Error-systematics of determining simultaneously the isotopic abundance ratios of natural lithium and natural boron as Li2BO2+

Recently, we have shown how the errors delta(j) and delta(k), that occur when measuring the two different isotopic molecular abundance ratios required for analysis, are transformed into the actual errors of elemental isotopic analysis, (deltaEi/Ei)'s. With a view to gain further understanding as to how the errors (deltaEi/Ei)'s are governed, we now evaluate theoretically the effects of selecting different isotopic molecular pairs as the monitor pairs (j and k) for measurements, and of the measurement errors (delta(j) and delta(k)), on the results of analysis (the 6Li/7Li and the 10B/11B abundance ratios), by considering all the constituent elements of Li2BO2+ at their natural isotopic abundances. It is shown that the ratio of measurement errors, delta(j)/delta(k), is a more fundamental parameter than either the individual errors (delta(j) and delta(k)), or their sum, absolute value(delta(j)) + absolute value(delta(k)), in governing deltaEi/Ei. The important implication of this observation is that it reveals the possibility of achieving not only a desired level of accuracy in analysis, but even absolute accuracy (i.e. deltaEi/Ei = 0) by causing mutual cancellation of the effects of individual measurement errors delta(j) and delta(k), through proper regulation of measurement parameters. However, as the measurement errors cannot be pre-set, it is shown how selection of proper monitor pairs (j and k) can help achieve the desired accuracy in analysis. The present work sets guidelines for the more general problem of selecting monitor pairs to avoid larger errors in analysis.     

Error-systematics of determining simultaneously the (6 )Li /(7)Li and the (10)B/(11)B abundance ratios as Li(2)BO(2)(+): considerations for analyte elements of varying isotopic abundance patterns

In the Li(2)BO(2)(+) ion beam method of isotopic analysis of elements, the (two) analyte isotopic abundance ratios (E( i)'s) are determined by solving a set of (two) simultaneous equations: f( i)(E( i)'s) = R( i) +/- delta( i), (i = 1, 2) where the R( i)'s are the measured isotopic molecular abundance ratios. We have shown recently that, in the process of solution of these equations, the experimental errors (delta( i)'s) are transformed into the actual errors of analysis, delta(E)'s, through some multiplication factors (MFs). For a given isotopic abundance distribution (IAD) of the Li(2)BO(2)(+) ions, the MFs are shown to depend on the combination of two molecular pairs (CTMPS) used as the monitor pairs for measurements, thereby indicating the requirement of careful selection of monitor pairs for avoiding large (propagated) errors in analysis. In this work, we study how the requirements for a correct analysis vary with the IADs of the elements to be analyzed. The results not only lay great emphasis on the need for proper selection, but also show that no CTMPS can be identified as the universal monitor pairs irrespective of molecular IAD. These observations are, moreover, independent of the actual isotopic abundances of the monitor ions and/or the achievable measurement accuracy (delta( i)). It is noted further that, depending on the IADs of the analyte elements and hence the specific IAD of the Li(2)BO(2)(+) ions, it may also happen that none of the different possible CTMPS really fits the criteria as the recommended monitor pairs, and then one has to ensure that the measurements are not only accurate but as perfect as possible (delta( i)'s --> 0) so as to achieve a reasonable accuracy in analysis. These findings are explained in terms of variations of MFs as a function of molecular IAD, and elaborated using experimental data. A comparative discussion on the determination of both (6)Li/(7)Li and (10)B/(11)B abundance ratios together, and that of either of them, as Li(2)BO(2)(+), is also presented with a view to highlighting different possible aspects of the involvement of computation steps in analysis. The important implication of the present investigation is that it sets guidelines for the general problem of accurately analyzing unknown samples, irrespective of their sources of origin. Copyright 2000 John Wiley & Sons, Ltd.     

