Chip-based microfluidic devices coupled with electrospray ionization-mass spectrometry

We present the current status of the development of microfluidic devices fabricated on different substrates for coupling with electrospray ionization-mass spectrometry (ESI-MS). Until now, much success has been gained in fabricating the ESI chips, which show better performances due to miniaturization when compared with traditional methods. Integration of multiple steps for sample preparation and ESI sample introduction, however, remains a great challenge. This review covers the main technical development of electrospray device that were published from 1997 to 2004. This article does not attempt to be exclusive. Instead, it focuses on the publications that illustrated the breath of the development and applications of microchip devices for MS-based analysis.     

On-line concentration of proteins in pressurized capillary electrochromatography coupled with electrospray ionization-mass spectrometry

Pressurized capillary electrochromatography (pCEC) and electrospray ionization-mass spectrometry (ESI-MS) have been hyphenated for protein analysis. Taken cytochrome c, lysozyme, and insulin as samples, the limits of detection (LODs) for absolute concentrations are 10(-11) mol (signal-to-noise ratio S/N = 3) with relative standard deviations (RSDs) of retention time and peak area, respectively, of less than 1.7% and 4.8%. In order to improve the detection sensitivity, on-line concentration by field-enhanced sample-stacking effect and chromatographic zone-sharpening effect has been developed, and parameters affecting separation and detection, such as pH and electrolyte concentration in the mobile phase, separation voltage, as well as enrichment voltage and time, have been studied systematically. Under the optimized conditions, the LODs of the three proteins could be decreased up to 100-fold. In addition, the feasibility of such techniques has been further demonstrated by the analysis of modified insulins at a concentration of 20 microg/mL.     

Processes that affect electrospray ionization-mass spectrometry of nucleobases and nucleosides

Due to the complexity of electrospray ionization processes and the many factors that affect the ion signal, optimization of electrospray ionization methods to gain ultimate sensitivity for analysis of nucleobases and nucleosides may not be straightforward. In this work, we investigated the effect of the pK _a and the gas-phase basicity of analyte and other electrolytes on the [M+H]⁺ ion signal for 11 select nucleobases and nucleosides in 50% methanol:water solution. Solution chemistry plays a role in the electrospray signal for all analytes, but gas-phase chemistry may be important for compounds with pK _a <3 depending on the solution composition. For compounds with pK _a <3, gas-phase proton transfer reactions can be promoted to increase analyte electrospray response by the addition of ammonium acetate to the solution.     

Fabrication of enclosed SU-8 tips for electrospray ionization-mass spectrometry

We describe a novel electrospray tip design for MS which is fabricated completely out of SU-8 photoepoxy. A three-layer SU-8 fabrication process provides fully enclosed channels and tips. The tip shape and alignment of all SU-8 layers is done lithographically and is therefore very accurate. Fabrication process enables easy integration of additional fluidic functions on the same chip. Separation channels can be made with exactly the same process. Fluidic inlets are made in SU-8 during the fabrication process and no drilling or other postprocessing is needed. Channels have been fabricated and tested in the size range of 10 microm x 10 microm-50 microm x 200 microm. Mass spectrometric performance of the tips has been demonstrated with both pressure-driven flow and EOF. SU-8 microtips have been shown to produce stable electrospray with EOF in a timescale of tens of minutes. With pressure driven flow stable spray is maintained for hours. Taylor cone was shown to be small in volume and well defined even with the largest channel cross section. The spray was also shown to be well directed with our tip design.     

