Mass spectrometry, the science and [610911]

Mass spectrometry, the science and
technology of gaseous ions (1), hasas its basis the measurement of
mass-to-charge ratios (m/z) of ions.
All atomic and molecular ions are,in principle, accessible by mass
spectrometry, making it a universal
method for chemical analysis. Itsimplementation requires suitable
methods of ion generation, ion
analysis,and iondetection. Wetreateach of these processes in turn and
show below that there are multiple
methods of accomplishing each.
The first step in recording a
mass spectrum is to convert analyte
molecules (or atoms)into gasphaseions. In biological applications, the
most common ionization tech-
niques are electrospray ionization(ES) (2), atmospheric pressure
chemical ionization (APCI) (3,4),
and matrix-assistedlaser desorptionionization (MALDI) (5). These are
soft ionization methods in the sense
that atleastsome analytemoleculesare converted, intact, into corre-
sponding ions. Solution phase sam-
ples are examined with ES andAPCI while MALDI is particularly
appropriate forsolid phasesamples.
Having successfully generated gasphase ions, they must then be mass
analyzed. There are several differ-
ent types of mass analyzers, allbased on theinteractions ofchargedparticles with electric and/ or mag-
netic fields.
T1summarizes the
most common mass analyzers and
lists some analytical performance
characteristics by which they canbe compared (6). As with the vari-
ous ionization methods, there is no
single right choice — the nature ofthe problem and the resources of
the laboratory will dictate which
mass analyzer is most appropriate.
Most uses of mass spectrome-
try are made in combination with
chromatographic separation, princi-pally in the form of the GC/MS or
LC/MS technique. These combina-
tions have been used, for example,in organic analysis in the environ-
mental sciences and in charac-
terization of biological compounds,including molecular weight (MW)
determinations, and sequence
analyses of biopolymers (7). In-creasingly important applications
have been found in drug metabo-
lism and protein sequencing due tothe high sensitivity and chemical
specificity of mass spectrometry.
These advantages apply even whenthe samples are presented to the
mass spectrometer as mixtures
since the two-stage tandem massspectrometry (MS/MS) experiment
serves as a method of separation as
well as characterizationof thesepa-rated components.Quadrupoles and Ion Traps
With the above background on
ionization and mass analysis, we
can now introduce a family of mass
analyzers whose operation is basedon ion motion in rf electric fields.
The quadrupole mass filter (8), or
linear quadrupole, consists of a lin-ear array of four symmetrically ar-
ranged rods (
F1) to which rf and dc
voltages are supplied. Forces areexerted in a plane normal to the di-
rection (z-direction) in which the
ions drift through the array in theirjourney from the ion source to the
detector. The rf potential gives rise
to a field which alternatively rein-forces and then dominates the dc
field, also applied by coupling op-
posite sets of rods. Ions oscillate inthe x,y-plane with frequencies
which depend on their m/z values
and with excursions which dependon the amplitudes of the applied
potentials andtheirinitial positions.
If the oscillations of an ion in thisplane are stable, the ion will con-
tinue to drift down the rod assem-
bly and reach the detector. Stableoscillations are only achieved by
ions of given m/z values for a given
rod assembly, oscillation frequency,rf voltages, and dc voltage. The
range of values of m/z which corre-
spond to stablemotion canbe madeIon Trap Mass Spectrometry
Philip S.H. Wong
Bioanalytical Systems
West Lafayette, IN
47906-1382
R. Graham Cooks*
Department of Chemistry
Purdue University
West Lafayette, IN
47907
*Corresponding author
E-mail:
cooks@purdue.eduThe operating principles of linear quadrupoles and quadrupole ion traps
are described, and the performance characteristics of triple quadrupolesand ion trap instruments are compared. The theoretical basis for massanalysis using quadrupole fields is also described. The highperformance of quadrupole ion traps is illustrated by introducing somenew developments, including mass range extension, high resolutionexperiments, MS
nexperiments, selective ion manipulation techniques,
and non-destructive ion detection.

very large (wide band pass) or it
can be a single m/z value (narrow
band pass). In practice, ions of aparticular m/z value are often se-
lected,and mass scanning is usually
achieved by sweeping the dc and rfvoltages, keeping their ratio and the
oscillator frequency constant.
