Desorption Of Analytes Research Articles

Kinetics and the on-site application of standards in a solid-phase microextraction fiber.

The kinetics of the desorption of analytes from a SPME fiber into an agitated sample matrix was studied, and a theoretical model was proposed to describe the dynamic desorption process, based on the steady-state diffusion of analytes in the extraction phase and in the boundary layer. It was found that the desorption of analytes from a SPME fiber into an agitated sampling matrix is isotropic to the absorption of the analytes onto the SPME fiber from the sample matrix under the same agitation conditions, and this allows for the calibration of absorption using desorption. The calibration was accomplished by exposing a SPME fiber, preloaded with a standard, to an agitated sample matrix, during which desorption of the standard and absorption of analytes occurred simultaneously. When the standard was the isotopically labeled analogue of the target analyte, the information from the desorption process, i.e., time constant a, could be directly used for estimating the concentration of the target analyte. When the standard varied from the target analyte, the mass-transfer coefficient of the analyte could be extrapolated from that of the standard. These predictions agree well with experimental results. This approach facilitates the full integration of sampling, sample preparation, and sample introduction, especially for on-site or in vivo investigations, where the addition of standards to the sample matrix, or control of the velocity of the sample matrix, is very difficult.

Determination of polycyclic aromatic hydrocarbons in water by solid-phase microextraction–gas chromatography–mass spectrometry

A solid-phase microextraction (SPME)–gas chromatography (GC)–mass spectrometry (MS) analytical method for the simultaneous separation and determination of 16 polycyclic aromatic hydrocarbons (PAHs) from aqueous samples has been developed, based on the sorption of target analytes on a selectively sorptive fibre and subsequent desorption of analytes directly into GC–MS. The influence of various parameters on PAH extraction efficiency by SPME was thoroughly studied. Results show that the fibre exposure time and the use of agitation during exposure are critical in enhancing SPME performance. The presence of colloidal organic matter (as simulated by humic acid) in water samples is shown to significantly reduce the extraction efficiency, suggesting that SPME primarily extracts the truly dissolved compounds. This offers the significant advantage of allowing the differentiation between freely available dissolved compounds and those associated with humic material and potentially biologically unavailable. The method showed good linearity up to 10 μg/l. The reproducibility of the measurements expressed as relative standard deviation (R.S.D.) was generally <20%. The method developed was then applied to extract PAHs from sediment porewater samples collected from the Mersey Estuary, UK. Total PAH concentrations in porewater were found to vary between 95 and 742 ng/l with two to four ring PAHs predominating. Results suggest that SPME has the potential to accurately determine the dissolved concentrations of PAHs in sediment porewater.

Study of multiple solid-phase microextraction combined off-line with high performance liquid chromatography: Application in the analysis of pharmaceuticals in urine

Solid-phase microextraction (SPME) was applied to the extraction of five common pharmaceuticals: quinine, naproxen, haloperidol, ciprofloxacin and paclitaxel. SPME combined off-line with HPLC analysis was studied independently for each analyte. Polydimethylsiloxane and polyacrylate fibres were used for the direct immersion SPME from isotonic analyte solutions and spiked human urine samples. Analyte desorption occurred in HPLC autosampler inserts containing 200 μL of methanol; next 80 μL of the resulting solution were injected on the HPLC. Extraction optimisation involved parameters such as addition of salts, pH value and saturation time. The pharmaceuticals were assayed independently by multiple extraction from isotonic aqueous solutions and urine samples. A protocol of multiple SPME provided higher yields for the four of the five examined analytes. The results showed the potential of using multiple SPME (but also single SPME) combined off-line to HPLC in bio-pharmaceutical analysis.

