Characterization of Birch Wood Pyrolysis Oils by Ultrahigh-Resolution

Article pubs.acs.org/EF

Characterization of Birch Wood Pyrolysis Oils by UltrahighResolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: Insights into Thermochemical Conversion Timo Kekal̈ aï nen, Tapani Venal̈ aï nen, and Janne Jan̈ is* Department of Chemistry, University of Eastern Finland, Post Office Box 111, FI-80101 Joensuu, Finland S Supporting Information *

ABSTRACT: A laboratory-scale reactor was used to produce pyrolysis oils from Finnish silver birch hardwood (Betula pendula). The resulting wood distillates were characterized by ultrahigh-resolution (12 T) Fourier transform ion cyclotron resonance (FTICR) mass spectrometry coupled with negative-ion electrospray ionization (ESI). Two different pyrolysis temperatures were tested: 300 and 380 °C (the resulting oil samples were named as Oil-300 and Oil-380, respectively). The detected species were sorted on the basis of heteroatom class, carbon number, and double bond equivalent (DBE). About 1200 and 1400 unique compounds were identified from the ESI FT-ICR spectra of Oil-300 and Oil-380, respectively. These were mainly oxygencontaining compounds (Ox heteroatom classes, with x = 2−14), comprising up to 90% of all identified compounds. The compounds in the O2, O3, and O4 classes comprised of different fatty acids, hydroxy/epoxy fatty acids, and diacids. The compounds in the O5−O8 classes comprised of mainly lignin degradation products and phenolic extractives. The compounds in the O9−O14 classes comprised of both low- and high-DBE compounds. Upon increasing the temperature from 300 to 380 °C, many compounds showed an overall decrease in their DBE and carbon number. The distribution of fatty acids in Oil-300 qualitatively matches the known lipid-derived fatty acid composition of silver birch. At a higher pyrolysis temperature (380 °C), hydrogenation of unsaturated C18 fatty acids toward fully saturated compounds was observed.

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INTRODUCTION As the world’s fossil fuel resources are diminishing, the use of biomass to generate renewable biofuels is gaining increasing interest. Biomass as the renewable energy resource includes, e.g., wood and forest residues, agricultural residues, algae, and overall, all organic waste material. Thermochemical decomposition of organic matter in the absence of oxygen, also known as pyrolysis, can be used to convert biomass into liquid, gaseous, and/or solid fuels. The liquid product is called pyrolysis oil (or pyrolysis liquid),1 which is the cheapest liquid fuel made from wood. The energy content of pyrolysis oil (ca. 15−20 MJ/kg) is nearly the same as that of wood and roughly half of the light/heavy fuel oil.2 Pyrolysis oil can be directly used to substitute heavy fuel oil in average-sized heating boilers, but it cannot be mixed with conventional transportation fuels. A high water content (up to 30 wt %) decreases the heating value of pyrolysis oil and makes it immiscible with hydrocarbons.2,3 The main methods to produce pyrolysis oils are slow, intermediate, and fast pyrolysis, which differ mainly by the hold time of the solid (typically 1−2 s in fast pyrolysis, while up to several hours in slow pyrolysis). Different amounts of gas, liquid, and solid (charcoal) are produced; the largest fraction of liquid is produced by fast pyrolysis (up to 75%). Thus, it is the main method for commercial pyrolysis oil production. In contrast, slow pyrolysis is normally applied in charcoal production or for more tailored pyrolysis experiments. In pyrolysis, depolymerization of the main wood constituents, cellulose, hemicellulose, and lignin, generates a lot of reactive, small and large oxygen-containing species.4 Thus, wood-based pyrolysis oil is a complex mixture of water, tar, and organic compounds, such as acids, alcohols, aldehydes, ketones, © 2014 American Chemical Society

