Analyte Internal Standardization Research Articles

An interactive R-based custom quantification program for semi-quantitative analysis of triacylglycerols in bovine milk.

The R programming language, RStudio, and open-source software solutions for analysis of liquid chromatography-mass spectrometry (LC-MS) data have been used with user-written R-based custom quantification programs (CQP) for semi-quantification of triacylglycerols (TAGs) in bovine milk lipid extracts. Using the peak-finding capabilities of the package "xcms" in RStudio, peaks were integrated, and retention times aligned, normalized, and then used for semi-quantitative analysis of a custom set of four extraction internal standards (EISs) and 29 TAG regioisomers using the choice of four analytical internal standards (AISs). Alternating stereospecific numbering (sn) 1,3 TAG regioisomers (standards 1, 3, and 5 of six calibration standards) and sn-1,2 TAG regioisomers (standards 2, 4, and 6 of six standards) were used to make a set of six calibration standards, which were used for quantification using a linear fit model, polynomial fit model, power fit model, level-bracketed linear fit, replicate-bracketed polynomial fit, replicate-bracketed power fit, and replicate- and level-bracketed linear fit and response factors. For example, the linear fit for EIS1 gave an unacceptable coefficient of determination (CoD), r2 = 0.9616, whereas the polynomial fit gave r2 = 0.9908 and the power fit gave r2 = 0.9928, while the double-bracketed linear fit gave CoDs of r2 = 0.9960, 0.9848, and 0.9781 for the three brackets, yet gave the least % difference to known calibration concentrations. For unparalleled transparency, the CQP produced webpages that allowed every step in the data processing and quantification sequence to be verified and reproduced, and contained interactive figures. The data are publicly available using a digital object identifier (DOI). The R code can be downloaded and used with the downloadable data to reproduce the results, to modify the code and further customize the results, or to copy and paste and adapt the code to other quantification applications.

Thermal degradation of metabolites in urine using multiple isotope-labelled internal standards for off-line GC metabolomics - effects of injector and oven temperatures

Thermal processes are widely used in small molecule chemical analysis and metabolomics for derivatization, vaporization, chromatography, and ionization, especially in gas chromatography mass spectrometry (GC/MS). An optimized derivatization protocol has been successfully applied using multiple isotope labelled analytical internal standards of selected deuterated and 13C selected compounds, covering a range of different groups of metabolites for non-automated GC metabolomics (off-line). Moreover, the study was also realized in a pooled urine sample, following metabolic profiling. A study of thermal degradation of metabolites due to GC inlet and oven programs (fast, slow) was performed, where the results indicated that both GC oven programs (fast and slow) negatively affected the thermal stability of the metabolites, while the fast-ramp GC program also suppressed MS signals. However, the use of multiple internal standards can overcome this drawback. The application of extended temperature ramp GC program presented identical behaviour on metabolite stability and better chromatographic separation combined with much lower signal suppression, compared to a short temperature ramp program. No effects were observed for organic acids, fatty acids, sugars and sugar alcohols, while significant differences were observed for amino acids. GC metabolomics is a strong tool that can facilitate analysis, but special attention is required for sampling handling and heating, before and during the GC analysis. The use and application of multiple multi-group internal standards is highly recommended.

A combined TIMS and ICP-MS study for Th0.5Np0.5O2 thorium neptunium mixed oxide analysis

This paper presented thorium and neptunium quantification of a new synthesized powder, a (Th,Np)O2 mixed oxide. Thorium was quantified by Thermal Ionization Mass Spectrometry using Isotope Dilution (ID-TIMS) which is one of the most accurate methodology to determine actinide mass fraction. The (sample-spike) mixture isotope ratio was measured using the TIMS reference method for major isotope ratio determination, i.e. the total evaporation method. An expanded uncertainty below 1% was estimated for the Th mass fraction. The study of the uncertainty sources showed that the main one comes from the spike mass fraction uncertainty. As Np quantification with ID-TIMS is difficult due to the lack of commercially-available spike solution, the Np mass fraction was obtained by ICP-MS measurements using a gravimetric standard addition method combined with an analytical internal standard. The developed method used an element initially present in the sample, here Th, as an internal standard for ICP-MS measurements. This helps reducing the number of manipulation as well as cross contamination, which is an important risk when experiments are performed in glove boxes. Also, equations are simplified. An expected uncertainty about 1.8% was estimated for Np. Finally, the Th–Np mixed oxide was found to have a Th0.4836(50)Np0.5164(50)O2 stoichiometry. This study can be extended to other complicated samples or to the characterization of in-house certified reference material.