The quadrupole ion trap,
(9,10), the subject of this article, is
the three dimensional analogue of
the linear quadrupole mass filter. Inthis devicetoo, ions aresubjectedto
forces applied by an rf field but the
forces occur in all three, instead ofjust two, dimensions. Stable motion
of ions in the linear quadrupole al-
lowed ions freedom of motion inone dimension (z-direction); in the
ion trap, stable motion allows no
degrees of freedom. Hence, ions aretrapped within the system of three
electrodes-a ring electrode and two
end-cap electrodes of hyperboliccross-section (
F2). The principal
advantages of the quadrupole ion
trap in chemical analysis can besummarized as follows:
(i) high sensitivity,
(ii) compactness and mechanical
simplicity in a device which is
nevertheless capable of highperformance,
(iii) tandem mass spectrometry ex-
periments are available by per-forming sequential mass analy-
sis measurements,
(iv) ion/molecule reactions can be
studied for mass-selected ions,
(v) high resolution (>10
6at m/z
>1000) is accessible throughslow scans, but mass measure-
ment accuracy is relatively poor,MethodQuantity
MeasuredMass/charge
(m/z) rangeResolution at
m/z = 1,000Dynamic
Range
Sector Magnet momentum/charge 104105107
Time of Flight flight time 106103-1 04104
Ion Cyclotron
Resonancecyclotron
frequency 105106104
Ion Trap frequency 104104104
Quadrupole
mass filterfilters
for m/z 103-1 04103-1 04105T1
Characteristics of Differ-
ent Mass Analyzers.
(Adapted from
reference 6.)
Ion Source Detector
Z
YXQuadrupoleMass FilterF1
Schematic diagram
showing the operation
of the quadrupole mass
filter. Note that as ions
drift through the array
of rods they are sub-
jected to forces which
cause oscillation in the
x,y-plane (shaded).
Ion SourceIons in
Endcap Endcap RingIons out
Detector
Endcap EndcapCeramic post
and spacer
RingZo
roF2
The ion trap consists of
three electrodes with hy-
perbolic surfaces, the
central ring electrode,
and two adjacent end-
cap electrodes. The
schematic of the assem-
bly shows how the elec-
trodes are aligned and
isolated using ceramic
spacers and posts. The
device is radially sym-
metrical, and r oand zo
represent its size.

(vi) ions of high mass/charge are
accessible using resonance ex-periments, and
(vii) non-destructive detection is
available using Fourier trans-form techniques.Comparisons of the Ion Trap
with the Triple Quadrupole
The differences in operating
principles of the linear quadrupole
and the ion trap have just been de-scribed. In comparing their per-formance characteristics, one im-
mediately notes that a unique fea-
ture of an ion trap is that MS/MSexperiments are possible. Even
when compared with a triple quad-
rupole MS/MS instrument, the iontrap can perform multiple stage
mass spectrometry (MS
n) simply
by the use of additional operationswhich are performed sequentially
in time. The triple quadrupole has
the advantage of access to parention and neutral loss scans, and ana-
lytically useful versions of the
MS/MS experiment; however, MS
n
experiments can only be performed
in multi-quadrupole instruments.
Although ion/molecule reactionscan be studied in both instruments,
the reaction time can only be varied
in the ion trap. This allows the ki-netics and equilibrium of ion-mole-
cule reactions to be studied. On the
other hand, the triple-quadrupoleinstrument provides good control
over the kinetic energies of the ionsF=f (x)
E=f (x)
Field = E = -2Φ
dΦ
dx
XF3
Potential and field
strength in a hypothetical
one-dimensional quadru-
pole field. The slope of
the plot of potential ( Φ)
against position (x) yields
the field strength E(x).
V
V(A)
(B)
roro
-Zo-Zo
ZoZo
-ro-ro
AxialDimensionAxialDimension
Radial DimensionRadial DimensionF4
Potential used for trap-
ping ions (A) in the radial
direction and (B) in the ax-
ial direction. An ion in the
position shown is acceler-
ated away from the trap
center in the axial direc-
tion at the rf phase
shown in (A) and towards
it in (B) (Adapted from ref-
erence 13).

which are important for thermo-
chemical studies.
