264
H. Schulz
components trans-crocetin and trans-crocin occurring in the plant tissue. Best normal Raman spectra were obtained applying excitation at 785 nm providing highest
signal intensity of the dye bands.
9.3.2 Alkaloids
Numerous alkaloids, showing a large range of different chemical structures of
which most possess a significant biological activity, have been analysed by Raman spectroscopy [63]. Especially caffeine, theophylline and theobromine show
characteristic strong features between 1,310 and 1,340 cm
−1
and between 1,690 and
1,710 cm
−1
, which are assigned as C-N and C = N stretching, respectively [64]. the
three purine alkaloids have been detected in cacao seeds and their extracts applying
Ft-Raman spectroscopy [65]. In this context it was also found that theobromine
seems to occur in its free form in cacao seeds because no spectral shift was registered compared to the pure standard.
main alkaloids occurring in poppy ( Papaver somniferum) and in related pharmaceutical products were successfully characterised by Raman spectroscopy [66].
In the fingerprint range between 700 and 1,500 cm
−1
various sharp bands can be
identified, which are mainly due deformation and stretching vibrations of the alkaloid ring system. generally, morphine, codeine, and thebaine show no significant
differences in their Raman spectral profile, however, some characteristic key bands
can be found in the ranges of 1,600–1,650 cm
−1
and 630–650 cm
−1
which may be
used to discriminate the four mentioned isoquinoline alkaloids.
Raman spectra obtained from poppy milk present characteristic well resolved
morphine bands (peaks at 631, 1,620 and 1,642 cm
−1
). Furthermore, the alkaloids
occurring in poppy capsules (morphine, codeine, papaverine, and noscapine) could
be reliably predicted. the total alkaloid content of poppy samples was determined
with comparatively high prediction quality as well (R
2
= 0.90) [66].
Quinine is the main alkaloid present in the bark of different Cinchona species
(Rubiaceae), especially Cinchona pubescens. the bark of trees of the Cinchona
genus is the source of a variety of alkaloids, the most familiar of which are the following pairs of stereoisomers: cinchonine and cinchonidine, quinine and quinidine,
as well as dihydroquinidine and dihydroquinine. Quinine and quinidine have a bitter taste and due to this property they are considerably used as food additives. the
high potential of resonance Raman spectroscopy was demonstrated by the detection
of small amounts of these alkaloids in plant materials, without any further sample
preparation. Non-resonant Raman microscopy with excitation wavelengths in the
visible (532 and 633 nm) and NIR (1,064 nm) regions was unsuccessful due to a
strong fluorescence background and due to noisy signals resulting from low laser
power, respectively. the use of higher laser power resulted in destruction of the
plant material. Non-resonant Raman spectroscopy suffers from low scattering cross
section, but sometimes for the active agents which are present at low concentration
in the biological environment the signals can be strongly enhanced if the excitation
H. Schulz
components trans-crocetin and trans-crocin occurring in the plant tissue. Best normal Raman spectra were obtained applying excitation at 785 nm providing highest
signal intensity of the dye bands.
9.3.2 Alkaloids
Numerous alkaloids, showing a large range of different chemical structures of
which most possess a significant biological activity, have been analysed by Raman spectroscopy [63]. Especially caffeine, theophylline and theobromine show
characteristic strong features between 1,310 and 1,340 cm
−1
and between 1,690 and
1,710 cm
−1
, which are assigned as C-N and C = N stretching, respectively [64]. the
three purine alkaloids have been detected in cacao seeds and their extracts applying
Ft-Raman spectroscopy [65]. In this context it was also found that theobromine
seems to occur in its free form in cacao seeds because no spectral shift was registered compared to the pure standard.
main alkaloids occurring in poppy ( Papaver somniferum) and in related pharmaceutical products were successfully characterised by Raman spectroscopy [66].
In the fingerprint range between 700 and 1,500 cm
−1
various sharp bands can be
identified, which are mainly due deformation and stretching vibrations of the alkaloid ring system. generally, morphine, codeine, and thebaine show no significant
differences in their Raman spectral profile, however, some characteristic key bands
can be found in the ranges of 1,600–1,650 cm
−1
and 630–650 cm
−1
which may be
used to discriminate the four mentioned isoquinoline alkaloids.
Raman spectra obtained from poppy milk present characteristic well resolved
morphine bands (peaks at 631, 1,620 and 1,642 cm
−1
). Furthermore, the alkaloids
occurring in poppy capsules (morphine, codeine, papaverine, and noscapine) could
be reliably predicted. the total alkaloid content of poppy samples was determined
with comparatively high prediction quality as well (R
2
= 0.90) [66].
Quinine is the main alkaloid present in the bark of different Cinchona species
(Rubiaceae), especially Cinchona pubescens. the bark of trees of the Cinchona
genus is the source of a variety of alkaloids, the most familiar of which are the following pairs of stereoisomers: cinchonine and cinchonidine, quinine and quinidine,
as well as dihydroquinidine and dihydroquinine. Quinine and quinidine have a bitter taste and due to this property they are considerably used as food additives. the
high potential of resonance Raman spectroscopy was demonstrated by the detection
of small amounts of these alkaloids in plant materials, without any further sample
preparation. Non-resonant Raman microscopy with excitation wavelengths in the
visible (532 and 633 nm) and NIR (1,064 nm) regions was unsuccessful due to a
strong fluorescence background and due to noisy signals resulting from low laser
power, respectively. the use of higher laser power resulted in destruction of the
plant material. Non-resonant Raman spectroscopy suffers from low scattering cross
section, but sometimes for the active agents which are present at low concentration
in the biological environment the signals can be strongly enhanced if the excitation