Electrospray Operating Research Articles

Development of a liquid chromatography–electrospray mass spectrometric method for the simultaneous analysis of benzoxazolinones and their degradation products

A new method for the simultaneous analysis of some benzoxazolinones, aminophenoxazinones and malonamic acids was developed based on liquid chromatography (LC) coupled to mass spectrometry (MS), using electrospray ionization (ESI) and operating in positive mode. Different ESI-MS parameters, such as fragmentor voltage, capillary voltage, drying gas flow, nebulizer gas pressure and drying gas temperature, were optimized in order to obtain structural information and to achieve maximum sensitivity. Chromatographic separation was performed by a reversed-phase LC column using a linear gradient of water and methanol. Quality assurance of the developed method was assessed by measuring parameters as linearity, sensitivity, repeatability and reproducibility. Quantification method based on the use of internal standard was developed, selecting synthetic 2-methoxy-2H-1,4-benzoxazin-3(4H)-one as internal standard. Good correlations were obtained for all analytes relative to this compound in the range of 0.05–1.5 ng/μL. Instrumental detection limits were between 0.02 and 0.2 ng/μL. Repeatability and reproducibility studies showed acceptable coefficient of variation values.

Charge competition and the linear dynamic range of detection in electrospray ionization mass spectrometry

An experimental investigation and theoretical analysis are reported on charge competition in electrospray ionization (ESI) and its effects on the linear dynamic range of ESI mass spectrometric (MS) measurements. The experiments confirmed the expected increase of MS sensitivities as the ESI flow rate decreases. However, different compounds show somewhat different mass spectral peak intensities even at the lowest flow rates, at the same concentration and electrospray operating conditions. MS response for each compound solution shows good linearity at lower concentrations and levels off at high concentration, consistent with analyte “saturation” in the ESI process. The extent of charge competition leading to saturation in the ESI process is consistent with the relative magnitude of excess charge in the electrospray compared to the total number of analyte molecules in the solution. This ESI capacity model allows one to predict the sample concentration limits for charge competition and the on-set of ionization suppression effects, as well as the linear dynamic range for ESI-MS. The implications for quantitative MS analysis and possibilities for effectively extending the dynamic range of ESI measurements are discussed.

Current/flow-rate characteristic of an electrospray with a small meniscus

A relation between the electric current and the flow rate of an electrospray operating in the cone–jet mode is proposed for the case of a very small meniscus, a long stationary jet, and a flow with important inertial effects. The finite size of the meniscus then affects the bulk-to-surface current transfer process, though the meniscus is still much larger than the drops of the spray. The result differs from the square-root law which is well-established for large menisci or very viscous flows.

Controlled Electrospray for Tissue Engineering Applications

AbstractThe development and status of a novel electrospray system, capable of a variety of applications, including the fabrication of tissue engineered structures is described. The system requirements dictate the following key components: a micro-fabricated silicon electrospray emitter, a mass spectrometer capable of selecting molecules having a charge to mass ratio up to 32, 000 with a resolution of 10, 000, and an integrated ion optics system permitting depositing the selected molecules ‘softly’ onto a target with a spatial resolution of 0.1μm. One critical feature of the system is the overall efficiency of transferring biomolecules from a buffer solution, typically comprising water, acetonitrile and ammonium acetate, into an ion flux. Commercial ES/MS systems are generally designed with simplicity of operation as a design driver, together with the low probability of transmitting spray droplets directly into the quadrupole mass selection system. This has focused attention onto the detailed design and characterization of the electrospray emitter and the environment into which the spray is launched. This has highlighted the need to evaluate in detail, an electrospray operating into a low pressure environment and to improve our understanding of the interaction between voltage and flow rate in the transition from nano-electrospray mode, wherein no positive pressure is applied to the fluid and normal electrospray mode, close to minimum flow rate. Early results of this investigation are presented. There is evidence that at reduced ambient pressure there is a small reduction in spray current, however this is offset by improved transmission of the beam into the first quadrupole stage.

A micromachined chip-based electrospray source for mass spectrometry.

A micromachining process is described for fabricating a mass spectrometry electrospray source on a silicon chip. The process utilizes polymer (parylene) layers to form a system of chambers, filters, channels, and hollow needle structures (electrospray emitters) that extend more than a millimeter beyond the edge of the silicon substrate. The use of photoresist as the sacrificial layer facilitates the creation of long channels. Access to the channel structures on the chip is through a port etched through the silicon substrate that also serves as a sample reservoir. A reusable chip holder consisting of two plastic plates and an elastomer gasket provides the means to mount the chip in front of the mass spectrometer inlet and make electrical and gas connections. The electrospray emitters have tapered tips with 5 microns x 10 microns rectangular openings. The shape of the tip can be varied depending on the shape of the mask used to protect the parylene structures during the final plasma etch. The parylene emitters are physically robust and require only a high electric field to achieve stable electrospray operation over a period of a few hours. Direct comparisons with conventional glass or fused silica emitters indicated very similar performance with respect to signal strength and stability, spectral quality, and endurance. The automated MS/MS analysis of a mixture of tryptic peptides was no more difficult and yielded nearly identical results as the analysis of the same sample using a conventional nanospray device. This work demonstrates that an efficient electrospray interface to mass spectrometry can be integrated with other on-chip structures and mass-produced using a batch process.

