1. Lecture 13. TOF-SIMS Mass Spectroscopy

2. Routine analytical technique
Detailed chemical structure
information
High sensitivity
New primary ion sources (Au,
Bi, & buckministerfullerene

3. 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

4. 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
                                                                                                                   
                                            
                                                                                

5. Static Mode Delicate organics (biomaterials)
Undamaged (opposed to X-ray fluorescence microscopy)
Surface sensitive (outermost couple of nm)

6. 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

7. 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

8. 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

9. 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

10. ToF-SIMS Images 10 micron diameter hair fibers
Distribution of materials
Nylon mesh – 10 micron depth
Plasma cleaned scalpel blade

11. 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 , 1985.
Kim, U., Kuga, S., Wada, M., Okano, T., Kondo, T. “Periodate oxidation of crystalline cellulose”, Biomacromolecules, 1: , 2000.

12. 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 (1997).

13. 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, (2000)

14. 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

15. ESCA 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

16. 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

17. Viscosity and physical testing
Chain scission count increases as degradation conditions become more favorable
Zero-span tensile tests show strength decrease as well. Perhaps we also see chemical refining

18. 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 zero-span tensile
Degradation appears to be a heterogeneous surface phenonemon

19. 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

20. 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 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.

21. What direction next?