Theoretical evaluation of error in the isotopic analysis of carbon and oxygen as CO (2)(+): considerations for determining two different isotopic abundance ratios simultaneously

The computations involved in the CO(2)(+) ion beam method of determining simultaneously a pair of constituent elemental isotopic abundance ratios P and Q (viz. (13)C/(12)C and (17)O/(16)O, or (13)C/(12)C and (18)O/(16)O, or (17)O/(16)O and (18)O/(16)O) are worked out, and the possible implications of their involvement as an analytical step are evaluated theoretically. It is shown, as an immediate consequence, that accurate measurements of the required isotopic CO(2)(+) abundance ratios (R(j) and R(k)) do not necessarily mean that the results (P; Q) are equally accurate. It is demonstrated that, and also explained why, the results can be far more inaccurate, or even in some cases more accurate, than the (R(j);R(k)) values themselves. It is clarified how the errors of analysis (delta(P) and delta(Q)) are actually governed, and elaborated further by evaluating their variations as a function of different possible parameters which control their magnitudes. The investigations thus help to predict the required analytical conditions for accurate isotopic analysis of carbon and/or oxygen samples of any origin as CO(2)(+). The considerations for the case of natural samples predict that, while it should be generally possible to simultaneously determine the isotopic abundance ratios of (13)C/(12)C and (18)O/(16)O with an accuracy better than the measurements themselves, the determination of either the ((13)C/(12)C and (17)O/(16)O) ratios or the ((17)O/(16)O and (18)O/(16)O) ratios, with an accuracy as good as that of the measurements, would be extremely difficult and may, in practice, be impossible.     

Amount-dependent isotopic fractionation during compound-specific isotope analysis

The performance of a gas chromatography-combustion-isotope ratio mass spectrometry system (GC-C-IRMS) with respect to the dependence of delta(13)C values on the amount of sample is presented. Particular attention is paid to the localization of the amount-dependent isotopic fractionation within the system. Injection experiments with varying amounts of gases (CO(2), n-hexane, and toluene) revealed that neither the detector unit nor the combustion reactor, but rather the conditions in the split/splitless injector, contributed to this effect. Although optimization of injector parameters was performed and a reduction of this adverse effect from 3 to 1 per thousand was achieved, it was not possible to eliminate isotopic fractionation completely. Consequently, additional injector parameters have to be considered and adjusted to achieve injection conditions free of fractionation. For routine analysis of the compound-specific delta(13)C analysis of different biomarkers in many environmental samples, perfect optimization may not always be reached. Therefore, in order to prevent systematic errors in the measured delta(13)C values due to different sample concentrations, it is suggested that correction for the remaining unknown amount-dependent fractionation can be made by means of co-analyzing standards of varying analyte concentrations and known delta(13)C values. Residual overall amount-dependent isotope-fractionation can thus be corrected mathematically.     

Isotopologs and isotopomers: New analytical aspects of isotopic mass spectrometry in biology

Metabolic products of biological systems in most cases are polyatomic molecular structures containing polyisotopic elements (hydrogen, carbon, nitrogen, oxygen, sulfur, chorine, etc.) that are different in genesis and biosynthetic pathways. In the molecules carrying two and more atoms of polyisotopic element in different positions (sites), the probability of detection of isotopes by sites in the pool (array) of analyzed molecules may be different, i.e., there is intramolecular isotopic heterogeneity by the specified element. Detection and quantitative estimation of isotopic heterogeneity of polyisotopic elements in molecules by the methods of isotopic mass spectrometry is a novel source of information about the processes involving the respective polyisotopic element. An isotopic equation has been proposed, the coefficients of which are normalized peak intensities of isotopically different molecular ions in the mass spectrum of the analyte. Solutions of this equation reflect the abundance ratios of isotopic atoms of polyisotopic element by its positions in molecules comprising the analyzed pool. During homogenous (equally probable) distribution of isotopic atoms of the element by all of its sites in molecules of the analyzed pool, solutions of the isotopic equation are equal to each other. In the case of non-homogenous isotope distribution of the polyisotopic element in the pool of molecules, solutions of the isotopic equation take on different values and may be presented by both real and complex numbers. Solutions of the isotopic equation reflect the peculiarities of distribution of element isotopes in a molecule and possible pathways of formation of a pool of analyzed molecules under laboratory and natural conditions.     

