Michael Barber, Robert S. Bordoll, Gerard J. Elllott, R. Donald Sedgwlck, and Andrew N. Tyler Deparbmlt of chemistry. University of Manchester Institute of Science and Technob& Sackville St.. Manchester M60 1QD United Kinmm
Fast Atom Bombardment Mass Spectrometry
-
lnstrumentation
/,
ot=o.OOps
For many years now mass spectrometry has held the position of an estahlished physical technique for moleeular weight and structure determination in the study of complex compounds. It is used either an a "stand alone" method, where purified materials are examined, or aa a structuresensitive detector in combination with some suitable chromatographic separation method. A typical mass spectrometric experiment consists of presenting the material under examination, in the gas phase, to the high vacuum region of the ion source of the instrument. Here the molecules are ionized, usually hy allowing them to interact with a beam of electrons, typically in the energy region of 7S100 eV. Radical molecular ions are formed with a range of excitation energies, and these can decompose to give structurally significant fragment ions as in Equation 1. AB+e
*AB+'+%
-+
J A' + B
I
(1) B' + A
Emanating from this ion source is a mixture of molecular ions, which give molecular weight information, and fragment ions, which contain the structural information. This mixture is then separated by the maea analyzer to give a spectrum of abundance of ions (ion current) v8. mass-to-charge ratio. An examination of this spec00052700/82/035 1845ASO 1.0010 @ 1982 American Chemical Society
Flgun 1. Twodimensional molecular dynamics calculation ( 8 ) of the sputtering of solid KCI by 1.33 keV Ar+ incident at 20' to the surfaceat t = 0 s (a).Note the eventual upward movement of some particles from the initial crater and the corn mencement of surface peeling away from the impact site after 0.37 ps (b) ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
645A
trum, using empirical rules and a knowledge of how related structures fragment under these conditions, can give rise to a considerable amount of information concerning the molecular structure of the original material. Other “softer” methods of ionization can he used; for example, the molecules in the gas phase can be subjected to very high electric field strengths, of the order of several volts per angstrom, which induces field ionization (1 ), to give mainly molecular ions with little fragmentation being observed. Alternatively, the molecules can be allowed to interact with reactive ions (such as CH5+), which can act as either Lowry or Bronsted acids. This “chemical ionization” (2) gives rise to “pseudomolecular” ions, in which the molecule has gained or lost a proton to produce a stable, evenelectron ion, with attack at structurally significant sites in the molecule giving rise to fragmentation. All these methods of ionization have one feature in common: They require the presentation of the sample to the ion source in the gas phase, so that vapor pressures of 10-6-iO-5 torr have to he obtained by some means, most commonly by heating the sample. Naturally this precludes many types of compounds because of thermal lability, their polar nature, general involatility, and molecular size of the suhstance. Chemical derivatization may be used in some cases to increase volatility and thermal stability but a t the expense of an increase in molecular weight. It is not surprising therefore that considerable effort has been devoted to this problem over the past 15 years or more, and a number of techniques have emerged that have succeeded, to a greater or lesser extent, in producing a “solid-state” ion source, where sample volatility is not a necessary criterion of success for mass spectrometric analysis. Of these techniques field desorption (FD) (3)has perhaps heen most widely adopted. In this method, the sample is deposited from solution onto a delicate tungsten wire. The wire has previously been activated by growth of a dense matrix of graphitic microneedles on its surface. The loaded emitter is then introduced into a region of very high electric field, as in field ionization, and the wire heated to assist desorption of ions. The technique characteristically gives molecular weight information but often little structural data, and it is notorious for the transient nature of the spectra. For all the experimental difficulties involved, it has neverthelesa been the mainstay of workers in the field of highly involatile, large molecules for a number of years. Other techniques such as laser-induced desorption ( 4 ) of 846A
