Control of electrochemical reactions at the capillary electrophoresis outlet/electrospray emitter electrode under CE/ESI-MS through the application of redox buffers

It was found that combining capillary electrophoresis (CE) and electrospray ionization mass spectrometry (ESI-MS) overlays two controlled current techniques to form a three-electrode system (CE inlet, CE outlet/ES emitter, and MS inlet electrodes) in which the CE outlet electrode and the ES emitter electrode were shared between the CE and the ESI-MS circuits. Depending on the polarities and magnitudes of the voltages at the CE inlet, CE outlet/ES emitter, and MS inlet electrodes, the nature of the two redox reactions at the shared electrode was the same or different (both reduction, both oxidation, or one oxidation and the other reduction). Several redox buffers were introduced for controlling electrochemical reactions at the shared electrode. By reacting at this electrode, redox buffers were able to maintain electrode potentials below the onset of water electrolysis, thereby eliminating gas bubble formation and/or pH drift. The volume of the gas generated due to water electrolysis was used to quantitate water oxidation or reduction at this electrode. Two types of redox buffers were used. A reactive electrode with an oxidation potential below that of water was used as the electrode under anodic conditions. Also, a reactive compound with a redox potential below that of water was added to the CE and/or ESI running buffer. When the shared electrode was the anode of both CE and ESI-MS circuits, the use of iron or etched and sanded stainless steel (ss) wire, instead of platinum wire, suppressed bubble formation at the shared electrode. Under these conditions, corrosion of the Fe wire and formation of Fe2+ replaced oxidation of water, eliminating O2 gas bubble and H+ formation. When mixtures of peptides were analyzed, iron adducts of peptides were observed. For a fresh wire, however, the intensities of adduct ions were less than 3% of the protonated molecules. After a few days of operation, the intensities of the adduct ions increased to approximately 50%, due to rust formation on the Fe wire. On-column rinsing with a 40% solution of citric acid rejuvenated the Fe wire and reduced the adduct peak intensities to less than 3%. Unmodified ss wire did not quench bubble formation, which was attributed to its passivated surface. When Fe, ss, and Pt wires were used as the shared electrode under forward polarity CE and positive ESI mode, where the shared electrode acted as a cathode with respect to CE inlet and as an anode with respect to MS inlet, reduction of water at the cathodic end of the electrode and, in the case of ss and Pt wires, oxidation of water at the anodic end of the shared electrode produced a significant amount of bubbles. Under these conditions, however, a buffer containing 50 mM p-benzoquinone completely suppressed both cathodic reduction and anodic oxidation of water for CE currents up to 4 microA. Reduction of p-benzoquinone at the cathodic end of the shared electrode to hydroquinone, and oxidation of this hydroquinone at the anodic end of the electrode, replaced reduction and oxidation of water, eliminating bubble formation. A 0.1% acetic acid solution saturated with I2 was also found to suppress bubble formation at the cathode for CE currents up to 3 microA; however, strong iodine adduct ions were observed under CE/ESI-MS when a mixture of peptides was analyzed. The application of iron as an in-capillary electrode for the analysis of a peptide mixture and a protein digest demonstrated a high separation efficiency similar to when hydroquinone was used as a redox buffer.     

Analysis of thin-wire ground penetrating radar systems for buried target detection, using a hybrid MoM-FDTD technique

A hybrid method combining the finite difference time domain (FDTD) method and the method of moments (MoM) in the frequency domain is proposed to model the electromagnetic behavior of a realistic ground penetrating radar (GPR) system. The GPR is a non-destructive testing (NDT) technique. It consists of a broadband thin-wire vee-dipole antenna located in the vicinity of a lossy ground containing unexploded ordnances. In the solution of the problem, we identify two sub-problems, namely, the antenna problem and the ground buried object problem. Capabilities of the MoM for solution of the governing thin-wire electric field integral equation of the antenna problem and general FDTD solution of Maxwell's equations posed by the scattering of arbitrary shape objects in multilayered media are combined to efficiently and accurately simulate the electromagnetic operation of the system. The proposed simulation technique is validated against the original MoM solution of the problem. It is shown that the proposed hybrid technique exhibits superior performance based on both computation time and numerical accuracy compared with the conventional FDTD and MoM solution techniques. It is also shown that the position and size of the target, as well as the electromagnetic characteristics of the target, can be determined using a simple signal processing technique.     

