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Macroscopic and microscopic spatially-resolved analysis of food contaminants and constituents using laser-ablation electrospray ionization mass spectrometry imaging.

Nielen MW, van Beek TA - Anal Bioanal Chem (2014)

Bottom Line: In this work, LAESI time-of-flight MSI was investigated for macroscopic and microscopic imaging of pesticides, mycotoxins, and plant metabolites on rose leaves, orange and lemon fruit, ergot bodies, cherry tomatoes, and maize kernels.Accurate mass ion-map data were acquired at sampling locations with an x-y center-to-center distance of 0.2-1.0 mm and were superimposed onto co-registered optical images.However, according to three-dimensional LAESI-MSI the penetration depth of imazalil into the peel has significant local variation.

View Article: PubMed Central - PubMed

Affiliation: RIKILT Wageningen UR, P.O. Box 230, 6700 AE, Wageningen, The Netherlands, michel.nielen@wur.nl.

ABSTRACT
Laser-ablation electrospray ionization (LAESI) mass spectrometry imaging (MSI) does not require very flat surfaces, high-precision sample preparation, or the addition of matrix. Because of these features, LAESI-MSI may be the method of choice for spatially-resolved food analysis. In this work, LAESI time-of-flight MSI was investigated for macroscopic and microscopic imaging of pesticides, mycotoxins, and plant metabolites on rose leaves, orange and lemon fruit, ergot bodies, cherry tomatoes, and maize kernels. Accurate mass ion-map data were acquired at sampling locations with an x-y center-to-center distance of 0.2-1.0 mm and were superimposed onto co-registered optical images. The spatially-resolved ion maps of pesticides on rose leaves suggest co-application of registered and banned pesticides. Ion maps of the fungicide imazalil reveal that this compound is only localized on the peel of citrus fruit. However, according to three-dimensional LAESI-MSI the penetration depth of imazalil into the peel has significant local variation. Ion maps of different plant alkaloids on ergot bodies from rye reveal co-localization in accordance with expectations. The feasibility of using untargeted MSI for food analysis was revealed by ion maps of plant metabolites in cherry tomatoes and maize-kernel slices. For tomatoes, traveling-wave ion mobility (TWIM) was used to discriminate between different lycoperoside glycoalkaloid isomers; for maize quadrupole time-of-flight tandem mass spectrometry (MS-MS) was successfully used to elucidate the structure of a localized unknown. It is envisaged that LAESI-MSI will contribute to future research in food science, agriforensics, and plant metabolomics.

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Positive-ion LAESI-TOF-MSI accurate ion maps of (a1, b1) the 35Cl35Cl isotope [M+H]+ ion at m/z 297.056 (±5 mDa), and (a2, b2) the 35Cl37Cl isotope [M+H]+ ion at m/z 299.056 (±5 mDa), on (a) orange slice and (b) lemon slice, showing the spatial distributions of the post-harvest fungicide imazalil. (c), 3D profiling of imazalil in a lemon-peel slice, represented by a stack of 2D ion maps of m/z 297.055; for details see text. The x–y center-to-center distance was 1 mm. Other conditions, see Fig. 1
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Fig2: Positive-ion LAESI-TOF-MSI accurate ion maps of (a1, b1) the 35Cl35Cl isotope [M+H]+ ion at m/z 297.056 (±5 mDa), and (a2, b2) the 35Cl37Cl isotope [M+H]+ ion at m/z 299.056 (±5 mDa), on (a) orange slice and (b) lemon slice, showing the spatial distributions of the post-harvest fungicide imazalil. (c), 3D profiling of imazalil in a lemon-peel slice, represented by a stack of 2D ion maps of m/z 297.055; for details see text. The x–y center-to-center distance was 1 mm. Other conditions, see Fig. 1