ions from surfaces and desorption chemical ionization (5) have been described, but as yet have not reached their full potential as routine analytical methods. The sputtering phenomenon, bowever, has always held out the promise that it could he used as a general method of ionizing materials from the solid state. Indeed it has heen used for some time as a means of snrface and bulk elemental analysis of solids in the ion microprohe (6).The phenomenon itself is not new, beiig first reported by Grove in 1852, and even the ubiquitous Langmuir dabbled in the subject (7). The phenomenon can be described very simply. If a solid is bombarded by high-velocity particles, say rare gas ions of about 8 keV energy, then material will he removed into the gas phase. This results from momentum transfer from the impinging particle to the target with the setting up of collision chains, some of which cross the snrface (8),as illustrated in Figure 1. Some of the sputtered material will he in the form of positively or negatively charged ions, and it is the m m speetrometric analyses of these species we are interested in. Early workers in the field, notably Robb and Lehrle (9),demonstrated the feasibility of such an ion source, but essentially it was left to Benninghoven (10) and coworkers, tcgether with Macfarlane (11) (who used uery heavy ions ohtained from the nuclear decay of 252Cf),to show that uniquely advantageous mass spectral results could be obtained with organic compounds from such a source. In each of these studies the quality of the results obtained was limited by the inherent performance of the mass spectrometers that were
--
It was a t this stage that we decided
to attempt to design a simple sputter ion source that could be attached to a large high-voltage double-focusing mass spectrometer, to use the full range of techniques available on such instruments, and to m e s s the m m range capabilities and limitations of such an ion source. Since with this type of instrumentation we are involved with accelerating voltages of -8 kV to extract free ions from the source region, we decided to use, instead of ion beams as the sputtering agent, molecular beams of fast neutral species. The technology of producing such fast beams of rare gas atoms with controllable kinetic energy (in the 3-10 keV region) is well known (12) and in all cases starts with the production of a beam of ions that can be accelerated to a known kinetic energy and focused into a beam of high intensity. These ions are allowed to undergo charge exchange with little change in forward momentum. We chose to use resonant gas-phase charge exchange of the ion with its neutral counterpart as in Equation 2, since this can he a highly efficient process. The residual ions, which have escaped charge exchange, can be cleansed from the resulting mixture by a simple electrostatic deflection method.
-
A,‘+ Ap-
AE=0
A,
+ A;
+
(2)
Fast ions, Fast atoms, KE=V KE=V Previous workers in the field have used separate charge exchange chamhers to carry out the above process (13).However, the source becomes extremely simple once it is realized that
I
Flgure 2. Schematicdiagram of me FAB ion source. (a) atom gun; (b) atom beam; (c)metal sample holder; (d) end of probe: (e) sample in low volatility solvent; (0 Ion beam; (g) Ion extraction plate; (h) lens system leading to the m a s analyzer
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
cantly reduce flow area
000,
InspectionGage Assures Sufficient Pull-up
Effective Metal-to-Metal
Interactionof precision parts -body, front ferrule, back ferrule and nut - produces a leak-free seal with simple 144 turn pull-up Works on thick or thin wall tubing Seals repeatedly under make-andbreak conditions Seals consistently over a wide range of pressures, temperatures and temperature cycling.
L,
Useful for both the installer and inspector If the gage will not fully enter the gap befween the nut and body shoulder, fining pull-up is sufficient If the gage enters the gap, you'll know pull-up is incomplete. Only SWAGELOK Tube Fittings offer gageabtlity of straight fittings, and forged shaped elbows and tees in Stainless steel and steel. Gages are available for 118"through 1" sizes.Torque.free swaging action, effective sealing and gageability are just three of the many elements of excellence for which SWAGELOK Tube Fittings are known and respected... a tradition of Excellence.
CnnwiORD FITTING COMPANY 29500 Soion Road, Solon, Ohio 44139 Crawford Fittings (Canada), Ltd., Ontario
e11081 MARKAOSERVICECO./aIIti~h~ rema
058
CIRCLE 33 ON READER SERVCE CARD
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
647A
A comprehensive handbook of laboratory safely equipment.
Your next lab accident may never happen because of this book
It's the new Arthur H. Thomas Lab Safety Book. It lists and describes 1,153 products that can improve the safety of your lab. In addition, it includes a preface that gives you ideas about the types of apparatus safety experts recommend. If you want to pursue safety further, the book contains a bibliography of publications you can consult. The 120-page Thomas Lab Safety Book is yours free. Just write for it on your letterhead to: Arthur H. Thomas Company, Vine Street at Third, P.O. Box 779,Philadelphia, PA 19105.
