Distribution by Scientific Domains
Distribution within Chemistry

Selected Abstracts

Characterization and Assessment of the Microjet Electrode as a Detector for HPLC

Susan Cannan
Abstract The microjet electrode (MJE) is characterized as a detector for high performance liquid chromatography (HPLC). Voltammetric measurements of the oxidation of hydroquinone (HQ) allow mass transport to be determined for the MJE detector configuration, and the factors controlling the conversion efficiency of the device to be well understood. The current-time response to the flow injection analysis of volumes of solution in the 10,80,,L range has been established, and the limit of detection of this method has been determined. The latter was found to approach that of UV absorbance measurements, which is particularly encouraging, given that HQ has a relatively strong chromophore (,=2,290.8,cm,1 mol,1,L). This detection system is a robust and simple arrangement with the capability of analyzing large volumes of eluent at typical analytical HPLC flow rates. [source]

Narrow-band fractionation of proteins from whole cell lysates using isoelectric membrane focusing and nonporous reversed-phase separations

Yi Zhu
Abstract Preparative isoelectric focusing (PIEF) is used to achieve narrow-band fractionation of proteins from whole cell lysates of Escherichia coli (E. coli). Isoelectric membranes create well-defined pH ranges that fractionate proteins by isoelectric point (pI) upon application of an electric potential. A commercial IsoPrime device (Amersham-Pharmacia BioTech) is modified for the PIEF separation to lessen run volumes significantly. Two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) analysis of chamber contents indicates that excellent pH fractionation is achieved with little overlap between chambers. PIEF pH fractions are further separated using nonporous reversed-phase high-performance liquid chromatography (NPS-RP-HPLC) and HPLC eluent is analyzed on-line by electrospray ionization-time of flight-mass spectrometry (ESI-TOF-MS) for intact protein molecular weight (MW) analysis. The result is a pI versus MW map of bacterial protein content. IEF fractionation down to 0.1 pH units combined with intact protein MW values result in a highly reproducible map that can be used for comparative analysis of different E. coli strains. [source]

Peptides of human gingival crevicular fluid determined by HPLC-ESI-MS

Elisabetta Pisano
The acidic-soluble protein content of human gingival crevicular fluid was analyzed by reverse-phase high-performance liquid chromatography (RP-HPLC), and the eluent deriving from the chromatography separation was directly introduced into an ion-trap mass spectrometer through electrospray ionization (ESI-IT MS). By this technique the molecular weight of peptides/proteins was determined with a precision of ,,1/10,000 amu. On the basis of the chromatographic behavior and the knowledge of the molecular mass value, some peptides and proteins soluble in acidic solution were unambiguously recognized. Besides high quantities of human serum albumin, , -defensins 1,4 and minor amounts of cystatin A, statherin, basic PB salivary peptide and other unidentified components were detected. The presence of , -defensins in gingival crevicular fluid is in agreement with their relevant contribution to protein composition deriving from granulocyte secretions. Other peptides and proteins abundant in human saliva, such as proline-rich proteins (PRPs) and histatins, were not detected in gingival crevicular fluid. Further investigations will be necessary to establish the origin of statherin and PB salivary peptide in gingival crevicular fluid. [source]

Alkylation of azoles: Synthesis of new heterocyclic-based AT1 -non-peptide angiotensin (II) receptor antagonists

Amal Al-Azmi
Several novel analogues of Losartan 2 were synthesized as potential non-peptide angiotensin (II) receptor antagonists. In these non-peptide analogues, the tetrazole and the substituted imidazole rings of Losartan 2 were replaced, respectively, by a carboxylic acid function or its methyl ester and substituted triazole, imidazole or benzimidazole moieties. The biphenyl bromide precursor 3 (BPE) used to introduce the linker between the acid/ester function and the heterocyclic moiety was synthesized using Suzuki biphenyl coupling and then incorporated into the target molecule by simple nucleophilic substitution. The fixed N-aryl isomeric forms of several azole and benzimidazole tautomers were successfully separated by HPLC using 50% aqueous acetonitrile as eluent. Intermediate reaction products and final target compounds were fully characterized spectroscopically. [source]

The synthesis of tritiated (R)-2-methoxy- N - n -propyl-nor-apomorphine (MNPA)

Jonas Malmquist
Abstract A method for the preparation of [3H]-MNPA ((R)-2-methoxy- N - n -propyl-norapomorphine) has been developed addressing the regioisomer problem as well as the oxidation problem. (R)-2, 10, 11-trihydroxy- N - n -propyl-norapomorphine was protected with 10, 11-dibenzyl or 10, 11-acetonide. The pure precursor was then methylated using [3H]-methyliodide. The product was isolated after deprotection and high-pressure liquid chromatography (HPLC) purification. Ascorbic acid was used as an antioxidant in the HPLC eluent and the stock solution. Characterization of the intermediates and products with 3H- and 1H-NMR was performed. A specific activity of 3.1,TBq/mmol (83.8,Ci/mmol) and 98.9% purity was obtained. Copyright 2007 John Wiley & Sons, Ltd. [source]

