Intact Glucosinolates by MALDI-TOF MS
J. Agric. Food Chem., Vol. 50, No. 5, 2002 987
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used to separate and identify the desulfoglucosinolates in L.
annua seeds and similar work performed on the vegetable
species (17). The spectrum obtained from the L. annua extract
clearly showed three strong peaks at m/z 360, 450, and 464 as
the only signals in the mass range 350 to 500 Da (Figure 2A).
These masses correspond to glucosinolates with isopropyl,
5-methylsulfinyl, and 6-methylsulfinyl side chains, which were
also the three glucosinolates identified by LC-MS, and in
previous work based upon the analysis of myrosinase-induced
breakdown products (31, 32). The sensitivity of the MALDI-
TOF MS technique is such that these species could be detected
from a sample of just 0.2 mg of seeds.
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The MALDI-TOF mass spectra of the samples from cauli-
flower, rutabaga, and turnip were also very clean, and the
glucosinolates could be clearly observed. The data from
MALDI-TOF MS again gave very good agreement with LC-
MS analysis (17), showing the same spread of glucosinolates,
and there was some correlation in terms of relative amounts.
MALDI-TOF mass spectrometry thus represents a quick and
simple method for the analysis of glucosinolates. Very little
purification is required, and the method also has the added
advantage that it does not require a desulfonation step. However,
its obvious disadvantage is that it will not distinguish glucosi-
nolates of near identical mass but different structure. For
example, glucoiberin (3-methylsulfinylpropyl glucosinolate) and
gluconasturtiin (phenylethyl glucosinolate) both give M-H ions
of molecular weight 422, differing by only 0.04 Da, and because
under the conditions employed only the intact molecular ion is
obtained, there is no fragmentation pattern to help distinguish
between these glucosinolates. Furthermore, without much more
extensive work comparing the relative ionization abilities, and
therefore signal intensities, of the glucosinolates found, no
detailed quantitative data can be obtained from this study.
However, there does seem to be a general correlation between
signal intensity and the amount of glucosinolate present, with
those described as the most abundant species in previous studies
giving strong signals here and those described as being at trace
levels giving weak signals. To obtain truly quantitative data the
use of labeled internal standards would be required. Each
individual glucosinolate would be required in a labeled form,
displaying a mass increase of at least 2 Da. In our laboratory
we have previously used deuterated desulfoglucosinolates as
internal standards for LC-MS analysis (17, 19), and the
analogous use of deuterated glucosinolates as internal standards
for MALDI-TOF MS is currently under investigation.
(14) Rosa, E. A. S.; Heaney, R. K.; Fenwick, G. R.; Portas, C. A. M.
Glucosinolates in crop plants. Hortic. ReV. 1997, 19, 99-215.
(15) Griffiths, D. W.; Birch, A. N. E.; Hillman, J. R. Anti-nutritional
compounds in the Brassicaceae: analysis, biochemistry, chem-
istry and dietary effects. J. Hortic. Sci. Biotechnol. 1998, 73,
1-18.
(16) Kiddle, G.; Bennett, R. N.; Botting, N. P.; Davidson, N. E.;
Robertson, A. A. B. R.; Wallsgrove, R. M. High performance
liquid-chromatography separation of natural and synthetic des-
ulphoglucosinolates and their chemical validation by UV, NMR
and chemical ionisation-MS methods. Phytochem. Methods 2001,
12, 226-242.
In conclusion, it has been demonstrated that MALDI-TOF
mass spectrometry is a sensitive technique for the analysis of
intact glucosinolates. The technique has been used to analyze
synthetically produced glucosinolates, glucosinolates which have
been isolated and purified, and those extracted in a crude form
directly from plant matter. It is therefore a useful addition to
the methods currently available for the analysis of glucosino-
lates.
(17) Griffiths, D. W.; Bain, H.; Deighton, N.; Botting, N. P.;
Robertson, A. A. B. Evaluation of liquid chromatography-
atmospheric pressure chemical ionisation-mass spectrometry for
the identification and quantification of desulphoglucosinolates.
Phytochem. Anal. 2000, 11, 216-225.
ACKNOWLEDGMENT
We thank Avril A. B. Roberston and David Milne for the
synthesis of gluconasturtiin.
(18) Mellon, F. A.; Chapman, J. R.; Pratt, J. A. E. Thermospray liquid
chromatography-mass spectrometry in food and agricultural
research. J. Chromatogr. 1987, 394, 209-222.
(19) Robertson, A. A. B.; Botting, N. P. Synthesis of deuterium
labeled desulfoglucosinolates as internal standards for LC-MS
analysis. Tetrahedron 1999, 55, 13269-13284.
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