Transfer Capillary Research Articles

Ion transmission in an electrospray ionization-mass spectrometry interface using an S-lens.

We present the design and performance of an in-house built electrospray ionization-mass spectrometry (ESI-MS) interface equipped with an S-lens ion guide. The ion source was designed specifically for our ion beam experiments to investigate the chemical reactivity and deposition of the clusters and nanoparticles. It includes standard ESI-MS interface components, such as nanoelectrospray, ion transfer capillary, and the S-lens. A custom design enables systematic optimization of all relevant factors influencing ion formation and transfer through the interface. By varying the ESI voltage and flow rate, we determined the optimal operating conditions for selected silica emitters. A comparison of the pulled silica emitters with different tip inner diameters reveals that the total ion current is highest for the largest tip, whereas a tip with the smallest diameter exhibited the highest transmission efficiency through the ESI-MS interface. Ion transmission through the transfer capillary is strongly limited by its length, but the loss of ions can be reduced by increasing the capillary voltage and temperature. The S-lens was characterized over a wide range of RF frequencies and amplitudes. Maximum ion current was detected at RF amplitudes greater than 50 V peak-to-peak (p/p) and frequencies above 750 kHz, with a stable ion transmission region of about 20%. A factor of 2.6 increase in total ion current is observed for 650 kHz as RF amplitudes reach 400 V p/p. Higher RF amplitudes also focus the ions into a narrow beam, which mitigates their losses when passing through the ion guide.

Offline analysis of secondary formation markers in ambient organic aerosols by liquid chromatography coupled with time-of-flight mass spectrometry

The present study provides a comprehensive assessment of the quantitative analysis by high-performance liquid chromatography coupled with dual orthogonal electrospray ionization time-of-flight mass spectrometry (HPLC-ESI-TOF-MS) of pinene markers, biomass-burning related phenols, and other relevant carboxylic acids in atmospheric aerosol samples. Significant insights into the quantitative determination are offered on the basis of systematic experiments targeting the optimization of chromatographic separation, ionization source, and mass spectrometer performance. After testing three analytical columns, the best separation of the compounds of interest was achieved on a Poroshell 120 ECC18 column (4.6 × 50 mm, 2.7 µm) thermostated at 35 °C, operating in gradient elution mode with 0.1% acetic acid in water and acetonitrile at a 0.8 mL min−1 flow rate. Optimal operational conditions for the ESI-TOF-MS instrument were identified as a 350 °C drying gas temperature, 13 L min−1 drying gas flow rate, 60 psig nebulizer pressure, 3000 V for the ion transfer capillary, 60 V for the skimmer, and 150 V for the fragmentor. Additionally, the matrix effect on the ESI efficiency and the spike recovery factors of the compounds were tested. Method quantification limits can go as low as in the 0.88–48.0 μg L − 1 (3.67–200 pg m − 3, at 120 m3 of sampled air) range. The developed method was shown to be reliable for the quantification of the targeted compounds in real atmospheric aerosol samples. The accuracy in the molecular mass determination of less than 5 ppm and the acquisition in the full scan mode were shown to bring additional insights into the organic constituents in atmospheric aerosols.

Photocatalytic reactive liquid microjunction surface sampling-mass spectrometry for rapid and selective in-situ analysis of alpha-unsubstituted amine metabolites or drugs in brain tissue

