Gas chromatography connected to multiple channel electrospray ionization mass spectrometry for the detection of volatile organic compounds

Thin layer chromatography/desorption flame-induced atmospheric pressure chemical ionization/mass spectrometry for the analysis of volatile and semi-volatile mixtures

Flame-induced atmospheric pressure chemical ionization (FAPCI) has been used to directly characterize chemical compounds on a glass rod and drug tablet surfaces. In this study, FAPCI was further applied to interface thin layer chromatography (TLC) and mass spectrometry (MS) for mixture analysis.

METHODS

A micro-sized oxyacetylene flame was generated using a small concentric tube system. Hot gas flow and primary reactive species from the micro-flame were directed toward a developed TLC gel plate to thermally desorb and ionize analytes on the gel surface. The resulting analyte ions subsequently entered the MS inlet for detection.

RESULTS

A 1-1.5-mm-wide light-brown line was observed on the TLC plate after the desorption FAPCI/MS (DFAPCI/MS) analysis, revealing that the gel surface withstood a high temperature from the impact of the micro-flame. Volatile and semi-volatile chemical compounds, including amine and amide standards, drugs, and aromatherapy oils, were successfully desorbed, ionized, and detected using this TLC/DFAPCI/MS. The limit of detection of TLC-DFAPCI/MS was determined to be 5 ng/spot for dibenzylamine and ethenzamide.

CONCLUSIONS

TLC/DFAPCI/MS is one of the simplest TLC-MS interfaces showing the advantages such as low costs and an easy set up. The technique is useful for characterizing thermally stable volatile and semi-volatile compounds in a mixture.

Probing surfaces of atmospherically relevant organic particles by easy ambient sonic-spray ionization mass spectrometry (EASI-MS)

EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Both ambient and laboratory-generated particles can have a surface composition different from the bulk, but there are currently few analytical techniques available to probe these differences. Easy ambient sonic-spray ionization mass spectrometry (EASI-MS) was applied to solid, laboratory-generated particles with core–shell morphologies formed from a variety of dicarboxylic acids. The soft ionization facilitated parent peak detection for the two compounds, from which the depth probed could be determined from the relative signal intensities. Two different configurations of a custom-made nebulizer are reported that yield different probe depths. In the “orthogonal mode,” with the nebulizer ∼10 centimeters away from the particle stream and at a 90° angle to the MS inlet, evaporation of the nebulizer droplets forms ions before interaction with the particles. The probe depth for orthogonal mode EASI-MS is shown to be 2–4 nm in these particle systems. In the “droplet mode”, the nebulizer and particle streams are in close proximity to each other and the MS inlet so that the particles interact with charged liquid droplets. This configuration resulted in full dissolution of the particles and gives particle composition similar to that from collection on filters and extraction of the particles (bulk). These studies establish that EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Sniffing with mass spectrometry

Gaseous compounds are usually on-line detectable on sensors. The limitations of conventional sensors are suffering from incapability for exactly identifying multiple components as well as incompatibility to possible toxicants in every odor sample. Herein, we discuss an inlet modification to the laboratory standard mass spectrometer, inspired by the sensitive olfactory systems of animals, for direct sniffing, established by connecting a mini pump to the nebulizer gas tubing. The modified mass spectrometry method-sniffing-mass spectrometry (sniffing-MS)-can acquire detailed fingerprint spectra of mixed odors and shows high tolerance to toxicants. Furthermore, the method has a low limit of detection in the order of parts per trillion and is a 'sampling-free' technique for analyzing various gaseous compounds simultaneously, thus offering versatility for smelling daily commodities, tracking diffusion, and locating position of odors. Sniffing-MS can mimic or even surpass the olfaction of animals and is applicable for analyzing gaseous/volatile compounds, especially those polar compounds, in a simple manner depending on the intrinsic molecular mass-to-charge ratio.

