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.     

New background correction method for liquid chromatography with diode array detection, infrared spectroscopic detection and Raman spectroscopic detection

A new method to eliminate the background spectrum (EBS) during analyte elution in column liquid chromatography (LC) coupled to spectroscopic techniques is proposed. This method takes into account the shape and also intensity differences of the background eluent spectrum. This allows the EBS method to make a better estimation of the background eluent spectrum during analyte elution. This is an advantage for quantification as well as for identification of analytes. The EBS method uses a two-step procedure. First, the baseline spectra are modeled using a limited number of principal components (PCs). Subsequently, an asymmetric least squares (asLS) regression method is applied using these principal components to correct the measured spectra during elution for the background contribution. The asymmetric least squares regression needs one parameter, the asymmetry factor p. This asymmetry factor determines relative weight of positive and negative residuals. Simulations are performed to test the EBS method in well-defined situations. The effect of spectral noise on the performance and the sensitivity of the EBS method for the value of the asymmetry factorp is tested. Two applications of the EBS method are discussed. In the first application, the goal is to extract the analyte spectrum from an LC-Raman analysis. In this case, the EBS method facilitates easy identification of unknown analytes using spectral libraries. In a second application, the EBS method is used for baseline correction in LC-diode array detection (DAD) analysis of polymeric standards during a gradient elution separation. It is shown that the EBS method yields a good baseline correction, without the need to perform a blank chromatographic run.     

Univariate method for background correction in liquid chromatography-Fourier transform infrared spectrometry

An univariate method is proposed for background correction in on-line gradient liquid chromatography-Fourier transform infrared (LC-FTIR) spectrometry using acetonitrile:water as mobile phase components. The method is based on the calculation of the ratio of absorbances (AR) at two characteristic wavenumbers for each spectrum. This parameter is subsequently used to locate the most appropriated eluent spectrum within a reference spectra matrix (RSM) to be subtracted from each spectrum included in the sample chromatogram. To correct minor changes in eluent spectra intensity during the elution of analytes, a correction factor (Kf), defined as the ratio of the absorbance of the sample and the selected eluent spectrum at a defined wavenumber was determined. The performance of the procedure was evaluated by correcting an on-line gradient LC-FTIR injection of a mixture of two pesticides (Atrazine and Diuron). Using the AR of the absorbance at 2248.6 and 2256.3 cm(-1) and a Kf at 2248.6 cm(-1), the correlation factors between FTIR spectra extracted at the peak apex from the LC-FTIR chromatogram and those obtained from pure pesticide standards were 0.975 and 0.94 for Atrazine and Diuron, respectively.     

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.     

Evaluation of different electrolyte systems and on-line preconcentrations for the analysis of haloacetic acids by capillary zone electrophoresis

The separation of various carboxylic acids was performed on a polymethacrylate-based weakly acidic cation-exchange resin (TSKgel OApak-A) using ion-exclusion chromatography under the acidic elution conditions. When a diluted sulfuric acid solution was used as the eluent, highly sensitive conductimetric detection of carboxylic acids was achieved without increasing the background conductance of the eluent. This method was more sensitive than using benzoic acid eluent and enabled a good resolution of dicarboxylic as well as monocarboxylic acids. The addition of 5–20% methanol to the eluent considerably reduced the retention times of carboxylic acids with hydrophobic nature.     

Rapid determination of Paeoniae Radix using near infrared spectroscopy

A rapid and nondestructive near infrared spectroscopy (NIRS) was used to differentiate different geographical Paeoniae Radix and quantitatively predict the content of main active components. Paeoniflorin, albiflorin and benzoylalbiflorin were analyzed simultaneously with an Agilent Zorbax SB-C₁₈ column by gradient elution under high-performance liquid chromatography-UV detection (HPLC-UV). Multiplicative scatter correction (MSC), first derivative and Savitsky-Golay were utilized together to correct the scattering effect and eliminate the baseline shift in all near infrared diffuse reflectance spectra in order to give a better correlation with the results obtained by HPLC-UV. Multiplicative regression methods were discussed. The spectra calibration equations produced highest correlation coefficient values (R²) and lowest root mean square error of prediction (RMSEP) were used for the determination of paeoniflorin, albiflorin and benzoylalbiflorin. The RMSEP of paeoniflorin, albiflorin and benzoylabiflorin were 0.866 mg/g, 0.369 mg/g and 0.084 mg/g, respectively, and the R² of cross validation were 0.986, 0.939 and 0.971, respectively. Furthermore with the use of principle component analysis (PCA), Paeoniae Radix was clustered according to different cultivation area. The results indicated that the NIRS method could be used for the quality control of Chinese herbal medicine.     

