Laser Fourier Transform Mass Spectrometry for Polymer

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4 Laser Fourier Transform Mass Spectrometry for Polymer Characterization J. Thomas Brenna , William R. Creasy2, and Jeffrey Zimmerman 1

2

Division of Nutritional Sciences, Cornell University, Savage Hall, Ithaca, NY 14853 I B M Corporation, P.O. Box 8003,D/T67, Endicott, NY 13760 1

2

This review focuses on laser-based Fourier transform ion cyclotron resonance mass spectrometry (FTMS) for polymer structure determi nation and identification of industrially important polymers on sur faces. In structural studies, laser desorption has been used as a gentle ionization technique to desorb and ionize intact polymer molecular ions of intractable polymers. Spatially resolved studies involving higher laser fluences cause more fragmentation and recombination but retain sufficient information to permit identification at spatial resolution of about 10 μm. Molecular fragments appear most often in negative ion spectra. Recombination products and carbon clusters are prominent in positive ion spectra. Three distributions of carbon clusters are ob served in separate mass ranges, the highest of which are identified as fullerenes first observed in laser ablation of graphite. Ongoing ad vances such as postionization techniques are expected to make laser FTMS an increasingly attractive and convenient tool for polymer analysis.

COMBINING L A S E R - I N D U C E D VAPORIZATION w i t h some f o r m o f mass spec trometry to analyze solids has b e e n practiced for over 20 years. I n fact, an exhaustive bibliography of the field, now 6 years old, contains 1461 references (1), 60 o f w h i c h are to polymer-related papers. I n a n u m b e r of fields, laser mass spectrometry has become the analytical m e t h o d o f choice including, for 0065-2393/93/0236-0129$07.50/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.


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instance, particle isotopic analysis. T h e technique continues to develop a n d provide answers to a w i d e n i n g array of questions even though it is not yet a routine tool i n p o l y m e r analysis. O n g o i n g investigations o f novel instrumenta tion w i t h strategic improvements over older designs show promise for diffi cult analyses. I n particular, the i n t r o d u c t i o n o f high performance analytical F o u r i e r transform mass spectrometry ( F T M S ) permits many types o f experi mental strategies to be executed for p o l y m e r analysis that were impossible w i t h previous systems. A l t h o u g h this w o r k is i n its early stages o f develop ment for p o l y m e r applications, experiments to date have y i e l d e d useful results. W e present here a representative (not exhaustive) review o f the field, i n c l u d i n g discussions o f instrumentation for spatially resolved analysis devel o p e d i n the recent past a n d several applications f r o m o u r laboratory and others.

Laser Mass Spectrometry. T w o b r o a d classes o f laser mass spec trometry experiments have developed independently. T h e y are most conve niently classified as (1) laser-microprobe experiments, w h i c h require spatial resolution and are a i m e d at identification a n d localization o f p o l y m e r i c materials o n the surface, a n d (2) laser-desorption experiments, the goal o f w h i c h is to maximize the amount o f structural information, i n c l u d i n g m o l e c u lar weight distributions, i n the mass spectrum. T h e two experiments are not mutually exclusive, but c o m p e t i n g analytical issues most often force a choice between the two analysis modes for optimization. F T M S has b e e n a p p l i e d to b o t h sorts o f experiments. T h e traditional c o m m e r c i a l instrumentation for laser microprobe, w h i c h is based o n time-of-flight analysis that has existed for a decade, w i l l be described briefly. L a s e r - m i c r o p r o b e mass spectrometry ( L A M M S ) has b e e n used for the past 20 years for elemental a n d molecular identification o f solids. T h e field is referred to by a n u m b e r o f names, w h i c h i n c l u d e the earliest, laser-micro probe mass analysis ( L A M M A ) a n d laser ionization mass analyzer, b o t h o f w h i c h are n o w associated w i t h c o m m e r c i a l instruments. Fundamentally, L A M M S is a spatially resolved surface or near-surface (few nanometers to micrometers) sensitive technique w i t h the chief advantage o f applicability to all solids regardless o f electrical conductivity. T h e technique was developed initially to serve the needs o f the b i o m e d i c a l research c o m m u n i t y for localiza tion o f easily i o n i z e d elements, such as the alkali metals, alkaline earths, or halides. A s c o m m e r c i a l instrumentation appeared i n the late 1970s a n d early 1980s, the technique was a p p l i e d to a w i d e variety of solids. Polymers were a m o n g the first class o f engineering materials to be analyzed by L A M M S ( 2 - 5 ) . These early applications focused o n the ability o f the technique to provide fingerprint mass spectra for identification o f small quantities o f p o l y m e r i c material (1 pg) o n surfaces as w e l l as to provide p o l y m e r structural information.



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F r o m the first instruments reported i n the early 1970s u n t i l 1988, L A M M S systems w e r e based almost exclusively o n time-of-flight ( T O F ) mass spectrometry. I n the L A M M S experiment, a b e a m o f laser fight (usually but not exclusively i n the U V ) is focused to a small spot [typically 10 μ m or less (6)] and directed onto a solid surface. A s a result o f this single laser pulse, ions are ejected f r o m the surface a n d extracted into a mass spectrometer. T h e laser pulse is typically about 10-ns duration, a n d i o n emission is usually o n this order. Therefore, scanning quadrupoles or magnetic sector (without array detectors) mass spectrometers cannot capture a complete mass spectrum f r o m a single pulse. T h e T O F instrument does acquire an entire mass spectrum, w i t h a theoretically u n l i m i t e d mass range, f r o m a single pulse. T o m i n i m i z e fragmentation, the laser p o w e r is often adjusted to the m i n i m u m level at w h i c h i o n emission occurs. H o w e v e r , because of the requirement for spatial resolution, ionization efficiency must be high, a n d the m i n i m u m p o w e r level usually causes extensive damage a n d molecular rear rangement d u r i n g the vaporization process. T h i s m o d e o f laser vaporization is often referred to as " p l a s m a i o n i z a t i o n " ( 7 ) or " a b l a t i o n " (8) to differentiate it f r o m gentler laser experiments. G e n t l e ionization, used i n " d e s o r p t i o n " experiments, tends to be less efficient and, therefore, must sample a w i d e r surface area to p r o d u c e sufficient signal. Positive i o n T O F - L A M M S spectra are generally characterized b y exten sive fragmentation; only major structural moieties, such as aromatic rings, appear i n the spectra. O f t e n , low-mass hydrocarbon ions appear u p to mass ~ 200 μ , w i t h scattered structure-specific peaks. Because the energy spread o f ions emerging f r o m the surface is h i g h (up to hundreds o f electronvolts), the mass resolution o f T O F - L A M M S spectra is rarely better than 1 μ . I n m a n y cases the poor mass resolution causes hydrocarbon peaks to obscure m i n o r peaks o f greater structural significance. It is p r i m a r i l y for this reason that T O F - L A M M S p o l y m e r spectra rarely are used for structural studies a n d only can be used to distinguish polymers w i t h major structural differences by gross fingerprint. I n fact, recent reports suggest pattern-recognition tech niques are most effective for this task ( 9 , 10). These limitations suggest that a laser m i c r o p r o b e interfaced to a more p o w e r f u l f o r m o f mass spectrometry w i l l y i e l d m o r e detailed i n f o r m a t i o n .