phenolics, sugars and sugar derivatives, and different wood extractives.4−6 The intrinsic acidity and complex chemistry of pyrolysis oils result in the oil instability. For example, aldehydes and ketones can react via aldol condensation to produce oligomeric species. These reactions cause changes in the physicochemical properties, such as increasing viscosity. Phase separation may also occur, resulting in two (or even three) distinct phases.7 In addition, acidity makes pyrolysis oils highly corrosive; pH is typically between 2 and 3 because of a large amount of different acids present, mainly formic and acetic acids as well as fatty and resin acids.8 However, pyrolysis oils are virtually free of sulfur, are carbon-neutral, and could be catalytically upgraded similar to fossil fuels, which makes them among the most attractive liquid biofuels in the future. Compositional studies on pyrolysis oils have resulted in a myriad of research papers in recent years. Knowledge of the composition of pyrolysis oils is of great importance for better understanding pyrolysis processes themselves as well as further refinement of the crude pyrolysis products. Process conditions and feedstock variations affect directly to the chemical composition and overall physicochemical properties of pyrolysis oil. Several conventional analytical methods have been applied in these studies, including thermogravimetry (TG),9,10 differential scanning calorimetry (DSC),11 Fourier transform infrared spectroscopy (FTIR),12,13 nuclear magnetic resonance (NMR),14,15 and ultraviolet/fluorescence spectroscopies.16,17 Despite the advanced methodologies, these Received: April 15, 2014 Revised: June 23, 2014 Published: June 23, 2014 4596

dx.doi.org/10.1021/ef500849z | Energy Fuels 2014, 28, 4596−4602

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the Supporting Information). The wood pieces (about 5 mm in diameter) were pre-dried 3 days in the air at 105 °C. The reactor was heated to the target temperature, and dried wood pieces (ca. 10 g per run) were dropped on a glass wool and placed in the top part of the pipe. The reactor was constantly flushed with nitrogen gas (50 mL/ min) during heating. The pyrolysis was performed at two different temperatures: 300 and 380 °C (the resulting oils were named as Oil300 and Oil-380, respectively). Relatively low pyrolysis temperatures were selected to prevent extensive carbonization of the starting material and further chemical changes of the initially released wood components, especially interesting extractives. At 300 °C, mainly cellulose and hemicellulose are degraded, while at 380 °C, the lignin degradation rate increases considerably. Evaporated vapors were condensed in a receiver vial, placed in an ice bath underneath the oven. The heating was applied for 10 min, after which the product formation was no longer observed. The hold time in the current reactor setup is estimated to be around a few minutes; thus, the conducted pyrolysis experiments are considered to be in the intermediate pyrolysis regime. The total liquid yield was ca. 1−2 mL per run. Mainly carbon dioxide and carbon monoxide were detected as the non-condensable gases by gas chromatography/mass spectrometry (GC/MS). The resulting pyrolysis oils were collected for further analysis. Elemental Analysis. Bulk elemental analysis was performed to determine the CHNOS content of the oil samples. All experiments were performed using a Vario MICRO Cube V1.7 instrument (Elementar Analysensysteme GmBH, Hanau, Germany). The amount of oxygen was defined as the difference of 100% − (CHNS). Mass Spectrometry. For negative-ion ESI FT-ICR MS analyses, the pyrolysis oil samples were dissolved in methanol to create measurement solutions at a concentration of 250 μg/mL. Ammonium hydroxide (28%) was added to the samples (1.0 vol %) to enhance ionization. All solvents used were high-performance liquid chromatography (HPLC)-grade. All measurements were performed with a hybrid quadrupole FT-ICR mass spectrometer (APEX-Qe, Bruker Daltonics, Billerica, MA), equipped with a 12 T superconducting magnet, an Infinity ICR cell, and an Apollo-II ESI source. The samples were directly infused at a flow rate of 1.5 μL/min. Dry nitrogen was used as drying and nebulizing gas. ESI-generated ions were externally accumulated for 1 s in the hexapole ion trap before being transferred to the ICR cell for trapping, excitation, and broadband (m/z 200− 1000) detection. A total of 500 time-domain transients of 2 Mword each were co-added and zero-filled twice (to provide final 8 Mword magnitude-mode data). The instrument was controlled, and the data were acquired with the use of Bruker XMASS 7.0.8 software. Mass spectra were first calibrated externally using sodium trifluoroacetate (STFA) ion clusters41 and recalibrated internally according to the known and highly abundant Ox class compounds, providing rootmean-square (rms) error below 220 ppb. DataAnalysis 4.0 software (Bruker Daltonics) was used to assign the peaks appearing at a signal-to-noise (S/N) greater than five standard deviations of the baseline rms noise (5σ). For molecular formula assignment, parameters were set as follows: double bond equivalent (DBE), 0−50; mass error, ±0.8 ppm; elemental formulas, 12C and 1H unlimited, 14N0−4, 32S0−2, 16O0−25, 23Na0−1, and 13C and 34S atoms were also taken into account; and only even-electron ions were considered. The data sorting and visualization were accomplished using Microsoft Excel (Microsoft Corporation, Redmond, WA) and SigmaPlot 12.0 software (Systat Software, Inc., San Jose, CA).

