Mass spectrometers: instrumentation

International Journal o f Mass Spectrometry and Ion Processes, 118/119 (1992) 1 - 3 6

1

E l s e w e r Science P u b h s h e r s B.V., A m s t e r d a m

Mass spectrometers: instrumentation* R.G. Cooks ~, S.H. Hoke II, K.L. Morand and S.A. Lammert Department of Chemistry, Purdue University, West Lafayette IN 47907 (USA) (Received 26 August 1991)

ABSTRACT Developments m mass spectrometry instrumentation over the past three years are reviewed. The subject is characterized by an enormous dwerslty o f designs, a high degree o f compehtlon between different laboratories working with e~ther different or slmdar techmques and by extremely rapid progress m improving analyhcal performance Instruments can be grouped into genealogical charts based on their physical and conceptual interrelationships This Is dlustrated using mass analyzers o f different types The Ume course of development of particular instrumental concepts Is d lustrated m terms of the s-curves typical of cell growth Examples are given of mstruments which are at the exponential, linear and mature growth stages The prime examples used are respectively: (i) hybrid instruments demgned to study reactive colhsions of ions with surfaces (n) the Paul 1on trap, and (in) the triple quadrupole mass spectrometer In the area of ion/surface colhslons, reactive collisions such as hydrogen radical abstraction from the surface by the impinging ion are studied. They are shown to depend upon the chemical nature of the surface through the use of experiments which utlhze self-assembled monolayers as surfaces. The internal energy deposited during surface-reduced dissociation upon colhslon with different surfaces in a BEEQ instrument is also discussed. Attention is also given to a second area o f emerging instrumentation, namely technology which allows mass spectrometers to be used for on-hne m o m t o r m g of fluid streams A s u m m a r y of recent Improvements in the performance of the rapidly developmg quadrupole ion trap instrument illustrates this stage of instrument development. Improvements in resolution and mass range and their apphcatlon to the characterization of blomolecules are described The interaction o f theory with experiment is illustrated through the role of simulations of ion motion m the ion trap. It is emphasized that mature instruments play a dominant role in most work using mass spectrometers. This is dlustrated with recent results on the chemistry of C~0" including the formation of covalent adducts with aromatic c o m p o u n d s Quantitative analysis of methylated nucleosides and structural studies of the anti-cancer drug taxol are also discussed A compendium of mass spectrometers constructed over the past three years is provided This includes a variety of hybrid instruments, combinations of sector mass spectrometers with traps, instruments designed to study collision dynamics, and m a n y more

INTRODUCTION

This paper reviews some of the developments which have taken place over the past three years in the area of instrumentation. The size of this subject is * P a p e r p r e s e n t e d at t h e 12th I n t e r n a t i o n a l M a s s S p e c t r o m e t r y C o n f e r e n c e , A m s t e r d a m , T h e N e t h e r l a n d s , 2 6 - 3 0 A u g u s t 1991 A u t h o r to w h o m c o r r e s p o n d e n c e s h o u l d be a d d r e s s e d . 0168-1176/92/$05.00

© 1992 Elsevier Science P u b h s h e r s B.V. All r i g h t s r e s e r v e d

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

Evolution of Instrumentation

~

Mature !

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o..

le Quad}

(PaulIonTrap)

E m e ~

(Hybridsfor Ion/SurfaceRxns) Time

Fig 1. Three main stages m the development of instrumentation and the principal examples chosen as illustrations of each

indicated by the fact that in this period more than 40 papers have appeared which describe the performance of new mass spectrometers. Similarly, its scope is indicated by the fact that some 60 companies are engaged in the production of mass spectrometers and ancillary equipment. A directory of these companies has been established [1]. In covering mass spectrometers, we have chosen to organize the material in terms of the stage of development of the instrumentation (Fig. 1). Emphasis is necessarily placed on a few particular systems with which the authors have close acquaintance, and the sequence followed is from emerging instrumentation through those undergoing rapid development and then on to relatively mature instruments. Instrumentation for ion/surface reactive collisions, Paul ion traps and triple quadrupole mass spectrometers are the principal types discussed in these respective sections. A remarkable feature of the instruments used in mass spectrometry is their wide variety. Different physical principles are used to separate ions by massto-charge ratio, including cyclotron frequency in a magnetic field, dispersion in a sector magnet or radial electric field, flight time after acceleration through a fixed potential, and motion in an r.f. electric field. In this respect, mass spectrometry is different from other forms of spectroscopy and this variety provides a degree of internal competition which is a source of strength to the subject. Examination of even a limited group of the types of instruments available reveals this rich variety; it also reveals the way instrument evolution has occurred and a sampling of this genealogy is shown in Fig. 2.

R.G. Coooks et al./Int. J. Mass Spectrorn. Ion Processes 118/119 (1992) 1-36

3

m 8Ej (MIKES)

Fig. 2 Evolution of more complex instruments from simpler types. Connections are suggested based on transfer of concepts as well as hardware

EMERGING INSTRUMENTATION

The most exciting part of the subject of mass spectrometric instrumentation concerns progress being made with new devices which are in the early stages of their evolution. There are a large number of possibilities from which to choose, some of which are listed in Table 1. Two of these topics are discussed here.

Ion~surface collisions A subject which appears to be ripe for rapid future development is that of collisions of ion beams with surfaces (Fig. 3). The study of collisions of kiloelectronvolt energy ion beams with surfaces has already generated a number of useful methods in mass spectrometry, including secondary ion

4

R.G. Cooks et al./Int. J. Mass Spectrom. 1on Processes 118/119 (1992) 1-36

TABLE 1 New mass spectrometers (1988-1991) Reference

Type

Purpose

P. Jonathan, M. Hamdan, A G. Brenton and G.D. Willet, Chem. Phys, 119 (1988) 159.

BE EE

Translational energy spectroscopy

C.C. Petty, D K. Smith and D.L. Smatlak, Rev. So. Instrum, 59 (1988) 601

TOF

Plasma impurity, exact charge state

A.K. Shukla, S G Anderson, S L. Howard, K W Sohlberg and J H. Futrell, Int. J Mass Spectrom. Ion Processes, 86 (1988) 61

