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ESPECTROMETRIA DE MASAS
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  1 Chapter Chemical Ionization (CI) The Ionization Process This chapter should be read in conjunction with Chapter 3, “Electron Ionization.” In electronionization (EI), a high vacuum (low pressure), typically 10 –5  mbar, is maintained in the ion sourceso that any molecular ions (M ã + ) formed initially from the interaction of an electron beam andmolecules (M) do not collide with any other molecules before being expelled from the ion sourceinto the mass spectrometer analyzer (see Chapters 24 through 27, which deal with ion optics).Decomposition (fragmentation) of a proportion of the molecular ions (M ã + ) to form fragmentions (A + , B + , etc.) occurs mostly in the ion source, and the assembly of ions (M ã + , A + , B + , etc.) isinjected into the mass analyzer. For chemical ionization (CI), the initial ionization step is the sameas in EI, but the subsequent steps are different (Figure 1.1). For CI, the gas pressure in the ionsource is typically increased to 10 – 3  mbar (and sometimes even up to atmospheric pressure) byinjecting a reagent gas (R in Figure 1.1).The substance being investigated (M) is present as only a small fraction of the reagent gaspressure. Thus, the electrons in the electron beam mostly interact with the reagent gas to formreagent gas ions (R ã + ) and not M ã +  ions. At the higher pressures, the initial reagent gas ions almostimmediately suffer multiple collisions with neutral reagent gas molecules (R). During this process,new ions (RH + ) are produced (step 2, Figure 1.1); these ions are reagent gas ions.Because many of these ions are produced, there is a high probability that they will collidewith sample molecules (M) and that a proton (H + ) will be exchanged to give protonatedmolecular ions [M + H] + , as shown in step 3, Figure 1.1. These quasi-molecular ions containlittle excess of internal energy following CI and therefore tend not to fragment. Whereas EIspectra contain peaks corresponding to both molecular and fragment ions, CI spectra are muchsimpler, mostly having only protonated molecular ion peaks. Negative reagent gases giveabundant [M –  H] −  or [M + X] −  ions. Example of the Chemical Ionization Process The ion source, across which an electron beam passes, is fi lled 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 (CH 4 ) withelectrons (e − ) gives methane molecular ions (CH 4 ã + ), as shown in Figure 1.2a. Copyright © 2003 by CRC Press LLC   Newly formed ions (CH 4 ã + ) collide several times with neutral molecules (CH 4 ) to givecarbonium ions (CH 5+ ) (Figure 1.2b). The substance (M) to be investigated is vaporized intothe ion source, where it collides with these carbonium ions (CH 5+ ). Proton exchange occursbecause organic substances (M) are usually stronger bases than is CH 4 , so protonated molec-ular   ions (written as MH +  or, better, as [M + H] + ) are formed (Figure 1.2c). These ions areexpelled from the ion source into the mass analyzer of the mass spectrometer (see Chapters24 through 27 for information on ion optics). Protonated molecular ions are also called quasi-molecular ions.Little or no fragmentation of [M + H] +  ions occurs, so CI spectra are very simple comparedwith EI spectra. A comparison of the two is shown in Figure 1.3, where it can be seen that the EI spectrum gives few molecular ions (M ã + ) but many fragment ions (A + , B + , etc.), whereas the CIspectrum shows many (abundant) quasi-molecular or protonated ions [M + H] +  and few fragmentions. Thus, CI and EI spectra are complementary and there is often considerable advantage inobtaining both, since EI gives structural information and CI con fi rms the relative molecular mass(molecular weight). Other Reagent Gases The example of methane as a reagent gas can now be extended to other species, some of whichare shown in Figure 1.4, along with the principal ions formed. The various reagent gases do not all act in the same proton-exchange manner described above, and some of the major variations aredetailed below. However, all of the ionization effected by the reagent gases is characterized by itspropensity to give spectra having very few fragment ions.Negative reagent ions, such as Cl −  (from CH 2 Cl 2 ), O ã−   (from NO), and OH −  (from NO plusCH 4 ), react with molecules (M) either by abstracting a proton to give [M −  H] −  ions or by Figure 1.1 Comparison of basic EI and CI processes showing different types of molecular ions and the formation of fragment ions in EI. Figure 1.2 Formation of reactive ions (CH 5+ ) from methane (CH 4 ) reagent gas and their reaction with sample molecules (M) to formprotonated molecular ions [M + H] + . IonizationStep 1EI:MM+IonizationStep 1CI:RR+M ,A ,B , etc.