Sub part-per-million mass accuracy by using stepwise-external calibration in fourier transform ion cyclotron resonance mass spectrometry

A new external calibration procedure for FT-ICR mass spectrometry is presented, stepwise-external calibration. This method is demonstrated for MALDI analysis of peptide mixtures, but is applicable to any ionization method. For this procedure, the masses of analyte peaks are first accurately measured at a low trapping potential (0.63 V) using external calibration. These accurately determined (< 1 ppm accuracy) analyte peaks are used as internal calibrant points for a second mass spectrum that is acquired for the same sample at a higher trapping potential (1.0 V). The second mass spectrum has a approximately 10-fold improvement in detection dynamic range compared with the first spectrum acquired at a low trapping potential. A calibration equation that accounts for local and global space charge is shown to provide mass accuracy with external calibration that is nearly identical to that of internal calibration, without the drawbacks of experimental complexity or reduction of abundance dynamic range. For the 609 mass peaks measured using stepwise-external calibration method, the root-mean-square error is 0.9 ppm. The errors appear to have a Gaussian distribution; 99.3% of the mass errors are shown to lie within three times the sample standard deviation (2.6 ppm) of their true value.     

The effect of the physical matrix on accurate measurements using fixed volume analytical techniques

In order to perform high accuracy analytical measurements most analytical techniques require some form of calibration using standards of the same quantity as that being measured. The highest accuracy calibration standards are those prepared by mass (gravimetrically) as opposed to by volume (volumetrically). The use of gravimetrically prepared standards to calibrate analytical techniques that rely on fixed volume injections can cause systematic errors, even when the analytical technique does not suffer from a chemical matrix interference. The origin of these errors is explained and is demonstrated experimentally for the analysis of sulphate in synthetic seawater samples, and the measurement of the anionic content of particulate matter following extraction with water and wetting agents; where average measurement biases of +2.7 and -3.2%, respectively, were observed. Proposals are offered for methods to overcome this 'physical matrix effect'.     

New Calibration Model: Combining Integrated Calibration Method and H-point Standard Addition Method to Detect and Avoid Interference Effects

A new calibration methodology based on the combination of integrated calibration method (ICM) and the H-point standard addition method (HPSAM) is presented. It allows the diagnosis and correction of errors caused in an analytical system by different kinds of interference effects. Six calibration solutions consisting of mixtures of sample, diluent, and one standard are prepared in accordance with the ICM principle to integrate the external calibration method with the standard addition method and thereby to detect and eliminate proportional interferences. Absorbance increments chosen according to the HPSAM principle are proposed to correct the errors caused by additive interferences. A set of as many as six apparent estimations of analyte concentration in a single calibration procedure is calculated for validating accuracy. As a consequence, doing calibration by the ICM-HPSAM method, it is possible to obtain the final analytical results with considerably improved accuracy. The determination of calcium in several different water samples (containing amounts between 4.9 and 127?mg?L^?1) with Arsenazo III has been chosen as an example because it is biased if the errors are not diagnosed and corrected. The results are characterized by small (not higher than 8%) relative error (RE), and good precision (RSD values smaller than 6%).     

Chromatographic quantitation at losses of analyte during sample preparation. Application of the modified method of double internal standard

Known methods of quantitative chromatographic analysis (calibration, external standard, internal standard and standard addition) require the application of sample preparation techniques without significant losses of analytes. If this condition cannot be satisfied, the compensation of these losses should be provided. The modification of known method of quantitative chromatographic analysis (double internal standard), implying the addition of two homologues (previous and following) of target analytes as internal standards into initial samples is considered. This approach permits us to compensate significant losses both analytes and standards at all stages of sample preparation. The advantages of this method are demonstrated on the examples of liquid-liquid extraction, head space analysis (HSA), distillation of volatile compounds with volatile solvents (concentration in condensates) and evaporation of volatile solvents (concentrating in the residues of solvents). In all cases the application of two homologues as internal standards provides accurate results (the typical relative errors are within 1-6%) at the values of a factor of composition distortion of initial samples (K', the definition is suggested) from 0.2 up to 4. These results are in accordance with general relationships between variations in any physicochemical properties of organic compounds within homologous series. The single found exception was the evaporation of volatile solvents (the open phase transition process) when to get the results with relative errors not more then +10% requires the minimal changes in the composition of initial samples (K' values should not be more then approximately 1.5).     