EB/EQ

Ion scattering

P A. Nalk, P D. Gupta and S.R. Kumbhare, Rev. So. Instrum., 59 (1988) 1076

TOF (X-ray source)

Plasma velooty distribution

R. Feng, C WesdemxotIs, M A. Baldwin and F.W. McLafferty, Int J. Mass Spectrom Ion Processes, 86 (1988) 95

EBEB

Neutrahzatlon-re~onlzation

E Leal-Qulros and M.A Prelas, Rev Scx Instrum., 59 (1988) 1738

Wlen filter

High temperature plasma diagnostics

D.T Young, S.J Bame, M F Thomsen, R.H Martin, J L Burch, J A. Marshall and B Remhard, Rev. Scl. Instrum., 59 (1988) 743

2n torol&al analyzer (360 ° × 10°) E sector

Satellite-borne mass spectrometer

K.L. Schey, R G. Cooks, A. Kraft, R Grix and H Wollnlk, Int. J Mass Spectrom. Ion Processes, 94 (1989) 1

TOF-TOF

Surface-reduced dissociation

D Schuetzle, T.J. Prater, S Kaberhne, J E deVrles, A Bayly and P Vohrahk, Rev Scl Instrum, 60 (1989) 53

SIMS

Surface analysis of complex samples

A G. Harrison and A B Young, Int J Mass Spectrom Ion Processes, 94 (1989) 321

BEQQ

Ion chemistry

L. Shl, H J Frankena and H Mulder, Rev Scl Instrum, 60 (1989) 332

Q/CMA

Energy and mass analysis of solids

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

5

TABLE 1 (continued) Reference

Type

Purpose

C.H. Lilhe, D.G. McMmn, H. Chambers and H.H. Hill, Jr, Int. J. Mass Spectrom Ion Processes, 95 (1989) 277.

Quadrupole

Ions in flames

K.-D. Rlnnen, D.A.V. Kliner, R S. Blake and R.N. Zare, Rev. So. Instrum., 60 (1989) 717

TOF

Shuttered TOF for selected-ion detection

H. Matsuda, T. Matsuo, Y. Fujlta and T Sakurai, Int. J. Mass Spectrom. Ion Processes, 91 (1989) 1.

BE

High resolution

A Montone, P. Morales and A Nard1, Rev Sci. Instrum., 60 (1989) 2639.

TOF

FragmentaUon processes in a collisional regime

C.-S. Su, Int. J. Mass Spectrom. Ion Processes, 88 (1989) 21,

Parallel-plate TOF

High resolution

B N. Eldndge, Rev. Sci Instrum., 60 (1989) 3160.

SIMS(EI)-TOF (dual source)

Reactive species interaction with sohd surfaces

P. Kofel, M. Allemann, H P. Kellerhals and K.P. Wanczek, Int J. Mass Spectrom. Ion Processes, 87 (1989) 237

Trapped source ICR

High sensitivity

C.C Hayden, S M. Penn, K J. Carlson Muyskens and F.F Cram, Rev. So. Instrum, 61 (1990) 775

Molecular beam-TOF

CharactenzaUon of molecular beams

A. Danon and A. Amlrav, Int J. Mass Spectrom. Ion Processes, 96 (1990) 139.

Quadrupole

Surface mnizatlon

D.P Land, C.L. Pett~ette-Hall, D Sander, R.T Mclver, Jr. and J.C. Hemmmger, Rev, Scl Instrum, 61 (1990) 1674

FTMS

Surface analysis AES, LEED, TDS

H. Wollnlk and M Przewoka, Int. J. Mass Spectrom Ion Processes, 96 (1990) 267.

TOF multiple reflection

High resolution

E Y. Wang, L. Schm~tz, Y. Ra, B. LaBombard and R.W. Conn, Rev Scl Instrum., 61 (1990) 2155

Omegatron

Analysis of a magnetized plasma

6

R.G. Cooks et al./lnt J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

TABLE 1 (contmued) Reference

Type

Purpose

M. Hawley, T.L. Mazely, L.K Randeniya, R S Smith, X.K. Zeng and M.A. Smith, Int. J Mass Spectrom. Ion Processes, 97 (1990) 55

Free-flow jet TOF

Ion/molecule reaction rates

D C. Hamdton, G. Gloeckler, F.M. Ipavlch, R A. Lundgren, R B. Sheldon and D. Hovestadt, Rev Scl. Instrum., 61 (1990) 3104

E sector

Solar wind analysis

D.L. Donohue, L.D Hulett, Jr., S.A McLuckey, G L. Glish and H.S. McKown, Int J. Mass Spectrom Ion Processes, 97 (1990) 227.

Positron ~omzation (TOF)

Thermo-chemmtry

J C Schwartz, K.L. Schey and R.G Cooks, Int J. Mass Spectrom. Ion Processes, 101 (1990) 1.

Pentaquadrupole

MS 3

M.E Bier, J C Schwartz, K L Schey and R.G Cooks, Int. J. Mass Spectrom Ion Processes, 103 (1990) I.

QQ (m hne)

Surface-reduced dissocmtlon

W Aberth, Anal. Chem., 6 (1990) 609.

Tandem Wlen filter (in hne)

Surface-reduced dissooation

J.C. Schwartz, R E Kaiser, R.G Cooks and P.J. Savlckas, lnt J Mass Spectrom Ion Processes, 98 (1990) 209.

BE/trap

MS"

C.G. Beggs, C.-H. Kuo, T Wyttenbach, P.R. Kemper and M T. Bowers, Int. J. Mass Spectrom Ion Processes, 100 (1990) 397.

FT-ICR

Radiative lifetimes

J.H.D. Eland and D.A Hagan, Int J. Mass Spectrom. Ion Processes, 100 (1990) 489.

Coincidence TOF

Charge separation

P Kofel, H. Remhard and U.P. Schlunegger, Org. Mass Spectrom, 26 (1991) 463.

Q/Trap/Q

MS"

R.G Cooks et al./Int. J. Mass Spectrom Ion Processes 118/119 (1992) 1-36

TABLE 1 (continued) Reference

Type

Purpose

C. Ma, C R Sporleder and R.A. Bonham, Rev. So Instrum, 62 (1991) 909.

TOF

Iomzation cross-sections

J.J. Stoffel(s) and H.-J Laue, Int. J. Mass Spectrom. Ion Processes, 105 (1991) 225.