Application of Electronic-Nose Technologies and VOC-Biomarkers for the Noninvasive Early Diagnosis of Gastrointestinal Diseases †

Conventional methods utilized for clinical diagnosis of gastrointestinal (GI) diseases have employed invasive medical procedures that cause stress, anxiety and pain to patients. These methods are often expensive, time-consuming, and require sophisticated chemical-analysis instruments and advanced modeling procedures to achieve diagnostic interpretations. This paper reviews recent applications of simpler, electronic-nose (e-nose) devices for the noninvasive early diagnosis of a wide range of GI diseases by collective analysis of headspace volatile organic compound (VOC)-metabolites from clinical samples to produce disease-specific aroma signatures (VOC profiles). A different "metabolomics" approach to GI disease diagnostics, involving identifications and quantifications of disease VOC-metabolites, are compared to the electronic-nose approach based on diagnostic costs, accuracy, advantages and disadvantages. The importance of changes in gut microbiome composition that result from disease are discussed relative to effects on disease detection. A new diagnostic approach, which combines the use of e-nose instruments for early rapid prophylactic disease-screenings with targeted identification of known disease biomarkers, is proposed to yield cheaper, quicker and more dependable diagnostic results. Some priority future research needs and coordination for bringing e-nose instruments into routine clinical practice are summarized.

Comprehensive Real-Time Analysis of the Yeast Volatilome

While yeast is one of the most studied organisms, its intricate biology remains to be fully mapped and understood. This is especially the case when it comes to capture rapid, in vivo fluctuations of metabolite levels. Secondary electrospray ionization-high resolution mass spectrometry SESI-HRMS is introduced here as a sensitive and noninvasive analytical technique for online monitoring of microbial metabolic activity. The power of this technique is exemplarily shown for baker’s yeast fermentation, for which the time-resolved abundance of about 300 metabolites is demonstrated. The results suggest that a large number of metabolites produced by yeast from glucose neither are reported in the literature nor are their biochemical origins deciphered. With the technique demonstrated here, researchers interested in distant disciplines such as yeast physiology and food quality will gain new insights into the biochemical capability of this simple eukaryote.

Atmospheric pressure soft ionization for gas chromatography with dielectric barrier discharge ionization-mass spectrometry (GC-DBDI-MS)

In this study, a gas chromatography (GC) system was interfaced to a high-resolution Orbitrap mass spectrometer by means of an active capillary plasma ionization source, based on dielectric barrier discharge ionization (DBDI).

Gas chromatography electrospray ionization mass spectrometry analysis of trimethylsilyl derivatives

A method based on the analysis of trimethylsilyl (TMS) derivatives by capillary gas chromatography electrospray ionization mass spectrometry (GC-ESI/MS) was proposed. To improve separation, analytes were derivatized to their TMS derivative. During ESI analysis, TMS derivatives may hydrolyze back to their polar native form and are thus suitable for ESI analysis. Several types of analytes were studied to investigate the potential of the approach. Not all TMS derivatives hydrolyzed back to their native form as anticipated. Incomplete hydrolysis was observed for TMS-organic acids and TMS-nonchlorinated phenols. For TMS-chlorophenols, the observation of only the [M - H]^- ion suggested that these phenols were hydrolyzed back to their native form. For TMS-beta agonists, the hydrolysis rate was low; therefore, the hydrolysis product was not detected. Both TMS-chlorophenols and TMS-beta agonists provide a sensitivity in the range of low parts per billion (0.25-5 ng/ml and 0.5-10 ng/ml respectively). Copyright © 2016 John Wiley & Sons, Ltd.

Letter: The formation of a M + 2 compound in the analysis of a trimethylsilyl derivative of monocarboxylic acids by gas chromatography electrospray ionization mass spectrometry

The trimethylsilyl (TMS) derivative is one of the most widely utilized derivatives for analyzing polar compounds by gas chromatography. An ion two mass units higher than the protonated molecular ion was observed in analyzing TMS-monocarboxylic acids by using gas chromatography electrospray ionization mass spectrometry (GC-ESI/MS). The structure of the M + 2 compound was investigated using tandem mass spectrometry and high-resolution mass spectrometry. The results suggest that one methyl group bound to the silicon atom was replaced by a hydroxyl group during the ESI process. One possible mechanism for the formation of the M + 2 compound is proposed. This observation suggests the possibility of synthesizing an organic compound by using ESI.

