Characterisation of humic substances using atmospheric pressure chemical ionisation and electrospray ionisation mass spectrometry combined with size-exclusion chromatography

Humic substances were analysed by atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) mass spectrometry in positive and negative modes. Using APCI the average m/z range of humic substances was reduced 5-fold compared to ESI. High-resolution time-of-flight mass spectrometry revealed the formation of multiply charged molecules in the ESI mode. Moreover, it was possible to obtain daughter ion mass spectra of humic substances by nanospray tandem mass spectrometry. The size-exclusion chromatography elution profile of humic substances was highly influenced by the pH of the analyte solution. By contrast, the pH had no significant influence on the observed mass spectra of humic substances.     

Size exclusion chromatography/mass spectrometry applied to the analysis of polysaccharides.

A novel method for analysing polysaccharide materials is described which employs size-exclusion chromatography (SEC) followed by detection by on-line electrospray ionisation mass spectrometry (ESI-MS) and off-line matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS). It is demonstrated through SEC/ESI ion trap mass spectrometry that the formation of multiply charged oligomer ions, which bind up to five sodium cations, allows the rapid analysis of polysaccharide ions with molecular weights in excess of 9 kDa. MALDI spectra generated from fractionation of the effluent collected from the same SEC separation are shown to be in good agreement with the ESI spectra with respect to molecular weight distributions and types of ions generated. ESI and MALDI mass spectra of samples obtained from sequential graded ethanol precipitation and SEC fractionation of acid and enzymatically digested arabinoxylan polysaccharides show important structural differences between polysaccharide fragments. In addition, a comparison is made between the mass spectra of native and permethylated SEC-separated fragments of acid and enzymatically treated arabinogalactan. Linkage information of the permethylated arabinogalactan oligomers can be rapidly established through the use of on-line SEC/ESI-MS( n) experiments.     

Molecular structure of fulvic acids by electrospray with quadrupole time-of-flight mass spectrometry

Characterisation of the molecular structure of aquatic fulvic acids (FA) has been performed using a quadrupole time-of-flight (Q-TOF) mass spectrometer equipped with an electrospray ionisation interface. Molecular masses centred around 450 Da and sinusoidal spectral distributions have been obtained for all fulvic acids. Tandem mass spectrometry (MS/MS) experiments showed losses of 18 Da (H(2)O) and 44 Da (CO(2)), and possible molecular structures were determined for the first time to our knowledge. A methodology is reported for evaluating the average elemental composition of FA from high-resolution mass spectra by processing post-acquisition data calculations using molecular size distributions and atomic compositions of ions. The results are found to be consistent with elemental analysis data.     

High-resolution analysis of a 144-membered pyrazole library from combinatorial solid phase synthesis by using electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry

A compound library consisting of 144 pyrazole carboxylic acids and six sublibraries consisting of 24 components was analysed using electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). The library was synthesised by the split-mix method and investigated by direct infusion analysis by which 134 compounds were detected. FTICR-MS is predestined for the direct characterisation of complex compound libraries because of its outstanding mass resolution and mass accuracy. However, discrimination within the electrospray ionisation process sometimes leads to signal suppression and thus to misinterpretation of the synthetic results. Using micro-HPLC/MS we were able to assign all 144 compounds including all pairs of isobaric pyrazoles. We also show that, due to partial separation, FTICR-MS is indispensable for proper detection of co-eluting compounds.     

Identification of compound classes in soil and peat fulvic acids as observed by electrospray ionization tandem mass spectrometry

Soil and peat fulvic acids obtained from the International Humic Substances Society were fractionated by their solubility in methanol and analyzed by electrospray ionization tandem mass spectrometry. Precursor and product ion experiments produced mass spectra that indicated the presence of benzene, phenol, dihydroxy benzene, furan and thiophene carboxylic acids. Standards were used to substantiate the fragmentation patterns observed in the product ion spectra of the fulvic acid samples. This study makes significant progress into the direct identification of individual compounds in humic substances using a non-degradation technique.     