Synthesis of N -(3-[18F]Fluoropropyl)-2, -carbomethoxy-3, -(4-iodophenyl)nortropane ([18F]FP- , -CIT)

R. P. Klok
Abstract N -(3-[18F]fluoropropyl)-2, -carbomethoxy-3, -(4-iodophenyl)nortropane ([18F]FP- , -CIT) was synthesized in a two-step reaction sequence. In the first reaction, 1-bromo-3-(nitrobenzene-4-sulfonyloxy)-propane was fluorinated with no-carrier-added fluorine-18. The resulting product, 1-bromo-3-[18F]-fluoropropane, was distilled into a cooled reaction vessel containing 2, -carbomethoxy-3, -(4-iodophenyl)-nortropane, diisopropylethylamine and potassium iodide. After 30 min, the reaction mixture was subjected to a preparative HPLC purification. The product, [18F]FP- , -CIT, was isolated from the HPLC eluent with solid-phase extraction and formulated to yield an isotonic, pyrogen-free and sterile solution of [18F]FP- , -CIT. The overall decay-corrected radiochemical yield was 25 5%. Radiochemical purity was > 98% and the specific activity was 94 50 GBq/mol at the end of synthesis. Copyright 2006 John Wiley & Sons, Ltd. [source]

Quantitative analysis of EO9 (apaziquone) and its metabolite EO5a in human plasma by high-performance liquid chromatography under basic conditions coupled to electrospray tandem mass spectrometry

Liia D. Vainchtein
A sensitive and specific LC-MS/MS assay for the quantitative determination of EO9 and its metabolite EO5a is presented. A 200-l human plasma aliquot was spiked with a mixture of deuterated internal standards EO9- d3 and EO5a- d4 and extracted with 1.25 ml ethyl acetate. Dried extracts were reconstituted in 0.1 M ammonium acetate,methanol (7 : 3, v/v) and 25 l-volumes were injected into the HPLC system. Separation was achieved on a 150 2.1 mm C18 column using an alkaline eluent (1 mM ammonium hydroxide,methanol (gradient system)). Detection was performed by positive ion electrospray followed by tandem mass spectrometry. The assay quantifies a range from 5 to 2500 ng/ml for EO9 and from 10 to 2500 ng/ml EO5a using 200 l of human plasma samples. Validation results demonstrate that EO9 and EO5a concentrations can be accurately and precisely quantified in human plasma. This assay will be used to support clinical pharmacologic studies with EO9. Copyright 2006 John Wiley & Sons, Ltd. [source]

Effect of eluent on the ionization efficiency of flavonoids by ion spray, atmospheric pressure chemical ionization, and atmospheric pressure photoionization mass spectrometry

Jussi-Pekka Rauha
Abstract The effect of nine different eluent compositions on the ionization efficiency of five flavonoids was studied using ion spray (IS), atmospheric pressure chemical ionization (APCI), and the novel atmospheric pressure photoionization (APPI), in positive and negative ion modes. The eluent composition had a great effect on the ionization efficiency, and the optimal ionization conditions were achieved in positive ion IS and APCI using 0.4% formic acid (pH 2.3) as a buffer, and in negative ion IS and APCI using ammonium acetate buffer adjusted to pH 4.0. For APPI work, the eluent of choice appeared to be a mixture of organic solvent and 5 mM aqueous ammonium acetate. The limits of detection (LODs) were determined in scan mode for the analytes by liquid chromatography/mass spectrometry using IS, APCI and APPI interfaces. The results show that negative ion IS with an eluent system consisting of acidic ammonium acetate buffer provides the best conditions for detection of flavonoids in mass spectrometry mode, their LODs being between 0.8 and 13 M for an injection volume of 20 l. Copyright 2001 John Wiley & Sons, Ltd. [source]

Robust design of countercurrent adsorption separation processes: 5.

