Atmospheric Pressure Photoionization for Enhanced Compatibility in On-Line Micellar Electrokinetic Chromatography−Mass Spectrometry

Atmospheric pressure photoionization versus electrospray for the dereplication of highly conjugated natural products using molecular networks

Natural products are sources of inspiration and reservoir of high valuable molecules. Recently, analytical tools based on liquid chromatography coupled to tandem mass spectrometry to generate molecular network became widely employed for dereplication. This strategy greatly accelerates the identification of known and structural hypothesis of unknown. Despite the availability of different ionization sources, alternatives to classical electrospray ionization (ESI), such as atmospheric pressure chemical ionization (APCI) or photoionization (APPI), have been neglected. In particular, APPI has been described for its ionization efficiency on non-polar molecules bearing no acid or basic groups. For that reason, we investigated APPI potential to generate molecular network and compare it to ESI on several criteria that are generation of ion species, sensitivity and signal-to-noise ratio (SNR) for different extracts rich in highly conjugated natural products. We first optimized APPI experimental conditions on crude extract from a fungus, Penicillium sclerotiorum, producing polyketones belonging to the azaphilone family. Then we compared APPI and ESI on different fractions of the fungus and on two plant extracts, French Guyanese Swartzia panacoco (Aubl.) R.S. Cowan (arial parts) and Indian Cassia auriculata L. (leaves) containing phenolic compounds, such as flavonoids. While ESI generated more ion species and displayed a better sensitivity, APPI generated only protonated adduct and better SNR. Comparing ESI and APPI generated species on molecular network reveal that both strategies overlap for the majority of protonated ions.

Two-dimensional capillary electrophoresis-mass spectrometry (CE-CE-MS): coupling MS-interfering capillary electromigration methods with mass spectrometry

Electromigration separation techniques often demand certain compounds in the electrolyte to achieve the required selectivity and efficiency. These compounds, including the electrolyte itself, ampholytes, polymeric compounds for sieving, complexing agents, tensides, etc. are often non-volatile. Thus, interference with the electrospray ionization process is a common issue, impeding direct coupling of such electrolyte systems to mass spectrometry. Still, several options exist to obtain mass spectra after separation, including offline fractionation, alternative ionization, dilution, or the change to volatile constituents. In the first part of this article, these methods are discussed. However, all of these options are a compromise of separation performance and sensitivity of mass spectrometric detection. Two-dimensional capillary electrophoresis-mass spectrometry (CE-CE-MS) systems represent a promising alternative to the aforementioned challenges, as they allow the use of existing methods with best separation performance in combination with sensitive mass characterization. In this context, the second part of this article is dedicated to the advantages, limitations, and applications of this approach. Finally, an outlook towards future developments is given.

Fully compatible and ultra-sensitive micellar electrokinetic chromatography-tandem mass spectrometry using sheathless porous-tip interfacing

The on-line coupling of micellar electrokinetic chromatography and mass spectrometry (MEKC-MS) is often hampered by incompatibility problems leading to reduced separation performance and unfavorable limits of detection (LODs). Here we propose a new selective and highly sensitive MEKC-MS/MS method employing a sheathless porous-tip interface in combination with a micellar phase comprised of semi-volatile surfactant molecules. Carbamate pesticides (CRBs) were selected as representative model compounds being neutral toxic pollutants potentially present at trace levels in environmental water samples. A background electrolyte of 75mM perfluorooctanoic acid adjusted to pH 9.0 with ammonium hydroxide allowed efficient separation of 15 CRBs and appeared fully compatible with electrospray ionization (ESI)-MS. Interfacing parameters, such as the distance between the capillary tip and mass-spectrometer inlet, ESI voltage, and dry gas temperature and flow were optimized in order to attain good spray stability and high analyte signal-to-noise ratios. For CRBs the LODs ranged from 0.2 to 3.9ngL^-1 (13nL injected, i.e., 2% of capillary volume), representing an improvement for certain CRBs of more than 300-fold when compared with conventional sheath-liquid interfacing. Good linearity (R²>0.99) and satisfactory reproducibility were obtained for all CRBs with interday RSD values for peak area and migration time of 4.0-11.3% and below 1.5%, respectively. Analysis of spiked mineral water showed that the new MEKC-MS/MS method allows selective and quantitative determination of CRB concentrations below the maximum residue limit of 100ngL^-1 without the need for sample preconcentration.

