Page 13 - Inorganic Mass Spectrometry : Fundamentals and Applications

Thermal Ionization Mass Spectromet~ 3

Most of these involved modifying the ionizing surface in some manner that made
production of positive ions more efficient. Some involved changing the surface
entirely, as in the use of silica gel [ 161. Other ionization-enhancing techniques
involved altering the surface to one of higher work function, as in introducing
carbon into a rhenium substrate [ 171. An overlayer of a material of high work
function, generated through either electroplating or other means, is another
method that has been successfully applied [18,19]. These topics are treated in
more detail later in the chapter.

To the best of the author's knowledge, no treatment of the theory underlying
thermal ionization has appeared since the description by Kaminsky over 30 years
ago [20]. As his book is no longer listed in Books in Print, it seems desirable to
include a description of it here, if only for reference. The following treatment
follows that of Kaminsky. In these equations, subscripts (+) and (0) are used to
refer to the state of the atom on the surface, representing, respectively, the singly
charged positive ion and the atom, and subscripts (i) and (a) are used to refer to the
species (ions and atoms, respectively) leaving the surface.
The theory of thermal ionization starts with the work of Langmuir and
Kingdon [ 151, who derived an equation from first principles that describes a gas-
phase atomic beam impinging on a hot metal surface; the atoms adsorb on the
surface and then desorb from it, partly as atoms and partly as singly charged
positive ions. The length of time the impinging species remains on the surface is
called the mean residence time, 7, and T~, for atoms and singly charged positive
ions, respectively. The desorption process is described by a first-order rate law.
Ntx = No exp (- thx) (1.1)
where No is the steady-state flux of the atomic beam impinging on the surface;
the subscripts x and i represent atoms and ions, respectively; and Ntx is the flux of
desorbing ions or atoms leaving the surface t seconds after interruption of the
impinging beam. A plot of log Ntx versus time yields a straight line whose slope
is 1hx.
To describe the efficiency of ion forrnation, it is useful to define two
parameters, the degree of ionization and the ionization coefficient. "hese are
defined as follows:
a = NilNa t 1 .2)

p = NilN t 1.3)
where a is the degree of ionization and f3 the ionization coefficient; Ni and Na are
the numbers of singly charged positive ions and atoms, respectively, leaving the
hot surface per unit area per second; N is the flux of atoms impinging on the