methods give only a rough overview about the bulk chemical composition (i.e., functional groups) and generally do not allow for identification of individual compounds. More detailed analysis has been performed via gas chromatography (GC) or gas chromatography−mass spectrometry (GC/MS).5,18 Thus, chemical composition of pyrolysis oils is only limitedly known; for example, in the recent work by Vispute et al., almost 50% of the compounds was unidentified for the water-soluble fraction of pinewood fast pyrolysis oil and 30% still remained unidentified after a two-stage catalytic hydrodeoxygenation.6 In particular, compounds of higher molecular mass have been out of range of conventional analytical methods, and also, the polarity and/or non-volatile nature of the compounds pose considerable challenges. In general, only those compounds that are found from the spectral database are unequivocally identified. Different solvent fractionation procedures have also been applied to extract different types of analytes in bio-oils, but these methods suffer from poor specificity.2 Ultrahigh-resolution mass spectrometry, such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS),19 has shown unparalleled performance in molecular fingerprinting of fossil fuels in many petroleomics studies.20−22 More recently, FT-ICR MS has also been applied to characterize different bio-oils.23−34 With FT-ICR MS, it is possible to detect and identify tens of thousands of different chemical constituents without chromatographic preseparation. A commonly used ionization technique, electrospray ionization (ESI),35 selectively ionizes polar, heteroatom-containing compounds, such as highly oxygenated species, thus being well-suited for these studies. A possibility to interface FT-ICR instruments with a number of other different ionization methods, such as atmospheric pressure photoionization (APPI) or atmospheric pressure chemical ionization (APCI), considerably increases the applicability of the method.30−33 Currently, high-resolution mass spectrometry with two-dimensional gas chromatography (GC × GC)36 represents the most promising combination for detailed chemical characterization of complex bio-oil samples.37,38 While pine wood is almost exclusively used for commercial production of (fast) pyrolysis oils, birch wood has traditionally been used for production of charcoal as well as different liquid products (e.g., methanol and acetic acid) through slow pyrolysis.1 Birch bark has already been used in the Middle Paleolitic (ca. 80 000 years ago) to produce birch tar (pitch). Birch wood vinegar (a water-soluble fraction of slow pyrolysis oil) has also shown high potential as a biodegradable pesticide.1,39 Moreover, birch bark suberin contains a large amount of interesting chemicals, such as rare hydroxy/epoxy fatty acids, that could be used as starting materials for cosmetics, biopolymers, or other specialty chemicals.40 Previously, Fagernäs et al. have analyzed overall chemical composition of Finnish silver birch (Betula pendula) slow pyrolysis liquids using different physicochemical and analytical methods (GC and GC/MS).5 The main aim of our work was to obtain a broader view on the compounds present in the birch wood pyrolysis oils by ultrahigh-resolution ESI FT-ICR MS.

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RESULTS AND DISCUSSION ESI FT-ICR Mass Spectra of Birch Pyrolysis Oils. Negative-ion ESI FT-ICR mass spectra of the birch wood pyrolysis oil samples are shown in Figure 1. The mass resolution (defined as m/Δm50%, where Δm50% is the peak full width at half-maximum peak height) was around 500 000 at m/z 300, which is close to the maximal theoretical value at a zero-pressure limit.19 This resolution is sufficient for resolving most isobaric ions present in complex bio-oils (see Figure S2 of

EXPERIMENTAL SECTION

Pyrolysis Experiments. The pyrolysis oil samples were produced from Finnish silver birch hardwood (B. pendula) using a laboratoryscale pyrolysis reactor. The reactor consisted of a vertical half-inch diameter quartz pipe placed in the central part of the heating oven (Cetal Four 300/3, PSR-Sotelem B.V., Netherlands) (see Figure S1 of 4597


