1. National Institute of Standards and Technology, Gaithersburg, Maryland
GC-MS determination of environmentally important phenols and their thio-analogs as chemical modification products
Levan A. Megutnishvili, Nino G. Todua, Stephen E. Stein, Anzor I. Mikaia
National Institute of Standards and Technology, Gaithersburg, Maryland
OVERVIEW
RESULTS AND DISCUSSION
Novelty
Initial compound
Derivative
[M-R] +
[M-R-CH3] +
Guaiacol
TMS
181 (26)
166 (100)
TBDMS
181 (78)
5-Methylguaiacol
195 (27)
180 (100)
195 (76)
4-Fluoroguaiacol
199 (24)
184 (100)
199 (49)
4-Bromoguaiacol
259/261 (32/32)
244/246 (100/100)
259/261 (63/63)
Syringol
211 (41)
196 (100)
211 (83)
2,5-Dimethoxyphenol
211 (11)
211 (39)
Isovanillyl alcohol
1-TMS
209 (22)
194 (100)
2-TMS
283 (23)
268 (41)
1-TBDMS
209 (90)
2-TBDMS
325 (100)
310 (3)
Vanillol
211 (33)
283 (43)
268 (58)
211 (70)
310 (.2)
Isovanillin
o-Vanillin
209 (100)
194 (32)
5-Aminoguaiacol
O-TMS
196 (15)
181 (100)
268 (40)
253 (100)
O-TBDMS
196 (11)
310 (15)
295 (100)
5-Nitroguaiacol
226 (75)
211 (100)
226 (100)
211 (91)
Normetadrenaline
O,O’-TMS
312 (3)
297 (100)
3-TMS
384 (5)
369 (.1)
4-TMS
456 (5)
441 (.4)
3-TBDMS
468 (3)
453 (0)
2-Methoxy-3-methyl-1,4-benzenediol
283 (3)
268 (100)
325 (36)
310 (100)
A GC-MS method for the determination of substituted phenols is developed.
It includes an employment of preliminary derivatization procedures.
Trifluoroacetates
Introduction. Phenols are largely present in plants and agro industrial byproducts, and they represent secondary metabolites in foods and natural health products. The food processing industries generate considerable quantities of various phenolic byproducts and the oil industry uses various phenols for production of oil field additives, fragrances, stabilizers and antioxidants for fuels and plastics. Identification of alkylphenols and differentiation of isomers in this series by GC-MS can be achieved with an employment of suitable chemical modification methods. A comparative study of trialkylsilyl and perfluoroacyl derivatives of tert-butylphenols has been conducted.
EXPERIMENTAL
Methods
Chemicals. Trialkylsilyl and perfluoroacyl derivatives were prepared for poly-substituted phenols (I-III). The initial chemicals, derivatization agents and solvents were commercially available.
Micro-synthesis. Well established reactions of the phenolic oxygen with bis(tert-butyldimethylsilyl) and bis(trimethylsilyl)trifluoroacetamides are employed for the synthesis of corresponding ethers. For micro- synthesis of perfluoroacyl phenol derivatives the reactions with trifluoroacetic, pentafluoropropionic and heptafluorobutyric anhydrides are utilized. The reactions of phenolic hydroxyls with methyl, ethyl, n-propyl and isopropyl chloroformates are used for the acquisition of corresponding alkoxycarbonyl derivatives.
Instrumentation. EI mass spectra were recorded on GC/MS systems with quadrupole analyzers (ionization energy 70 eV and ion source temperature 230 oC). Separation was achieved on a fused silica capillary columns (15m, 0.25mm i.d.; non-polar stationary liquid phase: polymethylsiloxane + 5% phenyl groups) and (60m, 0.25mm i.d.; non-polar stationary liquid phase: 100% dimethyl polysiloxane) with programming oven temperatures from 60 oC to 270 oC (40 min) at a rate of 10 oC/min and 60 oC to 270 oC (20 min) at a rate of 15 oC/min ; the injection temperature was 270oC.
The loss of water molecule from [M-CH3]+ ions is characteristic for trifluoroacetates of phenols containing tert-butyl at position 2.
This decomposition direction is also typical for pentafluoropropionates and heptafluorobutyrates of ortho-substituted isopropyl-, sec-butyl- and neopentyl-phenols.
Fig. 1 Mass spectra of ortho (A) and para (B) isomers of trifluoroacetyl derivatives of 4-tert-butylbenzenethiol
Trialkylsilyl esters
CONCLUSIONS
Trialkylsilylation and perfluoroacylation of phenols yield corresponding derivatization products with unique mass spectral characteristics. The spectra of derivatives allow reliable differentiation of ortho- phenols due to the presence of dominant peaks of diagnostically important ions.
Consecutive elimination of an alkyl radical and a water molecule is characteristic for trifluoroacetyl, pentafluoropropionyl and heptafluorobutyryl phenols containing branched alkyl substituents at ortho position.
Successive loss of two alkyl radicals in violation of the ‘even electron rule‘ is specific for the vicinal trimethylsilyl and tert-butyldimethylsilyl esters of methoxyphenols.
This process appears universal for TMS and TBDMS derivatives of 2-methoxyphenols, and results in the formation of dominant peaks in the spectra.
The exceptions are tri- and tetra-trialkylsilyl derivatives of normetadrenaline , and dialkylsilyl derivatives of isovanillyl and vanillyl alcohols due to more favorable fragmentation processes.
Fig. 2 Mass spectra of trimethylsilyl derivatives of 2,6-dimethoxyphenol (A), 2,6-dimethoxyphenol (B), 3,4-dimethoxyphenol (C), 2,5-dimethoxyphenol (D)