The Importance of Cationisation in SIMS Spectral Interpretation as Observed For
Surfactant Interaction With a Plant Leaf Surface
Mark Perkinsa, David Briggsa, Frank Ruttena, Clive Robertsa, Martyn Daviesa and Adrian Friedmannb
a
Laboratory of Biophysics and Surface Analysis, School of Pharmacy, The University of Nottingham, Nottingham NG7 2RD, UK
b
Syngenta, Jealotts Hill International Research Centre, Bracknell RG42 6EY, UK
Introduction:
Cuticle:
Major leaf component crucial in regulating the uptake of externally applied chemicals into the
plant
Prunus was used in this study for a number of reasons:
• No stomata so only diffusion through cuticle
• Smooth surface amenable to SIMS analysis
• Properties of bulk and esp. outer layer well studied in literature
• Prunus cuticle routinely used for penetration studies
• Plant leaf surfaces allow the leaf to ingress beneficial materials but also form an effective barrier against potentially harmful chemicals.
• This creates a barrier for agrochemicals: need to maximise uptake of active ingredients into the leaf and thus remainder of the plant.
• To aid hit process surfactants commonly added, e.g. Synperonic A7:
O C C
H2 H2
A7 consists of an oligomeric mixture of molecules with C13 and C15
hydrophobic tails, with a mass difference of 28 between oligomers with
the same number of Ethoxylate-moieties. Mean number of these
moieties is 7.
n
OH
Cuticle schematic :
Hydrophobic tail (C13/C15)
Mass = 44, Mean n = 7
Epicuticular Waxes
(ca. 100 nm)
Cuticular Waxes and Cutin
Cell Wall
cell
cell
Aim:
Experimental:
To investigate the interaction of a common active ingredient and surfactant with a model system leaf, from
which an isolated cuticle was prepared to allow determination of surfactant and active ingredient diffusing
through this barrier
• ToF-SIMS IV (ION-TOF GmbH, Münster, Germany) using Cs+ 10kV throughout, primary ion dose in static regime
• Surfactant concentration 0.125 % w/v in ultrapure water, active ingredient added at 0.25% w/v
• Prunus laurocerasus (rose family (Rosaceae), common names: English laurel, cherry laurel). Leaf used without pretreatment
• Cuticle isolated from leaf using well-established enzymatic methodology
I. Interaction of A7 with Prunus leaf
A7 on Leaf
○
44
When comparing spectra recorded after
depositing A7 on a Prunus leaf (top
spectrum) with a reference spectrum
on a Si-wafer several peaks were
strikingly absent, as marked with the
red arrows above. Peak positions all
concide with those for K-cationised C13
oligomers.
○
A7 Surfactant drop on leaf
Bare Prunus leaf
●
Area exposed to A7
o
Zo
Intensity
Intensity
M
= 45
M=
400
500
600
700
800
Co
m
pa
400
500
600
in
re
to
∆
re
f
er
en
c
e
5 × 5 mm2 stage raster
N2(l)-cooled to < 0ºC
on
(symbols denote series of peaks
separated by 44 amu)
Si
∆
540
560
580 mass / u
Spectral Interpretation:
560
580 mass / u
Deposition of both surfactant and active ingredient
yields very different spectra
■
▲
▲
500
520
⇒ alterations of the predominant ionisation
processes occurring on this surface compared to the
pure ingredients.
□
■
∆
∆
540
580 mass / u
560
(quasi-) molecular ions 500-600 amu
C13
• A7 consists of 2 different oligomers with C13 and C15 tail groups:
540
□
□ A7 & Active
∆
520
520
▲
▲
Intensity
Intensity
Spectra for both sides of the cuticle are identical
500
▲
▲
This was further investigated in II. by
repeating the experiment using an
isolated cuticle and investigating the
resulting chemistry at both sides after
exposure to A7.
∆
A7
∆
⇒ absence of peaks observed on the plant leaf
does not indicate that specific ingredients have
diffused through the cuticle, as might have been
inferred from solely comparing the spectra for
deposition on Si and the plant leaf!
∆
547
∆
III. Interaction of A7 and active ingredient on a Si wafer
Outside
Inside
591
503
500
II. Interaction of A7 with an isolated Prunus cuticle
●
28
A7 on Si
800 mass / u
700
m
16
6
Ioniser \ n
• C13 oligomers: C13H27-(O-CH2CH2)-OH at masses of approx. 200 + 44n amu
H
• C15 oligomers: C15H31-(O-CH2CH2)-OH at masses of approx. 228 + 44n amu
Na
K
503
C15
7
8
9
509
553
597
531
575
515
559
547
591
531
575
6
7
8
537
581
Using the data displayed in the table in combination with
the acquired spectra allows for a comprehensive
interpretation of all major oligomer peaks.
Peak denotations:
□ = protonation,
○ = Na-cationisation,
∆ = K-cationisation.
Open symbols denote C13- and closed symbols C15oligomers.
• Cationisation is prevalent for A7 spincast on Si – the Static SIMS Library shows predominantly
K-cationisation - but any Na present may also be involved in the ionisation process, yielding a
host of possible (quasi-) molecular ions which may be observed.
In summary:
• The difficulty in interpreting this data largely arises through some rather unfortunate peak
overlaps not separable with ToF-SIMS, which could, when not fully understood, lead to wrong
interpretation of these spectra:
¾ When only A7 is present on Si the spectrum is dominated by by K-cationised oligomers
¾ On the plant leaf K-cationisation is strongly suppressed, showing mainly Na-cationisation and some protonation
¾ Adding the active ingredient leads to suppression of cationisation and the spectrum is dominated by protonation
• Mass of CH2CH2O = 44 = difference in mass between oligomers with same tailgroup
• Mass difference between C13 and C15 chains with same n is 28
• Mass difference between identical Na- and K-cationised molecules is 16
• Unfortunately 28 + 16 = 44
Overlapping peaks are identified by identical colours in the following table displaying possible
(quasi-) molecular ions between 500 and 600 amu (H = protonation, Na and K denote
cationisation).
Conclusions:
¾The differences between the spectra recorded for A7 on Si and Prunus leaf can be wholly assigned to changes in ionisation
¾Whereas ToF-SIMS is a very powerful surface analytical tool, a thorough knowledge of the processes involved in generating
the secondary ions detected in the SIMS process is essential for a thorough and correct interpretation of spectra generated.
Reference:
The Static SIMS Library Version 3, Surface Spectra Ltd, Manchester, UK