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identified compounds in both oil samples. The rest of the identified compounds belonged to the NyOx, SzOx, and OxNam classes, albeit present only in very minor amounts. This result is consistent with the bulk elemental analysis (Figure 3), which

Figure 3. Elemental compositions (CHNOS) of produced pyrolysis oils. (∗) Oxygen by difference of 100% − (CHNS).

indicated that oils have very high oxygen content (62 and 55% for Oil-300 and Oil-380, respectively), only a very little amount of nitrogen, and no detectable sulfur present (detection limit > 0.1 p-%). It must be stated, however, that the results cannot be directly compared. First, the oxygen content determined by elemental analysis also includes oxygen from water, which should be corrected to obtain bulk elemental composition in dry weight. The oxygen content up to 62% suggests a high water content. It has been estimated that pyrolysis oils typically contain about 15−40% water.2,3 Second, because negative-ion ESI ionizes only polar heteroatom-containing compounds and, furthermore, the smallest compounds, such as small organic acids and aldehydes, are not efficiently ionized, the bulk elemental formula of the oil and the average sum formula of the detected compounds may differ. The ionization efficiency may vary considerably between analytes; i.e., signal abundance for a given analyte is dependent upon its ESI response factor. It is the highest for intrinsically acidic compounds (in negative-ion ESI), containing COOH and OH groups. Thus, the present results provide means for relative quantitation only. The pH values of the sample oils were ∼3, typical for most fast and slow pyrolysis oils. Ox Heteroatom Classes. A total of 13 different Ox classes were detected, namely, the O2−O14 classes (O14 observed only for Oil-300). For Oil-300, the most abundant Ox classes were O7, O8, O9, and O6, whereas for Oil-380, they were O6, O7, O5, and O8 (Figure 4). These results are consistent with some previous reports.27,28 A clear increase in the relative abundance of the O2, O4, O5, and O6 classes was observed upon increasing the pyrolysis temperature from 300 to 380 °C. In contrast, a decrease in the abundance of the O8−O13 classes was observed. This is consistent with the decrease in the average m/z observed in the mass spectra (Figure 1). To obtain a more detailed picture on the chemistry of each individual Ox class, isoabundance-contoured DBE versus carbon number (nC) plots were generated for each heteroatom class. Figure 5 shows DBE versus nC plots for the O2−O13 classes detected in the Oil-300 sample. The most abundant species in the O2, O3, and O4 classes were found with low DBE values (DBE = 2−4). The carbon number distributions for the O2, O3, and O4 classes

Figure 1. Broadband 12 T negative-ion ESI FT-ICR mass spectra of birch wood pyrolysis oil samples: (A) Oil-300 and (B) Oil-380.

the Supporting Information).26 More than 3000 peaks were observed in both mass spectra (at S/N ≥ 5σ). After deisotoping and molecular formula assignment, about 1200 and 1400 unique heteroatom-containing compounds could be identified from Oil-300 and Oil-380, respectively. For Oil300, most of the peaks appeared within the m/z range of 250− 500, whereas the peaks for Oil-380 appeared clearly shifted to the lower m/z (i.e., m/z ∼ 200−450) (Figure 1). Heteroatom Class Distributions. Figure 2 shows the heteroatom class distributions for Oil-300 and Oil-380. The Oxcontaining compounds comprised more than 90% of all

Figure 2. Relative abundance of heteroatom-containing compounds (Ox, NyOx, SzOx, and OxNam classes) identified in the pyrolysis oil samples by negative-ion ESI FT-ICR MS. 4598