Enantioselective capillary electrophoresis-mass spectrometry of amino acids in cerebrospinal fluid using a chiral derivatizing agent and volatile surfactant

The sensitivity of coupled enantioselective capillary electrophoresis-mass spectrometry (CE-MS) of amino acids (AAs) is often hampered by the chiral selectors in the background electrolyte (BGE). A new method is presented in which the use of a chiral selector is circumvented by employing (+)-1-(9-fluorenyl)ethyl chloroformate (FLEC) as chiral AA derivatizing agent and ammonium perfluorooctanoate (APFO) as a volatile pseudostationary phase for separation of the formed diastereomers. Efficient AA derivatization with FLEC was completed within 10 min. Infusion experiments showed that the APFO concentration hardly affects the MS response of FLEC-AAs and presents significantly less ion suppression than equal concentrations of ammonium acetate. The effect of the pH and APFO concentration of the BGE and the capillary temperature were studied in order to achieve optimized enantioseparation. Optimization of CE-MS parameters, such as sheath-liquid composition and flow rate, ESI and MS settings was performed in order to prevent analyte fragmentation and achieve sensitive detection. Selective detection and quantification of 14 chiral proteinogenic AAs was achieved with chiral resolution between 1.2 and 8.6, and limits of detection ranging from 130 to 630 nM injected concentration. Aspartic acid and glutamic acid were detected, but not enantioseparated. The optimized method was applied to the analysis of chiral AAs in cerebrospinal fluid (CSF). Good linearity (R(2) > 0.99) and acceptable peak area and electrophoretic mobility repeatability (RSDs below 21% and 2.4%, respectively) were achieved for the chiral proteinogenic AAs, with sensitivity and chiral resolution mostly similar to obtained for standard solutions. Next to l-AAs, endogenous levels of d-serine and d-glutamine could be measured in CSF revealing enantiomeric ratios of 4.8%-8.0% and 0.34%-0.74%, respectively, and indicating the method's potential for the analysis of low concentrations of d-AAs in presence of abundant l-AAs.

Recent advances in CE-mediated microanalysis for enzyme study

This review gives an overview of the recent developments and applications in the use of CE-mediated microanalysis for enzyme studies. The period covers mid-2011 until mid-2013. Both off-line and in-line enzyme assays with their applications using CE are described in this article. For the in-capillary enzyme reaction, the techniques using electrophoretically mediated microanalysis (EMMA) as well as immobilized enzyme reactor (IMER) are discussed. The applications include the evaluation of enzyme activity, enzyme kinetics, enzyme inhibition, screening of enzyme inhibitors, and the study of enzyme-mediated drug metabolism.

Carbon disulfide as a dopant in photon-induced chemical ionization mass spectrometry

RATIONALE

The addition of a dopant to an Atmospheric Pressure PhotoIonization (APPI) source of a mass spectrometer has been shown to enhance the degree of analyte ionization. A series of different dopants has been successfully utilized; however, there has been very little published on the characteristics of a good dopant. We have proposed carbon disulfide (CS2) as a novel new dopant based on its absorption cross-section for the VUV photon's energy used and its unique gas-phase ion chemistry, notably the fact that it does not contain a proton.

METHODS

The ability of CS2 to enhance the ionization effectiveness of APPI was tested by using a group of compounds that have different proton affinities (PAs) and electron affinities (EAs). These results were compared to results obtained using the commonly used dopants, toluene and anisole. Particular attention was paid to the formation of [M](+) ions relative to [M+H](+) ions. Mass spectra were collected using a Waters Quattro Premier liquid chromatography/tandem mass spectrometry (LC/MS/MS) system equipped with a commercial Photomate™ photoionization source.

