Transcript Laser Desorption Ionization Mass Spectrometry of Ink

Laser Desorption
Ionization Mass
Spectrometry of Ink
Katie Axwik
Jory Schlitt
Broc Glover
Laser Desorption Ionization
An analyte sample is moved under a pulsed laser.
Excitation of an analyte on that paper results in ionization of
that chemical species.
Ions are then taken into a Time-of-Flight mass spectrometer, and
the mass measured with high accuracy.
Not all compounds will readily ionize under LDI conditions.
For larger analytes, the use of a matrix may be required; this is
referred to as matrix-assisted laser desorption ionization
(MALDI).
A matrix is sprayed over a sample, and then crystalized as
the sample is placed under a vacuum.
This technique works very well for imaging studies of
compounds with very high mass-to-charge ratios.
MALDI is normally used for determining the spatial
distribution of proteins in a tissue cross-section.
Analysis of lower mass compounds is normally obscured by
solvent peaks, adducts and fragments making this
technique unsuitable for low molecular weight
compounds.
Artificial Techniques for Aging
There are currently three ways to artificially age pen inks:
1)Exposure to high intensity ultraviolet light.
2)Exposure to incandescent light.
3)Heating the sample to 100 C for a limited time.
Authors claim that exposure to both ultraviolet and
incandescent light have the same effect with respect to
sample breakdown.
As different chemical (dyes in particular) absorb at
different wavelengths, the photodegradation products
would not be expected to the be the same.
This study focused almost exclusively on methyl violet.
The photodegradation products may be the same for this
particular ink, but this observation cannot be extrapolated
to other dyes used in inks.
While this is a common ink, it does not account for all the
different pen inks available on the market, making this a
poor method to examine aging studies of inks in general.
Heating the sample would likely result in oxidation of the
compounds present on the paper, but none of these
products are accounted for in this study.
Paper is a highly complicated matrix, making samples taken
from it much more difficult to analyze.
The presented study does not account for handling and
other chemical exposures to the paper.
Analysis of Inks on Paper
The authors state that some of the samples tested required
the application of a matrix to facilitate ionization.
A standardized analysis method would be required for use
in forensic applications, but no method for determining
how this should be done is presented.
Which samples? Why is this the case?
If you don’t know the sample history, how can a decision
be made about how to analyze a sample?
Aging Studies
A study completed with samples obtained without
accelerated aging studies showed few breakdown peaks
from methyl violet.
The study completed with accelerated aging, however,
shows that rapid breakdown of methyl violet occurs.
How can it be possible to to generate a plot of
photodegradation as a function of time when there is so
much inconsistency in the rate of degradation of these
products?
This completely precludes the ability to date the
modifications to a document.
Yet another study shows that when three aliquots were
taken from different points in the same pen cartridge, the
degradation of the ink was inconsistent.
Degradation of Red Inks
Two isomeric red inks were tested, with nearly identical
mass spectra produced.
Although the spectra are quite similar, one actually gained
a methyl group in the degradation process.
This occurred after only 12 hours exposure to
incandescent light.
If methyl and ethyl group loss really is a product of aging,
how can this be explained?