Analysis of berberine and total alkaloid content in cortex phellodendri by near infrared spectroscopy (NIRS) compared with high-performance liquid chromatography coupled with ultra-visible spectrometric detection

This paper developed a rapid method using near infrared spectroscopy (NIRS) to differentiate two species of Cortex Phellodendri (CP), Cortex Phellodendri Chinensis (PCS) and Cortex Phellodendri Amurensis (PAR), and to predict quantitatively the content of berberine and total alkaloid content in all Cortex Phellodendri samples. Three alkaloids, berberine, jatrorrhizine and palmatine were analyzed simultaneously with a Thermo ODS Hypersil column by gradient elution with a new mobile phase under high-performance liquid chromatography-diode array detection (HPLC-DAD). Berberine content determined by HPLC-DAD was exploited as a critical parameter for successful discrimination between them. Multiplicative scatter correction (MSC), second derivative and Savitsky-Golay (S.G.) were utilized together to correct the scattering effect and eliminate the baseline shift in all near infrared diffuse reflectance spectra as well as to enhance spectral features in order to give a better correlation with the results obtained by HPLC-DAD. With the use of principal component analysis (PCA), samples datasets were separated successfully into two different clusters corresponding to two species. Furthermore, a partial least squares (PLS) regression method was built on the correlation model. The results showed that the correlation coefficients of the prediction models were R = 0.996 for the berberine and R = 0.994 for total alkaloid content. The influences of water absorption bands present in the NIR spectra on the models were also investigated in order to explore the practicability of NIRS in routine use. The outcome showed that NIRS possibly acts as routine screening in the quality control of Chinese herbal medicine.     

<Superscript>137</Superscript>Cs/<Superscript>137m</Superscript>Ba radioisotope generator based on 6-tungstocerate(IV) column matrix

Summary Barium-137m radioisotope generator of the chromatographic column elution mode based on loading 1.5 g 6-tungstocerate(IV) gel matrix with ~54 kBq of fission-produced ¹³⁷Cs is described. The elution performance of the generated ^137mBa radionuclide was investigated as a function of chemical composition of the eluent, flow rate, elution frequency, and age of the generator system. At comparable conditions, ^137mBa eluates with 0.9% NaCl-0.1M HCl eluent had higher elution yields and radionuclidic purity than with 0.1M NH₄Cl-0.1M HCl eluent. The generator has been repeatedly eluted for 311 days by passing 4810 ml of the saline eluent (10 ml × 481 elution operations) at a flow rate of 3.0 ml/min. Barium-137m eluates of high and reproducible elution yields, chemical and radionuclidic purities of (≥ 99.99%) were obtained.     

Application of wavelet transform to background correction in inductively coupled plasma atomic emission spectrometry

A background correction method based on wavelet transform was devised and applied to inductively coupled plasma atomic emission spectrometry (ICP-AES). The proposed approach separated background from analyte signal according to their different frequencies. Compared with the analyte signal, the background has a low frequency. By removal of the components attributed to the signal, the background over the spectral window of the analyte line can be fitted through wavelet reconstruction. The results showed that the wavelet transform technique could handle all kinds of background and low signal-to-background ratio spectra, and required no prior knowledge about the sample composition, no selection of suitable background correction points, and no mathematical assumption of the background distribution. This technique performed as well as the conventional three-point background correction method for linear backgrounds, and provided better results than the latter for curved backgrounds. The proposed procedure was illustrated, by processing real spectra, to be an effective and practical tool for background correction in ICP-AES.     

