Chemical Ionization in Mass Spectrometry*

Chemical Ionization in Mass Spectrometry Alex G Harrison, University of Toronto, Ontario, Canada & 1999 Elsevier Ltd. All rights reserved. This article is reproduced from the previous edition, volume 1, pp 207–215, & 1999, Elsevier Ltd.

Symbols etherm E m/z DH 1

electron of near-thermal energy energy mass to charge ratio enthalpy change

Introduction In chemical ionization mass spectrometry (CIMS), ionization of the gaseous analyte occurs via gas-phase ion– molecule reactions rather than by direct electron impact (EI), photon impact or field ionization. EI ionization of a reagent gas (present in large excess) is usually followed by ion–molecule reactions involving the initially formed ions and the reagent gas neutrals to produce the chemical ionization (CI) reagent ion or reagent ion array. Collision of the reagent ion(s) with the analyte (usually present at B1% of the reagent gas pressure) produces one or more ions characteristic of the analyte. These initial analyte ions may undergo fragmentation or, infrequently, react further with the reagent gas to produce a final array of ions representing the CI mass spectrum of the analyte as produced by the specific reagent gas. To a considerable extent, the usefulness of CIMS arises from the fact that a wide variety of reagent gases and, hence, reagent ions can be used to ionize the analyte; often the reagent system can be tailored to the problem to be solved. Problems amenable to solution by CI approaches include (a) molecular mass determination, (b) structure elucidation and (c) identification and quantification. In many instances, CI provides information that is complementary rather than supplementary to that obtained by EI, and often both approaches are used. After a brief discussion of instrumentation, the major approaches, in both positive ion and negative ion CI, to the solution of the above problems are discussed. The focus is primarily on the use of medium-pressure mass spectrometry; CI at atmospheric pressure or at much lower pressures in ion trapping instruments are discussed elsewhere.

Instrumentation Most commonly, CI studies have been performed at total ion source pressures of 0.1 to 1 torr using conventional

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sector, quadrupole or time-of-flight mass spectrometers with ion sources only slightly modified from those used for EI ionization. The essential change is that the ion source is made more gas-tight by using smaller electron beam entrance and ion beam exit slits; the latter is accomplished by having either exchangeable source volumes or a moveable ion exit slit assembly to change from EI to CI. Manufacturers now supply instruments capable of both EI and CI and incorporating enhanced pumping to maintain adequately low pressures in the remainder of the instrument when the source is operated at elevated pressures. Provision is made for introduction of the reagent gas and for introduction of the analyte by a heated inlet system, by a solids probe, by a direct exposure probe or by interfacing a gas chromatograph to the ion source. Capillary gas chromatography–CI mass spectrometry is a particularly powerful and sensitive approach to identification and quantification of the components of complex mixtures.

Brønsted Acid Chemical Ionization Gaseous Brønsted acids, BHþ, react with the analyte M primarily by the proton transfer reaction BH þ þ M-MH þ þ B

½1

Other, usually minor, reactions that are possible include hydride ion abstraction (eqn [2]) and charge exchange (eqn [3]) BH þ þ M-½M  H þ þ BH2 ðB þ H2 Þ

½2

BH þ þ M-M þ þ BH  ðB þ H  Þ

½3

The enthalpy change for eqn [1] is given by DH 1 ¼ PAðBÞ  PAðMÞ

½4

where the proton affinity of X, PA(X), is given by þ XðgÞ þ Hþ ðgÞ -XHðgÞ ; DH 1 ¼ PAðXÞ

½5

If PA(M)4PA(B), the reaction is exothermic and normally occurs at the ion–neutral collision rate determined by ion-induced dipole and ion–dipole interactions. Conversely, if the reaction is endothermic, the rate