Anal. Chem. 1988, 60, 279-282
1987. This work was supported in part by grants from the National Science Foundation (CHE-8210886) and the Office of Naval Research. Isiah M. Warner acknowledges support from an NSF Presidential Young Investigator Award (CHE-
279
8351675). Gregory Nelson acknowledges support from an American Chemical Society Division of Analytical Chemistry Summer Fellowship sponsored by the Society of Analytical Chemists of Pittsburgh.
Laser Desorption Fourier Transform Mass Spectrometry of Polymers: Comparison with Secondary Ion and Fast Atom Bombardment Mass Spectrometry Lydia
M.Nuwaysir a n d Charles L. Wilkins*
Department of Chemistry, University of California, Riverside, Riverside, California 92521
A series of alkoxylated pyrazole and hydrazlne polymers with average molecular weights between 600 and 1300 were studled by uslng laser desorption Fourler transform mass spectrometry. Spectra primarily contaln Kf- and Na+-attached Intact oilgomer Ions and show little evidence of fragmentatlon. LD-FTMS spectra are compared with previously reported secondary ion and fast atom bombardment spectra of the same polymers. I n general, laser desorptlon spectra show more regular polymer dlstributions, fewer fragment ions, and less mass dlscrlmlnatlon.
Characterization of polymers by mass spectrometry has become of increasing interest in recent years. In large measure, this is a result of instrumental advances that have provided improved capabilities for handling nonvolatile samples requiring extended mass range. Secondary ion mass spectrometry (SIMS) and fast atom bombardment (FAB) have been advocated for this purpose ( I , 2). It is therefore of interest to compare laser desorption mass spectrometry with these methods. Laser desorption Fourier transform mass spectrometry (LD-FTMS) was used previously for characterization of a number of polar polymers (3). Among those were poly(ethylene glycol) 6o00, poly(ethy1ene glycol methyl ether) 5o00, poly(caprolactonedio1) 2000, and two poly(ethy1enimine) polymers. The LD-FTMS spectra obtained after doping the samples with alkali salts were comprised primarily of potassium- or sodium-attached molecular ions of the oligomers present, with relatively little fragmentation being observed. For the poly(ethy1enimine) (PEI) samples, direct comparison with previous field desorption ( 4 ) , electrohydrodynamic ionization (5), and fast atom bombardment (FAB) (4) studies using sector instruments was possible. When the PEI sample spectra were compared, LD-FTMS appeared to more accurately reflect the starting polymer distribution, apparently because less fragmentation occurred under FTMS conditions or because FTMS discriminates against high-energy fragment species (6). Thus, it is of interest to compare LD-FTMS results to alternatives such as SIMS and FAB for other polymers, to determine whether reduced fragmentation is a general characteristic of this analytical procedure. When proton- or alkali-attached molecular ions of intact oligomers are the predominant species, determination of the molecular weight distribution is particularily easy. Both
weight-average molecular weight (&Zw)and number-average are simply calculated (eq 1and 2), as molecular weight (ATn)
Mw = CNiM:/CNiMi M,,= CNiMi/CNi
(1) (2)
is their ratio, the polydispersity. The relative number of moles of each species (Nifor species i) in the mixture is readily determined from integrating the areas of the peaks in the low-resolution mass spectra of the samples. Alternatively, the values may be estimated less accurately by using peak intensities, rather than integrated intensities. Recently, Doherty and Busch analyzed a series of alkoxylated pyrazole and hydrazine polymers of low volatility by using low-resolution SIMS and FAB (7).These compounds are corrosion inhibitors used in acids, antifreezes, and hydraulic fluids. These workers were interested originally in the possible use of such polymers as FAB matrices. However, the spectra proved to be too complex for this to be feasible. In the present study, laser desorption FTMS is used to obtain spectra of the same set of materials and the results are compared with those reported earlier. EXPERIMENTAL SECTION Mass spectra were obtained with a Nicolet 1000 Fourier transform mass spectrometer equipped with a 3-T superconducting magnet, a 5.08-cm cubic cell, and a laser desorption interface that has been described previously (8). The grating for the Tachisto 215G pulsed COz laser was replaced with a copper reflector that passes all COz laser wavelengths. Laser power was adjusted to a level where fragmentationwas minimal and spectral reproducibility was within a few percent. Survey mass spectra initially were obtained in the broad band mode with a high-frequency (low-mass)cutoff of 1.333 MHz (-33 amu). Then each sample was examined by using narrower bandwidth (i.e. higher resolution), chosen so that the polymer envelope filled the spectrum. For Oxypruf-20 a high-frequency cutoff (low mass) of 60 k H z (-745 amu) was sufficient, while for Oxypruf-12 and Oxypruf-E 140 kHz (-320 amu) was used. Oxypruf-6 and Oxypruf-P spectra were best obtained with a high-frequency cutoff of 300 kHz (- 145 amu). Normally, 64K transient data were collected and stored to disk. The 64K data point transient was base-line corrected, sine-bell apodized, or windowed with a Blackman-3-term window function (9),augmented by an additional 64K zeroes and a fast Fourier transform employed to obtain a magnitude mode spectrum. For analysis of each sample, nine spectra were signal averaged in the time domain. This was to ensure that the intensities of the peaks corresponding to each ion accurately reflected the true
0003-2700/SS/0360-0279$01.50/00 1988 American Chemical Society
280
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988 OXYPRUF-P
OXYPRJF-E 0 c
n54
t
Figure 1.
