Application of digital filters to process data for ion mobility spectrometry

Digital filters can be used to remove interferent and background spectra features from ion mobility spectra. A finite impulse response digital filter is used to improve the detection limit of bis-2chloroethyl sulfide when it is in the presence of an interferent species. Preliminary investigations on the use of narrow-bandpass digital filters for extracting signals of interest from other signals in IMS spectra are described.     

Spektrallinien und Koinzidenztafeln zur Bestimmung von Zirkonium, Niob und Tantal mit dem ICP

Summary Coincidence tables for ICP-AES are given for 20 Zr lines, 9 Nb lines and 7 Ta lines. They include 582 interferent lines of 49 elements. The specified interference data (IEC and CCR) hold for a bandwidth FWHM = 16 pm and an interferent concentration of 1,000 mg/l. For trace determinations of Zr, Nb, Ta in a matrix of Fe, W, Zr, Nb, Ta the dependence of the interference on the matrix concentration is specified up to 10,000 mg/l. Interference data CCR calculated for the ICP from the NBS tables often differ from the measured data by several orders of magnitude. The spectrum of Zr measured at a high concentration shows many weak lines most of which are not even listed in the MIT tables. They give rise to a quasi-continuous background the intensity of which increases nearly linearly with the matrix concentration. Therefore, an increase of sample concentration will not lead to an improved detection limit. The line intensities found in our investigations match well with those listed in the Wohlers tables.     

Peak purity assessment by matrix projection for spectral line selection and background correction in inductively coupled plasma optical emission spectrometry

This paper describes an improvement of the procedure, proposed previously (Spectrochim. Acta, Part B, 48 (1993) 1517), to provide information about multiplicative and spectral interferences at lines selected for the analysis of samples. The new version of the procedure only requires intensity scans across an analyte line when three solutions are aspirated: a standard solution, the sample solution and the sample spiked with the analyte standard solution. The third solution can be omitted if multiplicative interference is not of interest. Like the initial method, the present procedure does not require or assume information about the sample composition nor is the preparation of solutions of suspected interferents required. Figures of merit, namely the structured background factor, and the overall spectral interference have been devised and are used with the true detection limit at a line for line evaluation and selection. As the nonanalyte concomitant contribution to the measured spectrum is estimated, automatic background correction is possible. The proposed method not only allows selection of the spectral line, but can give a good estimate of the analyte concentration prior to a full quantitative analysis.     

Spectral interferences in the determination of traces of scandium,yttrium and rare earth elements in “pure” rare earth matrices by inductively coupled plasma atomic emission spectrometry Part III. Europium

This paper is the third part of a series of papers dealing with spectral interferences of rare earth elements (REEs) in inductively coupled plasma atomic emission spectrometry (ICP-AES). The present article shows: (a) the spectral data of europium interferent for 200 pm wide windows centred (±100 pm) about prominent lines of scandium, yttrium and REEs; (b) the database of Q values for line interference (Q_l) and Q values for (wing) background interference (Q_w); (c) the detection limits measured by using the “true detection limit” criterion as proposed by P.W.J.M. Boumans and J.J.A.M. Vrakking (Spectrochim. Acta Part B, 42 (1987) 819; 43 (1988) 69). Different possibilities for improvement of the true detection limits are discussed: the use of equipment of high resolving power, the application of multicomponent analysis (MCA) techniques and preliminary separation of the matrix component combined with preconcentration of trace REEs. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hard copy text is accompanied by a disk with data and text files. The data files comprise in particular the tabular material of this article in electronic form.     

Surface-assisted laser desorption/ionization mass spectrometry on titania nanotube arrays

Titania nanotube arrays (NTA) generated from anodizing processes are tested as the substrate for surface-assisted laser desorption/ionization mass spectrometry (SALDI MS). The background generated from titania NTA is very low, making the approach suitable for the analysis of small molecules. The upper detectable mass is approximately 29 kDa. Homogeneous sample deposition leads to good shot-to-shot reproducibility and suitability for quantitative analysis. Additionally, phosphopeptides can be selectively trapped on the titania NTA substrate, as illustrated by simply depositing a tryptic digest of beta-casein followed by titania NTA SALDI MS analysis. The detection limit for small organics and peptides is in low fmol.     

