New background correction approach based on polynomial regressions for on-line liquid chromatography–Fourier transform infrared spectrometry

In the present study a new approach for the chemometric background correction in on-line gradient LC–FTIR is introduced. For this purpose, the spectral changes of the elution mixture during gradient elution were analyzed applying 2D correlation spectroscopy. The fundamentals of the new background correction algorithm, based on polynomial fits calculated from a reference spectra matrix (Polyfit-RSM method) are explained. The Polyfit-RSM approach was applied on blank gradient runs as well as on LC–FTIR data obtained from the injection of a soft drink sample using acetonitrile:water as eluent. Results found were critically assessed and compared to those obtained by two previous background correction methods which are likewise based on the use of a reference spectra matrix (RSM). The Polyfit-RSM method provided lower noise levels throughout the whole spectral range than other alternative background correction methods, an excellent recovery of analyte spectra as well as chromatograms with a low noise level and also free from baseline shifts. A significant finding, which implies a major advantage for the practical applicability of the algorithm, is that the size of the RSMs can be reduced without affecting the accuracy of the correction method.     

Microwave-induced plasma emission spectrophotometer combined with a photodiode array monitor for capillary column gas chromatography

A microwave-induced plasma emission spectrophotometric detector (MIPD) was used as an element-specific detector for capillary column gas chromatography. The atmospheric pressure microwave helium plasma generated with an original device called a SURFATRON was used as an atomization and excitation source. Combining a photodiode array spectrophotometer with the above system made the emission spectrophotometric detector very powerful. A wide range of spectra could be instantly monitored without any mechanical device. However, the spectrum of atmospheric helium emission plasma was complicated by the presence of air around the plasma discharge. An on-line background correction scheme was developed to handle such complicated spectra.     

Some characteristics of an intensified photodiode array spectrometer system for use in plasma emission spectrometry

A 1024-element silicon photodiode array with a microchannel plate image intensifier was coupled to a Czerny-Turner spectrometer and basic characteristics of the spectrometer system were studied using an Hg lamp, a hollow cathode lamp and a d.c. argon plasma as emission sources. The intensifier proved to be useful to enhance signals without increasing the electronic background. The signal to dark current ratio of the cooled photodiode array was larger than that of a photomultiplier (HTV R457) in the wavelength region above 380 nm. Effects of entrance slit width, integration time per scan and intensifier gain on signal to background ratios and signal to noise ratio are presented and optimum conditions for emission measurements using the system are described.     

Application of point-to-point matching algorithms for background correction in on-line liquid chromatography-Fourier transform infrared spectrometry (LC-FTIR)

A new background correction method for the on-line coupling of gradient liquid chromatography and Fourier transform infrared spectrometry has been developed. It is based on the use of a point-to-point matching algorithm that compares the absorption spectra of the sample data set with those of a previously recorded reference data set in order to select an appropriate reference spectrum. The spectral range used for the point-to-point comparison is selected with minimal user-interaction, thus facilitating considerably the application of the whole method. The background correction method has been successfully tested on a chromatographic separation of four nitrophenols running acetonitrile (0.08%, v/v TFA):water (0.08%, v/v TFA) gradients with compositions ranging from 35 to 85% (v/v) acetonitrile, giving accurate results for both, baseline resolved and overlapped peaks.     

Use of a spectrally segmented photodiode-array spectrometer for inductively coupled plasma atomic-emission spectroscopy Examination of procedures for the evaluation of detection limits

The utility of a spectrally segmented photodiode array spectrometer was examined by using inductively coupled plasma atomic-emission spectrometry (ICP-AES). The spectrometer used in this study is capable of high resolution (reciprocal linear dispersion of approximately 0.08 nm/mm at 300 nm) over a wide spectral range (190-415 nm). The effect of using spectral peak areas instead of peak heights as a signal definition was examined by using the emission signals from 10 molybdenum lines obtained with various photodiode-array integration periods. In addition, a procedure to determine the detection limits obtainable with such a spectrometer is proposed. It was found that a signal definition involving a summation over a range of 5 pixels offered the best signal-to-noise ratio when the noise was defined as the standard deviation of the residual values from the line fitted to the sideband background level. A detection limit of 6 ng/ml was determined in this way for molybdenum. The multichannel capability of the spectrometer was found to permit continuous background correction, thereby reducing errors caused by low-frequency noise or plasma drift. The linearity of response was found to extend over three orders of magnitude with use of a single integration period. However, by use of different integration periods, the linear range of the detector could be extended to at least four orders of magnitude. The precision (RSD) of the spectrometer for a molybdenum concentration of 0.5 mug/ml was found to be about 3-4% for molybdenum peaks where the background emission was relatively low.     

