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Thermal Ionization (TI), Surface Emission of Ions

Thermal Ionization (TI), Surface Emission of Ions

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Mass Spectrometry Basics

Figure 7.1

A typical filament assembly. Positive ions from the filament are accelerated by a high negative potential of about 1000 V

placed on the first collimating plate. The other plates are used for further collimation and centering of the ion beam, which

is directed into a suitable mass analyzer. Some positive ions strike the edge of the first collimating slit and produce secondary

negative ions and electrons, which would be accelerated back onto the filament without a suppressor grid. This backbombardment of the filament would lead to the formation of extraneous positive ions. To minimize this process, a suppressor

grid (at a potential of about -300 V with respect to the filament) is included to deflect any backscattered ions.

The ion current resulting from collection of the mass-separated ions provides a measure of the

numbers of ions at each mlz value (the ion abundances). Note that for this ionization method, all

ions have only a single positive charge, z = 1, so that mlz = m, which means that masses are

obtained directly from the measured mlz values. Thus, after the thermal ionization process, mlz

values and abundances of ions are measured. The accurate measurement of relative ion abundances

provides highly accurate isotope ratios. This aspect is developed more fully below.

High Filament Temperatures

The high temperatures necessary to produce ions rapidly vaporize (evaporate) and thermally destroy

organic substances. Consequently, this surface ionization technique is generally not used to investigate them. Inorganic substances are generally much more stable thermal1y but also much less

volatile. Although an inorganic sample can be changed upon heating - consider, for example, the

formation of calcium oxide from calcium carbonate - the inorganic or metal (elemental) parts of

such samples are not destroyed. For example, if a sample of a cesium salt were to be examined,

the anionic portion of the sample might well be changed on the hot filament, but the cesium atoms

themselves would remain and would sti11 be desorbed as Cs-'. Sometimes, the desorbed ions appear

as oxide or other species, as with GdO·+.

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.


In Equation 7.1, n+/no is the ratio of the number of positive ions to the number of neutrals

evaporated at the same time from a hot surface at temperature T (K), where k is the Boltzmann

constant and A is another constant (often taken to be 0.5; see below). By inserting a value for k

and adjusting Equation 7.1 to common units (electronvolts) and putting A =0.5, the simpler Equation

7.2 is obtained.

n+/no = 0.5e l ' .6oo (qJ- I){f


Thermal Ionization (Tl), Surface Emission of Ions



Ionization Energy (eV)












i nonum















Figure 7.2

The table lists first ionization energies (electronvolts) for some commonly examined elements. Because only singly charged

ions are produced by surface emission from a heated filament, only first ionization energies are given. viz., those for M+

and not for higher ionization states in which mOre than one electron has been removed. Note that most elemental ionization

energies fall in the range of about 3-12 eV.

The expression 1-<1> in Equations 7.1 and 7.2 is the difference between the ionization energy

(I, sometimes known as the ionization potential) of the element or neutral from which the ions are

fanned and the work function (<1> or sometimes W) of the metal from which the filament is made.

The ionization energy and the work function control the amount of energy needed to remove an

electron from, respectively, a neutral atom of the sample and the material from which the filament

is constructed. The difference between I and <1> governs the ease with which positive ions can be

formed from sample molecules lying on the filament. Both I and <1> are positive and are frequently

reported in units of electronvolts (eV). Their importance is discussed in greater detail below. Some

typical values for ionization energies and work functions are given in Figures 7.2 and 7.3.

Thermochemistry of Surface Emission

Adsorption of a neutral (n'') onto a metal surface leads to a heat of adsorption of Q; as the electrons

and nuclei of the neutral and metal attract or repel each other. Partial positive and negative charges

are induced on each with the formation of a dipolar field (Figure 7.4).