In in-situ mass spectrometry (MS), different on-tissue derivatization methods have been developed to enhance the signals of poorly ionizable primary amines. However, those chemical derivatization methods are laborious and time-consuming, and are usually limited to detection of high-abundance amino acids which suppress the reaction of low-abundance monoamine neurotransmitters and drugs. Herein, A rapid and selective photocatalytic derivatization technique for alpha-unsubstituted primary amine was developed with 5-hydroxyindole as derivatization reagent and TiO2 as photocatalyst, and was introduced into liquid microjunction surface sampling (LMJSS)-MS system as online derivatization. The results showed that the photocatalytic derivatization method largely enhanced the signals of primary amines by 5–300 fold, and were selective to alpha-unsubstituted primary amines. Thus, the suppression effects from high-abundance amino acids to the reaction of monoamine neurotransmitters and benzylamine drugs proved to be largely reduced in the new method (matrix effect>50%) comparing with those in chemical derivatization method (matrix effect<10%). In addition, the optimal pH of the derivatization reaction was measured to be 7, which indicates the mild and physiologically compatible reaction conditions. By in-situ synthesis of TiO2 monolith in the transfer capillary of the LMJSS-MS system, rapid on-line photocatalytic derivatization was achieved and completed in 5 s during the transfer of sampling extract from the flow-probe to the MS inlet. With the new photocatalytic reactive LMJSS-MS method, detection limits of three primary amines on glass slides were in the range of 0.031–0.17 ng/mm2 with acceptable linearity (r=0.9815–0.9998) and relatively high repeatability (relative standard deviations <22.1%). Finally, endogenous tyramine, serotonin, two dipeptides and one doped benzylamine drug were identified and in-situ analyzed in the mouse cerebrum by the new method with largely enhanced signals comparing with LMJSS-MS without online derivatization. The new method provides a more selective, rapid and automated way to analyze alpha-unsubstituted amine metabolites and drugs in-situ comparing with traditional methods.

Selective Capture-Based Single-Cell Mass Spectrometry for Enhancing Sphingolipid Profiling of Neurons with Differentiation of Cell Body from Synapse.

Currently, single-cell lipidomic mass spectrometry (MS) techniques are mostly limited to detection of high-abundance phosphatidylcholines (PCs). Herein, for enhancing the coverage to low-abundance sphingolipids in single-cell analysis, in-tube solid-phase microextraction (SPME) was combined with a single-probe MS system for selective enrichment of sphingolipids during singe-cell sampling. From the results, a lab-made single probe with a 30 μm tip size proved to be able to resolve the axon from the cell body of neuron HT22 in ambient conditions. TiO2 was immobilized onto the inner wall of the transfer capillary of the single-probe device for online selective capture of sphingolipids in ammonia-acetonitrile and rapid desorption in formic acid-methanol. The results showed that the breakthrough volume of the capillary with sample loading flow rate at 500 nL/min was >14 μL. Standard experiments showed that the signals of cerebroside (CB), ceramide (Cer), and sphingomyelin (SM) were largely enhanced after selective capture in the coated capillary, while PCs were totally removed. The reusability (>10 times) and stability of the lab-made TiO2-coated capillary was verified. By introducing the coated capillary into the single-probe MS system, the new system proved to have low detection limits of SM, Cer, and CB (0.007-0.027 μg/mm2) and acceptable linearity (r > 0.98) and repeatability (RSD < 30%). Lipid coverage of the new method to SMs and CBs proved to be largely improved (SM, 21 vs 2; CB, 10 vs 0) with the new method in comparison to conventional single-probe MS without selective capture by ambient analysis of a single spot of rat cerebellum. Finally, the new system was used to perform single-neuron analysis of sphingolipids in the control and lipopolysaccharide (LPS)-treated HT22 with differentiation of the cell body from the axonal synapse. Results showed that 5 sphingolipids had significantly higher concentrations in the synapse than in the cell body, while 3 oxidized sphingolipids had significantly higher levels in the cell body than in the synapse. After LPS treatment, most of the sphingolipids largely decreased and became more accumulated in the synapse, providing new information on LPS-induced neuroinflammation.

Peak broadening caused by using different micro-liquid chromatography detectors.

