Liquid-phase microextraction based on solidified floating drops of organic solvents

Microextraction of organic or inorganic analytes using solidified floating drops of organic solvents is a fairly new method that is simple and rapid, and requires only small quantities of solvents and reagents. This review (with 109 references) covers published work up to Sep. 2012, and describes how the method was combined with analytical techniques such as GC, HPLC, ICP-OES and electrothermal atomic absorption spectrometry. We discuss basic principles and the main parameters that affect the extraction efficiency, and give specific applications of the technique.

Visualization of a protein-protein interaction at a single-molecule level by atomic force microscopy

Atomic force microscopy is unmatched in terms of high-resolution imaging under ambient conditions. Over the years, substantial progress has been made using this technique to improve our understanding of biological systems on the nanometer scale, such as visualization of single biomolecules. For monitoring also the interaction between biomolecules, in situ high-speed imaging is making enormous progress. Here, we describe an alternative ex situ imaging method where identical molecules are recorded before and after reaction with a binding partner. Relocation of the identical molecules on the mica surface was thereby achieved by using a nanoscale scratch as marker. The method was successfully applied to study the complex formation between von Willebrand factor (VWF) and factor VIII (FVIII), two essential haemostatic components of human blood. FVIII binding was discernible by an appearance of globular domains appended to the N-terminal large globular domains of VWF. The specificity of the approach could be demonstrated by incubating VWF with FVIII in the presence of a high salt buffer which inhibits the interaction between these two proteins. The results obtained indicate that proteins can maintain their reactivity for subsequent interactions with other molecules when gently immobilized on a solid substrate and subjected to intermittent drying steps. The technique described opens up a new analytical perspective for studying protein-protein interactions as it circumvents some of the obstacles encountered by in situ imaging and other ex situ techniques.

Analysis of flame retardants and elements of concern in printed wiring boards with respect to origin and year of construction

Thirty-one populated printed wiring boards, covering a range of 30 years of construction, and originating from various electronic devices, were investigated using different analytical procedures. Noble, precious and rare metals, as well as environmentally relevant elements were identified by EDXRF, and lead and the flame retardant (FR) indicator bromine were localised by means of microbeam EDXRF. A GC/MS procedure was developed to identify and quantify FR substances. Several sample preparation techniques were applied, optimised and compared. The method of first choice was ultrasonic extraction because it provided the best compromise between effort, cost and quality of the analytical results. Altogether, a wide variety of elements of concern, and halogenated and phosphate-based FRs were found in the investigated boards. Their occurrence is partially related to the origin and/or year of construction.

Application of normal-phase high-performance liquid chromatography followed by gas chromatography for analytics of diesel fuel additives

The paper presents the results of investigations on new procedures of determination of selected cleaning additives in diesel fuel. Two procedures: one-step analysis using gas chromatography with flame ionization detection (GC-FID) or mass spectrometry (GC-MS) and a two-step procedure in which normal-phase high-performance liquid chromatography (NP-HPLC) was used for preliminary separation of the additives, were compared. The additive fraction was collected using either simple elution or eluent backflush. Final determinations were performed by GC-FID and GC-MS. The studies revealed that it was impossible to determine the investigated analytes by one-step procedures, i.e. by using solely HPLC or GC. On the other hand, the use of a two-step procedure ensures reproducible results of determinations, and the limits of quantitation are, depending on the method of fraction collection by HPLC, from 1.4–2.2 ppm (GC-MS in SIM mode) to 9.6–24.0 ppm (GC-FID). Precision and accuracy of the developed procedures are compared, and possible determination errors and shortcomings discussed.

Electrochemical immunosensor for prostate-specific antigen using a glassy carbon electrode modified with a nanocomposite containing gold nanoparticles supported with starch-functionalized multi-walled carbon nanotubes

We report on a simple, rapid, and efficient method for the extraction of volatile organic compounds (VOCs; including methanol, tetrahydrofuran, 2-hexanone and benzene) from air and solid samples. The system is based on the use of a laboratory-made syringe as the extractor. The needle of the syringe is placed in a chamber cooled by liquid nitrogen. The tip of the needle is placed in the headspace of a vial containing the sample. The headspace components then are circulated with a pump to pass the needle, and this results in freeze-trapping of the VOCs on the inner surface of the needle. The circulation of the headspace components is continued for 15 min, and the syringe is then removed and placed in a GC injector. The effects of volume of the sample vial, headspace flow rate, temperature and time of extraction and desorption were optimized. The overall time for sampling and analysis is <30 min. The method displays an extraction efficiency of >80%) and a good sample transfer efficiency into the GC column due to the absence of a sorbent inside the needle. No carry-over was observed after 30?s desorption at 260?°C. An external standard method was used for quantitative analysis. The relative standard deviation values are below 10% and the limits of detection range from 1.3 to 4.6?ng?g^?1.

