Trilinearity deviation ratio: A new metric for chemometric analysis of comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry data

Comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC–TOFMS) is a well-established instrumental platform for complex samples. However, chemometric data analysis is often required to fully extract useful information from the data. We demonstrate that retention time shifting from one modulation to the next, Δ²t_R, is not sufficient alone to quantitatively describe the trilinearity of a single GC × GC–TOFMS run for the purpose of predicting the performance of the chemometric method parallel factor analysis (PARAFAC). We hypothesize that analyte peak width on second dimension separations, ²W_b, also impacts trilinearity, along with Δ²t_R. The term trilinearity deviation ratio, TDR, which is Δ²t_R normalized by ²W_b, is introduced as a quantitative metric to assess accuracy for PARAFAC of a GC × GC–TOFMS data cube. We explore how modulation ratio, M_R, modulation period, P_M, temperature programming rate, T_ramp, sampling phase (in-phase and out-of-phase), and signal-to-noise ratio, S/N, all play a role in PARAFAC performance in the context of TDR. Use of a P_M in the 1–2 s range provides an optimized peak capacity for the first dimension separation (500–600) for a 30 min run, with an adequate peak capacity for the second dimension separation (12–15), concurrent with an optimized two-dimensional peak capacity (6000–7500), combined with sufficiently low TDR values (0–0.05) to facilitate low quantitative errors with PARAFAC (0–0.5%). In contrast, use of a P_M in the 5 s or greater range provides a higher peak capacity on the second dimension (30–35), concurrent with a lower peak capacity on the first dimension (100–150) for a 30 min run, and a slightly reduced two-dimensional peak capacity (3000–4500), and furthermore, the data are not sufficiently trilinear for the more retained second dimension peaks in order to directly use PARAFAC with confidence.     

Two dimensional assisted liquid chromatography – a chemometric approach to improve accuracy and precision of quantitation in liquid chromatography using 2D separation,dual detectors,and multivariate curve resolution

Comprehensive two-dimensional liquid chromatography (LC × LC) is rapidly evolving as the preferred method for the analysis of complex biological samples owing to its much greater resolving power compared to conventional one-dimensional (1D-LC). While its enhanced resolving power makes this method appealing, it has been shown that the precision of quantitation in LC × LC is generally not as good as that obtained with 1D-LC. The poorer quantitative performance of LC × LC is due to several factors including but not limited to the undersampling of the first dimension and the dilution of analytes during transit from the first dimension (¹D) column to the second dimension (²D) column, and the larger relative background signals. A new strategy, 2D assisted liquid chromatography (2DALC), is presented here. 2DALC makes use of a diode array detector placed at the end of each column, producing both multivariate ¹D and two-dimensional (2D) chromatograms. The increased resolution of the analytes provided by the addition of a second dimension of separation enables the determination of analyte absorbance spectra from the ²D detector signal that are relatively pure and can be used to initiate the treatment of data from the first dimension detector using multivariate curve resolution–alternating least squares (MCR–ALS). In this way, the approach leverages the strengths of both separation methods in a single analysis: the ²D detector data is used to provide relatively pure analyte spectra to the MCR–ALS algorithm, and the final quantitative results are obtained from the resolved ¹D chromatograms, which has a much higher sampling rate and lower background signal than obtained in conventional single detector LC × LC, to obtain accurate and precise quantitative results. It is shown that 2DALC is superior to both single detector selective or comprehensive LC × LC and 1D-LC for quantitation of compounds that appear as severely overlapped peaks in the ¹D chromatogram – this is especially true in the case of untargeted analyses. We also anticipate that 2DALC will provide superior quantitation in targeted analyses in which unknown interfering compounds overlap with the targeted compound(s). When peaks are significantly overlapped in the first dimension, 2DALC can decrease the error of quantitation (i.e., improve the accuracy by up to 14-fold compared to 1D-LC and up to 3.8-fold compared to LC × LC with a single multivariate detector). The degree of improvement in performance varies depending upon the degree of peak overlap in each dimension and the selectivities of the spectra with respect to one another and the background, as well as the extent of analyte dilution prior to the ²D column.     