COMPANY Serving IabGlheworldoVerSinMl~ CIRCLE 209 ON READER SERVICE CARD
any high-pressure confined-discharge ion source will produce copious quantities of fast neutral species. These arise by charge exchange of the ions produced in the discharge with the high pressure of un-ionized gas in the source itself. Ion Tech saddle field ion sources (24) are extremely compact, simple, and efficient and have been widely adopted for this purpose. Their characteristics are well documented, and we have used them for this application for over two years with few, if any, problems (25).The total system, which is very simple in concept, is shown schematically in Figure 2. The stage, upon which the sample is mounted, consists of an inert heatable metal strip. This intercepts the atom beam a t an angle of about 20’. The ions nroduced hv the svutterine DIOcess h e then extractedby a s l i i l k system designed to collect ions emitted approximately normal to the samd e surface and to direct them into the optical system of the masa analyzer. The angle of incidence of the atom beam at the sample surface. which approaches grazing incidence, is somewhat critical, since sensitivity is improved at these shallow angles. Let us now discuss the performance of such an ion source, based on some criteria outlined below that could be proposed for an ideal routine mass spectrometric analysis method. The sputter source produces ions from the solid at room temperature, which fulfills the first criterion of eliminating the necessity to volatilize the sample prior to ionization. However, other equally important refinements are essential: The ionization process should give, in abundance, ions indicative of the molecular weight of the compound, and, additionally, structurally relatable fragmentation of the molecule should he in evidence. This latter feature is frequently absent in some of the newer “soft” ionization methods, in contrast with electron impact ionization. where the molecular species may fragment totally. The masa spectrum produced by the source should ideally also have a long lifetime, so that time-consuming techniques, such as metastable scanning, can he applied. The sensitivity of the method should he such that practical structural problems can be investigated where the amounts of material are very small (1pg). A final point, which is subjective, is that any new ion source should have the potential or demonstrable power, of easily and reliably producing structurally meaningful mass spectra from compounds that have proved difficult or intractable by other methods. We will examine these desirable features in turn, with relation to the FAB ion source.
574
(M + H)+
&
’
3bo
350
460
4io
$0
“I-r 550
L 512
(M - H)-
I
I
L
Flwre 3. Positive- tal and neoative-ion tbl FAB mass sDectra of underivatized met h b i n e enkephalin, Tyr-Gly-6ly-Phe-Met.’ mol wt 573. Reprinted from Reference 16 with permission
l I I
Figure 4. Positive- (a) and negative- ion (b) FAB mass spectra of the potassium salt of phenethicillin. mol wl402. (M H)-, m/z = 401: (M - K)-, m/z = 363; (M K)+, d z = 441
-
+
Molecular WelgM and Fragmentation Information
The positive- and negative-ion FAB mass spectra of the underivatized peptide, methionine enkephalin (26),are shown in Figure 3. I t will he noticed that a true molecular ion a t mlz 573 is not observed, but that even electron species (M H)+and (M - H)- predominate, in which the molecule forms a stable ion bv addition or loss of a proton. This is generally observed for organic molecules ionized by this method (27).Molecules that are alkali metal salts form similar stable species, in high abundance, by addition or loas of the alkali metal cation. The potassium salt of a penicillin ( I @ , RCOzK, will give RCOzKz+ and RCOz-, as il-
+
lustrated by phenethicillin in Figure 4. Cationized species can also he formed from mixtures of organic molecules with salts present as impurities in the sample. This facility can in some cases he used beneficially by judicious doping of the sample with alkali metal salts or by protonating agents. Thus the yield of the protonated molecular ion (M H)+ a t mlz 574 from the neutral underivatized pentapeptide, methionine enkephalin (26).can be increased 20-fold hy adding 5%of the strong protonating agent p-toluenesulphonic acid to the sample prior to atom bombardment. Alternatively, addition of potassium chlorid6 to a sample suspected to be a sodium salt will shift all peaks containing sodium ions by 16 u (the difference between
+
ANALYTICAL CHEMIST13Y, VOL. 54, NO. 4. APRIL 1982
649A