BBE

High sensitivity isotope ratios

K L. Morand, S R Hornmg and R.G. Cooks, Int. J. Mass Spectrom. Ion Processes, 105 (1991) 13.

Q/trap

Fundamentals of 1on rejection

R. Kutscher, R Gnx, G LI and H Wollnlk, Int. J. Mass Spectrom Ion Processes, 103 (1991) 117.

TOF with transverse and longitudinal focusing

High resolution and sensmwty

R. Snmvas, D. Sfilzle, T. Weiske and H Schwarz, Int. J. Mass Spectrom. Ion Processes, 107 (1991) 369.

BEBE

MS-MS; 1on structures

H.F Hemond, Rev. So Instrum., 62 (1991) 1420

180° B (cycloid tube)

Portable MSfor volatile compounds

F H Strobel, T. Solouk~, M.A. White and D.H. Russell, J. Am. Mass Spectrom, 2 (1991) 91.

EB-TOF

Low level detectton of peptides

mass s p e c t r o m e t r y ( S I M S ) and ion scattering s p e c t r o s c o p y (ISS). W h e n low e n e r g y ion b e a m s strike surfaces, elastic, melastic and reactive collisions are all possible. Inelastic collisions can lead to dissociation (surface-induced dissociation, SID) and hence the e x p e r i m e n t functions as a m e t h o d o f ion c h a r a c t e r i z a t i o n which can be used in analytical mass s p e c t r o m e t r y [2]. F i g u r e 4 illustrates the p e r f o r m a n c e o f a t a n d e m q u a d r u p o l e i n s t r u m e n t [3] in activating ions by a m o u n t s which d e p e n d o n the collision energy chosen. T h e S I D e x p e r i m e n t has been p e r f o r m e d successfully on various types o f mass s p e c t r o m e t e r , including F o u r i e r t r a n s f o r m i n s t r u m e n t s [4] and in q u a d r u p o l e ion traps [5] where collisions with internal surfaces are e m p l o y e d . In a related and p r o m i s i n g e x p e r i m e n t , an external surface has been used to study ion scattering s p e c t r o m e t r y , the scattered ions being collected and m a s s - a n a l y z e d in a F o u r i e r t r a n s f o r m mass s p e c t r o m e t e r ( F T M S ) [6] (Fig. 5). Similarly,

8

R.G. Cooks et al ~Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

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Surface Modification

Fig. 3. Some analytical procedures based on lOmC colhslons with surfaces. The higher energy processes are well estabhshed, the lower energy processes are now being developed.

Grizzi et al. [7] have developed effective instrumentation for surface characterization using time-of-flight analysis of scattered ions and neutrals. It is now well established that interfacial reactions accompany desorption ionization experiments, including the kiloelectronvolt ionic collisions which are the basis for the SIMS technique [8]. Ion/surface reactive collisions are a more recently discovered phenomenon [9] and are related to the earlier techniques as indicated in Fig. 6. One particular instrument [8] developed for

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R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

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Fig 6 Low energy reactive colhsions in relation to b o t h SID a n d to the mterfaclal r e a c u o n s which a c c o m p a n y SIMS

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12

R G. Cooks et al./Int J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

the study of ion/surface reactions is illustrated in Fig. 7. In this instrument, a mass-analyzed ion beam is decelerated before colliding with a surface at a selected energy and impact angle. The instrument is designed to allow the products to be angularly-selected, kinetic energy-analyzed using an electric sector, and mass-analyzed using a quadrupole mass analyzer. The ion optics are shown in Fig. 8a. It is worth remembering that most instrumentation for the study of ion/surface collisions is derived from experience in the study of gaseous collisions. Quian et el. [11] have shown that there is still much to be learned in this area with their studies of electronic excitation and de-excitation processes accompanying very low energy collisions. Their work has been greatly facilitated by effective instrumentation [12] for collecting and energyand mass-analyzing ions of laboratory energies as low as 0.2 eV (Fig. 9). The availability of surfaces comprised of self-assembled monolayers [13] means that well-defined organic surfaces are available to study organic ion/ molecular surface collision phenomena. Figure 10 illustrates such a surface and Fig. 11 is a schematic representation of one type o f ion/surface reactive collision process which has been observed [2]. It is of great interest to note that when the nature of the functional group at the interface is changed, there is a +,

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R.G. Cooks et al./Int. J Mass Spectrom. Ion Processes 118/119 (1992) 1-36

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dramatic change in the ~on/surface reaction products. As shown in Fig. 12, a carboxylic acid surface transfers a hydrogen atom to ionized pyrazine, whereas the corresponding nitrile-terminated surface does not [14]. It is known [15] that the nature of the surface affects the probability of neutralization; the data in Fig. 13 show that it also affects the internal energy deposited in SID [16].

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The limonene molecular ion was chosen as a model ion in this investigation because of the large amount of information available on its fragmentation [17]. The data show that much more extensive fragmentation is achieved in collisions from a CF3-terminated surface than from a conventional stainlesssteel surface. This is confirmed by the fact that the higher energy fragments m/z 91 and 77 are favored over the lower energy processes which yield m/z 93 and 79 respectively.