AICHE JOURNAL, Issue 7 2000
Nonconstant selectivity
Operating conditions for the separation of a binary mixture using a nonadsorbable eluent through simulated moving-bed technology were designed. The results obtained using equilibrium theory for adsorption described by Langmuir models lead to the definition of explicit constraints on the operating parameters to operate the unit in the desired regime of separation. A more general approach was able to produce the same result for a larger class of isotherms. The physical and mathematical conditions defining this larger class of isotherms are discussed, as well as the algorithms necessary to calculate the region of complete separation. Applications to the bi-Langmuir isotherm and the ideal adsorbed solution model, which are more flexible than the Langmuir model and can describe systems where selectivity changes with composition, are discussed. A shortcut method to get an explicit, though approximate, solution is proposed and its accuracy is discussed. [source]

Nano-level detection of naltrexone hydrochloride in its pharmaceutical preparation at Au microelectrode in flowing solutions by fast fourier transforms continuous cyclic voltammetry as a novel detector

P. Norouzi
Abstract An easy and fast Fourier transform continuous cyclic voltammetric technique for monitoring of ultra trace amounts of naltrexone in a flow-injection system has been introduced in this work. The potential waveform, consisting of the potential steps for cleaning, stripping and potential ramp, was continuously applied on an Au disk microelectrode (with a 12.5 m in radius). The proposed detection method has some of advantages, the greatest of which are as follows: first, it is no more necessary to remove oxygen from the analyte solution and second, this is a very fast and appropriate technique for determination of the drug compound in a wide variety of chromatographic analysis methods. The method was linear over the concentration range of 0.34,34000 pg/mL (r,=,0.9985) with a limit of detection 8.0,,10,4 nM. The method has the requisite accuracy, sensitivity, precision, and selectivity to assay naltrexone in tablets. The influences of pH of eluent, accumulation potential, sweep rate, and accumulation time on the determination of the naltrexone were considered. 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96:2009,2017, 2007 [source]

Determination of trace organophosphorus pesticides in water samples with TiO2 nanotubes cartridge prior to GC-flame photometric detection

Yunrui Huang
Abstract This article described a new method for the sensitive determination of organophosphorus pesticides in water samples using SPE in combination with GC-flame photometric detection. In the procedure of method development, TiO2 nanotubes were used as SPE adsorbents for the enrichment of organophosphorus pesticides from water samples. Several factors, such as eluent and its volume, sample pH, sample volume, sample flow rate, and concentration of humic acid, were optimized. Under the optimal conditions, the proposed method had good linear ranges as 0.1,40,,g/L for each of them, LOD of 0.11, 0.014, and 0.0025,,g/L, and LOQs of 0.37, 0.047, and 0.0083,,g/L for chlorpyrifos, phorate, and methyl parathion, respectively. The proposed method was validated with real environmental water samples and the spiked recoveries were over the range of 86.5,115.1%. All these results indicated that TiO2 nanotubes, as a new SPE adsorbent, would be used widespread for the preconcentraiton and determination of environmental pollutants in the future. [source]

Highly sensitive determination of tetrabromobisphenol A and bisphenol A in environmental water samples by solid-phase extraction and liquid chromatography-tandem mass spectrometry

Ru-Song Zhao
Abstract Using bamboo-activated charcoal as SPE adsorbent, a novel SPE method was developed for the sensitive determination of tetrabromobisphenol A and bisphenol A in environmental water samples by rapid-resolution LC-ESI-MS/MS. Important parameters influencing extraction efficiency, including type of eluent, eluent volume, sample pH, volume and flow rate, were investigated and optimized. Under the optimal extraction conditions (eluent: 8,mL methanol, pH: 7; flow rate: 4,mL/min; sample volume: 100,mL), low LODs (0.01,0.02,ng/mL), good repeatability (6.2,8.3%) and wide linearity range (0.10,10,ng/mL) were obtained. Satisfied results were achieved when the proposed method was applied to determine the two target compounds in real-world environmental water samples with spiked recoveries over the range of 80.5,119.8%. All these facts indicate that trace determination of tetrabromobisphenol A and bisphenol A in real-world environmental water samples can be realized by bamboo-activated charcoal SPE-rapid resolution-LC-ESI-MS/MS. [source]

Retention of arsenic species on zwitterionic stationary phase in hydrophilic interaction chromatography

Dan Xie
Abstract Zwitterionic hydrophilic interaction chromatography (ZIC -HILIC) was used to study the retention of selected organoarsenicals. The retention behavior of nine organic arsenic species on ZIC -HILIC was investigated to elucidate which is the driving force for their separation, hydrophilic partitioning or adsorption driven by hydrogen bonds with surface H-donor/acceptor groups of the stationary phase. For this, the retention factor of the compounds k was correlated with log PO/W and with the calculated strength of hydrogen bonding of the analytes. By examining aliphatic and phenylic compounds separately, improved correlation was received. This indicates that both phenomena contribute to the separation of these arsenic species on ZIC -HILIC. The results obtained evidence that considerable electrostatic interactions also occur on ZIC -HILIC. Retention behavior of arsenic species was investigated by varying the separation conditions, which shows that the composition of the eluent has a strong influence on the retention behavior. It is highly dependent on water/acetonitrile ratio, pH value and salt additives. Dissociation degree and polarity of arsenic species, which are varying with pH, regulate the distribution of arsenic species between stationary and mobile phases in HILIC. Increase in the ammonium acetate concentration leads to shortened or to prolonged retention depending on the structure of the arsenic species. [source]

