Differentially pumped interface

Plasma MS is usually based on quadrupole mass analysers. The atmospheric ICP, optimised for ion formation, is placed on its side facing a sample cone (Fig. 4.3). The mass spectrometer operates at reduced pressure and therefore a two- or three-stage differentially pumped interface is needed to transfer the ions from the plasma to the mass analyser. The interface for GC-ICP-MS is generally the same as for ICP emission systems. In one of the earliest GC-MS speciation studies (Chong and Houk, 1987) a packed GC column was used to obtain mass spectra of organic compounds with detection limits in the range 0.001-500 ngs The effects of isotopic fractionation by natural physico-chemical processes were also studied. 

The coupling of an ICP ionization source to a TOF-MS was first reported by Myers and Hieftje [24]. A schematic diagram of their instrument is provided as Fig. 12.9. Similarly to other ICP-MS instruments, it utilized a three-stage differentially pumped interface in order to realize the microtorr pressures required for TOF-MS from mean-free path considerations. This instrument was based on an orthogonal-extraction geometry and utilized a 1.6-m total flight path, which, at a 2-kV acceleration potential, dictated a maximum spectral repetition frequency of 20 kHz. 

Figure 11 - 2 shows schematically the components of a commercial ICPMS system. A critical part of the instrument is the interlace that couples the ICP torch, which operates at atmospheric pressure with the mass spectrometer that requires a pressure of less than lO tnrr. This coupling is accomplished hy a differentially pumped interface coupler that consists of a sampling cone, which is a vsater-coolcd nickel cone with a small... 

Fig. 4.69. Basic design of an ambient-pressure IMS-TOF mass spectrometer. Ions from an electrospray source are entering a desolvation chamber from where packets are pulsed into the ion mobility tube (uneven ion path to indicate diffusion of ions) by means of an ion gate. After ion mobility separation the ions are transferred into the W-type double reflector oaTOF analyzer via a differentially pumped interface. Courtesy of TofWerk AG, Thun, Switzerland. 

Sampling of ions from the atmospheric-pressure region into a two-stage differentially pumped interface, which enables sufficient pressure reduction and transfers the ions to the mass analyser in a high-vacuum chamber. 

A portion of the plasma (which is typically three sampler orifice diameters wide and two sampler orifice diameters deep) is sampled from the central channel of the plasma, and extracted through a differentially pumped interface (through two water-cooled metal cones with orifice diameter 1 mm, the sampler and the skimmer), which is maintained at approximately 1 torr using a mechanical roughing pump. This portion is then transmitted into the mass spectrometer where ions are separated according to their mass-to-charge ratios (m/z) and counted. 

Ions are transferred from the atmospheric pressure ICP into the high vacuum chamber of the mass spectrometer via a differential pumping interface. The ions are formed into a beam... 

Moving-belt (ribbon or wire) interface. An interface that continuously applies all, or a part of, the effluent from a liquid chromatograph to a belt (ribbon or wire) that passes through two or more orifices, with differential pumping into the mass spectrometer s vacuum system. Heat is applied to remove the solvent and to evaporate the solute into the ion source. 

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

Multi-element determination of dissolved metals at ultratrace level may be performed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). U.S. EPA s Methods 200.8 and 1638 present a methodology for measuring trace elements in waters and wastes by the above technique. Sample is acid digested and the solution is introduced by pneumatic nebulization into a radio-frequency plasma. The elements in the compounds are atomized and ionized. The ions are extracted from the plasma through a differentially pumped vacuum interface and separated by a quadrupole mass spectrometer by their mass to charge ratios. The mass spectrometer must have a resolution capability of 1 amu peak width at 5% peak height. 

With the successful implementation of differentially-pumped external ion sources, FTMS is rapidly becoming a routine mass spectrometric technique. Medium-pressure interfaces for the coupling of GC, LC, FAB, and liquid SIMS into the external ionizer are currently under development, and should become available in the near future. 

Thirdly, the use of differential pumping, as a general technique, is examined. It is widely applied in systems where regions having large pressure differentials are interfaced. Specific systems are discussed to indicate the usefulness of the technique. 

Differential pumping is a technique which is widely applied when an interface must be established between systems in which very large pressure differences exist. An excellent example is the coupling of mass spectrometers (normal working pressure 10 5mbar or below) to higher pressure systems such as GC columns (in GC-MS GC columns at 1 bar) or flow tubes (at a few mbar) in kinetics studies or molecular beam sampling. 

Chapter 6 examines what, in the authors opinion, are three important applications of vacuum technology in the chemical sciences. First, its use in chemical technology is clearly defined and, in many applications, the requirement for systems operating below 10 6 Pa is obvious. In both cases, typical systems are considered and quantified. The third topic concentrates on a technique (differential pumping) which is widely used in systems where high- and low-pressure areas must be interfaced. Specific systems are discussed to illustrate the usefulness of the technique. 

A miniature cylindrical ion trap mass spectrometer with APCI and ESI capabilities was developed [22], The system includes a three-stage, differentially pumped vacuum system and can be interfaced to many types of atmospheric pressure ionization sources. 

The interface used today between the atmospheric-pressure plasma and the low-pressure mass spectrometer is based on a differentially pumped two-stage interface similar to those used for molecular beam techniques [89-91]. The key to successful development of ICP-MS instruments was the use of a relatively large ( l-mm-diameter) sampling orifice so that continuum flow was attained with an unrestricted expansion of the plasma to form a free jet. When small orifices were used, a cold boundary layer formed in front of the orifice, resulting in substantial cooling of the plasma, including extensive ion-electron recombination and molecular oxide formation. The smaller orifices were also susceptible to clogging. 

This experiment presents the measurement of uranium with an inductively coupled plasma mass spectrometer (ICP-MS). In this system, a nebulizer converts the aqueous sample to an aerosol carried with argon gas. A torch heats the aerosol to vaporize and atomize the contents in quartz tubes. The atoms are ionized with an efficiency of about 95% by an RF (radiofrequency) coil. The plasma expands at a differentially-pumped air-vacuum interface into a vacuum chamber. The positive ions are focused and injected into the MS while the rest of the gas is removed by the pump. The ions are then accelerated, collected, and measured as a function of their mass. Losses at various stages, notably the vacuum interface, result in a detection efficiency of about 0.1 %, which is still sufficient to provide great sensitivity. The amounts of uranium isotopes in the sample are determined by comparisons to standards. Because different laboratories have different instruments, the instructor will provide instrument operating instmctions. Do not use the instrument until the instructor has checked the instrument and approved its use. 

Scheme of an atmospheric interface with differential pumping system using three stages. 

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