Anal. Chem. l9Q9, 65, 1947-1953
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Secondary Ion Emission from Perfluorinated Polyethers Using Megaelectronvolt and Kiloelectronvolt Ion Bombardment Herbert Feld, Angelika Leute, Derk Rading, and Alfred Benninghoven Physikalisches Znstitut der Universitht Miinster, Wilhelm-Klemm-Strasse 10,D-4400Miinster, Germany
M. Paul Chiarelli and David M. Hercules. Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and plasma desorption mass spectrometry (PDMS) were used to investigate the secondary ion emission from perfluorinated polyethers (PFPE; trade name Krytox) havingaverage molecular weights between 3800 and 10 800. The primary focus of the investigation was on comparison of peak patterns, molecular secondary ion yields, and stabilities of secondary ions formed by kiloelectronvolt (SIMS) and megaelectronvolt (PDMS) ion bombardment. Although both techniques produce the same secondary ion species from multilayer samples, secondary ion emission from monolayer PFPE films deposited on silver is observed only for SIMS; oligomer ions cationized with silver are seen up to m / z = 12 000. Metastable ions are observed both in PDMS and SIMS; the parent ions decay into charged and neutral fragments. The yields and half-lives of secondary ions emitted from multilayer samples are always larger in PDMS than in SIMS. However, the fragmentation processes observed are the same. Half-lives are independent of mass in the mass range studied. The yield ratio for ions in PDMS and SIMS is a n apparent function of mass because of ion half-lives. When half-lives are considered, the yield ratio of PDMS/SIMS is nearly constant at -10. INTRODUCTION Ion bombardment of large organic molecules on a surface results in the generation of characteristic fragment and molecular parent ions such as (M f HI*, M*, or (M + cation)+. Secondary ion (SI)generation depends on the characteristics of the bombarding particle (mass, energy, etc.) the chemical nature and size of the molecules on the surface, and the properties of the substrate itself. Perfluoropolyethers (PFPEs) are liquid polymers having a number of important applications, which generates interest in their mass spectra. Several investigators have studied the structure of the oligomers and have determined molecular weight distributions for these materials. Some investigations have been done by time-of-flight secondary ion mass spectrometry (TOF-SIMS).l-L TOF-SIMS spectra of PFPEs (1)Hum, S.M.; Colton, R. J.; Mowery, R. L.; Mc Grath, K. J.; Wyatt, J. R.Appl. Surf.Sci. 1988, 35, 507. (2) Bletsoe, I. V.; Hercules, D. M.; Fowler, D.; van Leyen, D.;
Benninghoven, A. Anal. Chem. 1990,62,1275. (3) Hues, 5. M.; Wyatt, J. R.; Colton, R. J.; Black, B. H. Anal. Chem. 1990.62.1074. - - - ., . - - - - -(4) Hagenhoff, B.; Benninghoven, A.; Barthel, H.; Zoller, W. Anal. Chem. 1991,63,2466.
feature intense fragment and oligomer peaks. The oligomer structure and molecular weight distribution of the polymer can be deduced from TOF-SIMS spectra for samples of molecular weight less than -7O00. It has been shown that TOF-SIMS provides accuracy for molecular weight distributions comparable to conventional gel permeation or size exclusion chromatography.3~4 In the present study we describe the results of detailed mass spectroscopicinvestigations of perfluoropolyethers using megaelectronvolt (PDMS) and kiloelectronvolt (SIMS) ion bombardment. Fragment and oligomer ions are observed; fragment ions have intrinsic charge while the oligomer ions are formed from neutral molecules cationized with silver. Therefore oligomer ions can only be observed from monolayer samples on silver substrates in SIMS. The yields and stabilities of fragment ions are compared for PDMS and SIMS as a function of mass. Polymers offer important advantages for such studies because they consist of distributions of chemically identical molecules covering a wide mass range. This means that, in contrast to peptides, the effect of size can be studied without interference from the effect of changing chemical composition. The Krytoxes studied had molecular weight distributions in the range M,,= 3 800-10 800. Emphasis will be on secondary ion emission from multilayer samples. For purposes of comparison, some results will be presented for monolayer samples. In the latter case, oligomer ions up to mass mlz = 12 O00 were detected.