A liquid chromatographic method optimization for the assessment of low and high molar mass carbonyl compounds in wines

Luciana C. de Azevedo
Abstract Carbonyl compounds (CC) play an important role in beverage aroma since they may affect flavor of wines, brandies, and beers, among others. For this reason, it is necessary to identify and quantify CC through adequate analytical techniques. This study is a proposal of both developing and optimization of a new analytical methodology that allows investigate C1,C8 CC in wines simultaneously by quantifying even those ones that are predominantly present in the adduct form hydroxylalkylsulfonic acids (HASA). The HASA dissociation is undertaken by specific alkaline media (pH 11). The developed methodology employed the LC with UV/VIS detection (, = 365 nm) technique under gradient elution in the way to reach both free-CC and bound-CC quantification. Results showed that binary gradient system using eluent A (MeOH/ACN/H2O 74.5:0.5:25% v/v/v) and eluent B (MeOH) reached the best separation condition of both lower and higher molecular mass CC. This proposed method allowed simultaneous quantification of formaldehyde, acetaldehyde, propanone, furfuraldehyde, butyraldehyde, benzaldehyde, hexanaldehyde, 2-ethyl-hexanaldehyde, E-pent-2-en-1-al, and cyclohexanone , all of them were found in white wine (Moscato Canelli) and red wine (Shiraz) produced in the So Francisco Valley, in the Northeastern Region of Brazil , although this optimized method may probably be suitable for quantification of propionaldehyde, isobutyraldehyde, heptanaldehyde, octanaldehyde, benzaldehyde, and E-hex-2-en-1-al as well. We could not prove if this method is also able to determine the latter CC group since we have not found these substances present in detectable levels in our real samples considered in this study. [source]

High-performance separation of small inorganic anions on a methacrylate-based polymer monolith grafted with [2(methacryloyloxy)ethyl] trimethylammonium chloride

Damian Connolly
Abstract A glycidyl methacrylate- co -ethylene dimethacrylate monolith in capillary format (100 ,m/id) has been grafted with chains of poly([2(methacryloyloxy)ethyl] trimethylammonium chloride (poly-META) and applied to the ion-chromatographic separation of selected inorganic anions. Grafting chains of META onto the generic monolithic scaffold resulted in a monolith with ,electrolyte responsive flow permeability', which manifested as increased permeability in the presence of electrolyte solutions. Using an eluent of 2 mM sodium benzoate and on-column contactless conductivity detection, a test mixture of six common anions was isocratically separated and detected within 12 min, with the first four anions baseline resolved within a retention time window of 3.2 min. Retention time precision was ,1.2% for all anions tested. Separation efficiencies of 15 000 N/m were achieved for fluoride at 1 ,L/min, with column efficiencies up to 29 500 N/m obtained at a lower flow rate of 100 nL/min. Furthermore, repeatability of the column modification procedure using photografting methods was acceptable, with retention times between replicate columns matching within 9%. [source]

Sensitive determination of phenols in environmental water samples with SPE packed with bamboo carbon prior to HPLC

Ru-Song Zhao
Abstract Bamboo carbon, an inexpensive, readily available material, has attracted great attention in recent years because of adsorptive properties. In this paper, the potential of bamboo carbon as a SPE adsorbent for the determination of phenols, was investigated. Phenols are important environmental contaminants that may adversely affect human health. Parameters influencing extraction efficiency, including type of eluent, eluent volume, amount of adsorbent, as well as sample pH, volume, and flow rate were investigated and optimized. The optimized results exhibited excellent linear relationships between peak area and phenol concentrations over the range of 2.0,100 ng/mL, with precision between 2.2,7.2%. The LODs were 0.06,0.4 ng/mL for the eight phenols tested. The proposed method has been successfully applied to the analysis of several real-world environmental water samples. These results indicate that bamboo carbon may be used as a novel SPE adsorbent for the concentration and determination of phenols in real environmental water samples. [source]

Confirmation and determination of carboxylic acids in root exudates using LC,ESI-MS