Centri-voltammetry and biocentri-voltammetry: a review

Centri-voltammetry and biocentri-voltammetry are techniques that combine centrifugation with voltammetry. This review covers (a) definitions; (b) experimental (cell) configurations; (c) effects of centrifugation parameters; and (d) applications of the method to the determination of heavy metals and biological molecules. Specific examples include applications to the determination of xanthine, acetylcholine esterase activity, and of acetyl choline.

Combination of TREF, high-temperature HPLC, FTIR and HPer DSC for the comprehensive analysis of complex polypropylene copolymers

A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method—high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method—HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene–propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.

Comparison between GC–MS and GC–ICPMS using isotope dilution for the simultaneous monitoring of inorganic and methyl mercury,butyl and phenyl tin compounds in biological tissues

The aim of this work is to compare simultaneous isotope dilution analysis of organotin and organomercury compounds by gas chromatography–mass spectrometry (GC–MS) and gas chromatography–inductively coupled plasma mass spectrometry (GC–ICP/MS) on certified bivalve samples. These samples were extracted by microwave with tetramethylammonium hydroxide (TMAH). Derivatization with both NaBEt₄ and NaBPr₄ was evaluated, and analytical performances were compared. Two CRM materials, BCR-710 and CRM-477, were analyzed by both techniques to verify accuracy. A mixed spike containing ²⁰¹Hg-enriched methylmercury (MeHg), ¹⁹⁹Hg-enriched inorganic mercury (iHg), ¹¹⁹Sn-enriched monobutyltin (MBT), dibutyltin (DBT), and tributyltin (TBT) as well as homemade ¹¹⁶Sn-enriched monophenyltin (MPT), diphenyltin (DPT), and triphenyltin (TPT) was used for the isotope dilution analysis of samples. The two techniques studied were compared in terms of classic analytical parameters: linearity, precision or repeatability (i.e., percent relative standard deviation, RSD%), limit of detection (LOD), and limit of quantification (LOQ), showing excellent linearity, precision below 12 % for all analytes, and LOQs of 0.06–1.45 pg for GC–MS and 0.02–0.27 pg for GC–ICP/MS.

Comparative investigation on chemical constituents of flower bud,stem and leaf of Lonicera japonica Thunb. by HPLC-DAD-ESI-MS/MS n and GC-MS

In approaches of HPLC coupled with diode array detector tandem multiple mass spectrometry (HPLC-DAD-ESI-MS/MS ⁿ ) and GC-MS techniques, efficient comparative methods were developed to profile the chemical constituents in flower bud, leaf and stem parts of Lonicera japonica Thunb. (LJT). 22 polar compounds of various chemical structures and 19 volatile or semi-volatile compounds were tentatively identified from aerial parts of the plant. A number of common composition groups in three separate parts of the plant were proposed, including phenolic acids, flavonoids, iridoids, alkanes, olefins and sterols, the leaf showing higher similarity with the flower bud part via their chemical constituents. Considering the fact that the leaf part has a lot of similar components with the flower bud, this study indicates the leaf part of LJT can be expected to be used as an alternative medicinal resource to the flower bud and stem of the plant.     

Gas chromatography/multiphoton ionization/time-of-flight mass spectrometry using a femtosecond laser

A laser can be used for the selective excitation and subsequent ionization of a molecule with an absorption band at the laser wavelength. This technique of multiphoton ionization (MPI), when combined with time-of-flight mass spectrometry (TOF-MS), permits the efficient detection of induced ions in mass analysis. This combination of MPI/TOF-MS can be coupled with gas chromatography (GC) to achieve even more enhanced selectivity. Thus, GC/MPI/TOF-MS can be employed for trace analysis of samples containing numerous chemical species. A variety of laser sources have been used for this purpose. Since molecules that are classified as environmental pollutants frequently contain chlorine and bromine atoms, the lifetime of the excited state can be decreased by energy transfer from the singlet to triplet levels by spin–orbit interaction. A high-power femtosecond laser with a pulse width shorter than the lifetime of the analyte molecule provides femtogram or even subfemtogram detection limits, which have not yet been achieved using the most sensitive high-resolution double-focus sector-type mass spectrometers. Numerous environmental pollutants such as dioxins in soils and pesticides in foods have been successfully quantified using GC/MPI/TOF-MS, and this technique has proven itself to be a useful and practical method for trace analysis.

