Closed ion source,

In order to curb - or avoid entirely - influences w/hich could stem from the sensor chamber or the cathode (e.g. disturbance of the CO-CO2 equilibrium by heating the cathode) a closed ion source (CIS) w/ill be used in many cases. 

Experimental evidence for the distortion of a kinetic growth curve due to a closed ion source was reported by Martinez et al. [42]. In an FPTRMS investigation of the infrared laser multiphoton dissociation of CF2HC1, these authors found that the risetime of the HC1 molecular elimination product, which was expected to be formed on a microsecond time scale, was approximately 2 ms. This they attributed to holdup in the ion source, which was a partially enclosed box. Gas entered the ion source chamber through a hole at one end and exited through several holes at either end or at its sides. 

It is useful to examine the consequences of a closed ion source on kinetics measurements. We approach this with a simple mathematical model from which it is possible to make quantitative estimates of the distortion of concentration-time curves due to the ion source residence time. The ion source pressure is normally low enough that flow through it is in the Knudsen regime where all collisions are with the walls, backmixing is complete, and the source can be treated as a continuous stirred tank reactor (CSTR). The isothermal mole balance with a first-order reaction occurring in the source can be written as... 

Ions formed in an electrospray or similar ion source are said to be thermolized, which is to say that their distribution of internal energies is close to that expected for their normal room-temperature ground state. Such ions have little or no excess of internal energy and exhibit no tendency to fragment. This characteristic is an enormous advantage for obtaining molecular mass information from the stable molecular ions, although there is a lack of structural information. 

As m increases, At becomes progressively smaller (compare the difference between the square roots of 1 and 2 (= 0.4) with the difference between 100 and 101 (= 0.05). Thus, the difference in arrival times of ions arriving at the detector become increasingly smaller and more difficult to differentiate as mass increases. This inherent problem is a severe restriction even without the second difficulty, which is that not all ions of any one given m/z value reach the same velocity after acceleration nor are they all formed at exactly the same point in the ion source. Therefore, even for any one m/z value, ions at each m/z reach the detector over an interval of time instead of all at one time. Clearly, where separation of flight times is very short, as with TOF instruments, the spread for individual ion m/z values means there will be overlap in arrival times between ions of closely similar m/z values. This effect (Figure 26.2) decreases available (theoretical) resolution, but it can be ameliorated by modifying the instrument to include a reflectron. 

In order to avoid such uncontrolled collisional activation, we chose to use apparatus which is closely related to the ion-source reaction chambers developed for thermal ion-molecule equilibria (see preceding Section A). In fact, sources like those shown in Figures 4 and 7 are well suited for providing thermalized ions however they provide somewhat low ion intensities, typically some 50,000 counts/s of a given major ion in a mass spectrum after mass analysis. However, such intensities are completely sufficient for CID threshold measurements and the source... 

The first part of this book is dedicated to a discussion of mass spectrometry (MS) instrumentation. We start with a list of basic definitions and explanations (Chapter 1). Chapter 2 is devoted to the mass spectrometer and its building blocks. In this chapter we describe in relative detail the most common ion sources, mass analyzers, and detectors. Some of the techniques are not extensively used today, but they are often cited in the MS literature, and are important contributions to the history of MS instrumentation. In Chapter 3 we describe both different fragmentation methods and several typical tandem MS analyzer configurations. Chapter 4 is somewhat of an outsider. Separation methods is certainly too vast a topic to do full justice in less than twenty pages. However, some separation methods are used in such close alliance with MS that the two techniques are always referred to as one combined analytical tool, for example, GC-MS and LC-MS. In effect, it is almost impossible to study the MS literature without coming across at least one separation method. Our main goal with Chapter 4 is, therefore, to facilitate an introduction to the MS literature for the reader by providing a short summary of the basic principles of some of the most common separation methods that have been used in conjunction with mass spectrometry. 

The most straightforward tool for the introduction of a sample into a mass spectrometer is called the direct inlet system. It consists of a metal probe (sample rod) with a heater on its tip. The sample is inserted into a cmcible made of glass, metal, or silica, which is secured at the heated tip. The probe is introduced into the ion source through a vacuum lock. Since the pressure in the ion source is 10-5 to 10-6 torr, while the sample may be heated up to 400°C, quite a lot of organic compounds may be vaporized and analyzed. Very often there is no need to heat the sample, as the vapor pressure of an analyte in a vacuum is sufficient to record a reasonable mass spectrum. If an analyte is too volatile the cmcible may be cooled rather than heated. There are two main disadvantages of this system. If a sample contains more than one compound with close volatilities, the recorded spectrum will be a superposition of spectra of individual compounds. This phenomenon may significantly complicate the identification (both manual and computerized). Another drawback deals with the possibility of introducing too much sample. This may lead to a drop in pressure, ion-molecule reactions, poor quality of spectra, and source contamination. 

Fig. 4.2. Schematic of a simple ion source. The ionizing event is preferably located close to the charged plate. After a neutral has been ionized (positive in this illustration), it is attracted by the opposite plate. Those ions passing through a hole of the grounded electrode create an ion beam emerging into the field-free region behind. The ion beam produced by such a primitive ion source is not parallel, but has some angular spread.

Most common ion sources are categorized as closed or tight, the inside of the cage typically being on the order of a 5-mm cube, the only passages to its interior being the electron entrance and exit slits, the positive ion exit slit, and one or more small holes for the sample inlet... 

Several other types of ion source under development should be watched closely for potential inorganic/organometallic use. Ionization by electrons from a Ni source has been used in an external ion source that is at atmospheric pressure 66,67) giving a reported sensitivity in the subpicogram range. 

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