光电二极管阵列等离子体原子发射光谱分析背景智能化实时校正研究

提出了用于光电二极管阵列等离子体原子发射光谱分析背景智能化实时校正的方法。采用二次微分结合原ICP光谱法,在不经微分噪声过滤的基础上,实现快速准确的谱线识别。简单背景采用多项式模型,结构背景及翼展干扰采用概率统计模型。通过对高浓度Ca存在下A₁及La的测定,表明本方法能够准确地校正背景干扰。     

Rapid Determination of Costunolide and Dehydrocostuslactone in Human Plasma Sample and Chinese Patent Medicine Xiang Sha Yang Wei Capsule Using HPLC‐DAD Coupled with Second‐order Calibration

A novel methodology that combines high performance liquid chromatography with photodiode‐array detector (HPLC‐DAD) coupled with second‐order calibration method based on alternating trilinear decomposition (ATLD) algorithm was used in determination of the effective constituents such as costunolide and dehydrocostuslactone, in plasma sample and Chinese patent medicine Xiang Sha Yang Wei (XSYW) capsule. Complicated systems such as plasma and Chinese patent medicine which have intricate components are tedious to isolate and purify. The problem that chromatographic peaks are heavily overlapped among the analytes and interferents from the background matrices can be resolved, and the satisfactory quantification results have been gained with the help of the ATLD algorithm which utilized "mathematical separation" instead of partial "physical or chemical separation". Meanwhile, HPLC‐MS/MS method was used to validate the accuracy of the proposed determination method.     

FTIR microscopy of biological cells and tissue: data analysis using resonant Mie scattering (RMieS) EMSC algorithm

Transmission and transflection infrared microscopy of biological cells and tissue suffer from significant baseline distortions due to scattering effects, predominantly resonant Mie scattering (RMieS). This scattering can also distort peak shapes and apparent peak positions making interpretation difficult and often unreliable. A correction algorithm, the resonant Mie scattering extended multiplicative signal correction (RMieS-EMSC), has been developed that can be used to remove these distortions. The correction algorithm has two key user defined parameters that influence the accuracy of the correction. The first is the number of iterations used to obtain the best outcome. The second is the choice of the initial reference spectrum required for the fitting procedure. The choice of these parameters influences computational time. This is not a major concern when correcting individual spectra or small data sets of a few hundred spectra but becomes much more significant when correcting spectra from infrared images obtained using large focal plane array detectors which may contain tens of thousands of spectra. In this paper we show that, classification of images from tissue can be achieved easily with a few (<10) iterations but a reliable interpretation of the biochemical differences between classes could require more iterations. Regarding the choice of reference spectrum, it is apparent that the more similar it is to the pure absorption spectrum of the sample, the fewer iterations required to obtain an accurate corrected spectrum. Importantly however, we show that using three different non-ideal reference spectra, the same unique correction solution can be obtained.     

Spectral detection in thin-layer chromatography by linear photodiode array spectrometry

Summary A new method of spectral detection in thin-layer chromatography (TLC) is given. Spectral reflectance and fluorescence can be measured by use of fiber optics and a photodiode array. The possibilities of the method with respect to quantitative evaluation, background subtraction, and recognition of overlapped spots are demonstrated at examinations of dyes in comparison to conventional single wavelength detection. The apparatus, the experimental limitations, and future developments are discussed.

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Spektrale Detektion in der Dünnschicht-Chromatographie durch lineare Photodiodenarray-Spektrometrie

     

Resolution of chemical species in flow injection analysis by using self-modeling curve resolution

Mixtures can be resolved into their components by separation, spectrometric and kinetic (chemical) procedures, which have varying degrees of efficiency. Chemometric methods of data processing have proved useful for increasing resolution, especially in spectral and chromatographic procedures. This study shows that chemometric methods are also useful for kinetic flow-injection methods based on chemical reactions combined with multichannel spectral detection, where neither the reaction nor the detector provides adequate resolution. The problem of background absorbance of a sample in a flow-injection system with an ultraviolet/visible photodiode array detector is solved by applying a self-modeling curve resolution method. The example used is a mixture of methyl violet and phenolphthalein.     

