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Fast-Atom Bombardment (FAB) and Liquid-Phase Secondary Ion Mass Spectrometry (LSIMS) Ionization

Fast-Atom Bombardment (FAB) and Liquid-Phase Secondary Ion Mass Spectrometry (LSIMS) Ionization

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18



Mass Spectrometry Basics



Atom or Ion Beams

A "gun" is used to direct a beam of fast-moving atoms or ions onto the liquid target (matrix).

Figure 4.1 shows details of the operation of an atom gun. An inert gas is normally used for

bombardment because it does not produce unwanted secondary species in the primary beam and

avoids contaminating the gun and mass spectrometer. Helium, argon, and xenon have been used

commonly, but the higher mass atoms are preferred for maximum yield of secondary ions.

In the gun, inert gas atoms are ionized to give positive ions that are immediately accelerated

by an electric potential to give a high-velocity beam of ions. As these ions collide with other inert

gas atoms, charge exchange occurs such that many of the fast-moving ions become fast-moving

atoms (Figure 4.1). Any residual ions are removed by an electric potential on a deflector pIate,

leaving a beam of fast-moving atoms that exits from the gun. The beam is somewhat divergent, so

the gun is situated near the target.

Instead of the fast-atom beam, a primary ion-beam gun can be used in just the same way. Generally,

such an ion gun emits a stream of cesium ions (Cs"), which are cheaper to use than xenon but still

have large mass (atomic masses: Cs, 139; Xe, 131). Although ion guns produce no fragment ions in

the primary beam, they can contaminate the mass spectrometer by deposition with continued use.



G



ionization



+e-







+



ze



-



(a)



slow ion



slow atom

acceleration



~



(0



(b)



fast ion



slow ion



-



(0+(0

fast

ion



g



slow

atom



charge

exchange



(c)





fast

atom



slow

ion



Figure 4.1

(a) Xe atoms are ionized to Xe using electrons. These ions are relatively slow and move in all directions. (b) Xe ions

are accelerated through a high electric potential so that they attain a high speed in one direction. (c) Charge exchange

between fast Xe ions and slow Xe atoms gives a beam of fast Xe atoms and slow ions. The latter are removed by electric

deflector plates, leaving just a beam of fast atoms.



The Ionization Process

When the incident beam of fast-moving atoms or ions impinges onto the liquid target surface,

major events occur within the first few nanometers, viz., momentum transfer, general degradation,

and ionization.



Momentum Transfer

The momentum of a fast-moving atom or ion is dissipated by collision with the closely packed

molecules of the liquid target. As each collision occurs, some of the initial momentum is transferred

to substrate molecules, causing them in turn to move faster and strike other molecules. The result

is a cascade effect that ejects some of the substrate molecules from the surface of the liquid

(Figure 4.2). The process can be likened to throwing a heavy stone into a pool of water - some



FAB and LSIMS Ionization



19



o



fast ion or atom



gas phase



liquid phase



Figure 4.2

A typical cascade process. A fast atom or ion collides with surface molecules, sharing its momentum and causing the struck

molecules to move faster. The resulting fast-moving particles then strike others. setting up a cascade of collisions until all

the initial momentum has been redistributed. The dots (e) indicate collision points. Ions or atoms (0) leave the surface.



of the water splashes upwards. Clearly, heavier and faster-moving atoms or ions will cause more

particles to be ejected from the surface through momentum transfer than will slower-moving, lighter

atoms or ions. This is the reason for preferring xenon or cesium.

If the liquid that is being bombarded contains ions, then some of these will be ejected from

the liquid and can be measured by the mass spectrometer. This is an important but not the only

means by which ions appear in a FAB or LSIMS spectrum. Momentum transfer of preformed ions

in solution can be used to enhance ion yield, as by addition of acid to an amine to give an ammonium

species (Figure 4.3).



Ionization by Electron Transfer

The close encounter of a fast-moving atom or ion with a neutral molecule can lead to charge

exchange in which an electron moves from one particle to another (Figure 4.4). In this way, positive

and negative ions are formed separately from any preexisting ions that might be present. Momentum

transfer again leads to the newly formed ions being ejected from the liquid.



RNH 3 +

acid



amine



+ A"



protonated



amine



I



I



I



I



I IIII I I



II



I



I



m/z



Figure 4.3

An example of enhanced ion production. The chemical equilibrium exists in a solution of an amine (RNH 2) . With little or

no acid present, the equilibrium lies well to the left, and there are few preformed protonated amine molecules (ions, RNH/);

the FAB mass spectrum (a) is typical. With more or stronger acid, the equilibrium shifts to the right, producing more

protonated amine molecules. Thus, addition of acid to a solution of an amine subjected to FAB usually causes a large

increase in the number of protonated amine species recorded (spectrum b).