Fourier Transform Ion Cyclotron Resonance Mass Spec trometry. F T M S i n its present f o r m was first demonstrated i n 1974 b y C o m i s a r o w a n d M a r s h a l l (11), a n d c o m m e r c i a l instrumentation first appeared i n 1981. Outstanding reviews o f the fundamentals o f the technique are available (12-17). Briefly, F T M S is an i o n - t r a p p i n g technique i n w h i c h ions f o r m e d b y any means are t r a p p e d i n an intense magnetic field [typically 3 tesla (T)] crossed w i t h a weak electric field ( 1 - 1 0 V ) . It is a fact of elementary physics that charged particles i n a u n i f o r m magnetic field orbit w i t h a characteristic m o t i o n . T h i s orbital frequency is t e r m e d the " c y c l o t r o n "


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frequency, and i n a static magnetic field it depends inversely o n the mass-tocharge ratio o f the particle as

w =

m

where w is the cyclotron frequency, q is the electrical charge carried b y the ion, m is the ionic mass, a n d B is the static magnetic field. I n the F T M S experiment, the frequency is independent o f the i o n kinetic energy a n d dependent only o n m/e, to the first order. (Accurate mass calibration requires that a second-order t e r m involving the electric field be included.) This fact permits h i g h - a n d ultrahigh-resolution measurements o f the cy clotron frequency a n d hence the i o n mass. C o m m e r c i a l instrumentation achieves mass resolution i n excess o f 1 part i n 1 m i l l i o n a n d accurate masses to < 1 p p m . These capabilities allow resolution o f ions at the same n o m i n a l mass a n d assignment o f elemental composition o n the basis o f mass alone. F u r t h e r m o r e , because the F T M S is an i o n trap, a complete mass spectrum can b e obtained f r o m a single p u l s e d event such as a laser pulse. C o m p l e x manipulations o f ions also are possible, for example, selective i o n ejection a n d collision-induced dissociation, or photodissociation w i t h a second laser. A l l these capabilities are used to derive structural information about ions.


e

P r i o r to the availability o f c o m m e r c i a l instrumentation, F T M S w i t h h o m e - b u i l t systems became one o f the methods o f choice i n the field o f gas phase i o n / m o l e c u l e chemistry. Since the advent o f c o m m e r c i a l instruments i n 1981, gas (18), l i q u i d , a n d supercritical f l u i d chromatography ( 1 9 ) inter faces have b e e n developed for c h e m i c a l applications. N u m e r o u s sources for h i g h molecular weight analysis have been designed a n d i m p l e m e n t e d for F T M S i n c l u d i n g C s i o n secondary i o n mass spectrometry ( S I M S ) (20, 21) and fast atom b o m b a r d m e n t ; C f fission fragment plasma desorption (22-24); a n d electrospray ionization (25-27). F o r the most part, these sources have b e e n a p p l i e d to biological samples, particularly peptides, n u cleotides, a n d oligosaccharides, although a polystyrene spectrum has b e e n reported for the S I M S source (20). 2 5 2

L a s e r sources have b e e n used i n conjunction w i t h F T M S since the 1970s, and a C 0 laser ( λ = 10.6 μ m) source was available soon after i n t r o d u c t i o n of the first c o m m e r c i a l instrument. This source directed a b r o a d b e a m o f ~ 1 0 - μ m diameter onto the surface, b u t h a d no provision for i n situ specimen viewing. 2

FTMS-LAMMS. I n 1988, three laboratories, w o r k i n g i n d e p e n dently, reported the design o f a laser-microprobe system f o r F o u r i e r trans f o r m mass spectrometry ( F T M S ) (28-30). T h e instruments at I B M - E n d i c o t t and I B M - S a n Jose have similar optical paths. T h e University o f M e t z system (30) is a very different design that uses an excimer laser ( K r F ; 247 n m ) w i t h


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a h e l i u m - n e o n pilot laser for visualization o f the laser ablation spot, Cassegrain optics for focusing the laser onto the specimen, a n d different provisions for specimen handling. W e w i l l discuss i n detail the I B M - E n d i c o t t instrument ( 2 8 ) , the optics o f w h i c h are similar to a c o m m e r c i a l system offered b y Extrel-Millipore F T M S (Madison, WI).

Instrumentation. A schematic o f the instrument is shown i n F i g u r e 1. A n N d : Y A G laser system ( Q u a n t e l International, w h i c h is n o w C o n t i n u u m , Santa C l a r a , C A ) operated at λ = 266, 532, or 1064 μιχι is directed through an optical attentuator, w h i c h permits continuous adjustment o f p o w e r level without m o v i n g the beam. T h e output passes through a telescope for fine focusing, a n d t h e n enters the v a c u u m system. T h e b e a m passes through a 7 5 - m m focal length objective lens, reflects o f f a m i r r o r , passes t h r o u g h the space between the c e l l plates, a n d is focused onto the sample surface. T h e sample is positioned about 3 m m f r o m one o f the trap plates, a n d , d u r i n g ablation, ions enter the F T M S c e l l a n d are t r a p p e d for analysis. A separate high-vacuum compatible fiber optic directs i l l u m i n a t i o n light through the c e l l and onto the sample at the corner opposite to the entry o f the laser light. For viewing, light reflected o f f the sample follows the laser optical path in the opposite direction. A f t e r leaving the v a c u u m chamber, light is d i r e c t e d to an ocular piece for viewing b y a sliding m i r r o r . Alternatively, a dichroic m i r r o r can be used instead o f the fully reflecting m i r r o r to allow the sample to be v i e w e d d u r i n g the 2 6 6 - n m ablation. Typically, an uninteresting area o f the sample is laser-ablated several times to make a crater to allow determina tion o f the precise position o f the b e a m . T h e crater then can b e located

Vacuum Chamber

Fiber Optic

FT1CR ι

75 mm lens

Eyepiece 4 ~ H

Attenuator

AW: YAG LASER

ELECTRONICS

IBM

PC AT

LASER ELECTRONICS

Telescope Figure 1. Schematic diagram of the IBM-Endicott FTMS -LAMMS instruments. Details given in the text. (Reproduced with permission from reference 46. Copyright 1989 San Francisco Press, Inc.)