EXPERIMENTAL SECTION Mass Spectrometer. All measurements were carried out using a PDMS/SIMS combination time-of-flight mass spectrometer: a diagram of which is shown in Figure 1. The spectrometercontainstwo different primary ion systems,so that the samplescan be irradiated either by megaelectronvolt (PDMS) or by kiloelectronvolt (SIMS)ions. The primary ions used were 10-keV Ar+ from an electron impact ion source and 100-MeV fission fragments from a 262Cfsource. Comparable conditions for the observation of secondaryions are assured by bombarding the same target area from the same side in both PDMS and SIMS. Additionally, the secondary ions from both techniques are analyzed by the same time-of-flight analyzer. Secondary ions generated at the target (ground potential) are accelerated to an energy of 4 keV. Two detectors (1and 2)are used on the instrument to define different drift paths for the secondary ions. The TOF analyzer can be operated with energy focusing (reflection-type analyzer mode) or without energy focusing (linear-typeanalyzermode). In the latter case (detector l), the effective length of the drift path is 0.8 m and is 2 m in the reflection mode (detector 2). Both detectors consist of identical components and have been tuned to yield the same detection probability for a given secondary ion species. Typical mass resolutions (R=MIAM) of 5000 in the reflection mode and about 500 in the linear mode were routinely obtained in the mass
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0003-2700/93/0365-1947$04.00/0
( 5 ) Feld, H. Thesis, Mlinster, 1991.
0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993 1
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fragments and their separation from the corresponding parent ion peak. Due to these separationeffects,it is in principle possible to determine the mass of the charged fragments in either mode of operation. By accumulating spectra with detector 1and simultaneously reflecting the charged particles with the ion mirror, it is possible to obtain spectra of only the neutral decay products (neutral mode). The different detection probabilities for neutral fragments and the corresponding charged particles are determined by accumulating spectra with and without postacceleration. If one assumes a first-order fragmentation process, it is possible, by measuring the yield of the neutral fragments and correcting for detection probability, to break down the SI peaks which normally consist of three components (charged and neutral fragments and parent ions). Therefore the yields of parent ions, Y,(t), can be estimated for different flight times, by combining the three modes of operation(linear,reflection, and neutral mode). The first-order rate constant k of the decay process can be determined by evaluating A In YJt) = -kAt or expressed as halflifetlp = In2/k.'* Thereproducibilityofthetl/zvaluesmeasured this way was about *20%. The accessible time range for measuring decay processes in the drift space ranges from about 100 ns to 500 ps. SamplePreparation. The Krytor samplesused had average molecular weights between 3800 and 10800 (n = 23-65). Multilayer Krytox sampleswere produced by directly depositing a drop of Krytox on aluminum foil. Monolayer samples were prepared by dissolving the Krytox in 1,1,2-trichloro-1,2,2trifluoroethane and depositing 5 p L of a 1od M solution on an area of about 30 mmzon a silver target that had been etched with nitric acid (20% volume).
RESULTS
Fburs 1. Diagram of the combination PDMS/SIMS TOF mass spectrometer.