Laser desorption Fourier transform mass spectrum of Ox-
ypruf-E.
Figure 2.
Laser desorption Fourier transform mass spectrum of Ox-
yrpuf-P.
abundance of that species within the polymer, because small changes in relative intensities occur from shot to shot of the laser (a few percent at most). Signal averaging was accomplished by rotating the probe after each laser shot to expose fresh sample for the next spectrum. It should be emphasized that reasonable spectra with signal-to-noiseratios ranging between 20_1and 1601 could be obtained from a single laser shot. For M , and M,, calculations, transients were truncated to 16K data and augmented by 16K zeroes. This use of deresolved spectra allows isotopic ion contributions to be integrated within the molecular ion region. Samples of polyalkoxylated hydrazines and pyrazoles were provided by Professor Kenneth L. Busch of Indiana University. These mixtures are manufactured by Olin Corp. under the trade name Oxypruf, and are highly viscous clear, yellow, or brown liquids, with boiling points between 169 "C (Oxypruf-6) and 252 "C (Oxypruf-20). Vapor pressures at room temperature are sufficiently low to prevent their convenient introduction as gaseous or probe samples. Samples studied were oxypruf-E (ethoxylated dimethylpyrazole), Oxypruf-P (propoxylated dimethylpyrazole), Oxypruf-6 [bis(2-hydroxypropyl)bis(2-hydroxypropoxypropyl)]hydrazine, Oxypruf-12 [tetrakis(hydroxypropoxypropoxypropyl)]hydrazine, and Oxypruf-20 [tetrakis(hydroxypropoxypropoxypropoxypropoxypropyl)]hydrazine. Samples were applied neat to a stainless steel probe tip mounted onto a direct insertion probe. Following sample insertion, pressure was allowed to drop to - 2 X Torr or less prior to analysis (30-60 min). Delays of up to 3 s were utilized after each laser pulse and before ion detection to allow neutral species to be pumped away. No salts were added to the samples.
OYYPRUF-6
Flgure 3. Laser desorption Fourier transform mass spectrum of Oxypruf-6. Inset shows resolution for n = 8 oligomer.
RESULTS AND DISCUSSION Alkoxylated Pyrazoles. T h e general formula for the alkoxylated pyrazoles (I) is indicated below, in which R = H CH
CH3
-C
\
-CH3
C' N-N
/
M A S S IN A M U
/
Figure 4.