Characterization of large,heterogeneous proteins by electrospray ionization-mass spectrometry

Characterization of heterogeneous proteins as large as 150,000 u was performed by a quadrupole mass spectrometer by using electrospray ionization (ESI). We were able to determine not only the molecular weight, but the detailed heterogeneity for the large glycoproteins as well. The successful application was facilitated by the optimization of the instrument in the high mass-to-charge range up to m/z 4000, where the multiply charged envelopes of the 150,000-u glycoproteins were found. For the analysis of clinically important monoclonal antibodies mass spectral data acquired by this method were consistent with the carbohydrate analysis and were useful in resolving the monosaccharide data into glycoform variations. In the case of the characterization of other large, heterogeneous proteins such as elongation factor 3 and bovine serum albumin, the quadrupole ESI mass spectrometer provided adequate mass resolution and high mass measurement accuracy to discern the modification and degradation of the proteins.     

Mass probe-assisted ionization method for total analysis of biomolecules with electrospray ionization-mass spectrometry

In this paper, we review the mass probes used for the derivation of a variety of biomolecules efficiently detected by the electrospray ionization-mass spectrometry and mass probe-assisted ionization method for total analysis and determination by consecutive detection with a single instrument. We describe mass probes for a variety of molecules including proteins, nucleobases, metallic cations, and other small molecules.     

Biases in ion transmission through an electrospray ionization-mass spectrometry capillary inlet

A heated capillary inlet for an electrospray ionization mass spectrometry (ESI-MS) interface was compared with shorter versions of the inlet to determine the effects on transmission and ionization efficiencies for low-flow (nano) electrosprays. Five different inlet lengths were studied, ranging from 6.4 to 1.3 cm. As expected, the electrospray current transmission efficiency increased with decreasing capillary length due to reduced losses to the inside walls of the capillary. This increase in transmission efficiency with shorter inlets was coupled with reduced desolvation of electrosprayed droplets. Surprisingly, as the inlet length was decreased, some analytes showed little or no increase in sensitivity, while others showed as much as a 15-fold gain. The variation was shown to be at least partially correlated with analyte mobilities, with the largest gains observed for higher mobility species, but also affected by solution conductivity, flow rate, and inlet temperature. Strategies for maximizing sensitivity while minimizing biases in ion transmission through the heated capillary interface are proposed.     

A review on the interrogation of peptide-metal interactions using electrospray ionization-mass spectrometry

Over the years, protein interactions have been studied by many techniques to obtain a wide breadth of information. The large size and complexity of the macromolecules have caused difficulties for studying them by some techniques. In some cases, peptides, smaller and less complex biomolecules, have been found to be suitable models to mimic the interactions of entire proteins. The study of peptide-metal interaction, in particular, has proven to be fruitful to researchers across the science fields. One technique in particular, electrospray ionization-mass spectrometry (ESI-MS), has been shown to provide a great deal of information to these studies. The speed, sensitivity, and selectivity of MS, along with the information that can be interpreted from MS-based experiments, has driven this technique to the forefront for understanding the nature of peptide-metal complexes. MS has allowed researchers to identify the stoichiometry of peptide-metal complexes or even mixtures of complexes. The specific amino acids in which the metal cations are bound and the degree of association in these complexes can also be determined by MS experiments. The following review discusses the ESI process and how it is ideal for studying noncovalent interactions between peptides and metals. An investigation of the qualitative and quantitative information that has been determined by ESI-MS follows for readers to realize the versatility of this technique and the diversity of information that can be obtained by a variety of related methods.     

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.     

Investigation of matched and mismatched duplex DNA by electrospray ionization-mass spectrometry

In this research, the gas-phase stabilities of matched and mismatched duplex DNA were investigated by electrospray ionization-mass spectrometry (ESI-MS). The wild-type p53 duplex DNA [ds1, perfectly-matched (PM) DNA] was successfully distinguished from its three mutated DNAs [double-base mismatched DNA (DM)]. Moreover, the three DM DNAs were also well discriminated from each other using ESI-MS. Results show that the gas-phase thermodynamic stability of the DM DNAs decreased as the two mismatch spots moved closer. This implies that the dissociation of DM duplexes into two single strands prefers the mode "from middle to terminals".     

