Determination of chlorothalonil in vegetables/fruits using liquid chromatography-tandem mass spectrometry with a discharge adapter interface

RATIONALE

Chlorothalonil (CHT), a broad-spectrum fungicide, has been employed widely to control foliar diseases, whereas with a major metabolite of polar 4-hydroxychlorothalonil (CHT-4-OH), only an acceptable nonpolar CHT residue is allowed by most countries. This study involves the method development for CHT residue in vegetables/fruits using liquid chromatography-tandem mass spectrometry (LC-MS/MS) with a novel modified discharge-adaptor (DA) interface.

METHODS

CHT residue was analyzed using LC-MS/MS with DA interface (LC-DA-MS/MS), developed in our previous works. A DA was placed on the electrospray tip to switch the ionization modes. A modified quick, easy, cheap, effective, rugged, and safe (QuEChERS) method was applied to extract CHT residue of vegetables/fruits efficiently with less sample preparation time and analysis cost.

RESULTS

CHT and CHT-4-OH spiked in four different vegetables/fruits were extracted using the modified QuEChERS method. After LC with isocratic elution, CHT and CHT-4-OH were separated within 3 min. Using LC-DA-MS/MS, the ion signals of CHT were improved two to three times, and the limit of quantification of 5 ng/g and linearity (r2 > 0.99) in the range of 5-200 ng/g were achieved using 10 g of vegetables/fruits. The precision and accuracy were within 15% each. The modified QuEChERS and LC-DA-MS/MS were applied to examine eight field-grown vegetables/fruits; 9.5 and 2588.9 ng/g of CHT were detected in two vegetables/fruits.

CONCLUSION

LC-DA-MS/MS combined with modified QuEChERS was successfully applied to determine CHT residue <10 ng/g in vegetables/fruits and with satisfied validation results. The developed method could reduce both analysis cost and time, attributing to simplifications in modified QuEChERS, isocratic elution, and DA interface in LC-DA-MS/MS.

Antibody-free colorimetric detection of chlorothalonil in cucumberviathe inhibition of an enzyme-triggered reaction

Currently known rapid determination of fungicides usually relies on antibody-based immunoassay. This paper reports a simple antibody-free colorimetric assay for chlorothalonil via the inhibition of an enzyme-triggered reaction. The enzymatic activity of glyceraldehyde-3-phosphate dehydrogenase was significantly inhibited by chlorothalonil, and the color change of NBT-PMS system induced from NADH formation was suppressed, which could be used indirectly to assay chlorothalonil. The limit of detection (LOD) was 0.05 μM with a linear range from 0.5 to 10 μM, and the detection of 1 μM chlorothalonil in solution was achieved with a naked-eye readout. In addition, the colorimetric measurement results of the cucumber samples showed a good recovery rate, although the sensitivity was less effective than the instrumental method. Nevertheless, the results demonstrates that the chlorometric method provides potential opportunities for reliable, cost-effective quantitative detection for chlorothalonil residues in vegetables.

Colorimetric detection of chlorothalonil in cucumber via the inhibition of an enzyme-triggered reaction.

Recent developments in atmospheric pressure photoionization-mass spectrometry

Recent developments in atmospheric pressure photoionization (APPI), which is one of the three most important ionization techniques in liquid chromatography-mass spectrometry, are reviewed. The emphasis is on the practical aspects of APPI analysis, its combination with different separation techniques, novel instrumental developments - especially in gas chromatography and ambient mass spectrometry - and the applications that have appeared in 2009-2014. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:423-449, 2017.

Development of an Immunosensor for Determination of the Fungicide Chlorothalonil in Vegetables, Using Surface Plasmon Resonance

An immunosensor based on surface plasmon resonance (SPR-sensor) was developed to analyze chlorothalonil residues and maximum residue limits (MRLs; 0.5-50 mg/kg) in vegetables in Japan. Conjugates of N-(pentachlorophenoxyacetyl)glycine and bovine serum albumin were covalently coated on the sensor chip. The SPR-sensor quantitatively determined chlorothalonil at concentrations ranging from 8.0 to 44 ng/mL, using TPN9A, a monoclonal antibody to chlorothalonil. The 50% inhibition concentration was 25 ng/mL. The reactivity was 10-fold lower than that of indirect competitive enzyme-linked immunosorbent assay (ic-ELISA). However, the SPR-sensor could determine chlorothalonil residues in vegetables at concentrations around the above MRLs. Chlorothalonil spiked in vegetables was recovered at 90-118% within 1 day and at 90-115% across 3 days, correlating with HPLC results. The sensor showed good performance for chlorothalonil residue analysis in vegetables with rapid determination, although the sensitivity and the cross-reactivity were less effective than with the ic-ELISA.

The use of isoprene as a novel dopant in negative ion atmospheric pressure photoionization mass spectrometry coupled to high-performance liquid chromatography

RATIONALE

As in the case with positive ion atmospheric pressure photoionization (PI-APPI), the addition of dopants significantly improves the sensitivity of negative ion APPI (NI-APPI). However, the research on dopant-assisted-NI-APPI has been quite limited compared to the studies on dopant-assisted PI-APPI. This work presents the potential of isoprene as a novel dopant for NI-APPI.

METHODS

Thirteen compounds, possessing suitable gas-phase ion energetic properties in order to make stable negative ions, were selected. Dopants were continuously introduced into a tee junction prior to the ion source through a fused-silica capillary, while analytes were directly injected into the same tee. Then both were mixed with the continuous solvent from high-performance liquid chromatography (HPLC), nebulized, and entered the source. The nebulized stream was analyzed by APPI tandem quadrupole mass spectrometry in the negative ion mode.

RESULTS

The results obtained using isoprene were compared with those obtained by using toluene as a dopant and dopant-free NI-APPI. Isoprene enhanced the ionization intensities of the studied compounds, which were found to be comparable and, in some cases, more effective than toluene. The mechanisms leading to the observed set of negative analyte ions were also discussed. Because in NI-APPI, thermal electrons, which are produced during the photoionization of a dopant, are considered the main reagent ions, both isoprene and toluene promoted the ionization of analytes through the same mechanisms, as expected.

CONCLUSIONS

Isoprene was shown to perform well as a novel dopant for NI-APPI. Isoprene has a high photoabsorption cross section in the VUV region; therefore, its photoionization leads to a highly effective production of thermal electrons, which further promotes the ionization of analytes. In addition, isoprene is environmentally benign and less toxic compared to currently used dopants.

Photoreduction of chlorothalonil fungicide on plant leaf models

Photodegradation is seldom considered at the surface of vegetation after crop spraying. Chlorothalonil, a broad-spectrum foliar fungicide with a very widespread use worldwide, was considered. To represent the waxy upper layer of leaves, tests were performed within thin paraffin wax films or in n-heptane. Laser flash photolysis together with steady-state irradiation in n-heptane allowed the determination of the photodegradation mechanisms Chlorothalonil ability to produce singlet oxygen was measured; noteworthy its efficiency is close to 100%. Additionally, chlorothalonil photodegradation mainly proceeds through reductive dechlorination. In these hydrophobic media, a radical mechanism was evidenced. Photochemical tests on wax films under simulated solar light show that formulated chlorothalonil is more reactive than pure chlorothalonil. The field-extrapolated half-life of photolysis on vegetation was estimated to 5.3 days. This value was compared to the half-lives of penetration and volatilization available in the literature. It appears that chlorothalonil dissipation from crops is ruled by both photodegradation and penetration. The relative importance of the two paths probably depends on meteorological factors and on physicochemical characteristics of the crop leaf cuticle.