Spraying Electrospraying

Figure 2.2 Schematics of (a) in-line and (b) Z-spray electrospray interfaces. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

Fig. 5.7 AChE-catalyzed hydrolysis of the fluorescent substrate AMQI in volatile buffer monitored by mass spectromet. Line 1 Start of the substrate pump delivering AMQI. Line 2 Start of the enzyme pump delivering AChE. Peak 3 Injection of 0.1 pM galanthamine. Peak 4 Injection of 1.0 pM galanthamine. MS instrument Q-ToF2 (Waters) equipped with a Waters Z-spray electrospray (ESI) source, (a) Mass chromatogram of m/z 288 (galanthamine) (b) mass chromatogram of m/z 104...

Fig. s.n On-line continuous-flow monitoring of biochemical interaction with (a) fluorescence and (b) MS SIM (m/z 390) detection. Fluorescein-biotin (96 nM), streptavidin (32 nM), 20-pL loop injections of 1000 nM biotin (n = 3). MS instrument Q-ToF2 (Waters) equipped with a Waters Z-spray electrospray (ESI) source. Point 1 Carrier pump, protein and reporter ligand pumps... 

In an interesting experimental protocol, Silvestro et al. (1993) utilized HPLC-mass spectrometry with an ion spray (electrospray) interface for determination of PAF and lysoPAF in human PMN (neutrophils). Both unstimulated and stimulated (with complement-activated zymosan) cells were used as starting material. The total lipids were isolated in the usual way, and the PAF was isolated and purified by a combination of thin-layer chromatography, HPLC, and silica chromatography. This final PAF preparation was subjected to a bioassay with the inclusion of 3H 16 0 PAF to monitor recoveries. 

HPLC column was directed into a triple quadropole mass spectrometer equipped with an atmospheric pressure articulated ion spray (electrospray) source. Using MS-MS conditions, dissociation of the parent ions yielded daughter ions comparable to the IM+H]+ of PAF and other molecules, such as O-phosphocholine. Quantitative analyses were obtained by selective ion monitoring. Amounts as low as 0.3 ng of PAF and similar compounds were detected. Interestingly with the lyso derivatives, the limited of detection was 3 ng. The primary alkyl chain lengths present in the neutrophil-derived PAF were 16 0 and 18 0. No mention was made of the presence of any 1-0-long chain acyl analogs. 

Figure 5.7 Z-spray electrospray source. Reprinted with courtesy from Waters.

Figure 9.6. Schematic diagram of the Micromas Z-spray electrospray source. (From ref. [54] Elsevier)...

Keywords Aerodynamic effects Charged droplets Cone jet Droplet evaporation Droplet deformation Electrohydrodynamic spray Electrospray Ion source Mass spectrometry Mass spectroscopy Rayleigh charge limit Spray modes Taylor cone... 

ESI-MS produces ions as a result of the application of a potential to a flowing liquid, which causes the liquid to charge and spray. Electrospray forms very small droplets of solvent containing the analytes. Usually, the solvent is removed by heat and multicharged ions are produced. As previously stated, ESI has the advantage over TSP of producing low fragmentation of flavonoid derivatives. 

Detector MS, PE Sciex API2000 turbo ion spray, electrospray ionization, positive ion mode... 

Schematic diagram of an electrospray inlet/ion source. A spray produced from the high electrical voltage (HT) on the capillary moves toward a hole in the electrical counter electrode. After removal of much solvent, sample ions continue under their momentum through the hole and then through the nozzle and skimmer, where most remaining solvent is removed.

A solution of an analyte in a solvent can be sprayed (nebulized) from an electrically charged narrow tube to give small electrically charged droplets that desorb solvent molecules to leave ions of the analyte. This atmospheric-pressure ionization is known in various forms, the one most relevant to this section being called electrospray. For additional detail, see Chapters 8, 9, and 11. 

The Z-spray inlet causes ions and neutrals to follow different paths after they have been formed from the electrically charged spray produced from a narrow inlet tube. The ions can be drawn into a mass analyzer after most of the solvent has evaporated away. The inlet derives its name from the Z-shaped trajectory taken by the ions, which ensures that there is little buildup of products on the narrow skimmer entrance into the mass spectrometer analyzer region. Consequently, in contrast to a conventional electrospray source, the skimmer does not need to be cleaned frequently and the sensitivity and performance of the instrument remain constant for long periods of time. 