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relative to crosshairs i n the ocular piece a n d interesting areas c a n b e positioned for analysis. Briefly, w e m e n t i o n t w o systems i n addition to the optics that are necessary for operation o f the m i c r o p r o b e . T h e electronics o f the laser a n d the F T M S must be t i m e d u n d e r c o m p u t e r control for o p t i m a l performance o f each device. C o m m u n i c a t i o n between the electronics is m e d i a t e d b y a personal c o m p u t e r ( I B M P C A T ) e q u i p p e d w i t h a laboratory interface b o a r d . T h e b o a r d contains programmable counter/timers to trigger the laser flashlamps a n d t h e Q - s w i t c h . T h e P C is interfaced to the F T M S c o m p u t e r ( N i c o l e t 1280) v i a an R S 2 3 2 line, a n d interfaced to the laser electronics b y three lines that trigger pulse sequences. T h i s arrangement permits the F T M S electronics to d e m a n d a laser pulse w i t h 1 - μ 8 accuracy, w h i c h is necessary f o r carefully t i m e d pulse sequences. S p e c i m e n m o t i o n is another issue o f importance f o r the F T M S m i c r o probe. F r o m a handle outside the v a c u u m system, the sample can b e m o v e d b y rotation o f the solids probe o n w h i c h it is m o u n t e d . T h i s m o t i o n does not affect the focus o f the laser b e a m o n the sample surface. A second degree o f f r e e d o m c a n b e conveniently obtained b y adjusting the sample distance f r o m the trap plate, w h i c h has the effect o f sweeping the probe b e a m across the surface because it impinges o n the surface at an angle. T h i s convenience comes at a price, however, because the b e a m focus is c o m p r o m i s e d w h e n this stage position is adjusted. W e have p u r s u e d a superior b u t mechanically m o r e complex approach i n w h i c h a miniature spring-loaded sample stage is m o v e d i n the lateral direction b y a p l u n g e r system. T h i s approach works w e l l i n practice b u t is somewhat delicate.

Structural Studies: Laser-Desorption FTMS R a p i d a n d sensitive analysis o f p o l y m e r molecular structure is critical to determination o f functional properties. Specific structural data provide infor mation for o p t i m i z i n g synthetic schemes a n d understanding the relationships of macroscopic properties to molecular characteristics. T h i s information is the p r i m a r y objective o f laser desorption ( L D ) experiments, although it is o b tained occasionally w i t h m i c r o p r o b e instruments. Instrumentation for L D - F T M S is similar to that previously described f o r F T M S - L A M M S except that n o provision is made f o r specimen v i e w i n g o r precise laser focusing.

Structure/Cure Conditions: Polycyclic Polymers. I n a series of several papers, C . B r o w n a n d co-workers (31-38) reported substantial structural a n d mechanistic i n f o r m a t i o n f o r a variety o f polymers analyzed i n L D - F T M S . These studies were conducted w i t h a c o m m e r c i a l system inter faced to a C 0 laser ( λ = 10.6 μ m ) a n d were specifically a i m e d at obtaining structural information without regard to spatial resolution. T h i s w o r k repre2


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sents the most comprehensive series o f p o l y m e r F T M S experiments p u b lished to date.


T h e most carefully studied p o l y m e r among those considered b y C . B r o w n et al. is p o l y ( p a r a - p h e n y l e n e ) ( P P P ) , an intractable highly conjugated p o l y m e r that is resistant to environmental attack a n d that attains electrical c o n d u c t i o n u p o n d o p i n g . T h e structure a n d molecular weight o f P P P s p r e p a r e d b y various routes w i t h various monomers were investigated b y L D - F T M S . Sample preparation consisted o f p r o d u c i n g a pellet o f P P P b y compression w i t h a 10-lb sledgehammer. P P P is desorbed p r e d o m i n a n t l y as molecular ions (without fragmenta tion), w h i c h means that oligomer distributions are directly d e t e r m i n e d w i t h each laser pulse a n d n u m b e r a n d weight average molecular weights c a n be calculated f r o m the mass spectra. F i g u r e 2 is a section o f the mass spectrum f r o m a P P P 12-mer, w h e r e i n the u n b r a n c h e d dodecamer (12-mer) Η - ( φ ) - Η appears at mass 914. T h e peaks appearing at masses 912 a n d 913 are apparently analogues o f this molecule, w h i c h are deficient i n t w o h y d r o gen atoms (at mass 912), plus its C isotope peak. T h e peaks provide evidence for a chain termination structure generated b y intramolecular ring closure as shown i n Structure 1. T h e p r o p o s e d P P P end-group configuration shown i n Structure 1 is based o n the mass spectrum i n F i g u r e 2. T h e data also show variable degrees o f halogenation dependent o n synthetic details. These studies l e d to the conclusion that electrical conductivities o f various 1 2

1 3

910

920

930

940

950

Figure 2. Section of the positive ion LD-FTMS mass spectrum of poly (paraphenylene) (PPP) in the region of the dodecamer (12-mer).


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polymers cannot be explained b y models of electron interaction that rely exclusively o n homogeneous linear P P P structures. Similar conclusions were reached i n studies o f heterocyclic aromatic polymers ( w i t h N , S, a n d Se substitution). I n related L D - F T M S spectra o f aromatic polymers containing heteroatoms [e.g., poly(phenylene-sulfide)], high-mass carbon clusters ( n = 1 5 0 - 4 0 0 ) that resemble fullerenes (discussed later) were observed. N o


anomalously stable ions were observed i n these spectra, although ions d i d appear at exclusively even masses as is observed i n the laser vaporization o f graphite.