Use,analysis, and regulation of pesticides in natural extracts,essential oils,concretes, and absolutes

Natural extracts used by the fragrance and cosmetics industries, namely essential oils, concretes, resinoids, and absolutes, are produced from natural raw materials. These are often cultivated by use of monoculture techniques that involve the use of different classes of xenobiotica, including pesticides. Because of these pesticides’ potential effect on public health and the environment, laws regarding permitted residual levels of pesticides used in cultivation of raw materials for fragrance and cosmetic products are expected to become stricter. The purpose of this review is to present and classify pesticides commonly used in the cultivation of these natural raw materials. We will summarize the most recent regulations, and discuss publications on detection of pesticides via chemical analysis of raw natural extracts. Advances in analytical chemistry for identification and quantification of pesticides will be presented, including both sample preparation and modern separation and detection techniques, and examples of the identification and quantification of individual pesticides present in natural extracts, for example essential oils, will be provided.

Fully automated isotopic dimethyl labeling and phosphopeptide enrichment using a microfluidic HPLC phosphochip

Quantitative detection of phosphorylation levels is challenging and requires an expertise in both stable isotope labeling as well as enrichment of phosphorylated peptides. Recently, a microfluidic device incorporating a nanoliter flow rate reversed phase column as well as a titania (TiO₂) enrichment column was released. This HPLC phosphochip allows excellent recovery and separation of phosphorylated peptides in a robust and reproducible manner with little user intervention. In this work, we have extended the abilities of this chip by defining the conditions required for on-chip stable isotope dimethyl labeling allowing for automated quantitation. The resulting approach will make quantitative phosphoproteomics more accessible.

Trends in liquid-phase microextraction, and its application to environmental and biological samples

Liquid phase microextraction (LPME) is a popular technique for sample pretreatment before the trace determination of target compounds from complex matrices, examples being pesticides in environmental and food samples, or drug residuals in biological samples such as blood or urine. LPME is simple, affordable, easy to operate, and highly sensitive. It is a miniaturized implementation of conventional liquid-liquid extraction in which only a few microliters of solvents are used instead of several hundreds of milliliters. This review focuses on newly developed LPME-based techniques, their application to environmental and biological samples, on their limitations, and on future applications.

Enzymatic hydrolysis-based absolute quantification of triacylglycerols in plant oil by use of a single marker

Absolute quantification of triacylglycerols (TAGs) in plant oils is a challenge for analysts, because most of the necessary chemical standards are unavailable. In this study, a new method for absolute quantification analysis of multi-components by use of a single marker (AQAMS), using two crucial technologies, evaluation of the collection recovery without chemical standards and enzymatic hydrolysis, was used for determining the absolute content of TAGs in brucea javanica oil (BJO), using glycerol as the marker. The TAGs in BJO were initially characterized using ultrafast liquid chromatography tandem atmospheric-pressure-chemical-ionization mass spectrometry. Then the TAGs in BJO were individually collected, by target-fraction collection via high-performance liquid chromatography coupled with an evaporative-light-scattering detector (HPLC-ELSD), and their recoveries were calculated by use of a novel non-standard evaluated recovery strategy (NSER). The results revealed that the collection procedure was feasible and reliable. Finally, modified commercial TAG assay kits using glycerol as the marker were used to determine the absolute abundance of individual TAGs in the plant oils. Comparing the result with that obtained by HPLC-ELSD analysis using triolein standard, the content of triolein determined by AQAMS was closely matched. The proposed strategy is a practical measure for solving the problem of the lack of chemical standards, and provides a new method for absolute quantification in natural products of multi-components with the same backbone.

A novel reversed-phase HPLC method for the determination of urinary creatinine by pre-column derivatization with ethyl chloroformate: comparative studies with the standard Jaffé and isotope-dilution mass spectrometric assays

Creatinine is an important biomarker for renal function diagnosis and normalizing variations in urinary drug/metabolites concentration. Quantification of creatinine in biological fluids such as urine and plasma is important for clinical diagnosis as well as in biomonitoring programs and urinary metabolomics/metabonomics research. Current methods for creatinine determination either are nonselective or involve the use of expensive mass spectrometers. In this paper, a novel reversed-phase high-performance liquid chromatographic (HPLC) method for the determination of creatinine of high hydrophilicity by pre-column derivatization with ethyl chloroformate is presented. N-Ethyloxycarbonylation of creatinine significantly enhanced the hydrophobicity of creatinine, facilitating its chromatographic retention as well as quantification by HPLC. Factors governing the derivatization reaction were studied and optimized. The developed method was validated and applied for the determination of creatinine in rat urine samples. Comparative studies with isotope-dilution mass spectrometric method revealed that the two methods do not yield systematic differences in creatinine concentrations, indicating the HPLC method is suitable for the determination of creatinine in urine samples.

