Anal. Chem. 1991, 63,903-910
of the background noise from the mirror M4 can be suppressed. Although the signal intensity is only one-f&h of that produced by the E,IIEfIIEb polarization configuration, much more background is suppressed and hence the signal-to-noise ratio is effectively enhanced. To demonstrate that the background noise from the backward pump beam can be efficiently rejected by using this polarization discrimination configuration with a circularly polarized probe beam, the signal-to-noise ratios are measured with and without a polarizer in front of the photomultiplier tube. The SIN with the polarizer in front of the photomultiplier tube is 50 times as high as that without the polarizer in front of the photomultiplier tube. A detection limit of 2.5 ng/mL lithium is observed while Doppler-free resolution is maintained by using this transient coherentgrating-based D4WM spectroscopy. Reasonably good accuracy and precision are also observed in measuring individual isotope contents. Relative standard deviations of 0.2%, 0.3%, 0.5%, 0.7%, and 1.0% are observed for 6Li isotopes in samples enriched with 50%,40%, 30%, 20%, and 10% (by weight) ‘jLi, respectively. In conclusion, D4WM offers not only high spectral resolution but also excellent detection sensitivity, as demonstrated here and with different atomizers reported previously (12-14). By using vertically polarized pump beams and a circularly polarized probe beam, one can improve the signal-to-noise ratio and also simplify fine or hyperfine spectra by selectively measuring only certain fine or hyperfine lines. A roompressure flame atomizer provides stable, fast, continuous, and convenient sample introduction.
903
LITERATURE CITED (1) Green, L. W.; Leppinen, J. J.; Eiiiot, N. L. Anal. Chem. 1988, 60, 34-37. (2) Chan, L.-H. Anal. Chem. 1987, 5 9 , 2662-2665. (3) Wheat, J. A. Appl. Spectrosc. 1970. 25, 328-330. (4) Pathiratne, K. A. S.; Lovett, R. J. Appl. Spectrosc. 1987, 4 7 , 206-218. (5) Tong, W. G.; Yeung. E. S. Talanta 1984, 37,659-665. (6) Tong, W. G.; Chen, D. A. Appl. Spectrosc. 1987, 4 7 , 586-590. (7) Optical Phase Conjugation;Fisher, R. A., Ed.; Academic Press: New York, 1983. (8) Hellwarth. R. W. J. Opt. SOC.Am. 1977, 67, 1-3. (9) White, J. 0.; Yarhr, A. Appl. Fhys. Lett. 1980, 37, 5-7. (10) Ewart, P.; O’Leary, S. V. Opt. Lett. 1988, 7 7 , 279-281. (11) Tong, W. G.; Andrews. J. M.; Wu, 2. Anal. Chem. 1987, 5 9 . 896-899. (12) Chen, D. A.; Tong, W. G. J. Anal. At. Spectrom. 1988, 3 , 531-535. (13) Andrews, J. M.; Tong, W. G. Spectrochim. Acta 1989, 448, 101-107. (14) Wu, 2 . ; Tong, W. G. Anal. Chem. 1989, 67, 998-1001. (15) Ducioy, M.; Bioch, D. Fhys. Rev. A 1984, 30, 3107-3122. (16) Saikan, S. J. Opt. Soc.Am. 1982, 72, 514-515. (17) Saikan, S.; Kiguchi, M. Opt. Lett. 1982, 7 , 555-557. (18) . . Lam, J. F.; Steel. D. G.; McFarlane, R. A,; Lind, R. C. ADD/. . . Phys. . Lett. 1981, 38,977-979. (19) Jabr, S. N.; Lam, L. K.; Hellwarth, R. W. Fhys. Rev. A 1981, 24, 3264-3267. (20) Nolan. T. G.; Koutny, L. 6.; Blazewicz, P. R.; Whitten, W. B.; Ramsey, J. M. Appl. Spectrosc. 1988, 42, 1045-1048. (21) Om,H.; Ackermann, H.; Otten, E . W. 2. Physik A 1975, 273, 221-232. (22) Mariella, R.. Jr. Appl. Phys. Lett. 1979, 35, 580-582. (23) Brog, K. C.; Eck, T. G.; Wieder, H. Phys. Rev. 1967, 153, 91-103.