A recent, instructive compari-
son of the Finnigan LCQ ion trapwith the Finnigan TSQ 700 triple
quadrupole mass spectrometer was
made using an LC/APCI/MS assayfor several spinosyns (11). The
overall sensitivityofthe LCQ in the
full-scan mode was found to be 5-10 times greater than the TSQ. In
contrast, in the selected ion moni-
toring mode, in which a single ionis monitored, the TSQ was found to
be 3-5timesmore sensitivethan the
LCQ. Similar results were obtainedin a comparative study of the LCQ
ion trap and the PE/Sciex API 300
triple quadrupole instrument usingLC/MS/MS quantitation of orlistat
in human plasma(12). Clearly,both
instruments have unique strengths.Given the small size, relatively low
cost, modest pressure requirements,
and experimental flexibility of thequadrupole ion trap, an increasing
number of analyses will be per-
formed with ion traps coupled with
ion sources (ES or APCI) which al-low solution analysis.
Operating Principle of
Quadrupole IonTraps
Quadrupole Fields
A quadrupole field is one in
which the field strength E varies
linearly with displacement x,
The applied potential Φwhich es-
tablishestheelectric field mustvary
quadratically in order that the field
strengthvary linearlywith x. Hence
andIf it were possible to employ a
system of electrodes and construct a
one-dimensional quadrupole field,then the potential distribution and
the field strength would be as
shown in
F3. Ions located an in-
creasing distance from the center
would be subjectedto a force which
would increase linearly with dis-placement and which would tend to
return the ions to the center of the
device. Ions could be trapped in sucha hypothetical field. If the field direc-
tion were reversed, a potential maxi-
mum would occur and ionswouldbeaccelerated away from the center.
In the three-dimensional quad-
rupole field present in an actual iontrap, ions are alternativelysubjected
to stabilizing and destabilizing
forces and oscillate in both the r-and z-directions.When the phase of
the rf signal is positive, the quadru-
pole potential surface is saddle-shaped as shown in
F4A. An ion
located as shown is on a potential
downhill in the z-direction and itwill be accelerated from the center
of the device. As the rf field
changes sign, the field inverts andthe same ion is accelerated towards
thecenterofthetrap(
F4B). Similar
considerations apply with respecttoan ion displaced in the radial (r) di-
rection. If the field inverts at an ap-
propriate rate, the ions will betrapped in both the r- and z-direc-
tions, in the volume defined by the
ring and the end-cap electrodes(13).
Mass Analysis Using Quadru-
pole Fields
Physically, ion traps are made
up of a rotationally symmetrical
ring electrode of hyperbolic shape
and two endcap electrodes of thesame cross-section. An rf voltage is
applied to generate an electric
quadrupole field. Because the elec-tric field is rotationally symmetric, it
is convenient to consider only radial
and axial (z) displace-
ments. The potential ( Φ
r,z)a ta n y
point in this field is given by-0.20.4
0.20.30.50.60.70.81.0
0.1
0.4
0.51.0 1.5
0.6
0.7
0.8
0.90.4az
qz
-0.6-0.40.2z stability
rstabilityq = 0.908eject
Operating linefor
mass selective stabilityOperating linefor
mass selective instabilityF5
The Mathieu stability
diagram for the quadru-
pole ion trap. Ions are
stable in both the r-
and the z-direction if
their Mathieu parame-
ters azand qzfall within
the shaded area in the
diagram. The common
mode of mass analysis
is the mass-selective in-
stability scan in which
the rf potential is raised
to increase the value of
qzto the instability point
qz= 0.908, while a z=0.
Ε=ΕΟ (1) x
Φ =f( x)2 (2)
Ε=dΦ
dx=f(x) (3)xy22+r=
r -2z + 2z22
020202
r+ 2 zΦωr,z= (U +Vcos t)( )
(4)

where the first term describes its
temporal variation and the second
its spatial dependence (9). Note
againthatrois the internalradiusof
the ring electrodeand zois theclos-
est distance from the center to the
end-cap, while U is the dc potential
and V is the rf potential (zero-to-
peak) applied between the ring and
end-cap electrodes, ωis its angular
frequency, and t is time. Ions of a
given m/zvaluemay undergo stable
motion in the trap for the reasons
already given qualitatively. The
quantitative solution to the stabilitycondition is described by a second
order differential equation of the
Mathieu form. The solutions to this
equation (actually two independent
equations which describe the un-
coupled motion of an ion in the r-
and z-directions) represent stability
conditions which are readily sum-
marized in the form of a stability
diagram (F5) expressed in terms of
the Mathieu coordinates azand qz
(EQ5-6).Radial stability, expressed in
terms of arand qr, must also be
maintained simultaneously with
stability in the z-direction. Note
that ions with identical Mathieu pa-rameters but different m/z values
behave identically. Optimum opera-
tion requires the ions have favor-able initial conditions, which is
achieved by using a helium buffer
gas (~1 mTorr) to remove kineticenergy from the ions and cause
them to occupy the central region
of the trap. Typically, the ion trapcan hold up to about 10
5-1 06ions
before coulombic repulsions sig-
nificantly affect their trajectories andgreatly reduce the mass resolution.