Circadian variation of the human metabolome captured by real-time breath analysis

Circadian clocks play a significant role in the correct timing of physiological metabolism, and clock disruption might lead to pathological changes of metabolism. One interesting method to assess the current state of metabolism is metabolomics. Metabolomics tries to capture the entirety of small molecules, i.e. the building blocks of metabolism, in a given matrix, such as blood, saliva or urine. Using mass spectrometric approaches we and others have shown that a significant portion of the human metabolome in saliva and blood exhibits circadian modulation; independent of food intake or sleep/wake rhythms. Recent advances in mass spectrometry techniques have introduced completely non-invasive breathprinting; a method to instantaneously assess small metabolites in human breath. In this proof-of-principle study, we extend these findings about the impact of circadian clocks on metabolomics to exhaled breath. As previously established, our method allows for real-time analysis of a rich matrix during frequent non-invasive sampling. We sampled the breath of three healthy, non-smoking human volunteers in hourly intervals for 24 hours during total sleep deprivation, and found 111 features in the breath of all individuals, 36–49% of which showed significant circadian variation in at least one individual. Our data suggest that real-time mass spectrometric "breathprinting" has high potential to become a useful tool to understand circadian metabolism, and develop new biomarkers to easily and in real-time assess circadian clock phase and function in experimental and clinical settings.

Secondary Electrospray Ionization

This chapter details the technique called extractive electrospray ionization (EESI) and describes its state-of-the-art developmental, mechanical and experimental aspects and shows its most important applications. EESI is a sensitive, matrix-tolerant secondary electrospray ionization technique, which is in the focus of ongoing investigations. The strength of EESI is its ability to ionize various compounds directly out of the sample without preparation or chromatographic separation. Although it appears to be not always the most sensitive method, it has shown enormous capabilities for various applications such as breath or skin analysis, the classification of perfumes, detection of melamine in milk and identification of the freshness of frozen meat or fruit.

Analysis of human breath samples using a modified thermal desorption: gas chromatography electrospray ionization interface

A two-stage thermal desorption/secondary electrospray ionization/time-of-flight mass spectrometry for faster targeted breath profiling has been studied. A new secondary electrospray ionization (SESI) source was devised to constrain the thermal desorption plume and promote efficient mixing in the ionization region. Further, a chromatographic pre-separation stage was introduced to suppress interferences from siloxanes associated with thermal desorption profiles of exhaled breath samples.In vitro tests with 5-nonanone indicated an increased sensitivity and a lowered limit-of-detection, both by a factor of ~4, the latter to an on-trap mass of 14.3 ng, equivalent to a sampled breath concentration of 967 pptv. Analysis of the mass spectrometric responses from 20 breath samples acquired sequentially from a single participant indicated enhanced reproducibility (reduced relative standard deviations (RSD) for 5-nonanone, benzaldehyde and 2-butanone were 28 %, 16% and 14% respectively. The corresponding values for an open SESI source were that 5-nonanone was not detected, with %RSD of 39% for benzaldehyde and 31% for 2-butanone).The constrained source with chromatographic pre-separation resulted in an increase in the number of detectable volatile organic compounds (VOCs) from 260 mass spectral peaks with an open SESI source to 541 peaks with the constrained source with pre-separation. Most of the observed VOCs were present at trace levels, at less than 2.5% of the intensity of the base peak.Seventeen 2.5 dm(3) distal breath samples were collected from asthma patients and healthy controls respectively, and subjected to comparative high-throughput screening using thermal desorption/SESI/time-of-flight mass spectrometry (TD-SESI-ToFMS). Breath metabolites were detected by using a background siloxane ion (hexamethylcyclotrisiloxane m/z 223.0642) as an internal lockmass. Eleven breath metabolites were selected from the breath research literature and successfully targeted. These data reinforce the proposition that TD-SESI-MS has potential for development as a rapid screening method for disease stratification and targeted metabolism profiling.

