Process mass spectrometer

Mass spectrometry is another widely used technology for hydrogen sensing in the industry. Process mass spectrometers such as gas chromatographs must be safety certified to operate in the plant environment. Mass spectrometers require special air-conditioned... 

Picture of process mass spectrometer. (From EXTEL, Extrel CMS, Pittsburgh, PA. With permission.)... 

AMETEK s ProMaxion process mass spectrometer can monitor up to 32 components. Automated sample switching at the multiport allows unattended analysis of process and calibration gases. Different calibration and analysis methods can be assigned to each sample port. A membrane inlet system is incorporated for ambient gas sampling. The available mass range is 1-200 amu. The detection range is 10 ppm to 100 % with a Faraday cup detector lower LODs are possible with an electron multiplier. 

Figure 9.15 The VC Prima process mass spectrometer (Reproduced by Kind permission of Thermo Electron Corp.).

Figure 19.1 is a block diagram of a typical process analyzer system, consisting of a sample collection and conditioning system, sample manifold, sample inlet, ion source, mass analyzer, detector, and a data analysis and output system that interfaces with the process control system. The dashed line indicates the parts of the overall system that are considered to comprise the analyzer itself (i.e., what is normally included when one purchases a process MS). Figure 19.2 is a photograph of a commercial process MS that incorporates these components. Aspects of these various components are described below, with emphasis on how they are applied in a process mass spectrometer. 

Fig. 19.1 Block diagram of a process mass spectrometer analyzer system.

Fig. 19.2 A commercial process mass spectrometer. Note rotary valve on the upper part of the analyzer, facing the computer. (Photograph used with permission ofThermoOnix Inc.).

These are used on most commerdal process mass spectrometers. Often aim long capillary of 10-100 pm inner diameter is suffident to provide the necessary pressure drop. Deactivated fused siHca is the most common capillary material, since it is reasonably inert and does not exhibit significant memory effects with most sample streams. Heating the capillary further reduces memory effects and pluggage due to condensation. Capillaries can also be made of other materials such as stainless steel or nickel if silica is problematic. Molecular leaks (pinhole orifices) are sometimes used, either in conjunction with or in place of the capillary. Porous frits, either of sintered glass or metal, are sometimes used to avoid pluggage problems with a capillary or molecular leak, but these can often exhibit greater memory effects. 

Modern process MS analyzers are controlled by microcontrollers and PC computers. Many incorporate internal processors that permit stand-alone operation, with a PC only required for initial configuration. The processor then handles all measurement and quantitation, as well as data interfacing, fault diagnosis and alarming, and calibration. Process mass spectrometers can be directly interfaced to plant distributed control systems, programmable loop controllers, or other process control systems. 

Process mass spectrometers are used predominantly for continuous quantitation of compounds that are included in the initial configuration. In the interest of speed, only the mfzs that are required for the analysis are measured. However, one of the most powerful aspects of process MS is the abihty to do qualitative analysis on process streams. This is particularly usefiil for pilot plant operation, or for diagnosing process upsets in scaled-up processes. Several commercial vendors provide instruments that can operate routinely in quantitative mode, then occasionally perform a full mass scan to be archived for future scrutiny if a problem is subsequently detected downstream of the MS sample point. These full mass scans can be compared with library spectra to identify new byproducts. Some systems incorporate rather sophisticated pattern recognition and deconvolution algorithms that can achieve a degree of semiquantitation with no a priori knowledge of sample compo-... 

Online mass spectrometry is very useful for obtaining process information concerning the molecular composition and structure of unknown compounds and for monitoring an analyte in a complex matrix, and has been used in many applications in the analysis of gaseous streams. Very little research has been carried out into the introduction of, and subsequent analysis of, liquid samples into a process mass spectrometer. 

Installing additional accessories in front of the ion source can render analytes amenable to ionization and subsequent mass spectrometric analysis. On-line sample treatment is especially important when analyzing liquid-phase, complex, and/or concentrated samples. For example a thermal vaporizer was used to enable analysis of liquid samples by a process mass spectrometer designed for gas analysis [196], This system has been successfully implemented in the monitoring of an esterification reaction [197]. The obtained data were in a good agreement with those recorded by in-line mid-infrared spectrometry. The setup incorporated a magnetic sector analyzer with two detectors an electron multiplier detector... 

Metastable Peaks. If the mass spectrometer has a field-free region between the exit of the ion source and the entrance to the mass analyzer, metastable peaks m may appear as a weak, diffuse (often humped-shape) peak, usually at a nonintegral mass. The one-step decomposition process takes the general form ... 

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

This chapter briefly discusses the advantages to be gained from the use of transputers in acquiring and processing data from an instrument like a mass spectrometer, which routinely deals with large-scale input and output at high speed. 

Most mass spectrometers for analytical work have access to a large library of mass spectra of known compounds. These libraries are in a form that can be read immediately by a computer viz., the data corresponding to each spectrum have been compressed into digital form and stored permanently in memory. Each spectrum is stored as a list of m/z values for all peaks that are at least 5% of the height of the largest peak. To speed the search process, a much shorter version of the spectrum is normally examined (e.g., only one peak in every fourteen mass units). 

For the naturally occurring elements, many new artificial isotopes have been made, and these are radioactive. Although these new isotopes can be measured in a mass spectrometer, this process could lead to unacceptable radioactive contamination of the instrument. This practical consideration needs to be considered carefully before using mass spectrometers for radioactive isotope analysis. 

A computer must communicate with a variety of peripheral devices (keyboard, mouse, printer, mass spectrometer). A central processing unit (CPU) controls the flow of information to each, rather like a choreographer directing complicated dance routines. 

Powerful mass spectrometer/computer systems can achieve simultaneous foreground/background operation, especially if transputers are used to provide the advantage of parallel processing. 

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