Spray capillary

The flow rate of liquid in the HPLC-electrospray system is paramount in determining performance both from chromatographic and mass spectrometric perspectives. The flow rate affects both the size and size distribution of the droplets formed during the electrospray process (not all droplets are the same size) and, consequently, the number of charges on each droplet. This, as we will see later, has an effect on the appearance of the mass spectrum which is generated. It should also be noted that the smaller the diameter of the spraying capillary, then... 

The loop for the 2nd-D was loaded with the effluent of the 1 st-D at 50 pL/min for 1 min 58 s, and then the injection valve was turned to inject the 100 pL fraction for 2 s onto the 2nd-D HPLC. The flow rate was 5 mL/min, and the valve was turned back for the next loading, resulting in fractionation of the lst-D every 2 min. In this case less than 2% of the effluent from the 1 st-D was wasted during sample injection. The 2nd-D effluent eluted at 5 mL/min from the 2nd-D column, passed through a UV detector, and then was split by using a T-joint at approximately a 1/140 split ratio, resulting in a flow rate of ca. 36 pL/min going into the spray capillary for ESI-TOF-MS detection. 

Fig. 11.7. Illustration of the chip-based Advion nanoESI system. The pictures stepwise zoom in from the pipetting unit to the spray capillary on the silicon chip. By courtesy of G. Schultz, Advion BioSciences, Ithaca, NY.

For static nanoESI-MS connect the static nanospray source (Protana, Odense, Denmark) to the mass spectrometer and use for each sample to be analyzed a new borosilicate spray capillary (ES380 Medium , PROXEON Biosystems AIS, Odense, Denmark see Note 3). 

The interest in ESI nebnlization in LC-MS results from the work of Perm and coworkers [11-13]. In an ESI interface for LC-MS, the coluttm effluent is nebulized into an atmospheric-pressure ion source. The nebuhzation is due to the application of a high electric field resulting from the 3-kV potential difference between the narrow-bore spray capillary, the needle , and a surrounding counter electrode. The solvent emerging from the needle breaks into fine threads which subsequently disintegrate in small droplets. Analyte ions are generated from these droplets by a variety of ionization process (Ch. 6.3). 

The 60° arrangement of the spray capillary in the ion source (Analytica of Bradforc Inc.) avoids contamination of the transfer capillary thus allowing permanent measure ments for days with the turbo pumped vacuum system. Therefore, the mass spectrometei can be immediately used after installation of the automatic sample injector. 

In the case of a negative-ion source setup (spray capillary placed at negative voltage), reduction reactions usually take place with the formation of deprotonated species. 

Spraying capillaries for ESI (a) Nebulization and droplet charging by the electric field only, (b) Pneumatically assisted electrospray or ionspray and (c) Tri-coaxial probe with sheath flow. 

This chapter is written for users of ESI-MS. It presents an account of how it all works. Such understanding is desirable because the observed mass spectra depend on a large number of parameters. These start with a choice of solvent and concentrations of the analyte, choice of additives to the solution that may be beneficial, choice of the flow rates of the solution through the spray capillary, the electrical potentials applied to the spray capillary (also called needle ) and the potentials of ion optical elements that are part of the mass analyzer. Proper choice of these parameters requires not only some understanding of conventional mass spectrometry but also of the electrospray mechanism. In early work on ESI-MS many of these parameters were established by trial and error, but now that a better understanding of the mechanism is at hand more rational choices are possible. The present chapter provides an up to date account of Electrospray. Eor a broader coverage, which is somewhat dated but still relevant, the review by Smith and coworkers is recommended [7]. 

The field when turned on, will penetrate the solution near the spray capillary tip. This will cause a polarization of the solvent near the meniscus of the liquid. In the presence of even traces of an electrolyte, the solution will be sufficiently conducting and the positive and negative electrolyte ions in the solution will move under the influence of the field. This will lead to an enrichment of positive ions on or near the surface of the meniscus and enrichment of negative ions away from the meniscus. The forces due to the polarization cause a distortion of the meniscus into a cone pointing downfield (see Figure 1.1). The increase of surface due to the cone formation is resisted by the surface tension of the liquid. The cone formed is called a Taylor cone (see Taylor [11] and Fernandez de la Mora [12]). If the applied field is sufficiently high. 