An Automated Hydrodynamically Mediated Technique for Preparation of Calibration Solutions via Capillary Electrophoresis System as a Promising Alternative to Manual Pipetting

In this paper, a novel procedure for preparing calibration solutions for capillary electrophoresis (CE)-based quantitative analysis is proposed. Our approach, named the automated hydrodynamically mediated technique (AHMT), uses a capillary and a pressure system to deliver the expected amount of working solution and diluent directly to a sample vial. As a result, calibration solutions are prepared automatically inside the CE instrument, without any or with minimal manual operation. Two different modes were tested: forward and reverse, differing in the direction of hydrodynamic flow. The calibration curves obtained for a model mixture of analytes using AHMT were thorough compared to the standard procedure based on manual pipetting. The results were consistent, though the volume of obtained calibration solutions and the potential risk of random errors were significantly minimized by AHMT. Its effectiveness was further enhanced by the application of SCIEX^® nanoVials, reducing the actual volume of calibration solutions down to 10 μL.     

Multivariate data analysis of dynamic amperometric biosensor responses from binary analyte mixtures-application of sensitivity correction algorithms

In this paper, it is demonstrated that a single-receptor biosensor can be used to quantitatively determine each analyte in binary mixtures using multivariate data analysis tools based on the dynamic responses received from flow injection peaks. Mixtures with different concentrations of two phenolic compounds, catechol and 4-chlorophenol, were measured with a graphite electrode modified with tyrosinase enzyme at an applied potential of −50 mV versus Ag/AgCl. A correction algorithm based on measurements of references in-between samples was applied to compensate for biosensor ageing as well as differences caused by deviations between biosensor preparations. After correction, the relative prediction errors with partial least squares regression (PLS-R) for catechol and 4-chlorophenol were 7.4 and 5.5%, respectively, using an analysis sequence measured on one biosensor. Additional validation mixtures of the two phenols were measured with a new biosensor, prepared with the same procedure but with a different batch of tyrosinase enzyme. Using the mixture responses for the first sensor as a calibration set in PLS-R, the relative prediction errors of the validation mixtures, after applying correction procedures, were 7.0% for catechol and 16.0% for 4-chlorophenol. These preliminary results indicate that by applying correction algorithms it could be possible to use less stable biosensors in continuous on-line measurements together with multivariate data analysis without time-consuming calibration procedures.     

Theoretical aspects of quantitative mass spectrometry of gas mixtures

The problem of calibration of mass spectrometers with standard gas mixtures is studied theoretically. The results obtained can be used in deciding the number and optimum composition of standard mixtures. An analogy is drawn between this calibration problem and the mathematical theory of experimental design when mixtures are considered. It is shown that calibration based on a number of standard mixtures is more accurate than calibration with pure gases. A procedure for correction of calibration coefficients is described; it can be applied during measurements on the composition of gas mixtures or gas flows. Application to gas mixtures containing CO, N₂ and CO₂ is discussed.     

Chemometric assisted simultaneous spectrophotometric determination of four-component nasal solutions with a reduced number of calibration samples

The use of multivariate spectrophotometric calibration for the simultaneous determination of three active components and one excipient in nasal solutions is presented. The resolution of four-component mixtures of phenylephrine, diphenhydramine, naphazoline and methylparaben in a matrix of excipients has been accomplished by using partial least-squares (PLS-1) and a variant of the so-called hybrid linear analysis (HLA) named net analyte preprocessing (NAP). Notwithstanding the presence of a large number of components and their high degree of spectral overlap, they have been rapidly and simultaneously determined with high accuracy and precision, with no interference, and without resorting to extraction procedures using non-aqueous solvents. A simple and fast method for wavelength selection in the calibration step is used, based on the minimisation of the predicted error sum of squares (PRESS) calculated as a function of a moving spectral window. The use of calibration designs of reduced size has been attempted. Satisfactory results were obtained when the number of calibration samples was reduced from 25 (full central composite) to 17 (fractional central composite) using the net analyte-based NAP method.     