23Na+ and 3 W + ) for each sodium ion present in the various species observed in the spectrum. It will he noted that both positiveand negative-ion m858 spectra are produced with equal facility and without the need to make any changes in the atom source conditions (19). Figure 5 shows the total positive-ion mass spectrum of an organo-silicon compound containingrhodium. This is a metal-containingmaterial, and the f i t point to note is that in this case we see a true molecular ion. M+at mlz 438. This is characteristic of some metal-containingsystems. The fragmentation is rich in structural information and is annotated on the diagram. Perhaps a better example of
the completeness of the structural information that can be obtained is exemplified in Figure 6. This is the FAB mass spectrum of a pore-forming peptide antibiotic (20).antiamoebin I11 (21).The molecular ion region is complicated by the presence of ubiquitous sodium salt impurities in the sample. However, if one considers the negative-ion spectrum, the problem of molecular weight determination is simplified since only one species is observed at mlz 1654-.This can be either (M - H)- from the neutral molecule or (M Na)- from the sodium salt, which hihliibts the usefulness of being able to easily obtain both polarities of mass spectra. The total interpretation of the spectra of this and re-
-
Fgure 5. Positive-ion FAB mass spectrum of an organo-siliconcompound Contaming rhodium. Reprintedfrom Reference 15 with permission
printed from Reference 21 with permission 65.0 A
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
lated molecules leads to a complete structure of the molecule in terms of the sequence of amino acid units in the chain. A similar approach was successful in the elucidation of the structure of 11related zervamicins and two emerimicin peptide antibiotics that had proved intractable by other solidstate ionization methods (20). The fragment ions present at reasonable abundance in most FAB mass spectra have heen shown to arise, at least in part, by gas-phase unimolecular decomposition reaction steps originating with the molecular ionic species. The fragmentation is not dependent on thermally induced reactions, since the Sample is usually maintained at ambient temperature throughout the analysis. Furthermore, since the sample lifetimes are long, typically 20 min (depending on the volume of the solution used), we are free to employ any of the established m a s spectrometric techniques that have heen developed to aid the interpretation of low-resolution mass spectra. Thus use can be made of accurate mass measurements (20) for the determination of the atomic compositions of both molecular and fragment ions, and metastable scanning (22) methods to confirm fragmentation pathways. Mass Spectral Litetlme and Sample Preparation The early experiments using charged particle sputter sources had simply deposited the sample from solution onto the sample stage and evaporated it to dryness before analysis (23). This method of preparation resulted in m&58 spectra of a transient nature. In our case, with atom fluxes sufficient to give adequate sputter ion yields, this lifetime was of the order of tens of seconds. However, it was noted by us that low vapor pressure liquids and oils gave spectra that lasted for hours. Examples frequently encountered were the common pumping fluids, Apiemn oils, Santovac 5, and Convalex 10,and siloxanes frequently found as contaminanta in organic samples. These observationsled us to study the use of solutions of materials in low vapor pressure viscous solvents to mimic this fluid behavior with solids (24).The first succesaful solvent found hy us was glycerol, used initially on small peptides (Z6).This gave enhanced sensitivity compared with solid sample preparations, and l i e times of the spectra could be extended to hours, provided there was sufficient sample and glycerol present to sustain a fluid condition. Of course, the relatively rare samples that were both intractable to conventional mass spectrometric investigation and were also liquids at room temperature yielded long-lived spectra without the need
for every fluorometric application. For over two decades, American Instrument Company has pioneered advances in fluorometry, making instruments which have helped make history. Today, we make instruments for virtually every fluorescence application. And this experience is part of every instrument we offer. Consider the !3PF-500TM Series Spectrofluorometers, for example; state-of-the-artanalytical tools which offer greater sensitivity and resolution, incorporatingsuch features as digitall readouts, automatic range selection, and decimal placemelit for unparalleledoperating ease. And, SPF-500’s are versatile. Even without optional accessories, each model includes application capabilities that are simply not available on competitive instruments. These capabilities; are further enhanced with our data processing accessory. For more informationabout the SPF-500, or any of our other fluorescence .- .- instruments. . -, mail this - - couoon - - -r - td -American Instrument Company, 810 West Anthony Drive, Urbana, IL 61801, U.S.A or call 800-637-7689toll-free. TELEX 20-6079.
