The mass spectrometer

To introdnce a chemical analyte into the high vacuum of the instrument  

The ion source. In an electron impact mass spectrometer, a sample of the compound is bombarded by electrons. The collision of a high-energy electron with a sample molecule converts it into a radical cation. 

The mass analyzer. The mass analyzer separates the fragments produced during and after the initial ionization process. 

The detector. The fragments that have been sorted are displayed on a computer monitor or printed as a mass spectrum that gives the mass of each fragment, and the relative abundance of the fragments. 

When we examine each function in detail, we see that the mass spectrometer is actually somewhat more complex than just described. Before the ions can be formed, a stream of molecules must be introduced into the ionization chamber where the ionization takes place. A sample inlet system provides this stream of molecules. 

With rather nonvolatile solids, a direct-probe method of introducing the sample may be used. The sample is placed on the tip of the probe, which is then inserted through a vacuum lock into the ionization chamber. The sample is placed very close to the ionizing beam of electrons. The probe can be heated, thus causing vapor from the sample to be evolved in proximity to the beam of electrons. A system such as this can be used to study samples of molecules with vapor pressures lower than 10 mm Hg at room temperature. 

Most of the sample molecules are not ionized at all but are continuously drawn off by vacuum pumps that are connected to the ionization chamber. Some of the molecules are converted to negative ions through the absorption of electrons. The repeller plate absorbs these negative ions. A small proportion of the positive ions that are formed may have a charge greater than one (a loss of more than one electron). These are accelerated in the same way as the singly charged positive ions. 

The energy required to remove an electron from an atom or molecule is its ionization potential. Most organic compounds have ionization potentials ranging between 8 and 15 eV. However, a beam of electrons does not create ions with high efficiency until it strikes the stream of molecules with a potential of 50 to 70 eV. To produce reproducible spectra, electrons of this energy range are generally used to ionize the sample. 

From the ionization chamber, the beam of ions passes through a short field-free region. From there it enters the mass analyzer, the region where the ions are separated according to their mass-to-charge ratios. 

---

The mass spectrometer tends to be a passive instrument in these applications, used to record mass spectra. In chemical physics and physical chemistry, however, the mass spectrometer takes on a dynamic function as a... 

TPD is frequently used to detenuine (relative) surface coverages. The area below a TPD spectrum of a certain species is proportional to the total amount that desorbs. In this way one can detennine uptake curves that correlate gas exposure to surface coverage. If tire pumping rate of the UHV system is sufiBciently high, the mass spectrometer signal for a particular desorption product is linearly proportional to the desorption rate of the adsorbate [20, 21] ... 

FIGURE 13 44 Diagram of a gas chromatograph When connected to a mass spectrometer as in GC/MS the effluent is split into two streams as it leaves the column One stream goes to the detector the other to the mass spectrometer (Adapted with permission from H D Durst and G W Gokel Experimental Organic Chemistry Inti eti McGraw Hill New York 1987)... 

Molecular Identification. In the identification of a compound, the most important information is the molecular weight. The mass spectrometer is able to provide this information, often to four decimal places. One assumes that no ions heavier than the molecular ion form when using electron-impact ionization. The chemical ionization spectrum will often show a cluster around the nominal molecular weight. 

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 ... 

In GC-MS effluent from the column is introduced directly into the mass spectrometer s ionization chamber in a manner that eliminates the majority of the carrier gas. In the ionization chamber all molecules (remaining carrier gas, solvent, and solutes) are ionized, and the ions are separated by their mass-to-charge ratio. Because each solute undergoes a characteristic fragmentation into smaller ions, its mass spectrum of ion intensity as a function of mass-to-charge ratio provides qualitative information that can be used to identify the solute. 

This chapter should be read in conjunction with Chapter 3, Electron Ionization. In electron ionization (El), a high vacuum (low pressure), typically 10 mbar, is maintained in the ion source so that any molecular ions (M +) formed initially from the interaction of an electron beam and molecules (M) do not collide with any other molecules before being expelled from the ion source into the mass spectrometer analyzer (see Chapters 24 through 27, which deal with ion optics). 