Extent of Cure: Polyimide. T h e extent o f cure is often m o n i t o r e d spectroscopically; for example, b y selected m o n i t o r i n g o f I R bands specific to the u n c u r e d or c u r e d material. These experiments often are l i m i t e d i n sensitivity and reveal differences i n cure state greater than 1 % ; however, u n w a n t e d selectivity based o n three-dimensional b o n d orientation i n space may be exhibited. A l t h o u g h laser m i c r o p r o b e F T M S p o l y m e r spectra are complex (to be discussed), they are also highly sensitive to subtle details o f p o l y m e r structure a n d may be used as a sensitive monitor o f cure state. A n example of the use o f 2 6 6 - n m F T M S - L A M M S to distinguish poly i m i d e cure state was reported b y Creasy a n d B r e n n a (39, 40). P o l y m e r i z e d [molecular weight ( M W ) » 25,000] but u n c u r e d polyamic a c i d o f pyromelfitic d i a n h y d r i d e - o x y d i a n i l i n e ( P M D A - O D A ) was c o m p a r e d to the c u r e d poly i m i d e . T h e structures o f polyamic a c i d a n d p o l y i m i d e are shown i n Structures 2 and 3, respectively. T h e negative i o n mass spectra are shown i n Figures 3a a n d 3b, respectively. T h e degree o f polymerization is identical for c u r e d a n d u n c u r e d p o l y m e r because the c u r i n g process involves a dehydration reaction w i t h accompanying changes i n intermolecular packing. H o w e v e r , the mass spectra are quite different because many higher mass peaks are present f r o m ablation o f polyamic a c i d than f r o m the p o l y i m i d e . T h e extra peaks i n F i g u r e 3a can b e assigned to fragments o f the p o l y m e r c h a i n smaller than a m o n o m e r (40). These negative ions may be stabilized b y a large n u m b e r o f carbonyl or carboxyl groups. T h e p o l y i m i d e spectrum i n F i g u r e 3b, o n the other h a n d , contains only the characteristic C N ~ , O C N ~ , a n d C N ~ peaks, n

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HO-f ^

H v

-

y

x

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Structure 2. Polyamic acid.

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Structure 3. Polyimide.



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100

I ' ' 200

1

' I 300

1

Mass (amu)

1

1

1

I

137

400

Figure 3. Negative ion LAMMS-FTMS spectra of pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyamic acid (a) and polyimide (h). High-mass ions can be assigned to fragments of the polymer. (Reproduced with permission from reference 39. Copyright 1988 Elsevier.)

along w i t h some carbon cluster peaks. T h e reason for the smaller n u m b e r o f fragment peaks for the i m i d e is not k n o w n , although it m a y be related to the greater structural rigidity o f the i m i d e c o m p a r e d to the acid, w h i c h may give rise to greater fragmentation. T h e s e studies show that molecular information about the p o l y m e r morphology is obtained f r o m the F T M S - L A M M S mass spectrum.

Molecular Weight Distributions. I n addition to the oligomer distributions described for P P P , several other groups have p u b l i s h e d mass spectra f r o m w h i c h molecular weights can be calculated. R. B r o w n a n d co-workers (36) have p u b l i s h e d spectra o f polyethylene glycols ( P E G ) o f several molecular weights (600, 1000, 1450, 3350, 6000), P E G m e t h y l ester ( P E G M E ; M W = 5000), polystyrene ( P S ; M W = 2000), poly(caprolactonediol) ( M W = 2000), p o l y p r o p y l e n e glycol) ( P P G ; M W = 4000), a n d poly(ethylenimine) ( P E I ; M W = 600, 1200). I n most cases, n u m b e r a n d weight average molecular weights are w i t h i n 1 0 % o f the values obtained b y



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conventional methods, a n d i n no case is the variance m o r e than 2 0 % . I n related work, molecular weight distributions o f aleoxylated pyrazole a n d hydrazine polymers (41) (with varying degrees o f polymerization) y i e l d molecular weights that compete favorably w i t h weights obtained v i a other methods. C o p o l y m e r distributions w e r e also r e p o r t e d b y this m e t h o d (42). It s h o u l d be emphasized that the molecular weight distributions ob tained i n these analyses reveal far m o r e than average molecular weight. Some oligomers appear m o r e abundant than their nearest neighbors, w h i c h may reveal subtle property differences i n d u c e d b y details o f the synthesis. W e conclude f r o m the literature that molecular weight distributions can be obtained for a w i d e range of polymers using C 0 laser d e s o r p t i o n - F T M S for polymers o f modest molecular weight. 2

T h e p r e c e d i n g studies w e r e c o n d u c t e d w i t h a 3-T magnet, w h i c h is the most c o m m o n magnet field e m p l o y e d i n F T M S at this time. I n a study specifically a i m e d at testing the mass range o f a 7-T magnet, molecular weight 8000 P E G was desorbed b y a C 0 laser (43). Ions u p to mass 10,000 were observed, a n d the spectrum peaks a r o u n d mass 8600 (molecular weight calculations were not reported). T h e mass l i m i t o f F T M S is more correctly reported as an m/e limit, w h i c h is o n the o r d e r o f about 38,000 for t h e r m a l ions i n a 3-T magnet (44). A t least one technique, electrospray ionization (to be discussed), routinely results i n m u l t i p l y charged ions (up to 1 5 0 have b e e n reported) that effectively extend the mass range b y that factor. F o r instance, ions o f mass 100,000 w i t h a charge state of 1 0 0 w o u l d appear at m/e = 1000. These techniques m a y p e r m i t m u c h higher molecular weight distributions to be d e t e r m i n e d . H o w e v e r , care must be taken for quantitative determinations o f distribution at h i g h accuracy using k n o w n standards o f the same p o l y m e r . Several effects, such as a dependence of cation attachment or volatility o n molecular weight, may shift the measured distributions systemati cally. Instrument-dependent artifacts, such excitation or detection conditions, and even the positioning o f the sample near the F T M S c e l l may affect the measurement. Investigations o f these issues are b e g i n n i n g to appear (45). 2

+

+

Structural Studies: Laser-Microprobe FTMS I n a series o f papers p u b l i s h e d over the past f e w years, w e have described mass spectra generated f r o m laser-microprobe analysis o f polymers w i t h the I B M - E n d i c o t t F T M S - L A M M S instrument (8, 39, 40, 46-49). T h e aim o f these studies is to characterize the distribution o f ions f o r m e d b y focused laser irradiation o f w e l l - d e f i n e d polymers, w i t h the goal o f identification of u n k n o w n p o l y m e r i c particulates. F o r this reason, the laser spot size is never larger than ΙΟ-μιτι diameter, a n d spectra are r e c o r d e d as a result o f the lowest irradiance level that generates sufficient signal. T h i s data is also o f interest i n studies o f the economically important but p o o r l y understood


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process o f laser a b l a t i o n / d r i l l i n g , w h i c h finds application i n fields as diverse as retinal surgery a n d p r i n t e d circuit b o a r d fabrication. W e discuss here the major features o f mass spectra observed i n these analyses, organized b y phenomenon.