FLUORESCENCE INSTRUMENTATION i I
I I
; 0Send more information on American Instrument’s I
I
fluorescence instrumentation.
FREE
;I I
Partners in progress.
I
Copyright 1980,1982. SLM Instruments, Inc.All rights resewed.
CIRCLE 188 ON READER SERVICE CARD
M
m 1 mE HEAT IS L, CAN YOU FIND THE ATOMS 3
. ..
.
.~ .
I
I
i
Zeeman Atomic
'I'M leaaer in
zeeman AA
SCIENTIFIC INSTRUMENTS
N I S I UNGYO AMERIUI. LlD. 460 Ecrst Middletleld Rood Mountah VleW.
CIRCLE 99 ON READER SERVICE CARD
I@ for any solvent as shown hy the example in Figure 5. Thii rhodium complex is a viscous liquid at room temperature. In contrast, the spectra in Figures 3,4,6,and 11were all obtained with the solid samples dissolved in glycerol. Other solvents found by us and by others have provided a useful backup for the relatively rare caws in which glycerol has proved ineffective. On their own these solvents gave characteristic maw spectra. Glycerol, for example, gives peaks a t maw numbers corresponding to (92n 1)+ and (92n - l)-, with values of n.up to 15 being detectable and with the relative intensities of the multimers decreasing with increasing n. This may be troublesome with low molecular weight materials and those samples that give a low surface concentration in the glycerol solution, and solvent spectrum subtraction in a data system may be advantageous. On the other hand the molecules under investigation may have the right surfactant properties and bulk solubility to give a maw spectrum in which tb solvent is totally suppressed. This is true for most free peptides in solution in glycerol, as exemplified in Figures 3.4, and 6. An example of the need for careful sample preparation is the case of chlorophyll A (25).Thii substance is totally insoluble in glycerol and consequently no FAB maw spectrum could he obtained in thi solvent. The addition of -1% of a solubilizing agent, Triton XlOO, to the glycerol gave good solubility as evidenced by formation of a deep green-colored solution, and this gave the m a e ~spectra shown in Figure 7.The positive-ion spectrum shows a strong spectrum of chlorophyll A above a weak spectrum of the solubilizing agent which, being a polyether, RO(C&IIO).H, gives a series ofpeaks separated hy 44 u. Being an organometallic complex, there is both an M+. and an (M H)+species present. The peaks due to fragmentation of the chlorophyll A molecule are easily distinguished and can be rationalimd with the structure. The negative-ion spectrum is much cleaner, since the Triton XlOO gives a very much weaker negative-ion spectrum than it does in the positive-ion mode. A further interesting example is shown by another metal-containing compound, the porphyrin whose structure is shown in Figure 8. It was necessary to run this in pure Triton X100, there being no response from a glycerol or from a glycerol plus Triton XlOO mixed substrate. The objective of the sample preparation should be to present the sample to the atom beam at a high mobile surface concentration. For maximum sen-
CH=CHz
CH3
I
I
+
+
(M
i
l
+
H 893
l
Figure 7. (a)Structure and (b) positive- and negative-ion FAB mass spectra of chlorophyll A
sitivity the sample should form a perfect monolayer a t the surface of a suhstrate having low volatility. This is a characteristic of compounds that exhibit high surface activity in aqueous media, attributable to the presence of highly polar or ionic groups, giving hydrophilic properties to an otherwise hydrophobic molecule. Monolayer formation at the surface of a dilute solution implies a constant surface excess concentration. Following Gihbs (26), this arises when the surface tension (7)depends linearly on the logarithmic hulk concentration (log. C) of the solution. This condition is achieved for cetylammonium bromide solutions in glycerol a t concentrations greater than 5 X 10-4 mol dm-3,as shown in Figure 9. Below this concentration we can always detect ions that are characteristic of both the solvent and the solute in the FAB maw spectra of the solutions. The ratio of peaks due to glycerol to those due to the sample cation falls to zero as the monolayer