反相高效液相色谱与紫外光谱对硝基苯类化合物的联合定性

结合高效液相色谱和紫外光谱各自的优点和信息，建立了有机化合物的高效液相色谱和紫外光谱联合定性方法。在高效液相色谱中，二极管阵列检测器实时在线地提供样品组分的紫外光谱，由此建立紫外光谱谱图库，并发展了谱图库检索算法，同时测定了硝基苯类化合物的反相色谱用热力学参数ａ、ｃ值。结果表明，该法是可行的，满足定性所需的精度。     

Aligning retention time shifts in HPLC three‐dimensional spectra by icoshift approach combined with data arrangement methods and the release of a graphical user interface

High‐performance liquid chromatography coupled with photodiode array detection has been extensively applied in many fields and the peaks among the analyzed samples can be shifted due to the variations of instrumental and experimental conditions. In multivariate analysis, retention time alignment is an important pretreatment step. Hence, the shifted peaks in high‐performance liquid chromatography coupled with photodiode array detection three‐dimensional spectra should be aligned for further analysis. Being motivated by this purpose, the interval correlated shifting method combined with the proposed data arrangement methods are recommended and employed on high‐performance liquid chromatography coupled with photodiode array detection data as a demonstration. We validate the alignment performance of the proposed method through comparison the consistency of the retention time before and after alignment. The obtained results demonstrated that the proposed method is capable of successful aligning the employed data. Additionally, the interval correlated shifting method combined with the data arrangement modes is implemented in an easy‐to‐use graphical user interface environment and so can be operated easily by users not familiar with programming languages.     

Determination of critical eluent composition for polyethylenglycols using on-line liquid chromatography-Fourier transform infrared spectrometry

In this work, it has been extended to methanol:water mobile phases, the use of a background correction method for on-line LC-FTIR measurements named Univariate background correction based on the use of a reference spectra matrix (UBC-RSM) and absorbance ratios. It permits to overcome the problem related to spectral changes occurring during the gradient elution, which in the past limited the on-line coupling of LC and FTIR to isocratic elutions. The combined use of the aforementioned background correction technique in on-line isocratic and gradient LC-FTIR, and partial least squares (PLS) has been applied for the search of the critical conditions for polymers. Polyethylenglycol (PEG) has been used as a model example and results found fitted well with previously published ones.     

Fused silica capillary tube isotachophoresis using a multichannel photodiode array detector

Isotachophoresis carried out in a 0.25 mm i.d. fused-silica capillary tube yielded high resolution, compared with that in a fluorinated ethylene-propylene polymer tube. The use of an ultraviolet-visible multichannel spectrophotometer with photodiode array as detector together with a cross flow cell (volume 0.01 μl) was investigated. The system was successfully applied to the analysis of cationic dyes such as neutral red, bismarck brown, and basic fuchsine.     

Determination of phosphorus, sulfur and the halogens using high-temperature molecular absorption spectrometry in flames and furnaces—A review

The literature about the investigation of molecular spectra of phosphorus, sulfur and the halogens in flames and furnaces, and the use of these spectra for the determination of these non-metals has been reviewed. Most of the investigations were carried out using conventional atomic absorption spectrometers, and there were in essence two different approaches. In the first one, dual-channel spectrometers with a hydrogen or deuterium lamp were used, applying the two-line method for background correction; in the second one, a line source was used that emitted an atomic line, which overlapped with the molecular spectrum. The first approach had the advantage that any spectral interval could be accessed, but it was susceptible to spectral interference; the second one had the advantage that the conventional background correction systems could be used to minimize spectral interferences, but had the problem that an atomic line had to be found, which was overlapping sufficiently well with the maximum of the molecular absorption spectrum. More recently a variety of molecular absorption spectra were investigated using a low-resolution polychromator with a CCD array detector, but no attempt was made to use this approach for quantitative determination of non-metals. The recent introduction and commercial availability of high-resolution continuum source atomic absorption spectrometers is offering completely new possibilities for molecular absorption spectrometry and its use for the determination of non-metals. The use of a high-intensity continuum source together with a high-resolution spectrometer and a CCD array detector makes possible selecting the optimum wavelength for the determination and to exclude most spectral interferences.     