Negative Ions: Structure. M o s t early studies o f L A M M S o f poly mers have focused o n positive i o n spectra, t h o u g h negative i o n spectra usually contain more structural information. A n excellent example o f this is the case o f p o l y (methyl methaerylate) ( P M M A ) . T h e ablation characteristics o f P M M A are somewhat unusual i n that no signal is obtained d u r i n g the first h u n d r e d or so laser pulses. T h i s is consistent w i t h observations o f U V - l a s e r etching o f P M M A (50). T h e negative i o n mass spectrum o f P M M A is shown i n F i g u r e 4. A l m o s t all the ions i n this spectrum can be assigned to fragments f o r m e d b y direct scission o f the p o l y m e r chain. Assignments are given i n T a b l e I. N o higher mass ions are observed i n this spectrum. T h i s spectrum is i n sharp contrast to that observed for positive ions, w h i c h is discussed i n more detail i n the f o l l o w i n g text. Odd Mass Ion Series and Stable Subunits. A positive i o n spec t r u m of polyethylene glycol ( P E G ) is shown i n F i g u r e 5. T h i s spectrum is very different f r o m many P E G spectra presented previously i n the literature and highlights the stark contrast between experiments that require spatially resolved i n f o r m a t i o n a n d those o p t i m i z e d for structural information. Previous

100

150

200

Mass

250

(amu)

350

Figure 4. Negative ion LAMMS-FTMS spectrum of polymethyl methaerylate (PMMA) showing primarily structural ions (Reproduced with permission from reference 49. Copyright 1991 Society for Applied Spectroscopy.)


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Table I. Fragment Ions Observed in Negative-Ion Spectrum of P M M A . a


M/E

Structure

59

CHC00

85

COOC(CH )CH

87

CH COOCH CH

3

3

3

2

2

101

C H C ( C H 3XCOOCH ) - H [ { R } - H ]

127

R-CH C

2

3

2

141

R-CH CCH

155

R-CH C(CH )CH

185

R-CH C(CH )COO

2

2

2

187

2

3

C(CH )COOCH -RH 3

197

3

HC(CH )(CO)-R-CH C C H 3

241

2

3

H-CCO-(R H) 2

255 1

3

3

C(CH XCO)-R 3

2

C H C ( C H ) ( C O O C H ) = R (repeat unit). 2

3

3

100

50

100

150 Mass ( a m u )

250

Figure 5. Positive ion LAMMS-FTMS spectrum of polyethylene glycol (PEG). The spectrum is dominated by an odd-mass ion series consisting of rearrangement products. (Reproduced with permission from reference 49. Copyright 1991 Society for Applied Spectroscopy.)

spectra concentrated o n desorption o f intact η-mers i n an attempt to observe distributions present i n the solid (36, 43). B y defocusing the laser material is gently desorbed. W e have accomplished desorption w i t h lower molecular weight P E G (Creasy, W . R.; B r e n n a , J. T . , u n p u b l i s h e d observations). D u e to differences i n absorption, laser wavelength ( U V versus I R ) probably plays a major role i n the way polymers are desorbed f r o m the surface. H o w e v e r , the


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requirement for fine focus i m p o s e d b y geometric constraints or the require ment for h i g h ionization efficiency because o f l i m i t e d sample size impose the n e e d for h i g h p o w e r densities that cause a h i g h degree o f fragmentation. T h e mass spectrum is d o m i n a t e d b y odd-mass ions i n the mass range m/z

= 4 0 - 1 7 0 . E a c h peak corresponds to the f o r m u l a H ( C H ) O * . 2

x

There

are no other combinations o f C , H , a n d O , the constituent elements o f P E G that fit this i o n series. T h i s f o r m u l a suggests that the laser-induced plasma consists o f major proportions o f C H

2

a n d O , perhaps i n radical f o r m , that


condense a n d protonate u p o n cooling a n d give rise to the observed spectrum. Similar effects, w h i c h give rise to different products, are observed i n positive ion spectra generated f r o m p o l y (vinyl acetate) ( P V A c ) , polystyrene (PS), p o l y (methyl methaerylate) ( P M M A ) , a n d u n d e r special conditions o f irradia t i o n o f P P S . I n each o f these cases the spectra are distinct a n d easily differentiated f r o m one another. T h e positive i o n spectrum o f P M M A is presented i n F i g u r e 6. T h i s spectrum is d o m i n a t e d b y a dense series o f rearrangement products o f the type observed for P E G , along w i t h intermediate-mass carbon clusters. T h e observed series

fits

the elemental f o r m u l a H C 0 ( C H ) 2

2

T O

-C ,

w h i c h is

n

w r i t t e n to suggest that an acid series may b e c o u p l e d to a bare carbon cluster series. O t h e r structures are likely, i n c l u d i n g a highly unsaturated series. Reports o f other positive i o n studies o f P M M A using a C 0

2

laser (no

spatial resolution) have appeared. C . B r o w n et al. m i x e d P M M A w i t h K B r to

100

200

Mass

300

(amu)

400

500

Figure 6. Positive ion LAMMS-FTMS spectrum of PMMA. The spectrum is dominated by odd-mass ions and a series of intermediate mass carbon clusters. (Reproduced with permission from reference 49. Copyright 1991 Society for Applied Spectroscopy.)


142



promote cation attachment, a n d oligomers u p to the 22-mer, w i t h some evidence o f hydrogen rearrangement ( 3 7 ) , w e r e reported. H s u a n d M a r s h a l l studied p o l y m e r dyes i n P M M A a n d obtained ions f r o m the host p o l y m e r (51). These authors report protonated a n d cationized d i m e r , trimer, and tetramer ions as the dominant species i n the spectrum. N u w a y s i r et al. (42) failed to observe odd-mass i o n series i n their studies o f methaerylate copoly mers. T h e absence o f odd-mass i o n series i n these spectra may reflect the higher energy density r e q u i r e d for efficient ionization conditions i n the laser-microprobe experiment.

Carbon Clustering. C a r b o n cluster formation is a c o m m o n observa tion i n L A M M S - F T M S of polymers. A l t h o u g h no theories have b e e n ad vanced to relate carbon cluster formation to p o l y m e r structure, it is o f utility for fingerprint purposes. T h e most representative case o f carbon cluster formation is that o f p o l y i m i d e (PI). T h e negative i o n spectrum o f P I was discussed previously; the positive i o n mass spectrum o f P I is shown i n F i g u r e 7. A l t h o u g h there are over 200 peaks i n this spectrum, no ions characteristic o f the structure o f P I are observed. T h e spectrum consists exclusively o f carbon cluster ions, some o f w h i c h appear associated w i t h H atoms. Separate distributions o f carbon clusters appear i n three distinct mass ranges. W e have labeled each region as low, m i d d l e - (or intermediate-), and high-mass distributions (39, 46). E a c h o f these distributions has b e e n observed i n spectra o f polymers o f w i d e l y varying covalent structure, although P I is the only p o l y m e r for w h i c h all three

500

1000

Mass

1500

2000

2500

(amu)

Figure 7. Positive ion LAMMS-FTMS

spectrum of PI.