Analysis of oligoguluronic acids with NMR,electrospray ionization-mass spectrometry and high-performance anion-exchange chromatography

Cystine and tyrosine were used as model sorbates to illustrate the design of sorbents with high sorption selectivity using two types of weak interactions that act synergistically. When two types of weak interactions are the driving forces in a sorption and they act synergistically, the second interaction would be effectively intramolecular. The entropy lost for the second interaction should be lower than that for the same interaction that occurs alone, and thus a significant enhancement of sorption should result. We designed an N-acetyl aminomethyl polystyrene resin (N-acetyl HC-D309), which was expected to sorb tyrosine through hydrophobic interaction and hydrogen bonding but not cystine. The chromatographic results for tyrosine and cystine indicate that the separation efficiencies on the N-acetyl HC-D309 column are higher than those on a styrene-divinylbenzene copolymer column, on which sorption should be driven by hydrophobic interaction only, and on an acrylamide-N,N'-methylene bisacrylamide copolymer column, on which sorption should be driven by hydrogen bonding only. Tyrosine as well as cystine had no retention at all on the acrylamide-N,N'-methylene bisacrylamide copolymer column. indicating the hydrogen bonding had little contribution to the sorption when it acted alone. The above results further indicate that hydrophobic interaction and hydrogen bonding contributed to the sorption of tyrosine on N-acetyl HC-D309 and they also acted synergistically. One of the conclusions of this paper is that some weak interactions which contribute little to the sorption when they act alone may contribute to the sorption when they act synergistically with other interactions.     

Investigation of novel sol-gel hydrophobic surfaces for desorption electrospray ionization-mass spectrometry analysis

Sol-gel-based materials were synthesized, characterized and finally tested as solid supports for desorption electrospray ionization-mass spectrometry (DESI-MS) analysis of a mixture of compounds of different polarity. Films with thickness in the 2-4 μm range were obtained by a dip-coating process using tetraethoxysilane (TEOS) and octyltriethoxysilane (OTES) as sol-gel precursors. Three types of surface with different hydrophobic character were obtained by varying the TEOS/OTES ratio in the sol-gel mixture. Each coating was characterized by atomic force microscopy investigations, gaining insight into homogeneity, smoothness and thickness of the obtained films. To study hydrophobicity of each surface, surface free energy measurements were performed. Different DESI-MS responses were observed when different solvent mixture deposition procedures and solvent spray compositions were investigated. Results were finally compared to those obtained by using commercial polytetrafluoroethylene-coated slides. It was found that surface free energy plays an important role in the desorption/ionization process as a function of the polarity of analytes.     

Identification of radiation-induced cross-linking between thymine and tryptophan by electrospray ionization-mass spectrometry

Upon exposure to ionizing radiation, DNA undergoes a variety of modifications including the production of a covalent bond between the nucleobase thymine and aromatic amino acids. In this work, electrospray ionization-mass spectrometry(ESI-MS) was used to identify the gamma radiation-induced covalent cross-linking of model peptides (sequence YPPW and pYPPW) with the nucleobase thymine. Tandem electrospray ionization-mass spectrometry (ESI-MSn) was employed to investigate the cross-linking sites. The results showed that irrespective of whether tyrosine was phosphorylated or not, the nucleobase thymine was cross-linked with the tryptophan residue. Possible cross-linking mechanisms are proposed by investigating the related mass peaks.     

Poly(dimethylsiloxane)-based microfluidic device with electrospray ionization-mass spectrometry interface for protein identification

An easy method to fabricate poly(dimethylsiloxane) (PDMS)-based microfluidic chips for protein identification by tandem mass spectrometry is presented. This microchip has typical electrophoretic microchannels, a flow-through sampling inlet, and a sheathless nanoelectrospray ionization (ESI) interface. The surface of the microchannel was modified with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and the generated electroosmotic flow under acidic buffer condition used for the separation was found to be more stable compared to that generated by the microchannel without modification. The feasibility of the device for flow-through sampling, separation, and ESI-MS/MS analysis was demonstrated by the analysis of a standard mixture composed of three tryptic peptides. Results show that four peaks corresponding to three peptide standards and acetylated products of the standard peptide were well resolved and the deduced sequences were consistent with those expected. Furthermore, the compatibility of this device with other miniaturized devices to integrate the whole process was also explored by connecting a miniaturized enzymatic digestion cartridge and a desalting cartridge in series to the sampling inlet of the microchip for the identification of a model protein, beta-casein.     