range of m/z = 200-5000. Postaccelerationof ions to an energy of 10 keV ahead of the detectors was used to enhance detection efficiency for large molecular ions. Further details of the instrument are given elsewherea6 Evaluation of Spectra. The secondary ion yields Y were determinedby dividing the background-corrected integrated peak area of secondary ions N(S1) by the primary ion dose N(P1): Y = N(SI)/N(PI). The size of the sample area analyzed is about 2 mm2. The primary ion dose density will be given as N(PI)/cm2 to allow comparison of different spectra. For comparison of PDMS and SIMS, the yield ratio Y! of the secondary ion species will be given as Y' = Y(PDMS)/Y(SIMS). The instrument as configured permits observationof products from metastable decay of secondaryions in the drift path between the extractor and the ion mirror. In these decay processes parent ions (mass m,) undergo fragmentation yielding charged (mass m,) and neutral (mass m,) fragments: (m, m, + md. The decrease of parent ion intensity with time will show exponential dependence (Le., will be first order). The kinetic energy of the parent ion is shared among the fragments (determined by their respective masses) such that each fragment has a lower kinetic energy than the parent ion. However,both fragmentsand parent ions will have the same velocity because the velocity of the center of mass is unchanged in the decay process. Therefore parent ions, neutrals, and charged decay products all will contribute to a single peak in spectra obtained with detector 1 (linear mode). However by postaccelerating the parent ions and charged fragments in front of detector 1,the two can be separated from both the neutrals and each other because they spend different times in the postacceleration gap. Discrimination of these ions in the spectrumwill be possible if their differencein mass is large enough to provide a sufficient time difference in the postacceleration region. In the reflection mode charged fragments originating from decay processes ahead of the ion mirror will penetrate less into the ion mirror than the correspondingparent ions. This will result in a shorter flight time of the charged
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(6) Feld, H.; Leute, A.; ZurmWen, R.; Benninghoven, A. Anal. Chem. 1991, 63, 903.
Secondary ion mass spectra for the perfluoropolyethers can be divided into three regimes: fingerprint, fragment, and oligomer regions. In the fingerprint region (low-mass range mlz < 500) fragment ions are observed derived mostly from one or two monomer units. Larger fragment ions, which are produced by particle bombardment-induced chain cleavages, are found in the fragment region, generally mlz = 500-3000. These fragment ions consist of multiple monomer units with or without one end group. In the oligomer region (high-mass range), intact oligomer molecules are observed. The present paper will focus only on secondary ions emitted with relatively high intensity so that they can be used to compare secondary ion yields and stabilities between PDMS and SIMS. Because all Krytox samples gave essentially the same secondary ion pattern, we will report results obtained for only one polymer, a Krytox having an average molecular weight M,,= 7800. Multilayer samples and monolayer samples will be discussed separately. Multilayer Samples. The PDMS and SIMS spectra from multilayer Krytox samples showed the same secondary ion species. No oligomer ions were observed for multilayer samples, so the following discussion will focus on the fingerprint and fragment regions. In the fingerprint region (mlz < 500) of the positive ion spectra the major secondary ion speces are (CF)+, (CF2)+, (C2Fd+, (C2F5)+, (CsFs)+,and (C3F7)+. Characteristic secondary ions are not observed in the negative ion spectra in this region. The major secondary ion species in the fragment region are (FR,CaFs)+ in the positive ion spectra and (FR,)- in the negative ion spectra. In addition, fragment ion series were observed having secondary ion peaks of lower intensity corresponding to (R,,CzFd)+, (R,CsFe)+, and (OR,CsF7)+ in the positive ion spectra and (OR,C2Fd- and (OR,,C$'6)- in the negative ion (7) Schueler,B.;Beavis,R.;Bolbach,G.;Ens,W.;Main,D.E.;Standiag, K. G. Secondary Zon Maas Spectrometry, SZMS V; Springer-Verlag:
Berlin, Heidelberg, 1986. (8)Schueler, B.; Breavis, R.; Ens,W.; Main, D.E.; Tang,X.; Standing, K. G.Znt. J. Mass Spectrom. Zon Processes 1989, 92, 186.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
Table I. Major Secondary Ion Species Found in the Krytox. Spectra
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-
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oligomer
positive ions
of broad Gaussian peak would require multiple fragmentation pathways for the parent ions, ranging from loss of a single fluorine atom from the parent ion (ck- F) at one extreme to one-carbonfragment ions ((CF)+,etc.) at the other. The most probable fragmentation process would correspond to production of charged and neutral fragments of approximately equal mass: m, m,, = m$2. However, such an idea is contrary to what is known about fragmentation of large chains. Figure 4 shows the measured half-lives as well as plots of Y* (Y* = YJ(Y, + Y,), where Y, is the yield of the charged fragments and Ypis the yield of the parent ions) for the positive fragment ion series ck. The half-lives are constant over the observed mass range mlz = 600-2400. The average values are tlp(S1MS) = 13 ps and tl/z(PDMS) = 27 ps. The reason for the larger derivations in tl/2 and Y* values for PDMS is the smaller absolute count rate observed for PDMS relative to SIMS. It is clear from Figure 3b that the ratio of the yields of charged fragments to parent ions vary as a function of mass. By neglecting the contribution of the neutral fragments in the linear spectrum, Y* varies as 1 - e-kt, where t is the flight time of the ion. Curves 1 and 2 in Figure 4 represent fits of the function 1- e-kt to the experimental data with the corresponding k values (k = In 2/t1/2);the agreement is quite good. These results indicate quite clearly that the increase in metastable ion intensity in spectrum 3b is not related to the mass of the parent ion per se, but is caused by longer flight times for increasingly heavier ions. The differences in metastability between the positive fragment ion series ck (tlp(S1MS) = 13 ps; tl/2(PDMS) = 27 ps) and the negative fragment ion series Ek (tl/2(SIMS) = 37 pa; tlp(PDMS) = 96 ps) explain the differences in the upper mass limit observed for the different types of fragment ion series. In principle, positive fragment ions c k are formed up to masses comparable to the negative ions. This is supported by the spectrum obtained for neutral fragments originating from the positive parent ions (ck)which shows peaks at masses up to mlz = 5000; this spectrum is shown in Figure 5. The peaks due to certain parent ions are identified in brackets. Note that postacceleration cannot be applied to neutral particles. Therefore not only the detection efficiency of neutrals decreases more distinctly d t h mass compared to the corresponding parent ions but also the flight time increases; thus the peaks are shifted to higher masses. The metastability and half-life of a parent ion strongly influences the relative secondary ion yields of SIMS and PDMS. The magnitude of the effect depends on the time between ion production and detection. Due to the longer half-lives in PDMS than in SIMS the yield ratio Y' = Y(PDMS)/Y(S1MS) increases with increasing flight time of the parent ion; generally Y' is higher when measured in the reflection mode (longer flight path) than in the linear mode. Figure 6a shows a comparison of the yield Y(PDMS) and Y(S1MS) for the most abundant fragment ion series Ek in the negative and ck in the positive ion spectra. The yields were calculated from spectra acquired in the reflection mode. A nearly exponential decrease in the yields of ck and Ek is observed for both Y(PDMS) and Y(S1MS). The slopes for the fragment ion intensity of Ek are nearly the same in PDMS and SIMS. This is expressedby the yield ratio Y'. Y' increases only slightly with increasing mass over the mass range mlz = 600-5000 for the negative fragment ions Ek as seen in Figure 6b. The situation is quite different for the positive fragment ion series ck. Due to the greater metastability of these ions in both PDMS and SIMS the decrease in yield for c k with mass is more pronounced than for Ek (Figure 6a). In addition, Figure 6b shows that Y' increases significantly as a function of mass for ck.