On-line monitoring There is a great need for on-line continuous monitoring of the molecular nature of fluid streams in several areas of science, including environmental science, biotechnology, and in vivo studies on living organisms. Two mass spectrometric technologies, both of which employ flow injection analysis techniques for sampling and control, have emerged recently in answer to this need. One employs the flow fast atom bombardment (FAB) method [18] of sample introduction and ionization, in conjunction with a microdialysis probe [19] for sampling. This experiment is particularly well-suited to the monitoring of biological compounds and it has been used in vivo [20]. The second technique employs a semi-permeable membrane as the interface between the fluid stream and the mass spectrometer [21]. This limits its response to lower molecular weight compounds but it has proved very effective in the continuous monitoring of fermentation vessels [22] and has extremely low detection limits for some compounds (sub-ppb in direct analysis [23]). Figure 14 illustrates the principle of this latter experiment and includes data for continuous monitoring of a fermentation. RAPIDLY D E V E L O P I N G I N S T R U M E N T A T I O N

For an area of science which has been in existence for almost a century, even the fact that rapid developments are occurring is cause for satisfaction. The past three years have seen significant progress in instrumentation in many areas including: (i) very rapid acquisition of complete GC-MS data using an integrating transient recorder and time-of-flight (TOF) mass spectrometer [24]; (ii) procedures for LC-MS and supercritical fluid chromatography-MS, including particle beam samphng methods [25], flow FAB [18] and, most notably, electrospray [26]; and 0ii) improvements in laser desorptlon through matrix enhancement which, when combined with TOF mass analysis, provides a powerful "molecular weight machine" for biochemistry [27]. There are many other areas which are also developing rapidly and their omission here is due only to considerations of space. It is worth taking one topic of emerging ~mportance and illustrating its

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

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recent progress. The subject of Paul (or quadrupole) ion traps [28-30] has been chosen because its fundamentals are well established, but there have been a large number of recent enhancements which have greatly improved its performance [31]. Similar comments might be made regarding Fourier transform ion cyclotron resonance (FT-ICR) spectrometry and other authors might have chosen that or yet another subject for detailed consideration. Three years ago, the quadrupole ion trap was almost universally regarded as a "low-end" G C - M S instrument of exceptional sensitivity but otherwise modest performance. The ability to perform chemical ionization and tandem mass spectrometry had recently been demonstrated [32] but much remained to be done to achieve high performance. An underlying feature which forms the basis for much of the enthusiasm for quadrupole ion traps is their high sensitivity. This is a consequence of the fact that ions are not lost (or better, are not necessarily lost) except in the course of mass analysis. Much of the enthusiasm for four-sector instruments equipped with imaging detectors and for FT-ICR has a simdar origin.

R G. Cooks et al./Int. J Mass Speetrom. Ion Processes 118/119 (1992) 1-36

18

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1.0

Fig. 15 Calculated mass range extension o f the ion t r a p achieved by resonance ejection as a function o f Math~eu value q at which the 1on is in resonance with a n a p p h e d dipole field.

The resonance ejection experiment, in which ions are ejected from the trap by acquiring kinetic energy from a dipole electric field of appropriate frequency, allows extension of the mass/charge range of the instrument by factors of 100 or more (Fig. 15) [33]. In order to utilize this extended mass range, methods of ionization appropriate for high molecular weight compounds or clusters are needed. Fortunately, ion injection from an external source into the trap is straightforward, given the fact that the helium buffer gas assists in removal of ion kinetic energy and so allows the injected ion to adopt a stable trajectory [34]. Hence the common desorption ionization (liquid SIMS [34], laser desorption [35]) and spray techniques (electrospray [36], thermospray [37] and SFC [38]) have all been coupled to ion traps. One consequence of this is that molecular weight determinations and structural studies on model biomolecules have commenced. The approach used is typified in the M S - M S spectrum of protonated actinomycin D, shown in Fig. 16. The experiment is done by Cs + ion bombardment of the analyte in a glycerol/thioglycerol matrix with ion injection into the trap [39]. Isolation of the protonated molecule is achieved by first scanning ~ons of higher mass out of the trap, then scanning from low mass to the selected ion to eject ions of lower m/z ratio. The isolated MH + ions are irradiated at their resonance frequency using a low amplitude supplementary a.c. signal in order to cause power absorption and then dissociation. The product spectrum shown was then recorded using resonance ejection. When more structural information is required, these steps can be repeated,

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

19

Actlnomyo/n D, 3 pmol (MY11255)

858

1 1 2 5 6 (MH*)

o

•

i

I

200

i

L, • I

I

i

400

l .....

I

,

600

~

I

i

800

I

,

I000

I

1200

, I

1400

m/z Fig 16. MS-MS spectrum of protonated actmomyc]n D showing products and their relat]onsh~p to the molecular structure [39]

Y7

SSEGESPDFPEELEK

Prepro VIP/PHM 156-170,2pmol -MS/MS Spectrum (M+H)

• 1680 (M'H)*

÷

,i

0

Ys , ,i

I

I

I

I

.ll.

I

y,Y,o

i ....

I

I

ils/

..... ;._,i.~ki _,/ J,; ~lh._~dh I

I

5OO -MS/MS/MS

Y,z

,

I

J

,

I000

I

]

f

,

,

1500

Spectrum

01680

$

• 891

Y~

0 ....

,

I

200

,..

i

....

I

.L,

....

f

400

_.,,

I

600

I

III

I

J

800

m/z Fig. 17 Use of MS ~ experiments to increase structurally dlagnost=c fragmentat=on from the pept~de shown. Expenment employed 7 keV Cs + aomzation and resonance ejection for mass range extension [39]

R.G. Cooks et al./lnt J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

20

namely MS" experiments can be performed [40]. Figure 17 shows such data for the case of the 15 amino acid peptide (SSEGESPDFPEELEK). Note that the majority of the fragment ions are assigned to amide bond cleavage with formation of C-terminal, protonated immonium ions (y-type ions). The MS 2 and MS 3 spectra, obtained on a total of 2pmol of material, exhibit high quality information on the amino acid sequence. Similar information can be obtained on the doubly- and triply-protonated forms ofpeptides generated by electrospray ionization [41] and this method is being used to elucidate the structures of unknown peptides [42]. A promising beginning [43] has also been made in obtaining spectra characteristic of oligonucleotides by ion trap MS-MS experiments using electrospray ionization with the instrumentation illustrated in Fig. 18. Perhaps the most exciting development of ion traps in the recent past is the demonstration that slow scan speeds lead to sharply increased resolution. This observation by Schwartz et al. [41] built on experiments [33] in which the scan speed was reduced to compensate for the loss of data when resonance ejection was employed for mass range extension. By slowing the scan speed by extreme amounts, for example by a factor of 2000 compared to the normal rate of 5555 Da s-1 (unit charge assumed), dramatic enhancements in resolution are achieved [44, 45]. Figure 19 demonstrates that such a procedure can yield a

Continuous Infusion

s~,~e

I ..............