Gadolinium isotopic measurements by surface ionization mass spectrometry

Gadolinium isotope ratio measurements were carried out by single filament thermal ion source, with the addition of colloidal carbon as surface and chemical modifier. The technique becomes an interesting alternative to classical single filament GdO⁺ and triple filament Gd⁺ analysis. The methodology developed has been applied to a sample containing Gd₂O₃ which was converted to nitrate before loading it on a rhenium filament. Optimum Gd⁺ ion signal is produced at a temperature of 1470C. The measured mean isotopic ratios agree with the representative isotopic composition reviewed and reported by IUPAC. Typical relative external standard deviations of 0.2% are attainable for the¹⁵⁷Gd abundance.     

Determination of boron isotopic variations in aquatic systems with negative thermal ionization mass spectrometry as a tracer for anthropogenic influences

A technique for precise boron isotope ratio measurements with a high detection power has been developed by negative thermal ionization mass spectrometry (NTIMS). Relative standard deviations in the range of 0.03-0.3% have been obtained for the determination of the (11)B/(10)B isotope ratio using nanogram amounts of boron. Ba(OH)(2) has been applied as ionization promoter for the formation of negative thermal ions. By adding MgCl(2) better reproducibilities of the measurement have been achieved. A possible interference of BO(-)(2) ions at mass number 42 by CNO(-) could be excluded by the sample preparation technique used. Contrary to other NTI techniques no dependence of the measured isotope ratio on the boron amount used has been observed. Anthropogenic and natural saline influences in ground water have been successfully identified by boron isotope ratio determinations with this NTIMS method, due to the different isotopic composition of boron in natural and anthropogenic substances. In sewage, the boron isotope ratio is substantially influenced by washing powder, which contains low (11)B/(10)B ratios (expressed in delta(11)B values normalized to the standard reference material NIST SRM 951). In contaminated ground water, low delta(11)B values are normally correlated with high boron and high chloride concentrations. On the other hand, delta(11)B shifts to higher values in less contaminated samples. For ground water with saline influences, only the delta(11)B determination, and not the boron or chloride content, allowed the correct identification of this natural source of contamination.     

Mass spectrometric isotopic and trace analysis of187Os

The application of mass spectrometric methods in the determination of isotopic abundance and of trace elements in highly enriched¹⁸⁷Os is described. The capability of ICP-MS in comparison with solid-state mass spectrometric techniques (SIMS, SNMS and GDMS) for the precise isotopic analysis of highly-enriched osmium has been investigated. The formation of cluster ions in several plasma types has been measured, and the problems of possible interferences from molecular and cluster ions is discussed.     

Cumulative reaction probabilities and transition state properties: a study of the F + H2 reaction and its deuterated isotopic variants

A comparative quantum mechanical (QM) and quasiclassical trajectory (QCT) study of the cumulative reaction probabilities (CRPs) is presented in this work for the F + H(2) reaction and its isotopic variants for low values of the total angular momentum J. The agreement between the two sets of calculations is very good with the exception of some features whose origin is genuinely QM. The agreement also extends to the CRP resolved in the helicity quantum number k. The most remarkable feature is the steplike structure, which becomes clearly distinct when the CRPs are resolved in odd and even rotational states j. The analysis of these steps shows that each successive increment is due to the opening of the consecutive rovibrational states of the H(2) or D(2) molecule, which, in this case, nearly coincide with those of the transition state. Moreover, the height of each step reflects the number of helicity states compatible with a given J and j values, thus indicating that the various helicity states for a specific j have basically the same contribution to the CRPs at a given total energy. As a consequence, the dependence with k of the reactivity is practically negligible, suggesting very small steric restrictions for any possible orientation of the reactants. This behavior is in marked contrast to that found in the D + H(2) reaction, wherein a strong k dependence was found in the threshold and magnitude of the CRP. The advantages of a combined QCT and QM approaches to the study of CRPs are emphasized in this work.     