Advancements in column technology resulted in smaller particles and more efficient phases. In parallel, the use of columns with reduced dimensions is becoming more common. This means the effective column volume is also decreased, thereby making the systems more susceptible to effects of band broadening due to extra-column volume. Despite these trends and the fact that a growing number of miniaturized liquid chromatography systems are being offered commercially, manufacturers often stick to the modular concept with dedicated units for pumps, column oven, and detectors. This modular design results in long connection capillaries, which leads to extra-column band broadening and consequently prevents the exploitation of the intrinsic efficiency of state-of-the-art columns. In particular, band broadening post column has a considerable negative effect on efficiency. In this study, mass flow and concentration-dependent detectors were examined for their influence on band broadening using a micro-LC system. A mass spectrometric detector, an evaporative light scattering detector, two UV detectors, and a previously undescribed fluorescence detector were compared. The influence on efficiency is compared using plate height vs linear velocity data and peak variance. It is shown that an increase in the inner diameter after the post-column transfer capillary leads to significant loss in plate height. Comparing the UV detectors, it could be shown that the dispersion was reduced by 38% by the reduction of the post-column volume. The largest variance was found for the evaporative light scattering detector, which was 368% higher compared to the variance of the detector with the least effect on band broadening.

Dehydration of gas-phase benzyl amine alcohols studied at atmospheric pressure

Gas-phase ion chemistry is a useful approach for the investigation of physical organic chemistry and the study of reactivity, structure, and thermochemical properties of ionic and neutral organic substrates. Herein, one-step dehydration of gas-phase benzyl amine alcohols to give benzazetidines was discovered without the use of catalysts. Mechanistic investigations of the gas-phase dehydration reactions were explored to study the possible influence of charged microdroplet acceleration during electrospray ionization (ESI) and collisions occurring during ion transfer through the atmospheric pressure interface (API) of the mass spectrometer. The product ion distribution was observed to be less sensitive to droplet effects (spray distance and voltage), and temperature of transfer capillary in the API. However, the product ion distribution exhibit high sensitivity to different S-lens radio frequency (RF) voltage of the API, providing evidence that the dehydration reaction is driven by collisions during ion transfer in the API. With this insight, we developed atmospheric pressure thermal dissociation platform that allowed thermal-induced collisions outside of the mass spectrometer when the benzyl amine alcohols are carried through a heated coiled tube. Up to 98% dissociation efficiency was achieved with the coiled tube heated to a temperature of 200 °C, yielding only the desired benzazetidine product. By performing the atmospheric pressure thermal dissociation experiment outside of the mass spectrometer, we believe it will be straightforward to collect the dehydration product.

Amine additives for improved in-ESI H/D exchange.

In-ESI H/D exchange is a convenient technique for analyzing small-molecular complex mixtures. However, such experiments do not yield sufficient levels of exchange or require an elevated temperature of the ion transfer capillary. Increased temperature may result in unexpected additional exchanges of -CH groups that may complicate the interpretation of the H/D exchange data used for identification. Gas-phase H/D exchange depends on the gas-phase basicity of the deuterating agent. In-ESI exchange involves both droplet-phase and gas-phase mechanisms, depending on a particular ion source setup and the deuterating agent used. Therefore, the addition of strong bases to the reaction mixture should facilitate in-ESI exchange. This work aimed to investigate the capabilities of different amines to improve in-ESI H/D exchange compared with pure D2O and to choose an amine modifier to increase the extent of H/D exchange. It was shown that such additives substantially enhanced the extent of H/D exchange in small molecules, peptides, and proteins even without heating the capillary. It was found that the extent of exchange increases in the following order: tertiary amines < secondary amines < primary amines. Therefore, we suggest that amines act as deuterating agents after being exchanged with D2O. These findings may improve H/D exchange applications, especially in small molecule analysis. The observation of improved H/D exchange with amine additives in peptides and proteins may become a subject of future research.

What affects the performance of POD for the simulation of heat transfer through building component?

The capability of the proper orthogonal decomposition for the simulation of heat transfer in building components is investigated via three applications: linear heat transfer (not coupled to mass transfer), mildly non-linear heat transfer (coupled to air and moisture transfer (hygroscopic)) and highly non-linear heat transfer (coupled to moisture transfer (capillary)). It is shown that increasing non-linearity leads to an increasing number of required construction modes. To further investigate the reason for this degrading performance of POD, the singular values’ decay progress from the different training snapshots is addressed in this paper. The results confirm that a fast decay of the singular values implies a high interrelation of the snapshots and a better performance of the POD method.