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distributions are observed i n the same spectrum. F u r t h e r , w e note that comparison o f these carbon clusters w i t h p u b l i s h e d spectra o f carbon clusters f o r m e d u p o n laser ablation o f graphite, w i t h o r without cooling using a p u l s e d valve, reveals a h i g h degree o f correspondence for the l o w - a n d high-mass distributions. Observation o f spectra f r o m a solid o f h i g h heteroatom content such as p o l y i m i d e , w h i c h is indistinguishable f r o m that for p u r e carbon, is evidence for similar mechanisms a n d a strong driving force f o r carbon cluster formation (48). Low-Mass Carbon Clusters. A n expanded section o f the positive i o n spectrum showing the low-mass distribution is shown i n F i g u r e 8. C a r b o n cluster ions appear at every carbon n u m b e r f r o m C through C , a n d are often associated w i t h a peak at 2 μ i n excess o f the p u r e carbon peak. Peak intensities show distinct m a x i m a w i t h a p e r i o d o f four carbons a n d peaks at 11, 15, 19, a n d 23. T h i s is precisely the same pattern observed w i t h carbon clusters generated b y graphite ablation. A b l a t i o n o f perdeuterated P I reveals that the peaks shifted out 2 μ are d u e to selective addition o f two hydrogen atoms, rather than substitution o f a nitrogen atom, w h i c h is also present i n abundance i n the laser-induced plasma (47). 1

0

2 5

Intermediate-Mass Carbon Clusters. T h i s distribution o f clusters is shown i n F i g u r e 9. T h e r e is a peak at every mass over the range m/bm = 1 5 0 - 4 0 0 . T h e intensities oscillate w i t h a p e r i o d equal to 12 μ . Valleys occur at mass numbers corresponding to bare carbon clusters, a n d peaks corre-



144


s p o n d to addition o f six to seven hydrogen atoms. T h e composition C H _ can t h e n be assigned to these ions. Identical experiments w i t h perdeuterated P I c o n f i r m these observations (47). T h i s distribution is not observed for graphite ablation because no h y d r o g e n atoms are present. It is interesting to note that the valleys i n the present spectrum occur at the bare carbon clusters, w h i c h is consistent w i t h the formation o f some type o f polyaromatic hydrocarbon. T h i s distribution is the most c o m m o n distribution observed i n F T M S - L A M M S p o l y m e r spectra. I n the P E G spectrum discussed p r e v i ously, the o d d mass i o n series merges at higher masses w i t h intermediate-mass carbon cluster ions similar to those observed for P I . Intermediate-mass carbon clusters also occur for P V A c , P M M A , a n d poly (vinyl chloride) ( P V C ) . n

1

1 2

High-Mass Carbon Clusters. F i g u r e 10 gives the distribution o f high-mass carbon clusters f r o m P I . Ions appear at exclusively even carbon numbers a n d range out past mass 5000, w h e r e the sensitivity o f the i n s t r u m e n t falls off. T h i s set o f ions appears to b e identical to the i o n set observed for laser ablation o f graphite, w h i c h are n o w c o m m o n l y referred to as fullerenes (52, 53). C . B r o w n a n d co-workers (38) also r e p o r t e d spectra that i n c l u d e d e v e n - n u m b e r e d clusters for some polymers. Magic Numbers. M a g i c numbers are d e f i n e d as those carbon cluster numbers that exhibit anomalous stability relative to n e i g h b o r i n g clusters. N o magic n u m b e r were observed for P I or any o f the polymers studied b y C . B r o w n a n d co-workers (38).



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3500

4000

4500

5000

Mass iamu) Figure 10. High-mass carbon clusters in the positive ion spectrum of PL (Reproduced with permission from reference 39. Copyright 1991 Elsevier.)

A positive i o n mass spectrum o f p o l y ( p h e n y l e n e sulfide) ( P P S ) is shown i n F i g u r e 11. T h e sole signal observed i n this spectrum arises f r o m the high-mass carbon cluster distribution. T h i s spectrum differs f r o m the P I spectrum because C at 720 is observed at anomalously h i g h intensity. T h i s molecule, whose structure has b e e n c o n f i r m e d to b e that o f a truncated icosahedron (or soccer ball), has b e e n n a m e d buckminsterfullerene a n d is currently the subject o f tremendous interest. Buckminsterfullerene was first observed d u r i n g laser ablation o f graphite ( 5 3 , 54), a n d is k n o w n to be very stable due to a closed shell structure. Its formation i n the presence o f heteroatoms (e.g., sulfur) is evidence f o r a strong driving force for formation, as w e l l as stability once generated. 6

0

I n over 20 polymers studied thus far, the variety o f p h e n o m e n a observed has p e r m i t t e d differentiation o f F T M S - L A M M S spectra b y inspection o f positive i o n spectra. I n T O F - L A M M S differentiation was p e r f o r m e d most successfully using computer-based pattern-recognition techniques, p r i m a r i l y because T O F - L A M M S positive i o n spectra are d o m i n a t e d b y low-mass hydrocarbons f o r most carbon-based polymers. Differences i n extraction conditions a n d plasma interactions i n the t w o experiments m a y cause this domination. It is clear that F T M S - L A M M S has u n i q u e capabilities for spatially resolved analysis o f p o l y m e r i c particles. T h e generation potential f o r oligomer distributions b y F T M S - L A M M S has not been fully investigated; it m a y be possible, w i t h only a modest sacrifice of spatial resolution, b y the use o f other wavelengths (e.g., 532 o r 1064 n m ) available f r o m a N d : Y A G laser. T h e f u l l capabilities o f F T M S , such


146



100

1000

2000

3000

Mass (amu)

4000

Figure 11. Positive ion LAMMS-FTMS spectrum of poly (phenylene sulfide) (PPS). The spectrum consists entirely of fullerenes. Buckminsterfullerene appears as a magic number. (Reproduced with permission from reference 49. Copyright 1991 Society for Applied Spectroscopy.)

as t a n d e m M S a n d ultrahigh-mass resolution, have not b e e n exploited to definitively assign i o n structure i n laser-microprobe studies. B r o a d i o n kinetic energy distributions resulting f r o m laser ablation m a y give rise to unexpected effects a n d m a y contribute to difficulties i n obtaining high-performance mass spectra. S u c h problems are k n o w n to exist for electrospray ionization F T M S o f proteins a n d w e r e addressed experimentally before high-resolution spectra were obtained b y this technique (26). M e a n s to reduce the i o n kinetic energies i n laser ablation studies are u n d e r investigation.

