Correspondence/Rebuttal pubs.acs.org/est
Comment on “MALDI-MS Imaging Analysis of Fungicide Residue Distributions on Wheat Leaf Surfaces”
T
sizes of the five spots were quite different; therefore, the fungicide residues on the leaf surface will vary after drying. Overall, the discussions of the LOD in this article is inaccurate. (3) The major perspective of this study is demonstrate a method for detection and mapping the distributions of fungicide residues after spray application. So, the diameter of the droplets should be considered when discussing the spatial resolution.5 The authors used spatial resolution of 300 μm × 300 μm without any explanations. In real spray applications, the diameter of droplets is determined by both the spray method and the size of the nozzle. For example, the electrostatic spraying can generate droplets in 20−50 μm scale.8 Therefore, the spray method and the nozzle type should be specified, and the diameter of the generated droplets should be evaluated. The spatial resolution and the laser scanning step size should be determined based on the above parameters. In addition, the spot-like imaging results shown in Figure 3 and Figure 4 should be discussed according to the spatial resolution, regarding whether they were caused by the deposition of the droplets or by measurement errors. (4) Figure 2, 3, 4, and 5 show brightness images based on the relative abundances of selected ions, but quantitative measurements are lacking. The credibility of the above results can be proved by a clearly discussion of quantitative calculation and repetition.7 Similar with the LOD discussion, a suitable specific mass spectra signal should be chosen, and a calibration curve should be built. Repeated mass spectra signals should also be acquired to show the credibility and repeatability of the imaging results.
he recent article by Annangudi et al. reports the use of MALDI-TOF-MS(matrix assisted laser desorption/ionization- time-of-flight- mass spectroscopy) for measuring the distributions of fungicide residues on leaf surfaces.1 Compared with previous works, the method in this study is more suitable for a field application. In the experiment part, three kinds of fungicide (epoxiconazole, azoxystrobin, and pyraclostrobin) were sprayed and their distributions on leaf surfaces were mapped. The results demonstrated that MALDI-MS can be used to monitor and evaluate the residue conditions after pesticide spay applications and thus has the potential to guide fungicide applications and may aid in environmental protection strategies. However, the article has various limitations, and the following issues need to be considered. (1) The manuscript does not discuss the influence of leaf surface conditions on the measurement results. MALDIMS is a soft ionization method, and the roughness of the surface conditions will therefore result in measurement errors.2 The roughness of the sample will lead to both inhomogeneous ionization states and a nonuniform matrix spray, thus inducing serious uncertainties into the measurements. In similar studies, biological samples, such as plants, are frozen and then cut to produce a smooth surface for measurements.2−4 The thickness of animal samples sections after cut were usually between 10 and 100 μm.5 Some studies even used a microscope to observe the sample surface to ensure a homogeneous spray.6 Therefore, it is necessary to discuss the influence of the flatness of the leaf surface. Some of the imaging results of the fungicide presented in the study (e.g., Figure 2) reveal obvious banding distributions that coincide with veins. It is worth discussing whether the banding shape is caused by the accumulation of fungicide on the leaf vein or is due to measuring errors that arise from the roughness of the leaf surface. (2) It is arbitrary to conclude that the LOD (limit of detection) is 0.06 μg/μL based on only five samples that contain 0.03, 0.06, 0.125, 0.25, and 0.5 μg pyraclostrobin spotted on the leaf surface and determined only via the imaging results. First, a standard LOD determination procedure should be applied. The authors could either determine the LOD using the specific mass spectra signals of the samples at different concentrations and constructing a calibration curve or by directly analyzing the LOD based on the signal-noise-ratio, considering the mass spectra of the samples at the lowest concentration.7 Second, the fungicide used in this study were pure compounds dissolved in emulsifiable concentrate (EC). Then, the EC solution with fungicide was sprayed onto the leaf surface and dried at room temperature for 1 h. The fungicide will gradually precipitate during the drying processes. Therefore, fungicide residues on the leaf surface are closely related to the spot size of the solutions. However, it is obvious from Figure 2 that the © 2015 American Chemical Society
D. Dong* W. Zheng C. Zhao
■
National Engineering Research Center for Information Technology in Agriculture, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. Published: August 12, 2015 10745
DOI: 10.1021/acs.est.5b02513 Environ. Sci. Technol. 2015, 49, 10745−10746
Environmental Science & Technology
■ ■
Correspondence/Rebuttal
ACKNOWLEDGMENTS The authors have been supported by National Natural Science Foundation of China (No. 31271614). REFERENCES
(1) Annangudi, S. P.; Myung, K.; Avila Adame, C.; Gilbert, J. R. MALDI-MS Imaging Analysis of Fungicide Residue Distributions on Wheat Leaf Surfaces. Environ. Sci. Technol. 2015, 49 (9), 5579−83. (2) Araujo, P.; Ferreira, M. S.; de Oliveira, D. N.; Pereira, L.; Sawaya, A. C.; Catharino, R. R.; Mazzafera, P. Mass spectrometry imaging: an expeditious and powerful technique for fast in situ lignin assessment in Eucalyptus. Anal. Chem. 2014, 86 (7), 3415−9. (3) Anderson, D. M.; Carolan, V. A.; Crosland, S.; Sharples, K. R.; Clench, M. R. Examination of the translocation of sulfonylurea herbicides in sunflower plants by matrix-assisted laser desorption/ ionisation mass spectrometry imaging. Rapid Commun. Mass Spectrom. 2010, 24 (22), 3309−19. (4) Anderson, D. M.; Carolan, V. A.; Crosland, S.; Sharples, K. R.; Clench, M. R. Examination of the distribution of nicosulfuron in sunflower plants by matrix-assisted laser desorption/ionisation mass spectrometry imaging. Rapid Commun. Mass Spectrom. 2009, 23 (9), 1321−7. (5) Vrkoslav, V.; Muck, A.; Cvacka, J.; Svatos, A. MALDI imaging of neutral cuticular lipids in insects and plants. J. Am. Soc. Mass Spectrom. 2010, 21 (2), 220−31. (6) Lunsford, K. A.; Peter, G. F.; Yost, R. A. Direct matrix-assisted laser desorption/ionization mass spectrometric imaging of cellulose and hemicellulose in Populus tissue. Anal. Chem. 2011, 83 (17), 6722−30. (7) Rankin, K.; Mabury, S. A. Matrix Normalized MALDI-TOF Quantification of a Fluorotelomer-Based Acrylate Polymer. Environ. Sci. Technol. 2015, 49, 6093. (8) Ganan-Calvo, A.M.; J, D.; A, B. Current and droplet size in the electrospraying of liquids. Scaling laws. J. Aerosol Sci. 1997, 28 (2), 249−275.
10746
DOI: 10.1021/acs.est.5b02513 Environ. Sci. Technol. 2015, 49, 10745−10746