Negative-ion field desorption revitalized by using liquid injection field desorption/ionization-mass spectrometry on recent instrumentation

Field ionization (FI), field desorption (FD), and liquid injection field desorption/ionization (LIFDI) provide soft positive ionization of gaseous (FI) or condensed phase analytes (FD and LIFDI). In contrast to the well-established positive-ion mode, negative-ion FI or FD have remained rare exceptions. LIFDI provides sample deposition under inert conditions, i.e., the exclusion of atmospheric oxygen and water. Thus, negative-ion LIFDI could potentially be applied to highly sensitive anionic compounds like catalytically active transition metal complexes. This work explores the potential of negative-ion mode using modern mass spectrometers in combination with an LIFDI source and presents first results of the application of negative-ion LIFDI-MS. Experiments were performed on two orthogonal-acceleration time-of-flight (oaTOF) instruments, a JEOL AccuTOF GCx and a Waters Micromass Q-TOF Premier equipped with LIFDI sources from Linden CMS. The examples presented include four ionic liquids (ILs), i.e., N-butyl-3-methylpyridinium dicyanamide, 1-butyl-3-methylimidazolium tricyanomethide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate), 3-(trifluoromethyl)-phenol, dichloromethane, iodine, polyethylene glycol diacid, perfluorononanoic acid, anionic surfactants, a tetraphosphazene silanol-silanolate, and two bis(catecholato)silanes. Volatile samples were delivered as vapors via the sample transfer capillary of the LIFDI probe or via a reservoir inlet. Condensed phase samples were applied to the emitter as dilute solutions via the sample transfer capillary. The compounds either yielded ions corresponding to their intact anions, A−, or the [M–H]− species formed upon deprotonation. This study describes the instrumental setups and the operational parameters for robust operation along with a discussion of the negative-ion LIFDI spectra of a variety of compounds.

Reagent and analyte ion hydrates in secondary electrospray ionization mass spectrometry (SESI-MS), their equilibrium distributions and dehydration in an ion transfer capillary: Modelling and experiments.

Secondary electrospray ionization (SESI) in a water spray environment at atmospheric pressure involves the reactions of hydrated hydronium reagent ions, H3 O+ (H2 O)n , with trace analyte compounds in air samples. Understanding the formation and dehydration of reagent and analyte ions is the foundation for meaningful quantification of trace compounds by SESI-mass spectrometry (MS). A numerical model based on gas-phase ion thermochemistry is developed that describes equilibria in H3 O+ (H2 O)n reagent cluster ion distributions and ligand switching reactions with polar NH3 molecules leading to equilibrated hydrated ammonium ions NH4 + (H2 O)m . The model predictions are compared with experimental results obtained using a cylindrical SESI source coupled to an ion-trap mass spectrometer via a heated ion transfer capillary. Non-polar isoprene, C5 H8 , was used to further probe the nature of the reagent ions. Equilibrium distributions of H3 O+ (H2 O)n ions and their reactions with NH3 molecules have been characterized by the model in the near-atmospheric pressure SESI source. NH3 analyte molecules displace H2 O ligands from the H3 O+ (H2 O)n ions at the collisional rate forming NH4 + (H2 O)m ions, which travel through the heated ion transfer capillary losing H2 O molecules. The data for variable NH3 concentrations match the model predictions and the C5 H8 test substantiates the notion of dehydration in the heated capillary. Large cluster ions formed in the SESI region are dehydrated to H3 O+ (H2 O)1,2,3 and NH4 + (H2 O)1,2 while passing through the heated capillary, and considerable diffusion losses also occur. This phenomenon is also predicted for other polar analyte molecules, A, that can undergo similar switching reactions, thus forming AH+ and AH+ (H2 O)m analyte ions.