Contaminant Analysis Analysis o f p o l y m e r products often involves the detection o f impurities o r contaminants that c o u l d affect performance. L A M M S methods allow contam ination analysis o f small spatial locations a n d c a n also p r o v i d e d e p t h profiling o f p o l y m e r films. E i t h e r technique can b e h e l p f u l for understanding p r o b lems associated w i t h manufacturing processes. A n example o f a d e p t h profile that was made b y F T M S - L A M M S is shown i n F i g u r e 12 (40). T h e figure shows d e p t h profiles o f K in a p o l y i m i d e film. T h e samples were p r e p a r e d b y i m m e r s i n g a free-standing film o f P M D A - O D A p o l y i m i d e i n a 1.0 M K C I solution. T h e water a n d K i o n diffused into the film f r o m the solution. T h e d e p t h profiles o f K were measured after 0.5 a n d 20 h i n the solution. T h e signal level o f the K i o n is +

+

+

+


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147


1.2

Ο 0.5 h in L0 M KC1

1.0 α 0.Θ bp ω 0.6


> eu 0 . 4 r—i ω PS 0.2

• 20 h -

-

0.0 120

Number of Laser Pulses Figure 12. Depth profiles of K ions in a polyimide film. The Κ signal was detected using LM-FTMS by signal-averaging every 10 laser pulses. Ten pulses corresponds to a depth of 0.5-1.0 μm. Eight runs were averaged for each curve. +

+

detected w i t h the F T M S b y ablating the p o l y i m i d e b y sequential laser pulses o n one spot. T e n laser pulses w e r e averaged f o r each data point. E i g h t runs f r o m different spots were averaged to obtain the data i n F i g u r e 12. B y measuring the crater depth, it was f o u n d that 10 laser pulses correspond to a d e p t h o f 0 . 5 - 1 . 0 μ τ η . F i g u r e 12 shows that after 0.5 h , the K is near the surface o f the film, a n d after 2 0 h , the K has penetrated more than 5 - 1 0 μ m into the film. Because the crater that is made b y the laser has a r o u n d e d rather than a flat b o t t o m , the d e p t h profile cannot b e used to obtain quantitative diffusion information, although it does demonstrate that d i f f u sion occurs o n such a time scale. +

+

L A M M S is quite sensitive to K a n d other alkali metal ions because o f their l o w ionization potential. Similarly, it is also sensitive to ionic organic species. F o r example, F i g u r e 13 shows a spectrum f r o m a film o f c a d m i u m arachidate ( C d C H C O O ~ ) . T h e film is a five-monolayer-thick L a n g m u i r - B l o d g e t t film ( 5 5 ) . Judging f r o m the signal-to-noise ratio o f the spec t r u m , it s h o u l d b e possible to detect a single monolayer. O t h e r i o n i c surfactants, such as alkyl sulfonates, can be detected w i t h good sensitivity. F o r example, w e have detected a perfluorinated alkyl sulfonate contaminant i n a 2 - n m p o l y i m i d e layer. O n the other h a n d , n o n i o n i c surfactants o r impurities are more difficult to detect i n l o w concentrations o r t h i n films, probably because they are laser desorbed as neutrals rather than as ions. Asamoto a n d co-workers ( 5 6 ) analyzed additives i n polyethylene b y extracting the additives w i t h solvent p r i o r to L D - F T M S . Blease a n d co-workers ( 5 7 ) studied p o l y m e r additives +

+

1 9

3 9


148


100

.-a


+3

BO

60

-A

40

H

20

50

100

150 Mass

200

250

300

350

(amu)

Figure 13. Negative ion LM-FTMS spectrum of a five monolayer Langmuir-Blodgett film of cadmium arachidate using 266-nm laser radiation. The peak at mass 311 corresponds to the arachidate anion.

w i t h L D - F T M S a n d r e p o r t e d collisionally activated dissociation ( C A D ) results. A s previously mentioned, H s u and M a r s h a l l reported the detection o f dyes i n P M M A to concentrations o f 0 . 1 % (51). L i a n g a n d co-workers d e t e r m i n e d the molecular weight distributions o f some p u r e samples o f nonionic ethoxylate-based surfactants (58). X i a n g a n d co-workers studied phosphite additives as antioxidants using laser d e s o r p t i o n - e l e c t r o n - i m p a c t ionization, b o t h as neat samples a n d i n polymers at concentrations o f 0 . 1 % (59). F i n a l l y , J o h l m a n a n d colleagues (60) analyzed a n u m b e r o f different additives, p r i n c i p a l l y b y m i x i n g pure or solvent-extracted samples w i t h a potassium salt for cation attachment. H o w e v e r , detection o f nonionic a d d i tives or c u r i n g agents at l o w concentration i n a p o l y m e r matrix is generally a difficult p r o b l e m that must be attempted o n a case-by-case basis. I n general, analysis o f u n k n o w n polymers or quantitative analysis using L A M M S involve some fundamental difficulties. O n e p r o b l e m is due to the different optical absorption characteristics o f different materials. T h e differ ent responses o f materials to laser radiation causes different amounts o f material to be ablated a n d can even lead to entirely different ionization mechanisms (7, 61, 62). T h e measurements o f p o l y m e r etch depths p e r laser pulse at various laser energies have b e e n measured only for a small n u m b e r o f polymers (63). F o r some polymers, it is possible to find laser conditions for w h i c h the laser ablation forms cleanly etched features a n d flat-bottomed craters, w h i c h are important for quantitative d e p t h profiling. H o w e v e r , the variations that exist between polymers i m p l y that an u n k n o w n or arbitrary


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p o l y m e r cannot be quantitatively analyzed without an extensive amount o f characterization o f its ablation properties. A n o t h e r factor associated w i t h the L A M M S technique is varying sensitiv ity to elements w i t h different ionization potentials. Sensitivity to atoms or molecules w i t h l o w ionization potentials can be orders of magnitude greater than for higher ionization potential species. T h i s effect causes the greater sensitivity to K or ionic organic molecules that was m e n t i o n e d previously. M a t r i x effects o n ionization probabilities analogous to those observed i n S I M S may be relevant (64). L A M M S studies have the additional disadvan tage that a dense gas-phase plasma is f o r m e d , w h i c h causes additional complications f r o m gas-phase ionization a n d reaction processes, some o f w h i c h have b e e n discussed.


+

D u e to these complicating effects, quantitative analysis using L A M M S is possible only i n specific cases i n w h i c h standards are available that closely resemble the u n k n o w n . I n many cases, this type of standard is difficult to accomplish. Various experimental approaches can be used to circumvent some o f these problems. O n e example is the use of a secondary ionization m e t h o d to ionize neutral species that are ablated f r o m the surface b y the laser. B y separating the ablation a n d the ionization steps, greater selectivity or control can be achieved. F o r F T M S , a simple m e t h o d is the use o f electron-impact ionization synchronized to the laser pulse (65). F o r example, F i g u r e 14 shows a spectrum of polystyrene that was ablated using 266-nm laser radiation and i o n i z e d b y electron impact ( E I ) (66). T h e mass spectrum corresponds closely to the E I spectrum o f styrene m o n o m e r . M c l v e r a n d co-workers used a similar technique to detect fractions o f a monolayer of molecular species adsorbed o n a metal surface (67, 68). T h u s , instrumental improvements can be used to allow greater selectivity, better reproducibility, or simpler i d e n t i fication o f unknowns.

