Nozzle-skimmer system

The momentum separator can be considered as a two-stage differentially-pumped nozzle-skimmer system, thus featuring a nozzle, a skimmer, and a colhmator. In this respect, the FBI resembles an atmospheric-pressure ionization system (Ch. 5.4). However, in the FBI, the analyte is sampled from a closed desolvation chamber, and the analyte is transferred to the high-vacuum region prior to ionization Generally, little attention is paid to the design characteristics of the momentum separator with respect to optimized molecular-beam performance. 

An electrodynamic ion furmel interface, positioned between the outlet of a heated stainless-steel capillary and an RF-only octapole ion guide, thus replacing the skimmer, was described by Shaffer et al. [41-42]. The ion furmel interface consists of a series of ring electrodes with decreasing internal diameter. Both RF and DC voltages are apphed to these electrodes. The device provides a more effective focussing and ion transmission. Abont ten-fold sensitivity improvement over a conventional nozzle-skimmer system was demonstrated. 

In early GC with packed columns several tens of milliliters of carrier gas per minute were eluted. Thus, most of the flow had to be separated before entering the ion source to prevent the mass spectrometer vacuum system from breakdown [4,33,38]. Flow reduction was either effected by a simple split to divide the effluent in the inlet system by a factor of about 1 100 or by means of a more elaborate separator, the jet separator being the best-known of those [57,58]. The advantage of separators over a simple split is the enrichment of the analyte relative to the carrier gas in case of the jet separator this effect is based on a principle related to that of the nozzle-skimmer system in ESI (Chap. 12.2.1). 

Ions, formed in the source, are transported into the high-vacuum system of the mass spectrometer by the use of a nozzle-skimmer arrangement. This acts as a momentum separator and heavier sample molecules tend to pass through, while lighter solvent and drying gas molecules can be more readily pumped away in this... 

Fig. 1. Schematic diagram of the multimass ion imaging detection system. (1) Pulsed nozzle (2) skimmers (3) molecular beam (4) photolysis laser beam (5) VUV laser beam, which is perpendicular to the plane of this figure (6) ion extraction plate floated on V0 with pulsed voltage variable from 3000 to 4600 V (7) ion extraction plate with voltage Va (8) outer concentric cylindrical electrode (9) inner concentric cylindrical electrode (10) simulation ion trajectory of m/e = 16 (11) simulation ion trajectory of rri/e = 14 (12) simulation ion trajectory of m/e = 12 (13) 30 (im diameter tungsten wire (14) 8 x 10cm metal mesh with voltage V0] (15) sstack multichannel plates and phosphor screen. In the two-dimensional detector, the V-axis is the mass axis, and V-axis (perpendicular to the plane of this figure) is the velocity axis (16) CCD camera.

The laboratory layout consists of a molecular beam apparatus and a laser system. NaK clusters are created in an adiabatic coexpansion of mixed alkali vapour and argon carrier gas through a nozzle of 70 pm diameter into the vacuum. Directly after the nozzle the cluster beam passes a skimmer. Next, the laser beam coming from perpendicular direction irradiates the dimers and eventually excites and ionizes them. The emerging ions are extracted by ion optics, mass selected by QMS and recorded by a computer. 

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