Figure 1.3 Different forms of Electrospray at the tip of the spray capillary, (a) cone jet mode. Relationship between radius of droplets and radius of jet RojR] 1-9- (b) and (c). Multijet modes result as the spray voltage is increased, and the flow rate imposed by the syringe is high. (After Cloupeau, Ref [13].)...

The cone-jet mode at the spray capillary tip described and illustrated in Figures 1.1 and 1.3a is only one of the many possible ES modes. For a qualitative description of this and other modes, see Cloupeau [13a-c]. More recent studies by Vertes and coworkers [15] using fast time-lapse imaging of the Taylor cone provide details on the evolution of the Taylor cone into a cone jet and pulsations of the jet. These pulsations lead to spray current oscillations. The current oscillations are easy to determine with conventional equipment and can be used as a guide for finding conditions that stabilize the jet and improve signal-to-noise ratios of the mass spectra. The cone-jet mode is the most used and best characterized mode in the electrospray literature [12, 13]. 

One expects that the reaction with the lowest oxidation potential will dominate, and that the oxidation reaction will be dependent on the material present in the metal electrode, the solutes/ions present in the solution, and the nature of the solvent. Proof of the occurrence of an electrochemical oxidation at the metal capillary was provided by Blades et al. [16]. When a Zn spray capillary tip was used, release of Zn to the solution could be detected. Furthermore, the amount of Zn release to the solution per unit time when converted to coulomb charge per second was found to be equal to the measured electrospray current (J) in amperes (coulomb/s. Figure 1.1). Similar results were observed with stainless steel capillaries [16]. These were found to release Fe " " to the solution. These quantitative results provided the strongest evidence for the electrolysis mechanism. These oxidation reactions introduce ions which were not previously present in the solution (see Eq. (1.2)). However, they also provide an opportunity to generate reactive intermediates that can be studied by mass spectrometry. 

The dependence of the total droplet current produced at the spray capillary on various parameters was given in Eq. (1.7). Relevant to the present discussion is the dependence of the current ) on the square root of the conductivity of the solution. At the low total electrolyte concentrations generally used in ESI, the conductivity is proportional to the concentration of the electrolyte. Thus, if a single electrolyte (E) was present in the sprayed solution, one would expect that the observed peak intensity Ie will increase with the square root of the concentration of that electrolyte (Cg, see Eq. (1.7)). At flow rates higher than that corresponding to the cone jet mode, the dependence on the concentration is lower than the 0.5 power [36]. Because ESI-MS is a very sensitive method, so that detection of electrolytes down to 10 M is easily feasible, one seldom works in practice with a single electrolyte system. The presence of electrolyte ions E leads to two concentration regimes for the analyte A ... 

With nanospray, the spray tip has a much smaller diameter. Also, for the by far most often used nonviscous solutions, the flow is not a forced flow due to a driven syringe as used in ESI (see Figure 1.2a). Instead, the entrance end of the spray capillary is left open and a self-flow results, which is due to the pull of the applied electric field on the solution at the capillary tip (see Section 1.2.2). This self-flow is controlled by the diameter of the tip of the spray capillary. 

An interesting commercially available PEEK mixing tee (Alltech, Figure 5.1) can be seen as a useful microreactor [28, 29]. It can be directly coimected to the ESI spray capillary, allowing reaction times from 0.7 to 28 s in a continuous-flow mode to be... 

Griep-Raming and Metzger [42] studied the thermal dissociation of the triphenyl-methyl dimer and of tetra-(p-anisyl) hydrazine, operating the ESI source as an electrolytic cell to ionize neutral species, for example, the triphenylmethyl radical. In this study, an electrospray ionization source able to heat the spray capillary was used. 

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