-
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Krylox
negative ions
(M+ CF2 + Ag)+ = 2; (M- C2F4 + Ag)+ = 3; (M+ Ag)+ = 4
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spectra. The major secondary ion species for the Krytoxes are listed in Table I along with the corresponding abbreviations. The negative fragment ion series Ek is observed up to very high masses (mlz= 6000),whereasthe positive fragment series ck can be detected only up to about m/z = 2500. Figure 2 shows a comparison of the negative SIMS (a) and PDMS (b) spectra measured in the reflection mode. The secondary ion peaks marked E*k correspond to charged fragments from metastable decay of the parent ions Ek in the drift space between the extraction lens and the reflector. Fragmentation of metastable negative ions causes the parent ion Ek to lose a neutral fragment corresponding to the mass of one repeat unit; therefore the composition will be the same as Ek-1. The peaks due to E*k and Ek-1 are separated in the spectra because of differences in flight time in the ion mirror. Thus, in Figure 2 the peak due to E*lo is at slightly lower apparent mass than Eg. The half-lives t l p of the parent negative ions were calculated to be tl/&IMS) = 37 ps and tlp(PDMS)= 96 ps. The half-lives of secondary negative ions were independent of mass for both desorption techniques. The shorter halflives for fragment ions Ek in SIMS are the reason that the peak widths in the SIMS spectra (Figure 2a) are broader than in the PDMS spectra (2b), resulting in lower mass resolution for the secondary ion peaks in SIMS. Figure 3 shows a comparison of negative (a) and positive (b) SIMS spectra for the Krytox measured in the linear mode. In this mode the negatively charged fragments produced by metastable decay are not separated from their parent ions by the time-of-flight effect of the postacceleration gap. Because the charged fragments E*k have nearly the same mass as the parent ions Ek (E*k/Ek = 0-8 to 0.91, the flight time difference between Ek and E*k is very small in this mode. Therefore the two peaks are not separated but appear as one in the negative ion spectrum in Figure 3a. The peaks labeled F k and Gk are due to other parent ions. The situation is quite different for the positive ion spectrum in Figure 3b. Here broad "hills" appear in front of the parent ion peaks ck. When the postacceleration voltage at detector 1is varied, it is easy to show that these broad peaks are due to charged fragments originating from fragmentation of the parent ions ck in the drift space. The processes involved here must be quite different from those observed for the negative parent ions Ek. The shape of the broad peaks was determined to be Gaussian in time; the shaded areas under the peaks represent computer-generated Gaussian peak fits. The skewed distribution of the peak shape relative to a regular Gaussian is due to the m = t2presentation of the spectrum. The fragmentation mechanism required to generate this type
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The yields of the positive fragment ions Ck taken in the linear mode are shown in Figure 6c in the mass range up to mlz = 2800. The yields of the parent ions ( Yp) and the charged fragments (Y,) can be separated; Y, = Y, + Y,. The s u m is the total envelope of the broad and sharp peaks in the spectra (Figure3b), and Y, is the component under the shaded part of the curve. Y,, and Yp show an exponential decrease with increasing mass for both PDMS and SIMS. The correspondingvalues of Y' are plotted in Figure 6d. Generally, Y' is smaller in the linear mode (compare y-axes of 6b and d) mirroring the differences in flight times and half-lives of the ions. Although Y', increases with mass, Ytsumis remarkably constant; the yield ratio is nearly constant over the entire mass range studied a t Y' = 10 when half-lives are considered.
Monolayer Samples. Both positive and negative SIMS spectra were obtained from monolayer preparations for all samples investigated. It is well-known that useful PDMS spectra cannot be obtained from thin organic layers on metal substrates, because the high conductivity of the substrate prevents effective transfer of the primary ion energy loss into movement of target atoms. Therefore we did not attempt a systematic PDMS investigation of monolayer samples,except to confiim the inability to obtain spectra. In the caae of SIMS, strong secondary ion emissionwas observed over a large mass range. The largest secondary ion masses are observed in the positive spectra up to mlz = 12 OOO. A complete study of the secondary ion peak pattern observedin SIMS from monolayer Krytox samples has been published;" therefore we will concentrate on the secondaryion species emitted in high yield. Generally the same secondary ion species are found in