Introduction
The use of gas chromatography – mass spectrometry (GC/MS) in the examination of writing inks for the purpose of determining the age of the writing has presented a powerful tool but has also raised numerous questions. [1, 2, 3, 4, 5, 6, 7] GC/MS is recognized throughout science as a useful and practical methodology for the analysis of volatile components. GC/MS is used routinely in forensic science to analyze toxicological samples, fire debris samples and illicit drugs. In each of these areas it was necessary to determine the limitations of GC/MS as it related to the particular analysis at hand.
GC/MS is a methodology that relies upon the interaction of a sample with the coating on a column over a range of temperatures. It is the ability to change and control temperature accurately that provides for the range of analyses that are possible. This range of useful temperature, however, also results in the deposition of material on various portions of the instrument. These materials can have a deleterious effect on the analysis, which is why it is normal practice to access this effect and perform the appropriate routine maintenance. In order to access the effects of the deposition of materials will have on an examination methodology for the determination of the age of writing ink, an investigation was undertaken that performed routine analysis of writing ink by GC/MS both before and after the performance of routine maintenance.
Methods and Materials
The GC/MS used in this investigation was an Agilent Model 6890 N Gas Chromatograph coupled with an Agilent Model 5975B Mass Selective Detector. The column used was an Agilent HP5-MS, 30 meters, 0.25 mm ID and 0.25µm film and the carrier gas was helium. Samples of PaperMate® blue and black ball pen ink were prepared by the author by writing on copy paper. Samples were removed from these papers with a 0.5 mm diameter Harris Punch and inserted into vials containing 200µl inserts. LCMS grade Acetonitrile (Fisher Scientific) was used for the preparation of standard phenoxyethanol (Sigma Aldrich) solutions, and for extraction of ink samples. Cresol (Sigma Aldrich) was used as the internal standard for all ink samples. Artificially aged ink samples were prepared by heating for 2 hours at 70º C.
The GC/MS method required an injection of 1µl of the sample of interest. Samples were injected manually, unless otherwise noted. The initial temperature was 75º C, which was held for 1.5 minutes, and the temperature was ramped to 250º C at 25º /min. The final temperature was held until 16.5 minutes had elapsed. The injector temperature was 250, the transfer line was 230° and the carrier gas flow rate was 1 ml/min. The mass spectrometer was set in positive EI and full scan mode between 50 and 400amu. It was engaged/online from 3 minutes until 13 minutes during the run.
The described GC/MS was used in the performance of listed analyses on three different occasions: initially prior to any maintenance, after maintenance to the injector, which included changing of the septum and liner, as well as removing a portion of the leading edge of the column, and after maintenance to the source, which included removal of a portion of the end of the column.
A series of PE standard solutions, (0.1, 1.0, 10 and 100 ng/µl) were analyzed using the above GC/MS methodology. This occurred initially prior to any maintenance, and then again after each maintenance procedure.
Multiple samples (5) from each of the PaperMate® writings were analyzed for the presence and quantitation of PE. Both neat samples and artificially aged samples were examined. The samples were made of 6 µplugs and the extraction volume was 5µl of acetonitrile with internal standard. The date of preparation of the writings was April 2018. The methodology used for this examination was the SLR method. [4, 5]
Results and Discussion
The results of the analysis of the PE standard solutions are listed in the following table. These values were generated by dividing the integrated peak area for the PE standard with the integrated peak area for the 100ng standard. Note the consistency both before and after maintenance indicating that the cleanliness of the instrument was ineffectual regarding the quantitation of PE.
The following table illustrates the average SLR’s for both the blue PaperMate® writings and the black PaperMate® writings. In several measurements the SLR was a negative number. As described in the literature this is due to the SLR not being a mass independent measurement, with the quantity of ink in individual samples found to vary. [7] The examination procedure is designed to minimize this factor by instructing the examiner to remove samples in close proximity to each other. [4, 5]
The examined writings were prepared in April of 2018 and the examinations were performed in June of 2018. One would expect the examination results of the SLR method to be either greater than 50, an indication of preparation date within 6 months of the examination date, or greater than 35, an indication of preparation date within 18 months of the examination date. It is clear from the data in this table that the level of cleanliness of the instrument is not the overriding factor in assessing the date of preparation of writing using this examination methodology. The presence of negative SLR’s impacts any set of data and makes a good argument for the inclusion of multiple samples when this procedure is employed.
Conclusion
The ability to reproduce a consistent relationship between multiple samples of PE standard solutions both before and after various routine maintenance procedures is demonstrated. This would indicate that any variability found in the measurement of PE is due to either the complexity of the ink in which the PE is found, or the performance of the examination methodology. It does not suggest that the cleanliness of the instrument has any impact upon these measurements. Any impact of the instrument cleanliness would involve any examination sequence, no matter the nature of the samples being examined.
The data generated regarding the examination of multiple samples of two different ink formulations illustrate the issues that must be addressed in performing the SLR Method. Precise sampling of a writing and the use of multiple samples in the examination of writings are two areas that require attention by an examiner performing this method.