Temperature dependence of dissociative electron attachment to molecules of gentisic acid, hydroquinone and p-benzoquinone

The temperature dependence of dissociative electron attachment to molecules of gentisic acid (2,5-dihydroxybenzoic acid) as well as to the products of its decomposition, namely molecules of hydroquinone and p-benzoquinone, has been investigated by means of electron capture negative ion mass spectrometry (ECNI-MS). We have obtained the negative ion yield as a function of incident electron energy for decay channels corresponding to m/ z 108 which are [M−CO 2−H 2] − anion in the spectrum of gentisic acid, [M−H 2] − anion in the spectrum of hydroquinone and [M] − parent long-lived anion in the spectrum of p-benzoquinone. It was shown that molecules of carbon dioxide may be formed at high temperature not only by decarboxylation of gentisic acid but also by capture of low-energy electrons (about 1 eV). Such a fact has a great importance for understanding of the problem of analyte desorption in matrix assisted laser desorption/ionization (MALDI) mass spectrometry in according to CO 2-MALDI model where the desorption of large biomolecules is explained in terms of gas-bubbles formation by thermal decomposition of molecules of MALDI matrix.

Phosphine emission measurements from a tobacco factory using cryogenic sampling and GC-ICP-MS analysis

A cryosampling system together with a low temperature GC-ICP-MS method have been developed for the determination of phosphine for industrial hygiene purposes. The effect of sampling temperature on the collection efficiency of PH3 was investigated. Two trapping mechanisms were differentiated for temperature ranges from −190 to −160 °C and from −110 to −35 °C. A cartridge filled with NaOH was used during the analytical desorption step to separate CO2 from phosphine. Phosphine recovery was found to be better than 98% with peak area and retention times RSD values better than 4% and 2%, respectively. The use of a Nafion® drying membrane before cryogenic trapping induced losses of PH3 of about 15%. Sample conservation in a cryocontainer at −190 °C over a period of 15 days did not lead to significant losses of PH3. For field experiments, air samples were collected in the fumigation room of a tobacco factory after fumigation, and outside the fumigation room during fumigation, using both cryogenic sampling and standard filters impregnated with silver nitrate. The results showed that phosphine concentrations in the tobacco factory were below the limit values for occupational exposure to phosphine (VME (France) and TLV-TWA (USA)) and also below the detection limit of the silver impregnated filter-ICP-AES method. Cryogenic trapping in combination with GC-ICP-MS allowed us to determine phosphine concentrations. The concentrations in ambient air are estimated to be about 1 ng m−3 and the detected concentrations in the vicinity of the fumigation room during the fumigation process are lower than 10 ng m−3.

Helical sorbent microtrap for continuous sampling by a membrane and trap interface for on-line gas chromatographic monitoring of volatile organic compounds.

A helical sorbent microtrap consisting of a helical sorbent fixed inside a silicosteel capillary tube is presented. The main parameters that affect the safe sampling time of the helical sorbent microtrap in continuous sampling by a membrane and trap interface for on-line gas chromatographic monitoring of organic volatiles in gaseous samples are examined, taking into account the helical configuration of the sorbent, the presence of the membrane in system, and the properties of the analytes. Thermal desorption of analytes from the helical sorbent trap was also examined having regard to the influence of the turbulent flow generated by the helical sorbent in the heat transfer and the effect of thermal backward flow on the peak shape. The practical application of the helical sorbent microtrap in a membrane and trap interface was demonstrated by on-line gas chromatographic monitoring of four volatile organic compounds in the fume hood air and of volatile organic compounds from a diesel engine exhaust. The limit of detection was in the picogram per milliliter range, depending on the time of trapping and the parameters that affect the permeation through the membrane.

A Gas Sample Pre‐concentration Device Based on Solid Phase Microextraction (SPME) and Temperature Programmed Desorption (TPD)

