Environ. Sci. Technol. 1907, 21, 349-357
Time-Resolved Pyrolysis Field Ionization Mass Spectrometry of Humic Material Isolated from Freshwater Hans-Rolf Schulten Department of Trace Analysis, Fachhochschule Fresenius, 0-6200 Wlesbaden, Federal Republic of Germany
Gudrun Abbt-Braun and Fritz Hartmann Frlmmel* Instltut fur Wasserchemie und Chemische Balneologie, Technische Universitat Munchen, D-8000 Munchen 70, Federal Republic of Germany
(1)It gives a summerized overview of the thermal degradation products of biomacromolecules that can be utilized for characterization and classification. The reproducibility and significance of the data obtained enable statistical evaluations by pattern recognition (8-10). (2) Thermally resolved degradation leads to ion formation, which can be related to the thermal degradation behavior of the original sample and thus gives insight on the structure of humic substances. (3) An assignment of natural biomacromolecules as precursors for HUS can be derived from the comparison with results of the application of FI-MS on well-defined natural products (11). (4)A fruitful comparison is possible with the results from other analytical techniques like EI-MS, CI-MS, 13C NMR, IR and X-ray or fluorescence UV-vis, etc. This study used humic material isolated from freshwater systems including standard samples from the International Humic Substances Society (IHSS) and reaction products of oxidative degradation and chemical derivatization. FI-MS is used to get structural information on the thermal behavior of the HUS and their degradation products. The data obtained are compared with GC/EI-MS and CI-MS results for similar humic materials.
rn The thermal degradation of aquatic humics in combination with mass spectrometric analyses using field ionization (FI) as a soft ionization method is reported. Since the FI mass spectra of the pyrolysis (Py) products consist almost exclusively of molecular ions and mass spectrometric fragmentation even of relatively polar substances is strongly reduced, the characterization of biomacromolecules is facilitated and the identification of direct, primary building blocks is achieved. Furthermore, the thermal behavior of characteristic classes of humic constituents is investigated by time/ temperature-resolved FI mass spectrometry (MS) with the commercially available directprobe introduction system. A general characterization of the Py products of humics and the ratios of volatilized matter and residual char are given. These methods permit insight into the thermal degradation processes of complex polymeric biomatter. In contrast to the humic acid (HA) investigated, for fulvic acid (FA) a larger amount of pyrolysates is formed, indicating less thermal stability. The Py mass spectra of the HA and FA show monomers and higher aggregated chemical species of furan, phenol, methoxyphenol, and dimethoxyphenol subunits. Chemical reactions such as alternating methylation and oxidation steps of FA lead to characteristically different results: in the Py mass spectrum of the methylated-oxidized FA the m / z values of benzenecarboxylic acids, methoxybenzenecarboxylic acids, and monobasic aliphatic acids show high intensities, whereas methyl esters of benzenecarboxylic acids and aliphatic dibasic acids dominate in the case of the mass spectrum of the oxidiqed-methylated FA. _____
~
~~
~
~~
~
Introduction Humic substances (HUS) in aquatic systems are of general interest because (1)they play an important role in ecosystems as the major part of the dissolved organic carbon (DOC) and as effective interaction agents for many inorganic and organic pollutants (I,2) and (2) they are the most important precursors for halogenated compounds in chlorinated drinking water (3). The question of the molecular structure of HUS is still unanswered, even though many contributions have been made assigning special characteristics ( 4 ) . Mass spectrometric investigations using electron impact (EI) and chemical ionization (CI) have focused mainly on the gas chromatographic fraction containing substances with up to 18 carbon atoms gained from samples after degradation and derivatization (4-6). In addition to the powerful tool of gas chromatography/mass spectrometry (GC/MS) coupling, thermal degradation with mass spectrometric characterization has been utilized (7). The advantages of the newly developed version of time/temperature-resolved field ionization (FI) mass spectrometry for the Characterization of aquatic humics are as follows: 0013-936X/87/0921-0349$01.50/0