RECENEDfor review October 31,1990. Accepted January 31, 1991. We gratefully acknowledge support of this work from the National Institute of General Medical Sciences, National Institutes of Health, under Grant No. 5-R01-GM41032.
Comparative and Complementary Plasma Desorption Mass SpectrometryISecondary Ion Mass Spectrometry Investigations of Polymer Materials Herbert Feld, Angelika Leute, Rudolf Zurmiihlen, and Alfred Benninghoven* Physikalisches Znstitut der Universitat Miinster, Wilhelm-Klemm-Strasse 10,0-4400 Miinster, FRG
Thick layers of various polymer materials have been investigated by static secondary Ion mass spectrometry (SIMS) and plasma desorption mass spectrometry (PDMS) with a new combination timeof-fllght (TOF) SIMSPDMS instnment. For ail polymers imestigat@, characteristic spectra could be obtained with both desorption techniques. The yield (number of detected secondary ions divided by number of primary ions) of characteristic secondary ions for PDMS turned out to be 1-2 orders of magnitude higher than for SIMS. Oligomers of PEG (cationized with sodium), of PMTFMA (protonated), and in the negative spectra of PMMA (methylene splitting) have been desorbed from thick layers with both desorption techniques, but only the PDMS measurements show correct distributions. The sensitivity for surface contaminatlon in SIMS is greater than in PDMS.
INTRODUCTION Static secondary ion mass spectrometry (SIMS) has been applied in analytical investigations of polymers for several years. Most of these SIMS investigations have been carried
out with quadrupole mass spectrometers (I). Recently, also time-of-flight (TOF) analyzers have been used (2,3). The polymer spectra obtained by static SIMS in general provide a lot of chemical information, especiallyfor optimally prepared SIMS samples, i.e. monolayer preparation on noble metal substrates. Characteristic secondary ions can be desorbed from polymers over a large mass range. Generally, the positive polymer secondary ion spectrum can be divided into three parts: oligomer, fragment, and fingerprint regions. This is illustrated by Figure 1, showing a positive SIMS spectrum of polystyrene prepared as a monolayer on an etched silver target. Intact oligomer chains consisting of several repeat units and two end groups are generally detected as positive cationized ions. Cations often originate from the substrate. As the chain length varies over a certain mass range, a corresponding oligomer distribution can be seen in the higher mass region of the spectrum, provided that the molecular weight is not too high. Fragment ions produced by bombardment-induced chain cleavage consist of a number of repeat units with one or without any end group. In general they appear in the positive spectrum as a series of cationized ions. Uncationized fragment
0003-2700/91/0363-0903$02.50/00 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 9, MAY 1, 1991 1
2 3 4 5
6 7
8 9 10 11 12 13 14
0
1000
2000
3000 mass/u
p r i m a r y ion source 90' - pulser buncher m a s s separation slit p r i m a r y ion lens target annular 2 5 2 c i - source channel iron extractor secondary ion lens o r e stage reflectron post - acceleratiop channelplate and scintWator photomultiplier
-
Figure 1. Positive SIMS spectrum of Polystyrene; monolayer prepa-
ration on an etched silver target.
