Reagent gases

If methane is introduced into the ion volume through the tube, the primary reaction with the electrons will be a classical El reaction  

This ion will fragment, mainly through the following reactions  

However, mostly, it will collide and react with other methane molecules yielding 

Other ion-molecule reactions with methane will occur in the plasma, such as 

Spectrum of methane ionization plasma at 20 Pa. The relative intensities depend on the pressure in the source. 

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Decomposition (fragmentation) of a proportion of the molecular ions (M +) to form fragment ions (A B+, etc.) occurs mostly in the ion source, and the assembly of ions (M +, A+, etc.) is injected into the mass analyzer. For chemical ionization (Cl), the Initial ionization step is the same as in El, but the subsequent steps are different (Figure 1.1). For Cl, the gas pressure in the ion source is typically increased to 10 mbar (and sometimes even up to atmospheric pressure) by injecting a reagent gas (R in Figure 1.1). 

The ion source, across which an electron beam passes, is filled with methane, the reagent gas. There is a high vacuum around the ion source, so, to maintain a high pressure in the source itself, as many holes as possible must be blocked off or made small. Interaction of methane (CH4) with electrons (e ) gives methane molecular ions (CH4 "), as shown in Figure 1.2a. 

Formation of reactive ions (CH," ) from methane (CH4) reagent gas and their reaction with sample molecules (M) to form protonated molecular ions [M + Hp. 

If the substrate (M) is more basic than NHj, then proton transfer occurs, but if it is less basic, then addition of NH4 occurs. Sometimes the basicity of M is such that both reactions occur, and the mass spectrum contains ions corresponding to both [M + H]+ and [M + NH4]. Sometimes the reagent gas ions can form quasi-molecular ions in which a proton has been removed from, rather than added to, the molecule (M), as shown in Figure 1.5c. In these cases, the quasi-molecular ions have one mass unit less than the true molecular mass. 

Some substances under El conditions fragment so readily that either no molecular ions survive or so few survive that it is difficult to be sure that the ones observed do not represent some impurity. Therefore, there is either no molecular mass information or it is uncertain. Under Cl conditions, very little fragmentation occurs and, depending on the reagent gas, ions [M + X]+ (X = H, NH4, NO, etc.) or [M - H] or [M - H]" or [M -1- X] (X = F, Cl, OH, O, etc.) are the abundant quasi-molecular ions, which do give molecular mass information. 

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... 

Further explanation of this Cl process can be found in Chapter 1. Briefly, Cl results from collision between sample molecules and specially produced reagent gas ions such that ions are formed from sample molecules by various processes, one of the most important of which is the transfer of a proton (H+, Figure 9.2). 

As each mixture component elutes and appears in the ion source, it is normally ionized either by an electron beam (see Chapter 3, Electron Ionization ) or by a reagent gas (see Chapter I, Chemical Ionization ), and the resulting ions are analyzed by the mass spectrometer to give a mass spectmm (Figure 36.4). 

Because this chemical reaction occurs between the and M species, the original methane (CH4) is called a reagent gas, the CH5+ species are reagent gas ions, and the process is known as chemical ionization (Cl). 

Other reactions between reagent gas ions and molecules (M) can occur. As examples, ions [M + X] can be formed, where X is Na, NO, or NH4. 

Molecular ion Chemical ionization using ammonia as reagent gas establishes the molecular weights of sugar acetates. 

An important additional feature of Cl spectroscopy is its ability to handle gas chromatographic (GC) effluents directly if a proper reagent gas is used as the carrier gas in the GC. 

Figure 3.2 Piocesses occurring in chemical ionization mass spectrometry using methane as the reagent gas.

The positive-ion Cl spectrum, using ammonia as a reagent gas, of an analyte containing carbon, hydrogen, oxygen and two nitrogen atoms, has a molecular species at m/z 222. What is the molecular weight of the compound Involved ... 

These arise either by an analogous process to that described above for Cl, i.e. the adduction of a negatively charged species such as Cl , and the abstraction of a proton to generate an (M — H) ion, or by electron attachment to generate an M ion. The ions observed in the mass spectrum are dependent on the species generated by the reagent gas and the relative reactivities of these with each other and with the analyte molecule. 

The Cl reagent-gas plasma is either generated by thermospray ionization if buffer is present or, as described above, by the filament or discharge electrode. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

Reagent gas A gas used in chemical ionization to produce species which react with molecules of the analyte of interest to produce a molecular species. 

Figure 5. Variation in cmicentration of products over Sr/La O, as a function of the reagent gas conqx>sitioa Results were obtained at 16 eV for CH, radicals and at 70 eV for stable products P(total) = 490 mTorr, T = 735 °C.

Reagent Gas Pred inant Reagent Ions at ca. One Torr... 

The appearance and reproducibility of chemical ionization mass spectra depends on the ionizing conditions, principally the source temperature and presstire and the purity of the reagent gas. Chemical ionization mass spectra are generally not as reproducible as electron impact spectra. 

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