Experimental Investigation of Scaling Laws for Electrospraying: Dielectric Constant Effect

ABSTRACT Experiments were performed to investigate the effect of liquid dielectric constant on existing scaling laws for the electrospraying process. The variations of the droplet size and the emitted current were measured as a function of the dielectric constant for the electrospray operating in the cone-jet mode. Eight different solvents with dielectric constants, κ, ranging from 12.5 to 182 were tested. The residue particle size distributions were measured using a TSI scanning mobility particle sizer, (SMPS). The produced liquid droplet sizes were then calculated from the known solution concentrations. The results show that: (1) For the produced droplet size, Dd , experimental data are in agreement with the scaling law (G(κ) = 1.66κ −1/6) proposed by Ganan-Calvo et al. (1994) for solvents with high dielectric constants. The derivation in low dielectric constant cases may be that the assumption of Ganan-Calvo (1994) on the characteristic length may not hold for the present system; (2) for the emitted cu...

Sensitivity enhancement of electrospray ionization-MS for aqueous solutions in the corona-assisted cone-jet mode

An experimental study is reported of the sensitivity obtained for the electrospray ionization of aqueous solutions in two different cone-jet modes. Six different polypeptides varying in molecular weight from ~600 to ~ 12 000 Da were analyzed using a quadrupole mass spectrometer. The sensitivity comparison suggests that more than an order of magnitude enhancement in signal intensity can be achieved in some cases by simply shifting the electrospray operation from the classic cone-jet mode to the corona-assisted cone-jet mode. The sensitivity enhancement in the corona mode spray appears to be independent of both liquid flow rate and sample molecular weight, suggesting that droplet field charging by the corona discharge is the dominant process. Comparison at the field charging limit and the Rayleigh limit indicates that the droplet charge can be effectively increased to the field charging limit for corona mode electrosprays of aqueous solutions at low flow rate.

A CE/ESI-MS interface for stable, low-flow operation.

A rugged, stable liquid sheath interfacing procedure for on-line CE/MS is described. This procedure combines optimized component sizes, tapered capillary tips, and adjustment of the capillary with respect to the liquid sheath tube during electrospray operation (active capillary positioning) to establish a stable electrospray quickly and reliably. The interface is especially effective at low flow rates, and CE/MS with a liquid sheath flow rate of 250 nL/min has been achieved. Active capillary positioning also allows on-line capillary isoelectric focusing mass spectrometry with in-probe focusing. In this application, the capillary is retreated into the liquid sheath tube, which becomes a microreservoir. After focusing, the capillary is returned to its proper position for electrospray, and focused zones are mobilized into the mass spectrometer for on-line detection. Both active capillary positioning and in-probe focusing readily lend themselves to automation.

Atmospheric pressure ionization liquid chromatography–mass spectrometry in the quantification of drugs in biological fluids

The application of atmospheric pressure ionization liquid chromatography–mass spectrometry to the analysis of drugs is described. The techniques are electrospray and atmospheric pressure chemical ionization. Examples are shown of quantitative analysis with electrospray operating in the ng ml–1 range and atmospheric pressure chemical ionization in the pg ml–1 range.

Charge and fission of droplets in electrostatic sprays

Measurements of the charge and size of heptane droplets generated by electrostatic sprays showed that the droplet charge-to-volume ratio is a monotonically decreasing function of size. In the useful range of electrospray operation, characterized by droplets smaller than the size of the orifice from which the liquid is issued, it was found that the larger were the droplets the closer they were to the Rayleigh limit. In particular, when droplets had charging levels between 70% and 80% of such limit, they were observed to rupture because the repulsive force due to surface charge evidently overcame surface tension. The rupture phenomenon, here termed Coulomb fission, was also captured in microphotographs that typically showed a droplet with one or two, diametrically opposed, conical protrusions terminating in a fine jet ejecting a stream of much smaller, apparently equisized offsprings. The process appeared swift and, yet, well ordered, quite different from the common view of a violent, convulsive explosion. Corroborating evidence on the disruption pattern was also gathered by quantitative measurements of the evolution of the droplet size distribution in evaporating sprays using phase Doppler anemometry (PDA). Implications of these findings are finally discussed in the context of a particular application of electrostatic sprays, electrospray ionization, a technique that is revolutionizing the mass-spectrometric analysis of large biomolecules.

Electrospray LC/MS--can it be used to determine lower molecular weight molecules?

A brief description of electrospray theory and operation is presented with emphasis on the determination of lower molecular weight molecules.