Achieving traceable chemical measurements: inter-laboratory evaluation of a simplified technique for isotope dilution mass spectrometry. Part 2. Methodology for high accuracy analysis of organic analytes

A high accuracy measurement procedure developed and validated at LGC has been transferred to a number of expert UK laboratories, and their experience in applying the technique has been evaluated by inter-laboratory comparisons. It is an “exact matching” calibration procedure for analysis of organic analytes using isotope dilution mass spectrometry (IDMS). This calibration procedure uses a calibration blend and a sample blend with closely matched isotope amount ratios, and is an iterative process, culminating in the calibration blend and sample blend having identical isotope amount ratios. It is capable of high accuracy, since systematic errors in the determination of the isotope amount ratios are cancelled out. A series of four inter-laboratory comparisons of increasing difficulty were carried out involving a number of expert laboratories. The first three comparisons used gas chromatography mass spectrometry (GC–MS) analysis of the pesticide metabolite (pp′-dichlorodiphenyl) dichloroethylene (pp′-DDE), involving both conventional calibration and IDMS exact matching procedures for pp′-DDE in a solvent and a complex liquid matrix (corn oil). The fourth comparisons utilised liquid chromatography mass spectrometry (LC–MS) and involved the analysis of sulphamethazine (4-amino-N-(4,6 dimethyl-2 pyrimidinyl) benzenesulphonamide) in solvent using IDMS and conventional calibration techniques. Following the first trial, a workshop for participants was held on the use of the exact matching procedure together with a short course on uncertainty estimation. The results of the comparisons clearly showed the superior accuracy of using IDMS with the exact matching procedure for both GC–MS and LC–MS applications. These comparisons and the workshop have enabled the methodology to be transferred to UK industry, helping to improve UK measurement capability.

     

Simultaneous Multivariate Spectrophotometric Analysis of Binary and Ternary Mixtures of Sulfamethoxazole,Trimethoprim and Phenazopyridine in Tablets

ABSTRACT

The use of multivariate spectrophotometric calibration is reported for the analysis of tablets containing the antibiotics sulfamethoxazole and trimethoprim, and a combination of the former two drugs with the analgesic phenazopyridine. The resolution of these mixtures has been accomplished without prior separation, derivatisation or use of nonaqueous solvents, with the aid of partial least-squares (PLS-1) regression analysis of electronic absorption spectral data. The analytes have been simultaneously determined with high accuracy and precision, and with no interference from tablet excipients.     

Achieving traceable chemical measurements: inter-laboratory evaluation of a simplified technique for isotope dilution mass spectrometry. Part 2. Methodology for high accuracy analysis of organic analytes

A high accuracy measurement procedure developed and validated at LGC has been transferred to a number of expert UK laboratories, and their experience in applying the technique has been evaluated by inter-laboratory comparisons. It is an “exact matching” calibration procedure for analysis of organic analytes using isotope dilution mass spectrometry (IDMS). This calibration procedure uses a calibration blend and a sample blend with closely matched isotope amount ratios, and is an iterative process, culminating in the calibration blend and sample blend having identical isotope amount ratios. It is capable of high accuracy, since systematic errors in the determination of the isotope amount ratios are cancelled out. A series of four inter-laboratory comparisons of increasing difficulty were carried out involving a number of expert laboratories. The first three comparisons used gas chromatography mass spectrometry (GC–MS) analysis of the pesticide metabolite (pp′-dichlorodiphenyl) dichloroethylene (pp′-DDE), involving both conventional calibration and IDMS exact matching procedures for pp′-DDE in a solvent and a complex liquid matrix (corn oil). The fourth comparisons utilised liquid chromatography mass spectrometry (LC–MS) and involved the analysis of sulphamethazine (4-amino-N-(4,6 dimethyl-2 pyrimidinyl) benzenesulphonamide) in solvent using IDMS and conventional calibration techniques. Following the first trial, a workshop for participants was held on the use of the exact matching procedure together with a short course on uncertainty estimation. The results of the comparisons clearly showed the superior accuracy of using IDMS with the exact matching procedure for both GC–MS and LC–MS applications. These comparisons and the workshop have enabled the methodology to be transferred to UK industry, helping to improve UK measurement capability.     