Application of chemometrics to resolve overlapping mass spectral peak clusters between trichloroethylene and its deuterated internal standard

Using one or two (2) H-atom-labeled analogs as internal standards (ISs) may cause a 'cross-contribution' problem (i.e., the overlap of ions from the IS and the analyte) especially for halogenated volatile organic compounds (VOCs). However, in this situation the overlapping peak clusters of the analyte and ISs can be resolved by multivariate chemometric methods such as classical least-squares (CLS) and inverse least-squares (ILS). Trichloroethylene (TCE) and its internal standard, deuterated TCE (TCE-d), as model compounds, were analyzed using portable gas chromatography/mass spectrometry. CLS and ILS were applied to resolve overlapping TCE and TCE-d mass spectral signals and evaluated for the determination of TCE. CLS and ILS models were constructed and used to predict concentration ratios of TCE to TCE-d. Calibration samples were prepared by adding TCE at different concentrations and TCE-d at 300 ng mL(-1) as an IS. The calibration curve was linear over a range of 10-1000 ng mL(-1) with a coefficient of determination (R(2)) of 0.993. A validation data set collected 2 weeks later was used to further test the model robustness. Lower prediction errors and higher correlation coefficients were obtained from TCE/TCE-d ratios predicted by the CLS model. This paper describes the first application of CLS to deconvolute overlapping peaks between an analyte and its corresponding isotopic internal standard for quantification. The proposed method enables simple isotopic analogs of analytes (one H or C atom is isotopically labeled) to be used as internal standards for analytes with isotopic distributions. It has wide application because of the environmental impact and prevalence of halogenated VOCs, especially when analytes have isotopic distributions that overlap with an internal standard or when sophisticated isotopic analogs of the analytes with three or more (2)H- or/and (13)C-atoms are prohibitively expensive or even impossible.

Simultaneous quantification and isotope ratio measurement of boron in uranium-silicon-aluminium compounds by inductively coupled plasma orthogonal acceleration time of flight mass spectrometry (ICP-oa-TOFMS) after its separation by pyrohydrolysis

A simple and efficient procedure has been developed for the rapid and quantitative extraction of boron (B) from uranium-silicon-aluminium compounds, aided by generation of in-situ accelerator, employing pyrohydrolysis (PH) technique. The PH operating conditions like choice of carrier gas, heating time and sample mass were optimized for rapid and quantitative recovery of B. Inductively coupled plasma orthogonal acceleration time of flight mass spectrometer (ICP-oa-TOFMS) was utilized for the simultaneous determination of B content and its isotope ratio (10B/11B). The ICP-oa-TOFMS operating parameters like acquisition time, mass bias, long- and short-term stabilities were optimized and common analyte internal standardization (CAIS) chemometric technique was applied to improve both the accuracy and precision of 10B/11B ratio. The detection limit of the method for B was found to be 0.7μgL-1. The proposed method for B quantification was validated by an independent spectrophotometric technique. The accuracy on 10B/11B ratio measurement was established by carrying out PH of a synthetic mixture of U3O8 (free from B) and NIST SRM 951 followed by ICP-oa-TOFMS analysis of the distillate. XRD study of the sample before and after PH revealed the auto-acceleration ability of in-situ generated U3O8 phase on faster and efficient removal of analyte. The developed methodology was successfully applied for the analysis of five real samples of uranium silicide dispersed in aluminium matrix (U3Si2-Al) and three real uranium-silicon-aluminium alloy U(Al,Si)3 samples. The relative standard deviations (1σ) on B content and 10B/11B ratio in these samples were found to be within 3% and 0.16-0.27% respectively.