Structural analysis of a compound despite the presence of an isobaric interference by using in-source Collision Induced Dissociation and tandem mass spectrometry.

The presence of an isobaric contaminant can drastically affect MS and MS/MS patterns leading to erroneous structural and quantitative analysis, which is a real challenge in mass spectrometry. Herein, we demonstrate that MS and MS/MS structural analysis of a compound can be successfully performed despite the presence of an isobaric interference with as low as few millidaltons mass difference by using pseudo-MS3 . To this end, in-source collisional excitation (in-source CID) and the Survival Yield (SY) technique (energy-resolved collision induced dissociation MS/MS) were performed on two different source geometries: a Z-spray and an orthogonal spray (with a transfer capillary) ionization sources on two different mass spectrometers. By using soft ionization conditions, the SY curve for the mixture is a linear combination of the SY curves from the pure compounds demonstrating the presence of two components in the mixture. In the case of harsher ionization conditions, the SY curve of the mixture perfectly overlaps the SY curve from the pure analyte. This observation demonstrates the isobaric interference has been completely removed by in-source CID fragmentation, independently of the source design, leaving then the analyte precursor ions only. Therefore, by measuring the MS spectrum in harsh ionization conditions and according to SY criterium, the compound of interest can be made free from isobaric interference paving the way for, for example, unequivocal HPLC-MS as well as HPLC-MS/MS structural and quantitative analysis despite the presence of a co-eluting isobaric interference.

Design and Evaluation of a Tethered, Open Port Sampling Interface for Liquid Extraction-Mass Spectrometry Chemical Analysis.

Presented is a tethered, liquid-extraction-sampling interface designed for the mass spectrometric surface sampling/analysis of 3D objects. The tethered, open port sampling interface (TOPSI) incorporates a vacuum line between the sampling probe and ionization source, which enables the ability for an extended, tethered sample transfer line. Herein, several designs of the hand-held TOPSI are presented and evaluated on the basis of the analytical metrics of analyte transport time, peak width, and analyte sensitivity. The best analytical metrics were obtained with capillary flow resistances arranged in a particular order and the vacuum region set at 6.2 kPa. This TOPSI design incorporated a transfer capillary 1 m in length, while retaining a fast analyte transport time (12 s), short signal peak width (5 s baseline-to-baseline), and high analyte signal at 90% of that obtained with a regular open port sampling interface (OPSI). The hand-held TOPSI was demonstrated for the characterization of extracted small molecules and metabolites from the surface of mint and rosemary leaves.

Avoiding H/D Scrambling with Minimal Ion Transmission Loss for HDX-MS/MS-ETD Analysis on a High-Resolution Q-TOF Mass Spectrometer.

Hydrogen/deuterium exchange monitored by mass spectrometry (HDX-MS) enables the study of protein dynamics by measuring the time-resolved deuterium incorporation into a protein incubated in D2O. Using electron-based fragmentation in the gas phase it is possible to measure deuterium uptake at single-residue resolution. However, a prerequisite for this approach is that the solution-phase labeling is conserved in the gas phase prior to precursor fragmentation. It is therefore essential to reduce or even avoid intramolecular hydrogen/deuterium migration, which causes randomization of the deuterium labels along the peptide (hydrogen scrambling). Here, we describe an optimization strategy for reducing scrambling to a negligible level while minimizing the impact on sensitivity on a high-resolution Q-TOF equipped with ETD and an electrospray ionization interface consisting of a glass transfer capillary followed by a dual ion funnel. In our strategy we narrowed down the optimization to two accelerating potentials, and we defined the optimization of these in a simple rule by accounting for their interdependency in relation to scrambling and transmission efficiency. Using this rule, we were able to reduce scrambling from 75% to below 5% on average using the highly scrambling-sensitive quadruply charged P1 peptide scrambling probe resulting in a minor 33% transmission loss. To demonstrate the applicability of this approach, we probe the dynamics of certain regions in cytochrome c.