Untersuchungen zur gleichzeitigen Bestimmung von Molybd?n und Wolfram in Proben mit schwierigen Matrices mit induktiv gekoppeltem Plasma

Summary Inaccurate values are often obtained in the determination of molybdenum and tungsten in difficult complex matrices — ores, processing and metallurgical products. The main reason for this effect is due to spectral interferences. Background equivalent concentrations caused by changes of background nearby the measured spectral line are calculated for the established interferent matrix elements. It is possible with the applied sample treatment to perform successful analyses of samples with very variable matrix composition.     

Determination of soluble phosphates in water samples using ytterbium(III) and dynamic measurements of light scattering intensity at long wavelength

A selective and fast method has been developed for the determination of phosphates by measuring the formation of ytterbium(III) phosphate through the variation of the light scattering intensity with time. The low solubility of this compound causes an efficient dispersion of the radiation at 490 nm, which is measured at 980 nm using the second-order grating effect. This approach minimizes potential background signals from the sample matrix. The initial rate of the system is automatically obtained in only 0.5 s by stopped-flow mixing technique. The variable optimization study has been carried out using univariate and multivariate methods. The dynamic range of the calibration graph is 0.09–7.9 mmol L⁻¹ (Pearson's correlation coefficient = 0.9999) and the detection limit is 0.03 mmol L⁻¹. The precision of the method, expressed as relative standard deviation, is 2.3%. The study of the potential interference of different inorganic anions showed that arsenate is the main interferent, although it is tolerated in a molar ratio of 5.5. The method has been satisfactorily applied to the determination of soluble phosphates in tap, ground and river water using a previous preconcentration step with a Dowex 1 (1 × 4–400) anionic resin. A 500-fold concentration has been achieved, which has allowed to decrease the detection limit up to 60 mmol L⁻¹. The recovery range is 97.5–102.5%. The results obtained are consistent with those obtained with the standard molybdenum blue method.     

Validation of chromatographic methods: evaluation of detection and quantification limits in the determination of impurities in omeprazole

To guarantee that an analytical procedure gives reliable, exact and interpretable information about a sample, it must be validated. Two ambiguous parameters are detection limit and quantification limit. The determination of these limits is still of great concern and there are still a variety of procedures described in the current literature. The fundamental objective of the present work is to apply the different recommendations suggested by official guidelines for the quantitative determination of omeprazole and its impurities (omeprazole sulphone and 5-hydroxy-omeprazole) in capsules and tablets using high performance liquid chromatography with UV detection. The importance of calibration linearity in the context of the quantification limit is considered, since one of the approaches, the estimated concentrations of this limit, are deduced from the regression line. The values of the detection limit and the quantification limit obtained show that, in chromatographic analyses, the best method is that based on the use of the parameters obtained from the analytical curve, which are statistically reliable. It was shown that smaller values of the detection limit and the quantification limit were obtained by the visual approach and by the method using the signal-to-noise ratio. However, these values may reflect a subjective evaluation, prone to error and large variations. This was confirmed by showing that these methods result in values that fall outside the linear range of the method.     

Determination of interferon-gamma in Chinese hamster ovary cell culture supernatant by coupled-column liquid chromatography

A robust tandem HPLC method coupling size-exclusion (Shodex Asahipak GS-320HQ) and reversed phase (Vydac 218TP54) columns with ultraviolet detection was developed for quantitative determination of interferon-gamma (IFN-gamma) in Chinese hamster ovary cell culture supernatant. The 2D-HPLC system was linked up by a 6-port 2-position low hold-up volume switch valve. Compared to a commercial ELISA kit for IFN-gamma, the coupled column LC approach was able to detect and quantify soluble IFN-gamma, regardless of the glycoprotein's molecular/conformational variability and sample background. Each LC-LC analysis took 90 minutes inclusive of column regeneration. The relative standard deviation of measurements (n = 5) was less than 3%. The limit of detection (LOD) was determined to be 0.35 microg IFN-gamma.     