(CH2-CHOkH
I
R
I
for Oxypruf-E, R = CH, for Oxypruf-P, and n varies between 2 and 20. Figures 1 and 2 show representative LD-FTMS spectra of Oxypruf-E and Oxypruf-P, respectively. The Oxypruf-E spectrum is characterized by abundant ions spaced 44 mass units apart (the mass of an ethoxy group), with masses corresponding to K+ attachment to individual oligomers. No potassium salts are added to the samples prior to analysis; thus, K+-attached ions must arise from adventitious salts. Laser desorption positive ion mass spectra frequently contain ions resulting from cation attachment. Previous spectra of
Laser desorption Fourier transform mass spectrum of Ox-
ypruf-12. Table I. Number-Average and Weight-Average Molecular Weights as Determined by Integration of Laser Desorption Fourier Transform Mass Spectra polymer
&in
M,
Mn,iMw
nna
nwb
Oxypruf-E Oxypruf-P Oxypruf-6 Oxypruf-12 Oxypruf-20
636.3 676.1 541.3 876.9 1334.6
656.7 754.5 557.2 908.3 1366.7
1.03 1.12 1.03 1.04 1.03
12.3 9.9 5.8 11.6 19.4
12.7 11.3 6.0 12.1 19.9
'nn is the average number of repeating units as computed from the number-average values. n, is the average number of repeating units as computed from the weight-average values.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
281
Table 11. Comparison of Average Degree of Polymerization of Oxypruf Polymersa sample
instrument
Oxypruf-E Oxypruf-E Oxypruf-E Oxypruf-P Oxypruf-P Oxypruf-P Oxypruf-6 Oxypruf-6 Oxypruf-12 Oxypruf-12 Oxypruf-20 Oxypruf-20 Oxypruf-20
Kratos MS80e FTMS-1000 QUAD kratos MS80 FTMS-1000 QUAD FTMS-1000 QUAD FTMS-1000 QUAD Kratos MS80 FMTS-1000
method
QUAD^
SIMS FAB LD SIMS E1 LD SIMS LD SIMS LD SIMS FAB LD
Mn*b
Mw*
Mw*/Mn*
n,*c
nw*c
500.5
537.7 513.5 661.0
1.07 1.09
10.0 9.5
1.03
9.2 8.5 12.4
1.07 1.10
8.7 4.7
1.12 1.07 1.03
9.7 4.0
9.4 5.4 11.1 4.5
5.8
6.1
1.05
7.2
1.03 1.07
11.5
7.7 12.0
472.8
641.7 601.1 370.3 660.9 439.0 541.8 621.5 873.9 723.2 485.3 1278.1
641.6
408.6 737.9 467.7 558.1 652.2 903.1 773.7
569.2 1314.6
8.9 4.8 18.5
1.17
1.03
12.8
9.8 6.2 19.1
4AllSIMS, EI, and FAB data calculated from spectra published in ref 7. All LD data calculated from spectra obtained in this study. bAn asterisk indicates spectral intensities of predominant isotopic species are used. cSeeTable I for definition of n, and n,. dExtrel quadrupole mass analyzer, see ref. 7. eKratos MS8O RFAQA mass spectrometer of EBQ geometry with mass analysis after the B sector, see ref. 7. OXYPRUF-PO
No values for Mw and Mn were reported from previous studies (ref 7). Therefore, estimates of average molecular weights and degrees of polymerization Mn*,nw*,nn*) were calculated from published spectra by substituting Ii for Ni in eq 1and 2, where Iiis the measured peak height of the most abundant isotope peak. Minor isotope and fragment peaks were neglected. The results of these calculations appear in Table 11. This procedure was repeated for the spectra obtained in the present study for comparison. Those values are included in Table 11. For both Oxypruf-E and Oxypruf-P the Mw*obtained by laser desorption is higher than the SIMS, EI, or FAB results. This is consistent with the earlier polyethylenimine analysis comparisons (3, 6). The LD-FTMS spectra also show no evidence of mass discrimination. Differences cannot be attributed to the samples used, as the samples were the same as those used in the previous study (7). Alkoxylated Hydrazines. Representative spectra for Oxypruf-6,Oxypruf-12, and Oxypruf-20 appear in Figures 3-5. The general formula for the hydrazine polymers (11)is as given below, where R is selected from hydrogen and lower alkyl groups and the sum of w ,x , y, and z is between 4 and 20. A spectrally more convenient structure (111)was proposed by Doherty and Busch, where n is between 4 and 20.