both the fingerprint and fragment region as for multilayer samples. Additionally some fragment ion peaks of lower intensity can be observed in the positive ion spectra. The secondary ion yield of the most prominent fragment ion series c b in the positive spectra (reflection mode) varies from 6 X lo" a t mass mlz = 501 to 2 X le7at mass mlz = 2493. These values are about 5 times higher than the corresponding values from multilayer samples. The secondary ion yield for the negative fragment ions Ek in the reflection mode decreases from about 6 X 1V at mass mlz = 517 to 1 X 1o-S at mass mlz = 4999. The decrease of secondary ion yield with mass is less pronounced for a monolayer sample than for a multilayer sample. While the ratio of the yields for monolayer and multilayer preparations (Y(monolayer)/Y(multi1ayer)) is about 2a t mass mlz = 517,it increases up to nearly 10at mass m/z = 4999. The half-lives of the positive fragment ions Ck
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were determined to be tl/z = 17 p s and that of the negative fragment ions E&tl/z = 79 NS. Again, both tl/z values were constant over the mass range studied. Note that the halflives are longer than those measured for multilayer Krytox samples (13 and 37 ps, respectively). Moreover, the same fragmentation processes are observed from monolayer and multilayer samples. Oligomer ions were detected only in positive ion spectra. The oligomers are cationized with silver ions, and the most prominent oligomer ion species are (M + CF2 + Ag)+, (MC2F4 + Ag)+,and (M + AB)+. M is the oligomer and is shown at the top of Table I. The high-mass region of a positive SIMS spectrum of a Krytox sample having an average molecular weight (M,) of 7800 is shown in Figure 7. Typically the yield of the most intense oligomer ion (M + Ag)+ at mlz = 7000 is Y = 2 X lo". It is important to note that metastable oligomer ions were not observed in any spectra. It is apparent from Figure 7 that several series of peaks are seen in the region of the oligomer ions; these are designated by 1-7. The inset shows how well these peaks can be separated
using the high resolution of the TOF-SIMS. The series labeled 1is due to the fragment ion series CC. The main oligomer peaks are labeled 4 and correspond to intact oligomer molecules cationized by silver. The series labeled 2 and 3 formally correspond to (M + CF2 + Ag)+ and (M - C2F4 + Ag)+,respectively. Because of their intensities and the shapes of their distributions it is unlikely that they are from fragment/ additive ions, but rather that they are due to different oligomer series. For example, it is easy to imagine that 2 is due to a propyl ether terminal group; this has been observed for a Krytox impurity.( Similary, 6 could be a doubly terminated perfluoroethylpolymer, as has been proposed in other TOFSIMS studies.2 Series 3, 5, and 7 could arise from combinations of perfluoromethylterminal groups and alternatively OCFzCFz units incorporated into the polymer chain.
DISCUSSION Fragmentation. It should be restated that the fragment ion spectra observed for PDMS and SIMS were essentially the same. This strongly supports the contention that ion-
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 15, AUGUST 1, 1993 ' ' ' ' ' ' ' ' '
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deposited by the primary ions, is distributed to internal and translational degrees of freedom of the target particles. Thus 5 3 I M - C,F4 + ApI' the emitted particles (here fragment ions) have on the average 4 tM + Apl' lower internal energy in PDMS than in SIMS. 1 = IM-C.F. + Apl' It is also interesting that the negative ions Ek are more 6 (M + C,F, + Ao1' 1 = IM-CF, + Apl' stable than the positive ions c k in both PDMS and SIMS. Both fragment ion speciesare due to ion formation by cleavage 3 of C-0 backbone bonds. The difference is that, in contrast to positive fragment ions, the oxygen atom retains the negative charge. Thus the charge of the emitted negative fragment 2 ion Ek is stabilized through this oxygen atom whereas the charge of the positive fragment ion ck is not. This is the 1 reason why the fragment ions Cr always show greater metastability in the gas phase. At this point it is interesting to note the low intensity of fragmentation products observed 0 2000 4000 6000 8000 10000 12000 for polymers other than the perfluoropolyethers.5*6 Metamh stable fragmentation would be expected for a variety of Figure 7. Positive SIMS spectrum of a monolayer Krytox sample homopolymers containing C, H, 0,and N. One can speculate deposlted on a silver substrate (reflection mode). Primary ion dose that large fragmentation products must be formed for other density 7 X 10I0/cm2. polymers, but if so, their lifetimes must be below 100 ns so that they decompose in the acceleration