Similarly, adsorption of ions (n") onto a metal surface leads to a heat of adsorption of Qj'

Generally, Qi is about 2-3 eV and is greater than Qa' which itself is about 1 eY. The difference

between Q i and Qa is the energy required to ionize neutrals (n") on a metal surface so as to give

ions (nt) or vice versa. This difference, Qi - Qa' can be equal to, greater than, or less than the

difference, J - <», between the ionization energy (I) of the neutral and the ease with which a metal

can donate or accept an electron (the work function, <»). Where Qi - Q, > I - <», the adsorbed


Work function (eV)

Melting point (K)













Figure 7.3

Values of the average work function (lj), electronvolts) for the commonly used filament metals. The melting points of the

metals are also shown to give some guidance as to the maximum temperature at which they can be used. Normally, the

practical maximum would lie a few hundred degrees below the melting point to prevent sagging of the filament.

Mass Spectrometry Basics













/ ////





Figure 7.4

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament. (a) As a

neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and

metal attract each other, causing dipoles to be set up in each. (b) When the neutral particle reaches the surface, it is attracted

there by the dipolar field with an energy Qa' (c) If the values of I and Ij) are opposite, an electron can leave the neutral

completely and produce an ion on the surface, and the heat of adsorption becomes Qt' Similarly, an ion alighting on the

surface can produce a neutral, depending on the values of I and Ij). On a hot filament the relative numbers of ions and

neutrals that desorb are given by Equation 7.1,which includes the difference, I -Ij), and the temperature, T.

particle will be an ion, whether it was originally a neutral particle or an ion that approached the

metal surface. Similarly, if Qi - Qa < 1 - $, the adsorbed particle will be a neutral species,

regardless if it was an ion or a neutral that approached the metal surface. If energy is now added

to the system by strongly heating the filament, desorption of ions and neutrals occurs. Clearly,

the numbers of desorbing neutral particles and ions must depend on the size and sign of the

difference in (I - $) and on the added energy, which is controlled by the absolute temperature,

T. As shown in Figure 7.5, the critical value for desorption of ions and neutrals at any given

temperature is governed by the relation, Q a = Qi - (1 - $). This criterion is used in the derivation

of Equation 7.1.

nO (at infinity)

rr' (at infinity)


nO (on metal surface)

n+ (at metal surface)

1- ~

K= [n' IC'] I [noICO]

At equilibrium,


n- C+







n+ C+ • (I -
- (I -
.', - = - e


no CO

Figure 7.5

On bringing a neutral (n") from infinity to the surface of a filament metal, it adsorbs with an energy Q. (Figure 7.4).

Similarly, bringing an ion (n") from infinity leads to a heat of adsorption of Qj. If the difference between Qa and Qt is equal

to I ~ 4>, the neutral particle can desorb again as a neutral Or desorb as an ion, depending on the value of I - 4>. Similar

arguments apply to an adsorbed ion (n"), which can desorb again as an ion or desorb as a neutral.

Thermal Ionization (TI), Surface Emission of Ions


On the surface of a heated filament metal, whether ions or neutrals are adsorbed initially,

an equilibrium will be set up between them, with equilibrium constant K (shown in Figure 7.5).

For the equilibrium constant, surface concentrations of desorbing neutrals (n") and ions (nt) must

be used. The surface concentration of ions is the proportion of ions actually desorbing to the

total number on the surface (C+), viz., the concentration of desorbing ions = [nt/C"]. Similarly,

the concentration of desorbing neutral particles = [nO/CO] (Figure 7.5). In the well-known thermodynamic equation that governs an equilibrium process, In K = -~GIRT, the gas constant R

can be replaced by its equivalent (the Boltzmann constant, k) and the total free energy change

(dG) by the critical energy change (I - <1». From these substitutions, the expression, K =

exp[- (I - '!> )/kTJ, is obtained. Combining this expression with the expression for K shown in

Figure 7.5, Equation 7.1 is revealed.

Therefore, the ratio of the number of ions to the number of neutrals desorbing from a heated

filament depends not only on the absolute temperature but also on the actual surface coverage

of ions and neutrals on the filament (C+, CO) and crucially on the difference between the

ionization energy and work function terms, I and «>. This effect is explored in greater detail in

the following illustrations.

