Sample inlet systems

Sample introduction system. A system used to introduce sample to a mass spectrometer ion source. Sample introduction system, introduction system, sample inlet system, inlet system, and inlet are synonymous terms. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

A mass spectrometer consists of four basic parts a sample inlet system, an ion source, a means of separating ions according to the mass-to-charge ratios, ie, a mass analyzer, and an ion detection system. AdditionaUy, modem instmments are usuaUy suppUed with a data system for instmment control, data acquisition, and data processing. Only a limited number of combinations of these four parts are compatible and thus available commercially (Table 1). 

As well as the methods discussed above headspace samplers, pyrolyzers, thermal desorbers, and column switching devices could be considered as specialized sample inlet systems. These are treated separately in Chapter 8. 

The essential components of a mass spectrometer include a sample inlet system, an ionization source and acceleration chamber where sample molecules are ionized, fragmented and accelerated into an analyser or separator, and an ion detection and recording system (Figure 9.51(a)). 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

For the purpose of sample introduction, any sample introduction system (also sample inlet system or inlet) suitable for the respective compound can be employed. Hence, direct probes, reservoir inlets, gas chromatographs and even liquid chromatographs can be attached to an El ion source. Which of these inlet systems is to be preferred depends on the type of sample going to be analyzed. Whatever type the inlet system may be, it has to manage the same basic task, i.e., the transfer of the analyte from atmospheric conditions into the high vacuum of the El ion source Table 5.1 provides an overview. 

Caldecourt, V.J. Heated Sample Inlet System for Mass Spectrometry. Anal. Chem. 1955, 27, 1670. 

The values of H and V are known, r is determined experimentally and e is assumed to be unity thus permitting us to determine the mass m. In practice the magnetic field is scanned so that streams of ions of different mass pass sequentially to the detecting system (ion collector). The whole system (Figure 4.1) is under high vacuum (less than 10 Torr) to permit the volatilisation of the sample and so that the passage of ions is not impeded. The introduction of the sample into the ion chamber at high vacuum requires a complex sample inlet system. 

The second box contains a peristaltic pump and a servoactuator. Both the pump and servo are controlled electrically from the sensing head and are powered from the same power supply. The separation of the sensing head from the pump and servo provide electrical and mechanical isolation and address space constraints associated with mounting the system on the autonomous underwater vehicle. The peristaltic pump enables operation at a variable flow rate and has bidirectional flow capability. The servo actuates a movable sample inlet tube that can be raised or lowered by remote control to enable precise positioning of the inlet relative to the source or in the source plume. 

The coupling of a GLC column with the sample inlet system of a mass spectrometer is relatively easy, as the effluents are already in gaseous form. The main problem is the relatively high pressure at which these effluents reach the spectrometer and the excess of carrier gas in the stream. Several experimental devices now allow separation of the sample from the carrier gas, either by an effusion process or with the help of a thin, semi-permeable membrane222,353. The use of capillary columns permits direct insertion of the GLC effluent into the ion source without overtaxing the pumping capacity of the mass spectrometer 311 3 5 5 >3 5 6. 

ESSENTIAL COMPONENTS OF A PREPARATIVE GC SYSTEM. The most important components for the system are (a) the sample inlet system, (b) the column, and (c) the collection system. 

Figure 4.5. Mass chromatograph. (A) Carrier A inlet (S) sample inlet (B) carrier B inlet (V) valve/trap system (C) chromatographic column (D) gas density detector (R) recorder.

Disposable pTAS will be ideal for medical use [14]. However, the high fabrication cost of sophisticated pTAS including micropumps and microvalves is a real problem. One of the basic components of medical pTAS taking this into account is illustrated in Fig. 2. A detector cell consists of micro sensors and a 3-way microvalve is placed at the sample inlet. Flow is controlled by a suction pump and an injection pump connected to the detector cell. The calibration solution flow is also controlled by an individual pump and a 3-way valve. In this system, only sample flow reaches the detector cell. The upper parts of the system are free from contamination and corrosion so that they can be reused many times, while the detector cell has to be disposed of. To realize this system, a 3-way microvalve which can handle whole blood is indispensable. A separable channel type microvalve whose channel part is disposable while actuator part is reusable is useful for the 3-way microvalve of the detector cell [15]. Mechanically fixed stack structures including disposable parts are useful in many medical pTAS. 

FIGURE 3.7 Diagram of the microfabricated electroosmotic pumping system. (1) Open-channel electroosmotic pump, (2) micropump inlet reservoir, (3) micropump outlet reservoir, (4) double-T sample injection element, (5) channel for sample infusion or separation, (6) sample inlet reservoir, (7) sample waste reservoir, (8) channels for sample inlet, (9) channels for sample outlet, (10) ESI emitter to a MS detector. The inset shows an expanded view of the micropump outlet reservoir (3) containing the porous glass disk [115]. Reprinted with permission from the American Chemical Society. 

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