Determination of Neodymium and Ytterbium by Isotope Dilution Laser Mass Spectrometry

The experimental conditions for minimizing errors in the laser mass spectrometric (LMS) analysis of isotopic ratios for Nd and Yb were found, and the optimum isotope pairs for determining these elements with the use of isotope dilution (ID) were chosen. The precision and accuracy of LMS data, which are characterized by relative random errors (RSD) and relative systematic errors (), can be improved using ID. The values for Nd or Yb were no higher than RSD = 2.9% and = 0.024 or RSD = 5% and = 0.097, respectively.     

单掺杂与共掺杂离子对Sr2Mg(BO3)2磷光体热释发光的影响

通过高温固相法合成了Sr2Mg(BO3)2磷光体, 并研究了Li＋, Bi3＋, Gd3＋, Ti4＋共掺杂对Sr2Mg(BO3)2∶Dy磷光体热释发光的影响. 研究发现: Li＋的共掺杂使Sr2Mg(BO3)2∶Dy磷光体的热释光主峰强度增加, 而 Bi3＋, Gd3＋或Ti4＋的掺入使样品的热释光强度降低. 在Li＋, Bi3＋, Gd3＋或Ti4＋共掺杂的Sr2Mg(BO3)2∶Dy磷光体高温热释光发射谱中, 我们观察到了480, 579, 662和755 nm的发射峰, 为特征Dy3＋离子的4F9/2→6H15/2, 4F9/2→6H13/2, 4F9/2→6H11/2和4F9/2→6H9/2跃迁, 与Sr2Mg(BO3)2∶Dy磷光体的发射一致. 利用峰形法, 我们评估了Sr2Mg(BO3)2∶ , ( )热释光磷光体234 ℃发光峰的动力学参数, 陷阱深度E＝1.1 eV, 频率因子s＝6.3×109 s－1, 遵循二级动力学.     

The effect of N2O on the isotopic composition of air-CO2 samples

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.     

Comparative analysis of ceramide structural modification found in fungal cerebrosides by electrospray tandem mass spectrometry with low energy collision-induced dissociation of Li+ adduct ions

Fungal cerebrosides (monohexosylceramides, or CMHs) exhibit a number of ceramide structural modifications not found in mammalian glycosphingolipids, which present additional challenges for their complete characterization. The use of Li+ cationization, in conjunction with electrospray ionization mass spectrometry and low energy collision-induced dissociation tandem mass spectrometry (ESI-MS/CID-MS), was found to be particularly effective for detailed structural analysis of complex fungal CMHs, especially minor components present in mixtures at extremely low abundance. A substantial increase in both sensitivity and fragmentation was observed on collision-induced dissociation of [M + Li]+ versus [M + Na]+ of the same CMH components analyzed under similar conditions. The effects of particular modifications on fragmentation were first systematically evaluated by analysis of a wide variety of standard CMHs expressing progressively more functionalized ceramides. These included bovine brain galactocerebrosides with non-hydroxy and 2-hydroxy fatty N-acylation; a plant glucocerebroside having (E/Z)-delta8 in addition to (E)-delta4 unsaturation of the sphingoid base; and a pair of fungal cerebrosides known to be further modified by a branching 9-methyl group on the sphingoid moiety, and to have a 2-hydroxy fatty N-acyl moiety either fully saturated or (E)-delta3 unsaturated. The method was then applied to characterization of both major and minor components in CMH fractions from a non-pathogenic mycelial fungus, Aspergillus niger; and from pathogenic strains of Candida albicans (yeast form); three Cryptococcus spp. (all yeast forms); and Paracoccidioides brasiliensis (both yeast and mycelium forms). The major components of all species examined differed primarily (and widely) in the level of 2-hydroxy fatty N-acyl delta3 unsaturation, but among the minor components a significant degree of additional structural diversity was observed, based on differences in sphingoid or N-acyl chain length, as well as on the presence or absence of the sphingoid delta8 unsaturation or 9-methyl group. Some variants were isobaric, and were not uniformly present in all species, affirming the need for MS/CID-MS analysis for full characterization of all components in a fungal CMH fraction. The diversity in ceramide distribution observed may reflect significant species-specific differences among fungi with respect to cerebroside biosynthesis and function.     