Multi-residue determination of organophosphorus and organochlorine pesticides in environmental samples using solid-phase extraction with cigarette filter followed by gas chromatography-mass spectrometry

A solid‐phase extraction (SPE) method was developed to extract 14 pesticides simultaneously from environment samples using cigarette filter as the sorbent before gas chromatography‐mass spectrometry (GC‐MS) analysis. Parameters influencing the extraction efficiency, such as the sample loading flow rate, eluent and elution volume, were optimized. The optimum sample loading rate was 3 mL/min, and the retained compounds were eluted with 6 mL of eluent at 1 mL/min under vacuum. Good linearity was obtained for all the 14 pesticides (r²>0.99) from 0.1 to 20 μg/L for water and from 2 to 400 μg/kg for soil samples. The detection limits (signal‐to‐noise=3) of the proposed method ranged from 0.01 to 0.20 μg/L for water samples and from 0.42 to 6.95 μg/kg for soil samples. The developed method was successfully applied for determination of the analytes in real environmental samples, and the mean recoveries ranged from 76.4 to 103.7% for water samples and from 79.9 to 105.3% for soil samples with the precisions (relative standard deviation) between 2.0 and 13.6%.     

Qualitative study of ethanol content in tequilas by Raman spectroscopy and principal component analysis

Using Raman spectroscopy, with an excitation radiation source of 514.5 nm, and principal component analysis (PCA) was elaborated a method to study qualitatively the ethanol content in tequila samples. This method is based in the OH region profile (water) of the Raman spectra. Also, this method, using the fluorescence background of the Raman spectra, can be used to distinguish silver tequila from aged tequilas. The first three PCs of the Raman spectra, that provide the 99% of the total variance of the data set, were used for the samples classification. The PCA1 and PCA2 are related with the water (or ethanol) content of the sample, whereas the PCA3 is related with the fluorescence background of the Raman spectra.     

Preparation of a graphene oxide/silica composite modified with nitro‐substituted tris(indolyl)methane as a solid‐phase extraction sorbent for the extraction of organic acids

This paper describes the use of graphene oxide/silica modified with nitro‐substituted tris(indolyl)methane as a solid‐phase extraction sorbent for the determination of organic acids. The resultant graphene oxide/silica modified with nitro‐substituted tris(indolyl)methane was characterized by FTIR spectroscopy and adsorption experiments. Solid‐phase extraction parameters such as sorbent type, sample solution pH, sample loading rate, eluent salt concentration, eluent methanol concentration, elution rate, sample loading, and elution volume were optimized. The method showed good precision, accuracy, sensitivity, and linear response for organic acids analysis over a concentration range of 1–100 μg/L for benzoic acid, p‐methoxybenzoic acid, and salicylic acid and 5–100 μg/L for the remaining organic acids (cinnamic acid, p‐chlorobenzoic acid, and p‐bromobenzoic acid) with coefficients of determination (r²) of higher than 0.9957. Limits of detection from 0.50 to 1.0 μg/L for six organic acids were achieved. The developed method was successfully applied to determine organic acids in real samples.     

Nondestructive determination of compound amoxicillin powder by NIR spectroscopy with the aid of chemometrics

Near-infrared (NIR) spectroscopy, in combination with chemometrics, enables nondestructive analysis of solid samples without time-consuming sample preparation methods. A new method for the nondestructive determination of compound amoxicillin powder drug via NIR spectroscopy combined with an improved neural network model based on principal component analysis (PCA) and radial basis function (RBF) neural networks is investigated. The PCA technique is applied to extraction relevant features from lots of spectra data in order to reduce the input variables of the RBF neural networks. Various optimum principal component analysis-radial basis function (PCA-RBF) network models based on conventional spectra and preprocessing spectra (standard normal variate (SNV) and multiplicative scatter correction (MSC)) have been established and compared. Principal component regression (PCR) and partial least squares (PLS) multivariate calibrations are also used, which are compared with PCA-RBF neural networks. Experiment results show that the proposed PCA-RBF method is more efficient than PCR and PLS multivariate calibrations. And the PCA-RBF approach with SNV preprocessing spectra is found to provide the best performance.     

