Equivalent isothermal curves for kinetic analysis of non-stoichiometry

The author proposed a method for analyzing kinetics of non-stoichiometry by observing cyclic mass change behavior under cyclic temperature change. Because relationship between the mass and the mass change rate is independent on the previous thermal history of the specimen, we can get equivalent isothermal curves (synthesized isothermal curves) by extracting datum pairs of mass and mass change rate at a given temperature and many conversions, changing the frequencies. The equivalent isothermal curves are really the same as curves of conversion and rate of conversion observed isothermally, so that conventional methods for kinetic analysis can be similarly applied to the equivalent isothermal curves. When multiple elementary processes are proceeding, they can be separately observed by changing the frequency, so that, the method proposed in this short communication is useful and effective to elucidate kinetics of non-stoichiometry as well as reversible reactions.     

Quality assessment of combinatorial pooled libraries using mass spectrometry

Parallel synthesis techniques aim to prepare collections of single compounds which, once tested, can easily be identified by their sole location in the synthesic array. On the other hand, true combinatorial chemistry produces libraries of compounds as mixtures of variable size which require a deconvolution procedure for identification of the active hits or leads. In the latter case, analytical methods are crucial for the success of the strategy and mass spectrometry plays a major role. If the goal is to identify all the library components, including expected products as well as by-products, various mass spectrometric techniques may be necessary. Library components can be separated according to their mass by increasing mass resolution or by their elution time by coupling liquid chromatography and mass spectrometry. The efficiency of such separation techniques are discussed as a function of the size and the degeneracy of the library. Library members possess common structural features which impart similar fragmentation patterns after ionization in the gas phase. This feature can be exploited by tandem mass spectrometry to specifically detect subfamilies of products. Examples of precursor ion scans, product ion scans and constant neutral loss scans will be shown that facilitate partial characterization of libraries. To solve the difficult problem of the quantitative analysis of libraries, i.e., to evaluate their equimolarity, the use of an evaporative light scattering detector (ELSD) or a chemiluminescent nitrogen detector (CLND) is suggested as more appropriate.     

Quantification of pharmaceutical compounds in tissue and plasma samples using selective ion accumulation with multiple mass isolation windows

Quantification of pharmaceutical compounds using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is an alternative to traditional liquid chromatography (LC)-MS techniques. Benefits of MALDI-based approaches include rapid analysis times for liquid samples and imaging mass spectrometry capabilities for tissue samples. As in most quantification experiments, the use of internal standards can compensate for spot-to-spot and shot-to-shot variability associated with MALDI sampling. However, the lack of chromatographic separation in traditional MALDI analyses results in diminished peak capacity due to the chemical noise background, which can be detrimental to the dynamic range and limit of detection of these approaches. These issues can be mitigated by using a hybrid mass spectrometer equipped with a quadrupole mass filter (QMF) that can be used to fractionate ions based on their mass-to-charge ratios. When the masses of the analytes and internal standards are sufficiently disparate in mass, it can be beneficial to effect multiple narrow mass isolation windows using the QMF, as opposed to a single wide mass isolation window, to minimize chemical noise while allowing for internal standard normalization. Herein, we demonstrate a MALDI MS quantification workflow incorporating multiple sequential mass isolation windows enabled on a QMF, which divides the total number of MALDI laser shots into multiple segments (i.e., one segment for each mass isolation window). This approach is illustrated through the quantitative analysis of the pharmaceutical compound enalapril in human plasma samples as well as the simultaneous quantification of three pharmaceutical compounds (enalapril, ramipril, and verapamil). Results show a decrease in the limit of detection, relative standard deviations below 10%, and accuracy above 85% for drug quantification using multiple mass isolation windows. This approach has also been applied to the quantification of enalapril in brain tissue from a rat dosed in vitro. The average concentration of enalapril determined by imaging mass spectrometry is in agreement with the concentration determined by LC–MS, giving an accuracy of 104%.     