5

~

r

~

-

-

-

~

SyNnge Pump Flow Injection F~g ]8 Method used to sample and lomze ohgonucleoudes by electrospray with a quadrupole ion trap [43]

R.G. Cooks et al ~Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36 Cs1411a+ Scan Speed Resolution Peak Width Total scans Filter

16.67 Da/charge-sec >1x10 e 0 0035 Da 3 Sample & Hold

i

3508.5

21

0s14113 +

i

=

i

i

i

3509

3509 5

3510

3510 5

3511

Mass-to-Charge

Fig 19. Extreme case of resolution enhancement achieved by 2000 x reduction m scan speed

resolution (m/Am where Am is measured at 50% of peak height) of 106 for a cesium iodide cluster ion. What is significant here is not the numerical value of resolution achieved or the definition chosen but the fact that unexpectedly high resolution is available. Figure 20 shows graphically the increase in resolution at slower scan speeds using protonated substance P. Improved resolution in ion traps provides desirable capabilities such as the ability to perform MS-MS experiments on peptides using unit mass resolution for both parent and product (daughter) ions, which was previously available only using four-sector instruments [46]. Figure 21 illustrates this capability in the case of gramlcidin S.

Resolution as a Function of Scan Speed (M+H) ÷ of Substance P

10,I "5

2

rr

I 04

e~ -~ ~

130

260

390

520

~

650

780

910

1040

1170

1300

Scan Speed(Da\second)

Fig 20. Relationship between resolution and scan speed for protonated substance P [39]

22

R G. Cooks et al./lnt J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

Gramacidin S 8 picomoles

//

Slow scan

1141 3~1 1~7

Irradmte mlz 1142 ~ I s o l a t e

m/z 1142

~,~

/MS

571

Ftg. 21. Umt resolution product spectra ofprotonated gramtc~din S. It is possible to isolate and selectively irradiate mdwldual Isotopic forms of the protonated molecule at unit resolution (center box) or fragment the entire ]sotopic cluster (left-hand box).

A limitation of quadrupole ion traps is the need for mass calibration since ion injection is dependent not only on the applied potentials but also on the properties of the ionic population in the trap. This problem is readily addressed by calibration against external standards--other peptides or even cesium iodide cluster ions can be used. However, at very high resolution the effects become increasingly severe and there is a need to develop procedures for calibration against internal standards. This problem throws into sharp relief questions about the details of ion motion m the trap. Two very different approaches are being taken to this problem. One employs simulations, although it is necessary that they be performed at a much higher level than done previously; in particular coulombic repulsions need to be accounted for in detail and ion/bath gas collisions need to be treated adequately. Several groups are working on these problems [47]. One program (NQS) developed in this laboratory involves massive parallel computing, handles up to 10 4 ions, and can produce simulated mass spectra of peptides [48]. Another program, ITSIM, employs PCs to follow the behavior of a smaller number of ions [49]. The latter program is useful in providing the investigator with qualitative insights into ion trap behavior. For example, it was used to characterize the phenomenon of surface-induced dissociation in ion traps [5]. Figure 22 illustrates the types of data obtained, showing how application of a d.c. pulse to the end caps causes the ions to acquire increased kinetic energy in the radial direction and to strike the ring electrode with the result that dissociation occurs.

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

23

nAAAAAAAAAI

RF'DClV V V V V V' V V V VI L dopulsebegins j . ~ AXIAL ~ POSITION

~

0.7 cm Icenter

~

0.7 crn KINETIC ENERGY

_

lcm

RADIAL L POSITIONI

center

I cm 150 eV

RADIALI KINETICI ENERGYI

~ j

t=llps

•

•

Fig. 22. PC simulation results (ITSIM program) for the ion trap surface-induced dissociation of the pyrene molecular ion (m/z 202) showing both the radial and axial positions and klneUc energies

Helium Stabilization of

Ion Trajectories 2000

.

.

.

.

.

.

.

.

C6Hs'CO'CH3 ~ ,,-1500 b-

.

.

.

.

.

.

.

C6Hs-CO+ ~

• NO Helium, tlon : 6 msec o He

/ ~

.

.

.

.

C6H~ + CO Psample: 1 X 10"7 torr

~ 1000 "1"

o. 500

-2.5

-1.5

-0.5

0.5

1.5

2.5

D i s t a n c e (ram) f r o m ~ = 0

Fig 23 Dlstr]butlon of ion density along the z-axis o f a quadrupole ion trap as determined by photodlssoclatton of acetophenone by a XeCI laser m a tomography experiment. The expected contraction of the ion cloud in the presence of hehum bath gas Js observed [50]

24

R G. Cooks et al ~Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

The second approach to obtaining information on ion motion within the trap is by direct physical measurements• Again, a considerable amount of work has been done in the past which assists in addressing the newer issues. In a recent experiment, Hemberger et al. [50] photodissociated gaseous ions with laser beams which intersect the trap volume along defined axes to map out the ion population in the trap. The experiment recognizes the presence of the ion of interest at a particular location by detecting a photodissociation product. This tomography experiment has been done as a function of several variables including helium pressure and axial modulation voltage with results in agreement with simulation and expectation• Figure 23 shows the distribution of ion density along the z-axis (towards the end caps) under two sets of operating conditions• MATURE INSTRUMENTATION

Significant contributions to major issues in science are being made through the use of instrumentation which is (apparently) near the crest of its development. This is the case for the use of four-sector instruments in peptide sequencing, a task recently made much more compatible with the small amounts of sample available through the use of imaging detectors [51]. Figure 24 illustrates the essentials of the ion optics needed for one such detector in which the electrostatic lens systems has the advantage of being able to focus ions of a variable range of masses onto the imaging detector. One striking feature of recent progress in mass spectrometry is that the results are of general interest to scientists from a wide range of disciplines. Increasingly, mass spectrometry is engaged in the major scientific issues of the times. This is illustrated in this section for several problems of current interest with results taken using the triple quadrupole [52], an instrument which has been in commercial production for a decade. These instruments have not changed in essential features since first introduced, although significant MAGNETIC FIELD

QD1

I

QD= COLLECTOR SLIT

In[

I

J

• ,{i L SEM COLD FINGER

Fig 24. Ion optical elements used to focus ions of a wide (and variable) range of masses onto an array detector. QD~ and QD2 are d.c. quadrupole doublets. The detector is mounted between the second and third sectors o f a JEOL HX 110/HX I l0 four-sector mass spectrometer. (From ref. 51 )

R G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

25

improvements have been made in the collision quadrupole (including its substitution by hexapoles or octapoles), in detection systems (higher gain conversion dynodes for improved sensitivity at h]gh mass), and in data acquisition and instrument control systems. Cumulatively, these and other enhancements have greatly improved the performance of these instruments. In addition, progress in sample introduction and ionization methods, sometimes revolutionary in impact, has greatly enhanced the capabilities of all mature instrumentation, including quadrupole mass filters and magnetic sector mass spectrometers.