Analysis of trace levels of domoic acid in seawater and plankton by liquid chromatography without derivatization,using UV or mass spectrometry detection

Quantitation of trace levels of domoic acid (DA) in seawater samples usually requires labour-intensive protocols involving chemical derivatization with 9-fluorenylmethylchloroformate and liquid chromatography with fluorescence detection (FMOC–LC–FLD). Procedures based on LC–MS have been published, but time-consuming and costly solid-phase extraction pre-concentration steps are required to achieve suitable detection limits. This paper describes an alternative, simple and inexpensive LC method with ultraviolet detection (LC–UVD) for the routine analysis of trace levels of DA in seawater without the use of sample pre-concentration or derivatization steps. Qualitative confirmation of DA identity in dubious samples can be achieved by mass spectrometry (LC–MS) using the same chromatographic conditions. Addition of an ion-pairing/acidifying agent (0.15% trifluoroacetic acid) to sample extracts and the use of a gradient elution permitted the direct analysis of large sample volumes (100 μl), resulting in both high selectivity and sensitivity (limit of detection = 42 pg ml⁻¹ by LC–UVD and 15 pg ml⁻¹ by LC–MS). Same-day precision varied between 0.4 and 5%, depending on the detection method and DA concentration. Mean recoveries of spiked DA in seawater by LC–UVD were 98.8% at 0.1–10 ng ml⁻¹ and 99.8% at 50–1000 ng ml⁻¹. LC–UVD exhibited strong correlation with FMOC–LC–FLD during inter-laboratory analysis of Pseudo-nitzschia multiseries cultures containing 60–2000 ng DA ml⁻¹ (r² > 0.99), but more variable results were obtained by LC–MS (r² = 0.85). This new technique was used to confirm the presence of trace DA levels in low-toxicity Pseudo-nitzschia spp. isolates (0.2–1.6 ng ml⁻¹) and in whole-water field samples (0.3–5.8 ng ml⁻¹), even in the absence of detectable Pseudo-nitzschia spp. cells in the water column.     

Development of a rapid and sensitive method for determination of cysteine/cystine ratio in chemically defined media

Two rapid, sensitive and quantitative methods for the determination of the cysteine and cystine ratio in complex defined media feedstock using monolithic reversed-phase liquid chromatography (RPLC) and RPLC–MS are presented. Cysteine is pre-derivatised with purified 2-chloro-1-methylquinolinium tetrafluoroborate (CMQT) and separated from other derivatisation products on a narrow-bore 50 mm × 2 mm I.D. monolithic C₁₈ column with UV detection at 355 nm. For reversed-phase LC (RPLC) the separation is carried out isocratically using a mobile phase of 50 mM trichloroacetic acid (TCA) adjusted to pH 2.5 with lithium hydroxide (LiOH) and acetonitrile (83:14) pumped at 1.5 mL/min with an elevated column temperature. For RPLC–MS an ammonium acetate and acetonitrile gradient method was developed with a reduced flow rate of 0.3 mL/min. The treatment of the samples consisted of dividing them into two aliquots, the first aliquot is analysed for cysteine and the second aliquot is analysed for cystine after its quantitative reduction to cysteine using tris(2-carboxyethyl)phosphine (TCEP). Both methods are linear, with R² > 0.999 for 0.25–500 μM for cysteine and 0.25–250 μM for cystine using the LC–UV method, sensitive, with detection limit of 36 nM for cysteine, and precise, with ≤1.1% RSD for both retention time and peak area (n = 6). Samples (n = 31) of an industry standard and supplied chemically defined media feedstock were analysed, finding cysteine ranging from 1.56 to 2.26 μg/mL and cystine from 1062.02 to 1348.13 μg/mL.     