Applications of high-pressure liquid chromatography and field desorption mass spectrometry in studies of natural porphyrins and chlorophyll derivatives.

Mixtures of porphyrins derived from natural sources can be readily separated by high-pressure liquid chromatography both analytically and on a preparative scale. A variety of procedures have been developed not only for the esters but also for free acids, and on the analytical scale quantitation is easily achieved by visible absorption. The retention times are largely characteristic of the number of carboxylic acid side chains (or other polar groups) but further information can be obtained by mass spectrometric studies of the various fractions. Field desorption mass spectrometry is particularly useful for this purpose because the emitter wire can be dipped directly into the eluates. The field desorption spectra of porphyrin free acids and esters as well as their metal complexes give essentially molecular ions with little or no fragmentation in most cases, whereas electron-impact mass spectrometry, particularly of free acids, is impeded by the low volatility of porphyrins. Mixtures can also be analysed by field desorption mass spectrometry, and this provides not only a rapid qualitative assessment of the components of a mixture, but also a check on the subsequent chromatographic separations.     

多接收器电感耦合等离子质谱精确测定钕同位素组成

报道了本实验室近两年来Neptune MC-ICP-MS测试Nd同位素的结果。测试结果显示样品化学分离中伴随的大量铈对钕同位素组成测定没有影响;而分离后残余少量钐,在一定范围内(钐/钕<0.04)可以直接扣除,获得准确的Nd同位素组成。Neptune MC-ICP-MS和热电离质谱(TIMS)平行测定实际地质样品表明,Neptune MC-ICP-MS可以精确测定Nd同位素组成,与经典的TIMS技术相比,MC-ICP-MS可以获得与TIMS相媲美的数据精度,而且分析时间缩短,效率明显提高。     

Affinity-based mass spectrometry using magnetic iron oxide particles as the matrix and concentrating probes for SALDI MS analysis of peptides and proteins

Silane-immobilized magnetic iron oxide particles were used as the assisting material in surface-assisted laser desorption/ionization (SALDI) mass spectrometric analysis. This approach can be used to analyze small proteins and peptides. The upper detectable mass range is approximately 16 kDa. The detection limit for peptides is about 20 fmol. Silanized iron oxide particles with negatively charged functionalities can also be used as the affinity probes to selectively trap oppositely charged species from sample solutions by adjusting the pH of the solution. A tryptic digest product of cytochrome C at a concentration as low as 10 nM can be enriched by the particles and directly analyzed by iron oxide SALDI MS without the need for elution steps. Affinity-based mass spectrometry using the bifunctional silanized magnetic iron oxide particles as the SALDI matrix and concentrating probe is demonstrated in this study.     

Determination of 3-Chloro-4-(Dichloromethyl)-5-Hydroxy-2(5H)-Furanone (Toxicant MX) in Drinking Water by Chromatography–Tandem Mass Spectrometry

It is shown that toxicant MX can be reliably identified and determined quantitatively by ion-trap tandem mass spectrometry with an accuracy as high as that for high-resolution mass spectrometry.     

Characteristic fragmentation of thromboxane B2 in thermospray high-performance liquid chromatography-mass spectrometry

Cyclo- and lipo-oxygenase metabolites of arachidonic acid have been reported to have very simple thermospray mass spectra. However, thromboxane B2 (TXB) in ammonium acetate-methanol and at interface temperatures below the point of total vaporization shows a mass spectral pattern characterized by abundant ions at low masses. The more abundant TXB fragment ions have been characterized as adducts from four principal fragments with molecular masses of 156, 170, 196 and 326. Positive- and negative-ion mass spectra, and mass spectra obtained with an alternative thermospray buffer (butylammonium acetate) support the molecular masses of these fragments. A tentative assignment of the fragments can be made by comparison of the thermospray mass spectra of TXB, 2,3-dinor-TXB and some of their methyl ester and methyl oxime derivatives. Interface temperature and solvent composition effects on the fragmentation, as well as in the electron-capture processes observed when working in the filament-on mode, are discussed. Fragmentation mechanisms can be related to those observed for monosaccharides, and imply retro-Diels-Alder as well as retroaldolic condensation-type rearrangements.     