Common analyte internal standardization as a tool for correction for mass discrimination in multi-collector inductively coupled plasma-mass spectrometry

It is well known that to achieve accurate isotope ratio data using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS), mass discrimination needs to be adequately corrected for. In this work, the capabilities and limitations of common analyte internal standardization (CAIS) as a method for correction for mass discrimination were assessed for two target elements, one in the low mass region (boron) and another in the medium mass region (antimony). CAIS has already been used in the context of element determination and, more recently, also in isotope ratio measurements with quadrupole-based ICP-MS, but it has, to the best of the authors' knowledge, never been applied to isotope ratio determination via MC-ICP-MS so far. Results obtained relying on CAIS were compared with those obtained via more established correction models, i.e. external correction in a standard-sample bracketing approach (SSB) and internal correction based on both the Russell's law (original and empirical) and the revised Russell's law. To the best of the authors' knowledge, such comparison has not been done before, especially not for low mass elements. Also the robustness of these approaches with respect to concomitant matrix elements was assessed using synthetic solutions, containing various concentrations of the matrix elements Be, Cs, Be+Cs or Fe. Whereas for B, the isotope ratio result did not seem to be significantly affected by the matrix, thus suggesting that complete separation of B from a matrix might not be necessary, at least in the cases studied, the mass discrimination observed for Sb was influenced by the presence of Cs. The experiments carried out demonstrate that the CAIS technique can be successfully applied for mass bias correction in MC-ICP-MS, providing data of the same quality as the revised Russell's law, while being more transparent and accessible. While for B, all mass bias correction approaches tested provided similar data quality (external correction with SSB gives slightly better precision), for Sb, CAIS and the revised Russell's law provide better precision and accuracy.

Analysis of [ 2H 7]methionine, [ 2H 4]methionine, methionine, [ 2H 4]homocysteine and homocysteine in plasma by gas chromatography–mass spectrometry to follow the fate of administered [ 2H 7]methionine

Homocysteine plays a key role in several pathophysiological conditions. To assess the methionine–homocysteine kinetics by stable isotope methodology, we developed a simultaneous quantification method of [ 2H 7]methionine, [ 2H 4]methionine, methionine, [ 2H 4]homocysteine and homocysteine in rat plasma by gas chromatography–mass spectrometry (GC–MS). [ 13C]Methionine and [ 13C]homocysteine were used as analytical internal standards to account for losses associated with the extraction, derivatization and chromatography. For labeled and non-labeled homocysteine measurements, disulfide bonds between homocysteine and other thiols or proteins were reduced by dithiothreitol. The reduced homocysteine and methionine species were purified by cation-exchange chromatography and derivatized with isobutyl chlorocarbonate in water–ethanol–pyridine. Quantification was carried out by selected ion monitoring of the molecular-related ions of N( O, S)-isobutyloxycarbonyl ethyl ester derivatives on the chemical ionization mode. The intra- and inter-day precision of the assay was less than 6% for all labeled and non-labeled methionine and homocysteine species. The method is sensitive enough to determine pharmacokinetics of labeled methionine and homocysteine.

Accurate and Precise Zinc Isotope Ratio Measurements in Urban Aerosols

We developed an analytical method and constrained procedural boundary conditions that enable accurate and precise Zn isotope ratio measurements in urban aerosols. We also demonstrate the potential of this new isotope system for air pollutant source tracing. The procedural blank is around 5 ng and significantly lower than published methods due to a tailored ion chromatographic separation. Accurate mass bias correction using external correction with Cu is limited to Zn sample content of approximately 50 ng due to the combined effect of blank contribution of Cu and Zn from the ion exchange procedure and the need to maintain a Cu/Zn ratio of approximately 1. Mass bias is corrected for by applying the common analyte internal standardization method approach. Comparison with other mass bias correction methods demonstrates the accuracy of the method. The average precision of delta(66)Zn determinations in aerosols is around 0.05 per thousand per atomic mass unit. The method was tested on aerosols collected in Sao Paulo City, Brazil. The measurements reveal significant variations in delta(66)Zn(Imperial) ranging between -0.96 and -0.37 per thousand in coarse and between -1.04 and 0.02 per thousand in fine particular matter. This variability suggests that Zn isotopic compositions distinguish atmospheric sources. The isotopic light signature suggests traffic as the main source. We present further delta(66)Zn(Imperial) data for the standard reference material NIST SRM 2783 (delta(66)Zn(Imperial) = 0.26 +/- 0.10 per thousand).