20



Mass Spectrometry Basics



M + A+



~



W+



A



(a)



M + A



~



M+ +



A-



(b)



M + A



~



M- +



A+



(c)



M(H) + M'+~



M(-H)" + MH+



(d)



Figure 4.4

Collision of a fast-moving ion (A+) or atom (A) with neutral molecules (M) can lead to an electron being stripped from

the molecule by charge exchange to give an ion (M+) or can lead to an electron being deposited in the molecule to give

M- (processes a, b, c). The initially formed ion (M+) can remove a proton from another molecule, M(H), to give protonated

molecular ions, [M + H]+ (process d).



Random Fragmentation

As well as the two specific effects discussed previously (momentum transfer and ionization by

electron transfer), bombardment of an organic liquid (solution) with a stream of atoms or ions

having very high kinetic energies leads to fairly random bond cleavages to produce radicals and

small neutral substances that, in turn, can be further fragmented or can recombine to form other

neutrals. All of these bits of fragmented or synthesized molecules can be ionized also. Therefore,

an FAB or LSIMS spectrum will have a background of peaks at almost every m/z value and of

fairly uniform height. The appearance is rather like viewing grassland by lying down and looking

along or through the grass. Indeed, the FAB background is sometimes called "grass."

A major advantage of using a liquid target in SIMS lies in the fact that these randomly produced

fragments appear first of all in the surface layers and then diffuse more or less rapidly into the bulk

of the liquid, i.e., they are greatly diluted as new surface is constantly formed. On the other hand,

sample molecules are distributed throughout the liquid at the start, and, as they disappear from the

surface during bombardment, they are continually replaced. Thus, the liquid (matrix) reservoir

disperses and dilutes the fragments that would otherwise give a large background of ions and

provides a flow of new sample (and solvent) molecules to the surface so that molecular and quasimolecular ions are continuously replenished.

Despite the high kinetic energy of the bombarding atoms or ions and transfer of some of this

energy during collision, the newly formed ions collide with other molecules and, before being

ejected from the surface of the liquid, lose most of any excess internal vibrational and rotational

energy. Therefore, these ions do not fragment, so any sample substance dissolved in the liquid

shows up as molecular or quasi-molecular positive or negative ions. The ionization process is mild,

with no additional heat needed for vaporization. Therefore thermally labile molecules like peptides

are readily amenable to FAB or LSIMS, giving good molecular mass information.



Properties of the Solvent (Matrix)

Liquids examined by FAB or LSIMS are moved on the end of a probe until the liquid becomes

( situated in the atom or ion beam. Because of the high-vacuum conditions existing in a mass

spectrometer ion source, there would be little point in trying to examine a solution of a sample

substance dissolved in one of the common solvents used in chemistry (water, ethanol, chloroform,

etc.). Such solvents would evaporate extremely quickly, probably as a burst, upon introduction into

the ion source. Instead, it is necessary to use a high-boiling liquid as solvent (matrix). A lowtemperature probe has been described, which does utilize low-boiling solvents. Finally, upon

bombardment, the solvent itself forms ions that appear as background in a mass spectrum. Very

often, protonated clusters of solvent ions can be observed (Figure 4.5).



FAB and LSIMS Ionization



21



93



185



I



369



II



I



m/z



Figure 4.5

A typical FAB mass spectrum of glycerol alone, showing a protonated molecular ion at mlz 93 accompanied by decreasing

numbers of protonated cluster ions (rn/z, 1 + nx92; n = 2, 3,4, ... ).



TABLE 4.1

Some Commonly Used Solvents for FAB or LSIMS

Solvent



Protonated molecular

(rn/z) ions



Glycerol



93



Thioglycerol



109



3-NOBN



154



NOpb



252



Triethanolamine



150



Diethanolamine



106



POlyethylene glycol (mixtures)

a



3-Nitrobenzyl alcohol



b



n-Octy l-3-nitrophenyl ether



C



Wide mass range depending on the glycol used.



In addition to low volatility, the chosen liquid should be a good all-around solvent. Since no

one liquid is likely to have the required solvency characteristics, several are in use (Table 4.1). If

a mass spectrum cannot be obtained in one solvent, it is useful to try one or more others before

deciding that an FAB spectrum cannot be obtained.



The Mass Spectrum

FAB or LSIMS leads to ions being formed from (a) the sample substance, (b) the matrix or solvent

(including clusters), and (c) general radiation-induced fragmentation. A typical example of a

substance (M; molecular mass, 1000) dissolved in glycerol is shown in Figure 4.6. Protonated

molecular ions at rn/z 1001 can be observed, together with cluster ions from the solvent (rn/z 92,

185, 277, ... ) and a background of randomly fragmented pieces of ionized solvent and substrate

in mainly small abundance. This background is sometimes referred to as "grass." It usually does

not prove to be a problem because the abundance of ions making up the "grass" is fairly uniform.

There can be a problem when the sample under investigation itself gives few molecular ions,

making it impossible or difficult to distinguish them against the background. At high molecular

mass of the sample, the momentum from the bombarding atoms becomes less effective in ejecting

molecular ions, which therefore are not abundant and are not easy to discern against the background.