A sample pre‐concentration device, based on solid phase microextraction (SPME) fiber and temperature programmed desorption (TPD), was developed and tested in the analysis of volatile organic compounds. The pre‐concentrator device consists of a low dead volume quartz tube that houses the SPME fiber during the extraction/ desorption steps, a Ni:Cr heating element, and a programmable power supply. Compounds are extracted from air into the SPME fiber according to their affinity and solubility to the fiber material. Following this extraction step, samples are desorbed by a pre‐programmed temperature ramp for their subsequent detection. The SPME fiber was exposed and sample extracted for a pre‐determined amount of time (1 min, in this work). Because of the limited specificity of the stationary phase in SPME fibers towards the analyte, background interfering compounds are unavoidably co‐extracted during this process. A TPD step was then applied to sequentially desorb and detect analytes and interferences. The temperature ramp was designed so as to maximize temporal resolution between target analytes and background chemical interferences. The SPME/TPD pre‐concentrator was tested with a binary chemical system consisting of dimethyl‐methylphosphate (DMMP, a chemical warfare simulant) and toluene (a common commercial solvent). Analyte desorption was monitored on‐line with a quadrupole ion trap mass spectrometer. Five SPME fibers with different stationary phases were tested: polyacrylate (PA), polydimethylsiloxane (PDMS), carboxen–polydimethylsiloxane (CAR/PDMS), polydimethylsiloxane–divinylbenzene (PDMS/DVB), and carbowax/divinylbenzene (CW/DVB). Best results were obtained with the PDMS (100 µm) fiber. Two other SPME fibers, CW/DVB and CAR/PDMS, along with the TPD step, were also successful in the temporal resolution the of DMMP and toluene. The approach is rapid in that the extraction and desorption steps take less than 3 min. Implications and applications of the SPME/TPD system as a sample pre‐concentration step in handheld devices are discussed. The simplicity of the device makes it amenable to miniaturization by available microfabrication techniques.

Thermal modulator array for analyte modulation and comprehensive two‐dimensional gas chromatography

AbstractSegmented thermal modulators were fabricated for the modulation of column effluent in one‐dimensional gas chromatography, comprehensive two‐dimensional gas chromatography, and as sample introduction devices in other analytical techniques. Thermal desorption of analytes from the modulator was achieved by computer‐controlled application of large DC currents at low voltages sequentially to consecutive segments or stages of the modulator heaters for periods between 100 and 400 ms per segment. This procedure produced sharply focussed analyte peaks. Unsegmented heaters and heaters having 2, 4, 6, 8, and 10 segments or stages were compared with respect to their focusing efficiency. The 10‐stage heater was found to give sharper focusing than heaters with a smaller number of segments. Analyte pulses with peak widths at half‐height of ca. 150 ms were generated by the 10‐stage heater. The application of a 10‐stage modulator in a comprehensive two‐dimensional analysis of a light petroleum condensate gave results of acceptable quality.

GC-MS of Ignitable Liquids Using Solvent-Desorbed SPME for Automated Analysis

Solid-phase microextraction (SPME) is well documented with respect to its convenience and applicability to sampling volatiles. Nonetheless, fire debris analysts have yet to widely adopt SPME as a viable extraction technique, although several fire debris studies have demonstrated the utility of SPME coupled with gas chromatography-mass spectrometry (GC-MS) to identify ignitable liquids. This work considers the expansion of SPME sampling from the customary thermal desorption mode to solvent-based analyte desorption for the analysis of ignitable residues. SPME extraction fibers are desorbed in 30 microL of nonaqueous solvent to yield a solution amenable to conventional GC-MS analysis with standard autosampler apparatus. This approach retains the advantages of convenience and sampling time associated with thermal desorption while simultaneously improving the flexibility and throughput of the method. Based on sampling results for three ignitable liquids (gasoline, kerosene, anddiesel fuel) in direct comparisons with the widely used activated charcoal strip (ACS) method this methodology appears to be a viable alternative to the routinely used ACS method.

Thermodynamic and kinetic models for the extraction of essential oil from savory and polycyclic aromatic hydrocarbons from soil with hot (subcritical) water and supercritical CO 2