Aerosols can be produced as a spray of droplets by various means. A good example of a nebulizer is the common household hair spray, which produces fine droplets of a solution of hair lacquer by using a gas to blow the lacquer solution through a fine nozzle so that it emerges as a spray of small droplets. In use, the droplets strike the hair and settle, and the solvent evaporates to leave behind the nonvolatile lacquer. For mass spectrometry, a spray of a solution of analyte can be produced similarly or by a wide variety of other methods, many of which are discussed here. Chapters 8 ( Electrospray Ionization ) and 11 ( Thermospray and Plasmaspray Interfaces ) also contain details of droplet evaporation and formation of ions that are relevant to the discussion in this chapter. Aerosols are also produced by laser ablation for more information on this topic, see Chapters 17 and 18. 

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

Many designs of nebulizer are commonly used in ICP/MS, but their construction and mode of operation can be collated into a small number of groups pneumatic, ultrasonic, thermospray, APCI, and electrospray. These different types are discussed in the following sections, which are followed by further sections on spray and desolvation chambers. 

Nebulizers are used to introduce analyte solutions as an aerosol spray into a mass spectrometer. For use with plasma torches, it is necessary to produce a fine spray and to remove as much solvent as possible before the aerosol reaches the flame of the torch. Various designs of nebulizer are available, but most work on the principle of interacting gas and liquid streams or the use of ultrasonic devices to cause droplet formation. For nebulization applications in thermospray, APCI, and electrospray, see Chapters 8 and 11. 

Electrospray uses an electric field to produce a spray of fine droplets. 

A sample to be examined by electrospray is passed as a solution in a solvent (made up separately or issuing from a liquid chromatographic column) through a capillary tube held at high electrical potential, so the solution emerges as a spray or mist of small droplets (i.e., it is nebulized). As the droplets evaporate, residual sample ions are extracted into a mass spectrometer for analysis. 

Z-spray is a novel kind of electrospray that functions as a combined inlet and ion source. Chapter 8 ( Electrospray Ionization ) should be consulted for comparison. 

For conventional electrospray, a solution of an analyte is sprayed from a narrow tube into a region where the solvent and other neutral molecules are pumped away and residual ions are directed into the analyzer of a mass spectrometer. 

The Z-trajectory ensures excellent separation of ions from neutral molecules at atmospheric pressure. In line-of-sight or conventional electrospray sources, the skimmer is soon blocked by ions and molecules sticking around the edges of the orifice. In Z-spray sources, the final skimmer, being set off to one side, is not subjected to this buildup of material. 

Z-spray sources require much less frequent maintenance than do conventional electrospray sources. 

The Z-spray inlet/ion source is a particularly efficient adaptation of the normal in-line electrospray source and gets its name from the approximate shape of the trajectory taken by the ions between their formation and their entrance into the analyzer region of the mass spectrometer. A Z-spray source requires much less maintenance downtime for cleaning. 

Z-spray. Z refers to the approximate shape of the trajectory of particles formed by electrospray ionization... 

Another big advance in the appHcation of ms in biotechnology was the development of atmospheric pressure ionization (API) techniques. There are three variants of API sources, a heated nebulizer plus a corona discharge for ionization (APCl) (51), electrospray (ESI) (52), and ion spray (53). In the APCl interface, the Ic eluent is converted into droplets by pneumatic nebulization, and then a sheath gas sweeps the droplets through a heated tube that vaporizes the solvent and analyte. The corona discharge ionizes solvent molecules, which protonate the analyte. Ions transfer into the mass spectrometer through a transfer line which is cryopumped, to keep a reasonable source pressure. 

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 electrospray process is susceptible to competition/suppression effects. All polar/ionic species in the solution being sprayed, whether derived from the analyte or not, e.g. buffer, additives, etc., are potentially capable of being ionized. The best analytical sensitivity will therefore be obtained from a solution containing a single analyte, when competition is not possible, at the lowest flow rate (see Section 4.7.1 above) and with the narrowest diameter electrospray capillary. 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

An involatile ion-pairing reagent would be deposited in the electrospray interface and lead to a reduction in performance. Some interfaces have been specifically designed to minimize this by removing the line-of-sight between the spray and the entrance to the mass spectrometer, and are thus more tolerant to involatile buffers. The performance of the interface will be improved by the use of volatile alternatives. 

Z-spray An electrospray source in which ions are extracted into the mass spectrometer at 90° to the direction in which the spray is produced. 

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