Mass spectra are normally re-
corded by operating the quadrupoleion trap in the mass selective insta-
bility scan mode (9). In this experi-
ment, the amplitude V of the ap-pliedrfisincreasedsoasto“move”
ions along the q
zaxis (F5) until
they become unstable at the bound-ary, where q
z= 0.908. As they ap-(B) Amplitude of rf signal
and supplementary ac signals
End Cap Electrode(A) Amplitude of rf signal
Ion Motion in r direction Ion Motion in r direction
10 mm7.1 mm,500V
12Vrf'
AC
Ion Motion inz direction Ion Motion inz direction
10 mm 10 mmRing
ElectrodeMicroseconds 100 0 Microseconds 0 100
7.1 mm10 mm
160 MHz7.1 mm,
78 kHz500V
1.1 MHzrf'
7.1 mmRing
ElectrodeF6
(A) A simulation of the tra-
jectory of an ion of m/z
100 in a r 0= 1 cm ion
trap operated at a rf volt-
age of 500 V and a fre-
quency of 1.1 MHz. The
first three boxes are time
plots of the instantaneous
rf amplitude, the excur-
sion of the ion from the
center in the r-direction,
and the z-excursion, re-
spectively. The last box is
a plot of r, z-motion. (B)
The same simulation in
which a supplementary
ac voltage is applied at
the time indicated to reso-
nantly excite ion motion.
(Adapted from reference
9.)
90100
50607080
40
30
20
10
0
1960 19611961.121961.351961.631961.881962.121962.36
1962.60
1962.87
1963.10
1963.34
1963.59
1963.85
1964.09+4 IL-8 (Rat)
1962
m/z1963 1964 1965RelativeAbundanceF7
Zoomscan showing part
of the ESI mass spec-
trum of rat interleukin-8,
including the isotope en-
velope around around the
[M+4H]4+ion at m/z
1,962. (Adapted from Fin-
nigan LCQ Operator’s
Manual, Revision B, July
1996.)
-16zU
0202m(r + 2z ) ω2a = -2a =zr (5)8zV
0202m(r + 2z ) ω2q= – 2 q=zr (6)

proach the region ofinstab ility,their
kinetic energies and z-direction ex-
cursions increase and they exit the
trap through a hole in the end-capelectrode and reach an external de-
tector. Ions of increasing m/z are
ejected and detected as the rf volt-age V is raised, so yielding a mass
(actually m/z) spectrum. The mass
analysis equation for a quadrupoleion trap operated in the mass-selec-
tive instability mode is obtained sim-
ply by rearranging the expression for
the Mathieu parameter q
z(EQ6)
Thisemphasizesthefactthatin
this mode of operation, ion motion
isconstrainedtotheaz=0 axis,i.e.,no dc voltages are applied to the
end-cap electrodes. For traps built
with the so-called ideal geometry, ,
EQ7 can be simplified
toEQ8
Trapped ions have charac-
teristic frequencies of oscillation,
known as secular frequencies, again
separately in both the r- and z-di-rections. The principal component
of these secular frequencies is
(ω/2)βradian per second, where β
is a parameter that varies with the
coordinates a and q of which it is a
continuing fraction. (At low valuesof a
zand qz,βz is approximately
given by . Motion is un-
coupled in the r- and z-directions
and the r-frequency is half that in
the z-direction. Because ions havethe characteristic frequencies just
noted, a supplementary ac potential
of frequency equal to the secularfrequency of motionof the ions will
c a u s ei o n st op i c ku pi n c r e a s i n g
amounts of kinetic energy. If thesignal is applied between the end-
cap electrodes, ions will be acti-
vated in the z-direction. If the reso-nant signal is strong enough, these
translationally activated ions can be
ejected from the trap in the z-direc-tion.