Direct aqueous supercritical fluid extraction coupled on-line with liquid chromatography–tandem mass spectrometry for the analysis of polyether ionophore antibiotics in water

A direct aqueous SFE system designed to extract water samples contained in vials has been coupled on-line with a reverse phase LC–MS–MS system using a single 10-port valve. An SFE trap system using C₁ stationary phase connected to a C₁₈ analytical HPLC column enabled the SFE–LC–MS–MS analysis of three polyether ionophore antibiotics in water using a step gradient. A quantitative SFE–LC–MS–MS method has been developed whereby the progress of SFE can be monitored directly on-line such that ionophore recovery profile data from a single water sample can be obtained. Using a continuous direct aqueous SFE period of 75 min, the SFE–LC–MS–MS recoveries of the ionophores were: monensin 76.2% with RSD 4.1%, lasalocid 84.6% with RSD 3.8% and narasin 91.2% with RSD 3.2%. With positive ion electrospray ionization, the SFE–LC–MS–MS system using a 4 mL water sample provided multiple reaction monitoring (MRM) limits of detection for monensin and lasalocid each equivalent to 90 ng/L whereas 30 ng/L for narasin. A two-way valve controlling carbon dioxide distribution to the SFE vessel has provided a means for the initial investigation of the recovery of ionophore sodium salts from water using static SFE.     

Separation of intact proteins on porous layer open tubular (PLOT) columns

Porous layer open tubular (PLOT) polystyrene divinylbenzene columns have been used for separating intact proteins with gradient elution. The 10 μm I.D. × 3 m columns were easily coupled to standard liquid chromatography–mass spectrometry (LC–MS) instrumentation with commercially available fittings. Standard proteins separated on PLOT columns appeared as narrow and symmetrical peaks with good resolution. Average peak width increased linearly with gradient time (t_G) from 0.14 to 0.33 min (t_G 20 and 120 min, respectively) using a 3 m column. With shorter columns, peak widths were larger and increased more steeply with gradient time. Theoretical peak capacity (n_c) increased with column length (tested up to 3 m). The n_c increased with t_G until a plateau was reached. The highest peak capacity achieved (n_c = 185) was obtained with a 3 m column, where a plateau was reached with t_G 90 min. The within- and between column retention time repeatabilities were below 0.6% and below 2.5% (relative standard deviation, RSD), respectively. The carry-over following injection of 0.5 ng per protein was less than 1.1%. The retention time dependence on column temperature was investigated in the range 20–50 °C. Proteins in a skimmed milk sample were separated using the method.     

High efficiency liquid chromatography techniques coupled to mass spectrometry for the characterization of mate extracts

There is growing interest related to rapid screening and full characterization of the constituents of plants with medicinal properties; among these, “Mate” or Yerba Maté is a tea-like beverage widely consumed in South America, obtained from the dried leaves of Ilex paraguariensis. The high content in polyphenols accounts for in vitro and in vivo antioxidant activity of the extracts obtained from this plant; on the other hand, the high complexity of the samples extracted, depending on the method employed, may preclude complete resolution by conventional HPLC techniques. For this purpose, a comprehensive two-dimensional liquid chromatography (LC × LC) system, comprised of an RP-Amide first dimension and a partially porous octadecylsilica column in the second dimension, has been compared with a one-dimensional system. The latter was operated using a partially porous octadecylsilica column, with diode array (DAD) and electrospray/ion trap-time of flight (ESI/IT-TOF) detection for the most complex extracts. The employment of the hybrid mass spectrometer allowed unequivocal identification of several compounds in the mate extracts. Using LC × LC–MS³, it was possible to discriminate between congeners of chlorogenic acids, along with monoacyl- and diacylchlorogenic acid esters.     

