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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: 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.It is envisaged that LAESI-MSI will contribute to future research in food science, agriforensics, and plant metabolomics.

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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Untargeted positive-ion LAESI-QTOF-MS–MS ion map of (a) m/z 438.244 (±5 mDa) on a tip cap of a maize kernel, showing the spatial distribution of an unknown. The x–y center-to-center distance was 0.2 mm. (b) centroided MS–MS spectrum of precursor ion m/z 438.2 (the MS–MS spectrum was lock-mass corrected after the data acquisition using the precursor ion m/z 438.241)
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Fig5: Untargeted positive-ion LAESI-QTOF-MS–MS ion map of (a) m/z 438.244 (±5 mDa) on a tip cap of a maize kernel, showing the spatial distribution of an unknown. The x–y center-to-center distance was 0.2 mm. (b) centroided MS–MS spectrum of precursor ion m/z 438.2 (the MS–MS spectrum was lock-mass corrected after the data acquisition using the precursor ion m/z 438.241)

Mentions: As a second example of untargeted ambient imaging, LAESI-TOF-MSI and LAESI-QTOF-MS–MS were applied to tip caps of maize kernels. Two different types of maize kernel were investigated: normal ones, and suspect kernels with darker areas in the tip cap of the kernel. The mass spectra and reconstructed ion chronograms obtained suggested the presence of a range of low-molecular-weight ions, including such common plant metabolites as a dihydroxycinnamic acid (e.g. caffeic acid), a trihydroxybenzoic acid (e.g. gallic acid), a hydroxybenzoic acid, and a dihydroxybenzoic acid. Interestingly, the tip-cap regions of both maize-kernel types also produced two highly localized signals having different relative intensities: an [M+Na]+ ion at m/z 463.183 and an [M+H]+ ion at m/z 438.241. Assuming an elemental composition containing only C, H, and O atoms, the [M+Na]+ ion at m/z 463.183 would correspond with the composition C18H32O12 (mass error 3.9 mDa). Several C6 fatty esters of disaccharides have that composition, among them sucrose caproate, which has been reported as a seed-germination inhibitor [33]. As expected, MS–MS experiments on the [M+Na]+ ion were not successful and consequently we were unable to discriminate between the different fatty-ester disaccharide options. The spatial distribution of the [M+H]+ ion at m/z 438.241 is shown in Fig. 5a. Note that this even-mass [M+H]+ ion must contain an odd number of nitrogen atoms. Assuming an elemental composition of C, H, N, and O atoms, many options are possible for this ion within a mass error of 5 mDa. The LAESI-QTOF-MS–MS spectrum of the [M+H]+ precursor ion m/z 438.2 is shown in Fig. 5b. Using this MS–MS spectrum, elemental compositions of fragment ions and neutral losses were calculated and (sub)structures proposed (Table 1). All assigned elemental compositions are within 0–6 mDa of their theoretical value. The ion at m/z 292.198 is directly formed from the precursor ion and fragments further, with neutral losses of 17.022, 74.087, and 88.095 Da corresponding with NH3, C3H10N2, and C4H12N2, respectively. Therefore, the unknown compound with an even-mass [M+H]+ ion does not contain only a single nitrogen but should contain (at least) three nitrogen atoms. Together with the neutral loss of 57.058 Da, a spermidine substructure is clearly suggested. Spermidine is a polyamine plant-growth regulator that occurs in, among others, corn. The neutral loss of 146.043 Da and the fragment ion at m/z 147.045 both support coumaroyl-like substructures. Altogether, N,N-bis-coumaroyl spermidine is a strong candidate for the identity of the highly localized unknown compound with an [M+H]+ ion at m/z 438.241. p-Coumaric acid amides of polyamines have been associated with the chemical defense of plants against arthropod pests. These compounds were found to be bioactive at the receptor level but not when ingested [34].Fig. 5


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)

Untargeted positive-ion LAESI-QTOF-MS–MS ion map of (a) m/z 438.244 (±5 mDa) on a tip cap of a maize kernel, showing the spatial distribution of an unknown. The x–y center-to-center distance was 0.2 mm. (b) centroided MS–MS spectrum of precursor ion m/z 438.2 (the MS–MS spectrum was lock-mass corrected after the data acquisition using the precursor ion m/z 438.241)
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4196196&req=5

Fig5: Untargeted positive-ion LAESI-QTOF-MS–MS ion map of (a) m/z 438.244 (±5 mDa) on a tip cap of a maize kernel, showing the spatial distribution of an unknown. The x–y center-to-center distance was 0.2 mm. (b) centroided MS–MS spectrum of precursor ion m/z 438.2 (the MS–MS spectrum was lock-mass corrected after the data acquisition using the precursor ion m/z 438.241)
Mentions: As a second example of untargeted ambient imaging, LAESI-TOF-MSI and LAESI-QTOF-MS–MS were applied to tip caps of maize kernels. Two different types of maize kernel were investigated: normal ones, and suspect kernels with darker areas in the tip cap of the kernel. The mass spectra and reconstructed ion chronograms obtained suggested the presence of a range of low-molecular-weight ions, including such common plant metabolites as a dihydroxycinnamic acid (e.g. caffeic acid), a trihydroxybenzoic acid (e.g. gallic acid), a hydroxybenzoic acid, and a dihydroxybenzoic acid. Interestingly, the tip-cap regions of both maize-kernel types also produced two highly localized signals having different relative intensities: an [M+Na]+ ion at m/z 463.183 and an [M+H]+ ion at m/z 438.241. Assuming an elemental composition containing only C, H, and O atoms, the [M+Na]+ ion at m/z 463.183 would correspond with the composition C18H32O12 (mass error 3.9 mDa). Several C6 fatty esters of disaccharides have that composition, among them sucrose caproate, which has been reported as a seed-germination inhibitor [33]. As expected, MS–MS experiments on the [M+Na]+ ion were not successful and consequently we were unable to discriminate between the different fatty-ester disaccharide options. The spatial distribution of the [M+H]+ ion at m/z 438.241 is shown in Fig. 5a. Note that this even-mass [M+H]+ ion must contain an odd number of nitrogen atoms. Assuming an elemental composition of C, H, N, and O atoms, many options are possible for this ion within a mass error of 5 mDa. The LAESI-QTOF-MS–MS spectrum of the [M+H]+ precursor ion m/z 438.2 is shown in Fig. 5b. Using this MS–MS spectrum, elemental compositions of fragment ions and neutral losses were calculated and (sub)structures proposed (Table 1). All assigned elemental compositions are within 0–6 mDa of their theoretical value. The ion at m/z 292.198 is directly formed from the precursor ion and fragments further, with neutral losses of 17.022, 74.087, and 88.095 Da corresponding with NH3, C3H10N2, and C4H12N2, respectively. Therefore, the unknown compound with an even-mass [M+H]+ ion does not contain only a single nitrogen but should contain (at least) three nitrogen atoms. Together with the neutral loss of 57.058 Da, a spermidine substructure is clearly suggested. Spermidine is a polyamine plant-growth regulator that occurs in, among others, corn. The neutral loss of 146.043 Da and the fragment ion at m/z 147.045 both support coumaroyl-like substructures. Altogether, N,N-bis-coumaroyl spermidine is a strong candidate for the identity of the highly localized unknown compound with an [M+H]+ ion at m/z 438.241. p-Coumaric acid amides of polyamines have been associated with the chemical defense of plants against arthropod pests. These compounds were found to be bioactive at the receptor level but not when ingested [34].Fig. 5

Bottom Line: 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.It is envisaged that LAESI-MSI will contribute to future research in food science, agriforensics, and plant metabolomics.

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