Ionization/inlet systems

The hybrid can be used with El, Cl, FI, FD, LSIMS, APCI, ES, and MALDI ionization/inlet systems. The nature of the hybrid leads to high sensitivity in both MS and MS/MS modes, and there is rapid switching between the two. The combination is particularly useful for biochemical and environmental analyses because of its high sensitivity and the ease of obtaining MS/MS structural information from very small amounts of material. The structural information can be controlled by operating the gas cell at high or low collision energies. 

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. 

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). 

Liquids that are sufficiently volatile to be treated as gases (as in GC) are usually not very polar and have little or no hydrogen bonding between molecules. As molecular mass increases and as polar and hydrogen-bonding forces increase, it becomes increasingly difficult to treat a sample as a liquid with inlet systems such as El and chemical ionization (Cl), which require the sample to be in vapor form. Therefore, there is a transition from volatile to nonvolatile liquids, and different inlet systems may be needed. At this point, LC begins to become important for sample preparation and connection to a mass spectrometer. 

Electrospray is both an atmospheric-pressure (API) liquid inlet system for a mass spectrometer, and, at the same time, it is an ionization source. 

Practical inlet systems for attaching a high-pressure liquid chromatography (HPLC) column to a mass spectrometer utilize atmospheric-pressure ionization (see Chapters 8 and 11). 

All mass spectrometers have different stages of pumping in order to maintain the analyzer and detector regions under high vacuum, i.e. 10 7 10 8 torr or higher. Depending upon the ionization technique, the inlet system and the ion source must be/... 

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)). 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Total pressure, required for detailed interpretation of the mass spectra, is determined with an ionization gauge (S). The gas inlet system (A, B, C) is used for calibration purposes. The relation between measured total pressure and the ion current of an injected specific gas permits calibration of the mass spectrometer in absolute partial pressure units or amps/torr. 

Another important feature of IRMS is the provision of a dual inlet system which permits the operator to alternate the gas flowing into the ionization chamber between... 

If high-resolution measurements are performed in order to assign elemental compositions, internal mass calibration is almost always required. The calibration compound can be introduced from a second inlet system or be mixed with the analyte before the analysis. Mixing calibration compounds with the analyte requires some operational skills in order not to suppress the analyte by the reference or vice versa. Therefore, a separate inlet to introduce the calibration compound is advantageous. This can be achieved by introducing volatile standards such as PFK from a reference inlet system in electron ionization, by use of a dual-target probe in fast atom bombardment, or by use of a second sprayer in electrospray ionization. 

Note In FI-MS, the ionization efficiency is very low, because of the low probability for a neutral effusing from any inlet system towards the field emitter to come close enough to the whiskers. Consequently, FI-MS produces very low ion currents. The application of FI-MS is therefore restricted to samples that are too volatile for FD-MS or require gas chromatographic separation before. 

The gas stream from the inlet system (Fig. 2.2) enters the ionization chamber (operated at a pressure of about 10-6 — 10"5 torr) in which it is bombarded at right angles by an electron beam emitted from a hot filament. Positive ions produced by interaction with the electron beam are forced through the first accelerating slit by a weak electrostatic field. A strong electrostatic field then accelerates the ions to their final velocities. To obtain a spectrum, the applied magnetic field is increased, bringing successively heavier ions into the collector slit. A scan from mass (strictly m/z see above) 12 — 500 may be performed in seconds. 

An extensive and detailed coverage of the MS of expls in general has been documented (Ref 163). Volatile constituents of Comp A-3, Comp B, pressed TNT and cast TNT were surveyed with a residual gas analyzer MS (Ref 40). The mass spectra of all possible TNT (except for 3,4,5-TNT) and DNT isomers in the vapor phase were obtained as a function of ionizing voltage (Refs 65 84). The use of membrane inlet systems for the separation of TNT vapor in trace vapor detection is described and an analysis of the membrane inlet system for quadrupole mass spectroscopy is presented (Refs 95 113). Estimations of the vapor pressure of TNT were made mass spectrometrically in the range of 50—... 

The stereospecificity of the RDA reaction can be used to produce unstable species and to determine their thermodynamic properties with the mass spectrometer. Such a project was undertaken by Turecek and collaborators571 to distinguish the Z- and -form of the dienol c given in Scheme 17. The 3-ew-vinylbicyclo[2.2.1]hept-5-en-2-ol is flash-pyrolized in the inlet system of a mass spectrometer and ionized within a few ms after decomposition. The mass spectra and the ionization energies of E-c and Z-c were determined. The electron impact spectra are similar, although some reproducible differences can be seen the CID spectra are identical. The same ions are produced by dissociative ionization of the precursors a and b. Their CID spectra are identical, however different from the isomeric aldehyde572. The ionization energies of the neutral dienols were measured as IIi(/i - c) = 8.51 0.03 and IE(Z - c) = 8.47 0.03 eV, respectively. Since the activation... 

The chemical ionization mass spectrum of cimetidine and the major fragmentation ions are presented in Figure 8 and Table 4. The spectrum was obtained using a Finnigan lYbdel 3200 quadrupole mass spectrometer fitted with a chemical ionization source. The sample, applied to the probe from an acetone solution, was introduced via the direct inlet system. Methane was used as the reactant gas. 

---