In vivo solid-phase microextraction for single rodent pharmacokinetics studies of carbamazepine and carbamazepine-10,11-epoxide in mice

The use of solid-phase microextraction (SPME) for in vivo sampling of drugs and metabolites in the bloodstream of freely moving animals eliminates the need for blood withdrawal in order to generate pharmacokinetics (PK) profiles in support of pharmaceutical drug discovery studies. In this study, SPME was applied for in vivo sampling in mice for the first time and enables the use of a single animal to construct the entire PK profile. In vivo SPME sampling procedure used commercial prototype single-use in vivo SPME probes with a biocompatible extractive coating and a polyurethane sampling interface designed to facilitate repeated sampling from the same animal. Pre-equilibrium in vivo SPME sampling, kinetic on-fibre standardization calibration and liquid chromatography–tandem mass spectrometry analysis (LC–MS/MS) were used to determine unbound and total circulating concentrations of carbamazepine (CBZ) and its active metabolite carbamazepine-10,11-epoxide (CBZEP) in mice (n = 7) after 2 mg/kg intravenous dosing. The method was linear in the range of 1–2000 ng/mL CBZ in whole blood with acceptable accuracy (93–97%) and precision (<17% RSD). The single dose PK results obtained using in vivo SPME sampling compare well to results obtained by serial automated blood sampling as well as by the more conventional method of terminal blood collection from multiple animals/time point. In vivo SPME offers the advantages of serial and repeated sampling from the same animal, speed, improved sample clean-up, decreased animal use and the ability to obtain both free and total drug concentrations from the same experiment.     

Comparison of several sorbents for continuous in situ derivatization and preconcentration of low-molecular mass aldehydes prior to liquid chromatography–tandem mass spectrometric determination in water samples

A comparative study of six SPE conventional and non-conventional sorbent materials (silica RP-C₁₈, LiChrolut EN, Amberlite XAD-2, C₆₀ fullerene, multiwall carbon nanotubes and graphitized carbon black) was carried out for the in situ derivatization/preconcentration of eight aldehydes with 2,4-dinitrophenylhydrazine. Although two of the sorbents, LiChrolut EN and RP-C₁₈, turned out to be the most suitable for ultratrace analysis of the aldehydes, LiChrolut EN showed higher capacity for 2,4-dinitrophenylhydrazine trapping (higher efficiency for the in situ derivatization reaction) and superior performance in terms of sensitivity (likely a result of its increased sample breakthrough volume). The LiChrolut EN-based method combined with LC–MS/MS allowed the determination of aldehydes over the linear range of 0.02–15 μg l⁻¹, with limits of detection at 6–24 ng l⁻¹ and precision of 3.2–7.2%. The method was applied to determine low-molecular mass aldehydes in water samples. These results indicate that the method proposed is a straightforward and sensitive tool for the determination of these aldehydes in water samples providing better results than those LC–MS/MS reported alternatives in terms of the limit of detection, sample requirements for analysis and cost.     

Absorption in regio-regular poly(3-hexyl)thiophene thin films: Fermi resonances,interband coupling and disorder

Absorption in conjugated polymer aggregates is studied theoretically, taking into account excitonic (intermolecular) coupling, exciton–phonon (EP) coupling and site-energy disorder, all treated on equal footing within a generalized Holstein Hamiltonian with numerically generated eigenmodes and energies. The analysis deals primarily with the weak excitonic coupling regime, which for polymers, corresponds to J₀ ? 0.4ω₀ where J₀ is the nearest neighbor excitonic coupling and ω₀ (≈1400 cm⁻¹) is the frequency of the ring breathing/stretching mode coupled to the molecular electronic transition with Huang–Rhys factor, λ² ≈ 1. Disorder is characterized by a Gaussian distribution of molecular transition frequencies of width σ. Absorption spectra are calculated under the two-particle approximation (TPA) as well as the less accurate, but computationally more efficient single-particle approximation (SPA). Fermi resonances (FRs) arise due to the coupling between the optically allowed single-particle states (vibronic excitons) and the dark two-particle states. In the motional narrowing limit FRs are clearly resolved. Further increases in σ cause the FRs to merge into a single peak for each vibronic band. At this point the SPA becomes accurate for the entire spectrum. The SPA also provides the basis for simplified analytical expressions for the peak intensity ratios, from which the free exciton bandwidth, W, can be determined. Analytical expressions are also obtained for the peak-positions within the vibronic progression. Application to regio-regular poly(3-hexyl)thiophene films shows the exciton bandwidth, W, to be in the range 0.8ω₀–ω₀. The model also accounts for the irregular peak spacings observed in experiment.     