Simple Procedures for Quantification and Reuse of Common Organic Solvent Mixtures

A recycling protocol is described for the recovery and reformulation of common solvents in teaching and research laboratories. Recovery of mixed solvents is shown to be facilitated by the use of solution density as a convenient and accurate assay method. The procedure is explicitly illustrated for mixtures of ethyl acetate/hexane. An effective procedure is also reported for recovery of wash acetone.     

Kinetic spectrophotometric determination of Ga(III)-Al(III) mixtures by stopped-flow injection analysis using principal component regression

A multivariate calibration method based on principal component regression analysis is applied to the resolution of mixtures by kinetic methodology. Gallium(III) and Al(III) were determined in mixtures on the basis of their different rate of reaction with 4-(2-pyridylazo)resorcinol in a slightly basic medium by using a stopped-flow injection procedure. Mixtures containing 1-5 mg/l. Ga(III) and 20-100 mg/l. Al(III) were successfully resolved with errors less than 10%.     

Density and refractive index in mixtures of ionic liquids and organic solvents: Correlations and predictions

Correlations between volumetric properties and refractive index of binary mixtures of room temperature ionic liquids (RTILs) and organic solvents were examined. To this end, the density and refractive index for a set of these systems were measured at atmospheric pressure at 298.15 K throughout the composition range. These data were used to calculate excess volumes and refractive index deviations by using expressions firmly based on the physical significance of each quantity, fact that allowed the expected relations between the two quantities to be confirmed. Based on these results, the molar refraction and free or void volume of the mixtures are calculated with a view to estimating the relative contribution of both quantities to the excess molar volume. Once molar refraction was confirmed to exhibit a near-ideal behaviour in all mixtures, a method for predicting the density and refractive index of RTIL + organic solvent mixtures was developed; the results show that this procedure can be a highly useful alternative to the usually complex experimental methods available for the thermophysical characterization of these systems.     

High-speed gas chromatographic separations with diaphragm valve-based injection and chemometric analysis as a gas chromatographic “sensor”

A high-speed gas chromatography system, the gas chromatographic sensor (GCS), is developed and evaluated. The GCS combines fast separations and chemometric analysis to produce an instrument capable of high-speed, high-throughput screening and quantitative analysis of complex chemical mixtures on a similar time scale as typical chemical sensors. The GCS was evaluated with 28 test mixtures consisting of 15 compounds from four chemical classes: alkanes, ketones, alkyl benzenes, and alcohols. The chromatograms are on the order of one second in duration, which is considerably faster than the traditional use of gas chromatography. While complete chromatographic separation of each analyte peak is not aimed for, chemical information is readily extracted through chemometric data analysis and quantification of the samples is achieved in considerably less time than conventional gas chromatography.

Calibration models to predict percent volume content of either alkanes or ketones were constructed using partial least squares (PLS) regression on calibration sets consisting of the five replicate GCS runs of six different samples. The percent volume content of the alkane and ketone chemical classes were predicted on five replicate runs of the 22 remaining samples ranging from 0 to 50 or 60% depending on the class. Root mean square errors of prediction were 2–3% relative to the mean percent volume values for either alkane or ketone prediction models, depending on the samples chosen for the calibration set of that model. The alkyl benzenes and alcohols present in the calibration sets or samples were treated as variable background interference. It is anticipated that the GCS will eventually be used to rapidly sample and directly analyze industrial processes or for the high throughput analysis of batches of samples.     