gap. This indicates formation processes are similar for the two techniques. that the presence of certain components in polymers stabilizes Fragmentation of the Krytox polymers can be explained by large ions having intrinsic charge so that they have lifetimes cleavage at the C-0 bond, accompanied by charge formation. which permit extraction into the flight tube of the instrument. An extensive study of the Krytox fragment ions has been It is important to consider the shapes of the metastable published;2 we will discuss only the major fragmentation peaks in Figure 3. The Gaussian fit implies that the most processes here. The two main fragment ions formed in both probable fragmentation process is cleaved into a charged and PDMS and SIMS are the series ck and Ek,shown in Table a neutral fragment having approximately equal masses. Such I. an idea is contrary to established ideas about fragmentation The major fragment ions formed in the positive and negative of large molecules; cleavage at the ends of the chain should ion spectra do not correspond to cleavage of a single bond; be favored,lOJZ or it should be totally random. One possible thus they are probably not produced by the same initial event. resolution of this dilemma is that the Krytox metastable ions This would rule out the possibility that the positive and do not undergo a single decay but split off multiple monomer negative ions are produced by one reaction, such as that shown units in the process of decaying. Such an idea is supported in eq 1. Rather, they must be produced by two separate by spectra observed for other polymers.l3 Sequential fragprocesses as shown in eq 2. These equations are intended to mentation would be indistinguishable from a single-chain show the overallreaction without any mechanistic implication. fracture by our instrument, because the center of mass would m3 m3 cF3 not change even for multiple losses, and the ion plus several .--Ohm2+o-L~0--$' I 01 neutrals would arrive at the detector together. Postaccel0 " ; + ocPao--(1) eration would operate on the remaining ion as though it had CP3 cF3 been produced in a single step. The Gaussian fit to our data p' -A& + &ao--@) would not be sensitive to the small differences between multiple monomer loss and multiple fragmentation possibilities. --ocpm2 + $ o > "I B &Fm0--(2b) P Molecular Size. The Krytoxes are an ideal system for OCFQO studying the effect of molecular size on secondaryion emission. Another interesting aspect of the fragmentation process is They have an advantage over other types of molecules such that the most intense positive and negative ions (Ck, Ek)both as biopolymers in that their chemical composition does not come from the same end of the chain. This corresponds to change with molecular size. One of the most important results preferential cleavage of fragments on the left of the structure of the present work was to show that the yield ratio Y' and in Table I. This means that retention of positive charge will the half-lives of polymer-derived ions are independent of be on the CF2 group rather than on CF(CFa),and the preferred molecularsize. Thus, the efficiencyof PDMS relative to SIMS negative ion is CF20- rather than (CF&FO-. Studies to does not increase with secondary ion mass, as in the case of confirm this idea are currently in progress. thick samples of biomolecules.14 Half-Lives. The different half-lives in PDMS and SIMS It has been observed that the yield ratio Y' increases with reflect the differences in internal energy imparted to the increasing mass for thick samples of poly(methy1 methacrysecondary ions by megaelectronvolt and kiloelectronvolt ion late) and poly(ethy1eneglycol). This results in a shift of the bombardment. The relationship between internal energy and oligomer distribution to lower masses in SIMS, whereas the ion stability can be expressed in terms of RRKM t h e ~ r y . ~ J ~ distribution in PDMS reflects the correct average molecular The longer lifetimes in PDMS are consistent with studies of weights.6 The reason for the difference from the Krytoxes biomolecules.11 When the megaelectronvoltprimary particles is probably related to the small intermolecular forces present impact the surface in PDMS, the area active for desorption in the fluoropolymers. We believe that it is the integrated is greater than for SIMS. Therefore, on the average more values of these weak binding forces that largely determine fragment ions are emitted from desorption active areas having the relative secondaryion emission efficienciesfor both PDMS low energy density in PDMS than in SIMS. This energy, XlO
I
--
'"4
-
+
(9) Marcus, R.A. J. Chem. Phys. 1962,20, 359. (IO) Levsen,K.~ndamentalAspectsof OrganicMass Spectrometry; Verlag Chemie: Weinheim, New York, 1978. (11)Cotter, R. J. Anal. Chem. 1988,60, 781A.