Low-resolution accurate mass measurement in self-chemical ionization mass spectrometry

Low-resolution (2000, 10% valley definition) accurate mass measurements using self-chemical ionization (self-CI) have been evaluated as an alternative to the conventional chemical ionization (CI) method. In conventional CI experiments a high pressure of reagent gas is required to induce ionization while in self-CI no reagent gas is used and the self-CI is produced presumably by molecular/fragment ion–molecule reactions. Nine compounds ranging in mass from 50–500 daltons were examined. Results obtained by the self-CI method indicate that the elemental composition assignment can be obtained simultaneously for the protonated molecule and/or molecular ion. It is also shown that perfluorokerosene can be used routinely in self-CI as an internal reference standard over a broad mass range (50–500 daltons). It is sometimes difficult to use a single reference standard in conventional low-resolution CI accurate mass spectrometry over a similar mass range.     

Classification of microheterogeneity in solid samples using µXRF

Micro X-ray fluorescence (microXRF) has been used nondestructively to investigate elemental heterogeneity by constructing two-dimensional maps of elemental concentrations in reference materials. microXRF probes sample sizes well below the 100 mg mass usually recommended for reference materials by NIST. Multivariate methods of analysis, such as principal-component analysis (PCA), show promise in identifying whether "nugget" effects exist within a material, where an element is enriched in small, isolated areas of the sample. The PCA model is built based on data taken in one location and compared with each elemental map. This methodology is shown for several reference materials including SRM 2702 and SRM 2703 to show how PCA treatment can be used to identify which elements exhibit nugget effects within the sub-mg mass range. A method of calculating the minimum recommended mass for solid samples is suggested using PCA iteratively on X-ray maps from which adjacent data points have been averaged. This is repeated until the mass sampled in a map is indistinguishable from data taken at a single location, suggesting no nugget effects can be detected. For SRM 1577c, a mass as low as 370 microg can be used without measurable nugget effects.     

Limitations of mass spectrometric methods for the characterization of polydisperse materials

This paper is a review of work on the characterization of coal liquids and petroleum residues and asphaltenes over several decades in which various mass spectrometric methods have been investigated. The limitations of mass spectrometric methods require exploration in order to understand what the different analytical methods can reveal about environmental pollution by these kinds of samples and, perhaps more importantly, what they cannot reveal. The application of mass spectrometry to environmental problems generally requires the detection and determination of the concentration of specific pollutants released into the environment by accident or design. The release of crude petroleum or coal liquids into the environment can be detected and tracked during biodegradation processes through specific chemicals such as alkanes or polyaromatic hydrocarbons (PAHs). However, petroleum asphaltenes are polydisperse materials of unknown mass range and chemical structures and, therefore, there are no individual chemicals to detect. It is necessary to determine methods of detection and the ranges of mass of such materials. This can only be achieved through fractionation to reduce the polydispersity of the initial sample. Comparison of mass spectrometric results with results from an independent analytical method such as size‐exclusion chromatography with a suitable eluent is advisable to confirm that all the sample has been detected and mass discrimination effects avoided. Copyright © 2010 John Wiley & Sons, Ltd.     

Novel strategy for the determination of illegal adulterants in health foods and herbal medicines using high‐performance liquid chromatography with high‐resolution mass spectrometry

The detection, confirmation, and quantification of multiple illegal adulterants in health foods and herbal medicines by using a single analytical method are a challenge. This paper reports on a new strategy to meet this challenge by employing high‐performance liquid chromatography coupled with high‐resolution mass spectrometry and a mass spectral tree similarity filter technique. This analytical method can rapidly collect high‐resolution, high‐accuracy, optionally multistage mass data for compounds in samples. After a preliminary screening by retention time and high‐resolution mass spectral data, known illegal adulterants can be detected. The mass spectral tree similarity filter technique has been applied to rapidly confirm these adulterants and simultaneously discover unknown ones. By using full‐scan mass spectra as stem and data‐dependent subsequent stage mass spectra to form branches, mass spectrometry data from detected compounds are converted into mass spectral trees. The known or unknown illegal adulterants in the samples are confirmed or discovered based on the similarity between their mass spectral trees and those of the references in a library, and they are finally quantified against standard curves. This new strategy has been tested by using 50 samples, and the illegal adulterants were rapidly and effectively detected, confirmed and quantified.     