ions can be detected in both polarities. In the low-mass region, positive and negative polymer backbone fragments are emitted without cation attachment. By this characteristic fingerprint, an identification of the sample is often possible. The investigation of thick samples is necessary, e.g. for the analysis of untreated original polymer surfaces or in the case of insoluble polymers which cannot be prepared as thin layers. Generally, it is difficult to obtain characteristic SIMS spectra in the high-mass region from a thick sample, because it seems to be impossible to obtain a correct oligomer distribution in this case ( 4 , 5). This may be due to the lack of substrate contact or to the entanglement between the polymer molecules that prevents the emission of intact oligomers. Another difficulty in SIMS may be the influence of surface contamination. Plasma desorption mass spectrometry (PDMS), developed by Macfarlane in 1974 (6), up to now has been applied t o polymer materials only in a few measurements obtained with low-resolution instruments (7,8).This desorption technique has a higher information depth than SIMS, as secondary ions are emitted not only from the first monolayer but also from deeper layers so that PDMS is better suited for the investigation of thick layers than for monolayer preparation. In PDMS for example the desorption of oligomer ions from a monolayer on a metal substrate seems to be impossible, whereas thick polymer samples provide good spectra. In this investigation thick layers of polymer materials have successfully been analyzed by SIMS and PDMS. The investigated substances are examples of various polymer classes with different molecular weight distributions. Table I shows a list of the analyzed polymers with their chemical structures and the masses of the corresponding repeat units. The study concentrates on a comparison of the PDMS measurements with the corresponding SIMS results obtained in the same spectrometer. A similar comparative study but for peptides has been published by Ens e t al. (9). In that investigation spectra were obtained from the same samples, in the same spectrometer, but with megaelectronvolt and kiloelectronvolt ion bombardment from different sides, 18keV Cs ions from the front side and Cf fission fragments from the backside. In our combination TOF SIMS/PDMS instrument, primary ion bombardment occurs from the front side for both desorption techniques. EXPERIMENTAL S E C T I O N Reliable comparative SIMS/PDMS experiments need instrumentation allowing (1) the use of identical targets for both techniques, (2) changing the mode of operation without breaking the vacuum, (3) identical analyzers for both PDMS and SIMS measurements, and (4) bombardment of the target from the same
Figure 2. Combined TOF SIMS/PDMS instrument.
side in both cases. The first criterion ensures that the influence of sample preparation is the same in both experiments, whereas the second one makes sure that there are no contamination effects or other variations in the targets between the measurements. The third criterion allows for equal transmission and detection probability in both modes. Finally, it is ensured that, if bombarding is done from the same side, there are no variations of primary ion energy due to different sample and substrate thicknesses. To meet the above requirements, in 1987 we developed a time-of-flight instrument that can be operated in both the SIMS and the PDMS modes (10). The corresponding primary particle sources irradiate the target from the same side and in the same target area. Changing the desorption mode takes only 2 min without breaking the vacuum. The SIMS part of the instrument was designed following the original TOF-SIMS I1 concept (11). Figure 2 shows a schematic of the instrument. Primary Ion Beam: SIMS Mode. In this mode an electron impact ion source (l).deliversa continuous beam of noble gas ions (neon, argon, xenon). The primary ion energy is normally 10 keV, but we can obtain higher energies up to 30 keV by using doubly or triply charged ions. These ions pass a 90' deflector (2) where a homogeneous electric field is switched on for a time that is correlated to primary ion mass and energy. Only ions that have experienced this electric field above a well-defined plane in the pulser can pass the exit slit and contribute to the primary ion packet. So the same momentum is added to all ions of this packet given by the time integral of the deflection pulse. Therefore at the exit slit the diverging angle of the incident ions is reduced by 2 orders of magnitude and different masses have different directions, so that mass separation'is possible by a slit in front of the target (4). Due to the transformation of radial velocities into axial velocities, the ions have to pass a second pulsed capacitor, the buncher (3), in which they are time-focused onto the target (6). The mass separation of this system is sufficient to