(12) Craig, A. G.; Derrick, P. J. J. Am. Chem. SOC.1985, 107, 6707. (13) Zimmerman, P. M.S. Thesis, University of Pittsburgh, 1991. (14) Ens, W.; Sundqvist, B. U. R.; Hakansson, P. In Secondary Zon Mass Spectrometry,SIMS VI;Benninghoven, A., Huker, A. M., Werner, H. W., Eds.; John Wiley & Sons: New York, 1987, p 623.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
and SIMS. This is consistent with results obtained for materials having small intermolecular binding energies. For example,similar valuesof Yfare observed for fluoropolymers16 and neutral metal complexes.16 Also use of nitrocellulose as a matrix apparently reduces surface binding energies,l'-lQ and a smaller Y' value is measured. The arguments presented here reinforce one of the assumptions which we make for interpretation of TOF-SIMS spectra of polymers: the desorption ionization event is caused by disruption of the surface by the incident particle, rather than by a direct hit of the incident projectile on a polymer molecule. It hinteresting to note that the yield ratio Yf is approximately the same as the ratio of stopping powers for megaelectronvolt and kiloelectronvolt ions. We believe that the value of Yf= 10will be constant for desorption of molecules having low intermolecular forces. In addition to stopping power, a second factor will become important as the binding energies of the molecules increase, namely, the difference in size of the desorption-active areas for SIMS and PDMS. The ratio of molecular size to the size of the desorption-active area will have a dramatic influence if there are significant (15) Feld, H.; Zurmahlen, R.; Leute, A.; Ilagenhoff,B.; Benninghoven, A. In Secondary Ion Mass Spectrometry, SIMS VII; Benninghoven, A., Evans, C. A., McKeegan, K. D., Storms,H., Werner, H. W., Eds.; John Wiley & Sons: New York, 1990, p 219. (16) Feld, H.; Leute, A.; Rading, D.; Benninghoven, A.; Henkel, G.; Kruger, T.; Krebs, B. 2.Naturforsch. B, in press. (17) Sundqvist, B. U. R. Nucl. Instrum. Methoda 1990, B48, 517. (18) Chait, B. T. Int. J. Mass Spectrom. Ion Processes 1987, 78,237. (19) Demirev, P.; Fenselau, C.; Cotter, R. J. Int. J. Mass Spectrom. Ion Processes 1987, 78, 251. (20) Feld, H.; Benninghoven, A., to be published. (21) Macfarlane, R. D. Acc. Chem. Res. 1982,15, 268. .
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molecule-molecule or molecule-matrix binding energies.6lm The desorption-active area is larger for PDMS than for SIMSFl so desorption will be limited by molecular size, more in SIMS than in PDMS, resulting in a higher accessible mass range for secondary ions in PDMS. Analytical Implications. There are three important analytical implications of this work. First, the observation of neutral spectra at masses greater than those observed for the positive ion spectra has implicationsfor mass spectrometry of polymers. For cases where metastables are observed, if the positive ion series falls off in intensity significantly with increasing mass, it may be possible to discern structural information from neutral spectra. It may also pay to look for neutral spectra even for cases where no fragmentation products are seen in the event that metastable ions may have very short lifetimes and not be seen in the reflection mode but still may be seen in the linear mode. Second, the inset of Figure 7 shows the potential value of TOF-SIMS for studying impurities in polymers and for establishing the nature of end groupsfor low molecular weight polymers. Third, the above discussion points out that the effective mass range of TOF-SIMS can be distinctly increased by minimizing surface-molecule interactions.
ACKNOWLEDGMENT This work was supported, in part, by the National Science Foundation under Grants CHE-9022135 and INT-9244276.
RECEIVED for review November 9, 1992. Accepted April 22, 1993.