Automated determination of precursor ion, product ion, and neutral loss compositions and deconvolution of composite mass spectra using ion correlation based on exact masses and relative isotopic abundances

After an accidental, deliberate, or weather-related dispersion of chemicals (dispersive event), rapid determination of elemental compositions of ions in mass spectra is essential for tentatively identifying compounds. A direct analysis in real time (DART)ion source interfaced to a JEOL AccuTOFmass spectrometer provided exact masses accurate to within 2 mDa for most ions in full scan mass spectra and relative isotopic abundances (RIAs) accurate to within 15-20% for abundant isotopic ions. To speed determination of the correct composition for precursor ions and most product ions and neutral losses, a three-part software suite was developed. Starting with text files of m/z ratios and their ion abundances from mass spectra acquired at low, moderate, and high collision energies, the ion extraction program (IEP) compiled lists for the most abundant monoisotopic ions of their exact masses and the RIAs of the +1 and +2 isotopic peaks when abundance thresholds were met; precursor ions; and higher-mass, precursor-related species. The ion correlation program (ICP) determined if a precursor ion composition could yield a product ion and corresponding neutral loss compositions for each product ion in turn. The input and output program (IOP) provided the ICP with each precursor ion:product ion pair for multiple sets of error limits and prepared correlation lists for single or multiple precursor ions. The software determined the correct precursor ion compositions for 21 individual standards and for three- and seven-component mixtures. Partial deconvolution of composite mass spectra was achieved based on exact masses and RIAs, rather than on chromatography.     

Atomicm absorption spectrometric determination of the isotopic composition of lithium by an ultimate absorbance-ratio technique

The conventional absorbance-ratio technique for determining the isotopic composition of lithium by atomic absorption spectrometry is improved by the use of “ultimate absorbance ratios” of sample solutions. These ratios are obtained by extrapolating the linear portion of lithium content/absorbance-ratio plots to the intercept at 0 mol m^?3 lithium. These graphs are obtained measuring the absorbances of solutions of known ⁶Li abundance and of various lithium contents with natural and ⁶Li-enriched lithium hollow-cathode lamps. Linear calibration is attained over the range 0.0–99.3% ⁶Li, and the lithium isotopic abundance can be determined with an absolute error of ±0.7% ⁶Li for > 0.01 mol m^?3 lithium solutions. The method requires neither prior measurement of the total lithium content in sample solutions nor adjustment of the content to match that in the standard solutions.     

Monodisperse and core-shell-structured SiO2@YBO3:Eu3+ spherical particles: synthesis and characterization

Y0.9Eu0.1BO3 phosphor layers were deposited on monodisperse SiO2 particles of different sizes (300, 570, 900, and 1200 nm) via a sol-gel process, resulting in the formation of core-shell-structured SiO2@Y0.9Eu0.1BO3 particles. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL), and cathodoluminescence (CL) spectra as well as lifetimes were employed to characterize the resulting composite particles. The results of XRD, FE-SEM, and TEM indicate that the 800 degrees C annealed sample consists of crystalline YBO3 shells and amorphous SiO2 cores, in spherical shape with a narrow size distribution. Under UV (240 nm) and VUV (172 nm) light or electron beam (1-6 kV) excitation, these particles show the characteristic 5D0-7F1-4 orange-red emission lines of Eu3+ with a quantum yield ranging from 36% (one-layer Y0.9Eu0.1BO3 on SiO2) to 54% (four-layer Y0.9Eu0.1BO3 on SiO2). The luminescence properties (emission intensity and color coordinates) of Eu3+ ions in the core-shell particles can be tuned by the coating number of Y0.9Eu0.1BO3 layers and SiO2 core particle size to some extent, pointing out the great potential for these particles applied in displaying and lightening fields.     

