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Mode of degradation in actual use

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mode of degradation in actual use
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Well documentes publication. The test substance is synthesized and handled as an aqueous sodium hydroxide solution (pH 13) only under inert gas atmosphere. It is instable in an aqueous phase at neutral pH or as solid material on air and oxidises rapidly to Indigo.

Data source

Reference Type:
Identification of Indigo and its Degradation Products on a Silk TextileFragment Using Gas Chromatography-Mass Spectrometry
Poulin J
Bibliographic source:
Journal of the Canadian Association for Conservation 2007;32:48-56

Materials and methods

Test guideline
no guideline available
GLP compliance:
not specified
Type of study / information:
Preparation of Textile and Paper Samples
Fibre samples of approximately 2 mm lengths were cut from a Schweppe indigo on wool reference sample. Very small samples from the warrior motif silk textile were removed by a conservator and sent to the Canadian Conservation Institute for analysis. For the yellowed tissue paper wrapping from the warrior motif textile, a strip of approximately 3 mm by 50 mmwas removed from an area of concentrated discoloration andsubsequently cut into smaller pieces. All samples were treatedwith a 1:1 solution of TMTFTH and toluene and heated at 65ºC for 45 minutes in a capped 2 mL glass vial. The samples were then centrifuged at 1500 rpm for 2 minutes and the resulting liquidpipetted into a microvial insert, replaced in the original vial and capped.

An Agilent 6890 gas chromatograph (GC) with 7683 auto injector was used with an HP-5MS capillary column ((5% phenyl)-methylpolysiloxane; 30.0 m, 0.25 mm internal diameter, 0.25 µm film thickness), interfaced to a 5973 turbo pump massspectrometer (MS) (Agilent Technologies, Inc., Santa Clara , CA95051, USA). The GC inlet temperature was set at 250ºC and the MS interface at 280ºC. The GC oven was programmed from 50ºCto 200ºC at a rate of 10ºC/min and then from 200ºC to 300ºC ata rate of 6ºC/min and a final hold time of 15 min; total run timewas 46.67 min. Ultra-high purity helium carrier gas was used witha constant flow of 1.0 mL/min. The MS was run in scan modefrom 50-500 amu, with the source and quadrapole temperaturesset at 230ºC and 150ºC respectively. The MS was operated in theelectron impact (EI) positive ion mode (70 eV). Data were processed using Agilent Chemstation software (v.D.02.00.275).

Test material

Constituent 1
Chemical structure
Reference substance name:
EC Number:
EC Name:
Cas Number:
Molecular formula:
Details on test material:
Fibre samples of indigo dyed textiles
Tissue paper wrapping of ancient indigo dyed textile

Results and discussion

Any other information on results incl. tables

Analysis by TMTFTH extraction and derivatization followed by GC-MS allowed indigo and its degradation products to be

identified on a silk Sultanate warrior motif textile fragment and its discoloured tissue paper wrapping.

Table: Compounds and Mass Spectral Data Corresponding to Indigotin and its Degradation Products Identified by GC-MS after Derivatization with TMTFTH.

Peak  Label  Compound Molecular weight  m/z (mass-to-charge ratio)Values of Characteristic Fragment Ions; % Abundance given in parenthesis  
1  anthranilic acid, methyl ester   151  119 (100), 151 (59), 92 (46), 120 (31), 65 (18)  
2  N-methyl anthranilic acid, methyl ester   165  165 (100), 105 (93), 104 (88), 132 (60), 77 (44)  
3  2,3-dihydro-2,3-dimethoxy-1-methyl-indole (isomer)   193  146 (100), 161 (32), 130 (31), 118 (28), 178 (16), 193 (13)  
4  2,3-dihydro-2,3-dimethoxy-1-methyl-indole (isomer)   193  161 (100), 118 (91), 146 (68), 132 (28), 178 (1), 193 (1)  
5  2,3-dimethoxy-1-methyl-indole   191  161 (100), 160 (92), 132 (68), 148 (65), 191 (57), 176 (46)  
6  2-benzyl-3-indolinone   223  132 (100), 164 (33), 77 (29), 105 (26), 223 (24), 104 (23)  
7  1,2-dihydro-2,4-dimethoxy-1-methyl-3,1-benzoxazine   209  146 (100), 177 (36), 209 (25), 90 (12), 120 (8)  
8  N-methyl isatin   161  104 (100), 161 (68), 105 (62), 78 (33), 133 (32), 77 (19)  
9  2-bis-(N-methylindole-3-methoxy)   320  320 (100), 305 (86), 275 (43), 290 (25), 146 (25), 145 (23)  