Both of the terms I and o have positive values. Examination of Equation 7.1 or 7.2 reveals

that, for <1> > I, then <1> - I is positive, and the proportion of positive ions to neutrals diminishes with

increasing temperature. For example, with a sample of cesium (ionization energy, 3.89 eV) on a

tungsten filament (work function, 4.5 eV) at 1000 K, the ratio of n+/no = 591. Thus, for every

cesium atom vaporized, some 600 atoms of Cs" ions are produced. At 2000 K, the ratio of nr/n"

becomes 17, so only about 20 ions of cesium are evaporated for every Cs atom (Figure 7.6a). For

$ < I, as with lead (I = 7.42 eV) on tantalum (<1> = 4.2 eV), the corresponding figures are 3 x 10-16

at 1000 K and 1 x 10-8 at 2000 K (Figure 7.6b).

Clearly, the lower the ionization energy with respect to the work function, the greater is the

proportion of ions to neutrals produced and the more sensitive the method. For this reason, the

filaments used in analyses are those whose work functions provide the best yields of ions. The

evaporated neutrals are lost to the vacuum system. With continued evaporation of ions and

neutrals, eventually no more material remains on the filament and the ion current falls to zero.

Changing the Work Function (Activators)

For an element of ionization energy I, Equation 7.2 shows that at any given temperature, the work

function of the surface from which particles are emitted is clearly crucial to the proportion of ions

produced in relation to the number of neutrals. As 1-<1> changes from negative to positive, the ion

yield becomes progressively smaller. Figure 7.3 indicates that platinum would be the filament metal

ofchoice in most applications because it has the biggest work function of the four metals commonly

used(Pt, Re, W, or Ta). However, platinum also has the lowest melting point, and to reach the high

temperatures needed to effect suitable evaporation rates, it may be necessary to use a metal such

as tantalum or rhenium, for which the work functions are smaller. Thus, there is a trade-off between

work function and temperature in maximizing ion yield.

For difficult cases, this dilemma can be solved by using activators on the surface of the

filaments. The activators commonly used are colloidal or very finely dispersed (high surface area)

silicon dioxide or carbon. These substances are much more electronegative than the filament metal

and produce a dipolar field (Figure 7.7). This field induces a positive image charge in the filament

surface, thereby making removal of electrons more difficult and increasing its effective work

function. Since <1> is increased, the difference I - <1> must change and, therefore, the ion yield ratio

n+/no. Activators are used to improve ion yield when examining metals of high ionization energy,

as with uranium, lead, or plutonium on tungsten filaments.

Mass Spectrometry Basics















T (K)







10. 8



10. 12



10. 18


Figure 7.6

Graphs showing the influence of work function and ionization energy on the efficiency of ionization. Using Equation

7.1, the ratio nr/n'' was calculated for uranium (1 = 6.08 eV) on either (a) a platinum filament ( = 6.2 eV) or (b) a

rhenium filament ( = 4.8 eV) at different temperatures. For platinum (a), a good yield of ions is obtained, but the

ratio n+/nu falls with increasing temperature. For rhenium (b), the relative ion yield is small but increases with

increasing temperature. The best ion yields are given by the use of platinum with uranium, for which 1- is negative

by about 0.1 eV.

carbon surface layer


Figure 7.7

Schematic diagram showing how placing a thin layer of highly dispersed carbon onto the surface of a metal filament

leads to an induced dipolar field having positive and negative image charges. The positive side is always on the metal,

which is much less electronegative than carbon. This positive charge makes it much more difficult to remove electrons

from the metal surface. The higher the value of a work function, the more difficult it is to remove an electron. Effectively,

the layer of carbon increases the work function of the filament metal. Very finely divided silicon dioxide can be used

in place of carbon.

Thermal Ionization (TI), Surface Emission of Ions


Amount of Sample

The rate of evaporation of ions from a heated surface is given by Equation 7.3, in which Qi is the

energy of adsorption of ions on the filament surface (usually about 2-3 eV) and C, is the surface

density of ions on the surface (a complete monolayer of ions on a filament surface would have a

surface density of about 1015 ions/cm-).


Similarly, the rate of evaporation of neutral species from a filament surface is given by Equation

7.4, in which Co is the surface density of atoms on the surface (a complete monolayer of atoms

would have a surface density of about 1015 atoms/crrr").


n ::: Coe

-(Qj - q> + I)/kT


Dividing Equation 7.3 by 7.4 yields Equation 7.1, in which A::: C/Co.