Shifting unoccupied spectral space in mass spectrum of peptide fragment ions

Ions near the high-end border of a mass defect distribution plot for native peptide fragment ions have potential as signature markers that are based on mass-to-charge ratio determination. The specificity of these marker ions, including phosphoryl ions, can be improved by removing interfering isobaric ions from the border region on the distribution plot. These interfering ions are rich in Asp and Glu content. The masses of amino acid residues and peptides are rescaled from the IUPAC scale (¹²C=12 u as the mass reference) to the averagine scale (averagine mass=111 u* as the mass reference with zero mass defect; u*: the mass unit on the averagine scale), using a scaling factor of 0.999493894. It is theoretically predicted that esterification of Asp and Glu side-chain carboxylates with n-butanol can achieve a sufficient retreat of the high-end border on a mass defect distribution plot based on the use of mass spectrometers with better-than-medium resolution. Theoretical calculations and laboratory experiments are performed to examine effects of various esterifications on the averagine-scale mass defect distribution of peptide fragment ions and on the specificity of two positive phosphoryl ions: the phosphotyrosine immonium ion and a cyclophosphoramidate ion.     

Charge remote fragmentation of fatty acids cationized with alkaline earth metal ions

Fatty acids can be collisionally activated as [M ? H + Cat]⁺, where Cat is an alkaline earth metal, by using tandem mass spectrometry. High-energy collisional activation induces charge remote fragmentation to give structural information. In the full scan mass spectra molecular ions are easily identified, particularly when barium is used as a cationizing agent; ions are shifted to a higher mass, lower chemical noise region of the mass spectrum. Moreover, the isotopic pattern of barium is characteristic, and the high mass defect of barium allows an easy separation of the cationized analyte from any remaining interfering ions (chemical noise), provided medium mass-resolving power is available. An additional advantage is that most of the ion current is localized in [M ? H + Cat]⁺ species. Structural analysis of fatty acids can be performed when the sample size is as low as 1 ng.     

A secondary ion microprobe ion trap mass spectrometer

An ion trap mass analyzer has been attached to an organic secondary ion microprobe. A pressure differential >100 can be maintained between the ion trap and microprobe. The well-focused secondary ion beam can transit a small (2 mm) diameter tube, but gas flow from ion trap to microprobe is impeded. This pressure differential allows the microprobe to retain imaging capability. Ion trap and microprobe data systems are integrated by taking advantage of the highly reproducible periodicity of the ion trap operating in resonant ejection mode and asynchronous signal and data acquisition afforded by commercially available interface cards. Secondary ion mass spectra and images obtained indicate an approximately 10-fold improvement in sensitivity, although preliminary evidence indicates low (<1%) trapping efficiency. Image data acquisition using the ion trap for mass analysis requires at least 10 times as much time compared to using a quadrupole mass filter because the mass-selected instability mode is used for mass analysis, i.e., mass resolution in the ion trap is not continuous as it is in the quadrupole.     

Differential electrochemical mass spectrometry

Differential electrochemical mass spectrometry (DEMS) can be used not only to identify products or intermediates of continuous faradaic reactions, but also to characterize submonolayer amounts of adsorbates on polycrystalline and single crystal electrode surfaces by means of their desorption, because of its high sensitivity. One possibility to achieve this is to oxidize a carbonaceous species to CO(2), which is quantitatively detected in the mass spectrometer. Many adsorbates can also be desorbed at certain potentials as such, or as the hydrogenated product, allowing a more direct characterization of the adsorbate. In some cases, a nonreactive desorption can be induced by displacement with a second adsorbate, yielding additional information. Interfacing an electrochemical cell to a mass spectrometer via a porous Teflon membrane can be achieved with a variety of cells. These will be described together with their specific advantages and characteristics.     