Much of the energy deposited in a sample by a laser pulse or beam ablates as neutral material and not ions. Ordinarily, the neutral substances are simply pumped away, and the ions are analyzed by the mass spectrometer. To increase the number of ions formed, there is often a second ion source to produce ions from the neutral materials, thereby enhancing the total ion yield. This secondary or additional mode of ionization can be effected by electrons (electron ionization, El), reagent gases (chemical ionization. Cl), a plasma torch, or even a second laser pulse. The additional ionization is often organized as a pulse (electrons, reagent gas, or laser) that follows very shortly after the... 

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). 

After ions have been formed by El, they are examined for mass and abundance by the analyzer part of the mass spectrometer, which can incorporate magnetic sectors, electric sectors, qua-drupoles, time-of-flight tubes, and so on. The region in which the ions are first formed is called... 

Instead of the fast-atom beam, a primary ion-beam gun can be used in just the same way. Generally, such an ion gun emits a stream of cesium ions (Cs ), which are cheaper to use than xenon but still have large mass (atomic masses Cs, 139 Xe, 131). Although ion guns produce no fragment ions in the primary beam, they can contaminate the mass spectrometer by deposition with continued use. 

If the liquid that is being bombarded contains ions, then some of these will be ejected from the liquid and can be measured by the mass spectrometer. This is an important but not the only means by which ions appear in a FAB or LSIMS spectrum. Momentum transfer of preformed ions in solution can be used to enhance ion yield, as by addition of acid to an amine to give an ammonium species (Figure 4.3). 

A positive ion formed at a positive electrode tip is repelled and travels toward the negative counter electrode, which has a slit in it so that the ion can pass into the mass spectrometer. 

These thin wires are supported on a special carrier that can be inserted into the ion source of the mass spectrometer after first growing the whiskers in a separate apparatus. Although the wires are very fragile, they last for some time and are easily renewed. They are often referred to as emitter electrodes (ion emitters). 

A further consequence of the high temperatures is that much of the sample is simply evaporated without producing isolated positive ions. There is a competition between formation of positive ions and the evaporation of neutral particles. Since the mass spectrometer examines only isolated charged species, it is important for maximum sensitivity that the ratio of positive ions to neutrals be as large as possible. Equation 7.1 governing this ratio is given here. 

Although simple solutions can be examined by these electrospray techniques, often for a single substance dissolved in a solvent, straightforward evaporation of the solvent outside the mass spectrometer with separate insertion of the sample is sufficient. This situation is not true for all substances. Peptides, proteins, nucleotides, sugars, carbohydrates, mass organometallics, and many... 

For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

The advent of atmospheric-pressure ionization (API) provided a method of ionizing labile and nonvolatile substances so that they could be examined by mass spectrometry. API has become strongly linked to HPLC as a basis for ionizing the eluant on its way into the mass spectrometer, although it is also used as a stand-alone inlet for introduction of samples. API is important in thermospray, plasmaspray, and electrospray ionization (see Chapters 8 and 11). 

The mix of ions, formed essentially at or near ambient temperatures, is passed through a nozzle (or skimmer) into the mass spectrometer for mass analysis. Since the ions are formed in the vapor phase without having undergone significant heating, many thermally labile and normally nonvolatile substances can be examined in this way. 

The Z-spray inlet causes ions and neutrals to follow different paths after they have been formed from the electrically charged spray produced from a narrow inlet tube. The ions can be drawn into a mass analyzer after most of the solvent has evaporated away. The inlet derives its name from the Z-shaped trajectory taken by the ions, which ensures that there is little buildup of products on the narrow skimmer entrance into the mass spectrometer analyzer region. Consequently, in contrast to a conventional electrospray source, the skimmer does not need to be cleaned frequently and the sensitivity and performance of the instrument remain constant for long periods of time. 

At the target, clusters are broken up and sample molecular ions, accompanied by some remaining solvent ions, are extracted by an electrical potential through a small hole into the mass spectrometer analyzer (Figure 11.1), where their mass-to-charge (m/z) ratios are measured in the usual way. The mass spectrometer may be of any type. 

Liquids examined by FAB are introduced into the mass spectrometer on the end of a probe inserted through a vacuum lock in such a way that the liquid lies in the target area of the fast atom or ion beam. There is a high vacuum in this region, and there would be little point in attempting to examine a solution of a sample in one of the commoner volatile solvents such as water or dichloromethane because it would evaporate extremely quickly, probably as a burst of vapor when introduced into the vacuum. Therefore it is necessary to use a high-boiling solvent as the matrix material, such as one of those listed in Table 13.1. 

---