Response to Comment on 'MALDI-MS Imaging Analysis of Fungicide Residue Distributions on Wheat Leaf Surfaces'

摘要

We thank Dong et al. (2015) for their comments on our paper. Dong et al., requested clarity regarding the (1) effect of variations in the leaf surface on MALDI-MS, (2) measurement of limits of detection, (3) correlation between droplet size of the spray and spatial resolution of the MALDI image, and (4) MALDI image resolution and quantitative aspects of the measurement. MALDI imaging of leaf surfaces to determine spatial distribution of endogenous and exogenous compounds has been very well documented in the literature. As Dong et al. pointed out, uneven surfaces can affect the mass accuracy of the data obtained. In our study, we ensured that the sample had a uniform matrix coating using an optical microscope as described by Araujo et al. in addition the sample was flattened to the extent that any surface height variations had a minimal effect on the mass accuracy of the measurement. To demonstrate the quality of our measurements, we overlaid 256 individual mass spectra obtained from MALDI imaging of a leaf applied with a 0.5 μL droplet containing pyraclostrobin as described in our paper. The overlaid MS spectra in Figure 1 illustrates that the shift in the observed protonated pyraclostrobin peak (m/z 388.1) was within the mass window used to generate images in this study (±0.25%), and any observed mass shift could be attributed to the uneven surface of the leaf. The importance of choosing the appropriate mass window for generating the MALDI images was illustrated in Figure 2, wherein we show that using a smaller mass window (higher mass resolution) can skew the results due to variability associated with mass measurement under the experimental conditions which were used in these studies. The "banding effect" which was observed in some of the images may be an artifact of variation in matrix deposition caused by uneven surface of the leaf (before drying), or could be due to spreading of the fungicide formulation on the leaf surface after application. Since the authors used a finer nozzle for matrix application than for formulation application, there was less probability for this process to contribute to banding effect, increasing the likelihood that the "banding effect" was a result of formulation spreading.