REPOSITORY > RESULTS

Abstract

Time-of-flight secondary ion mass spectrometry using MeV ions (MeV ToF SIMS) has
been around for a few decades as a reliable, surface-sensitive method for the detection of
molecular ions having masses up to 1000 Da. It provides excellent lateral resolution of a
few μm for imaging of organic samples, making it a superb mass spectrometry technique
in fields such as biology, forensics, cultural heritage, etc. In conventional keV SIMS,
nuclear stopping predominates and causes a collision cascade that primarily results in
the dissociation of larger molecules but is very effective at desorbing atomic ions and
small molecules. In contrast, MeV atomic-ion bombardment causes energy loss in the
material that is primarily caused by electronic stopping, which is reflected in the ”soft”
desorption of larger organic molecules. The secondary ion yield is enhanced at higher
masses (100-1000 Da) compared to the typical ToF SIMS, and fragmentation of molecules
is decreased, although the underlying desorption mechanisms are still not fully understood
or explainable by a general theory.
This work investigates the idea of a low energy range (100 keV – 5 MeV) primary
ion beam mode in MeV SIMS and its potential for exploiting the capabilities of both
conventional keV SIMS and MeV SIMS simultaneously – the analysis of inorganic species,
while still being able to sputter and analyze larger organic molecules. This energy mode
was named LE (Low Energy) MeV SIMS. Secondary ion yield dependence on the primary
ion energy of leucine and various inorganic targets was studied. Next, imaging of a hybrid
organic/inorganic target made of Cr and leucine was performed, showing that the contrast
between the organic and inorganic area is almost completely diminished when lowering the
primary ion beam energy. LE MeV SIMS depth profiling of a dual-layer Cr-ITO sample
in a dual-beam mode was also explored, and the obtained depth profile was compared
against well-established keV SIMS. Depth profiles demonstrated solid chemical sensitivity
to inorganic secondary ions and satisfactory depth resolution, given that a simple sputter
source was used for etching. Systematic investigation of MeV ToF SIMS in the low energy
regime opens up new possibilities in both the fundamental understanding of the impact
of the primary ion stopping power on the detection of secondary ions of organic and
inorganic species, and expanding the application of MeV SIMS to analysis, imaging, and
depth profiling of inorganic species with increased efficiency.
Furthermore, the increasingly complex, multivariate nature of MeV ToF SIMS datasets
often makes the analysis and comparison very hard, possibly leaving crucial information
overlooked. This can be tackled by implementing multivariate analysis algorithms standardly
used in various other analytical techniques, including the most similar keV SIMS,
in order to extract critical, often latent information, and reveal underlying patterns in the
data. This proved to be especially beneficial in the MeV SIMS application in the forensics
of questioned documents, specifically the imaging of ink intersections.
A novel application of MeV SIMS coupled with particle induced X-ray emission (PIXE)
is presented to determine the deposition order of intersecting lines made by various types
of writing tools. Principal component analysis (PCA) employed in image processing of the
data from both MeV SIMS and PIXE yielded excellent image contrast that is needed to
identify the inks and to determine their deposition order. Only the cases that could not be
solved with MeV SIMS were further analyzed by PIXE, which detects elemental information
from greater depths. In challenging cases involving liquid-based inks, the combination
of approaches proved to be crucial in disclosing the deposition mechanisms and assisted in
the resolution of some of them. Next, intersections of several optically identical ballpoint
pen inks were studied using non-destructive optical techniques (microscopic and infrared
luminescence) that are standardly applied for questioned documents’ examination at the
Forensic Science Centre “Ivan Vučetić”, and MeV SIMS, which is applied at the Ruđer
Bošković Institute. The obtained results were compared and discussed. An emphasis is
placed on the success of MeV SIMS assisted by t-stochastic neighbor embedding (t-SNE)
in differentiating two very similar ballpoint pen inks and determining their deposition order.
In general, MeV SIMS also provides chemical information about the studied writing
tools, which is an added value. The traces from all the intersections studied so far during
this work proved to be distinguishable.