Automated mass analysis of low-molecular-mass bacterial proteome by liquid chromatography-electrospray ionization mass spectrometry

Due to the presence of a large number of proteins in cell extracts, ion chromatograms of cell extracts obtained by electrospray ionization mass spectrometry (ESI-MS) can be quite complicated. It is found that the elevated baseline in an ion chromatogram contains many protein signals. One deficiency of current commercially available LC-ESI-MS data interpretation software is found to be the lack of functional operation that allows automated mass spectral integration and interpretation over signals hidden in the baseline. This current limitation can be overcome by a technique that involves the introduction of artificial pulses to an ion chromatogram by removing the solvent mixer in the HPLC pump. These artificial pulses are treated as chromatographic peaks by the software, thereby allowing automated spectral integration over the duration of a pulse. The reliability of mass analysis from the integrated spectra is shown to be dependent on spectral interpretation parameters such as mass spectral baseline threshold. The application of this method is demonstrated for rapid detection and mass analysis of low-molecular-mass proteins from cell extracts of Escherichia coli or Bacillus globigii.     

Multiplexed screening of cellular uptake of gold nanoparticles using laser desorption/ionization mass spectrometry

Gold nanoparticles (AuNPs) are highly promising candidates as drug delivery agents into cells of interest. We describe for the first time the multiplexed analysis of nanoparticle uptake by cells using mass spectrometry. We demonstrate that the cellular uptake of functionalized gold nanoparticles with cationic or neutral surface ligands can be readily determined using laser desorption/ionization mass spectrometry of cell lysates. The surface ligands have "mass barcodes" that allow different nanoparticles to be simultaneously identified and quantified at levels as low as 30 pmol. Using this method, we find that subtle changes to AuNP surface functionalities can lead to measurable changes in cellular uptake propensities.     

A miniaturized matrix-assisted laser desorption/ionization time of flight mass spectrometer with mass-correlated acceleration focusing

Mass correlated acceleration (MCA) has now been integrated into a 4 in (10.2 cm) matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer to achieve high resolving power across a broader mass range, without sacrificing detection of higher mass ions. The goal was to combine MCA with a custom-built miniaturized instrument such as those that might be used for field-portable applications. Unlike other pulsed extraction methods, MCA is not mass dependent and mass spectra can be achieved with a single tuning of instrument parameters. Additionally, the multi-channel recording advantage is better realized because ions of all masses can be brought into focus simultaneously. The MCA dual-stage ion source compensates dynamically for the mass dependence by incorporating an extraction pulse region followed by an acceleration region that contains a time-dependent waveform correlated with mass. The technique was validated with applications in peptide mixtures and protein digestions. Diagnostic studies for the instrument include m/z range and limits of detection.     

Analysis of polyamides by size exclusion chromatography and laser light scattering

The molar mass analysis of polyamides is complicated due to the fact that only a limited range of solvents can be used and association and aggregation phenomena have to be screened by adding electrolytes to the mobile phase. Optimum SEC behaviour is obtained when hexafluoroisopropanol + 0.05 mol/L potassium trifluoroacetate is used as the mobile phase. The calibration of the SEC system can be conducted in different ways. While a calibration with narrow disperse polymethyl methacrylate standards does not yield accurate molar mass information, the quantification can be done using an “artificial” calibration curve. This calibration curve is obtained by correcting the PMMA calibration curve with polyamide molar mass data from light scattering. The resulting molar mass distributions for different types of polyamides are compared with molar masses that are determined by size exclusion chromatography with a light scattering detector and an excellent correlation is obtained.