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magnesium salts of euxanthic acid have been known for a long (Steinheim, Germany). Dimethyl sulfoxide (DMSO) (reagent
period of time as a kind of “mysterious” colorant named the ACS) and n-hexane (anhydrous, 95%) were obtained from Merck
Indian Yellow, which came probably from Persia to India KGaA (Darmstadt, Germany). Ethyl acetate, dichloromethane,
th
around the 15 century, and reached Europe through the Dutch hydrochloric acid, tartaric acid, oxalic acid, citric acid, acetyla-
th
trade in East Indies not later than in the 17 century. The cetone and thionyl chloride (analytical grade) were purchased
Indian Yellow, being quite transparent, was said to be most from POCh (Gliwice, Poland). Hydroquinone, 2,6-dihydrox-
suitable for the watercolor and tinted drawings, yet it was also ybenzoic acid and diphenyl ether were obtained from Lancaster
th
used in the oil technique. In the late 19 century it was believed (Eastgate, England). All aqueous solutions were prepared using
that the colorant was made from the urine of cows fed with deionized Milli Q water.
mango leaves and its use was prohibited for humanitarian
reasons in 1908.
Synthesis of 1,7-dihydroxyxanthone (euxanthone)
Most of the natural dyestuffs are bound to a mordant thus
forming a dye lake through co-precipitation with an inorganic
substrate by strong chelate bonds. The process of dyes extrac-
tion is yet more complicated by presence of aged binding
media, varnishes, and other constituents of the samples. The
conventional methods of extracting dyes from lake paints are
based on utilization of strong inorganic acids such as H SO
Methyl 2,6-dihydroxybenzoate has been prepared according to
17
ꢀ
1
the reported procedure. Yield 55%, mp 57–60 C; H NMR d
H
(
200 MHz, MeOD-d ) 4.05 (3H, s, Me), 6.45 (2H, dd, J 9.2; 1.6,
4
Ar-H), 7.30 (1H, m, Ar-H).
Euxanthone has been synthesized in accordance with the
18
modied procedure reported by Trivedi et al. A mixture of
2
4
hydroquinone (1.62 g, 0.015 mol), methyl 2,6-dihydrox-
ybenzoate (2.5 g, 0.15 mol) and diphenyl ether (7.5 mL, 0.05
mol) was reuxed under an atmosphere of nitrogen for
and HCl at elevated temperatures in order to facilitate hydro-
lysis of metal–dye complexes, which are very stable and difficult
to decompose. Unfortunately, the strong mineral acids not only
hydrolyze the metal–dye complex, which is a desirable reaction,
but also decompose glycosidic dye components into their
48 hours. The removal of solvent by distillation under reduced
pressure gave brown solid which was puried by ash chro-
matography (SiO2, n-hexane/AcOEt (85 : 15, v/v)). Further
recrystallization from methanol gave a yellow crystals (yield
4,5
parent aglycons. Another negative effect of the use of the
strong acids for isolation of a dye from the lake matrix are its
ꢀ
1
5
.1%, mp 236–239 C; H NMR d (200 MHz, acetone-d ) 6.85
H 6
6
decarboxylation (e.g. pseudopurpurin to purpurin) and dehy-
(
(
1H, dd, J 8.1; 2.2, Ar-H), 6.92 (1H, dd, J 8.3; 2.2, Ar-H), 7.01–7.03
2H, m, Ar-H), 7.38 (1H, t, J 8.18, Ar-H), 7.55 (1H, d, J 1.9, Ar-H),
7
dration. Although this procedure can at times be actually quite
useful for identication of the aglycone part of the molecule, the
inadvisable reactions accompanying the extraction alter the
original composition of coloring substances making the iden-
tication of the dye problematic. Alternative method of dis-
rupting the paint medium and solubilizing the dyestuffs based
1
2.7 (2H, br, OH)).
Equipment
The analysis were performed using Agilent LC-MS/MS liquid
chromatograph series 1290 (Agilent Technology, Waldbronn,
Germany) consisting of binary pump G4220A, autosampler
G4226A, thermostated column compartment G1316C, diode-
array detector G1315C, and triple quadrupole mass spectrom-
eter G6460 with AJS electrospray ionization source. The chro-
matographic system was controlled with Agilent MassHunter
soware version B 06.01. For conrmation of identied
dyestuffs structures HPLC-QTOF-MS analysis were carried out
using an Agilent 1290 HPLC system coupled to a hybrid quad-
rupole time-of-ight (QTOF) mass spectrometer G6540 with
Dual ESI interface operated in scan mode in the same chro-
matographic conditions.
3
on methylation of carboxyl moieties in the dyes with a BF /
8
MeOH mixture was also used. Mild extraction procedures with
use of chelating agents such as oxalic or ethyl-
enediaminetetraacetic acid (EDTA) and of moderate strength
acids have been tried on textiles dyed by various authors but
have generally been found less efficient than the reference HCl
9
–11
method.
Lastly, J. Sanyova et al. used hydrouoric acid for
12,13
extraction of mordant-type dyestuffs.
HF is both a weak acid
and a strong aluminium-complexing agent, and it preserves
glycoside bonds. Very limited systematic studies on dyes
extraction methods, mainly from textile bers, have been
14–16
reported in the literature.
To the best of our knowledge, no
Microscopic photographs were performed using the Optical
microscope Nikon, Optiphot-2. The SEM images were carried
out using the scanning electron microscope, type 1430 VP (LEO
Electron Microscopy Ltd., England), equipped with detectors of
secondary electrons (SE), backscattered electrons (BSE) and an
energy dispersive X-ray spectrometer (EDX) Quantax 200 with a
detector XFlash 4010 (Bruker AXS Microanalysis GmbH,
Germany).
systematic study has been performed to compare the efficien-
cies of different protocols on the Indian Yellow dyestuff's
extraction from the paint binder. For this reason, nine different
extraction methods were systematically examined and
compared.
Experimental
Chemicals and standards
Sample
Acetonitrile and methanol used as mobile phase components
were of HPLC grade and were purchased from Merck (Darm- The historical sample of a naturally aged paint dating back to
th
stadt, Germany). Formic acid, triuoroacetic acid and hydro- the 19 century has been kindly offered by the National
uoric acid (48% in water) were purchased from Sigma-Aldrich Museum in Krakow, Poland. The oil paint tube had formerly
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 48786–48792 | 48787