This resonance experiment is
extremely valuable in causing par-ticular ions to be excited so that
they can be made to dissociate or
eject so that the population of ionsin the trap can be controlled.
F6A
shows a simulation of the trajectoryof anion ofm/z =100 in anion trapoperated at an rf voltage of 500 V
and a frequency of 1.1 MHz. Be-
cause of the particular initial condi-tions chosen, the center of the trap
is not visited; instead, a “donut” of
space is accessed.
F6Bshows the
simulated ion trajectory when a
supplementary ac potential, in reso-
nance with the frequency of ionmotion in the z-direction, is applied
across the endcap electrodes. It can
be seen thatthereis no effect on ionmotion in the r-direction. However,
the excursion in the z-direction in-MS/MS
1363
10871249
-TFA
MS5779
-Glc
or
-RhaMS4925
941
-Rha
779-Glc
MS6617
455-Glc
-Glc
250 500 1000 750
m/zRelativeAbundance
RelativeAbundanceRelativeAbundanceRelativeAbundanceRelativeAbundanceRelativeAbundance
1250 1500COO Glc
Rha Glc
GlcGlc OMS
MS3(M+TFA) –
MW12501363
1087
-GlcF8
Sequential MS6analy-
sis of an oleanolic acid
glycoconjugate per-
formed using a Finni-
gan LCQ ion trap instru-
ment showing control
over loss of the sugar
monomers, to allow sim-
ple, rapid elucidation of
a complex structure.
(Adapted from Finnigan
LCQ Catalog 1996.)
8V
0202q (r + 2z )z ω2m/z = (7)4V
02qrzω2m/z = (8)2zo ro=
aq / 2z+2
z

creases, and the ion is energized
and ejected through the aperturesin
the end-cap electrodes after a fewcycles of application of the ac po-
tential. Other ions of different m/z
values are not affected, so the ex-periment can be used for selective
ejection or activation (see section
on Ion Population Control).
High Performance and
Biological Applications
Mass Range Extension By
Resonant Ejection
The mass range of an ion trap
can be calculated by substitutingappropriate values into
EQ7. Note
that qz= 0.908 is the qzvalue at
which instability occurs in the nor-
mal mass-selective instability modeof operation. However, under reso-nance conditions, q
zbecomes a
variable provided the supplemen-tary resonance frequency can bevaried. Since it appears in the de-nominator of
EQ7, it can be de-
creasedto increasem/z. Typicalop-erating conditions for the FinniganLCQ are V
0-p= 0 – 8500 V, ro=
0.707 cm, zo= 0.783 cm, ω= 0.76
MHz and (qz)eject=0 . 8 3 .T h e
maximum m/z range that can beachieved under these conditions isabout 2000 dalton/charge.
Decreasing the size of the trap
or lowering the frequency has been
used to extend the mass range of
the ion trap. However, the moststraightforward method is to lower
(q
z)ejectby modulating the ion mo-
tion ata chosenfrequency using thedipolar electric field applied across
the end caps. Ions of a particular
m/z value in resonance with the ap-plied frequency then pick up trans-
lational energy and are ejectedfrom
the trap. Conceptually, the resonantejection experiment creates a
“hole” in thestability region(
F5)at
that value of qzwhich corresponds
to the frequency applied to the end-
cap electrodes.Byscanningtheam-
plitude of the main rf voltage in thenormal way, ions of different mass
sequentially acquire q
zvalues that
give them the frequency which cor-responds to this hole and causes
resonant ejection. They exit the ion
trap in sequence of m/z values but atalowerrfamplitudethanwouldordi-
narily be required for ion ejection.
Slow Scans and High Resolution
In the usual mode of operation
(mass selective instability scan) ofan ion trap mass spectrometer, ionsof different m/z values arrive at thedetector separated in time. Ions ofincreasing mass are ejected in turnas increasing rf voltages are appliedto the ring electrode. If the rate atwhich the amplitude V of the mainrf is changed is too fast, ions willfail to respond to the instabilitycondition completely before ions ofnext value begin to be ejected. Lossof resolution will result. By slow-ing the rate atwhich V is increased,an improvement in resolution is ex-pected (14, 15). In fact, this type ofexperiment has been shown to pro-duce extremely high resolution: inexcess of 10
6at m/z 3000 (15). It is
most appropriately applied in azoom-scan mode in which ions ofinterest of a narrow mass window(<10 dalton/charge) are examined.The zoom-scan mode is able to re-solve the isotopic forms of multi-ply-charged ions observed in elec-trospray ionization mass spectra.