becomes established. Under monolayer conditions, glycerol ions are ahsent from the spectrum, and the solute exhihits a maximum sputter ion yield that is independent of the hulk concentration. Additionally, since there is no increase in ion yield as we increase the solution concentration in this range, we observe an “apparent” decrease in sensitivity measured relative to this concentration. In this case, which is not atypical, if we place 10r L of glycerol as solvent on the probe, then the sample sensitivity would increase with increasing amounts of cetylammonium bromide, achieving a maximum for a solution containing 5 nmol of solute. Useful mass spectra would be recorded with only 100 pmol on the probe, and a detectable response would exist at even lower levels. Since a monolayer of material is completely sputtered in a matter of seconds in a typical FAB ion source, it is esaential that the sample surface be
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
653A
End mess and clutter: Identit crucial materials: orotect idei tification from's'mearing an smudglng qulckly and ea611 wlth the new BebArP Iabetln system. The dlspenssr accon modates 3 slzes 01 tape an color coded labels. Ideal f i labeling everythlng from sma test tubes to large bottles an beaker, -.-
The complete laboratory labeling system When ordering specify Bel-Art Numbers: F13450. Dispenser: F13453 .Tape; F13455. Labels. Available through local laboratory Supply dealt
L--
CiRa
BeCArt Products Pequam
w Jersey 07440 USA
pN READER SERVICE CARD
THE ONLY PLACE YOU HAVE TO LOOK FOR FINE LABORATORY EQUIPMENT I
UBOFiATORY OVENS America's most complete line. Fromsimpie budget models at a b u t 3250 to large floor models with proportional controls. Mechanical and gravity. temperatures to 704% including: Batch, Vacuum, Aging, Drying-plus FRICTION-AIREB. the patented electric ovens with no heating elements. Over. temperature protection on most models. CIRCLE 13
+
FURNACES Dozens of models . . . bench andfloortypes ... to +162o'C. All but budget models - which start at well under $4W have built-in overtemperature prctection. All have solid-state instrumentation i n c l u d i n g StabilGlaw and RAD.0-GLOW GlobaP Furnaces with patented POWER-0-MATIC 808 SCR Proportioning Control lor superb performance. CIRCLE 14
BATHS Water. oil, relrigerated . . . dozens 01 proven models. in. cluding Shaker (standard and relrigerated), Hydrometer. FulI Visibility Jar. Tissue. Temperatures from 0%. l o + m C . Most feature patented MAGNIWHIRLa agitation that made conventional circulating sys. terns obsolete. Quality-built easily the finest baths you can buy! CIRCLE 15
For lull information. Contact: Blue M. A Unit of General Signal; Blue Island, Illinois 60406. Telephone: (312)3859000.
I
BLUE M I
A U N I T OF OENERAL S l O N A L
654A
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
-
ENViRONMENTALCHAMBERS Acknowledged as the finest ever built lor controlling tem. perature and humidity. For testing to MIL, and even tougher in.house specs. Starting with economics jar-type chamber for the most modest budget, models include unique air-cooled design. . . mechan. ically refrigerated steady-itate units , , . high temperaturelhumidity chambers. CIRCLE 16
5
4
3
2
log. C (C in mol dm-1)
Flgure 9. Dependence of the surface tension y on the bulk concentration C for solutions of cetylammonium bromi& in glycerol. Ratios of ion currents due to glycerol (b)relative to those due to the solute (Is) obtained from the FAB mass spectra of these solutions are also shown. Note that Ig fallsto zero and Is is m s t a n t when the solute forms a monolayer at the surface as indicated by the constancy of dy/d(log.C)
Figure 8. Structure of a porphyrin compound continuously regenerated during prolonged examinations. This replenishment occurs automatically by diffusion of sample to the surface of the solution. However, it is essential that the sample have some solubility in the low volatility solvent, not only to provide the diffusion mechanism but also as a reservoir of unirradiated material. We see then that the ionic groups that make compounds involatile and hence rule out conventional methods of ionization are also those groups that frequently bestow both solubility in polar solvents and associated surfactant properties that facilitate good sample preparation for FAB ionization. The detection of solvent substrate peaks in a FAB mass spectrum implies that optimal sample preparation has not been achieved. Either the sample concentration is insufficient to achieve monolayer formation or an alternative solvent system may be required. The former condition may occur as the solvent is pumped away preferentially during a prolonged experiment and the sample spectrum “grows”at the expense of that of the solvent.
