Lecture 13.
TOF-SIMS Mass Spectroscopy
• Routine analytical technique
• Detailed chemical structure
information
• High sensitivity
• New primary ion sources (Au,
Bi, & buckministerfullerene
Principles of ToF-SIMS
• Pulsed primary ion
beam
• Emission of particles –
SECONDARY IONS
• Ions are mass analyzed
by FLIGHT TIMES
• Two modes of analysis:
static & spectroscopic
TOF-SIMS
 Bombard surface with
gallium and run through
mass spectrometer
 Gives both chemical
composition of surface and
“SEM-like” image of where
chemicals are located
 Lightly used on
lignocellulosic materials
 Find concentrations as low
as 10 ppm
http://www.phi.com/genf.asp?ID=83
Static Mode
• Delicate organics (biomaterials)
• Undamaged (opposed to X-ray
fluorescence microscopy)
• Surface sensitive (outermost couple of
nm)
Spectroscopic Mode
• ONLY mass spectral (MS) data provided
• Chemical imaging is POSSIBLE
• Raster a micro-focused ion beam (sound
familiar???) over surface
• Collect MS
• Map distribution of species
Organic imaging technique
• Previously, limted by most significant
signals – polyatomic clusters
• For example, most biomaterials dominated
by fragments (CxHy+/-) at low mass (< m/z
100) – MORE than one species
• NOT DIAGNOSTIC
Higher-order chemical imaging
• Larger masses  (m/z > 200) more structurally
assignable and unique
• Chemical mapping possible
• Ga+ bombardment doesn’t allow for sufficient
sensitivity for good imaging
• Polyatomic primary ion sources overcomes
deficiency (Aun+, Bin+, C60+)  100x increases in
secondary ion yields
Chemical imaging of
pharmaceuticals
• Drug-loaded particles can be visualized with
Bi3+, whereas with Ga+ they cannot
• Due to low intensity of molecular ion peak
• Tablet formulations can be studied – distribution
of drug, excipient(s), lubricant(s) on surface and
in bulk
• Thickness & uniformity can be assessed
ToF-SIMS Images
10 micron diameter hair fibers
Distribution of materials
Nylon mesh – 10 micron depth
Plasma cleaned scalpel blade
Lignocellulosic biomaterials
• The work of Thompson with superoxide (Potassium
superoxide) in DMSO found attack in amorphous
regions first
• Hemicellulose and lignin removed more rapidly than
cellulose
• The work of Kim with periodate oxidation suggests
that the attack on crystalline cellulose proceeds
highly heterogeneously
• Once an area is damaged, however, the area
becomes more susceptible to damage due to loss of
crystalline order
Thompson, N.S., Corbett, H.M, “The effect of potassium superoxide on cellulose”, TAPPI, 68:12, pp. 68-72, 1985.
Kim, U., Kuga, S., Wada, M., Okano, T., Kondo, T. “Periodate oxidation of crystalline cellulose”, Biomacromolecules, 1:488-492, 2000.
TOF-SIMS hypotheses
• Lind studied the ability of hydroxyl radicals to induce viscosity
loss in cellulose fibers
• In their work they found that the decrease in viscosity was
proportional to the imparted irradiation dose
• This can be read to mean that as the number of hydroxyl
radicals increases, so does the cellulose degradation
• The work of Lind studied the role of hydroxyl radicals in
viscosity loss using ionizing radiation
• In their work they found that almost no amount of radical
scavenger could protect against depolymerization of the
cellulose
• This means that hydroxyl radicals produced outside the
cellulose surfaces have a minimal effect on degradation and
are more likely produced very near to their consumption point
Lind, J., Merényi, G., “Hydroxyl radical induced viscosity loss in cellulose fibers”,