separate xenon isotopes. Typically, a primary ion pulse width of about 1ns and an intensity of 5000 ions/pulse is obtained. By an Einzel lens (5) a minimal focus of about 50 pm in diameter can be achieved. The repetition rate can be varied from 1 to 5 kHz. Primary Ion Beam: PDMS Mode. The sample is bombarded with the megaelectronvolt particles from the front side comparable to the SIMS mode. This is achieved by a retractable apertured disk that is moved between target and extractor and is connected to the extraction potential. On the target side this apertured disk is annularly activated with Californium-252 (7). These atoms emit (Y particles (97%) and two heavy fragments caused by spontaneous fission (3%). The energy and mass of these fragments are distributed over a wide range. The maxima of the energy distri-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 9, MAY 1, 1991
005
Table I. List of Investigated Polymers
polymer
repeat unit mass/u
structure
t %ft
polytetrafluoroethylene (PTFE)
polyvinylidene difluoride (PVDF)
+pi
polyvinylidene dichloride (PVDC)
preparation
100
aluminized foil
64
aluminized foil
96
aluminized foil
308
aluminized foil
254
aluminized foil
366
solution, electrospray on aluminum foil
100
solution, direct deposition on aluminum foil
154
solution, direct deposition on aluminum foil
H CI
0
' H H O
I
polyethylene terephthalate (PET)
I II I
I
H
H
polycarbonate (PC) HeO-!iO-@&O
- " f o e
H,
n
polyamide, PA 6.6 (a-10) (NYLON)
polymethyl methacrylate (PMMA) d-CH, H CF,
polymethyl trifluoromethacrylate (PMTFMA)
A L o I
6-CH,
polyethyleneglycol (PEG)
44 H
H
polystyrene (PS)
perfluorinated polyether
1::
1:
i
104
solution, direct deposition on aluminum foil
166
liquid, direct deposition on aluminum foil
74
liquid, direct deposition on aluminum foil
(KRYTOX)
polydimethylsiloxane (PDMSO)
butions are around 80 and 100 MeV whereas the mass distributions have maxima around 100 and 140 u. The source is completely shielded and the targets used are controlled to avoid radiation contamination. The source activity is 15 FCi, which gives a primary particle flux of 300 s-l in a sample area of 2 mm2. The sample thickness has to be small enough for fission fragments to be able to penetrate sample and substrate. Secondary electrons emitted from the backside are extracted into a channeltron (8). This signal is amplified, capacitively coupled to the ground and used as the start trigger for the flight time measurement. Secondary Ion Analyzer: PDMS/SIMS Mode. The analyzers are identical for both desorption techniques. The secondary ions generated at the target (ground potential) are accelerated to an energy of 4 keV by the extractor (9). The target acceptance area is projected on the detectors by an Einzel lens (10) that provides an acceptance angle of f2' in both directions.
solution, electrospray on aluminum foil
The spectrometer can be operated in different detection modes. In the reflection mode (detector 2) the analyzer consists of the ion znirra (11) and an effective drift path of 2 m. Energy focusing is achieved by different penetration depths in this mirror for secondary ions with different energies. For the usually small energy distributions found in organic mass spectrometry, the one-stage reflector is advantageous compared to the two-stage reflector. For the used acceptance angle a mass resolution of about 8000 can be achieved, 5000 is routinely obtained. In the linear mode the ions reach detector 1 after a flight path of 0.8 m. In this case the ion mirror is a t the extraction potential and belongs to the drift space. Fragments produced by the decay of metastable secondary ions in the drift space are also detected. In the linear mode they have similar flight times to their parent ions and therefore contribute to the parent ion peak (12). Neutral decay products of metastable secondary ions can be detected separately
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 9, MAY 1, 1991
Table 11. List of Characteristic Secondary Ions Detected from the Different Polymers in SIMS and PDMS oligomer ions
fragment ions
PTFE PVDF PVDC
fingerprint
(RnCF)+, (RnCF)(R,CF3 - kHF)+ k = 0, ..., n (R,CCl - kHCI)+ (R,CC13 - kHCI)+ k = 0, ..., n
PET PC PA 6.6 (n-10)
PMMA
(HR,H - CH3)-
PMTFMA (C6H4NRnH+ H)+ (OHR,H + Li)+, (OHR,H PEG Na)+, (OHR,H + K)+ PS KRYTOX PDMSO
+
(R,H + H)' (R, + Li)+, (R,
+ K)+
+ Na)+, (R,
with detector 1,if all ions are suppressed by the ion mirror. This operation mode is referred to as the neutral mode. In front of both detectors the secondary ions can be postaccelerated (12) up to an energy of 10 keV, increasing the detection efficiency for large molecules. The detection system consists of a single channel plate and a scintillator (13) on the vacuum side of the instrument with a photomultiplier (14) placed outside the vacuum window. In this way the high-potentialsignal is optically coupled to the ground and fed into a 100-MHz-signalaverager (13).