Mass discrimination during MC-ICPMS isotopic ratio measurements: Investigation by means of synthetic isotopic mixtures (IRMM-007 series) and application to the calibration of natural-like zinc materials (including IRMM-3702 and IRMM-651)

A long known way of anchoring isotope ratio values to the SI system is by means of gravimetrically prepared isotopic mixtures. Thermal ionization mass spectrometry (TIMS) is the traditionally associated measurement technique, but multi-collector double focusing inductively coupled plasma (MC-ICP)-MS now appears to be an attractive alternative. This absolute calibration strategy necessitates that mass discrimination effects remain invariant in time and across the range of isotope ratios measured. It is not the case with MC-ICPMS and the present work illustrates, in the case of Zn isotopic measurements carried out using locally produced synthetic Zn isotope mixtures (IRMM-007 series), how this calibration strategy must be adjusted. First, variation in mass discrimination effects across the measurement sequence is propagated as an uncertainty component. Second, linear proportionality during each individual measurement between normalized mass discrimination and the average mass of the isotope ratios is used to evaluate mass discrimination for the ratios involving low abundance isotopes. Third, linear proportionality between mass discrimination and the logarithm of the isotope ratio values for n(67Zn)/n(64Zn) and n(68Zn)/n(64Zn) in the mixtures is used iteratively to evaluate mass discrimination for the same ratios in the isotopically enriched materials. Fourth, ratios in natural-like materials (including IRMM-3702 and IRMM-651) are calibrated by external bracketing using the isotopic mixtures. The relative expanded uncertainty (k = 2) estimated for n(68Zn)/n(64Zn) and n(67Zn)/n(64Zn) ratio values in the synthetic isotopic mixtures and the natural-like zinc samples was in the range of 0.034 to 0.048%. The uncertainty on the weighing (0.01%, k = 1) was the largest contributor to these budgets. The agreement between these results and those obtained with a single detector TIMS and with another MC-ICPMS further validated this work. The absolute isotope ratio values found for IRMM-3702-material also proposed as "delta 0" for delta-scale isotopic measurements-are n(66Zn)/n(64Zn) = 0.56397 (30), n(67Zn)/n(64Zn) = 0.082166 (35), n(68Zn)/n(64Zn) = 0.37519 (16), and n(70Zn)/n(64Zn) = 0.012418 (23). The derived Zn atomic weight value Ar(Zn) = 65.37777 (22) differs significantly from the current IUPAC value by Chang et al. [1]. Remeasurement, with isotopic mixtures from the IRMM-007 series, of the Zn isotope ratios in the same Chang et al. [1] material have revealed large systematic differences (1.35 (27)% per atomic mass unit) that suggest unrecognized measurement biases in their results.     

Automated analysis of 13C/12C ratios in CO2 and dissolved inorganic carbon for ecological and environmental applications

Stable carbon isotope ratios (13C/12C) are a valuable tool for studying a wide range of environmental processes, including carbon cycling and subsurface microbial activity. Recent advances in automated analysis provide the opportunity to increase greatly the ease and consistency of isotopic analysis. This study evaluated an automated headspace sampler linked to a commercially available CO2 preconcentration system and continuous flow isotope ratio mass spectrometer. Field sampling and analysis methods are illustrated for delta13C of soil respired CO2, from both tracer and natural abundance experiments, and dissolved inorganic carbon from contaminated groundwater. The automated system demonstrated accuracy, precision, and linearity, with standard errors below 0.1 per thousand for replicate gas standards run at concentrations varying five-fold. It measured 40 samples per 10-hour run, with concentrations ranging from ppb to percentage levels. In the field, gas samples were injected into nitrogen-filled autosampler vials, thereby allowing use of small sample volumes, control of analyte concentration, and direct analysis by the automated system with no further preparation. A significant linear relationship between standard concentrations and peak area allows for accurate estimates of sample CO2 concentration from the mass spectrometric data. The ability to analyze multiple small-volume samples with minimal off-line preparation should enhance the application of isotopes to well-replicated field experiments for process-level studies and spatial and temporal scaling.     