Transfer conditions and transmission bias in capillaries of vacuum interfaces

A detailed study of the transport of ions in transfer capillaries of electrospray ion sources is presented. The laminar flow field for various capillary sizes and wall temperatures is calculated. It forms the base of ion transfer simulation of a large number of ions with space charge. This allows to study the thermodynamical conditions of the ions during transfer, which are found to vary strongly with the capillary dimensions. The dependence of mass flow and ion current on the size of the capillary is presented. Simple scaling relations are derived and tested. The method also allows to predict a transmission bias between different ion species depending on the difference in ion mobility and the composition of transferred ion clouds.

A nebulizer interface for liquid chromatography - Flame ionization detection: Development and validation

Within this study, a novel liquid chromatography (LC)/flame ionization detector (FID) interface was improved. In contrast to previously published interface concepts, the main nebulizer body and the transfer capillary was made of stainless steel. Previously reported problems such as blocking of the transfer capillary were investigated. The simple design of the here presented nebulizer interface allows a convenient handling and the exchangeability of all nebulizer parts targets fast maintenance during routine analysis.A significant advantage is the capability to implement the novel interface into most common gas chromatography (GC)/FID systems. The effects of the instrumental parameters such as backpressure, gas flow, distance between nebulizer nozzle and FID collector or FID temperature on the signal were analyzed and optimized. The influence of the nebulizer material on flame stability and capillary blockage, a well-known problem of former coupling systems, was investigated, too. Finally, the novel interface was validated for the analysis of selected compounds known from literature. Obtained results for chromatographic separation of the alcohols propanol, butanol, pentanol and hexanol, used within previous studies in the field of LC/FID coupling are shown.Limits of detection (LODs), sensitivity and linearity found within this work are compared with LC/FID interfaces developed in the past.

NanoBlow: A Simple Device To Limit Contaminants during NanoLC-MS.

Limiting contamination of liquid chromatography/tandem mass spectrometry (LC-MS) systems and reducing the downtime associated with maintenance and cleaning is essential for productivity. We developed a simple device that creates a gas curtain barrier to prevent ions from entering the MS inlet. The gas can be quickly and easily applied when certain contaminant ions are known to elute. We show that the device can prevent the buildup of contaminants on the heated transfer capillary following >100 injections of a crude tissue lysate and improves peptide identifications. The device may provide a promising approach toward improving instrument robustness.

Analysis of steroids in urine by gas chromatography-capillary photoionization-tandem mass spectrometry

A new heated capillary photoionization (CPI) ion source design was developed to photoionize analytes inside a transfer capillary between a gas chromatograph (GC) and a mass spectrometer (MS). The CPI setup included a wide, oval-shaped vacuum-ultraviolet (VUV) transparent magnesium fluoride (MgF2) window to maximize photoionization efficiency and thus sensitivity. The source contained a nitrogen housing around the ionization chamber inlet to avoid undesirable hydrolysis and oxidation reactions with ambient air and to maximize the proportion of formed molecular radical cations of analytes. The feasibility of the ion source was studied by analyzing 18 endogenous steroids in urine as their trimethylsilyl (TMS) derivatives with gas chromatography-tandem mass spectrometry (GC–MS/MS). The method was validated and applied to human urine samples. To our best knowledge, this is the first time that a capillary photoionization ion source has been applied for quantitative analysis of biological samples. The GC-CPI-MS/MS method showed good chromatographic resolution (peak half-widths between 3.1 to 5.3 s), acceptable linearity (coefficient of determination between 0.981 to 0.996), and repeatability (relative standard deviation (RSD%) between 5 to 18%). Limits of detection (LOD) were between 2 to 100 pg mL−1 and limits of quantitation (LOQ) were between 0.05 to 2 ng mL−1. In total, 15 steroids were quantified either as a free steroid or glucuronide conjugate from the urine of volunteers. The new CPI source design showed excellent sensitivity for analysis of steroids in complex biological samples.

Capillary photoionization: interface for low flow rate liquid chromatography-mass spectrometry.