On-line gradient liquid chromatography-Fourier transform infrared spectrometry determination of sugars in beverages using chemometric background correction

An on-line gradient reversed phase liquid chromatography-Fourier transform infrared spectrometry (LC-FTIR) method was developed for the determination of fructose, glucose, sucrose and maltose in beverages. Improved chromatographic resolution was obtained using a linear gradient from 75 to 55% (v/v) acetonitrile in water in 15 min. Changes in the background spectra were corrected employing “univariate background correction based on the use of a reference spectra matrix” (UBC-RSM) and using the ratio of absorbance (AR) at 2256 and 2253 cm⁻¹ for the identification of the eluent spectra within the RSM. The method provided limits of detection in the order of 0.75 mg ml⁻¹. The precision (as relative standard deviation) ranged between 3.3 and 4.1% for glucose and fructose, respectively at a concentration level of 3.0 mg ml⁻¹. Quantitative recovery values on spiked samples confirmed the accuracy of the method. A set of samples from the Spanish market were analysed to test the suitability of the procedure.     

In-line respeciation: an ion-exchange ion chromatographic method applied to the separation of degradation products of chemical warfare nerve agents in soil

The natural background of anions encountered when analyzing soil samples by ion chromatography (IC) present significant problems in the separation, detection and quantification of isopropyl methylphosphonic acid (IMPA) and methylphosphonic acid (MPA), the degradation products of sarin, a chemical warfare nerve agent. Using chemically-suppressed IC with conductivity detection, a commercially available ion-exchange column, and an isocratic binary eluent system, IMPA and MPA were determined in aqueous extracts of soil at sub-ppm (μg/g) concentrations without the need for gradient elution or organic solvent eluent modifiers. Common soil anions such as chloride, nitrate, sulfate and phosphate do not interfere with the analysis method due to the composition of the binary eluent allowing for greater mobilization of multivalent anions (e.g., MPA, carbonate, and sulfate) while monovalent anions (e.g., IMPA and nitrate) are relatively unaffected. Carbonate is selectively removed by in-line respeciation to bicarbonate.     

Solvent impregnated hollow fibre for a selective preconcentration of Pb(II) in an on-line determination by flame atomic absorption spectrometry

A solvent impregnated hollow fibre (SIHF) module has been developed for the preconcentration of lead by using bis(2-ethylhexyl) phosphoric acid (DEHPA) dissolved in kerosene as extractant. The module has been designed for an on-line determination of trace amounts of lead(II) at mg l⁻¹ (ppm) level by flame atomic absorption spectrometry (FAAS).

The SIHF system is based on the metal liquid–liquid distribution between aqueous solutions of different acidity and the mentioned organic solution. The highest enrichment factor of Pb(II) was determined at pH=4.0 using a formic acid/formiate buffer solution.

Preconcentration experiments were carried out at low lead(II) concentration (mg l⁻¹ level) by using the SIHF module. This study includes the influence of hydrodynamic and chemical conditions on the loading and elution of Pb(II) on the SIHF, i.e., flow rate through the fibres, acidity of the eluent (as nitric acid concentration) and the chemical nature of the acid used in the elution. Breakthrough curves were determined for different sampling flow rates, 0.54 ml min⁻¹ was selected to minimise the loading volume of Pb(II) sample. 0.1 M nitric acid was chosen as eluent solution, and perchloric acid also shows appropriate elution characteristics. The degree of concentration obtained for Pb(II) are of 10 fold the original concentration. The quantification limit for Pb(II) achieved with this preconcentration system is 0.17 mg l⁻¹.

The results obtained indicate that the SIHF system can be applied for on-line determination of trace amounts of lead(II) by FAAS.     

Role of Hansen solubility parameters in solid phase extraction

The sorbent–eluent systems combined from eight polymeric sorbents and seven solvents as eluents were used for the extraction of phenol and its oxidation products from water samples. The individual interactions between sorbents, eluents and analytes were characterized by Hansen solubility parameters. Principal components analysis (PCA) was used for revealing the dominant interactions (dispersive, polar, and hydrogen bonding type) in sorbent–analyte–eluent systems. The importance of solubility parameters was also determined by a novel procedure based on sum of ranking differences (SRD). Although PCA and ranking by SRD are based on different principles and calculations, they have provided very similar results. The recovery in a given system has been predicted from the magnitudes of mutual interactions (sorbent–analyte, sorbent–eluent, analyte–eluent) by multiple linear regression.     