Zuliang Chen
Abstract Reversed-phase liquid chromatography with UV detection is of limited applicability in the separation and identification of carboxylic acids because of the column's poor separation efficiency and the non-selective nature of the UV detector. To address this issue, RP-LC with electrospray ionization mass spectrometry has been explored for the confirmation and determination of carboxylic acids in plant root exudates, with ESI-MS providing structural information, high selectivity, and high sensitivity. The separation of 10 carboxylic acids (pyruvic, lactic, malonic, maleic, fumaric, succinic, malic, tartaric, trans -aconitic, and citric acid) was performed on a C18 column using an eluent containing 0.1% (v/v) acetic acid within 10 min, where the acidic eluent not only suppressed the ionization of the carboxylic acids to be retained on the column, but was also compatible with ESI-MS detection. In addition, an additional standard was used to overcome the matrix effect. The results showed that peak areas correlated linearly with the concentration of carboxylic acids over the range 0.05,10 mg/L. The detection limits of target acids (signal-to-noise S/N ratio of 3) ranged from 20 to 30 ,g/L. Finally, the proposed method was used for the confirmation and determination of low-molecular-weight carboxylic acids in plant root exudates, and provided a simple analytical procedure, including sample processing, fast separation, and high specificity and sensitivity. [source]

Direct enantiomeric separations of tris(2-phenylpyridine) iridium (III) complexes on polysaccharide derivative-based chiral stationary phases

Xiaoming Chen
Abstract A convenient method is presented for the first time for the direct separation of enantiomers of a tris(2-phenylpyridine) iridium (III) and an analog substituted with long alkoxy chains on polysaccharide derivative-based chiral stationary phases by HPLC. Tris(2-phenylpyridine) iridium (III) was separated on the immobilized amylose 3,5-dimethylphenylcarbamate (Chiralpak IA) using hexane/CHCl3/CH2Cl2 (75:20:5) as an eluent, and the analog could be separated on the coated cellulose 3,5-dimethylphenylcarbamate (Chiralcel OD) and cellulose 4-methylbenzoate (Chiralcel OJ) using hexane/2-propanol (96:4) as the eluent. CD spectra of the eluted HPLC fractions were also recorded, and the observed mirror image patterns confirm their enantioseparations. [source]

A review of the background, operating parameters and applications of microemulsion liquid chromatography (MELC)

A. Marsh
Abstract Microemulsions are dispersions of nanometre-sized droplets of an immiscible liquid within another liquid. Droplet formation is facilitated by the addition of surfactants and often also cosurfactants. Microemulsions are classified as either oil-in-water (O/W) (oil droplets such as octane dispersed throughout aqueous buffer) or water-in-oil (W/O) (aqueous droplets in oil such as hexane). Both microemulsion types have been used as mobile phases for separation in microemulsion HPLC (MELC). There has been a recent increase of interest in this area with new applications and developments such as gradient elution and optimisation of methods using experimental design. O/W microemulsions have been employed as eluents for RP-HPLC while W/O microemulsions have been used for normal phase chromatography. Separations can have superior speed and efficiency to conventional HPLC modes while offering a unique selectivity with excellent resolution. The capability for quantitative and stability-indicating analysis has also been demonstrated. Specific advantages include the ability to operate at low UV wavelengths and elimination of the need for an equilibration rinse between gradients. Operational issues associated with the use of MELC have been identified including the need to add salt to the gradient eluent, relatively high back-pressures and increased need for equipment cleaning compared to conventional RP eluent. This report details the different microemulsion types and compositions used and their reported applications. The use of gradient and isocratic elution is described. The effects on separations of varying operating parameters such as temperature, oil type and concentration, surfactant type and concentration, sample solvent, column type, and organic solvent addition will be discussed and illustrated. [source]

Simultaneous determination of carotenoids, tocopherols, and ,-oryzanol in crude rice bran oil by liquid chromatography coupled to diode array and mass spectrometric detection employing silica C30 stationary phases

Wolfgang Stggl
Abstract Crude rice bran oil contains tocopherols (vitamin E), carotenoids (vitamin A), and phytosterols, which possess antioxidant activities and show promising effects as preventive and therapeutic agents. The aim of this work was to establish methods and to compare C18 and C30 silica stationary phases in order to separate and detect tocopherols, carotenoids, and ,-oryzanol in one single run. Comparing RP-LC on silica C18 and C30, higher resolution between all target compounds was obtained using the C30 stationary phase. Methanol was used as eluent and the elution strength was increased by the addition of tert -butyl methyl ether for highly hydrophobic analytes such as ,-oryzanol. Detection was accomplished by diode array detection from 200 to 500 nm. Absorbance maxima were found at 295 nm for tocopherols, 324 nm for ,-oryzanol, and 450 nm for carotenoids. Furthermore, compounds were characterized and identified on the basis of their UV-spectra. Both RP systems were coupled to MS (LC-MS) by using an atmospheric pressure chemical ionization interface. [source]

Separation and determination of small amounts of sulfur in technical thiophanate-methyl by reversed-phase high-performance liquid chromatography