Mechanisms that control the extraction rates of essential oil from savory ( Satureja hortensis) and polycyclic aromatic hydrocarbons (PAHs) from historically-contaminated soil with hot water and supercritical carbon dioxide were studied. The extraction curves at different solvent flow-rates were used to determine whether the extractions were limited primarily by the near equilibrium partitioning of the analyte between the matrix and solvent (i.e. partitioning thermodynamics, or the “elution” step) or by the rate of analyte desorption from the matrix (i.e. kinetics, or the “initial desorption” step). Two simple models were applied to describe the extraction profiles obtained with hot water and with supercritical CO 2: (1) a model based solely on the thermodynamic distribution coefficient K D, which assumes that analyte desorption from the matrix is rapid compared to elution, and (2) a two-site kinetic model which assumes that the extraction rate is limited by the analyte desorption rate from the matrix, and is not limited by the thermodynamic ( K D) partitioning that occurs during elution. For hot water extraction, the thermodynamic elution of analytes from the matrix was the prevailing mechanism as evidenced by the fact that extraction rates increased proportionally with the hot water flow-rate. This was also confirmed by the fact that simple removal calculations based on a single K D (for each essential oil compound) gave good fits to experimental data for flow-rates from 0.25 to 4 ml/min. In contrast, supercritical CO 2 extraction showed only minimal dependence on flow-rate, and the simple K D model could only describe the initial 20–50% of the extraction. However, a simple two-site kinetic model gave a good fit for all CO 2 flow-rates tested. The results of these investigations demonstrated that very simple models can be used to determine and describe extractions which are limited primarily by partitioning thermodynamics, or primarily by desorption kinetics. Furthermore, these results show that the time required for the recovery of essential oil from savory with hot water can be minimized by increasing flow-rate, with little change in the total volume of water required. In contrast, raising the flow-rate of supercritical CO 2 has little effect on the mass of essential oils recovered per unit of time, indicating that optimal recovery of these compounds with supercritical CO 2 (amount recovered for the lowest amount of CO 2) requires longer extraction times rather than faster flow-rates.

Characterization of an insoluble poly(9,9-diphenyl-2,7-fluorene) by solvent-free sample preparation for MALDI-TOF mass spectrometry.

The application of solvent-free sample preparation for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) allowed the characterization of an insoluble fraction of poly(9,9-diphenylfluorene) that was previously hindered by the lack of suitable characterization methods. The MALDI mass spectrometric analysis gives valuable mechanistic information about the heterogeneous polymerization process of the insoluble high molecular weight fraction of the polymer. The fragmentation appearing even under moderate desorption and ionization conditions of this rigid backbone analyte is identified as a multiple loss of the bulky phenyl side groups and can be avoided by applying the new MALDI matrix 7,7,8,8-tetracyanoquinodimethane. A specialized fragmentation study by postsource decay MALDI-TOF MS reveals a molecular weight dependent change in fragmentation mechanism from an exclusive cleavage of side groups from long polymer chains to an additional cleavage of the polymer backbone of short polymer chains.

Automated headspace solid-phase dynamic extraction for the determination of amphetamines and synthetic designer drugs in hair samples

The technique of automated headspace solid-phase dynamic extraction (SPDE) coupled with gas chromatography–mass spectrometry was evaluated for the determination of amphetamines and synthetic designer drugs in hair samples. Headspace SPDE is a novel method for the solventless extraction of organic compounds in aqueous samples. In a so-called inside needle capillary absorption trap a hollow needle with an internal coating of polydimethylsiloxane is used as extraction and preconcentration medium. Sampling is performed on the solution headspace by passing the gas through the device actively by a syringe. Analytes present in the sample are sorbed onto the deposited stationary phase. The syringe needle is placed into the injection port of a GC and rapid heating of the metal needle induces the desorption of analytes. For the determination of amphetamine, methamphetamine, 3,4-methylendioxyamphetamine (MDA), 3,4-methylendioxymethamphetamine, 3,4-methylendioxyethylamphetamine (MDEA), 3,4-methylendioxyphenyl-2-butanamine and N-methyl-1-(3,4-methylendioxyphenyl)-2-butanamine in human hair samples, 10 mg of hair were hydrolysed with sodium hydroxide. After absorption of analytes for an on-coating derivatization procedure the SPDE needle was directly placed into the headspace of a second vial containing N-methyl-bis(trifluoroacetamide). A validation procedure revealed absolute analyte recoveries between 10.2 and 16.7%. Linearity was obtained from 0.1 to 20 ng/mg with coefficients of correlation between 0.992 and 0.999. Intra- and inter-day precision were determined at two different concentrations and resulted in ranges between 1.4 and 4.1% (intra-day) and 4.2–14.6% (inter-day). Limits of detection between 0.03 ng/mg (MDA) and 0.19 ng/mg (MDEA) were achieved. Results indicated that SPDE is a rapid and sensitive method for the analysis of biological samples. Compared to solid-phase microextraction we found a higher extraction rate coupled with a faster automated operation.