This is the first report on capillary photoionization (CPI) interfacing a liquid chromatograph (LC) and mass spectrometer (MS). A new heated CPI ion source was developed, including a heated transfer capillary, a wide oval-shaped and low-depth ionization chamber with a vacuum ultraviolet (VUV) transparent magnesium fluoride (MgF2) window to increase the photoionization efficiency and thus the sensitivity. As both analytes and eluent are first vaporized and then photoionized inside the CPI ion source between the atmosphere and the vacuum of MS, the ion transfer efficiency into the MS and thus the sensitivity is improved. The effect of the most important operation parameters, the eluent flow rate and temperature of the CPI source, on the signal intensity was studied with selected steroids. The feasibility of LC-CPI-MS/MS for the quantitative analysis of steroids was also studied in terms of linearity, repeatability, and limits of detection. The method showed good quantitative performance and sensitivity down to the low femto-mole level.

Splitless hyphenation of SFC with MS by APCI, APPI, and ESI exemplified by steroids as model compounds

A systematic evaluation of splitless hyphenation of supercritical fluid chromatography (SFC) with mass spectrometry (MS) was performed using different techniques for ambient pressure ionization. Interfaces commonly known from HPLC-MS/MS, i.e. electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo ionization (APPI), were tested for their suitability in SFC-MS/MS. A triple quadrupole MS was used for data evaluation in a targeted multi-analyte design using endogenous steroids as model compounds. Individual optimization of the ionization parameters was performed in multi-dimensional design for best support of ionization in all three techniques. A post-column make-up was used to avoid analyte precipitation in the transfer capillary but also to support ionization independently from mobile phase composition. Buffer choice and concentration as well as temperature were found crucial in ESI and APCI. Best results for the multi-analyte method were obtained in both techniques using ammonium fluoride as make-up buffer. Instead of buffer solutions different organic solvents were used as dopants in APPI to support ionization. The mobile phase constituent isopropanol was already found to support ionization in APPI, however, for many analytes the addition of toluene resulted in superior results in terms of intensity. Comparing the optimized methods in terms of limit of detection (LOD), limit of quantification (LOQ), and sensitivity (slope of calibration curve) ESI was the best choice for the multiple analyte design. Only a few analytes resulted in a different optimum ionization, if focused on separately. In terms of linear dynamic range, APCI and APPI proved superior to ESI, where calibration over the whole range of concentrations (from LOD up to 5000 pg ∗ μL−1) required quadratic regression.

Lab-In-Syringe automation of stirring-assisted room-temperature headspace extraction coupled online to gas chromatography with flame ionization detection for determination of benzene, toluene, ethylbenzene, and xylenes in surface waters

Online coupling of Lab-In-Syringe automated headspace extraction to gas chromatography has been studied. The developed methodology was successfully applied to surface water analysis using benzene, toluene, ethylbenzene, and xylenes as model analytes. The extraction system consisted of an automatic syringe pump with a 5 mL syringe into which all solutions and air for headspace formation were aspirated. The syringe piston featured a longitudinal channel, which allowed connecting the syringe void directly to a gas chromatograph with flame ionization detector via a transfer capillary. Gas injection was achieved via opening a computer-controlled pinch valve and compressing the headspace, upon which separation was initialized. Extractions were performed at room temperature; yet sensitivity comparable to previous work was obtained by high headspace to sample ratio VHS/VSample of 1.6:1 and injection of about 77% of the headspace. Assistance by in-syringe magnetic stirring yielded an about threefold increase in extraction efficiency. Interferences were compensated by using chlorobenzene as an internal standard. Syringe cleaning and extraction lasting over 10 min was carried out in parallel to the chromatographic run enabling a time of analysis of <19 min. Excellent peak area repeatabilities with RSD of <4% when omitting and <2% RSD when using internal standard corrections on 100 μg L−1 level were achieved. An average recovery of 97.7% and limit of detection of 1–2 μg L−1 were obtained in analyses of surface water.