Buckminsterfullerene The properties of mass spectrometry--high sensltivity, molecularly-specific information, and applicability to samples in impure states--have made it an essential tool in exploring the chemistry of C60 and its analogs. Mass spectrometric experiments using laser ablation provided the first evidence for the existence of this class of compounds [53]. Preparation of derivatives has become a key activity in fullerene chemistry [54]. In addition, the ion chemistry of these molecules is fascinating, including the formation of inclusion com-

÷

C6o

keV

+

"

C6oHe

eV

÷

_- C58He

MIKES Ceo*

Q - rf only

o,ecu,.r,

MIKES C~o+ Q = Css*

7~

765O

770O

775O

tON ENERGY (eV)

Fig. 25. Use of a hybrid BEQQ mass spectrometer to cause helium capture by C~+ in a high energy collision and subsequent dissociation by C2 loss. (From ref. 56.)

R G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

26

'°°]1 8O

'°t

6O

!

o 34363840

424446

485052

545658

606264, 666870

CarbonOuster

ioo~ 6O

'°t 6O ,~ 5O ,~ 3O 2O

34363840424446485052545658606264666870 Csrbon Cluster

60

,,~ 50" 4,01

~ 3o-

2010' . . . . . . . . . . . , , , 34363840424446485052545656606264666870

,

,

,

,

Carbon(3uster Ftg 26 C o m p a r i s o n o f M S - M S p r o d u c t spectra o f C60+ recorded by photodissoclatlon, 8 keV C I D a n d C I D m the ion trap ( D a t a are from (top) ref. 57, (middle) ref. 58, ( b o t t o m ) ref 59.)

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

27

plexes with rare gases [55,56]. The ion C60l-Ie"+, remarkably, is observed during high energy collisions, simply in the attempt to perform CID on C60+ using He as the target gas. Use of 3He demonstrates the process unambiguously [55]. The capture of helium can occur even without fragmentation of the ion and further fragmentation by C2 loss (Fig. 25) provides evidence that the gas atom is held within the sphere [56]. Additional M S - M S - M S experiments recorded using a hydrid instrument showed that the high energy collision yields the intact helium inclusion product and a subsequent low energy collision causes it to fragment by C2 loss, not by loss of helium, i.e. "°

C60He "÷

xo 200 eV

, CssHe "÷

(1)

The fragmentation of the molecular ions C~0+ and C70+ is also fascinating. The latter undergoes preferential fragmentation to generate C60+, in addition to dissociating by the C2 loss processes which characterize this class of molecules. Comparable behavior is seen upon photodissociation [57], kiloelectronvolt energy collisional dissociation [58] and remarkably, upon collisional activation in an ion trap [59] (Fig. 26). As illustrations of the way m which mass spectrometry is contributing to the development of the chemistry of the fullerenes, consider the formation of C60 and C70 derivatives which bear epoxide or methylene substituents. These derivatives are generated upon UV irradiation of impure C60 samples in solution and are recognized by electron attachment mass spectrometry [60]. This mild ionization method is effected by moderating electron energies in a chemical ionization source using a suitable buffer gas (e.g. NH 3) while sample vaporization is achieved by rapidly heating the sample to high temperatures on a desorption probe. Figure 27 shows a spectrum of product mixture including the molecular ions of C60 + O + (CH2),, where n = 1-6. One can propose the conversion of the starting material ("buckyball") to its derivative ("spikyball"):

Buckyball

Spikyball

28

R.G. Cooks et al /lnt J Mass Spectrom. Ion Processes 118/119 (1992) 1-36

Molecular Anions of Fullerene Derivatives after Irradiation

I )(20

720 100"

I xl

I xl00

c,; 80-

c

60-

<

n=l

[C~+O+(CH2)R]"

e~ 840

40cc ;~

(Cro+O+CH2)-

(C~o+O)" 736 20-

I

[ ,

, 750

I.

700

857 870 ~ ' n=5 n=3 806 n=2 778 n=6 764 " ~ rl=4 82O 792 h, .J [,. .h, ,,I II, ,.h,. 800 mass-to-charge

,

I,, , 850

hJli,... IL,Ib...h,,,h,..,lll . . . i

.

900

Fig 27 FormaUon of oxo and cyclopropenyl denvaUves (up to s~x groups) of C60 and C70 as shown by electron attachment mass spectrometry (From ref. 60 )

Electron Attachment Mass Spectrum of Toluene Denvatwes of C6o 812

100"

(C6o + CrHs)

C6o 72O

(C~ + n(C,H6) - m(H2))

80"

(n= 2,3,4,5) (m= 0,1,2)

8 ~OO,< n=2 902 o~ 40n-, n=3 994

n=4 1084

n=5 1176

20-

, 7OO

800

L.......... 900

.1. ..... ----i--

1000 mass-to-charge

11O0

-

1200

1300

F~g 28. Toluene denvatwes of C60generated m the presence of FeCI~ and observed by electron attachment [61].

R.G. Cooks et al./lnt J Mass Spectrom. Ion Processes 118/119 (1992) 1-36

29

Energy Resolved Fragmentation Efficiency (Ceo + CTHs)-

* C6o

1.0

*6 ¢)

08-

0 1:7.

0.6

IS g

04-

l"__

.90 "6 *6

¢J i,

0.2-

0.0

=

,

20

_-

,

40

=

,

60

m.