RESULTS

The results show that CS2 increases the ionization efficiency of most of the analytes studied in this work comparably to toluene and anisole. CS2 promotes both ionization routes of [M](+) and [M+H](+). In addition, due to the higher ionization energy (IE) of CS2 (10.01) compared to the IEs of toluene (8.83) and anisole (8.20), CS2 can enhance the ionization efficiency of analytes that cannot be enhanced with toluene and anisole.

CONCLUSIONS

We have determined that CS2 is a viable dopant for use in APPI sources. For some analytes, significant [M+H](+) ion signals are observed; therefore, the donated proton must come from either water clusters or solvents. In addition, CS2 promotes the ionization of analytes with low PAs and higher IEs than that of toluene and anisole.

Recent methodological and instrumental development in MEKC

The review gives an update about the methodological and instrumental developments in micellar electrokinetic capillary chromatography as a type of CE analytical technique. Here, the last two years development of the technique are particularly presented. Recent approaches to improve sensitivity are discussed. Newly introduced concentration techniques and experimental methods for verification of the different mechanisms and processes of micellar electrokinetic chromatography analysis are highlighted. A theoretical model to explain changes in separation and electrophoretic mobility order of fully charged analytes are demonstrated. Modern approaches for improving compatibility of micellar electrokinetic capillary chromatography to mass spectrometry are also reported.

Development of a chiral micellar electrokinetic chromatography-tandem mass spectrometry assay for simultaneous analysis of warfarin and hydroxywarfarin metabolites: application to the analysis of patients serum samples

The enantioseparation of warfarin (WAR) along with the five positional and optical isomers is challenging because of the difficulty to simultaneously separate and quantitate these chiral compounds. Currently, no effective chiral CE-MS methods exist for the simultaneous enantioseparation of WAR and all its hydroxylated metabolites in a single run. Polymeric surfactants (aka. molecular micelles) are particularly compatible with micellar electrokinetic chromatography-mass spectrometry (MEKC-MS) because they have a wider elution window for enantioseparation and do not interfere with the MS detection of chiral drugs. Using polysodium N-undecenoyl-L,L-leucylvalinate (poly-L,L-SULV) as a chiral pseudophase in MEKC-MS baseline separation of WAR, its five metabolites along with the internal standard was obtained in 45 min. This is in comparison to 100 min required for separation of the same mixture with packed column CEC-MS using a vancomycin chiral stationary phase. Serum samples were extracted with mixed-mode anion-exchange (MAX) cartridge with recoveries of greater than 85.2% for all WAR and hydroxywarfarin (OH-WAR) metabolites. Utilizing the tandem MS and multiple reaction monitoring mode, the MEKC-MS/MS method was used to simultaneously generate calibration curves over a concentration range from 2 to 5000 ng/mL for R- and S-warfarin, 5 to 1000 ng/mL for R- and S-6-, 7-, 8- and 10-OH-WAR and 10 to 1000 ng/mL for R and S-4'-OH-WAR. For the first time, the limits of detection and quantitation for most WAR metabolites by MEKC-MS/MS were found to be at levels of 2 and 5 ng/mL, respectively. The method was successfully applied for the first time to analyze WAR and its metabolites in plasma samples of 55 patients undergoing WAR therapy, demonstrating the potential of chiral MEKC-MS/MS method to accurately quantitate with high sensitivity.

Capillary electrophoresis and small molecule drug discovery: a perfect match?

Capillary electrophoresis (CE) is an analytical technique based on the separation of the analytes within a capillary owing to their different electrophoretic mobilities. It is widely used in pharmaceutical analyses owing to its versatility and high separation power. However, its penetration into the drug discovery scene has been relatively limited until recent years. Several factors have contributed to this low implementation, including the maturity of liquid chromatography, the scarcity of experienced CE practitioners, and certain limitations intrinsic to the technique. Recently, instrumental improvements and the growing demand for analytical information have lead to a continuously expanding range of routine electrophoretic applications throughout pharmaceutical discovery and development. In this article we review CE fundamentals, review well-established CE methodologies in drug discovery of small molecules and discuss trends that, in our opinion, might emerge in the coming years.