A method for lactate and pyruvate determination in filter-paper dried blood spots

Lactic acidemia is commonly associated with severe diseases in pediatric patients. Quantitation of blood lactate and pyruvate is important for the diagnosis and clinical management. A liquid chromatography–tandem mass spectrometry (LC–MS/MS) method using dried blood spots (DBS) was developed and could be used for simultaneous quantification of blood lactate and pyruvate. The applicability of the developed method was tested and confirmed by the regression analysis between LC–MS/MS method and enzymatic assay. Lactate and pyruvate were extracted from DBS obtained from 580 full-term, 120 pre-term infants (gestations ranging from 24 to 36 weeks), and 65 patients with suspected lactic acidemia, with methanolic internal standard (IS) solutions of sodium l-lactate-¹³C₃ and pyruvate-¹³C₃. An API-2000 LC–MS/MS system with multiple reaction monitoring (MRM) mode was applied. The within-run and between-run precisions (CV%) were determined and the results were 1.9% and 3.9% for lactate (n = 20) and 5.7% and 7.3% for pyruvate (n = 20). The linearity of lactate (r = 0.9986) and pyruvate (r = 0.9973) based on the IS was excellent. The parameter r squared (r²) of linear regression between LC–MS/MS method and enzymatic assay was 0.9405 for lactate and 0.9447 for pyruvate, respectively, and the agreement between these methods was consistent and acceptable. The stability of lactate and pyruvate on DBS was also confirmed. The LC–MS/MS method we developed is a specific, sensitive, and reproducible method for measuring blood lactate and pyruvate concentrations. The use of DBS in this method makes it particularly attractive for pediatric patients.     

Mobile phase selection for the combined use of liquid chromatography–inductively coupled plasma mass spectrometry and electrospray ionisation mass spectrometry

Four different organic solvents: dimethylformamide, 1,4-dioxane, n-propanol and ethanol were evaluated as alternative organic modifiers to acetonitrile for liquid chromatography (LC) separations. The aim was to establish common sets of chromatographic conditions that could be applied for LC hyphenation to inductively coupled plasma mass spectrometry (ICPMS) as well as to electrospray ionization MS (ESIMS). The approach was to evaluate candidate solvents that, compared to acetonitrile, potentially could give improved analytical performance (low solvent vapor loading, maximized analyte sensitivity and minimized carbon depositions on instrumental parts) in ICPMS analysis while retaining chromatographic and ESIMS performances. The study showed that dimethylformamide, 1,4-dioxane, n-propanol and ethanol all can be advantageous chromatographic modifiers for LC–ICPMS analysis, giving superior performance compared to acetonitrile. For the combined use of LC–ICPMS and LC–ESIMS with a common set of chromatographic conditions, n-propanol gave the best overall performance. The ¹⁹⁵Pt⁺ signal in ICPMS was continuously monitored during a 0–60% organic solvent gradient and at 25% of organic modifier, 100% of the signal obtained at the gradient start was preserved for n-propanol compared to only 35% of the signal when using acetonitrile. Platinum detection limits were 5–8 times lower using n-propanol compared with acetonitrile. Signal-to-noise ratio in continuous ESIMS signal measurements was 100, 90 and 110 for a 100 μg/ml solution of leucine–enkephaline using acetonitrile, ethanol and n-propanol, respectively. Chromatographic efficiency in reversed phase separations was preserved for n-propanol compared to acetonitrile for the analysis of the whole protein cytochrome C and the peptide bacitracin on a column with particle and pore sizes of 5 μm and 300 Å, but slightly deteriorated for the separation of the peptides leucine–enkephaline and bacitracin on a 3 μm and 90 Å column as the peak width at half height for both peptides increased by a factor of two. The performance on the smaller dimensioned column could however be improved by running the separations at 40 °C.     