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likely phenolic derivatives. The decomposition of lignin mostly produces phenolic species, such as guaiacols and syringols; lignin is an aromatic polymer consisting of hydroxy- and methoxy-substitued phenylpropane units (the most common units are p-coumaryl, coniferyl, and sinapyl structures) linked through ether and condensed types of linkages but with no exact structure.4 Lignin is more thermostable than cellulose or hemicellulose; however, lignin begins to undergo exothermic decomposition at a temperature as low as 200 °C, and thermal degradation continues even up to 900 °C.42 On the basis of the DBE/nC ratios, the main compounds in the O5 (high-DBE region), O 6 , and O 7 classes are likely lignin-derived depolymerization products, e.g., di- and trilignols. Such compounds contain, at least, two phenyl groups (DBE ≥ 8) and, depending upon the lignin type, at least 4−6 oxygen atoms. Lignin pyrolysis has been previously observed to produce mainly monomers, dimers, and trimers of lignin with a molecular mass centered around 400 Da.43 The main compounds detected in the O9−O13 classes are likely to be hemicellulose/cellulose degradation products (e.g., anhydrosugars) or phenolic extractives (e.g., catechin or beluloside types of compounds44) based on their DBE and nC values. For example, the compound peaking out in the O10 class is most likely cellobiosan (anhydrocellobiose, C12H20O10; DBE = 3), one of the most abundant cellulose pyrolysis end products. On the basis of the literature, the most abundant cellulose degradation product is usually levoglucosan (anhydroglucose) based on its easy identification by GC/MS. Without appropriate derivatization, some oligosaccharides present in pyrolysis oils might thermally degrade during the GC run, thus making their analysis difficult and probably increasing the amount of the observed monomer (levoglucosan). The main compound observed in the O11 class is most likely 2/3-O-acetyl-xylopyranosyl-4-O-methylglucuronic acid (C14O11H22; DBE = 4). Birch has a high content of glucuronoxylan (∼90% of the total hemicellulose content), which is heavily methylated/acetylated.45 However, the analysis of exact structures would require further analysis, for example, tandem mass spectrometry (MS/MS) experiments. These findings are not surprising because hemicellulose is a mixture of various polymerized and partially branched monosaccharides (about 150 monomer units on average). Hemicellulose degradation occurs typically at temperatures of 200−260 °C. In addition, cellulose is a high-molecular-weight linear polymer consisting of between 2000 and 14000 glucose units, and it decomposes at temperatures between 240 and 350 °C.4 The DBE versus carbon number plots for the O2−O13 classes detected in the Oil-380 sample are shown in Figure 6. The O2 class was dominated by saturated long-chain fatty acids (i.e., DBE = 1), as compared to Oil-300, for which non-saturated fatty acids were also observed. The O3 class was a similar turning point as the O5 class in the case of Oil-300 (Figure 5); the compounds at both low and high DBE were observed. In comparison to Oil-300, both higher abundances of the O4−O6 classes (Figure 2) as well as the appearance of high-DBE compounds in the O4 class of Oil-380 indicate an increasing rate of lignin degradation at higher temperatures. However, the most abundant compounds with x = 5−8 were found within the DBE range of 7−11; this observation is similar to that observed for Oil-300. These compounds are likely lignin degradation products. The amount of different species is rather large; the DBE versus nC plots for the O5, O6, O7 and O8 classes

Figure 4. Distribution of oxygen-containing compound classes in the pyrolysis oil samples (scaled to identified Ox classes).

Figure 5. Isoabundance-contoured DBE versus carbon number plots for the O2−O14 heteroatom classes for Oil-300.

were quite narrow, and the most abundant compounds had very similar carbon chain lengths. The species in these classes likely correspond to mainly different carboxylic acids, such as lipid-derived fatty acids (O2 class), hydroxy/epoxy fatty acids (O3 class), and diacids and/or dihydroxy fatty acids (O4 class and low-DBE region of the O5 class). A number of different hydroxy/epoxy fatty acids and diacids have been identified from birch, some of which are especially rich in bark.40 The most abundant species in Oil-300 were detected with x ≥ 5 and DBE in the range of 7−11. These compounds are 4599