Liquid and gas chromatography coupled to isotope ratio mass spectrometry for the determination of 13C-valine isotopic ratios in complex biological samples

On-line gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) is commonly used to measure isotopic ratios at natural abundance as well as for tracer studies in nutritional and medical research. However, high-precision (13)C isotopic enrichment can also be measured by liquid chromatography-isotope ratio mass spectrometry (LC-IRMS). Indeed, LC-IRMS can be used, as shown by the new method reported here, to obtain a baseline separation and to measure (13)C isotopic enrichment of underivatised amino acids (Asp, Thr-Ser, Glu, Pro, Gly, Ala, Cys and Val). In case of Val, at natural abundance, the SD(delta(13)C) reported with this method was found to be below 1 per thousand . Another key feature of the new LC-IRMS method reported in this paper is the comparison of the LC-IRMS approach with the conventional GC-C-IRMS determination. To perform this comparative study, isotopic enrichments were measured from underivatised Val and its N(O, S)-ethoxycarbonyl ethyl ester derivative. Between 0.0 and 1.0 molar percent excess (MPE) (delta(13)C= -12.3 to 150.8 per thousand), the calculated root-mean-square (rms) of SD was 0.38 and 0.46 per thousand and the calculated rms of accuracy was 0.023 and 0.005 MPE, respectively, for GC-C-IRMS and LC-IRMS. Both systems measured accurately low isotopic enrichments (0.002 atom percent excess (APE)) with an SD (APE) of 0.0004. To correlate the relative (delta(13)C) and absolute (atom%, APE and MPE) isotopic enrichment of Val measured by the GC-C-IRMS and LC-IRMS devices, mathematical equations showing the slope and intercept of the curves were established and validated with experimental data between 0.0 to 2.3 MPE. Finally, both GC-C-IRMS and LC-IRMS instruments were also used to assess isotopic enrichment of protein-bound (13)C-Val in tibial epiphysis in a tracer study performed in rats. Isotopic enrichments measured by LC-IRMS and GC-C-IRMS were not statistically different (p>0.05). The results of this work indicate that the LC-IRMS was successful for high-precision (13)C isotopic measurements in tracer studies giving (13)C isotopic enrichment similar to the GC-C-IRMS but without the step of GC derivatisation. Therefore, for clinical studies requiring high-precision isotopic measurement, the LC-IRMS is the method of choice to measure the isotopic ratio.     

Are liquid chromatography/electrospray tandem quadrupole fragmentation ratios unequivocal confirmation criteria?

Multiple reaction monitoring (MRM) ratios as provided by tandem mass spectrometers are used to confirm positive residue findings (e.g. veterinary drugs or pesticides). The Commission Decision 2002/657/EEC defines tolerance levels for MRM ratios, which are intended to prevent the reporting of false positives. This paper reports findings where blank sample extracts have been spiked by a drug (difloxacin) and the corresponding measured MRM ratios significantly deviated from MRM ratios observed in matrix‐free solution. The observation was explained by the formation of two different [M+H]⁺ analyte ions within the electrospray ionization (ESI) interface. These two ions vary only by the site of analyte protonation. Since they are isobaric, they are equally transmitted through the first quadrupole, but are differently fragmented in the collision chamber. The existence of two isobaric ions was deduced by statistical data and the observation of a doubly charged analyte ion. It was hypothesized that the combined presence of [M+H]⁺ and [M+2H]²⁺ implies the existence of two different singly charged ion species differing only by the site of protonation. Low‐ and high‐energy interface‐induced fragmentation was performed on the samples. The surviving precursor ion population was mass selected and again fragmented in the collision chamber. Equal product ion spectra would be expected. However, very different product ion spectra were observed for the two interface regimes. This is consistent with the assumption that the two postulated isobaric precursor ions show different stability in the interface. Hence the abundance ratio among the two types of surviving precursor ions will shift and change the resulting product ion spectra. The existence of the postulated singly charged ions with multiple chargeable sites was finally confirmed by successful ion mobility separation. Copyright © 2009 John Wiley & Sons, Ltd.