Real-time monitoring of exhaled drugs by mass spectrometry

Future individualized patient treatment will need tools to monitor the dose and effects of administrated drugs. Mass spectrometry may become the method of choice to monitor drugs in real time by analyzing exhaled breath. This review describes the monitoring of exhaled drugs in real time by mass spectrometry. The biological background as well as the relevant physical properties of exhaled drugs are delineated. The feasibility of detecting and monitoring exhaled drugs is discussed in several examples. The mass spectrometric tools that are currently available to analyze breath in real time are reviewed. The technical needs and state of the art for on-site measurements by mass spectrometry are also discussed in detail. Off-line methods, which give support and are an important source of information for real-time measurements, are also discussed. Finally, some examples of drugs that have already been successfully detected in exhaled breath, including propofol, fentanyl, methadone, nicotine, and valproic acid are presented. Real-time monitoring of exhaled drugs by mass spectrometry is a relatively new field, which is still in the early stages of development. New technologies promise substantial benefit for future patient monitoring and treatment.

Low-sample flow secondary electrospray ionization: improving vapor ionization efficiency

In secondary electrospray ionization (SESI) systems, gaseous analytes exposed to an elecrospray plume become ionized after charge is transferred from the charging electrosprayed particles to the sample species. Current SESI systems have shown a certain potential. However, their ionization efficiency is limited by space charge repulsion and by the high sample flows required to prevent vapor dilution. As a result, they have a poor conversion ratio of vapor into ions. We have developed and tested a new SESI configuration, termed low-flow SESI, that permits the reduction of the required sample flows. Although the ion to vapor concentration ratio is limited, the ionic flow to sample vapor flow ratio theoretically is not. The new ionizer is coupled to a planar differential mobility analyzer (DMA) and requires only 0.2 lpm of vapor sample flow to produce 3.5 lpm of ionic flow. The achieved ionization efficiency is 1/700 (one ion for every 700 molecules) for TNT and, thus, compared with previous SESI ionizers coupled with atmospheric pressure ionization-mass spectrometry (API-MS) (Mesonero, E.; Sillero, J. A.; Hernández, M.; Fernandez de la Mora, J. Philadelphia PA, 2009) has been improved by a large factor of at least 50-100 (our measurements indicate 70). The new ionizer coupled with the planar DMA and a triple quadrupole mass spectrometer (ABSciex API5000) requires only 20 fg (50 million molecules) to produce a discernible signal after mobility and MS(2) analysis.

Dual nano-electrospray for probing solution interactions and fast reactions of complex biomolecules

A novel nano-electrospray emitter has been developed containing two separated channels running throughout the length of the emitter. The emitters have been fabricated from "theta-shaped" borosilicate capillaries. Loading of different solutions into the two different channels opens up the possibility to study short timescale interactions within a Taylor cone common to both channels. The common Taylor cone constitutes an extremely small "mixing volume" of the order of femtolitres. The products of electrospray from the dual-channel emitters have been analysed by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Results are presented for interactions of vancomycin with diacetyl-L-lysyl-D-alanyl- D-alanine and interactions of vancomycin with deuterated vancomycin. On the basis of these results, it is concluded that, during electrospray, specific non-covalent adducts have been formed and that there have been exchange reactions involving making and breaking of covalent bonds.

Formation of iron complexes from trifluoroacetic acid based liquid chromatography mobile phases as interference ions in liquid chromatography/electrospray ionization mass spectrometric analysis

Two unexpected singly charged ions at m/z 1103 and 944 have been observed in mass spectra obtained from electrospray ionization mass spectrometric analysis of liquid chromatography effluents with mobile phases containing trifluoroacetic acid (TFA) that severely interfered with sample analysis. Accurate mass measurement and tandem mass spectrometry studies revealed that these two ions are composed of three components; clusters of trifluoroacetic acid, clusters of mass 159 and iron. Formation of these ions is inhibited by removing TFA from the mobile phases and using formic acid in its place, replacing the stainless steel union with a titanium union or by adding a small blank fused-silica capillary column between the chromatography column and the electrospray tip via a stainless steel union without any adverse effects to chromatographic separation, peak broadening or peptide identifications.