Ganoderma species discrimination by dual-mode chromatographic fingerprinting: A study on stationary phase effects in hydrophilic interaction chromatography and reduction of sample misclassification rate by additional use of reversed-phase chromatography

Acetonitrile–water extracts of several Ganoderma species – a mushroom being used in Traditional Chinese Medicine – were analysed by liquid chromatography–UV detection in hydrophilic interaction chromatography (HILIC) and reversed-phase (RP) elution modes. A set of six polar stationary phases was used for HILIC runs. These columns had remarkably different separation properties under binary gradient conditions as evinced by hierarchical cluster analysis on retention patterns of seven test compounds. Complementary measurements of RP chromatograms were carried out on a C₁₈ packing. Injection precision (n = 5) and intra-day precision (n = 5) were each <2.0% RSD (HILIC) and <0.7% RSD (RP) for relative retention times of main characteristic peaks of a sample extract while for relative peak areas RSD values were max. 6.8%. Repetitive analysis (n = 7) of a processed sample stored in the autosampler tray for 48 h was used to confirm within-sequence sample stability. Eleven Ganoderma lucidum samples served as training set for the construction of column-specific simulated mean chromatograms. Validation with twelve samples comprising G. lucidum, Ganoderma sinense, Ganoderma atrum, and Ganoderma tsugae by correlation coefficient based similarity evaluation of peak patterns showed that a discrimination of G. lucidum from other Ganoderma species by means of chromatographic fingerprints is conceptually possible on all columns, except of a bare silica packing. The importance of the combined use of RP and HILIC fingerprints to improve the rate of correct sample classification was demonstrated by the fact that each one G. sinense specimen was wrongly assigned being G. lucidum by all HILIC fingerprints but not the RP fingerprint and vice versa. The present data revealed that (i) the analysis of complex biological materials by quasi orthogonal chromatographic modes such as HILIC and RP may deliver more discriminative information than single-mode approaches which strengthens the reliability of fingerprint-based sample classification and (ii) different retention and selectivity characteristics of polar bonded silica packings in the HILIC elution mode may only have a minor impact on chemometric sample discrimination capabilities in such kind of pattern-oriented metabolomics separation problems.     

Development and optimization of a method for the separation of platycosides in Platycodi Radix by comprehensive two-dimensional liquid chromatography with mass spectrometric detection

Comprehensive two-dimensional chromatography (LC × LC) using combinations of two columns (C₁₈ × CN and C₁₈ × NH₂) was employed with electrospray (ESI) mass spectrometry to analyze platycosides from root extract. Based on the capability of the C₁₈, CN and NH₂ columns to separate the platycosides, the orthogonality in two-dimensional space according to each combination of columns was predicted from the correlation coefficients between the retention times of the 17 compounds separated by the independent CN and C₁₈ columns, and NH₂ and C₁₈ columns. The expected distribution of the peaks was also compared with the two-dimensional plots obtained by practical separation in an LC × LC system. The increased peak capacities using C₁₈ × NH₂ allowed three minor components and five isomers of the platycosides to be newly separated, which were not identified with 1D-LC using the individual C₁₈ column, whereas the combination of C₁₈ × CN did not result in any improvement of the separation performance.     