Fast urinary screening for paracetamol using on-line microwave assisted hydrolysis and spectrophotometric detection

A fully automated urinary screening system for paracetamol and its metabolites is proposed. The method comprises on-line acid microwave assisted hydrolysis of the drug to p-aminophenol followed by reaction with o-cresol in alkaline medium. The indophenol blue dye formed can be continuously monitored at 620 nm. The detection limit achieved, 0.1 microgram ml-1, allows a high dilution of the samples, thus reducing potential interferences from the sample matrix (mainly protein degradation during urine hydrolysis). The proposed screening system also possesses an adequate selectivity, as the major interferent, epinephrine, is tolerated at concentrations higher than those that could be found in the positive urine samples. The reproducibility, expressed as relative standard deviation, was 3.0% and the sample frequency 20 h-1. The reliability of the method was established at five concentrations (between 0.5 and 4 times the detection limit). Finally, it was applied to the screening of several human urine samples. The results obtained were compared with those provided by batch acid hydrolysis, and were similar in all instances.     

Development of an HPLC method with electrochemical detection of femtomoles of 8-oxo-7,8-dihydroguanine and 8-oxo-7,8-dihydro-2'-deoxyguanosine in the presence of uric acid

A selective method based on high performance liquid chromatography with electrochemical detection (HPLC-ECD) was developed to enable simultaneous detection of 8-oxo-7,8-dihydroguanine (8-oxoGua) and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodGuo), products of DNA oxidative damage, in the presence of uric acid (UA), a strong interferent in their electrochemical detection. The method developed consists of HPLC isocratic elution with amperometric detection on a glassy carbon electrode, enabling a detection limit for 8-oxoGua and 8-oxodGuo lower than 1 nM in standard mixtures. Detection of low concentrations up to 25 nM of 8-oxoGua and 8-oxodGuo in the presence of UA in a 10⁴-fold higher concentration was achieved after one-step solid phase extraction (SPE). The method was tested with urine samples and it was possible to detect and quantify the presence of 8-oxoGua, and to confirm that UA was eliminated after uricase degradation and SPE. The LOD found in urine samples was about 80 nM, a value higher than in standard mixtures, due to the increase of background current in the urine matrix. The results presented here contribute to the development of a methodological approach to simultaneous determination of 8-oxoGua and 8-oxodGuo in urine samples.     

Image background removal in comprehensive two-dimensional gas chromatography

This paper describes a new technique for removing the background level from digital images produced in comprehensive two-dimensional gas chromatography (GCxGC). Background removal is an important first step in the larger problem of quantitative analysis. The approach estimates the background level across the chromatographic image based on structural and statistical properties of GCxGC data. Then, the background level is subtracted from the image, producing a chromatogram in which the peaks rise above a near-zero mean background. After the background level is removed, further analysis is required to determine the quantitative relationship between the peaks and chemicals in the sample. The algorithm is demonstrated experimentally to be effective at determining and removing the background level from GCxGC images. The algorithm has several parametric controls and is incorporated into an interactive program with graphical interface for rapid and accurate detection of GCxGC peaks.     

Validation of PCR methods for quantitation of genetically modified plants in food.

For enforcement of the recently introduced labeling threshold for genetically modified organisms (GMOs) in food ingredients, quantitative detection methods such as quantitative competitive (QC-PCR) and real-time PCR are applied by official food control laboratories. The experiences of 3 European food control laboratories in validating such methods were compared to describe realistic performance characteristics of quantitative PCR detection methods. The limit of quantitation (LOQ) of GMO-specific, real-time PCR was experimentally determined to reach 30-50 target molecules, which is close to theoretical prediction. Starting PCR with 200 ng genomic plant DNA, the LOQ depends primarily on the genome size of the target plant and ranges from 0.02% for rice to 0.7% for wheat. The precision of quantitative PCR detection methods, expressed as relative standard deviation (RSD), varied from 10 to 30%. Using Bt176 corn containing test samples and applying Bt176 specific QC-PCR, mean values deviated from true values by -7to 18%, with an average of 2+/-10%. Ruggedness of real-time PCR detection methods was assessed in an interlaboratory study analyzing commercial, homogeneous food samples. Roundup Ready soybean DNA contents were determined in the range of 0.3 to 36%, relative to soybean DNA, with RSDs of about 25%. Taking the precision of quantitative PCR detection methods into account, suitable sample plans and sample sizes for GMO analysis are suggested. Because quantitative GMO detection methods measure GMO contents of samples in relation to reference material (calibrants), high priority must be given to international agreements and standardization on certified reference materials.     