F7illustrates a zoomscan of the +4
charged state of rat interleukin-8,i.e. [M+4H]
+4. The resolved iso-
topic cluster reveals a one-fourthm/z unit difference between carbonisotopes, which are separated inmass by one dalton due to the
12C/13C difference. Therefore, the
charge on the ion must be +4.
MS/MS and MSn
Mass spectrometry/mass spec-
trometry (MS/MS), or tandem mass
spectrometry (16), is a procedure
for examining individual ions in amixture of ions.The ions of interest
are isolated by their characteristic
m/z values, activated by collision,and allowed to dissociate. The re-
sulting product ions are examined
in asecondmassmeasurementstep.In an ion trap mass spectrome-
ter, MS/MS is achieved by the use
of an additional sequence of opera-tions in the scan function. The scan
function begins with ionization and
is followed by selection of a parention in a step that involves ejecting
all other ions from the trap. The
parention isthen translationally ex-cited, typically by applying a sup-
plementary rf voltage to the end
caps. The product ions resultingfrom collision-induced dissociation
of these excited ions with the he-
lium buffer gas are recorded byscanning the rf voltage to perform a
second mass-analysis scan. The
main advantage of the MS/MS ex-periment is its enhanced specificity.
This is useful in isomer distinction,
sequencing of biopolymers, andmost particularly in the analysis of
complex mixtures. Tandem mass
spectrometry experiments eliminateor greatly reduce signals due to
other matrix components or instru-
mental background (chemical noise).
A unique featureoftheiontrap
is the MS
ncapability which has un-
precedented powerinstructuralelu-cidation. The selectivity of MS
n
means that a compound can befragmented, and the resulting frag-ments further isolated and analyzed
to yield structural information
about complex molecules in thepresence of mixtures.
F8shows a
sequential MS6experiment per-
formed on an oleanolic acid gly-conjugate demonstrating dramatic
control over the loss of the sugar
monomers. The experiment allowssimple, rapid elucidation of a com-
plex molecular structure.
Ion Population Control By Selec-
tive Ion Manipulation Techniques
A significant weakness of ion
traps is the poor dynamic range dueto space charge effects, defined aschanges in ion motion which resultfrom the mutual coulombic interac-
tions of ions. Space charge effects
can be alleviated by removing ma-
trix ions. Similar reasons for devel-oping a capability for exciting ionsof specified m/z values exist in ion

cyclotron resonance instruments.
The basis for selective excitation is
the resonance experiment describedabove. It can be used to extend the
m/z range, as already noted, or to
allow only specified ions to frag-ment by collision induced dissocia-tion. Resonance excitation experi-ments are most effectively imple-mented by applying a mixture offrequencies so as to manipulateions of different m/z values simul-taneously. This can be done using atime domain signal in a techniqueknown as SWIFT (stored waveform
inverse Fourier transform) (17).
This was the first procedureapplied
to quadrupole ion traps for the gen-
eral purpose of ion population con-trol. In trace level analysis, it is de-
sirable to selectively fill the trap
with the analyte ions while the ma-trix ions are ejected (18). The abil-ity to selectively store ions is ex-pected to provide a substantial im-provement in limits of detection.This and related selective ion stor-age techniques have been used inultra-trace-level analysis of volatileorganic compounds at levels as lowas parts-per-quadrillion (pg/L) (19).
Non-Destructive Ion Detection
A new and simple method of
ion detection in the quadrupole ion
trap is that in which ions approach
an electrodeand polarize it sothat acurrent flows toward and away
from the electrode through an ex-
ternal conductor in response to theoscillating ion motion. The induced
current flow has a frequency equal
to that of the coherently movingions,which in turn depends ontheir
m/z value. Since the ions are not
destroyed, they can be remeasured.A non-destructive method of ion
detection (20) is achieved in ion
traps by impulsive excitation of acollection of trapped ions, typically
of different m/z values. The ion im-
age currents are induced on a smalldetector electrode embedded in, but
isolated from, the end-cap elec-
trode. The image currents are di-rectly measured using a differential
preamplifier, filter, and amplifier
combination and then Fourier ana-lyzed to obtain the broad-band fre-
quency domain spectra charac-
teristic of the sample ions. An ad-vantage of non-destructive ion de-
tection is the ability to measure a
single-ion population multipletimes.