As ions and neutrals evaporate from a heated filament surface, the amount of sample decreases

and the surface densities (C; Co) must decrease. Therefore, Equation 7.1 covers two effects. The

first was discussed above and concerns the changing value for the ratio nt/n" as the temperature of

the filament is varied, and the other concerns the change in the total number of ions desorbing as

the sample is used up. The two separate effects are shown in Figure 7.8a,b. Combining the two

effects (Figure 7.8c) reveals that if the temperature is increased to maintain the flow of ions, which

drops naturally as the sample is used up (time), then eventually the flow of ions and neutrals

becomes zero whatever the temperature of the filament because the sample has disappeared from

the filament surface.

Measurement of Ratios of Isotopic Abundances

Foranyone ion type (e.g., Cs"), measurement of its abundance in a sample requires the sample to

be evaporated over a period of time. The total yield of ions is obtained by integrating the area

under the ion-yield curve (Figure 7.8c).





time I temperature

Figure 7.8

(a) The effect of increasing temperature on the ion yield, which increases as the temperature rises. (b) The effect of decreasing

surface coverage of the filament surface as ions and neutrals evaporate; as the surface densities of ions and neutrals decrease.

the ion yield falls off with time. (c) An example of the shape of a curve resulting from the two effects. As the temperature of

the surface is increased to improve ion yield, the surface is depleted of sample more and more rapidly until no sample remains

and therefore no ion current. The area under curve (c) represents the total ion yield.


Mass Spectrometry Basics

Generally, ratios of isotopic abundances need to be obtained and not individual total ion yields.

Experimentally, for two isotopes M 1 and M z, obtaining the ratios entails the simultaneous measurement of their abundances as given by the ion current for the two masses arriving at the ion collector.

For two isotopes, the ion yields are given by Equations 7.5 and 7.6, which are obtained simply

from Equation 7.1 by inserting the relevant values for C, Co, and Q. Not only are C, and Codifferent

(because the relative amounts of isotopes are different), but they vary with time, as discussed above.

Dividing Equations 7.5 by 7.6 gives Equation 7.7, from which it is clear that at any given temperature, since the ratio of C, to C, changes with time, the ratio of ion yields for isotopes M, and M2

must change with time.



(n , +/n I O)/(n2 +/n2 0) -- (C I IC2 )e-(~Q + M)/kT


Figure 7.9 shows a schematic representation of this effect, in which the ratio of the two

isotopes changes with time. To obtain an accurate estimate of the ratio of ion abundances, it is

better if the relative ion yields decrease linearly (Figure 7.9) which can be achieved by adjusting

the filament temperature continuously to obtain the desired linear response. An almost constant

response for the isotope ratio can be obtained by slow evaporation of the sample, viz., by

keeping the filament temperature as low as is consistent with sufficient sensitivity of detection

(Figure 7.9).

The previous discussion demonstrates that measurement of precise isotope ratios requires a

substantial amount of operator experience, particularly with samples that have not been examined

previously. A choice of filament metal must be made, the preparation of the sample on the filament

surface is important (particularly when activators are used), and the rate of evaporation (and

therefore temperature control) may be crucial. Despite these challenges, this method of surface

ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other

chapters in this book discuss practical details and applications.

l-----'....-~------- (c)

..... - - - - ( a )



-, -, (b)

time / temperature

Figure 7.9

Schematic illustrations of the effect of temperature and surface density (time) on the ratio of two isotopes. (a) shows

that, generally, there is a fractionation of the two isotopes as time and temperature change; the ratio of the two isotopes

changes throughout the experiment and makes difficult an assessment of their precise ratio in the original sample. (b)

illustrates the effect of gradual1y changing the temperature of the filament to keep the ratio of ion yields linear, which

simplifies the task of estimating the ratio in the original sample. The best method is one in which the rate of evaporation

is low enough that the ratio of the isotopes is virtually constant; this ratio then relates exactly to the ratio in the original


Thermal Ionization (TI), Surface Emission of Ions



Precise measurement of isotope ratios can be obtained by comparing the yields of isotopic ions

desorbing from a sample placed on a strongly heated filament that is generally made from platinum,

tantalum, rhenium, or tungsten.