A novel simulation approach for particulate flows during filtration

This paper presents a novel approach for simulation of filtration process when velocity gradient within pore space cannot be neglected. The new model is useful for accurate prediction of the filtration performance and particle retention efficiency. Artificial porous media such as filters, by design, have a large surface-to-volume ratio because of an inherent homogeneity present within their structure; the homogenous structure is realized due to organized packing of grains as building blocks, which leads to a significant velocity gradient inner pore space. In this work, the inner-pore flow characteristics of two different homogeneous packing patterns (cubic and oblique hexagonal packing) were examined. The multiple constricted tubes analogy was adopted to model porous media to simplify the inner-pore geometrical structure. A new integrated simulation approach was utilized through implementing the particle trajectory model to every unit bed element of the simulation domain. The accuracy of the numerical simulations used in this study was verified by comparing the particle distribution pattern and penetration depth obtained from simulations to those monitored via a visual experiment. A sensitivity analysis was carried out to study parameters that may affect the particle distribution and penetration length, such as grain-to-particle size ratio, flow rate, and fluid viscosity. The simulation method utilized in this paper provides an in-depth understanding of the fine particle migration during filtration process through artificial porous media, and, thus, provide useful insights for improved filtration design.     

A fuzzy alignment approach to sizing surface cracks by the AC field measurement technique

In real world applications, one major issue with most of non-phenomenological methods is the need for a large and complete databanks. In most practical cases, it is impossible to obtain a complete and representative database that includes sufficient number of representative examples. This fact renders most of available defects and cracks databases useless. In this paper, an aligning method is formalized by a fuzzy recursive least square algorithm as a learning methodology for electromagnetic alternative current field measurement (ACFM) probe signals of a crack (data). This method along with a set of fuzzy linguistic rules, including adequate adaptation of different crack shapes (knowledge) for combining knowledge and data whenever the superposition theory can be utilized, provide a means to compensate for the lack of sufficient samples in available crack databases. We have shown that the combination of this fuzzy inference method and the method of the adaptation for different crack shapes provides sufficient means as a priori empirical knowledge for the training system. This approach significantly reduces the need for a large and complete crack databases. The validity of the proposed methodology is demonstrated by examining the sizing errors in the case of several surface cracks with elliptical depth profile when inverting their respective ACFM signals.     

Using a wavelet network for reconstruction of fatigue crack depth profile from AC field measurement signals

This paper presents a wavelet-network-based technique for reconstructing the crack depth profile of a fatigue crack in a metal from the output signal of an alternating current field measurement (ACFM) probe. The main feature of this technique is that it requires only the ACFM probe output signals along the crack opening. The database for training the network is established by developing a random crack-depth generator, using a fast pseudo-analytic ACFM probe output simulator. The validity of the proposed technique is demonstrated by comparing the actual and reconstructed depth profiles of several simulated and machine-made cracks with no predetermined geometries.     

Numerical simulation of electromagnetic fields radiated by lightning return stroke channels: a wavelet-based approach

The antenna theory model is widely employed to numerically simulate the propagation of current wave along lightning return stroke channels and compute the radiated electromagnetic fields. In this model, the channel is approximated as a vertically straight or a horizontally bent thin-wire antenna above perfectly conducting ground for which the numerical solution of the governing electric field integral equation in frequency domain by the conventional method of moment is prohibitively slow. This paper proposes an efficient algorithm to substantially reduce the computation time of the numerical process for the entire frequency components of the excitation current. In this algorithm, a class of predefined wavelet packet transform is first used to effectively sparsify the resulting moment matrix equations. A proper iterative solver is then utilized to take the full advantages of manipulatory sparse matrices. To accelerate the construction of the original moment matrix, the reciprocal closed-form mutual impedance of sinusoidal electric dipoles and the symmetry of the model are fully exploited. A good agreement is observed with computed data found in technical literature while the overall computational time is reduced remarkably.     

A provable secure pairing-free certificateless identification scheme

Certificateless identification (CLI) schemes offer an alternative solution to the certificate management problem of traditional identification schemes, as well as remove the key escrow from key generation, an inherent property of identity-based identification. In this paper, we provide a pairing-free CLI scheme, provable secure against passive and active/concurrent attacks for both Type-1 and Type-2 adversaries. This shows that our scheme is computationally efficient because no bilinear pairings are involved.     

Analysis of complex protein mixtures with improved sequence coverage using (CE-MS/MS)n