Electrospray ionization mass spectrometry of terrestrial humic substances and their size fractions

Electrospray ionization mass spectrometry (ESI-MS) was used to evaluate the average molecular mass of terrestrial humic substances, such as humic (HA) and fulvic (FA) acids from a soil, and humic acid from a lignite (NDL). Their ESI mass spectra, by direct infusion, gave average molecular masses comparable to those previously obtained for aquatic humic materials. The soil HA and FA were further separated in size-fractions by preparative high performance size exclusion chromatography (HPSEC) and analyzed with ESI-MS by both direct infusion and a further on-line analytical HPSEC. Unexpectedly, their average molecular mass was only slightly less than for the bulk sample and, despite different nominal molecular size, did not substantially vary among size-fractions. The values increased significantly (up to around 1200 Da) after on-line analytical HPSEC for the HA bulk sample, at both pH 8 and 4, and for the HA size-fractions when pH was reduced from 8 to 4. It was noticed that HA size-fractions at pH 8 were separated by on-line HPSEC in further peaks showing average masses which progressively increased with elution volume. Furthermore, when the HA and NDL bulk samples were sequentially ultracentrifuged at increasing rotational speed, their supernatants showed mass values which were larger than bulk samples and increased with rotational speed. These variations in mass values indicate that the electrospray ionization is dependent on the composition of the humic molecular mixtures and increases when their heterogeneity is progressively reduced. It is suggested that the dominance of hydrophobic compounds in humic supramolecular associations may inhibit the electrospray ionization of hydrophilic components. Our results show that ESI-MS is reasonably applicable to humic substances only after an extensive reduction of their chemical complexity.     

Ionisation in the absence of high voltage using supercritical fluid chromatography: a possible route to increased signal

Supercritical fluid chromatography (SFC) is fast becoming a technique of choice for the analysis of a wide range of compounds and has been found to be complementary to high-performance liquid chromatography (HPLC). The combination of SFC and mass spectrometry (MS) affords a very useful tool in the separation and analysis of compounds. In this study the ionisation of samples in the absence of an applied electrospray voltage has been observed when using SFC/MS, with some compounds showing increased sensitivity when all ionisation source high voltage (HV) is removed. In an attempt to understand the mechanism of ionisation, a series of test compounds were analysed using standard electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI) source configurations and also different API source designs. In both cases, data were acquired with the applied high voltage on (normal conditions) or with the high voltage off, i.e. no voltage spray (novo-spray). The standards were analysed with a range of pressure and modifier percentage conditions. To understand the nature of the ionisation process observed, this was compared with three established liquid-to-gas ionisation mechanisms. These were thermospray (TSP), charge residue model (CRM) of ESI and sonic spray ionisation (SSI). Experiments were undertaken in an attempt to explain this ionisation phenomenon and quantify any observed change in sensitivity. The most important point to note is that enhanced ionisation was observed under novo-spray conditions in a SFC/MS configuration, which in certain cases provides a lowering in the overall limit of detection (LOD).     

Effects of sample properties and mass spectroscopic parameters on electrospray ionization mass spectra of size-fractions from a soil humic acid

Size-fractions from a soil humic acid were separated by preparative size-exclusion chromatography (SEC), desalted, and concentrated by ultrafiltration and vacuum centrifugation without being subjected to any freeze-drying process. After having assessed the lack of formation of any multiple-charged ions by high-resolution Fourier transform ion cyclotron resonance electrospray ionization (ESI) mass spectrometry (MS), the size-fractions were used by direct infusion to compare the molecular ion distribution by both atmospheric pressure chemical ionization (APCI)- and ESI-MS in negative mode. The weight- (Mw) and number-averaged (Mn) molecular weight obtained by ESI-MS were invariably larger than by APCI-MS for all size-fractions, thereby indicating that ESI is more efficient than APCI to evaluate the molecular mass distribution of humic samples. No substantial difference was observed when concentration and pH of unfreeze-dried humic size-fractions were varied. The negative mode was applied to assess the effect of cone voltage from −20 to −60 V on ESI of the humic size-fractions further separated through an on-line SEC column. The resulting mass spectra and Mw and Mn values suggested that the variation of cone voltage in ESI-MS affects the ionization potential of associated humic molecules more in solution rather than their fragmentation. These findings agree with previous observations which indicated a limitation of ESI in providing consistent mass detection for a complex mixture of heterogeneous humic molecules, especially when they are aggregated by a freeze-drying process.     

A study of human coagulation factor XIII A-subunit by electrospray ionisation mass spectrometry

A study of factor XIII A-subunit has been made by electrospray ionisation mass spectrometry. Factor XIII has been isolated and rigorously purified, and the intact A-subunit protein has been analysed to confirm its molecular weight accurately for the first time. Thrombin proteolysis of the A-subunit has been monitored and the resulting cleavage products, the activated A-subunit protein and the Activation Peptide, have been analysed by electrospray ionisation mass spectrometry to confirm the site of proteolysis. Facile and rapid separation of the Activation Peptide from the activated A-subunit protein has been achieved using simple on-line separation techniques. The molecular weight measurements have been repeated numerous times to provide an indication of the precision and variability of the analyses.     