In general, FAB and LSIMS will give excellent molecular mass information in the range

(approximately) of m/z 100-2000. Above this value, the abundance of molecular ions tends to

diminish until, in the region of m/z 4000-5000, they become either nonexistent or very difficult to



22



Mass Spectrometry Basics



93



(a)

185



1001



m/z

(b)



1001



m/z



Figure 4.6

(a) A typical FAB mass spectrum in glycerol with protonated sample molecular ions at m/z 1001, protonated glycerol at

rnJz 93, protonated glycerol clusters at m/z 185, 277, ... , and general background ions. (b) The spectrum illustrates the

different appearance of recording and expanding from above m1z 300 to m/z 1100. Note the fairly uniform appearances of

the background peaks and the absence of solvent cluster ion peaks.



discern against the background. Because background ions above about mlz 200-300 tend to have

similar and small abundances, FAB or LSIMS mass spectra are often recorded above, say, mlz 200;

this practice also eliminates most of the solvent cluster ion peaks (Figure 4.6b). Alternatively,

computer-aided background subtraction can be used to enhance the visibility of molecular ion peaks.



Conclusion

By using a beam of fast atoms or ions incident onto a nonvolatile liquid containing a sample

substance, good molecular or quasi-molecular positive and/or negative ion peaks can be observed

up to about 4000-5000 Da. Ionization is mild, and, since it is normally carried out at 25-35°C, it

can be used for thermally labile substances such as peptides and sugars.



Chapter



Field Ionization (FI) and Field

Desorption (FD)

Introduction

The main difference between field ionization (FI) and field desorption ionization (FD) lies in the

manner in which the sample is examined. For FI, the substance under investigation is heated in a

vacuum so as to volatilize it onto an ionization surface. In FD, the substance to be examined is

placed directly onto the surface before ionization is implemented. FI is quite satisfactory for volatile,

thermally stable compounds, but FD is needed for nonvolatile and/or thermally labile substances.

Therefore, most FI sources are arranged to function also as FD sources, and the technique is known

as FIIFD mass spectrometry.



Field Ionization

If an electric voltage (potential) is placed across an arrangement such as that shown in Figure 5.1, the

linesof equipotential in the resulting electric field crowd in around the needle tip. In this region the field

is more intense than, say, near the plate electrode where there are fewer lines of equipotential per unit

area. When the needle tip is very fine and the applied potential is large, then very intense electric fields

can be generated at the surface (point) of the tip. It is in these high-field regions that ionization occurs.

Unless extremely high potentials are to be used, the intense electric fields must be formed by

making the radius of curvature of the needle tip as small as possible. Field strength (F) is given by

Equation 5.1 in which r is the radius of curvature and k is a geometrical factor; for a sphere, k = 1,

but for other shapes, k < 1. Thus, if V = 5000 V and r = 1~ m, then, for a sphere, F = 5 x 109 VIm;

with a larger curvature of, say, 10-4 m (0.1 mm), a potential of 500,000 V would have to be applied

to generate the same field. In practice, it is easier to produce and apply 5000 V rather than 500,000 V.

F=VIkr



(5.1)



When a neutral molecule settles onto an electrode bearing a positive charge, the electrons in the

molecule are attracted to the electrode surface and the nuclei are repelled (Figure 5.2), viz., the electric

field in the molecule is distorted. If the electric field is sufficiently intense, this distortion in the molecular

fieldreduces the energy barrier against an electron leaving the molecule (ionization). A process known



23



Mass Spectrometry Basics



24



--------8



Figure 5.1

An electric potential placed across a needle and a flat (plate) electrode. The lines of equipotential in the resulting electric

field are focused around the tip of the needle, where the electric field becomes very large.



as quantum tunneling occurs by which one of the molecular electrons finds itself on the electrode side

of the barrier and is promptly neutralized by the positive charges in the electrode. The molecule (M)

has then been turned into a positive ion. Because positive charges repel each other, the newly formed

positive ion on a positive electrode is repelled by the electrode and flies off into the vacuum of the ion

source toward the negative counter electrode (Figure 5.3). A slit in the counter electrode (or a grid

electrode) allows the ion to pass into the mass analyzer part of a mass spectrometer.

The electrical reverse of the above arrangement produces negative ions. Thus, a negative needle

tip places an electron on the molecule (M) to give a negative ion (M--), which is repelled toward

a positive counter electrode.



Design of the Needle Tip Electrode

If there were only one such tip electrode, the yield of ions would be very limited (small surface

area and small numbers of ions formed per unit time). To increase ion yield, it is better to use



®



~

Electrode



(a)



Molecule



Molecule on the

surface of

electrode



®

~



@



===>

Electron transfer

to the eleetode



Electrode



(b)



Ion is

repelled by the

electrode



Figure 5.2

In (a), a molecule alights onto a positive electrode surface, its electrons being attracted to the surface and its nuclei repelled.

In (b), an electron has tunneled through a barrier onto the electrode, leaving a positive ion that is repelled by and moves

away from the positive electrode.



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