Laser Desorption Ionization Mass Spectrometry of InkKatie AxwikJory SchlittBroc 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?