Determination of steroid hormones, hormone conjugates and macrolide antibiotics in influents and effluents of sewage treatment plants utilising high-performance liquid chromatography/tandem mass spectrometry with electrospray and atmospheric pressure chemical ionisation

In this study we present a high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) method which has been elaborated to analyse steroid hormones, hormone conjugates, oral contraceptives and macrolide antibiotics unchanged in unfiltered influents and effluents of sewage treatment plants (STPs). HPLC separation of the steroid hormones was achieved in 35 min, as well as those of the antibiotics. The analytes were extracted by solid-phase extraction, followed by clean-up using size exclusion chromatography (SEC). For the final quantification HPLC/MS/MS was used. The two ionisation modes, electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI), in HPLC/MS/MS were compared for the analysis of steroid hormones. For quantitative results drastic matrix effects were observed while using ESI. These effects were less pronounced while using APCI. These pitfalls were additionally reduced by clean-up using SEC as well as isotope dilution. Additionally, two multiple reaction monitoring (MRM) transitions per compound were used to prevent false positive results. Recovery experiments with spiked tap water with concentrations varying from 1 to 1000 ng/L gave constant recovery rates: The recovery rates for the hormones and conjugates ranged from 58 to 107%, those of the contraceptives ranged from 83 to 109%. The relative standard deviation was found to be 7 to 24% and the limits of detection were 0.1 to 4.5 ng/L. The recovery rates of the macrolide antibiotics ranged from 76 to 103%, while the relative standard deviation was found to be 7 to 14% and the limits of detection ranged from 0.6 to 1.8 ng/L. The maximum concentrations found in influents of a STP was 470 ng/L for estriol and 1200 ng/L for erythromycin.     

The Effects of a Strong Disaggregating Agent on Sec-Page of Aquatic and Soil Humic Matter

Abstract

Size-exclusion chromatographic (SEC) fractionation and electrophoretic separation of aquatic humic matter samples from a Finnish lake using Sephadex G-75 with 7 M urea solution as eluent and 10% polyacrylamide gel (PAGE) with urea and sodium dodecyl sulphate solution (SDS), respectively, were performed and compared to similar analyses performed on a Russian chernozem soil humic acid sample and Nordic reference fulvic and humic acid samples. The integrated whole of aquatic humic solutes and soil humic acids were found to exhibit similar SEC-PAGE behaviours. Humic matter was not excessively disaggregated by the 7 M urea and hence SEC-PAGE can with confidence be applied as a coarse, initial fractionation procedure or for certain predeterminations of the structural composition.     

ESI-MS analyses of lake dissolved organic matter in light of supramolecular assembly

A high-performance size-exclusion chromatography (SEC) system was coupled on-line to an electrospray ionization (ESI) interface to detect gas-phase ions by an API 365 LC/MS/MS triple quadrupole analyzer. The SEC fractions of a strongly coloured freshwater solution containing dissolved organic matter-humic substances (DOM-HS) were screened both by UV₂₅₄ and by ESI mass spectrometry (ESI-MS) in the full-scan mode within the m/z range of 100–2,900 amu in negative and positive polarities. The ESI-MS spectra were also collected by direct infusion of the DOM-HS solution in both polarities. ESI-MS spectra did not primarily favour low mass compounds, and negative and positive total ion chromatograms were parallel to the SEC elution profile obtained by UV₂₅₄ detection from DOM-HS solution. The UV₂₅₄ detection overestimated the SEC portion of higher size/mass solutes and underestimated that of solutes of smaller sizes/masses as compared with the total ion chromatogram intensities in negative or positive polarities. The change of mass-weighted and number-weighted average sizes/masses (M _w and M _n) of different SEC fractions was fairly small, in contrast to UV₂₅₄ detection, with increasing elution volume. A reasonable explanation for the great differences between M _w and M _n values, obtained by UV₂₅₄ and ESI-MS detections for eight different SEC fractions, seems to be a supramolecular-type association of relatively small components through weak dispersive forces. M _n values obtained by vapour-pressure osmometry for different SEC fractions were to some extent analogous with those of negative and positive ESI-MS. The shapes obtained by either negative or positive polarities and calculated M _w and M _n values indicated a close structural similarity between each SEC fraction. Positive ion and negative ion spectra of different humic fractions represented quite similar components, and there was no evidence for noteworthy occurrence of multiply charged ions being able to lower mass distributions of negative ion spectra. The effect of nitrogen on the mass spectra seemed to be unimportant, and the weak ions observed at even m/z values correspond most likely to the ¹³C counterparts of the more abundant ¹²C odd ions. No uncontrolled ESI fragmentation was observable and humic solutes seemed to be quite heat-resistant. Direct infusion of the untreated DOM-HS solution and statistical calculation verified that the SEC-separated different fractions really represent distinct entities of the original DOM-HS mixture. ESI-MS results support the opinion that the structural composition of humic solutes in their original combined mixture resembles supramolecular-type associations of smaller molecular size entities possessing similar structural functionalities.     

