THE DEVELOPMENT AND INITIAL APPLICATION OF A GAS CHROMATOGRAPHIC METHOD FOR THE CHARACTERIZATION OF GUM MEDIA
SARAH L. VALLANCE, B.W. SINGER, S. M. HITCHEN, & J. H. TOWNSEND
Initially, specimens of standard monosaccharides and gum materials were studied in order to compile a library of standard chromatograms for comparison with samples from works of art. The monosaccharides used (arabinose, fucose, galactose, galacturonic acid, glucose, glucuronic acid, mannose, rhamnose, and xylose) were in chromatographically pure forms, and the gum materials used (arabic, tragacanth, cherry, guar, ghatti, locust bean, and karaya) were all in powdered form except arabic, which was obtained in both powdered and preprepared forms.
Aqueous solutions (approximately 10%) of the gums were prepared (the gums being allowed to swell in water for 24 hours before filtering to remove insoluble material), painted onto glass microscope slides, and allowed to dry naturally for at least 1 week. One set of gum samples was used unaged, while the other set was artifically aged for comparative purposes. The samples were first subjected to light aging for 4 weeks at 18,000 lux using ultraviolet filtered, daylight color rendering fluorescent tubes (custom built), then thermally aged for 4 weeks at 60°C, 55% relative humidity in a climate-controlled oven.
Replicates of these standad media, a minimum of 6 samples (both aged and unaged) from various sources, were analyzed in order to form a library of standard chromatograms to assist with identification of gum media removed from works of art.
The samples investigated were from the collection of works by William Blake at the Tate Gallery, London. With the aid of a surgical scalpel, minute samples of paint were removed from the surface of works, taking care to include as little of the support as possible. Then the samples were immediately placed in chemically clean Reactivials, which were sealed with caps containing polytetrafluoroethene-coated septa.
Standard gums (1–2 mg in Reacti-vials) and individual monosaccharide samples (<1 mg in Reacti-vials) were heated in a Reacti-Therm heating module at 105°C, for 30 minutes and 10 minutes respectively, with trifluoroacetic acid (0.1 ml, analytical grade, undiluted). The samples were allowed to cool to room temperature before pyridine (0.2 ml, analytical grade) was added to the vials.
Hexamethyldisilazane (0.3 ml, derivatization grade) and trifluoroacetic acid (5 drops, analytical grade, undiluted) were added to the samples, which were vigorously shaken for 30 seconds and then allowed to stand for 1 hour prior to analysis.
Samples removed from works of art (0.1 mm2 approximately) were placed directly into Reacti-vials then heated for 30 minutes at 105°C with trifluoroacetic acid (0.1 ml, analytical grade, undiluted) in a Reacti-Therm heating module. The samples were allowed to cool to room temperature before pyridine (0.1 ml, analytical grade) was added.
Hexamethyldisilazane (0.2 ml, derivatization grade) and trifluoroacetic acid (3 drops, analytical grade, undiluted) were added to the samples, which were then shaken vigorously for 30 seconds and allowed to stand for 1 hour before being analyzed.
A 5890 Series II Gas Chromatograph fitted with a quadrupole mass selective detector was used. The detector temperature was 280°C and the injector temperature was 250°C. The column was a DB-5 (length 30 m; internal diameter 0.25 mm; film 0.25 microns; Fisher Scientific), and helium was used as the carrier gas (pressure 2.5 psi). Program control and data processing were achieved using an MS-Chemstation with the G1034B software package.
Separation and analysis were achieved using GC-MS. A completely splitless injection (12-minute solvent delay) of sample (1 ml) was made onto the column. The 43-minute temperature program had an initial setting of 100°C, held for 2 minutes, and was then ramped, at a rate of 6°C min−1, to a final temperature of 285°C, which was held for 10 minutes.
Detection was achieved via selected ion monitoring (SIM). Parameters were as detailed below, the selected ions having been chosen as suitable for monitoring purposes after careful study of a large number of chromatograms obtaine for standard monosaccharides:
- dwell time per ion: 100 milliseconds
- ions monitored: 73.0, 147.0, 204.0, 217.0