R. Nageswara Rao
Abstract A simple and rapid reversed phase high-performance liquid chromatographic method for separation and determination of elemental sulfur in technical thiophanate-methyl using a reversed-phase C18 column and methanol-water-tetrahydrofuran (90 : 8 : 2 v/v/v) as eluent with UV detection at 254 nm has been developed and validated with respect to accuracy, precision, specificity, linearity range, and limits of detection and quantification. The method was found to be suitable not only for detection but also determination of elemental sulfur at levels of 510,9 g. [source]

Physicochemical parameters and magnetic speciation of Iron in Nakivubo Channel and Lake Victoria waters

Naziriwo Betty Bbosa
Abstract Magnetic speciation technique was used for the determination of iron species in Nakivubo Channel and Lake Victoria waters. The method consisted of a column of supported Dowex 1-X18, 20,50 US mesh (Cl) surrounded by movable permanent magnets. Dowex was supported by a porous material to permit adequate passage of the eluent through the column. In the presence of an external magnetic field, enhanced capacity for adsorption of iron III was observed. The enhanced capacity is primarily due to the magnetic field produced and ion exchange sorption mechanism. The results show that, most of the Iron in Nakivubo Channel waters is in reduced ferrous form while, in the Lake Victoria waters, it exists in the oxidized ferric form. Physicochemical parameters for the field samples are discussed in this study. Turbidity levels in catchments with substantial vegetation were significantly lower than those without. pH values up to 13 was observed for some of the point source. The physicochemical parameters along Nakivubo Channel waters were relatively higher than the Lake Victoria waters indicating slow increasing pollution load along the Nakivubo Channel. [source]

Liquid Chromatography of Synthetic Polymers under Limiting Conditions of Insolubility III

Application of Monolithic Columns
Abstract Summary Performance was evaluated of silica based commercial monolithic rod-like columns in liquid chromatography of synthetic polymers under limiting conditions of enthalpic interactions (LC LC). LC LC employs the barrier effect of the pore permeating and therefore slowly eluting small molecules toward the pore excluded, fast eluting macromolecules. Phase separation (precipitation) barrier action was applied in present study. The barrier was created either by the narrow pulse of an appropriate nonsolvent injected into the column just before the sample solution (LC LC of insolubility , LC LCI) or by the eluent itself. In the latter case, the polymer sample was dissolved and injected in a good solvent (LC LC of solubility , LC LCS). In LC LCI, polymer species cannot break thru the nonsolvent zone while in LC LCS they cannot enter eluent, which is their precipitant. Therefore, polymer species keep moving in the zone of their original solvent. Macromolecules eluting under the LC LC mechanism leave the column in the retention volume (VR) roughly corresponding to VR of the low molar mass substances and can be efficiently separated from the polymer species non-hindered by the barrier action. The known advantages of monoliths were confirmed. From the point of view of LC LCI and LC LCS the most important quality of monolithic columns represents their excellent permeability, which allows both working at high flow rates and injecting very high (in the range of 5%) sample concentrations. Monolithic column tolerate also extremely high molar mass samples (M>10,000 kg,,mol,1). On the other hand, the mesopores (separation pores) of the tested monoliths exhibited rather small volume and wide size distribution. These shortcomings partially impair the permeability advantage of monoliths because in order to obtain high LC LC separation selectivity a tandem of several monolithic columns must be applied. Presence of large mesopores also reduces applicability of monolithic columns for molar masses below about 50 kg,,mol,1 because VRs of polymers eluted behind the barrier are similar to that of freely eluting species. The non- negligible break-thru phenomenon was observed for the very high polymer molar masses largely eluting behind the barrier. It is assumed that the fraction of very large mesopores present in the monoliths or association/microphase separation of macromolecules may be responsible for this phenomenon. This is why the presently marketed SiO2 monolithic columns are mainly suitable for the fast purification of the LC LC eluting macromolecules from the polymeric admixtures non-hindered by the barrier-forming liquid. Still, monolithic columns have large potential in the LC LCI and LC LCS procedures provided size (effective diameter) of the mesopores can be reduced and their volume increased. [source]

HPLC,SPE,NMR hyphenation in natural products research: optimization of analysis of Croton membranaceus extract,