Influence of humic substances on sorption and methylation processes of inorganic- and organo-tin species

The influence of humic substances on sorption and methylation processes for inorganic- and organotin species is presented. Four sediment samples from different locations of the Rivers Elbe, Mulde and Spittelwasser, Germany, with different organotin and humic contents were selected to extract the humic and fulvic acids. The various fractions—the original sediment, the humic acid, the fulvic acid and the residual sediment—were analysed for their organotin content. The individual buyltin species show quite different distribution patterns. Monobutyltin is found mostly associated with humic acids. Dibutyltin shows a nonunique behaviour. At low total organotin content, dibutyltin is found bonded to humic and fulvic acids, whereas at high organotin content dibutyltin is distributed more with the residual sediment. Most of the tributyltin remains in the sediment unextracted; only small quantities of it are in the fulvic acid fraction. Tetrabutyltin is only in the humic acid fraction when it binds to humic matter; it mostly remains in the sediment. General observations indicate that ionic butyltin species bind to fulvic acids whereas the non-polar tetrabutyltin is not found in the fulvic acid fractions in any of the samples. The appearance of monomethyl- and dimethyl-tin species in the humic and fulvic acid fractions after the alkaline extraction was surprising. There is a correlation between the humic content of the sample and the formation of methyltin species. Evidence is provided by experiments that humic substances act as methylation agents.     

Desorption electrospray ionisation mass spectrometric analysis of chemical warfare agents from solid‐phase microextraction fibers

Desorption electrospray ionisation mass spectrometry (DESI‐MS) was recently reported for the direct analysis of sample media without the need for additional sample handling. During the present study, direct analysis of solid‐phase microextraction (SPME) fibers by DESI‐MS/MS was evaluated with indoor office media that might be collected during a forensic investigation, including wall surfaces, office fabrics, paper products and Dacron swabs used for liquid sampling. Media spiked at the µg/g level with purified chemical warfare agents and a complex munitions grade sample of tabun, to simulate the quality of chemical warfare agent that might be used for terrorist purposes, were successfully analysed by DESI‐MS/MS. Sulfur mustard, a compound that has not been successfully analysed by electrospray mass spectrometry in the past, was also sampled using a SPME fiber and analysed for the first time by DESI‐MS/MS. Finally, the overall analytical approach involving SPME headspace sampling and DESI‐MS analysis was evaluated during a scenario‐based training live agent exercise. A sarin sample collected by the military was analysed and confirmed by DESI‐MS in a mobile laboratory under realistic field conditions. Copyright © 2007 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.     

Desorption electrospray ionisation mass spectrometry and tandem mass spectrometry of low molecular weight synthetic polymers

A range of low molecular weight synthetic polymers has been characterised by means of desorption electrospray ionisation (DESI) combined with both mass spectrometry (MS) and tandem mass spectrometry (MS/MS). Accurate mass experiments were used to aid the structural determination of some of the oligomeric materials. The polymers analysed were poly(ethylene glycol) (PEG), polypropylene glycol (PPG), poly(methyl methacrylate) (PMMA) and poly(alpha-methyl styrene). An application of the technique for characterisation of a polymer used as part of an active ingredient in a pharmaceutical tablet is described. The mass spectra and tandem mass spectra of all of the polymers were obtained in seconds, indicating the sensitivity of the technique.     

Direct sample fraction deposition using electrospray in narrow-bore size-exclusion chromatography/matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for polymer characterization

A direct sample fraction deposition method was developed for off-line size-exclusion chromatography (SEC)/matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. By using electrospray, the SEC eluent, together with a suitable matrix solution added coaxially, was directly deposited on the MALDI plate. Owing to the formation of very small droplets in electrospray, solvent evaporation is much faster. The fractionation volume in narrow-bore SEC, which can directly be collected in one MALDI spot, can easily be optimized in the range of a few microlitres. In addition, fairly homogeneous sample spots were obtained. The possible influence of composition variation of the SEC effluent on the analytical results using direct fraction deposition was investigated; no substantial effects were observed. The applicability of the method was demonstrated by characterizing a broad poly(methyl methacrylate) sample. Copyright 2000 John Wiley & Sons, Ltd.     