Direct analysis of liquid samples by desorption electrospray ionization-mass spectrometry (DESI-MS)

Desorption electrospray ionization-mass spectrometry (DESI-MS) was evaluated for the direct analysis of liquid samples. Several interesting results were found. First, in contrast to the previous DESI analysis of dried solid samples that was limited to proteins with MW ≤25 kDa (Anal. Chem. 2007, 79, 3514), bovine serum albumin (BSA, 66 kDa) was successfully ionized from solutions by DESI with observation of corresponding multiply charged ions. Second, direct DESI analysis of protein tryptic digest solutions without chromatographic separation, sample clean-up, and the sample drying step was demonstrated, providing reasonably good sequence coverage of 52% to 97%. Third, direct analysis of biofluids such as an undiluted urine sample without sample pretreatment is possible, emphasizing the high tolerance of DESI with salt. These results suggest that a charged droplet pick-up mechanism is responsible for desorption and ionization of liquid samples by DESI. Also, unlike in electrospray ionization (ESI), inhibition of electrochemical reduction in the negative ion mode was observed for liquid sample DESI. In addition, reactive DESI can be performed with ion/ion reactions of Zn(II) complexes for the selective binding of phosphoserine in the presence of serine. DESI experiment can also be carried out directly to liquid samples flowing out of a pumped syringe needle tip, allowing rapid analysis. Furthermore, on-line coupling of electrochemical cell with DESI-MS was demonstrated, in which perylene radical cations generated in the cell were successfully transferred to the gas-phase for MS detection by DESI. This study extended the scope of DESI-MS applications, which could have potentials in bioanalytical and forensic analysis.     

高效液相色谱-电喷雾质谱法测定环境大气中的六溴环十二烷

优化了环境大气样品前处理步骤中复合硅胶柱的净化条件,建立了高效液相色谱-电喷雾-质谱(HPLC-ESIMS)测定环境大气中六溴环十二烷(hexabromocyclododecanes,HBCDs)的分析方法。样品经正己烷提取后,采用复合硅胶柱净化,以50 mL正己烷和100 mL正己烷-二氯甲烷(9∶1,v/v)为淋洗液,以180 mL正己烷-二氯甲烷(4∶1,v/v)为洗脱液。采用UF-ODS柱(150 mm×2.1 mm,3.0μm),以乙腈-甲醇-水为流动相进行梯度洗脱,在电喷雾负离子源、选择离子监测(SIM)模式下检测。在优化的条件下,α-HBCD、β-HBCD和γ-HBCD能很好地分离,在1~100μg/L范围内,α-HBCD、β-HBCD和γ-HBCD与进样内标D_(18)-γ-HBCD峰面积的比值与对应的质量浓度均具有良好的线性关系,相关系数(R)≥0.998 8。α-HBCD、β-HBCD和γ-HBCD的仪器检出限(S/N=3)分别为0.4、0.5和0.4μg/L;定量限(S/N=10)分别为1.4、1.6和1.3μg/L;方法检出限(MDL)分别为0.13、0.17和0.13pg/m~3(n=5);实际样品的加标回收率为74.8%~95.8%。该法灵敏度高,选择性好,可以满足大气样品中HBCDs的监测和分析需求。     

Negative mode sheathless capillary electrophoresis electrospray ionization-mass spectrometry for metabolite analysis of prokaryotes