Mentions: Next, we analyzed slices of citrus fruit in 2D and 3D by LAESI-TOF-MSI for the presence and the penetration of the post-harvest fungicides imazalil and thiabendazole in the peel. Thiabendazole was absent but, in contrast, imazalil was found on all regular citrus fruit samples analyzed. In Fig. 2 accurate ion maps of m/z 297.056 (±5 mDa) and m/z 299.056 (±5 mDa) are shown for orange (Fig. 2a) and lemon slices (Fig. 2b) revealing the [M+H]+ ions of the 35Cl35Cl and 35Cl37Cl isotopes of imazalil. C14H15N2OCl2 is the first theoretical elemental-composition option corresponding with the accurate mass measurement and the Cl2 isotope pattern observed. The ion maps of both isotopes are very similar and provide an additional confirmation of identity. As can be seen from the superimposed figures, the fungicide is located on the peel only (except for the cutting artefact in Fig. 2a). Next we performed a 3D LAESI experiment by firing 10 individual laser shots on each x–y location at 1 Hz and correlating the MS data thus obtained not only by location but also by the shot number. As a result a 3D profile can be obtained represented by a stack of individual 2D accurate-mass ion images superimposed on the camera image of the slice (Fig. 2c). The 3D profile confirms that imazalil is mainly present in the first layers and a gradual decrease is observed from the top (sample surface) to deeper layers. According to literature [7, 11], the penetration depth per laser shot is in the order of 40 μm, but highly dependent on the tensile strength of the sample surface. Provided the literature estimate applies here, imazalil residues are not yet absent at 400 μm below the peel surface. Apple slices were also analyzed by LAESI-TOF-MSI to investigate the presence of diphenylamine residues. Diphenylamine is used as a pre or postharvest scald inhibitor for apples but this pesticide, although occasionally reported on apples from non-EU countries, was absent on the apples investigated here, and only ion maps of natural sugars were obtained. Note that spiked apple slices did reveal the accurate mass of the [M+H]+ ion of diphenylamine, within 5 mDa of its theoretical exact mass (Electronic Supplementary Material Fig. S2).Fig. 2


Macroscopic and microscopic spatially-resolved analysis of food contaminants and constituents using laser-ablation electrospray ionization mass spectrometry imaging.

Nielen MW, van Beek TA - Anal Bioanal Chem (2014)

Positive-ion LAESI-TOF-MSI accurate ion maps of (a1, b1) the 35Cl35Cl isotope [M+H]+ ion at m/z 297.056 (±5 mDa), and (a2, b2) the 35Cl37Cl isotope [M+H]+ ion at m/z 299.056 (±5 mDa), on (a) orange slice and (b) lemon slice, showing the spatial distributions of the post-harvest fungicide imazalil. (c), 3D profiling of imazalil in a lemon-peel slice, represented by a stack of 2D ion maps of m/z 297.055; for details see text. The x–y center-to-center distance was 1 mm. Other conditions, see Fig. 1
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Related In: Results  -  Collection