Cubic smoothing splines background correction in on-line liquid chromatography–Fourier transform infrared spectrometry

A background correction method for the on-line coupling of gradient liquid chromatography and Fourier transform infrared spectrometry (LC–FTIR) is proposed. The developed approach applies univariate background correction to each variable (i.e. each wave number) individually. Spectra measured in the region before and after each peak cluster are used as knots to model the variation of the eluent absorption intensity with time using cubic smoothing splines (CSS) functions. The new approach has been successfully tested on simulated as well as on real data sets obtained from injections of standard mixtures of polyethylene glycols with four different molecular weights in methanol:water, 2-propanol:water and ethanol:water gradients ranging from 30 to 90, 10 to 25 and from 10 to 40% (v/v) of organic modifier, respectively. Calibration lines showed high linearity with coefficients of determination higher than 0.98 and limits of detection between 0.4 and 1.4, 0.9 and 1.8, and 1.1 and 2.7 mg mL⁻¹ in methanol:water, 2-propanol:water and ethanol:water, respectively. Furthermore the method performance has been compared with a univariate background correction approach based on the use of a reference spectra matrix (UBC-RSM) to discuss the potential as well as pitfalls and drawbacks of the proposed approach. This method works without previous variable selection and provides minimal user-interaction, thus increasing drastically the feasibility of on-line coupling of gradient LC–FTIR.     

Use of an o-Benzyl-(S)-Serine Containing Eluent for the Efficient Ligand-Exchange Chromatography-Based Enantioseparation of Constrained Glutamate Receptor Ligands

Four dihydroisoxazole prolines and four dihydroisoxazole cyclopentane derivatives were submitted to chiral ligand-exchange chromatographic analysis in the presence of O-benzyl-(S)-serine, as the chiral mobile phase additive to the eluent. The 1.0 mM O-benzyl-(S)-serine and 0.5 mM Cu(NO₃)₂ eluent flowed at 1.0 mL/min through a conventional octadecylsilica-based stationary phase maintained at 25°C and provided excellent levels of enantioselectivity and resolution for all the species. By using enantiomers as model compounds, the method was validated revealing that the mixed ternary diastereomeric eluates present slightly different spectroscopic properties. The selected chiral ligand-exchange chromatography method was applied for semi-preparative enantioisolation that allowed the establishment of the k(?) < k(+) enantiomeric elution order.     

Comparative Chiral Separation of Thalidomide Class of Drugs Using Polysaccharide-Type Stationary Phases with Emphasis on Elution Order and Hysteresis in Polar Organic Mode

The enantioseparation of four phthalimide derivatives (thalidomide, pomalidomide, lenalidomide and apremilast) was investigated on five different polysaccharide-type stationary phases (Chiralpak AD, Chiralpak AS, Lux Amylose-2, Chiralcel OD and Chiralcel OJ-H) using neat methanol (MeOH), ethanol (EtOH), 1-propanol (PROP), 2-propanol (IPA) and acetonitrile (ACN) as polar organic mobile phases and also in combination. Along with the separation capacity of the applied systems, our study also focuses on the elution sequences, the effect of mobile phase mixtures and the hysteresis of retention and selectivity. Although on several cases extremely high resolutions (R_s > 10) were observed for certain compounds, among the tested conditions only Chiralcel OJ-H column with MeOH was successful for baseline-separation of all investigated drugs. Chiral selector- and mobile-phase-dependent reversals of elution order were observed. Reversal of elution order and hysteresis of retention and enantioselectivity were further investigated using different eluent mixtures on Chiralpak AD, Chiralcel OD and Lux Amylose-2 column. In an IPA/MeOH mixture, enantiomer elution-order reversal was observed depending on the eluent composition. Furthermore, in eluent mixtures, enantioselectivity depends on the direction from which the composition of the eluent is approached, regardless of the eluent pair used on amylose-based columns. Using a mixture of polar alcohols not only the selectivities but the enantiomer elution order can also be fine-tuned on Chiralpak AD column, which opens up the possibility of a new type of chiral screening strategy.