Electrospray Ionization (ESI)


Fora more detailed description of the ionization process inherent in electrospray, please see Chapter

9, which discusses atmospheric pressure ionization (API). The reader also should compare electrospray with thermospray (see Chapter 11).

In many applications in mass spectrometry, the sample to be analyzed is present as a solution

in a solvent that could be organic (as with methanol or acetonitrile) or aqueous (as with body

fluids). The solution could also be an effluent from a liquid chromatography (LC) column. In any

case, a solution must flow into the front end of a mass spectrometer (MS), but, before it can provide

a mass spectrum, the bulk of the solvent must be removed without losing the sample (solute). If

thesolvent were not removed, then its vaporization as it entered the vacuum of the mass spectrometer

would produce a large increase in pressure and stop the instrument from working. At the same time

that this excess of solvent is removed, the dissolved sample must be retained so that its mass

spectrum can be measured, viz., there must be differentiation between solvent and solute (sample)

molecules. There are several means of effecting this differentiation between carrier solvent and the

solute of interest, and electrospray is just one of them. However, there is an additional important

consideration in electrospray. Unlike the other methods of introducing a liquid into a mass spectrometer, electrospray frequently produces multicharged ions that make accurate measurement of

large masses easier and gives this inlet/ion source a considerable advantage in areas such as peptide

and protein research (see below).

One of the first successful techniques for selectively removing solvent from a solution without

losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The

drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual

solute on the belt was carried into a normal EI (electron ionization) or CI (chemical ionization)

ion source, where it was heated more strongly so that it in turn volatilized and could be ionized.

However, the moving-belt system had some mechanical problems and could be temperamental.

The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray

inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which

is described in Chapter 9.


Mass Spectrometry Basics


Differential Solvent Removal

A sample for which a mass spectrum is required may well be dissolved in an organic or aqueous

solvent. For example, in searching for drugs in blood plasma, the plasma itself may be investigated

(aqueous) or its active components may be first extracted into an organic solvent such as dichloromethane. Alternatively, the sample can first be separated into its components by passage through

a liquid chromatographic instrument (see Chapter 37); upon emerging from the column, the sample

of interest is present as a solution in the solvents used in the chromatography. In either case, the

sample to be examined is in solution and cannot be put straight into a mass spectrometer without

first removing most of the solvent and without, of course, removing the dissolved sample also!

Electrospray is one method for effecting this differential solvent removal. The solution is passed

along a short length of stainless steel capillary tube, to the end of which is applied a high positive

or negative electric potential, typically 3-5 kV (Figure 8.1). When the solution reaches the end of

the tube, the powerful electric field causes it to be almost instantaneously vaporized (nebulized)

into a jet or spray of very small droplets of solution in solvent vapor. Spraying efficiency can be

increased by flowing a gas past the end of the charged capillary tube. Before entering the mass

spectrometer proper, this mist of droplets flows through an evaporation chamber that can be heated

slightly to prevent condensation. As the droplets move through this region, solvent evaporates

rapidly from the surfaces and the droplets get smaller and smaller. In addition to producing the

spray, this method of rapid vaporization leaves no time for equilibrium to be attained, and a

substantial proportion of the droplets have an excessive positive or negative electrical charge on

their surfaces. Thus as the droplets get smaller, the electrical surface charge density increases until

the natural repulsion between like charges causes the release of ions and neutral molecules. The

end of the capillary tube is aimed at a small hole (target) at the opposite end of this evaporation

region. After vaporizing from the surface of a droplet, solvent molecules of low molecular mass

quickly and conveniently diffuse away from the line-of-sight trajectory to the inlet target. A Zspray ion source operates slightly differently (see Chapter 10).