Identification of proteins, in a complex protein mixture, using one-dimensional high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) analysis of its digest, usually suffers from low sequence coverage. There are several reasons for the low coverage including undersampling, wide concentration dynamic range of the proteins in a complex protein mixture, and wide range of electrospray ionization efficiency of peptides under each mobile-phase composition. To address this low sequence coverage, we introduce a novel technique, (CE-MS/MS)n, which utilizes the most significant advantages of CE-MS/MS, including economy of sample size, fast analysis time, and high separation efficiency, to increase the sequence coverage of a complex protein mixture. Based on these characteristics, (CE-MS/MS)n can be performed in which multiple CE-MS/MS subanalyses (injections followed by analyses) are analyzed and experimental variables are manipulated during each CE-MS/MS subanalysis in order to maximize sequence coverage. (CE-MS/MS)n is a practical technique since each CE-MS/MS subanalysis consumes <10 nL, and each CE-MS/MS subanalysis takes approximately 10 min; therefore, several subanalyses can be performed in approximately 1 h consuming only nanoliters of the sample. Two techniques have been introduced to address the undersampling: (1) (CE-MS/MS)n using dynamic exclusion. In this technique, several CE-MS/MS analyses (injection followed by separation) were performed in one run using the dynamic exclusion capability of the mass spectrometer until all peptide peaks were analyzed by MS/MS. (2) Gas-phase fractionation. In this technique, (CE-MS/MS)n is performed by scanning a narrow mass range (every approximately 100 m/z) during each CE-MS/MS subanalysis without using dynamic exclusion. Under this condition, in each subanalysis, the number of peptides available for MS/MS analysis is significantly reduced, and peptides with the same nominal masses are analyzed, thereby increasing sequence coverage. Additionally, to address the lack of detection of low-level peptides in a mixture containing a wide concentration dynamic range, the concentration of the sample was systematically increased in each subanalysis (while utilizing dynamic exclusion) so that low-intensity peptides would rise above the mass spectrometer threshold and, consequently, undergo MS/MS analysis. Moreover, to alter the ionization efficiency of peptides with low electrospray ionization efficiency, and to change the migration behavior of comigrating peptides under a specific liquid composition, the CE background electrolyte was modified in several subanalyses to further improve sequence coverage. The combination of the above-mentioned techniques was applied to the analysis of the tryptic digests of three well-characterized protein mixtures: a six-protein mixture with average MW of approximately 26,000 (standard I), a six-protein mixture with an average MW approximately 49,000 (standard II), and a more complex protein mixture containing 55 proteins (E. coli ribosomal proteins). In approximately 1 h, when the MS/MS of the peptides were manually checked, all peptides that produced peaks under electrospray ionization in the scanned range of the analysis (500-2000 m/z) and within the practical fragmentation capability of the MS (peptides with MW <3500) were identified for standard I by consuming only 200 fmol of each protein. When searched against a Swissprot database, the average sequence coverage for the standard I, II, and E. coli's ribosomal proteins were 57, 34, and 15%, respectively.     

Analysis of carbonic anhydrase in human red Blood cells using capillary electrophoresis/ electrospray ionization-mass spectrometry

Capillary electrophoresis/electrospray ionization-mass spectrometry (CE/ESI-MS) was applied to the analysis of human red blood cells (RBCs) using the split-flow technique for interfacing CE to MS. By using a long (approximately125-cm) and narrow (approximately 15-microm-i.d.) capillary, the four major proteins of the RBC, which are hemoglobin (Hb, alpha- and beta-chains, 900 amol/chain), carbonic anhydrase I (CAI, approximately 7 amol/cell), and carbonic anhydrase II (CAII, approximately 0.8 amol/cell), were separated from each other and detected at low-attomole levels in one run and minimal sample preparation. Under these conditions, the detection limits for CAI and CAII in lysed RBCs were approximately 20 and approximately 44 amol, respectively. The approximately 20-amol detection limit of CAI was confirmed by the CE/ESI-MS analysis of three intact RBCs that had been drawn into the capillary under a microscope. A shorter capillary (approximately 55 cm long) provided faster analysis time but did not separate CAII from the beta-chain of hemoglobin, causing the CAII signal to be masked by the background chemical noise generated by the approximately 1,000 x molar excess of the beta-chain. Under this condition, the CAII detection limit increased to approximately 500 amol. From three methods of sample introduction (injection of lysed blood, injection of intact cells under microscope, and injection of intact cells suspended in saline solution), injection of lysed blood provided the optimum sensitivity. It was found that a background electrolyte (BGE) containing 0.1% acetic acid in water worked best for the analysis of intact cells, while a BGE containing 0.1% acetic acid in water + acetonitrile (50/50 by volume) worked best for the analysis of lysed blood.     

Experimental investigation on solid dispersion,power consumption and scale-up in moderate to dense solid–liquid suspensions

Detailed particle concentration distribution in dense solid–liquid suspension was measured by means of fiber optic probes. The effect of solid loading, impeller speed, and impeller type and clearance was investigated. Results were compared with modeling approaches to show the accuracy of sedimentation–dispersion model and its capability to describe complex phenomena taking place in dense liquid–solid mixing systems. Variation of power numbers by changing impeller clearance and solid loading were also investigated. It was shown that the impeller power number for a slurry system exhibited different trends in a moderate or dense liquid–solid system. In addition, scale-up rules to achieve the same level of homogeneity on a large scale as the laboratory scale were evaluated.