Enhancing the selectivity of liquid chromatography–mass spectrometry by using trilinear decomposition on LC‐MS data: An application to three‐way calibration of coeluting analytes in human plasma

The high selectivities of liquid chromatography and mass spectrometry make liquid chromatography–mass spectrometry one of the most popular tools for quantitative analysis in complex chemical, biological, and environmental systems, while the potential mathematical selectivity of liquid chromatography–mass spectrometry is rarely investigated. This work discussed the mathematical selectivity of liquid chromatography–mass spectrometry by three‐way calibration based on the trilinear model, with an application to quantitative analysis of coeluting aromatic amino acids in human plasma. By the trilinear decomposition of the constructed liquid chromatography–mass spectrometry‐sample trilinear model and individual regression of the decomposed relative intensity versus concentration, the proposed three‐way calibration method successfully achieved quantitative analysis of coeluting aromatic amino acids in human plasma, even in the presence of uncalibrated interferent(s) and a varying background. This analytical method can ease the requirements for sample preparation and complete chromatographic separation of components, reduce the use of organic solvents, decrease the time of chromatographic separation, and increase the peak capacity of liquid chromatography–mass spectrometry. As a “green analytical method”, the liquid chromatography–mass spectrometry three‐way calibration method can provide a promising tool for direct and fast quantitative analysis in complex systems containing uncalibrated spectral interferents, especially for the situation where the coelution problem is difficult to overcome.     

Spectral interferences in inductively coupled plasma atomic emission spectrometry-I: A theoretical and experimental study of the effect of spectral bandwidth on selectivity,limits of determination,limits of detection and detection power

This article links up with recent work on high resolution spectroscopy in this laboratory [1] and primarily deals with the effect of the spectral resolution on the “selectivity” in the case of samples that emit line-rich spectra. The concept of selectivity, as developed by KAISER [8] on the basis of the set of calibration equations for a multicomponent system, is considered as a useful starting-point but is rejected as a meaningful analytical figure of merit. Instead the concept of “line selectivity” is introduced as a criterion and related to the analytical error. This approach leads to a definition of the limit of determination such that its dependence on the spectral resolution can be clearly and unambiguously revealed in any concrete situation, that is, once the sample type has been specified. Such a specification is necessary since the numerical values of quantities related to selectivity are inherently linked with sample composition.Thus the theory is illustrated with practical examples including the results of a multiplicity of simulated line overlap situations using representative experimental line profiles measured at two extreme levels of resolution, referred to as “medium” and “high” resolution.It is shown that in the case of line overlap the limit of determination may exceed the limit of detection by one or even two orders of magnitude, unless line selection is based on a selectivity criterion so that the limit of detection is inherently coupled to the limit of determination. It is also shown that the prime benefit of high resolution spectroscopy is the reduction of the limit of determination, not that of the limit of detection. This benefit is found only if the spectral resolution can improve the selectivity, thus if there exist situations of line overlap.     

Novel Immunochromatography Assay Based on Background Fluorescence Quenching for the Sensitive Determination of Serum Cystatin C

It is recognized that cystatin C is an effective marker for monitoring the glomerular filtration rate and the clinical diagnosis of various diseases. In this study, a novel immunochromatographic method has been established to achieve quantitative detection of serum cystatin C. Unlike conventional and traditional gold immune chromatographic assays (GICAs), a uniform layer of a fluorescent film was added to the solid phase, which has been designated as the background fluorescence quenching immune chromatographic assay (bFQICA). Under the optimized conditions, there was a good correlation between the fluorescence signal ratio (F1/F2) of the background fluorescence (F1) to the fluorescence signal at the detection line (F2) for cystatin C at concentrations from 0.0?ng/mL to 100?ng/mL with a correlation coefficient of 0.9977. The detection limit was 0.69?ng/mL, and the recovery values were 87.9–105%. The differences between the intra- and interbatch precision were less than 15% in three batches. In addition, after 120 serum samples were detected, there were no significant differences in the results obtained by bFQICA and immunoturbidimetry (t?=?0.963, p?=?0.338?>?0.05). This work demonstrates that bFQICA is a simple, sensitive, and accurate approach for the determination of serum cystatin C, providing a new approach for the clinical diagnosis of cystatin C.     