Determination of methyltin species in sediments using a pervaporation–gas chromatographic approach

AbstractA method for speciation of monomethyltin (MMT), dimethyltin (DMT) and trimethyltin (TMT) in sediments based on the coupling between a pervaporation module, a preconcentration system and a gas chromatograph coupled with a pulsed flame photometric detector is reported. All the species are derivatized using sodium tetraethylborate as an appropriate reagent in order to produce volatile compounds. The experimental approach consists of a high‐pressure injection valve. In one of the valve positions, the volatile tin species are driven to an appropriate minicolumn as a sorptive trap placed at the valve loop to preconcentrate the analytes. After a fixed time, the current valve position is changed for the injection mode and desorption of the preconcentrated analytes are obtained by heating in a furnace. Estimated detection limits (of tin) for MMT, DMT and TMT using 500 mg of sample are in all cases lower than 21 ng g−1. Excellent recoveries (between 90 and 98%) for tin species were obtained by this extremely simple and easily automated setup. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Solid-phase microextraction-high performance liquid chromatography and diode array detection for the determination of mycophenolic acid in cheese

SPME, using a carbowax/templated resin fiber, interfaced with HPLC–UV/DAD has been optimized for the determination of the mycotoxin mycophenolic acid (MPA) in cheese samples. All the parameters influencing the efficiency of the analyte extraction and desorption have been carefully explored. The procedure has been applied to the analysis of blue-cheese samples such as Gorgonzola and Danablu. Samples were subjected to a preliminary short sonication in bicarbonate buffer (0.2 M, pH 9.7); the subsequent SPME was capable of a selective extraction of MPA, characterized by high recovery yields and detection limits of 50 and 100 ppb for Danablu and Gorgonzola, respectively. The present method is faster and simpler than any other existing method for the extraction of MPA from cheese and does not involve the use of toxic organic solvents.

Formation of artefacts during air analysis of volatile amines by solid-phase micro extraction.

Solid-phase micro extraction (SPME) is a promising technique for fast and low cost trace analysis. However, some limitations of the technique were encountered when using a PDMS (polydimethylsiloxane)/Carboxen fibre for sampling a mixture of volatile aliphatic amines in air. On the GC chromatogram, two supplementary peaks were noticed in addition to the analyte peaks, thus limiting qualitative and quantitative analysis in this particular case. This paper presents the investigations to identify the artefacts and determine the origin of their formation. First, GC-MS identification, by both electron impact and chemical ionisation modes, demonstrated that the two artefacts were unsaturated amines assumed to be formed by a dehydrogenation reaction of the target amines. This reaction was found to occur during thermal desorption of analytes in the GC injection port and to be catalysed by temperature and by metals consisting of the inox (stainless-steel) needle of the SPME device. It was also demonstrated that artefact formation was not significant when using PDMS or PDMS/divinylbenzene fibres. This difference with PDMS/Carboxen fibre can be explained by the high desorption temperature required for this fibre. Moreover, the microporosity of Carboxen induces a longer desorption time which increases the contact between analytes and inox and thereby enhances artefact formation.

Membrane-Enclosed Sorptive Coating. An Integrative Passive Sampler for Monitoring Organic Contaminants in Water

An integrative sampler that consists of a bar coated with poly(dimethylsiloxane) (PDMS) enclosed in a dialysis membrane bag has been developed combining the advantages of the passive sampling approach with solventless preconcentration of organic solutes from aqueous matrixes and subsequent desorption of the sequestered analytes on-line with a capillary GC/MS system. The performance of the sampler was tested for integrative sampling of hydrophobic persistent organic pollutants including gamma-hexachlorocyclohexane, hexachlorobenzene, 2,2'-bis(4-chlorophenyl)-1,1'-dichloroethylene, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls in the laboratory in a continuous-flow system. Linear uptake of all test analytes during exposure periods up to one week has been observed, and concentration proportionality of response of the sampler has been demonstrated. Over the range of controlled laboratory conditions, the magnitude of sampling rate values varied from 47 to 700 microL h(-1) per sampler. The uptake rate of chemicals was dependent on their molecular mass, as well as on the partition coefficient between the PDMS and water. A decrease in sampling rates with decreasing water temperature was observed. The sampling device has the potential to detect low aqueous concentrations (ng to pg L(-1)) of test substances.

