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Field Ionization (FI) and Field Desorption (FD)

Field Ionization (FI) and Field Desorption (FD)

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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.



25



Field Ionization (FI) and Field Desorption (FD)



To mass

spectrometer



Positive tip

electrode



Jon repelled

towards negative

electrode



Negative

counter

electrode



Figure 5.3

A positive ion formed at a positive electrode tip is repelled and travels toward the negative counter electrode, which has a

slit in it so that the ion can pass into the mass spectrometer.



manytips or microneedles. This might be achieved by using a sharp edge like a razor blade. Indeed,

this was one of the first types of ionization sources to be used, because even the sharpest razor

blade, on a molecular scale, is very rough and has many small tips on its surface (Figure 5.4). Such

an edge is better than a single needle, but it still does not provide an efficient ion source.

Eventually, methods were found for growing microneedles or "whiskers" on the surface of a thin

wire. For example, by maintaining a narrow wire at a high temperature in the vapor of benzonitrile,

decompositionof the nitrile on the hot wire produces fine, electrically conducting growths or whiskers

(microneedles) having tips of very small radius of curvature (Figure 5.5). Application of a high electric

potential to such a wire produces many ionization points and a high yield of ions.

These thin wires are supported on a special carrier that can be inserted into the ion source of

the mass spectrometer after first growing the whiskers in a separate apparatus. Although the wires

are very fragile, they last for some time and are easily renewed. They are often referred to as emitter

electrodes (ion emitters).



Practical Considerations of Field Ionization/Field Desorption

Although there has been some controversy concerning the processes involved in field ionization

mass spectrometry, the general principles appear to be understood. Firstly, the ionization process

itself produces little excess of vibrational and rotational energy in the ions, and, consequently,

fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods

available for producing excellent molecular mass information. The initially formed ions are either

simple radical cations or radical anions (M·-).



Figure 5.4

Magnification of a sharp edge showing the many tips and valleys on a molecular scale.



26



Mass Spectrometry Basics



(a)



(b)



Figure 5.5

A narrow wire (a) heated in the vapor of an organic compound such as benzonitrile causes decomposition of the nitrile and

the formation of whiskery growths on the surface of the wire (b). The sizes of the growths are exaggerated for purposes

of illustration and are. in fact. very small in relation to the diameter of the wire.



However, in both FI and PD, there are other neutral molecules on or close to the surface of

the emitter and, in this region, ion/molecule reactions between an initial ion and a neutral (M(H»)

can produce protonated molecular ions ([M + HJ+), as seen in Equation 5.2.

M(H) + MO+



~



M(-HO) + (M + H]+



(5.2)



For simple FI, the substance to be mass measured is volatilized by heating it close to the

emitter so that its vapor can condense onto the surface of the electrode. In this form, an FI source

can be used with gas chromatography, the GC effluent being passed over the emitter. However, for

nonvolatile andlor thermally labile substances, a different approach must be used.

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied

to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the

solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally

labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide

excellent molecular mass information. Although still Fl, this last ionization is referred to specifically as

field desorption (FD). A comparison of PI and FD spectra of D-glucose is shown in Figure 5.6.

~



100



181



(a) FI



80

60

163



121

127



40

20



OL-...+_--,,--_~'--"""+--u"'---,,,,,_-,-.lL-_-1"-_



60



~



100



80



100



120



140



160



180

181



(b) FD



80

60



40

20



0'--+---,,----,----r--_...--_..........L--_-jU-_

60



80



100



120



140



160



180



mlz

Figure 5.6

A comparison of the (a) PI and (b) FD spectra of D-glucose. Note the greater fragmentation in PI (heat applied for

volatilization) and the appearance of ions at mlz 180 in the FD spectrum as well as the [M + H]+ ions at mlz 181 in

both spectra.



27



field Ionization (FI) and Field Desorption (FD)



By intentionally adding inorganic salts to the solution used for FD, cationated molecular ions

can be produced in abundance. Equation 5.3 illustrates how addition of NaCI can give rise to [M

+ Na]' ions.

M + Nat + Cl



~



[M + Na]! + Cl-



(5.3)



Sometimes, in FD, the emitter electrode is heated gently either directly by an electrode current

or indirectly by a radiant heat source to aid desorption of ions from its surface.



Types of Compounds Examined by FI/FD

Newer developments in ionization methods have tended to overshadow FI, particularly in view of

the fragile nature of the emitters and the need for a separate apparatus in which to form the

microneedles. In contrast, FD still offers advantages, being able to ionize a wide range of mass

spectrometrically difficult substances (peptides, sugars, polymers, inorganic and organic salts,

organometallics).

Because there is little fragmentation on FD, it is necessary to activate the molecular or quasimolecular ions if molecular structural information is needed. This can be done by any of the methods

used in tandem MS as, for example, collisional activation (see Chapters 20 through 23 for more

information on tandem MS and collisional activation).