(aw*,
800
1060
1260 1460 MASS I N A M U
1660
1SdO
Flgure 5. Laser desorptlon Fourler transform MSS spectrum of Oxypruf-20. Inset shows resolution for n = 24 oligomer.
the same samples obtained by Doherty and Busch show no evidence of cation-attached peaks, suggesting the source of alkali salts is the FTMS instrument. The low-mass region shows peaks characteristic of H+-attached oligomers. There was no fragmentation when using low laser powers (- lo6 W/cm2), and very soft desorption/ionization could be accomplished. With laser powers higher than this, fragmentation did occur. The major ions corresponded to K+-attached chain fragments [fCH2CH20),H + K]+. For the spectra in Figures 1-5, low laser power was used to eliminate fragmentation and simplify the analysis. Values for and A?, are given in Table I. The LD-FTMS of Oxypruf-P also is characterized by features representing K+-attached oligomers 58 mass units apart (the mass of a propoxy group). (M H)+ oligomer ions appear in the low-mass region and are less abundant at higher masses (n> 7), as with Oxypruf-E. An ion of high relative abundance appears a t nominal mass 367, corresponding to n = 4. This pattern of a high abundance n = 4 species, along with a second envelope centered around an n = 11 oligomer is very reproducible. To test whether mass selectivity was involved in the analysis, the same sample was also analyzed by laser desorption with a dual-cell 7.2-T FTMS. The spectra were qualitatively similar, showing abundant ions with m / z lower than 400. Comparison with the previously reported SIMS and E1 spectra reveals similar behavior, although more fragmentation is obvious in those spectra. Those observations suggest that ions with n = 4 in the Oxypruf-P LD-FTMS spectrum accurately reflect the sample properties. Fragmentation could be induced by employing higher laser powers. Mw and Mn values are also included in Table I.
+
HeOHCCH2)
(CHzCHOvH
’
I
R
\N-N/
H --$- OHCCH?)
I R
‘(CH2CHO);--H R
11
H(C3HeO)
’
(C3H6O)H
‘N-N’
H[C3HeO)
‘(C3HeO+H
I11
All spectra are characterized by features indicating abundant K+-attached oligomers with less abundant Na+-attached oligomers. No fragmentation was observed when using low laser power. At higher laser power major fragments correspond to K+ attached chain portions, [f6’&HsO)$ + K]+, spaced 58 mass units apart. The spectra obtained under low power conditions were used for calculations, and M wand Mn values are given in Table I. Table I1 presents the values for average degree of polymerization as calculated from spectral abundances of the predominant molecular ion species for each oligomer. Mw*, n,* are all higher for samples analyzed by LD-FTMS than by other methods. Furthermore, polydispersity (Mw*/an*) values are closer to 1 for LD-FTMS
an*,
Anal, Chem. 1988, 60, 282-283
282
results, reflecting the greater regularity of polymer distribution as determined by this method. Of course, the comparative values of Table I1 are different than those in Table I because isotopic contributions (of necessity) were neglected. Although these materials were unsuitable as FAB matrices due to their spectral complexity under FAB conditions, the regular spacing of the spectral peaks and the mass range which they cover, as well as the ease by which spectra are obtained, make these compounds candidates for use as calibration compounds for LD-FTMS.
ACKNOWLEDGMENT We thank Kenneth Busch for supplying the Oxypruf samples and for helpful discussions. Registry No. Oxypruf-E,81042-97-9;Oxypruf-P, 81065-92-1; Oxypruf-6, 108764-62-1;Oxypruf-12, 108764-61-0;Oxypruf-20, 108778-62-7.
LITERATURE CITED (1) Cochran, R. L. Appl. Spectrosc. Rev. 1988, 22, 137-187. (2) Bletsos, I.V.; Hercules, D. M.; Greifendorf, D.: Benninghoven, A. Anal. Chem. 1985, 8 5 , 2384-2308. (3) Brown, R. S.;Weil, D. A.; Wilkins, C. L. Macromolecules 1986, 19, 1255-1 260. (4) Lattimer, R. P.; Schulten, H.-R. Int. J. Mass Spectrom. Ion Processes 1985, 67,277-284. ( 5 ) Callahan, J. H.; Cook, K. D. 32nd Annual Conference on Mass Spectrometry and Allied Toplcs, Sen Diego, CA, May 1985. (6) Cotter, R. J.; Honovich, J. P.; Oithoff, J. K.;Lattimer, R. P. Macromolecules 1986, 19, 2996-3001. (7) Doherty, S. J.; Busch, K. L. Anal. Chim. Acta 1986, 187, 117-127. (8) Wllkins, C. L.; Weil, D. A.; Yang, C. L. C.; Ijames, C. F. Anal. Chem. 1985, 5 7 , 520-524. (9) Aarstol, M.; Comisarow, M. B. Int. J. Mass Spectrom. Ion frocesses 1987, 76,287-297.