Secondary electrospray ionization-mass spectrometry: breath study on a control group

A series of fatty acids among other compounds have recently been detected in breath in real time by secondary electrospray ionization mass spectrometry (SESI-MS) (Martínez-Lozano P and Fernández de la Mora J 2008 Anal. Chem. 80 8210). Our main aim in this work was to (1) quantify their abundance in breath calibrating the system with standard vapors and (2) extend the study to a control group for several days, both under fasting conditions and after sucrose intake. For the quantitative study, we fed our system with controlled amounts (∼140-1440 ppt) of fatty acid vapors (i.e. propanoic, butanoic, pentanoic and hexanoic acids). As a result, we found sensitivities ranging between 1 and 2.2 cps/ppt. Estimated concentrations of these particular acids in the breath of a fasting subject were in the order of 100 ppt. These values were in reasonable agreement with those expected from reported typical plasma concentrations and Henry constants. A second set of experiments on three fasting individuals before and after ingesting 15 g of sucrose showed that the concentration of propionic and butanoic acids increased rapidly in breath for two subjects. This response was attributed to bacterial activity in mouth and pharynx. In contrast, a third subject showed no response to the administration of sucrose. In addition, we performed a survey among six fasting subjects comparing nasal and mouth exhalations during 11 days, 4 months apart. The signal intensity was comparable for mouth and nose breath. This observation, in conjunction with the quantitative study, suggests that these compounds are mostly systemic when measured under fasting conditions. We finally used the NIST MS search algorithm to evaluate the possibility of recognizing a breathing subject based on his/her breath signature. The global recognition score was 63% (41 out of 65), while the probability by chance alone was 6 × 10(-17). This indicates that (i) there are statistically recognizable differences in individual breath patterns and (ii) the breath pattern for a given subject is relatively stable in time. This is consistent with previous NMR-based studies indicating the existence of stable individual metabolic phenotypes.

Study on reactions of long-lived phenoxathiin radical cation with aliphatic alcohols, phenol and phenyl halides in ambient condition by fused-droplet electrospray ionization mass spectrometry

he reactions of phenoxathiin radical cations with diverse organic compounds in ambient conditions were realized by using fused-droplet electrospray ionization mass spectrometry. In the investigation, the phenoxathiin radical cation was prepared by electrospray ionization. The reactants included aliphatic alcohols, phenol and phenyl halides and the reaction studies showed the unique reactivity the of phenoxathiin radical cation towards neutral organic compounds in ambient conditions, which has not been revealed in previous studies.

Fast detection of volatile organic compounds from bacterial cultures by secondary electrospray ionization-mass spectrometry

We propose a novel application of secondary electrospray ionization-mass spectrometry (SESI-MS) as a real-time clinical diagnostic tool for bacterial infection. It is known that volatile organic compounds (VOCs), produced in different combinations and quantities by bacteria as metabolites, generate characteristic odors for certain bacteria. These VOCs comprise a specific metabolic profile that can be used for species or serovar identification, but rapid and sensitive analytical methods are required for broad utility. In this study, the VOC profiles of five bacterial groups from four genera, Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, Salmonella enterica serovar Typhimurium, and Salmonella enterica serovar Pullorum, were characterized by SESI-MS. Thirteen compounds were identified from these bacterial cultures, and the combination of these VOCs creates a unique pattern for each genus. In addition, principal component analysis (PCA) was applied for the purpose of species or serovar discrimination. The first three principal components exhibit a clear separation between the metabolic volatile profiles of these five bacterial groups that is independent of the growth medium. As a first step toward addressing the complexity of clinical application, in vitro tests for mixed cultures were conducted. The results show that individual species or serovars in a mixed culture are identifiable among a biological VOC background, and the ratios of the detected volatiles reflect the proportion of each bacterium in the mixture. Our data confirm the utility of SESI-MS in real-time identification of bacterial species or serovars in vitro, which, in the future, may play a promising clinical role in diagnosing infections.

Ambient ionization mass spectrometry

Mass spectrometric ionization methods that operate under ambient conditions and require minimal or no sample pretreatment have attracted much attention in such fields as biomedicine, food safety, antiterrorism, pharmaceuticals, and environmental pollution. These technologies usually involve separate ionization and sample-introduction events, allowing independent control over each set of conditions. Ionization is typically performed under ambient conditions through use of existing electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) techniques. Rapid analyses of gas, liquid, and solid samples are possible with the adoption of various sample-introduction methods. This review sorts different ambient ionization techniques into two main subcategories, primarily on the basis of the ionization processes, that are further differentiated in terms of the approach used for sampling.