Sample Preparation. PTFE, PVDF, PVDC, PC, and PET were analyzed as polymer foils with thicknesses between 2 and 10 pm, aluminized from the backside to ensure a well-defined electric potential for the extraction. NYLON, PMMA, PMTFMA, PS, PEG, KRYTOX, and PDMSO were deposited from solution. Thick films (>100 monolayers) were produced by electrospraying (NYLON and PEG) or by direct deposition (PMMA, PMTFMA, and PS) of the solutions onto aluminum targets. In the cases of PEG and NYLON sodium chloride was mixed into the polymer solution to enhance the cationization. Thick layers of KRYTOX and PDMSO were prepared by direct deposition of the liquid polymer without solvent onto the aluminum targets. Measurements. For all polymers investigated, positive and negative secondary ion spectra were obtained with both desorption techniques and in the different detection modes. In SIMS and PDMS the targets were bombarded in the same area of approximately 2 mm2. For the Cf-fission fragments this is the acceptance area determined by measuring the secondary ion yield in dependence on the acceptance angle of the start trigger system. For SIMS the primary ion beam diameter can easily be varied by adjusting the potential of the Einzel lens (5). Primary ion doses (PID) ranged between 103and lo8 in the PDMS and between l@ and 10" in the SIMS mode.
RESULTS AND DISCUSSION General Results. Secondary Zon Species. In general SIMS and PDMS provide very similar spectra showing the same characteristic secondary ions. Table I1 gives a survey of the most important characteristic secondary ions detected from each polymer with both desorption techniques. Fingerprint Region. All polymers investigated emit characteristic fragment ions in the fingerprint region. Normally, the fingerprint spectra obtained with SIMS and PDMS contain sufficient information for identification of the polymer. Besides the characteristic polymer fragment ions, most of the spectra show carbon cluster ions in the low-mass range. In all cases positive and negative carbon clusters could be observed with both desorption methods, but they were much more intense with PDMS than with SIMS. The results concerning carbon cluster emission from polymer foils are discussed in detail in another article (24).
(CO)+,(CH2CcF3CO)+ (CHsCHOH)+,(CHCO)-, (CH,CHO)-, (CH,CHOCHCHO)-, (ROHCHvCHv0)-
Higher Mass Region. The polymers PEG, PS, PMMA, PMTFMA, PDMSO, and KRYTOX have nominal molecular weight distributions between 1000 and 11000 u. Generally, all of them emit cationized oligomers provided that the appropriate cations are available. As silver cationization from the substrate is not possible for thick polymer layers, this study is limited to alkali-metal cationized or protonated oligomers or oligomer ions without any ion attachment. The same is the case for the emission of fragments which are also cationized, with the exception of KRYTOX. The polymer foils of PTFE, PVDF, PVDC, PC, PET, and NYLON have high molecular weight distributions, so that no intact polymer molecules could be detected. It is impossible to determine whether or not oligomer ions are formed in the desorption process because of limitations in the technique used. Therefore only fragment ions were detected from these materials. In our study characteristic secondary ions in the higher mass region (>500 u) were detected from PEG and NYLON (alkali-metal cationized), from PMTFMA (protonated), and from KRYTOX, PTFE, and PMMA (without cation attachment). In all cases both desorption techniques provided the same fragment and oligomer ions. Carbon cluster series in the higher mass range (480-2400 u) were detected from PVDF and PVDC only with PDMS, and not with SIMS (14). Metastability. Measurements in the linear and in the neutral modes reveal that fragment ions in the higher mass region of some polymers (KRYTOX, PEG, NYLON, PTFE, PMTFMA, PDMSO) are metastable; i.e. a certain fraction of the desorbed secondary ions decay during their flight times (about 100 ps). With one exception (positive KRYTOX fragments (15)),however, this fraction is small (