Molecular features of organic matter in diagnostic horizons from andosols as seen by analytical pyrolysis

Andosols are usually formed from volcanic substrates, with thick A horizons high in organic carbon mainly in the form of stabilized humic fractions. The peculiar properties of these soils are, to large extent, affected by poorly crystalline materials like allophanes, imogolite and other Fe and Al oxyhydroxides that induce intense organo-mineral interactions which are considered to play a relevant role in OM protection against biodegradation as regards other soils developed under similar climatic conditions. Little is known about the molecular composition of this stabilized OM in a soil scenario often considered as an efficient C-sink in terms of C sequestration processes.

Whole soil samples in addition to isolated humic and fulvic acids from organic A horizons of three soils with andic properties and one non-andic soil (Sodic Cambisol) from the island of Tenerife (Canary Islands, Spain) were analysed by double shot pyrolysis-gas chromatography–mass spectrometry (Py-GC/MS) in order to get some insight on the molecular composition of different structural domains of progressive structural stability.

Clear differences were found between soils, both in thermal desorption and pyrolysis behaviour of humic substances. In general the results suggest large yields of aliphatic (both alkyl and carbohydrate-derived pyrolysis compounds) pointing to high-performance processes of incorporation of aliphatic humic constituents in andosols probably favoured by interactions with amorphous minerals, whereas in non-andic soils the latter are comparatively less stabilized and are removed in early pyrolysis stages as if they occurred as loosely joined, thermoevaporation-released products.

In fact, compared to Cambisol, humic and fulvic acids from andic soils led to comparatively richer pyrograms and compound assemblages. The lack of similar amounts of these loosely joined, mainly aliphatic structures after thermal desorption of the whole andic soils indicate an origin for humic substances based on rapid sequestration of C-forms of recent (litter or microbial) origin.     

Reversed-phase ion-pair chromatography coupled to electrospray ionisation mass spectrometry by on-line removal of the counter-ions.

A strong anion-exchanger was used as a trapping column to perform on-line coupling of ion-pair chromatography with electrospray ionisation mass spectrometry. Ion-pairing reagents were used to retain polar analytes of low molecular mass away from the solvent front in reversed-phase LC. The trapping column enabled removal of the non-volatile counter-ions from the mobile phase prior to detection, so that the electrospray process could be performed with favourable ionisation conditions and without contamination of the interface. The efficiency of the trapping process was studied for 1-octanesulfonic acid and sodium dodecyl sulfate as ion-pairing reagents. Using this on-line trapping method, biopterin and guanidine could be retained with a k'>2 and detected by electrospray mass spectrometry with a stable signal.     

Coupling desorption electrospray ionisation and neutral desorption/extractive electrospray ionisation with a travelling-wave based ion mobility mass spectrometer for the analysis of drugs

Desorption electrospray ionisation (DESI) and neutral desorption/extractive electrospray ionisation (EESI) have been coupled to a hybrid quadrupole travelling-wave (T-Wave)-based ion mobility time-of-flight mass spectrometer for the direct accurate mass analysis of active ingredients formulated into pharmaceutical samples. The collision cross-section measurements of polyethylene glycol (PEG) excipients detected in one formulation were estimated and compared with published data. These estimated collision cross-sections of the PEG species showed good agreement with published data.     

Residue analysis of 15 penicillins and cephalosporins in bovine muscle, kidney and milk by liquid chromatography–tandem mass spectrometry

A sensitive and specific liquid chromatographic–tandem mass spectrometric (LC–MS–MS) method for most of those penicillins and cephalosporins for which EU maximum residue limits (MRL) were set in Council regulation (EEC) 2377/90 was developed and validated in bovine muscle, kidney and milk. The analytes were extracted with acetonitrile/water and cleaned-up by a single reversed-phase solid-phase extraction step. Highest sensitivity for the analytes was obtained when amoxicillin, ampicillin, cephalexin, cephapirin, desacetylcephapirin, cephalonium, cefquinome and cefazolin were measured in the positive electrospray ionisation mode (ESI (+)) and cefoperazone, benzylpenicillin, phenoxymethylpenicillin, oxacillin, cloxacillin, dicloxacillin and nafcillin in the negative electrospray ionisation mode (ESI (−)). Chromatography was performed with a formic acid/methanol gradient. Collision-induced dissociation (CID) with argon was used for fragmentation of the pseudomolecular ions to achieve the required specificity. Possible adverse matrix effects on the electrospray ionisation process caused by co-eluting matrix components were investigated. The method was validated closely to the new EU guidelines and applied to positively screened samples from official food control allowing the identification and quantification of the residual β-lactams.