallest amounts of pesticides in water or other biological matrices to understand the effect on health and the environment, this new method supports analytical laboratories in this challenge.

LC-MS Method

The different pesticides are separated on a Supelco Ascentis Express C18 (15 cm x 2.1 mm i.d., 2.7 μm particle size) column using a gradient from 95% 5 mmol/L formic acid/water and 5% 5 mmol/L formic acid/methanol to 5% aqueous buffer within 20 min at 0.4 mL/min. The ion-pair reagent is mixed to the LC flow post-column via a mixing tee with a flow rate of 0.1 mL/min. Injection volume of water samples is 25 μL (fixed loop). This method demonstrates a very good sensitivity with a standard LC-MS system. Table 1 shows the list of analytes analyzed with this technique.

In Table 1 also the results for 19 acidic pesticides using three different ion-pairing reagents are presented. The limits
of detection (LOD) of 19 pesticides obtained were in the range from 0.6 to 19 pg. The fourth ion-pairing reagent
applied (1,9-Nonanediyl-bis(3-methylimidazolium) difluoride) did not show any improvement in terms of LOD. In total, this approach shows an increased sensitivity of one to three orders of magnitude for the pesticides analyzed.

A novel method based on paired-ion electrospray ionization (PIESI) mass spectrometry has been developed for
determination of acidic pesticides at ultratrace levels in surface and ground waters. The proposed approach provides
greatly enhanced sensitivity for acidic pesticides and overcomes the drawbacks of the less sensitive negative ion
mode ESI-MS. The limits of detection (LODs) of 19 acidic pesticides were evaluated with four types of dicationic ionpairing reagents (IPR) in both single-ion monitoring (SIM) and selected-reaction monitoring (SRM) mode. The LOD of 19 pesticides obtained with the use of the optimal dicationic ion-pairing reagent ranged from 0.6 pg to 19 pg,
indicating the superior sensitivity provided by this method. The transition pathways for different pesticide-IPR complexes during the collision-induced dissociation (CID) were identified. To evaluate and eliminate any matrix effects and further decrease the detection limits, off-line solid-phase extraction (SPE) was performed for DI water and a river water matrix spiked with 2000 ng/L and 20 ng/L pesticide standards respectively, which showed an average percent recovery of 93%. The chromatographic separation of the acidic pesticides was conducted by high-performance liquid chromatography (HPLC) using a C18 column (15 cm x 2.1 mm i.d., 2.7 μm particle size) in the reversedphase mode using linear gradient elution. The optimized HPLC–PIESI-MS/MS method was utilized for determination of acidic pesticides at ng/L level in stream/pond water samples. This experimental approach is one to three orders of magnitude more sensitive for these analytes than other reported methods performed in the negative-ion mode.

Chemicals Used to Carry Out HPLC-PIESI-MS Experiments
Cat. No. Name (Abbreviation) Exact Mass Package Size
31518 2,4-dichlorophenoxyacetic acid (2,4-D) 219.97 250 mg
45555 4-chloro-o-tolyloxyacetic acid (MCPA) 200.02 250 mg
45667 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) 253.93 250 mg
45420 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) 248.00 250 mg
36145 4-(4-chloro-o-tolyloxy)butyric acid (MCPB) 228.06 100 mg
S412325 4-(2,4,5-trichlorophenoxy)butyric acid (2,4,5-TB) 281.96 25 mg
233013 2-(3-chlorophenoxy)propionic acid (cloprop) 200.02 50 g
45436 2-(2,4-dichlorophenoxy)propionic acid (dichlorprop) 233.99 250 mg
45691 2-(2,4,5-trichlorophenoxy)propionic acid (fenoprop) 267.95 250 mg
36147 2-(4-chloro-o-tolyloxy)propionic acid (mecoprop) 214.04 100 mg
45430 3,6-dichloro-2-methoxy-benzoic acid (dicamba) 219.97 250 mg
R169676 2,3,6-trichlorobenzoic acid (2,3,6-TBA) 223.92 25 mg
36758 3,6-dichloro-2-pyridine-carboxylic acid (Clopyralid) 190.95 250 mg
36521 3,7-dichloro-8-quinoline-carboxylic acid (Quinclorac) 240.97 250 mg
36522 7-chloro-3-methyl-8-quinoline-carboxylic acid (Quinmerac) 221.02 250 mg
402923 2-chloroacetic acid (MCA) 93.98 100 g
T6399 2,2,2-trichloroacetic acid (TCA) 161.90 5 g
35562 2,2-Dichloropropionic acid (dalapon) 141.96 250 mg
56618 1,5-Pentanediyl-bis(1-butylpyrrolidinium) difluoride solution   100 mL
75128 1,9-Nonanediyl-bis(3-methylimidazolium) difluoride solution   100 mL
42341 1,3-Propanediyl-bis(tripropylphosphonium) difluoride solution   100 mL
76507 1,5-Pentanediyl-bis(3-benzylimidazolium) difluoride solution   100 mL
53802-U Supelco Ascentis Express™ C18 (15 cm x 2.1 mm i.d., 2.7 μm particle size) 1 EA
94318 Formic acid   50 mL, 250 mL
34485 Methanol   2.5 L, 4x2.5 L


[1] Köhling, R.; Reichlin, N.; Wille, G. Analytix No. 9.
[2] Köhling, R.; Reichlin, N. Analytix No. 3.
[3] Chengdong X.; Armstrong, D. W. Analytica Chimica Acta, 792. 2013 1–9.


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