Conclusion

PI and FD are mild or "soft" methods of ionization that produce abundant molecular or quasimolecular positive or negative ions from a very wide range of substances. In the FD mode, it is

particularly useful for high-molecular-mass and/or thermally labile substances such as polymers,

peptides, and carbohydrates.



,



,,~

,



I(



I



Chapter



Coronas, Plasmas, and Arcs

Background

Charged particles from the sun, usually protons, encounter the earth's magnetic field, spiral down

toward the negatively charged earth, and meet the atmosphere above the magnetic north and south

poles (Figure 6.1). The charged particles from the sun are moving at high speed and begin to collide

with molecules of oxygen, nitrogen, and other gases in the upper atmosphere. These high-energy

collisions cause electrons in the gas molecules to be excited into higher energy orbitals to form

excited atoms or ions (Figure 6.2). Additionally, nitrogen and oxygen ions are formed if an electron

is ejected from a molecule by the energy of collision. During this process, the remaining electrons

in the ions may also be excited. Other gases are present in the atmosphere, such as carbon dioxide,

and argon, and these also form excited molecules and ions. When the electrons in the excited atoms

or ions return to their original (ground) state, light is emitted. The light can be green, red, or other

colors, depending on which molecules have been excited (Figure 6.3).

Such events account for the appearance of the very pretty, mysterious displays of lights in the

sky in the polar regions of the northern and southern hemispheres, viz., the aurora borealis and

aurora australis. (Sometimes the light can be observed in areas more distant from the poles and,

in the north, they are called the "northern lights.") These natural phenomena are manifestations of

electric discharge physics, namely the passage of charged particles through a gas. Excitation of

atoms or molecules in an electric field by electrons and recombination of ions and electrons causes

the formation of excited species, which emit light. Other examples of the discharge can be seen in

lightning flashes, the common yellow sodium street lights, fluorescent lighting, and arc welding.

In the laboratory, it has been found that similar effects can be produced if a voltage is applied

between two electrodes immersed in a gas. The nature of the laboratory or instrumental discharge

depends critically on the type of gas used, the gas pressure, and the magnitude of the applied

voltage. The actual electrical and gas pressure conditions determine whether or not the discharge

is called a corona, a plasma, or an arc.

Although the discharges attract interest because of the emitted light, the major components of

thedischarges are ions and electrons. Such electrons can be utilized in mass spectrometry to enhance

ionization of sample molecules, and the ions themselves can be used to gain information about the

sample (m/z values and abundances of ions). This latter use of discharges is the subject of this

chapter. The effects of electrons in these discharges can be greatly enhanced by the application of

an external high-frequency electromagnetic field, which leads to the plasma discharge reaching very

high temperatures, as in plasma torches (see Chapter 14 for more information on plasma torches).



29



Mass Spectrometry Basics



30



Mag;;;iC field

/



/

I:



~



::J



7



/



til

Cl)



::



/



/



E



~

til



Cl)



~



\



o

<11

Q.



\



'Cl




~



<11



1



\

<,



.c



o



-



lines "



/



<,



---- --



~



.../



..........



Figure 6.1



/

/



Schematic representation of the movement of positively charged particles (mostly protons) from the sun entering the earth's

magnetic and electric fields. Under the influence of the magnetic component, the particles spiral down toward the north

and south poles of the negatively charged earth, clockwise at one pole and counterclockwise at the other. When the incoming

charged particles collide with molecules of oxygen, nitrogen, argon, water, and other gas molecules in the upper atmosphere,

the resultant excitation leads to emission of light, known as the aurora borealis in the northern hemisphere and the aurora

australis in the southern.



(M)



t



Total

atom

energy



(M')



,COllision



*"

"*"

"*"



with

electrons



Orbitals

occupied

by pairs

of electrons

(ground state)

(a)



::>



(M')



.,..

-1-



(M·...,



4-



-1-



"*"

* "*

* "*

"*



Electronically

excited

atom



Ground

state

positive

ian



Electronically

excited

positive

atom



(b)



Figure 6.2

(a) Occupied and unoccupied orbital energy levels in an atom. Some of these are occupied by pairs of electrons (the electron

spins in the bonding levels are indicated by the arrows), and some are empty in the normal ground state of the atom (M).

After collision with a high-energy (fast-moving) electron, one or more electrons are promoted to higher unoccupied orbitals.

(b) One electron is shown as having been promoted to the next higher orbital energy to give an electronically excited atom

(M'). Alternatively, the promoted electron may be ejected from the atom altogether, leaving a ground-state positive ion

(M'). Finally, an ion can be formed by loss of an electron but, at the same time, another electron may be promoted to a

higher orbital to give an electronically excited ion (M").



Electric Discharges in a Gas

In this discussion, only inert gases such as argon or neon are used as examples because they are

monatomic, which simplifies description of the excitation. The introduction of larger molecules into

a discharge is discussed in later chapters concerning examination of samples by mass spectrometry.

If a gas such as argon is held in a glass envelope that has two electrodes set into it (Figure 6.4),

application of an electric potential across the electrodes leads to changes in the gas as the electrons

flow from the cathode (negative electrode) to the anode (positive electrode). This passage of electrons



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