A simple approach to reduce dimensionality from comprehensive two-dimensional liquid chromatography coupled with a multichannel detector

Comprehensive two-dimensional liquid chromatographic (LC × LC) systems play an ever increasing role in separation and characterization of complex samples. When coupled with multichannel detectors, such as the diode array detector, these LC × LC systems become especially useful for non-target analysis and identification of patterns based on the information extracted from those complex samples. Nevertheless, due to the large amount of data generated by these systems, the extraction of useful information for the identification of patterns still is one of the major drawbacks for a wider application of this technique. As a preliminary step in data treatment, we have developed a simple and fast way to deal with this large amount of multi-dimensional data by identifying the three-dimensional (3D) regional maxima of each chromatographic peak generated in a LC × LC–DAD system: retention times at the peak maximum in the first- and second-dimensions and the wavelength of the maximum UV absorption. This dataset is then used to build a 3D fingerprinting of the given sample, which alongside the 3D fingerprinting of other samples, can be used to identify different patterns associated with the specific properties of every sample under study. The applicability of the developed methodology was further assessed by performing a non-target LC × LC–DAD analysis of four Portuguese red wine samples.     

Measurement and modeling of osmotic coefficients of aqueous solution of ionic liquids using vapor pressure osmometry method

Osmotic coefficients of binary mixtures containing an ionic liquid, (1-butyl-3-methylimidazolium tetrafluoroborate, [BMIm]BF₄, 1-ethyl-3-methylimidazolium ethyl sulfate, [EMIm]ES, and 1-butyl-3-methylimidazolium methyl sulfate, [BMIm]MS) with water were measured until about 3 molal concentrations using vapor pressure osmometry method (VPO) at temperature ranges 298.15–328.15 K and modeled using different electrolyte excess Gibbs free energy models including electrolyte non-random two liquids (NRTL), modified NRTL (MNRTL), mean spherical approximation NRTL (MSA-NRTL), non random factor (NRF), and extended Wilson models. The results show that osmotic coefficient data increase with increasing temperature. The calculated standard deviations of the studied systems show that the applicability of these models for the correlation of VLE properties of ionic liquid solutions. The average standard deviations for the models have the order σ(?) MNRTL < σ(?) Wilson < σ(?) NRTL < σ(?) MSA-NRTL < σ(?)NRF. The results show MNRTL model is able to reproduce experimental osmotic coefficients of aqueous solution of studied ionic liquids with good precision.     

Probability of failure of the watershed algorithm for peak detection in comprehensive two-dimensional chromatography

The watershed algorithm is the most common method used for peak detection and integration in two-dimensional chromatography. However, the retention time variability in the second dimension may render the algorithm to fail. A study calculating the probabilities of failure of the watershed algorithm was performed. The main objective was to calculate the maximum second-dimension retention time variability, Δ²t_R,crit, above which the algorithm fails. Several models to calculate Δ²t_R,crit were developed and evaluated: (a) exact model; (b) simplified model and (c) simple-modified model. Model (c) gave the best performance and allowed to deduce an analytical expression for the probability of failure of the watershed algorithm as a function of experimental Δ²t_R, modulation time and peak width in the first and second dimensions. It could be demonstrated that the probability of failure of the watershed algorithm under normal conditions in GC × GC is around 15–20%. Small changes of Δ²t_R, modulation time and/or peak width in the first and second dimension could induce subtle changes in the probability of failure of the watershed algorithm. Theoretical equations were verified with experimental results from a diesel sample injected in GC × GC and were found to be in good agreement with the experiments.     

Determination of cyromazine and melamine in chicken eggs using quick,easy, cheap,effective, rugged and safe (QuEChERS) extraction coupled with liquid chromatography–tandem mass spectrometry