Maja Lambert
Abstract The HPLC,SPE,NMR technique was used for the analysis of a root-bark extract of Croton membranaceus. The components of the extract were separated on an analytical-size reversed-phase HPLC column, the chromatographic peaks were trapped on SPE (solid-phase extraction) cartridges after post-column dilution of the eluate with water and the compounds were eluted from the cartridges with acetonitrile- d3 into a 30 l 600 MHz NMR probe in a fully automated procedure. The trapping efficiency of scopoletin (1), the major extract constituent, was much higher on a GP (general phase, a polystyrene-type polymer) SPE phase than on a C18 phase. Thus, under the conditions used, up to 100 g of scopoletin per cartridge could be accumulated linearly after repeated trappings. The maximum achievable NMR signal-to-noise ratio using the GP cartridges was at least four times higher than that achievable with the C18 cartridges. It was shown that excessively long T1 relaxation times may compromise experiments in which acetonitrile- d3 is used as the cartridge eluent. Nevertheless, the sensitivity gain provided by the HPLC,SPE,NMR technique through repeated peak trappings allowed the acquisition of good-quality proton-detected 2D NMR spectra without the need for solvent suppression. Copyright 2005 John Wiley & Sons, Ltd. [source]

Identification of desulphoglucosinolates in Brassicaceae by LC/MS/MS: Comparison of ESI and atmospheric pressure chemical ionisation-MS

Nadine S. Zimmermann
Abstract In order to develop a sensitive method for the detection of desulphoglucosinolates by HPLC-MS, the two most common interfaces for HPLC-MS, atmospheric pressure chemical ionisation (APCI) and ESI, were compared. While working with the APCI-interface the evaporation temperature and corona amperage were optimised. In doing so 300C and 6 ,A proved to be most suitable for aliphatic and indole desulphoglucosinolates. The use of formic acid instead of water in the eluent in HPLC-ESI-MS measurements increased the sensitivity for the indole desulphoglucosinolates in the presence of 1 mM formic acid, while the sensitivity for the aliphatic desulphoglucosinolate desulphoglucoraphanin was substantially increased by the presence of 5 mM formic acid. Using an Agilent ion trap, two optimisation procedures for the MS parameters, smart and expert mode, were available. In smart mode the software optimises several parameters automatically, which is much more time efficient than expert mode, in which the optimisation is done manually. It turned out that ESI-MS is most sensitive in smart mode, while for APCI-MS a higher sensitivity could be gained using the expert mode. Comparing both interfaces, APCI-MS was more sensitive than ESI-MS. However, no additional information, in terms of structure determination, was obtained by APCI-MS. [source]

Limitations of mass spectrometric methods for the characterization of polydisperse materials,

Alan A. Herod
This paper is a review of work on the characterization of coal liquids and petroleum residues and asphaltenes over several decades in which various mass spectrometric methods have been investigated. The limitations of mass spectrometric methods require exploration in order to understand what the different analytical methods can reveal about environmental pollution by these kinds of samples and, perhaps more importantly, what they cannot reveal. The application of mass spectrometry to environmental problems generally requires the detection and determination of the concentration of specific pollutants released into the environment by accident or design. The release of crude petroleum or coal liquids into the environment can be detected and tracked during biodegradation processes through specific chemicals such as alkanes or polyaromatic hydrocarbons (PAHs). However, petroleum asphaltenes are polydisperse materials of unknown mass range and chemical structures and, therefore, there are no individual chemicals to detect. It is necessary to determine methods of detection and the ranges of mass of such materials. This can only be achieved through fractionation to reduce the polydispersity of the initial sample. Comparison of mass spectrometric results with results from an independent analytical method such as size-exclusion chromatography with a suitable eluent is advisable to confirm that all the sample has been detected and mass discrimination effects avoided. Copyright 2010 John Wiley & Sons, Ltd. [source]

Semi-online nanoflow liquid chromatography/matrix-assisted laser desorption ionization mass spectrometry of synthetic polymers using an octadecylsilyl-modified monolithic silica capillary column

Takehiro Watanabe
We have designed a semi-online liquid chromatography/matrix-assisted laser desorption/ionization mass spectrometry (LC/MALDI-MS) system to introduce eluent from a octadecylsilyl (ODS) group modified monolithic silica capillary chromatographic column directly onto a sample plate for MALDI-MS analysis. Our novel semi-online system is useful for rapidly and sensitively examining the performance of a monolithic capillary column. An additional advantage is the small elution volume of a monolithic capillary column, which allows delicate eluents, such as 1,1,1,3,3,3,-hexafluoroisopropyl alcohol (HFIP), to be used to achieve cost-effective analysis. Using the semi-online LC/MALDI-MS system, chromatographic separation of polymers by the monolithic column with different eluents was studied. Separation of poly(methyl methacrylate) and Nylon 6/6 showed that the column functioned via size-exclusion separation when tetrahydrofuran or HFIP eluent was used. On the other hand, the separation behavior of Nylon 11 indicated a reversed-phase mode owing to the interaction of the polymer with the modified ODS group in the column. Using tetrahydrofuran/methanol (1:1, v/v) as the eluent, the LC/MALDI-MS spectra of poly(lactic acid), which contains both linear and cyclic polymer structures, showed that the column could separate the hydrophobic cyclic polymer and elute it out relatively slowly. The monolithic column functions basically via size-exclusion separation; the reversed-phase separation by interaction with the ODS functions may have less influence on column separation. The semi-online monolithic capillary LC/MALDI-MS method we have developed should provide a means of effectively analyzing synthetic polymers. Copyright 2010 John Wiley & Sons, Ltd. [source]