Experimental Methods Aquatic Humic Substances Preparation. Aquatic humic materials were isolated from a bog lake near the Brunnensee (BM), in south Bavaria. A modification of Mantoura and Riley's XAD-2 method (12) was used to obtain humic material from the brown water as described previously (13). The eluted HUS were separated into humic acid (HA) and fulvic acid (FA) fractions by precipitating the HA at pH 2 (HC1; 24 h in the dark). The dissolved FA (m/z 170) as revealed in the FI mass spectra cannot be matched with the reported components of HUS described in Table 11. Due to the pyrolysis procedure employed here and soft ionization MS, it should be possible to detect not only the basic subunits of HUS structure but also higher molecular building blocks containing two or more basic units. Haider and Schulten (11)showed by using time-resolved pyrolysis FI-MS that dimeric guaiacyl fragments occur from lignins at m/z 260,272,284,298,312, 326,342,358, and 372 and thus gave first evidence for the direct mass spectrometric detection of higher, aggregated structural subunits of soil polymers. The following linked molecules can explain some of the more abundant mass numbers: (a) alkylphenol-alkylphenol units, e.g., at m / z 214, 228, 238, 252, 266, 268, 280, 282, and 294; (b) methoxyphenol-alkylphenol units, e.g., at m/z 216, 230, 242, 256, 270, 284, 296, and 310; (c) dimethoxyphenol-dimethoxyphenol units, e.g., at m/z 304, 306,318,320, and 332; (d) furan-furan units, e.g., at mlz 162, 164, 176, 178, 190, 192, and 204; (e) furan-dimethoxyphenol units, e.g., at m/z 220, 234, 248, 260, 274, 288, and 300; (f) alkylphenol-alkylphenol-alkylphenol units, e.g., at m/z 346, 360, 372, 374, 388, and 390; (g) alkylphenol-alkylphenol-methoxyphenol units, e.g., at m/z 336, 350, 364, 376, 390, and 402. Temperature-Resolved Pyrolysis. Thermogravimetric experiments of soil HUS (28,29) have shown that Environ. Sci. Technoi., Voi. 21, No. 4, 1987
351
Table 11. Py-MSa n d Substances
Py-GCData from Soil a n d Aquatic
class of compds polysaccharides furans
mlz
68 82 84 96
dihydroxybenzenes
ref
110
furan methylfuran hydroxyfuran furfural, dimethylfuran dihydropyrone, furfuryl alcohol methylfurfural
7, 11, 20-24 7,20-24 7, 20, 21, 24 7, 11, 20, 21, 23-26 7, 11, 20-24, 26 7, 11, 20, 21, 24-26
94 108
phenol cresol
122
xylenol
136 150 110 124
C3-alkylphenol methoxyvinylphenol dihydroxybenzene dihydroxymethylbenzene
7, 11, 20-26 7, 11, 20, 21, 23-26 7, 11, 20, 21, 25, 26 7, 11, 20, 21 7, 11, 20, 21, 24 7, 11, 20, 21 7, 11, 20, 21, 24
78 92 106 120 134 148 162
benzene toluene Cz-alkylbenzene C3-alkylbenzene C,-alkylbenzene C6-alkylbenzene C6-alkylbenzene
7, 20-25 7, 20-25 7, 20, 21, 23-25 7, 20, 21 7, 21 7 7
94 108
phenol cresol
120
vinylphenol
122
xylenol
134 136 148 124
methylvinylphenol C8-alkylphenol Cz-alkylvinylphenol 2-methoxyphenol
138
2-methoxymethylphenol 2-methoxyvinylphenol Cz-alkyl-2-methoxyphenol allylmethoxyphenol
7, 11, 20-26 7, 11, 20, 21, 23-26 7, 11, 20, 21, 24, 25 7, 11, 20, 21, 25, 26 7, 11, 20, 21 7, 11, 20, 21 7, 11, 20, 21 7, 11, 20, 21, 24, 25 7, 11,20, 21, 24-26 7, 11, 20, 21, 24, 25 7, 11, 20, 21, 24 7, 11, 20, 21, 24,25 7, 11, 20
98
phenol precursors alkylphenols
tentatively assigned compd
(0H)p
aromatic @R hydrocarbon precursors alkvlbenzenes
lignins alkylphenols
methoxyahenols
150 152 164 166
dimethoxyphenols
178 154 168 180 182
194 196 210 lipids alkenes Cnbn
352
C3-alkyl-2-methoxyphenol coniferylaldehyde 2,6-dimethoxyphenol 2,6-dimethoxymethylphenol coniferyl alcohol, 2,6-dimethoxyvinylphenol Cz-alkyl-2,6dimethoxyphenol 2,6-dimethoxypropenylphenol 2,6-dimethoxypropylphenol sinapyl alcohol
56, 70, 84, C,-C8-alkenes 98, 112 54. 68, 82. C&-alkadienes 96, 110
Environ. Scl. Technol., Vol. 21, No. 4, 1987
7, 11, 20 7, 11, 20, 24-26 7, 11, 20, 25 7, 11, 20 7, 11, 20
7, 11, 20 7,11 7, 11
7, 20-22, 25 7, 20-23
two steps of thermal degradation can be observed. Pyrolysis at temperatures at