The sputtering of neutral species, which are mass spectrometridy undetected, is probably the major process occurring at the surface. The simple sputtering of ions that are naturally present in the sample is undouhtedly an important source of ions in FAB mass spectra. Consequently, sample preparation methods such as the addition of acids, which lead to an increase in the amount of“preionization” (27) in the sample, can lead to an enhanced sensitivity. Degpite considerable speculation on the subject, the mechanism of ionization in the sputtering of nonionic compounds is uncertain. G o d FAB maaS spectra may be obtained with ease from compounds that do not contain ionizable groups. Interest has hitherto centered naturally on those compounds that had previously been considered diffcult or intractable, but that does not mean that “ordinary” organic molecules cannot be studied. Completely nonionic compounds, such as aliphatic hydrocarbons, do give
good FAB mass spectra when the sample is in R condensed phase (%), one example of which is shown in Figure 10. This is completely at variance with previous reports that preionization is essential (28). The complex solution and surface chemistry that operate in a typical sample can lead to large apparent differential sensitivities between quite similar compounds (29,30).Until these factors have been quantified, the analysis of unseparated mixtures of compounds must be approached with caution. Finally, the FAB ion source has proved to be a rich source of mass spectra from compounds previously considered to be too ionic (24), and
I LercAla-Glu-Val-Leu-HisSer4iy~~eu-Hial2he-H I
Tyr4ercVal-Cy4ly-Glu4rg-Giy-Phe-Phe-Tyr-Thr2r~Lys+%l&OH I S03H (M +ti)+
\
3480
Flgure 10. Positiveion FAB mass spectrum in the molecular weight region of a mixtwe of saturated aliDhatic Wdrocarbons
-
L
Fwre 11. Positiveion spectrum of mc u ion region of oxidized bovine insulin B chain. Calculated mol wl3493.6. measured m/z of (M H)+ = 3494.5. Ion anergy 6 keV on VG Analytical MM-ZAB mass spectrometer wlih a high field magnet
+
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
655A
.
.
,.
i
...,
. .
'
..
A.
,
Ir
..
anewdofbw-cost (Aadsimplearfthmetw
Let it guide your imaginationand arithmetic.
TheaedvesouKe. e-dealerr handling KODAK LaboratoryChemi
hence too involatile, for other ionization methods to sucreed. Involatility also increases with mnlecular size. and an index of the power of an ionization method is the upper limit of the mo. lecular weight \,aIues attrihutahle to its use. Without dnuht the 2sTf source is the leader in this respert. and this situatinn is likely t u rontinue in view of the large amounts of energy available in thin sputter prnress. Monomeric mulerular inn species have heen deterted from sume mudifled nligunurleutides (31)with molecular weights up to 6980 u. The perfor. manre of the FAH ion sourre has so far heen limited hy the mass range of the availahle magnetir mass annlvzers. operating at full sensitivity and at least unit mass resnlving power. N e v ertheless. a high-efficiency FAR im source. using xennn atoms as the primary beam to improve sensitivity. combined with a \I(; Analytical ZAR mass spectrnmeter fitted with a high field magnet has been used to give spertra from many compounds with mulecular weights up to 3tKK) u (Y2J. Heduction of the inn energy has a l m permitted resolved mnss spectra to he recorded for mnlecules such as glucagon, a free uligupeptide uf molecular weight 3481 u and the oxidized form of the R chain of hovine insulin (3494 U I as shown in Figure 1 I . T h i s molecule is a linear chain of 30 amino arid residues w i t h t w o free sulphonic acid groups formed hy oxidatim uf the sulphur bridges in the original insulin molecule. Acknowledgmenl Theauthors wish t o thank the Srience Hesearrh Cnunril and the National Researrh Development (.’orporation for financial support and V(; Analytiral I.td. fur providing instru. mental facilities. References ( 1 ) H. D.