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Fig2: Positive-ion LAESI-TOF-MSI accurate ion maps of (a1, b1) the 35Cl35Cl isotope [M+H]+ ion at m/z 297.056 (±5 mDa), and (a2, b2) the 35Cl37Cl isotope [M+H]+ ion at m/z 299.056 (±5 mDa), on (a) orange slice and (b) lemon slice, showing the spatial distributions of the post-harvest fungicide imazalil. (c), 3D profiling of imazalil in a lemon-peel slice, represented by a stack of 2D ion maps of m/z 297.055; for details see text. The x–y center-to-center distance was 1 mm. Other conditions, see Fig. 1
Mentions: Next, we analyzed slices of citrus fruit in 2D and 3D by LAESI-TOF-MSI for the presence and the penetration of the post-harvest fungicides imazalil and thiabendazole in the peel. Thiabendazole was absent but, in contrast, imazalil was found on all regular citrus fruit samples analyzed. In Fig. 2 accurate ion maps of m/z 297.056 (±5 mDa) and m/z 299.056 (±5 mDa) are shown for orange (Fig. 2a) and lemon slices (Fig. 2b) revealing the [M+H]+ ions of the 35Cl35Cl and 35Cl37Cl isotopes of imazalil. C14H15N2OCl2 is the first theoretical elemental-composition option corresponding with the accurate mass measurement and the Cl2 isotope pattern observed. The ion maps of both isotopes are very similar and provide an additional confirmation of identity. As can be seen from the superimposed figures, the fungicide is located on the peel only (except for the cutting artefact in Fig. 2a). Next we performed a 3D LAESI experiment by firing 10 individual laser shots on each x–y location at 1 Hz and correlating the MS data thus obtained not only by location but also by the shot number. As a result a 3D profile can be obtained represented by a stack of individual 2D accurate-mass ion images superimposed on the camera image of the slice (Fig. 2c). The 3D profile confirms that imazalil is mainly present in the first layers and a gradual decrease is observed from the top (sample surface) to deeper layers. According to literature [7, 11], the penetration depth per laser shot is in the order of 40 μm, but highly dependent on the tensile strength of the sample surface. Provided the literature estimate applies here, imazalil residues are not yet absent at 400 μm below the peel surface. Apple slices were also analyzed by LAESI-TOF-MSI to investigate the presence of diphenylamine residues. Diphenylamine is used as a pre or postharvest scald inhibitor for apples but this pesticide, although occasionally reported on apples from non-EU countries, was absent on the apples investigated here, and only ion maps of natural sugars were obtained. Note that spiked apple slices did reveal the accurate mass of the [M+H]+ ion of diphenylamine, within 5 mDa of its theoretical exact mass (Electronic Supplementary Material Fig. S2).Fig. 2

Bottom Line: In this work, LAESI time-of-flight MSI was investigated for macroscopic and microscopic imaging of pesticides, mycotoxins, and plant metabolites on rose leaves, orange and lemon fruit, ergot bodies, cherry tomatoes, and maize kernels.Accurate mass ion-map data were acquired at sampling locations with an x-y center-to-center distance of 0.2-1.0 mm and were superimposed onto co-registered optical images.However, according to three-dimensional LAESI-MSI the penetration depth of imazalil into the peel has significant local variation.

View Article: PubMed Central - PubMed

Affiliation: RIKILT Wageningen UR, P.O. Box 230, 6700 AE, Wageningen, The Netherlands, michel.nielen@wur.nl.

ABSTRACT
Laser-ablation electrospray ionization (LAESI) mass spectrometry imaging (MSI) does not require very flat surfaces, high-precision sample preparation, or the addition of matrix. Because of these features, LAESI-MSI may be the method of choice for spatially-resolved food analysis. In this work, LAESI time-of-flight MSI was investigated for macroscopic and microscopic imaging of pesticides, mycotoxins, and plant metabolites on rose leaves, orange and lemon fruit, ergot bodies, cherry tomatoes, and maize kernels. Accurate mass ion-map data were acquired at sampling locations with an x-y center-to-center distance of 0.2-1.0 mm and were superimposed onto co-registered optical images. The spatially-resolved ion maps of pesticides on rose leaves suggest co-application of registered and banned pesticides. Ion maps of the fungicide imazalil reveal that this compound is only localized on the peel of citrus fruit. However, according to three-dimensional LAESI-MSI the penetration depth of imazalil into the peel has significant local variation. Ion maps of different plant alkaloids on ergot bodies from rye reveal co-localization in accordance with expectations. The feasibility of using untargeted MSI for food analysis was revealed by ion maps of plant metabolites in cherry tomatoes and maize-kernel slices. For tomatoes, traveling-wave ion mobility (TWIM) was used to discriminate between different lycoperoside glycoalkaloid isomers; for maize quadrupole time-of-flight tandem mass spectrometry (MS-MS) was successfully used to elucidate the structure of a localized unknown. It is envisaged that LAESI-MSI will contribute to future research in food science, agriforensics, and plant metabolomics.

Show MeSH
Related in: MedlinePlus