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earlier by Piispanen and Saranpäa.̈ 46 In their report, the most abundant fatty acids observed were palmitic acid (hexadecanoic acid, 16:0), linoleic acid (octadeca-9,12-dienoic acid, 18:2), oleic acid (9-octadecanoic acid, 18:1), linolenic acid (octadeca9,12,15-trienoic acid, 18:3), stearic acid (octadecanoic acid, 18:0), arachidic acid (eicosanoic acid, 20:0), behenic acid (docosanoic acid, 22:0), and lignoceric acid (tetracosanoic acid, 24:0), with the latter three being present in only trace amounts. This is consistent with our observations. We also observed a clear signal for condoic acid (11-eicosenoic acid, 20:1; C20H38O2; DBE = 2) (see Figure S2 of the Supporting Information), a monounsaturated C20 fatty acid, which could not be observed by GC.46 As stated above, all unsaturated C18 fatty acids were clearly saturated because stearic acid (18:0; C18H36O2; DBE = 1) was predominantly observed in Oil-380. Also, the compounds in the O3 heteroatom class showed partial hydrogenation (to lower DBE), suggesting the presence of a mixture of unsaturated hydroxy and epoxy fatty acids. The same was also observed with the O4 class. Such tendency for unsaturated fatty acid hydrogenation to fully saturated fatty acids thus increases substantially when the pyrolysis temperature rises from 300 to 380 °C, at least in the intermediate pyrolysis regime. The O4 heteroatom class of Oil-380 also showed the presence of the dilignol type of compounds (DBE ≈ 7−12 and nC ≈ 14−20), which are due to an increased lignin degradation rate at higher pyrolysis temperatures. NyOx, SzOx, and OxNam Heretoatom Classes. The most abundant NyOx compound classes present were N1O7, N1O8, N1O6, and N1O9 for Oil-300 and N1O7, N1O6, and N1O5 classes for Oil-380 (Figure 8). Considering the results between the Figure 6. Isoabundance-contoured DBE versus carbon number plots for the O2−O14 heteroatom classes for Oil-380.

show quite an unresolvable mixture of compounds at nC = 12− 22 and DBE = 5−14 (Figure 6). The most interesting observations were made with fatty acids. Figure 7 shows a more detailed comparison of the O2, O3, and O4 heteroatom classes in both oil samples. The fatty acid composition observed in Oil-300 is qualitatively similar to the lipid-derived fatty acid composition of silver birch, as reported

Figure 8. Distribution of NyOx heteroatom compounds identified in the pyrolysis oil samples.

NyOx and Ox classes, it can be seen that the number of oxygen atoms for the most abundant species are very similar. The DBE varied from 6 to 11 for both oil samples. The carbon numbers of the homologous series in the NyOx compounds were, in most cases, between C12 and C22. The relative abundance of these species halved as the temperature of the pyrolysis process was raised from 300 to 380 °C (Figures 2 and 8). Even though the measurements were accomplished by negative-ion ESI, sodium-containing compounds (OxNam heteroatom classes) were also seen in the samples, the O8Na1−O12Na1 classes in Oil-300 and the O7Na1−O11Na1 classes in Oil-380. These species are presumably sodium salts of the corresponding Ox

Figure 7. Comparison of the O2, O3, and O4 heteroatom classes in (top) Oil-300 and (bottom) Oil-380. 4600


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class compounds because they occupy roughly the same nC/ DBE space. Similar sodium-containing species were also reported by Jarvis et al., who used ESI FT-ICR MS to characterize peanut hull bio-oil.26 The detected SzOx class compounds (mainly S1O3 and S1O4 classes) are most likely sulfonic acids or their derivatives.

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CONCLUSION The present results show that chemical composition of pyrolysis oil is very complex and requires the use of highlevel analytical methods. Here, we used ultrahigh-resolution FT-ICR MS coupled with negative-ion ESI to characterize Finnish silver birch pyrolysis oils. The method was shown to be especially good in fingerprinting high-molecular-weight, oxygen-containing heteroatom compounds (e.g., wood extractives, lignin degradation products, and different anhydrosugars), which are difficult or even impossible to characterize with more traditional techniques (like GC or GC/MS). However, identification of individual compounds may be difficult solely based on the determined elemental formula (and DBE), which may overlap for different types of compounds, and the other methods, such as solvent fractionation, could be used to facilitate better identification. In addition, MS/MS still remains quite unexplored for further structural analysis of many detected species in complex bio-oil samples and will be addressed in the future.

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ASSOCIATED CONTENT

* Supporting Information S

Schematic diagram of the micropyrolyzer used in the study (Figure S1) and mass-scale expanded view (0.5 Da window at m/z 309) of the 12 T (−)ESI FT-ICR spectrum of Oil-300 (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: janne.janis@uef.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Decentralized Biorefineries Project (European Social Fund) and the Academy of Finland (Grant 259901) is gratefully acknowledged. Taina Nivajärvi is thanked for performing bulk elemental analysis. Prof. Juha Rouvinen is thanked for critical reading of the manuscript and valuable comments. The FT-ICR MS facility is supported by Biocenter Finland.

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REFERENCES

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