Methylated derivatives of the indigo oxidation products isatin, anthranilic acid and isatoic acid anhydride were identified. In addition, a new degradation product was identified as 2-benzyl-3-indolinone. The identified degradation products of indigo are smaller and evidently more volatile than the parent compound. Although analysis shows that they form and remain, to an extent, on the originating textile fibre, they also volatilise over time and deposit

on surfaces in close proximity, such as tissue paper wrappings and neighbouring fibres.

During the period of time in which the yellow discolorations were first observed, the warrior motif textile had been stored in the dark, in a humidity-controlled environment. This, of course, did not preclude the natural oxidative degradation that presumably occurred in the outer most layers of dye on the fibre. From previous research into this process, it is likely that these dark conditions slowed the rate of degradation.

One can clearly see from the photograph of the textile that it has retained its vibrant blue and dark blue hues for over 500 years, and it is likely to do so for many years into the future. The fact that blue-dyed textiles are still so prevalent in museum collections is perhaps partially a product of

the dyeing method of layering colour upon colour, a process unique to indigo, and partially due to the manner in which indigo can enter into the pores of some textile. Thus the loss of indigo due to degradation is generally not very noticeable. Analysis of natural dyes on textiles using TMTFTH derivatization and traditional GC-MS instrumentation has led to an increased understanding of the degradation of indigo and the volatile

compounds formed through the oxidative process. It has also given valuable insight into the cause and identity of yellow discolorations on materials in close proximity to indigo-dyed fibres. It is useful for conservators to understand that the discolorations originate from indigo dye on

a textile, and not from foreign chemical interaction. Because both isatin and anthranilic acid are water soluble, yellow discolorations could potentially be removed by aqueous treatments.

The analysis method described in this paper has proven to be an efficient and fast method for identifying indigo dye on textiles. As preliminary studies in our laboratory have shown, a key advantage to this procedure centres on its effectiveness in the identification of many other natural dyes

on textiles as well, including flavonoids and anthraquinones.

Therefore, when analysing textiles for dye identification, only one fibre sample and one sample preparation step is required. The sample preparation procedure is very simple, utilizing readily attainable reagents, a short extraction time, and classic GC-MS instrumentation. The TMTFTH reagent has the added advantages of determining further information on dye compound degradation, and identifying additional organic compounds.

Applicant's summary and conclusion

Indigo gegrades in light mainly to isatin, anthranilic acid and isatoic acid anhydride. However this degradation process takes also place in the dark at albeit at a slower rate.
Executive summary:

The use of m-(trifluoromethyl)-phenyltrimethylammonium hydroxide (TMTFTH) extraction and derivatization, followed by gas chromatography-mass spectrometry (GC-MS), has proven to be a simple and fast technique for the analysis of indigo dye on textiles. Not only does the procedure allow for the identification of the main dye component, indigo, but it also provides information on the degradation of indigo on textiles. Compounds formed through the reactions of TMTFTH with indigo and the main degradation products of indigo, 2-aminobenzoic acid, isatin and isatoic anhydride were investigated. In addition, a new degradation product has been tentatively identified as 2-benzyl-3-indolinone. The occurrence of these degradation products was investigated in samples obtained from an Indian textile from the Sultanate period (13th - 15th centuries) belonging to the Fine Arts Museums of San Francisco, woven, in large part, from indigo-dyed blue silk fibres. During a period of storage, pale yellow discolorations gradually appeared on tissue paper used to wrap the textile. Analysis of the tissue paper determined an abundance of indigo degradation products, in particular anthranilic acid (2-aminobenzoic acid). These compounds formed on the textiles fibres through oxidative degradation and subsequently volatilised to the surface of the paper, and also to the neighbouring fibres.