On-line detection of human skin vapors

Vapors released by the skin in the hand of one human subject are detected in real time by sampling them directly from the ambient gas surrounding the hand, ionizing them by secondary electrospray ionization (SESI, via contact with the charged cloud from an electrospray source), and analyzing them in a mass spectrometer with an atmospheric pressure source (API-MS). This gas-phase approach is complementary to alternative on-line surface ionization methods such as DESI and DART. A dominating peak of lactic acid and a complete series of saturated and singly unsaturated fatty acids (C(12) to C(18)) are observed, in accordance with previous off-line studies by gas chromatography-mass spectrometry. Several other metabolites have been identified, including ketomonocarboxylic and hydroxymonocarboxylic acids.

Characterization of the chemical components on the surface of different solids with electrospray-assisted laser desorption ionization mass spectrometry

In this study we demonstrate that electrospray-assisted laser desorption ionization (ELDI) mass spectrometry (MS) can be used to rapidly characterize major chemical components on the surfaces of different solids under ambient conditions. The major chemical components in (a) dried milks with different fat contents, (b) different color-regions of a painting, (c) the thin coating on a compact disc, (d) drug tablets, and (e) porcine brain tissue were rapidly characterized as protonated molecules [M+H](+) or sodiated molecules [M+Na](+) by ELDI-MS with minimum sample pretreatment. The ionized ions of synthetic polymer and dye standards were detected directly from dried sample solutions using either positive or negative ion mode. Further structural information for the FD&C Red dye was obtained through tandem mass spectrometric (MS/MS) analysis using an ion trap mass analyzer attached to the ELDI source.

Extractive electrospray ionization for direct analysis of undiluted urine, milk and other complex mixtures without sample preparation

On-line droplet-droplet extraction occurs when a sample spray intersects a reagent electrospray; this allows continuous analysis of trace amounts of compounds directly in complex matrices including undiluted urine, milk and polluted water over extended periods of time.

Chemical analysis in the submillimetre spectral region with a compact solid state system

A new analytical system that uses the rotational signatures of gas phase molecules is described and demonstrated. It uses a solid state source to probe molecular systems in the millimetre and submillimetre wave range, the only region of the electromagnetic spectrum not yet used extensively for analytical purposes. It employs the FAst Scan Submillimetre Spectroscopy Technique (FASSST), which leads to an especially simple system architecture. Among the attributes of the system are generality, sensitivity, 'absolute' specificity, small size, simplicity, and the potential for very low cost. Applications to problems of analytical interest are also discussed.

Characterization of Synthetic Polymers by Electrospray-Assisted Pyrolysis Ionization-Mass Spectrometry

Rapid characterization of synthetic polymers based on the differences in the appearance of the polar pyrolysate ions was achieved by electrospray-assisted pyrolysis ionization-mass spectrometry. The pyrolytical products produced by a commercial Curie point pyroprobe were conducted to the tip of a capillary, where charged methanol droplets were generated continuously by electrospray. Polar components in the pyrolysates may react directly with the protonated methanol ions or fuse with the charged methanol droplets; electrospray ionization proceeds from the fused droplets to generate protonated analyte ions. The mass spectra obtained through this approach were used to rapidly distinguish the polymer standards that differ in the nature of building units, degrees of polymerization, and copolymerization coefficients.

Electrospray-assisted laser desorption/ionization mass spectrometry for direct ambient analysis of solids

A new method of electrospray-assisted laser desorption/ionization (ELDI) mass spectrometry, which combines laser desorption with post-ionization by electrospray, was applied to rapid analysis of solid materials under ambient conditions. Analytes were desorbed from solid metallic and insulating substrata using a pulsed nitrogen laser. Post-ionization produced high-quality mass spectra characteristic of electrospray, including protein multiple charging. For the first time, mass spectra of intact proteins were obtained using laser desorption without adding a matrix. Bovine cytochrome c and an illicit drug containing methaqualone were chosen in this study to demonstrate the applicability of ELDI to the analysis of proteins and synthetic organic compounds.