Quantitative fluorescence kinetic analysis of NADH and FAD in human plasma using three- and four-way calibration methods capable of providing the second-order advantage

The metabolic coenzymes reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) are the primary electron donor and acceptor respectively, participate in almost all biological metabolic pathways. This study develops a novel method for the quantitative kinetic analysis of the degradation reaction of NADH and the formation reaction of FAD in human plasma containing an uncalibrated interferent, by using three-way calibration based on multi-way fluorescence technique. In the three-way analysis, by using the calibration set in a static manner, we directly predicted the concentrations of both analytes in the mixture at any time after the start of their reactions, even in the presence of an uncalibrated spectral interferent and a varying background interferent. The satisfactory quantitative results indicate that the proposed method allows one to directly monitor the concentration of each analyte in the mixture as the function of time in real-time and nondestructively, instead of determining the concentration after the analytical separation. Thereafter, we fitted the first-order rate law to their concentration data throughout their reactions. Additionally, a four-way calibration procedure is developed as an alternative for highly collinear systems. The results of the four-way analysis confirmed the results of the three-way analysis and revealed that both the degradation reaction of NADH and the formation reaction of FAD in human plasma fit the first-order rate law. The proposed methods could be expected to provide promising tools for simultaneous kinetic analysis of multiple reactions in complex systems in real-time and nondestructively.     

Performance evaluation of an axially viewed horizontal inductively coupled plasma for optical emission spectrometry

A performance evaluation of a horizontal axially viewed inductively coupled plasma (ICP) for optical emission spectrometry is presented. The main contribution of this work is the elucidation of the sources of analytical performance differences using practical diagnostics in the comparison of axial and conventional radial viewing of the ICP. Figures of merit such as detection limit, background equivalent concentration, precision, and dynamic range are compared for both viewing arrangements. The detection limit improvements with axial viewing, known from previous work in the literature, are shown to be understood in the context of the signal-to-background-ratio relative-standard-deviation-of-the-background (SBR-RSDB) theory. The usefulness of the SBR-RSDB approach as a diagnostic tool for understanding the detection limit improvement and identifying performance differences is demonstrated. This approach can be further utilized for quality control and quality assurance of instrument performance and detection limit results. Other characteristic differences between axial and radial viewing are presented including matrix effects on line signals and the magnitudes of spectral interferences from OH bands. An overall improvement factor of five in detection power was observed when using axial viewing compared with radial viewing.     

Application of central composite design in the optimisation of thermal desorption parameters for the trace level determination of the chemical warfare agent chloropicrin

A quantitative trace-level thermal desorption gas chromatography mass spectrometry method was developed for chloropicrin CCl3NO2 using central composite design. Factors influencing the thermal decomposition were elucidated and optimum conditions for maximum response deduced. Four factors were investigated: desorption time, desorption temperature, valve temperature and line temperature. Only valve and line temperature influenced the response. The storage stability of chloropicrin on Tenax TA was investigated. Only the storage conditions affected recovery: no significant loss of chloropicrin was observed for spiked tubes stored in a refrigerator for up to 30 days. The application of central composite design to study thermal degradation of chloropicrin has not been described in the literature. The benefits in adopting this approach are reflected in the limit of detection, 22 ng on the sorbent tube (equivalent to 3.2 ppbv), the lowest atmospheric detection limit reported to date.     

Conversion of “Tables of Spectral-Line Intensities” for NBS copper arc into table for inductively coupled argon plasmas

This paper describes an approach used to convert the Tables of Spectral-Line Intensities for the copper arc explored by Meggers, Corliss and Scribner at the National Bureau of Standards (NBS Tables) into a table appropriate for inductively coupled plasmas (ICP). Previous work of this author was extended to determine “definitive” factors for converting the intensities listed in the NBS Tables into ICP sensitivities using the ICP detection limits of more than 800 prominent lines published by Winge, Peterson and Fassel. A computer iteration procedure was devised to find simultaneously the desired conversion factors and the true relative intensity distribution in the background spectrum of the argon ICP explored by Winge et al. This approach proved to be viable and resulted in a complete translation of the tables for the NBS copper arc into a table of ICP sensitivities, which, together with a previously described approach by this author to quantify line coincidences in terms of critical concentration ratios, formed the basis of new Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry involving 896 prominent lines of 67 elements. The Appendix of the present work produces a list of these prominent lines arranged according to element and wavelength, along with sensitivities, detection limits, and ratios of the detection limits to the best detection limit of the element. A second list of some 200 supplementary computer predicted prominent lines is also appended.