Ionization techniques in capillary electrophoresis-mass spectrometry: principles, design, and application

A major step forward in the development and application of capillary electrophoresis (CE) was its coupling to ESI-MS, first reported in 1987. More than two decades later, ESI has remained the principal ionization technique in CE-MS, but a number of other ionization techniques have also been implemented. In this review the state-of-the-art in the employment of soft ionization techniques for CE-MS is presented. First the fundamentals and general challenges of hyphenating conventional CE and microchip electrophoresis with MS are outlined. After elaborating on the characteristics and role of ESI, emphasis is put on alternative ionization techniques including sonic spray ionization (SSI), thermospray ionization (TSI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), matrix-assisted laser desorption ionization (MALDI) and continuous-flow fast atom bombardment (CF-FAB). The principle of each ionization technique is outlined and the experimental set-ups of the CE-MS couplings are described. The strengths and limitations of each ionization technique with respect to CE-MS are discussed and the applicability of the various systems is illustrated by a number of typical examples.

MEKC as a powerful growing analytical technique

This review summarizes the principle and the developments in MEKC in terms of separation power, sensitivity, and detection approaches more than 25 years after its appearance. Newly used surfactants are mentioned. Classical and new sample concentration techniques in MEKC are described. The different detection approaches in MEKC with advantages, limitations, and future prospects are also discussed. This review highlights the wider application of MEKC in different analytical fields. Various recent selected applications of this technique in different analytical fields are reported.

Electrokinetic chromatography and mass spectrometric detection using latex nanoparticles as a pseudostationary phase

The utility of novel latex nanoparticles as pseudostationary phases for electrokinetic chromatography with UV and mass spectrometric detection is demonstrated. The nanoparticles are synthesized using ab initio RAFT (reversible addition-fragmentation chain transfer) in emulsion polymerization, which yields small (63 nm) particles with a narrow size distribution, a hydrophobic core, and an ionic shell. The nanoparticles are shown to provide efficient and selective separations, with retention and separation selectivity dominated by hydrophobic interactions. The nanoparticles are highly retentive, such that they are effective at relatively low concentrations. Addition of the nanoparticles to the background electrolyte at these concentrations has a minor effect on the noise with UV detection, no measurable effect on the separation current, and minor effects on analyte ionization efficiency during electrospray ionization. The nanoparticles do not cause fouling or degradation of the electrospray-mass spectrometer interface even after several weeks of use. The combination of online sample preconcentration via sweeping and selective mass spectrometric detection yields low detection limits (10-16 ppb), particularly for more hydrophobic compounds.

Efforts to improve detection sensitivity for capillary electrophoresis coupled to atmospheric pressure photoionization mass spectrometry

Electrospray ionization performs best with volatile buffers. However, generally the best separation performance for capillary electrophoresis (CE) is achieved with non-volatile buffers. Hyphenation of CE with mass spectrometry (MS) utilizing atmospheric pressure photoionization (APPI) enables use of a wider range of separation buffers without compromising detection sensitivity. As APPI is considered to be mass flow sensitive, the use of a larger inner diameter separation capillary (75 microm) allows larger volumes to be injected, without decreased separation performance, thus providing improved sensitivity (approx. a factor of 10), compared to the use of a 25 microm capillary. However, nebulizing gas flow and position of capillary tip in the sprayer have to be carefully optimized to prevent excessive band broadening. Further improvement in sensitivity (approx. a factor of 2) was obtained by decreasing the distance between the sprayer and ionization region, indicating that a specially designed CE/APPI-MS interface for low flow rates will be favourable.