+Mass SpectrumStep 2++Step 2RH+Mass SpectrumMH + R+Step 3M CH + e –++4 CH + CH 44 (a)(b)(c) + CH + M 5+ CH + CH 53+ CH + MH 4 CH + 2e –4 Copyright © 2003 by CRC Press LLC    D  o  w  n   l  o  a   d  e   d   b  y   [   U  n   i  v  e  r  s   i   d  a   d   I  n   d  u  s   t  r   i  a   l   D  e   S  a  n   t  a  n   d  e  r   ]  a   t   0   9  :   0   2   0   8   A  u  g  u  s   t   2   0   1   4    addition to form [M + X] −  ions. Of these, OH −  is particularly useful because its high protonaf  fi nity leads to hydrogen abstraction from most classes of organic compounds (except alkanes)with little fragmentation. As with the EI/CI combinations, it is often convenient for structureelucidation to generate both positive and negative ion data by CI. Instruments are available tomeasure both positive and negative CI spectra simultaneously. For example, a reagent gasmixture of CH 4  and NO gives two corresponding reagent gases, CH 5+  and OH − . Thus, both[M + H] +  and [M −  H] +  quasi-molecular ions can be formed from sample molecules (M). Byalternately injecting (pulsing) positive ions and negative ions (a process termed PPINICI) intothe mass analyzer (usually a quadrupole or ion trap), almost simultaneous positive and negativeCI spectra can be measured. Other Ionization Routes Molecules of substrate M may not be ionized simply by the proton-transfer mechanism shown inFigure 1.2c. For example, with ammonia reagent gas, either [M + H] +  ions or [M + NH 4 ] +  ions canbe formed, depending on the nature of the substrate (Figure 1.5). Process 5a is proton exchange, butFigure 1.5b shows an example in which the whole of the reagent gas ion attaches itself to the substrateto form a quasi-molecular ion [M + NH 4 ] + . In Figure 1.5a, the protonated molecular ion has a mass Figure 1.3 Comparison of EI and CI mass spectra illustrating the greater degree of fragmentation in the former and the greater abundanceof quasi-molecular ions in the latter. Figure 1.4 Some types of reagent gases and their reactive ions. EI Spectrum    I  o  n   A   b  u  n   d  a  n  c  e   I  o  n   A   b  u  n   d  a  n  c  e CI Spectrumm/zm/zM + MH+ . + Reagent gasMolecular ionReactive reagent ion HC HNHCH OHNONONO 241033 H 2 C H 410+ NH 3++ CH OH 3+ CH OH 3+2 NH 4++ H 3 C H 411+ Copyright © 2003 by CRC Press LLC    D  o  w  n   l  o  a   d  e   d   b  y   [   U  n   i  v  e  r  s   i   d  a   d   I  n   d  u  s   t  r   i  a   l   D  e   S  a  n   t  a  n   d  e  r   ]  a   t   0   9  :   0   2   0   8   A  u  g  u  s   t   2   0   1   4   that is one unit greater than the true relative molecular mass, since the mass of H is one. In Figure 1.5b,the adduct ion is greater than M by 18 mass units: (N = 14, 4 ×  H = 4, total NH 4  = 18).If the substrate (M) is more basic than NH 3 , then proton transfer occurs, but if it is less basic,then addition of NH 4+  occurs. Sometimes the basicity of M is such that both reactions occur, andthe mass spectrum contains ions corresponding to both [M + H] +  and [M + NH 4 ] + . Sometimes thereagent gas ions can form quasi-molecular ions in which a proton has been removed from, ratherthan added to, the molecule (M), as shown in Figure 1.5c. In these cases, the quasi-molecular ionshave one mass unit less than the true molecular mass. Uses of CI Some substances under EI conditions fragment so readily that either no molecular ions survive orso few survive that it is dif  fi cult 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 CI conditions,very little fragmentation occurs and, depending on the reagent gas, ions [M + X] +  (X = H, NH 4 ,NO, etc.) or [M −  H] + or [M −  H] −  or [M + X] −  (X = F, Cl, OH, O, etc.) are the abundant quasi-molecular ions, which do give molecular mass information.Fragmentation under EI conditions yields structural information, but CI yields little or nonebecause it gives few fragment ions. Thus, CI is used mostly for molecular mass information andis frequently used with EI as a complement. Because there is little structural information, in contrastto EI, there are no extensive libraries of CI spectra. CI spectra are apparent also in atmospheric-pressure ionization systems (see Chapters 9 and 11). CI is often called a soft   ionization methodbecause little excess energy is put into the molecules (M) when they are ionized. Therefore,substances that might not otherwise give mass spectra containing molecular ions will give molecularmass information under CI conditions. Use of CI/EI in Tandem in GC/MS As shown above, CI and EI spectra complement each other and they are used frequently insuch techniques as gas chromatography/mass spectrometry (GC/MS), where successive massspectral scans are recorded. Alternate scans can be arranged to be either EI or CI by alternateevacuation of reagent gas from or pressurization with reagent gas into the ion source throughwhich the GC ef  fl uent is fl owing. With modern pumping systems, this switchover is completewithin a few seconds. Figure 1.6 illustrates the EI/CE switching process and the sort of  information obtained. Figure 1.5 Typical CI processes in which neutral sample molecules (M) react with NH 4+  to give either (a) a protonated ion [M + H] + or (b) an adduct ion [M + NH 4 ] + ; the quasi-molecular ions are respectively 1 and 18 mass units greater than the true mass(M). In process (c), reagent ions (C 2 H 7+ ) abstract hydrogen, giving a quasi-molecular ion that is 1 mass unit less than M. M + NH +4 [M+H] +NH 3+ [M+NH ] 4+ C H + M[M - H] + C H + H 22267++ (a)(b)(c) Copyright © 2003 by CRC Press LLC    D  o  w  n   l  o  a   d  e   d   b  y   [   U  n   i  v  e  r  s   i   d  a   d   I  n   d  u  s   t  r   i  a   l   D  e   S  a  n   t  a  n   d  e  r   ]  a   t   0   9  :   0   2   0   8   A  u  g  u  s   t   2   0   1   4
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