Beckey and D. Sehuelte. Z. In-
strum.. 68, :lo2 (1960). (2) M. S. H. Munsen and F. H. Field, J . Am. Chem. .Snc..XR.2021 (1966). (3) H. D. Beckey, Inf.J. Mass Sperfrom. Ion Phys., 2.500 (1969). (4) M. A. Posthumus. P. G. Kistemaker, H. L. C. Muezelaar, and M. C. T. N. de Brauw. Anal. (‘hem.. SO. 985 (1978). (5) M. A. Baldwin and F. W. MeLafferty, Ow. Mars Spertrnm.. 7.1353 (1973). ( 6 ) R. Castaing and G. Slndzian, C . R . Hebd. Seances. Acod. Sri. Paris, 255, 1893 (1962). (7) G. Carter and J. S. Colligon. “Ion Bombardment of Solids.” Heinemann Ltd., London, 1968. (8)D. M. Heyes. M. Barber. and J. H. R. Clarke. Sur\. Sci.. 105.225 (1981). (9) A. F. Dillon, H. S. Lehrle, and J. C. Robb. Ado. Mosr Sperlrom., 4,477 (1968).
A. Benninghoven,D. Jaspers. and W. Siehtermann. Appl. Phys., I I.35 (1976). (11) R. D. Maefarlaneand D. F. Torgerson,S c i m r c . 191.920(1976). (12) H. Lew in”Methcdsof Experimental (10)
Physics 4A,” V. W. Hughes and H. L. Schultz. Fds.. Academic Press. New York, l k 7 . (13) 0. Heeck, Ann. Physik, 19,121 (1934). (14) J. Franks and A. M. Ghsnder. V o w um, 24,489 (1974). (15) M. Barber, R. S.
Bordoli, R.D. Sedg-
wick.and L.W. Tetler. O m Mors Soeetrom:, 16,256 (1981). (23) M. Barber and J. C. Vickerman, Surf, Defeef Prop. Solids, 5.162 (1976). (24). M. Barber, R. S. Bordoli, R. D. Sedgwick. and A. N. Tvler. J.C.S. Chem. ~
wick. and A. N. Tyler. Nofure. 293.270
Barber, R. S. Bordoli, G. V. Gar-
ner, D. R. Gordon, R.D. Sedgwiek, L. W. Tetler, and A. N. Tyler. Riochem. J.. 197,401 (1981). (17) M. Barber. R. S. Bordoli, R. D. Sedgwick. A. N. Tyler. and E.T. Whslley, Hiamed. Moss Speefrorn.,8.337 (1981). (18) M. Barber, R. S. Bordoli. R. D. Sedgwick, A. N. Tyler. B. N. Green. V. C. Parr, and J. L. Cower, Riomed.Moss Speetrom., in press.
M. Barber, R. S.Bordoli. R.D. Sedgwick, and A. N. Tyler. Riomed. Moss
(19)
Jr.. M. Bar&, R. D. SedGiek, R. S. ’ Bordoli. A. N. Tyler, and R. N. Green,J.
Am. Chem. Soe., 103,6517 (1981). (21) K.L. Rinehsrt, .Jr.. M. L. Moore, and A. N. Tvler. Inviehl 11. V.G. Analvtieal Ltd., Jine 198l.” (22) M. Barber, R. S. Bordoli. R. D. Sedg-
~
~~
~
~~~~
.
li, R. D. Sedgrw. ’. Humphreys.
1191111 l_l___,.
(16) M.
~
332). itefano. Anol.
Chem., 53.25 (1981). (28) D. H. Williams. C. Bradley, G. Bojesen, S. Santikam. and L.C. E. Taylor,J. Am. Chem. SOC., 103,5700 (1981). (29) H. R. Morris, M. Panieo, M. Barber,
R. S. Hordoli. H.I). Sedgwiek, and A. N. Tyler. Hiorhem. Hiophys. Res. Cumm., 101,62:1 (l9Hl).
(30) M. Barber, R.S. Fbrdoli. R.D. Sedgwick. A. N. ‘I‘yler,andB. W. Bycroft. Hiochem. Riaphys. Rer. Comm., 101,632 ( 1 981). (31) R. D. Macfarlane. Symposium on Fast Atom and Ion Induced Mass Swetrome-
Medicik and En