RECEIVED for review July 13,1987. Accepted October 25,1987. Support under National Institutes of Health Grant GM-30604 is gratefully acknowledged. We also thank the Shell Development Co. for support.
CORRESBU#DENCE Removal of the Samarium Isobaric Interference from Promethium Mass Analysis Sir: One of the primary advantages of resonance ionization mass spectrometry (RIMS) is the removal of isobaric interferences. Isobaric interference is common in isotope ratio measurements by mass spectrometry and occurs when different analyte elements possess isotopes of the same mass. In our application of RIMS, a tunable laser is employed to excite (usually via a nearly degenerate, multistep process involving two photons for red laser dyes) and subsequently ionize (with a third photon) vapor phase atoms that are produced from a hot filament in the source region of a mass spectrometer ( I ) . In many cases, a single color method requiring only one laser can effect this process. The tunable laser line width can be chosen such that it selectively excites and ionizes all of the isotopes of a single element. The mass spectrometer then separates and measures the isotopic abundances of that element. Isobaric interferences are thus eliminated. Such a method has already been demonstrated for cerium, neodymium, and samarium isobars ( I , 2), uranium and plutonium (3), and ytterbium and lutetium ( 4 ) . Promethium is unique among the lanthanides in that it has no naturally occurring isotopes. It can be synthesized by neutron capture using samarium as starting material. Those promethium samples usually contain residual samarium. Furthermore, all of the promethium isotopes are radioactive, with many decaying by /3 emission to samarium daughters. /3 decay always results in a daughter element that interferes in mass spectrometric analysis because the daughter is isobaric with the parent. For any measurement of promethium nuclear properties (e.g., half-life, reaction cross section, etc.) it is necessary to correct for the samarium content of the sample. In the past, this correction has been difficult and inaccurate using mass spectrometric methods because of isobaric interference (5). EXPERIMENTAL SECTION The instrumentation for RIMS has been described ( I ) and will only be outlined briefly here. A flashlamp-pumped dye laser 0003-2700/88/0360-0282$01.50/0
(Model CMX-4, Chromatix Corp.) was used for excitation and ionization. Rhodamine 6G laser dye was used, producing approximately 2 mJ/pulse. The nominal laser bandwidth was 3 cm-' for the 1-ks pulses. The beam was focused in front of the source entrance slit of a mass spectrometer with a 25-cm focal length lens. The mass spectrometer consisted of a single 90' sector magnet with a 30-cm central radius of curvature. The beam interacted with the vapor from a 1400 "C rhenium filament that contained a small charcoal fragment. Samarium and promethium had been loaded on the charcoal from dilute nitric acid solutions. The charcoal provided a convenient medium for solution loading and a reducing atmosphere at operating temperature for efficient production of neutral atoms. The samarium sample was of natural abundance; the promethium was isotopically pure (mass 147). The ions created by resonance ionization were accelerated into the mass spectrometer by a 4.9-kV field applied to a modified Nier source (6). An electron multiplier (Model R515, Hamamatsu Photonics Corp.) at 3 kV was used for ion detection. For mass analysis, a PDP-11/34 computer counted and logged single ion events as the spectrometer was stepped through prescribed masses. For resonance ionization optical spectra, the ion burst correlated with each laser flash was integrated with an amplifier (Model 575, EG&G Ortec) and measured with a boxcar averager (Model SR250, Stanford Research Corp.) while the mass spectrometer was tuned to a selected mass.
RESULTS AND DISCUSSION The resonance ionization spectra obtained for an interesting portion of the R6G tuning range are shown in Figure 1. Trace A was acquired after loading the sample filament with 600 ng of samarium. The mass spectrometer was tuned to mass 152, the most abundant samarium isotope. This mass was chosen to obtain the optimum signal-to-noise ratio and to assure that the spectra acquired represented a true blank, even on the off chance that the samarium was contaminated with promethium. Promethium-152 has an 18-min half-life. The lines a t 587.55 and 587.8 nm are among the strongest RIMS transitions for samarium in the R6G range. The wavelength axis was calibrated by use of mercury emission lines and a 1988 American Chemical Society