Conclusions
Ion trap mass spectrometry has
recently undergone very rapid de-
velopment and is emerging as a
high performance technique whichshow signs of becoming one of the
leading tools in the discipline.
These instruments allow tandemmass spectrometry experiments
which are possible only using com-
binations of multiple quadrupolessuch as the highly successful triple
quadrupole instrument. Extension
to high mass/charge measurements,the development of high resolution
capabilities and the very recent
demonstration of non-destruct ive,
broad-band Fourier transform capa-
bilities, all suggest an increased
role in the future. Limitations occurin dynamic range, accurate mass
measurement, and quantitative pre-
cision.
Acknowledgement
The work at Purdue is sup-
ported by the Department of En-ergy, Officeof BasicEnergySciences.
References
1. R.G. Cooks, G. Chen, P. Wong and
H. Wollnik, Encl. Appl. Phys. 19
(1997) 289.
2. J.B. Fenn, M. Mann, C.K. Meng,
S.F. Wong and C.M. Whitehouse,
Mass Spectrom. Rev. 9 (1990) 37.
3. J.R. Chapman, Practical Organic
Mass Spectrometry, Chichester:
Wiley, 1993.
4. R.K. Mitchum and W.A. Korfmacher,
Anal. Chem. 55 (1983) 1485A.
5. M. Karas, U. Bahr and U. Giess-
mann, Mass Spectrom. Rev. 10
(1991) 335.6. J.B. Lambert, H.F. Shurvell, D.A.
Lightner and R.G. Cooks, Introduc-
tion to Organic Spectroscopy,
Macmillian Publishing Company,
New York (1987).
7. A.L. Burlingame, R.K. Boyd and S.J.
Gaskell, Anal. Chem. 68 (1996)
599R.
8. P.H. Dawson, Quadrupole Mass
Spectrometry and Its Applications,
New York: Elsevier Scientific Pub-
lishing Company, 1976.
9. R.G. Cooks, G.L. Glish, S.A.
McLuckey and R.E. Kaiser, Chem.
Eng. News 69 (1991) 26.
10. R.E. March and R.J. Hughes in
“Quadrupole Storage Mass Spec-
trometry” Wiley, New York 1989.
11. J. Gilbert, J. Balcer, L.T. Yeh and
S. Erhardt-Zabik, Qualitative and
Quantitative Performance of a
Benchtop LC/MS Ion Trap Verses
Traditional Research-Grade LC/MS
Systems, Presented at the 45th
ASMS Conference on Mass Spec-
trometry and Allied Topics, Palm
Springs, CA, June 1-5, 1997.
12. R. Wieboldt, D. Campbell and J.
Henion, LC/MS/MS Quantitation of
Orlistat in Human Plasma with an
Ion Trap and a Triple Quadrupole
Spectrometer: A Comparative
Study, Presented at the 45 th
ASMS Conference on Mass Spec-
trometry and Allied Topics, Palm
Springs, CA, June 1-5, 1997
13. C. Weil, M. Nappi, C.D. Cleven, H.
Wollnik and R.G. Cooks, Rapid
Commun. Mass Spectrom. 10
(1996) 742.
14. J.D. Williams, K.A. Cox, R.G.
Cooks, R.E. Kaiser, Jr. and J.C.
Schwartz, Rapid Commun. Mass
Spectrom. 5 (1992) 327.
15. J.C. Schwartz, J.E.P. Syka and I.
Jardine, J. Am. Soc. Mass Spec-
trom. 2 (1991) 198.
16. K.L. Busch, G.L. Glish and S.A.
McLuckey in “Mass Spectrometry/
Mass Spectrometry: Techniques
and Applications of Tandem Mass
Spectrometry” VCH Publishers,
Inc., New York, 1988.
17. A.G. Marshall, T.-C.L. Wang and
T.L. Ricca, J. Am. Chem. Soc. 107
(1985) 7893.
18. L.D. Bowers and D.J. Borts, Clin.
Chem. 43 (1997) 1033.
19. M. Soni, S. Bauer, J.W. Amy, P.
Wong and R.G. Cooks, Anal.
Chem. 34 (1995) 1409.
20. M. Soni, V. Frankevich, M. Nappi,
R.E. Santini, J.W. Amy and R.G.
Cooks, Anal. Chem. 68 (1996)
3314.