Phospholipids in liquid chromatography/mass spectrometry bioanalysis: comparison of three tandem mass spectrometric techniques for monitoring plasma phospholipids, the effect of mobile phase composition on phospholipids elution and the association of phospholipids with matrix effects

Yuan-Qing Xia
Because plasma phospholipids may cause matrix effects in bioanalytical liquid chromatography/tandem mass spectrometry (LC/MS/MS) methods, it is important to establish optimal mass spectrometric techniques to monitor the fate of phospholipids during method development and application. We evaluated three MS/MS techniques to monitor phospholipids using positive and negative electrospray ionization (ESI). The first technique is based on using positive precursor ion scan of m/z 184, positive neutral loss scan of 141 Da and negative precursor ion scan of m/z 153. The second technique is based on using class-specific positive and negative selected reaction monitoring (SRM) transitions to monitor class-representative phospholipids. The third technique, previously reported, utilizes in-source collision-induced dissociation (CID)-based positive SRM of m/z 184,,,184. We recommend the all-inclusive technique 1 for use in qualitative assessment of all classes of phospholipids and technique 2 for use in quantitative assessment of class-representative phospholipids. Secondly, we evaluated the elution behaviors of the plasma phospholipids under different reversed-phase mobile phase conditions. The phospholipid-eluting strength of a mobile phase was mainly dependent on the type and amount (%) of the organic eluent and the strength increased in the order of methanol, acetonitrile and isopropyl alcohol. Under the commonly used gradient and isocratic elution schemes in LC/MS/MS bioanalysis, not all the phospholipids are eluted off the column. Thirdly, we investigated the association between phospholipids and matrix effects in positive and negative ESI using basic, acidic and neutral analytes. While the phospholipids caused matrix effects in both positive and negative ESI, the extent of ionization suppression was analyte-dependent and was inversely related to the retention factor and broadness of the phospholipids peaks. The lysophospholipids which normally elute earlier in reversed-phase chromatography are more likely to cause matrix effects compared to the later-eluting phospholipids in spite of the larger concentrations of the latter in plasma. Copyright 2009 John Wiley & Sons, Ltd. [source]

A versatile method for stable carbon isotope analysis of carbohydrates by high-performance liquid chromatography/isotope ratio mass spectrometry

H. T. S. Boschker
We have developed a method to analyze stable carbon isotope (13C/12C) ratios in a variety of carbohydrates using high-performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS). The chromatography is based on strong anion-exchange columns with low strength NaOH eluents. An eluent concentration of 1,mM resulted in low background signals and good separation of most of the typical plant neutral carbohydrates. We also show that more strongly bound carbohydrates such as acidic carbohydrates can be separated by inclusion of NO as an inorganic pusher ion in the eluent. Analyses of neutral carbohydrate concentrations and their stable carbon isotope ratios are shown for plant materials and marine sediment samples both at natural abundance and for 13C-enriched samples. The main advantage of HPLC/IRMS analysis over traditional gas chromatography based methods is that no derivatization is needed resulting in simple sample treatment and improved accuracy and reproducibility. Copyright 2008 John Wiley & Sons, Ltd. [source]

Segmented post-column analyte addition; a concept for continuous response control of liquid chromatography/mass spectrometry peaks affected by signal suppression/enhancement

Anton Kaufmann
A novel technique, "segmented post-column analyte addition", is proposed to visualize and compensate signal suppression/enhancement effects in electrospray ionization tandem mass spectrometry (ESI-MS/MS). Instead of delivering a constant flow of analyte solution between the liquid chromatography (LC) column exit and the ESI interface into the eluent resulting from LC separation of analyte-free matrix in order to determine retention time widows in which suppression/enhancement is unimportant (King et al., J. Am. Soc. Mass Spectrom. 2000; 11: 942), segmented packets of analyte-containing solvent and analyte-free solvent were infused into an LC eluent resulting from separation of an analyte-containing sample. The obtained, superimposed, periodic spikes are much narrower than the analyte peak eluting from the column. The height of the spikes is affected by signal suppression phenomena to the same extent as the analyte signal, and hence variations of the spike height can be used to correct the peak area of analyte peaks affected by signal suppression/enhancement. Copyright 2005 John Wiley & Sons, Ltd. [source]