Interfaces to connect thin-layer chromatography with electrospray ionization mass spectrometry

This study has developed two interfaces to connect small-size thin-layer chromatography (TLC) with electrospray ionization mass spectrometry (ES-MS) for the continuous analysis of organic mixtures. The interfaces are (1) two bound optical fibers inserted into the C18-bonded particles at the exit of a small TLC channel and (2) a small commercial TLC strip with a sharpened tip. A reservoir continuously supplied a makeup solution to the tip of the TLC channel. The high voltage required for electrospray ionization was introduced into the makeup solution or mobile phase through a Pt wire, and electrospray was generated at the tip of the bonded optical fibers and at the sharp end of the TLC strip. Since small-size TLC channels were used, the elution time was short and less than 0.2 microL of the sample solution and 200 microL of the eluting solvent were required. Organic mixtures were separated successfully and detected on-line using the TLC/ES-MS techniques.

Volatile polydimethylcyclosiloxanes in the ambient laboratory air identified as source of extreme background signals in nanoelectrospray mass spectrometry

Polydimethylcyclosiloxanes, an almost ubiquitous air contaminant, can interfere with nanoelectrospray analysis. The sensitivity of nanoelectrospray to these volatile air contaminants was demonstrated in this study. The intensity of the interfering ion signals caused by these compounds can be decreased by changing the position of the nanoESI needle and almost completely suppressed by applying a flow of pure nitrogen around the needle and the sample cone. The nitrogen flow causes a slight shift in charge distribution, but does not influence the sensitivity for peptide detection.

Electrospray ionization from a droplet deposited on a surface-modified glass rod

This paper reports development of a non-mechanical electrospray ionization (ESI) method to generate electrospray from a droplet deposited on an optical fiber coated with a thin gold or Nafion film. Modification of the surface of the optical fiber in this manner increases its wettability, such that a droplet of the aqueous sample solution can adhere sufficiently strongly to the tip of the fiber. The aqueous sample solution was deposited near the tip of the fiber with a micropipette. When a high voltage (2,000 V) was applied to the fiber by electrical connection through the gold film, the sample solution moved and hung at the tip of the fiber. Simultaneously, ESI was generated from the sample droplet. Multiply charged peptide and protein ions were detected by connecting the ESI source to a quadrupole mass analyzer.

Detecting large biomolecules from high-salt solutions by fused-droplet electrospray ionization mass spectrometry

A novel fused-droplet electrospray ionization (FD-ESI) source was developed to generate peptide and protein ions. The sample solution was first ultrasonically nebulized to form fine aerosols. The aerosols were then purged into a glass reaction chamber via nitrogen. Charged methanol droplets were continuously generated through electrospraying the acidic methanol solution from a capillary, which was located at the center of the reaction chamber. As the sample aerosols entered the reaction chamber, they fused with the charged methanol droplets from which electrospray proceeded continuously. The mass spectra of peptide and protein that FD-ESI-MS produced were practically identical to those that conventional ESI-MS produced. However, FD-ESI-MS resulted in an extremely high salt tolerance. Cytochrome c ions were detected in the solutions that contained 10% (w/w; 1.709 M) NaCl or 2.5% (425 mM) NaH2PO4. As with those obtained from the solution that lacked NaCl and NaH2PO4, the width of cytochrome c ion peaks remained nearly unchanged.

Generating multiply charged protein ions by ultrasonic nebulization/multiple channel-electrospray ionization mass spectrometry

An ultrasonic nebulization/multiple channel electrospray ionization (USN/MC-ES) source, which generates multiply charged peptides and proteins ions, was developed. The source is an ultrasonic nebulizer that is connected to a multiple channel electrospray ionization source. Aerosols were formed by ultrasonically nebulizing the sample solution. The aerosols were then purged into the central channel of a seven-channel ES source via nitrogen gas. A methanol solution that contained 1% trifluroacetic acid was electrosprayed through the outlying six electrosprayers. Detection of multiply charged peptide and protein ions indicated that electrospray was generated from the charged droplet containing analyte. The sample aerosol appeared to fuse with the charged methanol droplet in the air. Then electrospray ionization of the analyte occurred from the newly formed droplet. The peptide and protein prepared in deionized water were detected by this USN/MC-ES-MS. By varying the electrospray solvents, the signals of certain components in the mixture were selectively suppressed.