Parallel Synthesis: A New Approach for Developing Analytical Internal Standards. Application to the Analysis of Patulin by Gas Chromatography−Mass Spectrometry

The polymer-assisted reaction of 4-(hydroxymethyl)furan-2(5H)-one (4HM2F) with 21 carboxylic acids using polystyrene-carbodiimide (PS-carbodiimide) yielded an ester library. Four of the esters, (5-oxo-2,5-dihydrofuran-3-yl)methyl acetate (IS-1), (5-oxo-2,5-dihydrofuran-3-yl)methyl butyrate (IS-2), (5-oxo-2,5-dihydrofuran-3-yl)methyl 2-methylpropanoate (IS-3), and (5-oxo-2,5-dihydrofuran-3-yl)methyl chloroacetate (IS-4), were tested as internal standards for the quantification of patulin in apple juice by gas chromatography-mass spectrometry in the selected ion monitoring mode (GC-MS-SIM). The developed method combines an AOAC official extractive step and a GC-MS-SIM analysis. Using a chromatographic column containing trifluoropropylmethylpolysiloxane as the stationary phase and IS-1 as the internal standard, it was possible to perform an accurate and precise quantification of underivatizated patulin in apple juice at concentrations down to 6 microg/L. A detection limit of 1 microg/L was established.

Simultaneous determination of endogenous and 13C-labelled thyroid hormones in plasma by stable isotope dilution mass spectrometry

This study describes a capillary gas chromatography–mass spectrometry (GC–MS) method for the simultaneous determination of endogenous thyroid hormone (thyroxine, T 4) and its 13 C -labelled analogue ( 13 C 6 -thyroxine) in plasma. 13 C 9 -thyroxine was used as analytical internal standard. A double derivatization (CH 3OH/HCl and HFBA) inducing good GC mobility was used for the GC–MS analysis of the thyroid hormones. Quantification was carried out by selected ion monitoring (SIM) of specific ions of the fragment ions ( m/ z 970/976/979). The detection limit of the present GC–MS–SIM method was found to be 100 pg per injection for thyroxine (S/N=3.0). A first implementation in in vivo tests of 13 C 6 -T 4 like metabolic tracer was carried out under veterinary control on one cat and one rabbit. The thyroxine follow-up was done by GC–MS and based on double isotopic dilution with two different regio-selective 13 C -labelled molecules of the same hormone. The present paper discusses the possibilities and limitations of this methodology. The in vivo experiment demonstrated that the use of stable isotopes and mass spectrometry provide a reliable methodology for hormonal monitoring.

Simultaneous determination of endogenous and 13C-labelled thyroid hormones in plasma by stable isotope dilution mass spectrometry

This study describes a capillary gas chromatography–mass spectrometry (GC–MS) method for the simultaneous determination of endogenous thyroid hormone (thyroxine, T4) and its 13C-labelled analogue (13C6-thyroxine) in plasma. 13C9-thyroxine was used as analytical internal standard. A double derivatization (CH3OH/HCl and HFBA) inducing good GC mobility was used for the GC–MS analysis of the thyroid hormones. Quantification was carried out by selected ion monitoring (SIM) of specific ions of the fragment ions (m/z 970/976/979). The detection limit of the present GC–MS–SIM method was found to be 100 pg per injection for thyroxine (S/N=3.0). A first implementation in in vivo tests of 13C6-T4 like metabolic tracer was carried out under veterinary control on one cat and one rabbit. The thyroxine follow-up was done by GC–MS and based on double isotopic dilution with two different regio-selective 13C-labelled molecules of the same hormone. The present paper discusses the possibilities and limitations of this methodology. The in vivo experiment demonstrated that the use of stable isotopes and mass spectrometry provide a reliable methodology for hormonal monitoring.