,

80 1()0 120 1~,0 160 180 200

Lab Colhslon Energy (eV)

F~g. 29. D~ssoclaUonefficiencycurve (Ar target) of the toluene adduct showing that toluene brads strongly to C60. Carbon-carbon bond formation is of particular interest and the formation of derivatives of buckminsterfullerene with simple aromatic compounds in the presence of FeC13 (Fig. 28) is therefore of note [61]: ¢'~ TT FeCI~ C60 Ji- k-~71--18--------~ ---~ C60" C 7 H 8

(2)

The aromatic adducts are formed with H 2 loss, when multiple aromatic rings are added. The fact that covalent bonding is involved is demonstrated by M S - M S experiments which show that vigorous conditions are needed to dissociate the adducts (multiple collisions, ~ 100 eV collisions) and even then only limited dissociation occurs as shown by energy-resolved data (Fig. 29). Taxol

A flesh look at the contributions which mass spectrometry continues to make in the area of discovery of therapeutic agents is available by considering the agent taxol [62, 63] now in phase III clinical trials in the US. The exper]ments described below used standard commercial triple quadrupole instrumentation, although as with buckminsterfullerene and its derivatives careful attention to detail was needed to optimize sample introduction/ionization. The great promise of taxol as a chemotherapeutic agent, its scarcity and ecological concerns regarding destruction of Taxus brevlfolia, the tree in whose bark it occurs most abundantly, together with the lack of a synthetic route to the compound, all make rapid methods of screening plant material

30

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36 0

H3C

~

O~CH 3 .0

o

o"

R°"

0

0

R =

H3C"

Taxol M.W.= 853

"CH3

Cephalomannine M.W.= 831

Baccatin III M.W.= 586

Scheme 1 Chemical structures of taxanes.

desirable. Such a screen has been developed based on tandem mass spectrometry experiments on the molecular radical anion [64]. This experiment has a remarkable sensitivity (one can detect picomoles of the pure compound using a product ion spectrum). Even more significantly, one can use parent Electron Attachment Parent Scan of m/z 526 Selected from Crude Plant Extract

100-

Ix5

526

]

8O-

586 Baccatm III

~60-

"~ r~ 40-

831 Cephalomannme I l 853

i Taxol

20-

. ,ILl. ,~,, L,L. 5O0

600

700 mass-to-charge

800

900

Fig 30 Screen of plant extract for taxol and its analogs using the M S - M S parent scan capabdlty of a triple quadrupole mass spectrometer

31

R.G. Cooks et al./Int J Mass Spectrom. Ion Processes 118/119 (1992) 1-36

scans to screen for analogous compounds which contain the basic fused ring skeleton but have different side-chains. Figure 30 illustrates this experiment and Scheme 1 shows the species which are involved. The possibility that analogs can be discovered using this approach is another driving force in the work. Chemical modifications to DNA

There can hardly be a more important task in science than the discovery of the molecular processes by which carcinogenesis proceeds. This ~s one area in which the impressive capabilities of mass spectrometry with respect to sensitivity, speed and molecular specificity of analysts still fall far short of what is required [65]. The action on D N A of chemical carcinogens such as methylating agents or polynuclear aromatic hydrocarbons (suitably metabolically-activated) depends on low levels of modification at specific sites (both within the individual nucleotides and as a function of position in the polymer). Capabilities for sequencing oligonucleotides and thereby recognizing the modified nucleotides

.ro,.El" Laser Beam /

~g A

~C

G

I

0

I

T

O

I

O

.~,, ~-..., ~-~,j ~;~ I

939

I

.0 U tD c 0

1321

610

d(AGCT)

0 13

Co.

C

o

Magnetic field

d3t13 o

oY.

321 o

g

(M-H)

,f

610

1172

5.[ o

'

. . . . . . . . .

400

~ . . . .

73, I

600

. . . .

'

. . . .

/ I

. . . .

8/OzO

'

. . . .

I

. . . .

1000

'

. . . .

I

. . . .

o

1200

Fig 31. Laser desorptlon FTMS spectrum of the tetranucleotide d(AGCT). The spectrum is recorded from a sohd matrix of pyrazlne carboxyhc acid and displays abundant sequence ions. (Adapted from ref 66.)

R G. Cooks et al./Int. J Mass Spectrom. Ion Processes 118/119 (1992) 1-36

32

Ion Chromatograms of Product Ions from m6dG and d3-m6dG 103

M/Z

12512

,°° 1] 166

i ~

974] M/Z 169

]_

1024] "'°

'

-

'12192

_

' (,-'---~

/

!

'

' "

'

\

""

'

12816

Scan 80

90

100

110

120

130

140

Time .07 (Sec)

08

09

10

11

12

13

Fig 32. DesorpUon chemical ionization experiment (.~ 1 s desorption time) with multiple reaction momtorlng (m/z 282 ~ 166 and m/z 285 --* 169) to quantltate methyldioxyquanosme (m6dG) generated from chemical treatment of D N A in vitro

are being developed and Fig. 31 illustrates recent FTMS data [66]. Analysis of the results even in in vitro chemical modification of DNA therefore presents a staggering problem. One approach is to perform the analysis at the nucleoside level and to seek to quantify the individual modified nucleosides in the complex mixture of isomers, unmodified nucleosides, and enzymes used to perform digestion. An off-line reverse-phase LC procedure, coupled with desorption chemical ionization (DCI) and triple quadrupole MS-MS, has been developed for this purpose [67]. Figure 32 illustrates results of the TABLE 2 Quantltat~on of methylated nucleosldes Nucleoslde

Ratio CH3/CD3

Percent mo&ficatlon

Limit of quantltation ~ (pmol)

m3T (MeMS) m3T (MENU) m4T (MeMS) m4T (MENU) m6dG (MeMS) m6dG (MENU) mTdG (MENU)

2.60 0 97 0 04 2 40 0 80 0.86 0.95

6.8 18 0 0.01 0.17 0.02 2.8 25.0

3.8 0.4 7.6 0.4 38 04 0.4

a Estimated.