Comparison of two different multidimensional liquid–gas chromatography interfaces for determination of mineral oil saturated hydrocarbons in foodstuffs

This investigation focused on direct comparison of two popular multidimensional liquid–gas chromatography (LC–GC) systems, the Y-interface (retention gap approach) and the syringe-based interface (programmed temperature vaporizer approach). Such transfer devices are structurally very different, and could potentially have a substantial effect on the outcome of a specific application. In this work the application was a topic of much current interest, determination of mineral oil saturated hydrocarbon (MOSH) contamination of a series of food products (rice, pasta, icing sugar, olive oil); the final results were then compared. The two LC–GC methods developed were validated for linearity over the calibration range, analyte discrimination, precision, accuracy, and limits of detection and quantification. No significant differences were found between the two approaches.

Recent advances in on-line concentration and separation of amino acids using capillary electrophoresis

This article highlights recent methodological developments in the on-line concentration and separation of amino acids and their enantiomers using capillary electrophoresis. Sections are dedicated to recent contributions to on-line concentration strategies such as field-amplified sample stacking, large-volume sample stacking, dynamic pH junction, transient isotachophoresis, sweeping, and the combination of two methods. The main applications, advantages, and limitations of these procedures in the biological, food, and pharmaceutical fields are addressed. Comprehensive tables listing on-line techniques for the concentration and separation of amino acids and their enantiomers, categorized by the stacking strategies used, background electrolytes, sample matrix, limit of detection, and enhancement factor, are provided.

Asymmetrical flow field-flow fractionation as a novel technique for the analysis of PS-b-PI copolymers

Asymmetrical flow field-flow fractionation (AF4) was used as a fractionation technique to investigate the molecular heterogeneity of poly(styrene-b-isoprene) diblock copolymers synthesized by either sequential living anionic polymerization or coupling of living precursor blocks. AF4 coupled to multi-angle laser light scattering (MALLS), refractive index (RI), and ultraviolet (UV) detectors was used to separate the diblock copolymers from the homopolymers and coupling products, and the molar masses of the different components were analyzed. In order to get more information about the separated block copolymers, homopolymers, and coupling products, fractions were collected directly after the AF4 channel. The collected fractions were analyzed offline by ¹H NMR to provide identification of the different species and additional information on the true chemical composition, and the microstructure of the diblock copolymer was obtained.

Spectral Reproducibility and Quantification of Peptides in MALDI of Samples Prepared by Micro-Spotting

Previously, we reported that MALDI spectra of peptides became reproducible when temperature was kept constant. Linear calibration curves derived from such spectral data could be used for quantification. Homogeneity of samples was one of the requirements. Among the three popular matrices used in peptide MALDI [i.e., α-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid (DHB), and sinapinic acid (SA)], homogeneous samples could be prepared by conventional means only for CHCA. In this work, we showed that sample preparation by micro-spotting improved the homogeneity for all three cases.

Electrochemical behavior of methyl parathion and its sensitive determination at a glassy carbon electrode modified with ordered mesoporous carbon

Ordered mesoporous carbon (OMC) was synthesized and used to modify the surface of a glassy carbon (GC) electrode. Due to the unique properties of OMC, a decrease in the overvoltage of the reduction potential of methyl parathion (MP) (to ca. 219 mV) and a 76-fold increase in the peak current are observed (compared with a bare GC electrode). The absorption capacity of the surface of the electrode for MP was determined by chronocoulometry. The results show that the Г value of the modified electrode (2.34?×?10^–9 mol cm^–2) is 9.5 times as large as that of the GC electrode (2.47?×?10^–10 mol cm^–2). The new electrode exhibits synergistic electrocatalytic and accumulative effects on MP. MP can be determined by linear sweep voltammetry (LSV) which displays a linear relationship between peak current and MP concentration in the range from 0.09 to 61 μM, with a detection limit as low as 7.6 nM (at an S/N of 3) and after an accumulation at 0 V for 5 min. The electrode was successfully applied to the determination of MP in spiked lake water samples.