The Integrated Calibration Method: Comparison of Various Flow Approaches

The integrated calibration method (ICM) is a novel calibration approach merging interpolative and extrapolative modes in a single procedure. Thanks to this strategy, chemical analysis can be performed along with a diagnosis of systematic calibration errors that leads to better accuracy of analytical results. The paper describes how the ICM can be adapted to different flow techniques: continuous flow, flow-injection, and sequential-injection ones. Instrumental systems dedicated to each flow technique have been presented and their operation has been explained. The systems were used for UV/VIS chromium determination in water samples and compared with each other in terms of precision and accuracy of the obtained results along with time and reagent consumption.     

Development of a Unified X-ray Fluorescence Procedure for Determining Heavy Metals in Biological Materials

A unified procedure for the X-ray fluorescence determination of Pb, Mo, Rb, As, Zn, Ni, Fe, and Mn in biological materials was developed using the internal standard technique (gallium was the reference element). The samples were prepared for analysis by ashing the materials. The effects of variations in the chemical composition and of the surface density of unsaturated radiators on the accuracy of the analytical results were studied with the use of theoretical intensities; this effect varied from 2.5 to 22%. Based on this information, an optimum calibration function was chosen. Ashes of plant and animal tissues can be analyzed with the use of this function. The errors due to sample preparation for analysis and the reproducibility and accuracy of the results of X-ray fluorescence analysis were evaluated.     

Determination of nicotine in tobacco with second-order spectra data of charge-transfer complex in ethanol–water binary solvents processed by parallel factor analysis

A new spectrophotometric method for the determination of nicotine in mixtures without pre-separation has been proposed. Nicotine could react with 2,4-dinitrophenol through a charge-transfer reaction to form a colored complex. The second-order data from the visible absorption spectra of the complex in a series of ethanol–water binary solvents with various water volume fractions could be expressed as the combination of two bilinear data matrices. With the bilinear model, the second-order spectra data of mixtures containing nicotine and other interferents could be analysed by using second-order calibration algorithms, and the determination of nicotine in the mixtures could be achieved. The algorithm used here was parallel factor analysis. The method has been successfully used to determine nicotine in tobacco samples with satisfactory results.     

Evaluation of the calibration errors on cooling of a differential scanning calorimeter using different sets of standard metals

The temperature calibration on cooling of thermal analysis instruments is important for the accurate study of the non-isothermal crystallization kinetics of semi-crystalline polymers. In previous works, a methodology was proposed for performing the calibration on cooling of differential scanning calorimeters (DSCs) with standard metals, and the calibration errors were checked using transitions of high-purity liquid crystals. In this work, alternative, physically meaningful, procedures for carrying out the calibration on cooling are analyzed and validated. The calibration errors are evaluated also with liquid crystalline transitions, when the calibration is performed with standard metals, in a wide temperature range, and when due account is taken for the need of isothermal corrections to the temperature measurements. It is shown that any pair of standard metals may be used to calibrate on cooling, that the calibration errors increase for wider working temperature ranges and that, providing that certain conditions are fulfilled, both calibration procedures yield similar results.     

Simultaneous spectrophotometric-multivariate calibration determination of several components of ophthalmic solutions: phenylephrine, chloramphenicol, antipyrine, methylparaben and thimerosal

The use of multivariate spectrophotometric calibration for the simultaneous determination of several active components and excipients in ophthalmic solutions is presented. The resolution of five-component mixtures of phenylephrine, chloramphenicol, antipyrine, methylparaben and thimerosal has been accomplished by using partial least-squares (PLS-1) and a variant of the so-called hybrid linear analysis (HLA). Notwithstanding the presence of a large number of components and their high degree of spectral overlap, they have been determined simultaneously with high accuracy and precision, with no interference, rapidly and without resorting to extraction procedures using non aqueous solvents. A simple and fast method for wavelength selection in the calibration step is presented, based on the minimisation of the predicted error sum of squares (PRESS) calculated as a function of a moving spectral window.