Atmospheric pressure thermospray ionization using a heated microchip nebulizer

When a standard atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) ion source is used without applying the corona discharge or photoirradiation, atmospheric pressure thermospray ionization (APTSI) of various compounds can be achieved. Although largely ignored, this phenomenon has recently gained interest as an alternative ionization technique. In this study, this technique is performed for the first time on a miniaturized scale using a microchip nebulizer. Sample ionization with the presented microchip-APTSI (microAPTSI) is achieved by applying only heat and gas flow to a nebulizer chip, without any other methods to promote gas-phase ionization. To evaluate the performance of the described microAPTSI setup, ionization efficiency for a set of test compounds was monitored as the microchip positioning, temperature, nebulizer gas flow rate, sample solution composition, and solvent flow rate were varied. The microAPTSI mass spectra of the test compounds were also compared to those obtained with ESI and APCI. The microAPTSI produces ESI-like spectra with low background noise, favoring the formation of protonated or deprotonated molecules of compounds that are ionizable in solution. Multiple charging of peptides without in-source fragmentation was also observed. Unlike ESI, however, the microAPTSI source can tolerate the presence of mobile phase additives like trifluoroacetic acid (TFA) without significant ion suppression. The microAPTSI source can be used with standard mass spectrometer ion source hardware, being a unique alternative to the present interfacing techniques.

Capillary electrophoresis-atmospheric pressure chemical ionization-mass spectrometry using an orthogonal interface: set-up and system parameters

The feasibility of atmospheric pressure chemical ionization (APCI) as an alternative ionization technique for capillary electrophoresis-mass spectrometry (CE-MS) was investigated using a grounded sheath-flow CE-MS sprayer and an orthogonal APCI source. Infusion experiments indicated that highest analyte signals were achieved when the sprayer tip was in close vicinity of the vaporizer entrance. The APCI-MS set-up enabled detection of basic, neutral, and acidic compounds, whereas apolar and ionic compounds could not be detected. In the positive ion mode, analytes could be detected in the entire transfer voltage range (0-5 kV), whereas highest signal intensities were observed when the corona discharge current was between 1000 and 2000 nA. In the negative ion mode, the transfer voltage typically was 500 V and the optimum corona discharge current was 6000 nA. Analyte signals were raised with increasing nebulizing gas pressure, but the pressure was limited to 25 psi to avoid siphoning and current drops. Signal intensities appeared to be optimal and constant over a wide range of sheath liquid flow rate (5-25 microL/min) and vaporizer temperature (200-350 degrees C). APCI-MS signals were unaffected by the composition of the background electrolyte (BGE), even when it contained sodium phosphate and sodium dodecyl sulfate (SDS). Consequently, BGE composition, sheath-liquid flow rate, and vaporizer temperature can be optimized with respect to the CE separation without affecting the APCI-MS response. The analysis of a mixture of basic compounds and a steroid using volatile and nonvolatile BGEs further demonstrates the feasibility of CE-APCI-MS. Detection limits (S/N = 3) were 1.6-10 microM injected concentrations.

Capillary separation: micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC), a separation mode of capillary electrophoresis (CE), has enabled the separation of electrically neutral analytes. MEKC can be performed by adding an ionic micelle to the running solution of CE without modifying the instrument. Its separation principle is based on the differential migration of the ionic micelles and the bulk running buffer under electrophoresis conditions and on the interaction between the analyte and the micelle. Hence, MEKC's separation principle is similar to that of chromatography. MEKC is a useful technique particularly for the separation of small molecules, both neutral and charged, and yields high-efficiency separation in a short time with minimum amounts of sample and reagents. To improve the concentration sensitivity of detection, several on-line sample preconcentration techniques such as sweeping have been developed.