Simultaneous determination of endogenous and stable isotope-labelled 6β-hydroxycortisols in human urine by stable isotope dilution mass spectrometry

This study describes a capillary GC–MS method for the simultaneous determination of endogenous 6β-hydroxycortisol (6β-OHF) and its stable isotope-labelled analogue, 6β-hydroxy-[1,1,19,19,19- 2H 5]cortisol (6β-OHF- 2H 5), in human urine. 6β-Hydroxy-[1,2,4,19- 13C 4,1,1,19,19,19- 2H 5]cortisol (cortisol- 13C 4, 2H 5) was used as an analytical internal standard. The methoxime trimethylsilyl ether (MO-TMS) derivatization was employed for the GC–MS analysis of 6β-OHF. Quantitation was carried out by selected-ion monitoring (SIM) of the characteristic fragment ion ([M−31] +·) of the MO-TMS derivative of 6β-OHF. The sensitivity limit of the present GC–MS-SIM method was found to be 25 pg per injection for 6β-OHF ( S/ N ratio=5.6). The within-day reproducibility in the amounts of unlabelled and labelled 6β-OHFs determined were in good agreement with the actual amounts added, the relative errors being less than 5.30%. The inter-assay RSDs were less than 4.95% for unlabelled and labelled 6β-OHFs.

Syntheses of stable isotope-labeled 6β-hydroxycortisol, 6β-hydroxycortisone, and 6β-hydroxytestosterone

A method is described for the preparation of two types of multi-labeled 6β-hydroxycortisol containing either five deuterium atoms at C-19 methyl and C-1 methylene or four 13 C atoms at C-1, C-2, C-4, and C-19 in addition to the five deuterium atoms for use as analytical internal standards for gas chromatography-mass spectrometry (GC-MS). BMD derivatives of [1,1,19,19,19- 2 H 5 ]cortisone and [1,2,4,19- 13 C 4 ,1,1,19,19,19- 2 H 5 ]cortisone (cortisone- 2 H 5 -BMD and cortisone- 13 C 4 , 2 H 5 -BMD) were first synthesized via indan synthon method starting from optical active 11-oxoindanylpropionic acid and labeled isopropenyl anion ([1,1,3,3,3- 2 H 5 ]- or [1,3- 13 C 2 ,1,1,3,3,3- 2 H 5 ]isopropenyl anion). The labeled isopropenyl anion was prepared from commercially available [1,1,1,3,3,3- 2 H 6 ]- or [1,3- 13 C 2 ,1,1,1,3,3,3- 2 H 6 ]acetone. Ultraviolet (UV) irradiated autoxidation at C-6 position of 3-ethyl-3,5-dienol ether derivatives of the labeled cortisone-BMDs gave 6β-hydroxy-[1,1,19,19,19- 2 H 5 ]cortisone-BMD and 6β-hydroxy-[1,2,4,19- 13 C 4 ,1,1,19,19,19- 2 H 5 ]cortisone-BMD, respectively, as a mixture of 6β- and 6α-epimers in a ratio of 4:1. Separation of 6β- and 6α-epimers by thin-layer chromatography (TLC) and subsequent hydrolysis of the BMD group at C-17 gave pure labeled 6β-hydroxycortisone. After protecting the keto group at C-3 of the labeled 6β-hydroxycortisone-BMD as semicarbazone, reduction of 11-keto group with NaBH 4 and subsequent removal of the C-3 and C-17 protecting groups gave 6β-hydroxy-[1,1,19,19,19- 2 H 5 ]cortisol (6β-hydroxycortisol- 2 H 5 ) and 6β-hydroxy-[1,2,4,19- 13 C 4 ,1,1,19,19,19- 2 H 5 ]cortisol (6β-hydroxycortisol- 13 C 4 , 2 H 5 ), respectively, as a mixture of 6β- and 6α-epimers (6β:6α=4.4:1). The isotopic compositions of 6β-hydroxycortisol- 2 H 5 and 6β-hydroxycortisol- 13 C 4 , 2 H 5 were 90.9 and 92.1 at.%, respectively. Furthermore, 6β-hydroxy-[1α,16,16,17α- 2 H 4 ]testosterone was synthesized by the UV irradiated autoxidation at C-6 position of 3-ethyl-3,5-dienol ether derivative of deuterium-labeled testosterone ([1α,16,16,17α- 2 H 4 ]testosterone) obtained by using catalytic deuteration and hydrogen–deuterium exchange reactions.