A rapid and sensitive method has been developed for the simultaneous detection of cyromazine and melamine in chicken eggs using the quick, easy, cheap, effective, rugged and safe (QuEChERS) method coupled with liquid chromatography–tandem mass spectrometry (LC–MS/MS). The optimal extraction solvent for the liquid–liquid extraction was 5 mL of acetonitrile with a 0.1 M hydrochloric acid aqueous solution (99.5:0.5, v/v). The extract was cleaned with 0.5 g of anhydrous magnesium sulfate and 10 mg of graphitized carbon black. The analysis of cyromazine and melamine was accomplished by combining the use of an anion exchange LC column with tandem mass spectrometry in the positive electrospray ionization mode with selected reaction monitoring mode (SRM). The detection limits were 1.6 ng g⁻¹ for cyromazine and 8 ng g⁻¹ for melamine, and the quantitation limits were 5.5 ng g⁻¹ for cyromazine and 25 ng g⁻¹ for melamine. The recoveries of cyromazine and melamine in the spiked egg samples were 83.2% and 104.6%, respectively, with an relative standard deviation (RSD) of less than 18.1%. The intra-day and inter-day precisions, represented by the RSD, ranged from 1.5% to 8.8% and 6.8% to 14.3%, respectively. The proposed method was tested by analyzing chicken eggs from the markets and from the veterinary medicine laboratory. The concentrations of cyromazine and melamine detected in these samples were in the range of 20–94 ng g⁻¹. The results demonstrated that the QuEChERS method combined with LC–MS/MS is a simple, rapid and inexpensive method for the analysis of cyromazine and melamine in eggs.     

Prediction of gas collection efficiency and particle collection artifact for atmospheric semivolatile organic compounds in multicapillary denuders

A modeling approach is presented to predict the sorptive sampling collection efficiency of gaseous semivolatile organic compounds (SOCs) and the artifact caused by collection of particle-associated SOCs in multicapillary diffusion denuders containing polydimethylsiloxane (PDMS) stationary phase. Approaches are presented to estimate the equilibrium PDMS–gas partition coefficient (K_pdms) from a solvation parameter model for any compound, and, for nonpolar compounds, from the octanol–air partition coefficient (K_oa) if measured K_pdms values are not available. These estimated K_pdms values are compared with K_pdms measured by gas chromatography. Breakthrough fraction was measured for SOCs collected from ambient air using high-flow (300 L min⁻¹) and low-flow (13 L min⁻¹) denuders under a range of sampling conditions (−10 to 25 °C; 11–100% relative humidity). Measured breakthrough fraction agreed with predictions based on frontal chromatography theory using K_pdms and equations of Golay, Lövkvist and Jönsson within measurement precision. Analytes included hexachlorobenzene, 144 polychlorinated biphenyl congeners, and polybrominated diphenyl ethers 47 and 99. Atmospheric particle transmission efficiency was measured for the high-flow denuder (0.037–6.3 μm diameter), and low-flow denuder (0.015–3.1 μm diameter). Particle transmission predicted using equations of Gormley and Kennedy, Pich, and a modified filter model, agreed within measurement precision (high-flow denuder) or were slightly greater than (low-flow denuder) measured particle transmission. As an example application of the model, breakthrough volume and particle collection artifact for the two denuder designs were predicted as a function of K_oa for nonpolar SOCs. The modeling approach is a necessary tool for the design and use of denuders for sorptive sampling with PDMS stationary phase.     

Identification of degraded products of aldicarb due to the catalytic behavior of titanium dioxide/polyacrylonitrile nanofiber

Photocatalytic properties of fibers containing TiO₂ nanoparticles were explored for use as a self-decontaminating material using degradation of the pesticide aldicarb as the model toxin. During the analysis of the aldicarb treated sample by liquid chromatography (LC) with diode array detector (DAD), an unidentified peak was found at relative retention time (RT) 3.9 min when compared to aldicarb and major metabolites, aldicarb sulfoxide, and aldicarb sulfone. An analytical method was developed to confirm and identify this degradation product. LC–APCI/MS techniques were used first to analyze molecular ions and major fragments comparing retention times and spectra with those of known standards. FTIR and LC–MS/MS techniques were used to confirm the identity of the degradation product as 2-propenal, 2-methyl-, O-[(methylamino)carbonyl]oxime.