J. Wood Chem. Technol., 17,(1,2), pp. 111-117 (1997).
TOF-SIMS hypotheses, continued
• Metal-induced peroxide cellulose degradation causes the creation of
carboxylic acid content
• Work of Lind shows that hydroxyl radicals are formed and react very
near to the carbohydrate surface
• Work of Kleen has been used to measure metals on fiber surface as
compared to bulk during bleaching, Found majority of metals, 5 to
55 times bulk, on surface
• Not likely to be precipitates due to the fact that the sheets were
made at a pH of 5
M. Kleen, Sixth European Workshop on Lignocellulosics and Pulp, 41-44 (2000)
Central hypothesis of work
Hypothesis: We are seeing metals bound to carboxylic acid groups
caused by radical degradation of cellulose
 Metal distribution begins rather homogeneous in the unbleached
case, but becomes heterogeneous in the bleached cases
 Attack appears heterogeneous concentrated and surface orientated
due to the fact it does not appear “deep” enough to be seen by
SEM under comparable resolutions
Total Ion Image
Unbleached Mg
Bleached Mg
Bleached + 50ppm iron Fe
ESCA
ip0212_101.spe: sample 2124*
02 Feb 12 Al mono 350.0 W 0.0 45.0° 23.50 eV
C1s/Full/1 (SG5 Shft)
4
4
3.5325e+004 max
Evans PHI
30.00 s
ip0212_101.spe
x 10
3.5
3
2.5
C-C/C-H
1.5
C-O
C=O/O-C-O
2
1
O-C=O
c/s
• Used to identify carbon and
oxygen and the oxidation level
• Has been utilized for the
detection of carboxyl content
(mostly fiber modification work)
in literature and is comparable
to other methods
• We appear to have a
difference between the
bleached and unbleached
samples in COOH content
0.5
0
300
298
296
294
292 290 288
Binding Energy (eV)
286
284
282
280
Viscosity and physical testing
• As the load bearing structure of a fiber, the
cellulose chains, are being cleaved or
“peeled” the mechanical strength of a fiber
should decrease
• This change should manifest itself in test
to include the zero-span tensile and the
standard tensile test
Viscosity and physical testing
Chain scission count increases as
degradation conditions become
more favorable
0.25
Chain Scission Count
•
0.2
0.15
0.1
3.0% H2O2, 90C, 50 ppm Fe
3.0% H2O2, 90C
0.05
1.5% H2O2, 70C
Zero-span tensile tests show
strength decrease as well.
Perhaps we also see chemical
refining
Dry Zero-Span Breaking Length
25
20
15
km
•
m
at
ur
e
ju
ve
ni
le
0
Series1
10
5
0
Mature
Bleached
Sample
Bleached + iron
Summary
• TOF-SIMS appears to visualize degradation through
the indirect measurement of COOH groups
• The analysis of this degradation can be coupled with
other techniques including ESCA, viscosity, and zerospan tensile
• Degradation appears to be a heterogeneous surface
phenonemon
Other work
Phytic Acid Chelation
• Relatively unstudied chelant that is a product of
unwanted by-products of corn
• Current data shows performance on par with
DTPA/EDTA, but effectiveness is very pulp
dependent
• Agriculture literature says excellent chelant for iron
• Studying it as a bleaching additive and chelant on
many different pulp samples and at differing pH
Depth Profiling of Organic Films using the Time-of-Flight SIMS
S.V. Roberson
Objective: To independently optimize the parameters for analysis and erosion in depth profiling of organic films by using a dual beam
approach
Problem: Conventional depth profiling is performed with either quadrupole or magnetic sector SIMS instruments. In these instruments, a
direct current, monatomic beam is utilized, with a single ion gun eroding the sample surface and providing secondary ions that are
characteristic of its surface composition. In Time-of Flight (ToF) SIMS, a pulsed ion beam is utilized. The duty cycle of this pulsed beam is
in the range of 10-3-10-5. This low duty cycle results in very small sample consumption. For this reason, ToF SIMS is generally
considered to be solely a surface analysis tool. An additional complication ensues where conventional depth profiling of organic films is
concerned. In previous work, we have demonstrated that monatomic depth profiling of varied organic films under dynamic (high dose)
SIMS conditions results in rapid decay of characteristic molecular ion signals. To circumvent this problem, we utilized polyatomic primary
ion sources. Utilizing polyatomic ions as primary projectiles in SIMS provides several potential advantages over the use of atomic
projectiles. Among these are: increased secondary ion signals, surface localization of damage, sputtering beyond the static SIMS dose
limit, and the potential to optimize the primary beam for a given experiment.
Approach: In dual beam depth profiling, two pulsed ion guns are utilized. A low energy sputter gun erodes a square crater in the sample.
A second, pulsed ion gun is used to analyze a small region in the center of the sputtered crater. The ToF-SIMS IV has been utilized for
dual beam, molecular depth profiling. Ga+ (generated with the liquid metal ion source) was used for analysis, and the polyatomic ion,
SF5+, (generated in the dual source column) was used for sputtering. The analysis beam was operated at relatively high energies (15
keV) so that short pulses and small spot sizes could be generated. Long pulses and low energies (3 keV) were generally utilized with the
sputter gun to attain the highest depth resolution. This dual-beam approach was investigated in terms of its usefulness for studying
positive molecular ion emission from thin films of glutamate and PMMA on silicon substrates.
Results and Future Plans: Figure 1 shows a dual beam depth profile of a glutamate thin film. (The Ga+ analysis gun was operated at a
current of ~3 pA, while the SF5+ sputtering gun was operated at ~ 5 nA). The quasi-molecular ion (m/z 148) and the diagnostic fragment
at mass 84 were monitored over the depth of this film. We intend to optimize the parameters of both ion guns so that the secondary ion
signal is maximized. Once optimized, we will utilize this technique for profiling of ultra thin layers and for interface analysis.
What direction next?