Simultaneous correction for drift and non-spectroscopic matrix effect in the measurement of geological samples by inductively coupled plasma-mass spectrometry using common analyte internal standardization chemometric technique

The common analyte internal standardization (CAIS) chemometric technique is extended to correct for drift in signal intensity in inductively coupled plasma-mass spectrometry. The CAIS technique is used because, unlike the conventional internal reference method, it allows for the analyte to behave differently from the internal reference under the influence of drift. Thus, only one internal reference element is sufficient to correct for drift effect for all the analytes irrespective of the difference of their mass and ionization potential from that of the internal reference element. Experimental test with 15 analytes, as a representative of the whole mass range, in different geological matrices, using only indium as internal reference element, demonstrates that the developed drift correction method is efficient in correcting up to 25% drift error and is easy to use. Further, a CAIS scheme was developed to correct for both drift and non-spectroscopic matrix effects simultaneously in a single run, using standard containing no matrix. This scheme is based on the use of two internal reference elements for simultaneous monitoring and correction of drift and matrix effects for all the analytes irrespective of their difference in mass and ionization potential from the two internal reference elements. The developed scheme was validated using river water samples containing appreciable matrix concentration and was measured under the influence of drift. The results of the experimental validation indicate that the proposed scheme is capable of removing error as large as 43% that arises from drift and non-spectroscopic matrix effects.

Investigation of non-spectroscopic interference and internal standardization method in axially and radially viewed inductively coupled plasma optical emission spectrometry using cross-flow and ultrasonic nebulization

The matrix effects on radially and axially viewed inductively coupled plasma optical emission spectrometry (ICP-OES) equipped with cross-flow (CFN) and ultrasonic nebulizer (USN) were evaluated for the determination of multi-elements in various sample matrices. Na, Ca, Mg, Al, Mn, Cu, Si and Fe were used as matrix elements and the change in light intensities of selected analyte elements with a wide range of spectral lines differing in energy sum (summation of excitation energy and first ionization energy) were compared. From the correlation of the change in line intensity and energy sum of analyte elements and the second ionization energy of matrix elements, a novel observation on non-spectroscopic interference is described. Based on our experimental results, two types of non-spectroscopic interferences resulting from different matrix elements were observed, regardless of the sample introduction systems or observation modes. Moreover, besides the fact that the second ionization energy of matrix elements shows unique effect on the change in the analyte spectral lines, bigger matrix effect was found for analytical lines possessing energy sum larger than 14.03 eV in the presence of matrix elements with second ionization energy lower than 15.64 eV. The applicability of RF power adjustment and internal standardization were investigated for the compensation for matrix interferences. It was found that satisfactory corrected results can be achieved by the use of single internal standard as the second ionization energy of matrix element is higher than 15.75 eV. However, in view of the different depressive effect occurring in the analyte spectral lines, the the similarity in energy sum of analytical lines and internal standard has been confirmed by using two internal standards for compensating the matrix interferences caused by matrix elements with second ionization energy lower than 15.64 eV.

Comparison of mass bias correction models for the examination of isotopic composition of mercury using sector field ICP-MS

Six mass bias correction models, viz., linear law, power law, exponential law, Russell equation, common analyte internal standardization (CAIS) and polynomial function, were evaluated for the measurement of the isotopic composition of a natural abundance mercury standard solution using a sector field ICP-MS instrument. Thallium, Os and Ir were tested as internal standards. Significant differences in resultant isotope abundances were noted when the various mass bias correction models and different internal standards were used for instrument mass bias correction. The isotopic abundances calculated from Hg ratios corrected for mass bias using a polynomial function with Os, Ir and Tl as internal standards resulted in data having the smallest relative difference (Df) from IUPAC values, which were selected as common reference points. No significant Df in results arose when the power law, exponential law and Russell equation models for mass bias correction were used in conjunction with the Tl internal standard, these exhibiting the next smallest Df values. Hg isotope abundances having the third smallest Df values arose with a linear law model. Of the three internal standards examined, use of Tl for mass bias correction provided the smallest Df values with these four models, which only require one pair of isotopes from a reference standard. The CAIS model produced the largest Df values in this study.

