The three-dimensional structure of humic substances and soil ...

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Table 2 (continued) o CH2-CH2-C~H 0H CH2-CH:-CHO H3C0 OCH 3 OCH3 OH (210) CH=CH2 (180) 22--(CH2) s-CH3 H3CO~0CH 3 ~ (218) OKcI{2__CH3(180) ~ ~CH2) 7-cn3 / ~ 0H (220) H3CO OCH 3 OH ~H2--( CH2 ) 9-CH3 1182) © (232) CH2--( CH 2 ) io-CH3 OH OK (186) ~--~H3 --( CH 2 ) ~-CH3 (246 ) (1901 ~ -(CH2) II--CH3 H O ~ 12601 1194) ~C6H14 CH2-CHO TC6HI4 H3C0 0CH 3 (264) OH (196) ~--(CH2) 12-CH3 .~--( CH2 ) 7 -CH3 (204) (274) ¢ CH2--(CH2) 13-CH3 12881 -4CH 2114-CH3 (302) -qCH 2 ) 15-CH3 1316) -~CH 2 ) 16-CH3 (330) romolecules be simulated at atomic and molecular level? Experimental Pyrolysis-fieM ionization mass spectrometry (Py-FIMS) For temperature-resolved Py-FIMS, about 100~tg of humic substances such as humic acid (HA), fulvic acid (FA), humin or 5 mg of whole soil samples, respectively, were thermally degraded in the ion source of a MAT 731 (Finnigan, 28127 Bremen, Germany) modified high-per- formance (AMD Intectra GmbH, 27243 Harpstedt, Ger- many) mass spectrometer. The samples were weighed be- Table 3. Aliphatic compounds characteristic for soil organic matter (see Table i) [11 CH2=CHCHO CH3-C0--CH 3 CH3-C00H CHa--CO--CHO CH2-C--CO--CH 3 CN3-CH--CH--CH--CH--CH 3 3 H C ~ CH3 65 (56) (58) (601 (72) (84) (96) 0 (i12) Qo 1114) 3HC-~ OH 11281 0 0 0 II II II CH3-C--CH2-C--CH2-C--CH 3 11421 ~H3 ?H3 ~H3 CH3-CH--(CH2) 3--CH--(CH2) 3-CH--CH2=CH--CH2 (224) CH CH 3 CH 3 3 i i CH3--CH--~CH2)3--CH---~CH 2 3~CH~--~CH2 2-C~3 CH3--~ CH 2 )IT"CH20H CH 3 CH 3 CH 3 CH 3 I I I CH3-CK--~CH 2 3-CN--CH 2 3-CH--CN 2 3-C=CH2 n=2-30 series CH3-~ CH 2 )~--CH3 n=i-28 series CH2=CH--( CH 2 )~--CH 3 CH3--( CH 2 )~4 COOH (226) 12281 (266) (58-450) (56-4341 (256) 14101 fore and after Py-FIMS (error + 0.01 mg) to determine the pyrolysis residue and the produced volatile matter. The heatable/coolable direct introduction system with elec- tronic temperature-programming, adjusted at the + 8 kV potential of the ion source and the field ionization emitter, was used. The slotted cathode plate serving as count- er electrode was on - 6 kV potential. Thus, at 2 mm dis- tance between the emitter tips and the cathode, in total a potential difference of 14 kV is applied resulting in an ex- tremely high electric field strength which is the essential basis for soft ionization. All samples were heated in high vacuum (1.3 × 10 -4 Pa) from 323 K to 973 K at a heating rate of approximately 0.5Ks -1. About 60magnetic
66 (CH3)o-3 (CHs)o-2 0 ~ @O ~ (CH3) 0-5 @ OH H O~'.1 oH°'f -° oH°-.-r "°H ,o, OH OH scans were recorded for the mass range 16 to 1000 Daltons. In general, at least three replicates were performed for each sample. The total ion intensities (TII) of the single spectra were normalized to 1 mg sample weight, averaged for replicate runs, and plotted versus the pyrolysis temperature, resulting in Py-FIMS thermo- grams. For the selection of biomarkers and quantitative evaluations, in particular of whole soils and soil particle- size fractions, detailed descriptions of the method have been given recently [6-9]. HO OH Curie-point pyrolysis-gas chromatography~mass spectrometry (Py-GC/MS) The humic substances and soils were pyrolyzed in a type 0316 Curie-point pyrolyzer (Fischer, 53340 Meckenheim, Germany). The samples were not pretreated except drying and milling. The final pyrolysis temperatures (Tf) employed were 573 K, 773 K and 973 K, respectively. The total heating time (THT) was varied between 3 and 9.9 s. Following split injection (split ratio 1:3; flow rate 1 ml 20 s -1) the pyrolysis products were separated on a gas chromatograph (Varian 3700, 64289 Darmstadt, Ger- many), equipped with a 30 m capillary column (DB5), coated with 0.25 gm film thickness and an inner diameter of 0.32 mm. The starting temperature for the gas chro- matographic temperature program was 313 K, and the end temperature was 523 K, with a heating rate of 10 K min -1. The gas chromatograph was connected to a nitrogen-selective, thermosensitive detector (TSD) and a double-focusing Finnigan MAT 212 mass spectrometer. Conditions for mass spectrometric detection in the electron ionization mode were +3 kV accelerating voltage, 70eV electron energy, 2.2kV multiplier voltage, 1.1 s/ OH HO HO (CH3)0-2 /.(N ~.,.,2 (CH3)0-2 H C=N OH ~0/ ~C_. ? O HO =N [~OH /CH2OH Fig. 2. Drawing of the proposed humic acid structure using Indian ink and a chemical stencil as published in Ref. [3] mass decade scan speed and a recorded mass range between m/z 50 and m/z 500. A detailed description of the principle, potential and limitations of Py-GC/MS of humic fractions and soils has been given [10]. Further- more, studies of organic nitrogen-containing compounds in soils using the TSD detector were reported [11] which expanded the knowledge on the structure of 'unknown' soil nitrogen beyond the status reported in Tables 1- 3. Structural modelling and geometry optimization For all described 2D- and 3 D-work, model construction, chemical interaction studies and semi-empirical calcula- tions, the HyperChem ® software (release 2 for windows) [12] was used which is assisted by a comprehensive manu- al. In the present text some main software commands are indicated in brackets (in italics). The program is supplied by Computer Aided Animation (CAA) GmbH, Kenner- weg 15, 72622 Ntirtingen, Germany. The employed PC consisted of the Escom local-bus tower 486DX2/66, VLB 34 in combination with 8 MB memory, Escom 17" colour monitor with AVGA VL bus/1MB graphic card, Samsung 250 MB disk, 250 MB powerstreamer, and peripheric hardware plus utility pro- grams (Escom office, Rheinstr. 41, D-65185 Wiesbaden, Germany). Results and discussion Building molecules Building a 3 D model of the 2D humic acid structure (Fig. 2) is performed in five steps using the HyperChem ® software.
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