Capillary electrophoresis (CE) was coupled to negative mode electrospray ionisation-mass spectrometry (MS) for separation and detection of phosphorylated and acidic metabolites in extracts of prokaryotes. Unlike previous CE-MS systems for metabolite analysis, a sheathless interface was used to improve sensitivity. To accomplish this, the separation capillary was modified by creating a porous junction near the outlet where the electrospray voltage and cathodic voltage for CE were applied. The outlet of the capillary was pulled to a 5 microm inner diameter to form an electrospray emitter and had a frit fabricated near the exit to prevent clogging. During analysis pressure was applied at the inlet of the separation column to create sufficient flow towards the detector. Limits of detection for 19 metabolites in full scan mode ranged from 20 nM for ADP ribose to 2.5 microM for alpha-ketoglutarate for 40 nL injections. Extracts of Escherichia coli, strain DH5-alpha, were analyzed using this system. In full scan mode, 118 different metabolites were detected. Tandem mass spectrometry was also employed to attempt identification. Reproducible fragmentation of 19 parent peaks was found and 10 of these produced spectra that were consistent with identification obtained from matching to compounds in the MetaCyc database. These results demonstrate the utility of a sensitive CE-MS system for large scale metabolite detection in biological samples.     

Investigation of monovalent and bivalent enantioselective molecular recognition by electrospray ionization-mass spectrometry and tandem mass spectrometry

In this work is described the investigation of bivalent versus monovalent enantioselective molecular recognition in the context of enantioselective separations. Electrospray ionization-mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) are used for evaluating enantioselective systems through the measurement of (1) relative solution-phase binding constants via titration and (2) relative gas-phase binding via collision threshold dissociation. In HPLC, a cinchonane-type chiral stationary phase (CSP) based on tert.-butylcarbamoylquinine provides vastly increased retention and enantioselectivity for separation of bivalent versus monovalent alkoxy-benzoyl-N-blocked leucine enantiomers. The bivalent enantiomers are able to span and simultaneously interact with multiple interaction sites on the CSP surface, leading to enhanced separation. ESI-MS titration measurements also show an increased avidity for binding between bivalent selector and bivalent selectand, compared with the monovalent system. However, enhanced enantioselectivities measured in HPLC for the bivalent system cannot be reproduced by MS due to inherent mechanistic differences. Assumed discrepancies in relative response factors also give rise to systematic errors which are discussed. The results of MS/MS gas-phase experiments show that enantioselectivity is essentially lost in the absence of solvation, but that dissociation thresholds can provide a measure of relative dissociation energy in the bivalent interaction system compared to the monovalent counterpart. Such measurements may prove useful and efficient in better understanding multivalent interactions, in line with current theoretical considerations of effective concentrations and ion trap effects. This is the first application of mass spectrometric methods for assessing increased avidity of binding in multivalent enantioselective molecular recognition.     

Comparison of sheathless and sheath-flow electrospray interfaces for the capillary electrophoresis-electrospray ionization-mass spectrometry analysis of peptides

Capillary electrophoresis coupled to mass spectrometry via an electrospray interface provides a powerful system for separation and characterization of a high number of biomolecules. The present paper describes a home-made sheathless interface and compares it with a commercial sheath-flow interface, using a separation method based on a peptide hormone mixture of therapeutic interest. In a previous work, we optimized the parameters involved in a sheath-flow interface and obtained good results in sensitivity and reproducibility. The sheathless interface is performed with a graphite-coated electrospray ionisation (ESI) tip attached to the separation capillary. We demonstrate that electrolyte composition is the main parameter affecting signal sensitivity and separation resolution. The effect of the nature and concentration of the organic solvent added to the separation electrolyte is carefully studied. Furthermore, a general comparison of both interfaces is made in terms of separation, reproducibility, and sensitivity obtained under the optimized conditions described. Advantages and disadvantages of both coupling setups have been evaluated.