R.G. Cooks et al./Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 1-36

33

experiment and Table 2 summarizes the performance achieved for particular methylated nucleosides. Note that multiple reaction monitoring is used to maximize sensitivity, while the choice of DCI over other ionization methods is based on the same requirement. Distinction between isomeric methylated nucleosides is achieved by chromatography but the tandem mass spectrometry step in the procedure provides a considerable measure of separation from other mixture components. The principal instrumental requirement when using DC1 is an extremely fast response time since analyte desorption occurs for times on the order of 1-3 s. Since MS-MS capabilities are also required the triple quadrupole fulfills these requirements and other less demanding ones regarding mass range and resolution. Further improvements in sensitivity, without loss in quantitative accuracy, are required to extend the analysis to in vivo chemical modification. It is not clear whether this can be done with the relatively mature triple quadrupole instrumentation or whether further developments of an alternative type will be needed. It is clear that many challenges lie ahead in terms of development of instrumentation in mass spectrometry. CONCLUSION Instrumentation for mass spectrometry continues its vigorous development. Not only are new instruments being introduced, but established devices continue to be improved and, increasingly, to be applied to problems at the center of attention in science. The examples of biopolymer characterization, C60 chemistry and quantitation of D N A adducts illustrate this. An area with great potential for future developments is material and surface science. Mass spectrometry has a role to play both as a method of characterization and as a means of materials preparation. Low energy collisions of mass-selected ions with surfaces have a role in both these endeavors and the whole phenomenon of low energy ion/surface reactive collisions now seems ripe for the type of systematic development which ion/molecule reactions in the gas phase has already experienced. The increased mass/charge range and mass resolution of the quadrupole ion trap are noteworthy. There appears now to be developing an increasing degree of overlap of this instrument and the FT-ICR mass spectrometer in capabilities, some aspects of instrumentation and in applications. A merging of these two technologies is one long-term possibility. Increasingly, the center of gravity in mass spectrometry is moving away from its traditional base in physical organic chemistry. This is not only true for applications, which now extend across the earth and atmospheric and hfe sciences, as well as covering the molecular sciences. It is also true of fundamental aspects of mass spectrometry which are less and less accurately

34

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described as gas-phase ion chemistry. A wider definition is needed which embraces also solution b i o c h e m i s t r y , materials o r g a n i c c h e m i s t r y a n d aspects o f i n t e r f a c i a l science, a m o n g others. In this direction one can perceive a s t r o n g future for o u r science. ACKNOWLEDGMENT This w o r k was s u p p o r t e d by the N a t i o n a l Science F o u n d a t i o n g r a n t s C H E 87-21768 a n d D I R - 8 9 1 2 6 1 2 . REFERENCES 1 S.A. Lammert and R.G. Cooks, Rapid Commun. Mass Spectrom., 5 (1991) 452. 2 S.R. Hornmg, J.M. Wood, J.R. Gord, B.S. Freiser and R.G Cooks, Int J. Mass Spectrom Ion Processes, 101 (1990) 219. 3 V.H. Wysockl, J.M. Ding, L.L. Jones, J.H. Callahan and F.L King, J. Am. Soc. Mass Spectrom., m press. 4 (a) C J. Ijames and C.K. Wdkens, Anal Chem., 62 (1990) 1295. (b) E.R Wllhams, K D. Henry, F.W. McLafferty, J Shabanowltz and D.F Hunt, J. Am Soc. Mass Spectrom., 1 (1990) 361. 5 S.A. Lammert and R.G. Cooks, J. Am. Soc. Mass Spectrom., 2 (1991) 487. 6 E.N Nikolaev, A.V Mordehal and V.E. Frenckevich, Jr., Rapid Commun. Mass Spectrom., 5 (1991) 260. 7 0 . Gnzzl, M. Shl, H. Bu and J.W Rabalais, Phys Rev. B, 40 (1989) 10129. 8 L.D. Detter, O.W. Hand, R.G Cooks and R.A. Walton, Mass Spectrom. Rev, 7 (1988) 465 9 T. Ast, Md.A Mabud and R.G. Cooks, Int J. Mass Spectrom. Ion Processes, 82 (1988) 131. 10 (a) B.E. Winger, Ph.D. Thesis, Purdue University, 1990 (b) B.E. Winger, H.J Laue, S.R. Horning, R.K. Julian, Jr., S.A. Lammert, D E Rlederer, Jr and R.G. Cooks, submitted for pubhcation. 11 K Quian, A. Shukla, S.L Howard, S G. Anderson and J.H. Futrell, J. Phys. Chem., 93 (1989) 3889 12 A.K Shukla, S,G. Anderson, S.L. Howard and I.H Futrell, Int. J. Mass Spectrom Ion Processes, 86 (1988) 61 13 C.D. Baln, E B. Troughton, Y.-T Tao, J. Evad, G.M. Whiteslde and R.G. Nuzzo, J. Am. Chem. Soc., 111 (1989) 321 14 D.E. Riederer, Jr., C Frednck, C.E. Chidsey and R.G. Cooks, to be published. 15 T. Ast, K.L. Schey and R.G Cooks, J Serb. Chem. Soc., 55 (1990) 247. 16 B.E. Winger, D.E. Rlederer, Jr, R.G. Cooks and C.E. Chidsey, J. Am. Chem. Soc., m press. 17 M. Vmcentl, S.R Hornmg and R.G. Cooks, Org Mass Spectrom., 23 (1988) 585. 18 R.M, Caprloh (Ed), Continuous-flow Fast Atom Bombardment Mass Spectrometry, Wiley, Chlchester, 1990. 19 C E. Lunte, D O. Scott and P.T. Klssmger, Anal. Chem., 63 (1991) 773A. 20 R.M. Capnoh and S.N Lm, Proc. Natl. Acad. Sci. U.S.A, 87 (1990) 240 21 (a) M.A. LaPack, J.C. Tou and C.G. Enke, Anal. Chem, 62 (1990) 1265. (b) M.J. Hayward, A.K. Lister, T. Kotlaho, R G. Cooks, G.D. Austin, R. Narayan and G.T Tsao, Blotech Tech., 3 (1989) 361. 22 (a) F.R. Launtsen, Int. J. Mass Spectrom. Ion Processes, 95 (1990) 259.

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