Investigation of chromatographic peak broadening in supercritical fluid chromatography/atmospheric pressure chemical ionization mass spectrometry

Multimode ionization source allows for switching between different ionization techniques, for example, electrospray and atmospheric pressure chemical ionization, within a single analysis. Supercritical fluid chromatography can handle a wide polarity range of substances from hydrophilic to lipophilic in a single run and can undoubtedly benefit from versatility of this ion source. Nevertheless, we observed a significant chromatographic peak broadening effect in atmospheric pressure chemical ionization mode during supercritical fluid chromatography‐mass spectrometry analysis of volatile flavor compounds with a dual ion source named ESCi (Waters). Surprisingly, this effect was not related to the separation process but was triggered solely by the ion source conditions. Neither of photodiode array detector, electrospray mode nor a dedicated atmospheric pressure chemical ionization source suffered from such a phenomenon. Chromatographic peak profiles of ten test substances obtained with the dual ion source were compared with photodiode array detector data as a reference. The broadening effect was more pronounced for volatile compounds with low polarity. Dependence of peak broadening on the ion source settings was systematically investigated. Tuning of desolvation gas flow and its temperature dramatically reduced peak distortion and increased detection sensitivity.     

Capabilities of a photodiode array based system for cataloguing the atomic emission spectra of the inductively coupled plasma

This paper describes a photodiode array based system and discusses its capabilities for cataloguing the atomic emission spectra emitted from an inductively coupled plasma. Dynamic range and line coverage are assessed and compared with those achievable with Fourier transform spectrometry.     

A flexible trilinear decomposition algorithm for three-way calibration based on the trilinear component model and a theoretical extension of the algorithm to the multilinear component model

There is a great deal of interest in decompositions of multilinear component models in the field of multi-way calibration, especially the three-way case. A flexible novel trilinear decomposition algorithm of the trilinear component model as a modification of an alternating least squares algorithm for three-way calibration is proposed. The proposed algorithm (constrained alternating trilinear decomposition, CATLD) is based on an alternating approximate least-squares scheme, in which two extra terms are added to each loss function, making it more efficient and flexible. The analysis of simulated three-way data arrays shows that it converges fast, is insensitive to initialization, and is insensitive to the overestimated number of components used in the decomposition. The analysis of real excitation–emission matrix (EEM) fluorescence and real high performance liquid chromatography–photodiode array detection (HPLC–DAD) data arrays confirms the results of the simulation studies, and shows that the proposed algorithm is favorable not only for EEMs but also for HPLC–DAD data. The three-way calibration method based on the CATLD algorithm is very efficient and flexible for direct quantitative analysis of multiple analytes of interest in complex systems, even in the presence of uncalibrated interferents and varying background interferents. Additionally, a theoretical extension of the proposed algorithm to the multilinear component model (constrained alternating multilinear decomposition, CAMLD) is developed.     

Signal-to-noise characteristics of photodiode array fourier transform spectrometers

The signal-to-noise characteristics of two configurations of photodiode array Fourier transform spectrometers are compared with that of a conventional grating spectrograph using an identical photodiode array. One configuration uses a Sagnac-type common path triangle interferometer, the other uses a stationary tilted mirror Michelson interferometer. While the common path triangle configuration has the property that the resolving power is independent of the field position, theS/N for both is reasonably comparable to the grating spectrograph.     

Progress in direct solid sampling analysis using line source and high-resolution continuum source electrothermal atomic absorption spectrometry

The literature about direct solid sample analysis of the past 10–15 years using electrothermal atomic absorption spectrometry has been reviewed. It was found that in the vast majority of publications aqueous standards were reported as having been used for calibration after careful program optimization. This means the frequently expressed claim that certified reference materials with a matrix composition and analyte content close to that of the sample have to be used for calibration in solid sample analysis is not confirmed in the more recent literature. There are obviously limitations, and there are examples in the literature where even calibration with certified reference materials did not lead to accurate results. In these cases the problem is typically associated with spectral interferences that cannot be corrected properly by the systems available for conventional line source atomic absorption spectrometry, including Zeeman-effect background correction. Using high-resolution continuum source atomic absorption spectrometry, spectral interferences become visible owing to the display of the spectral environment at both sides of the analytical line at high resolution, which makes program optimization straightforward. Any spectrally continuous background absorption is eliminated automatically, and even rapidly changing background absorption does not cause any artifacts, as measurement and correction of background absorption are truly simultaneous. Any kind of fine-structured background can be eliminated by “subtracting” reference spectra using a least-squares algorithm. Aqueous standards are used for calibration in all published applications of high-resolution continuum source atomic absorption spectrometry to direct solid sample analysis. This contribution is based on a presentation given at the Colloquium for Analytical Atomic Spectroscopy (CANAS ‘07) held March 18–21, 2007 in Constance, Germany.