Coupling CE with atmospheric pressure photoionization MS for pharmaceutical basic compounds: Optimization of operating parameters

The use of CE coupled with MS (CE-MS) has evolved as a useful tool to analyze charged species in small sample volumes. Because of its sensitivity, versatility and ease of implementation, the ESI interface is currently the method of choice to hyphenate CE to MS. An alternative can be the atmospheric pressure photoionization (APPI) source, however, numerous parameters must be optimized for its coupling to CE. After evaluation of the sheath liquid composition and the CE capillary outlet position, an experimental design methodology was assessed for optimizing other ionization source parameters, such as sheath liquid flow rate, drying gas flow rate and temperature, nebulizing gas pressure, vaporizer temperature, and capillary voltage. For this purpose, a fractional factorial design (FFD) was selected as a screening procedure to identify factors which significantly influence sensitivity and efficiency. A face-centered central composite design (CCD) was then used to predict and optimize sensitivity, taking into account the most relevant variables. Sensitivity was finally evaluated with the optimized conditions and height-to-noise ratios (H/N) around 10 were achieved for an injection of 200 ng/mL of each analyte.

Microemulsion electrokinetic chromatography with on-line atmospheric pressure photoionization-mass spectrometric detection of medium polarity compounds

In this paper, we present the determination of pharmaceuticals employing microemulsion electrokinetic chromatography (MEEKC) with atmospheric pressure photoionization-mass spectrometric (APPI-MS) detection. This recent hyphenated technique allows to overcome some disadvantages of MEEKC, namely its inherent incompatibility with MS detection. Important parameters like microemulsion (ME) composition, the composition of the sheath liquid and APPI-MS detection parameters have been investigated. Using the optimized set of parameters, the eight selected substances could be detected down to concentrations between 3 mg L(-1) (phenacetin) and 41 mg L(-1) (diltiazem). Switching to the MS2 mode, the use of specific transitions for the detection of each analyte provided improved detection limits in the range of 0.6 mg L(-1) (carbamazepine) to 6 mg L(-1) (metoprolol). Calibration curves were linear over one to two orders of magnitude with correlation coefficients better than 0.98.

Continuous full filling capillary electrochromatography: Nanoparticle synthesis and evaluation

Reversed phase continuous full filling capillary electrochromatography with electrospray ionisation mass spectrometric detection was performed with highly sulphated poly[styrene-co-(lauryl methacrylate)-co-(divinylbenzene)] nanoparticles. The nanoparticles that contained a hydrophobic core and a hydrophilic surface were prepared in a one step synthesis using soap free emulsion polymerisation. By changing the concentration of monomers, the polymerisation temperature, and the polarity of the dispersive phase, the size of the nanoparticles could be controlled. With the optimised conditions, nanoparticles with an average size of 157 nm were obtained. These nanoparticles were dispersed in the background electrolyte and used for reversed phase continuous full filling. An orthogonal electrospray ionisation interface was used to separate the eluting nanoparticles from the eluting analytes prior to mass spectrometry detection. Compared to previous studies on reversed phase continuous full filling, the retention, the separation efficiency, and the resolution of a homologous series of dialkyl phthalates were greatly improved.

Comparison of atmospheric pressure photoionization and ESI for CZE-MS of drugs

The performance of atmospheric pressure photoionization (APPI) and ESI for CZE was compared using a set of seven drugs (basic amines, quaternary amines and steroids) and four different BGEs. The influence of volatile and nonvolatile BGEs of acidic and neutral pH on the MS responses of test compounds was evaluated by infusion of test solutions into the respective ion sources, and by actual CZE-MS experiments. The infusion experiments indicate that sodium phosphate buffers cause ionization suppression in ESI-MS, although for the amines the suppression was modest (25-60% signal reduction). By contrast, APPI-MS responses were not affected by nonvolatile BGEs. With phosphate buffers, ESI-MS responses for the basic amines were still a factor 3-13 higher than the APPI-MS signals, whereas the steroids yielded similar responses in ESI-MS and APPI-MS. The quaternary amines could readily be detected in ESI-MS, but detection in APPI-MS required specific interface conditions. Using typical CZE-APPI-MS settings, quaternary amines remained undetected. Remarkably, the S/Ns observed in CZE-ESI-MS for the test compounds, were generally similar when using volatile and nonvolatile BGEs. For basic compounds, the S/Ns obtained in CZE-ESI-MS were a factor 2-5 higher than in CZE-APPI-MS, whereas steroids yielded equal S/Ns in both methods. Overall, it is concluded that when using relatively low BGE concentrations, the sensitivity of ESI-MS detection in CZE is more favorable than APPI-MS detection, even when nonvolatile BGEs are employed.