Sample molecular ions and cluster ions have much greater molecular mass (and therefore

momentum) than those of the solvent and tend to carry straight on toward the target at the end

of the inlet region (Figure 8.1). To assist evaporation of the droplets and the breaking up of

unwanted cluster ions, a drying gas (nitrogen) flows along and past the end of the capillary

(Figure 8.1). If the gas is arranged to flow between the counter electrode and the nozzle, it is





gas{N)2 ~

LC Flow

- - -.....-





Counter electrode

To rotary pump

To high

vacuum pump

Figure 8.1

Schematic diagram of an electrospray inlet/ion source. A spray produced from the high electrical voltage (HT) on the

capillary moves toward a hole in the electrical counter electrode. After removal of much solvent, sample ions continue

under their momentum through the hole and then through the nozzle and skimmer, where most remaining solvent is removed.

Electrospray Ionization (ESI)


sometimes referred to as a curtain gas. At the target hole, the heavier ions pass through, but most

of the lighter solvent molecules have by this time diffused away and thus do not pass through.

In effect, the device is a momentum separator between solvent and solute (sample) molecules.

After passing through this hole, the ions pass through two evacuated regions via a nozzle and a

skimmer (Figure 8.1). These conically shaped holes refine the separation of sample ions from

solvent ions, still mainly on the basis of momentum but also by an extraction and focusing effect

of electrical potentials applied to the nozzle and skimmer. Finally, sample ions pass into the

analyzer of the mass spectrometer, where their mass-to-charge (mlz) ratios are measured in the

usual way. The mass analyzer can be of any type.

The result of the above process means that sample molecules dissolved in a solvent have been

extracted from the solvent and turned into ions. Therefore, the system is both an inlet and an ion

source, and a separate ion source is not necessary.

The ions passing into the mass spectrometer analyzer from electrospray have little excess

internal energy and therefore not enough energy to fragment. Many of the ions are of the form [M

+ X]' or [M - H]-, with X representing hydrogen or some other element, such as sodium or

potassium. While these quasi-molecular ions are an excellent source of accurate molecular mass

information - which may be all that is required - they give little or no information concerning

the actual molecular structure of the substance being investigated, which is provided by fragment

ions. This situation is entirely analogous to the problem with simple chemical ionization, and similar

solutions are available. To give the quasi-molecular ions the extra energy needed to induce fragmentation, they can be passed through a collision gas and the resulting spectra analyzed for

metastable ions (MS/MS methods). An alternative arrangement uses the potential difference

between the electrodes (cone voltage) to accelerate the ions. If the voltage difference is increased,

collisions between the faster moving ions and neutral molecules lead to fragmentation, as in CI.

If the cone voltage is reduced, the ions slow and the resulting collisions have insufficient energy

for fragmentation.

Multicharged ions

Another type of ion is formed almost uniquely by the electrospray inlet/ion source which makes

this technique so valuable for examining substances such as proteins that have large relative

molecular mass. Measurement of rn/z ratios usually gives a direct measure of mass for most mass

spectrometry because z = I and so mlz = mil = m. Values of z greater than one are unusual.

However, for electrospray, values of z greater than one (often much greater), are quite commonplace.

For example, instead of the [M + H]+ ions common in simple CI, ions in electrospray can be [M

+ n-H]» where n can be anything from 1 to about 30.

Thus the rn/z value for such ions is [M + n-Ij/n, if the mass of hydrogen is taken to be one.

As a particular example, suppose M = 10,000. Under straightforward CI conditions, [M + H]:' ions

will give an rn/z value of 10,001/1 = 10,001, a mass that is difficult to measure with any accuracy.

In electrospray, the sample substance can be associated with, for example, 20 hydrogens. Now the

ion has a mass-to-change ratio of [M + 20·HFO+ and therefore rn/z = 10,020/20 = 501. This mass

is easy to measure accurately with a wide range of instruments.

Normally, a range of values for n is found, each molecule (M) giving a series of multicharged

ions. For example, a series

[M + nH]n+, [M + (n + l)H]


might be observed, each successive quasi-molecular ion having one more hydrogen and one more

electrical charge than the preceding one (Figure 8.2). The only difficulty lies in knowing the value

of nl Fortunately, this value is relatively easy to extract from the mass spectrum (Figure 8.3).

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