Column silylation method for determining endocrine disruptors from environmental water samples by solid phase micro-extraction

In solid phase micro-extraction (SPME), the analyte is partitioned between the coating and the sample and then desorption of the concentrated analyte is followed by GC-MS, where the analytes are thermally desorbed and subsequently separated on the column and quantified by the detector. The SPME method preserves all the advantages, such as simplicity, low cost, on site sampling and does not require solvents. Poly(acrylate) coating fibers have been developed for the extraction of phenols (such as 4- tert-butylphenol, 2,4-dichlorophenol, 4- n-pentylphenol, 4- n-hexylphenol, 4- tert-octylphenol, 4- n-heptylphenol, 4- n-nonylphenol, 4- n-octylphenol, pentachlorophenol and bisphenol A) in different water samples. The precision of the HS-SPME method ranges from 3–12% RSDs, depending on the compounds analyzed. More accurate results were obtained by HS-SPME with acidification and salting out, where the fiber is located above the liquid sample. The extraction period was 60 min, followed by desorption for 5 min at 300°C. After the analytes were completely desorbed, 1 μl of bis(trimethylsilyl)trifluoroacetamide (BSTFA) was injected by ordinary GC-MS injection. The trimethylsilylate peaks were improved significantly compared with free phenol peaks. The addition of salt (saturated sodium chloride) and acidification by hydrochloric acid (pH 2.0) were found to be very important for enhancing the partitioning of the polar phenols into the polymer coating and preventing ionization of the analytes. The method is capable of limits of detection of subparts per billion of the total phenols extracted from environmental water samples.

Speciation without chromatography : Part I. Determination of tributyltin in aqueous samples by chloride generation, headspace solid-phase microextraction and inductively coupled plasma time of flight mass spectrometry

An analytical procedure was developed for the determination of tributyltin in aqueous samples. The relatively high volatility of the organometal halide species confers suitability for their headspace sampling from the vapour phase above natural waters or leached solid samples. Tributyltin was collected from the sample headspace above various chloride-containing matrices, including HCl, sodium chloride solution and sea-water, by passive sampling using a polydimethylsiloxane/divinylbenzene (PDMS/DVB)-coated solid-phase microextraction (SPME) fiber. Inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) was used for detection following thermal desorption of analytes from the fiber. A detection limit of 5.8 pg ml–1(as tin) was realized in aqueous samples. Method validation was achieved using NRCC PACS-2 (Sediment) certified reference material, for which reasonable agreement between certified and measured values for tributyltin content was obtained.

Solid-phase micro-extraction in bioanalysis, exemplified by lidocaine determination

Solid-phase micro-extraction (SPME) is a never sample preparation technique that can be used for gaseous, liquid or solid samples in conjunction with GC, HPLC or CE (e.g. [1]). The use of SPME for the analysis of drugs in biofluids is also becoming popular (e.g. [2]). The principle is that a fused silica fibre coated with a polymer such as polydimethylsiloxane (PDMS) is put directly into the sample or placed in the headspace above it (e.g. [3]), analyte desorption is done thermally (for GC) or with liquid. As a model compound to study conditions we have investigated lidocaine in human urine using GC or LC [4] and, with attention to protein binding, in plasma mainly using GC [5].

An improved procedure for the preparation of the endothelin-B-receptor ligand sarafotoxin 6c.

Sarafotoxin 6c (S6c) is a specific agonist for the endothelin-B- (ETB) receptor, being largely used as a standard compound in pharmacological studies related to ETB-receptors. We have developed an improved peptide synthesis prncedure. Using the Fmoc chemistry methodology and an appropriate orthogonal protection scheme, we obtained a crude peptide with a major component identified as [Cys(Acm)(1,15)]S6c. A silver salt (AgBF4) treatment of this crude material completely removed the Acm moieties without the appearance of any major side-products. The free peptide was cyclized using a Fe3+/Fe2+ oxidizing method. Besides the formation of the bridges, no other modification was observed. Thus, the peptide purification was facilitated and an excellent yield of pure sarafotoxin was obtained. The material was characterized by analytical reverse phase-high performance liquid chromatography (RP-HPLC) and matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Pharmacological properties of the peptide were evaluated in an ETB preparation (guinea pig lung parenchyma strips) and were identical to those of an S6c standard.