CE-MS of antihistamines using nonvolatile phosphate buffer

Antihistamines were analyzed by CE-ESI-MS using phosphate buffer. The separation was performed in an acidic environment so that phosphate ions had a net velocity flowing toward the inlet reservoir instead of the ESI source. To further reduce the effect of ion suppression, the sodium ion in sodium phosphate was replaced with an ammonium ion. Furthermore, with the combination of reducing the concentration of acid added to the sheath liquid and the use of a low-flow interface, phosphoric acid could be added to the sheath liquid. Because of the use of the same counterion (phosphate ion) in running buffer and in sheath liquid, the separation integrity (resolution, elution order, and peak shape) was preserved. In addition, ion suppression was also greatly alleviated because a minimal amount of phosphate flowed into the ESI source.

Metabolome analysis by capillary electrophoresis-mass spectrometry

Capillary electrophoresis (CE)-mass spectrometry (MS), as an analytical platform, has made significant contributions in advancing metabolomics research, if still limited up to this time. This review, covering reports published between 1998 and 2006, describes how CE-MS has been used thus far in this field, with the majority of the works dealing with targeted metabolite analyses and only a small fraction using it in the comprehensive context. It also discusses how some of the key features of CE-MS were exploited in selected metabolomic applications.

Ammonium perfluorooctanoate as a volatile surfactant for the analysis ofN-methylcarbamates by MEKC-ESI-MS

Ammonium perfluorooctanoate (APFOA) was investigated as an MS-friendly surfactant for the analysis of a mixture of ten N-methylcarbamates with MEKC-ESI-MS. Because of the relatively low boiling point of perfluorooctanoic acid ( approximately 190 degrees C), APFOA can be introduced into a mass spectrometer without the adverse effects of less volatile surfactants such as SDS. With a BGE consisting of 50 mM APFOA/isopropanol (IPA) 98:2 and with 30 kV applied, a very fast separation ( approximately 6 min) was possible with only one pair of analytes comigrating. Using an experimental design with four factors (voltage, nebulizer pressure, concentration of APFOA, and concentration of IPA) we were able to resolve all analytes in just over 11 min. Sheath liquid composition and flow rate, drying gas temperature and flow rate, and fragmentor voltage were then optimized for maximum signal intensity and S/N. It was found that the faster method gave better S/N because of narrower peak widths, and detection limits in SIM mode were between 0.01 (aldicarb) and 0.08 mg/L (methomyl). Calibration curves were prepared with standards of 0.50, 1.00, and 2.00 mg/L for the analysis of samples obtained after SPE of tap water spiked with the ten N-methylcarbamates at a level of 10 microg/L. All analytes showed very good recoveries (>86%), except for the most polar analyte aldicarb sulfone (recovery of 73%), testifying for the potential use of APFOA for this kind of analyses.

Capillary electrophoresis-mass spectrometry, an attractive tool for drug bioanalysis and biomarker discovery

The coupling of CE with MS detection, a relatively recent hyphenated technique, has gained increasing respect in the field of bioanalytical applications over the past few years. The first part of this review presents CE-MS applications dealing with drug bioanalysis, including forensic analysis and metabolism studies. Practical considerations to achieve a robust and sensitive CE-MS coupling are also presented. It is indeed essential to strictly control some critical electrospray parameters, such as the sheath liquid composition and flow rate, the nebulizing gas pressure as well as the capillary outlet position. The second part of the review critically describes the applications of CE coupled on-line to MS for the identification of biomarkers in body fluids for diagnostic purposes. Since the sample preparation procedures strongly differ according to the intended use (drug bioanalysis or biomarker discovery), they are discussed separately, taking into account the particular properties of plasma and urine matrices.