Future Work F r o m the foregoing discussions it should be clear that this novel approach to p o l y m e r analysis has demonstrated a n u m b e r o f unique capabilities. O n g o i n g instrumental developments a i m e d at high-mass analysis, generally focused at biological polymers (usually proteins), have not b e e n a p p l i e d yet to synthetic polymers. W e m e n t i o n here a few instrumental approaches currently b e i n g p u r s u e d vigorously i n biological areas. T h e major difficulty i n fitting F T M S instruments w i t h novel i o n sources is the position o f the trapping cell, typically s u r r o u n d e d b y a 1-m-long dewar for a superconducting magnet. A solution to this p r o b l e m is to generate ions f r o m the sample outside the magnetic field a n d inject t h e m into the F T M S cell. T w o instrument manufacturers, IonSpec (Irvine, C A ) a n d B r u k e r


150



100

50

100

150

Mass famu)

Figure 14. Laser ablation-electron-impact mass spectrum of polystyrene. A 266-nm laser pulse was used to ablate neutral material and a simultaneous electron beam was used for ionization. The mass spectrum corresponds closely to an EI spectrum of the styrene monomer. (Billerica, M A ) , offer a n " e x t e r n a l " i o n source (69) for their F T M S systems. A s far as w e are aware at this writing, no laser m i c r o p r o b e has b e e n i m p l e m e n t e d f o r an external i o n source system, although such a device should b e relatively straightforward because external i o n sources are made specifically to p e r m i t novel i o n source applications for F T M S . T h e spectra made i n this fashion may b e dependent o n the extraction field e m p l o y e d , a n d so spectra presented here m a y differ i n some details f r o m such a source. D e t e r m i n a t i o n o f molecular weight distributions b y L D - F T M S is inher ently l i m i t e d i n ability to p r o d u c e sufficient populations o f high-mass ions a n d b y the u p p e r mass limitation o f the mass spectrometer. Recent developments i n the field o f matrix-assisted laser desorption show promise toward f o r m i n g high-mass ions b y direct laser interaction a n d have b e e n demonstrated i n the F T M S (70). T h i s m e t h o d couples w e l l w i t h the l o w operating pressures necessary i n F T M S , a n d thus no external i o n source o r differential p u m p i n g is r e q u i r e d . T h e successful i m p l e m e n t a t i o n o f electroscopy ionization sources to study p o l y m e r i c species may also lead to a decrease i n the degree o f p o l y m e r fragmentation as w e l l as an effective decrease i n the mass-to-charge ratio o f ions d u e to the addition o f m u l t i p l e charges. Spectra o f P E G w i t h average molecular weights u p to 17,500 (71) have b e e n obtained a n d electrospray ionization recently has b e e n executed o n F T M S (25). Unfortunately, electrospray requires an external source w i t h differential p u m p i n g a n d b o t h t e c h n i q u e s — m a t r i x assisted desorption a n d electrospray—require extensive sample preparation that can alter the original nature o f sample, whereas



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direct laser vaporization c a n examine samples i n situ. T h e ability to trap a n d mass-isolate ions w i t h i n the F T M S enables the identification o f u n k n o w n ions f o r m e d b y laser vaporization w i t h a variety o f dissociative a n d reactive methods. Collisionally i n d u c e d dissociation ( C I D ) ( 1 7 , 72, 73), photodissocia tion (74-76) a n d i o n - m o l e c u l e reactions ( 7 7 ) a l l can b e used i n conjunction w i t h accurate mass analysis to provide conclusive identification o f u n k n o w n ions. C I D has traditionally p r o v i d e d structural information at laboratory frame kinetic energies u p to several M o e l e c t r o n volts (72) a n d has b e e n used successfully w i t h laser desorbed ions ( 1 7 , 73). Quantitative determination o f b o n d i n g a n d b i n d i n g energies f o r a n u m b e r o f ionic species have also b e e n obtained (78-80). T h e m a x i m u m kinetic energy available is dependent o n the magnitude o f the magnetic field, the size o f the i o n cyclotron resonance c e l l (the m a x i m u m radius o f the excited ions), a n d the mass-to charge ratio o f the i o n (44). T h i s limits the technique for large-mass ions because the center-ofmass kinetic energy available is greatly r e d u c e d a n d the ability to introduce sufficient energy to induce fragmentation is decreased. L a s e r photodissociation offers i o n identification using fragmentation i n formation a n d wavelength-dependent spectra. These approaches c a n over c o m e some o f the shortcomings o f C A D (81), although they require some a p r i o r i knowledge o f the i o n structure i n order to select the p r o p e r laser wavelengths (74-76). These methods have b e e n used successfully to differ entiate isomeric species. Infrared photodissociation, w h i c h often yields the lowest energy dissociation pathway, has b e e n a p p l i e d to laser desorbed ions as large as 1500 u i n the F T M S ( 8 2 , 83). H o w e v e r , as the mass-to-charge ratio increases, radiative emission competes favorably w i t h p h o t o n absorption, thereby l i m i t i n g the u p p e r mass limit (74). T h e fragmentation o f laser-desorbed ions via U V radiation has also b e e n demonstrated (82). I o n - m o l e c u l e reactions c a n also b e e m p l o y e d to a i d i n the identification o f ions through determination o f electron affinities, ionization potentials, a n d the selective reactivity o f one isomer relative to another ( 7 7 ) . A l l the foregoing techniques aid i n the identification o f i o n structure, b u t are specific o n a case-by-case basis (84). Laser-desorption a n d laser-microprobe methods c o m b i n e d w i t h F T M S have b e e n used f o r a variety o f p o l y m e r studies. T h e literature suggests that the technique possesses a u n i q u e capability to discern p o l y m e r structure i n a r a p i d a n d precise manner. F u r t h e r improvements i n instrumentation a n d , particularly, the application o f existing strategies to p o l y m e r problems w i l l greatly expand the flexibility o f laser F T M S for p o l y m e r studies.

Acknowledgments W i l l i a m Creasy thanks J o h n Rabolt, I B M A l m e d a n Research C e n t e r , f o r p r o v i d i n g the L a n g m u i r - B l o d g e t t film o f c a d m i u m arachidate. T h e authors thank an anonymous reviewer for many h e l p f u l comments.


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