Improving isotope ratio precision in inductively coupled plasma quadrupole mass spectrometry by common analyte internal standardization

The common analyte internal standardization (CAIS) chemometric technique is extended to improve isotope ratio precision obtained by inductively coupled plasma-quadrupole mass spectrometry (ICP-QMS). A correction function for isotope ratio measurement is derived from a relationship of mass sensitivity as a function of element mass after considering the degree of ionization and isotopic abundance. An analyte isotope ratio correction factor can be determined by simultaneously measuring several internal reference isotopic ratios and the analyte. The CAIS approach corrects for isotopic mass bias and isotope ratio variation with sample matrix concentration. Isotope ratio precision is improved from about 0.4 to 0.05%, because long-counting times allow correcting mass bias and isotope ratio drift during extended data-acquisition periods. Isotope ratio accuracy is improved from 0.2 to 0.02%.

Improving boron isotope ratio measurement precision with quadrupole inductively coupled plasma-mass spectrometry

A method was developed to improve the precision of inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) for the determination of boron isotope ratios ( 11B/ 10B) in various environmental materials including seawater. This approach is based on the common analyte internal standardization (CAIS) chemometric algorithm. The sample solution obtained after digestion is spiked with lithium, and both 7Li/ 6Li and 11B/ 10B values are measured using long-counting periods (20 min). The CAIS algorithm corrects the measured 11B/ 10B values for (a) statistical fluctuations resulting from short-term noise; (b) drift in 11B-to- 10B ratio as a result of long-term deviation in instrumental parameters likely to occur during long counting times; (c) change in 11B-to- 10B ratio caused by variation in matrix elements concentrations; and (d) drift in mass bias correction factor. Comparing boron isotopic ratios in seawater measured by conventional and the new isotope ratio methods validates the procedure. A synthetic isotopic mixture of boron SRM 951 and enriched 10B SRM 952 also was examined. The CAIS method provided a measured boron isotopic ratio precision of 0.05% R.S.D. while eliminating 5.1% matrix concentration error and 0.25% instrumental drift error.

Simultaneous determination of endogenous and 13C-labelled cortisols and cortisones in human plasma by stable isotope dilution mass spectrometry

This study describes a capillary GC–MS method for the simultaneous determination of endogenous cortisol and cortisone and their 13C-labelled analogues, [1,2,4,19-13C4]cortisol (cortisol-13C4) and [1,2,4,19-13C4]cortisone (cortisone-13C4), in human plasma. [1,2,4,19-13C4,1,1,19,19,19-2H5]Cortisol (cortisol-13C4,2H5) and [1,2,4,19-13C4,1,1,19,19,19-2H5]cortisone (cortisone-13C4,2H5) were used as analytical internal standards. A double derivatization (bismethylenedioxy-pentafluoropropionate, BMD-PFP) with good GC behavior was employed for the GC–MS analysis of cortisol and cortisone. Quantitation was carried out by selected-ion monitoring of the molecular ions ([M]+·) of the BMD-PFP derivatives of cortisol and cortisone. The sensitivity limit of the present GC–MS–SIM method was found to be 150 pg per injection for cortisol (s/n=5.0) and 50 pg for cortisone (s/n=8.1). The within-day reproducibility in which the amounts of unlabelled and labelled cortisols and cortisones determined were in good agreement with the actual amounts added, the relative errors being less than 3.07%. The inter-assay coefficients of variation (C.V.) were less than 1.80% for unlabelled and labelled cortisols and cortisones.

Correction for non-spectroscopic matrix effects in inductively coupled plasma-mass spectrometry by common analyte internal standardization

The Common Analyte Internal Standardization (CAIS) chemometric technique is extended to correct for non-spectroscopic matrix effects in inductively coupled plasma-mass spectrometry (ICP-MS). The approach is based on using an internal reference element to correct for the matrix effect. Unlike the conventional internal reference method, the CAIS technique allows for the analyte to behave differently from the internal reference under the influence of the matrix. With the CAIS technique a single internal reference element is sufficient to correct for all the analytes. Experimental tests with 13 analytes in four different matrices using different ICP-MS instruments demonstrate that the CAIS is efficient and general for matrix effect correction. Not only is the corrected concentration more accurate, but the precision is significantly better. The capability of CAIS to correct for the effect of a mixture of two matrices was also established experimentally, and 20–30% matrix suppression was eliminated. Furthermore, the developed technique was used as a simple diagnostic quality assurance procedure to evaluate the performance of the mass spectrometer.