Anal. Chem. 1992, 64, 1879-1884
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Determination of Molecular Weight Distributions of Polymers by Desorption Chemical Ionization Mass Spectrometry Marco Vincenti' and Ezio Pelizzetti Dipartimento di Chimica Analitica, Universith degli Studi di Torino, Via P. Giuria 5, 10125 Torino, Italy
Alessandro Guarini Istituto Guido Donegani, Via Fauser 4, 28100 Novara, Italy
Silvestro Costanzi EniChem Synthesis, Via Maritano 26, 20097 S. Donato Milanese (Milano),Italy
Ikaorptlonchemlcai ionlzatlonmars spectrometry (DCI-MS) has been investigated as a technique for polymer analysis. Under approprlate experimental conditlons, protonated molecular ions of ollgomerlc specks are produced, without fragmentation. Thus, it b poulbk to obtain a dlstrlbutlon of molecular welghts for varlour polymers (polystyrenes, poly(ethylone glycols), polysiloxanes, and polynorbornene) by recordlngtheir positive ion DCI mars spectra. Number- and wdght-average mdecuiar welghts determined from these dktrlbutlona were repeatable, reproducible (two different Instruments were used In different laboratories) and in good agreement with the average molecular weight declared by the manufacturer, and checked by gd-permeatlon chromatography. Oligomeric molecular ions up to m/z 8000 were obtainedfor a branchedpdyrlloxane. A k o the mars spectra of negative ions were found to represent correctly the molecular wdght dktributlon of polystyrenes. Upon systematic study, the following critical parameters were identifled: (I) ion-crourcetemperature, (ii) amount of sample, (Mi) gradlent of the DCI wire current, (Iv) analyzer scan t h e , and (v) type and pressure of the CI reagent gas. The modiflcatlonsto the m a u spoctra inducedby varying the preceding experimental parameters lead us to propose a mechanism in which the neutral oligomors are vaporized by rapld heating, thermally stablllmd by collklonswith the reagent gas, and subsequently ionized by proton transfer.
INTRODUCTION Desorption (or direct) chemical ionization (DCI) is an ionization technique in which a wire, loaded with the sample and placed in the center of a mass spectrometer (MS) ion source, is rapidly heated in an atmosphere of reagent gas, under the conditions commonly used for chemical ionization. DCI was introduced in 1973l and was soon utilized for the study of involatile biological substances.'-10 In the same time, the overall mechanism of evaporation and ionization was (1)Baldwin, M. A.; McLafferty, F. W. Org. Mass Spectrom. 1973,7, 1353-1356. - -. . - -. .
(2)Beuhler, R. J.; Flanigan, E.; Greene, L. J.; Friedman, L. J. Am. Chem. SOC. 1974,96,3990-3999. (3)Hunt, D. F.;Shabanowitz, J.; Botz, F. K.; Brent, D. A. Anal. Chem. 1977,49,1160-1163. (4)Daves, G. D., Jr. Acc. Chem. Res. 1979,12,359-365. (5) Cotter, R. J. Anal. Chem. 1979,51,317-318. (6)Cotter, R. J.; Fenselau, C. Biomed. Mass Spectrom. 1979,6,287-
---. 9Q2
(7)Hansen, G.;Munson, B. Anal. Chem. 1980,52,245-248. (8)Bruins, A. P. Anal. Chem. 1980,52,605-607. 0003-2700/92/0364-1879$03.00/0
investigated,llJZ but it has remained partially unclear. Soon after its introduction, the DCI technique was recommended for the analysis of highly polar and nonvolatile compounds and was considered a convenient and easy to use alternative to field desorption (FD) i ~ n i z a t i o n . ~ ~ ~ ~ * ~ Q ~ l ~ ~ ~ * With the introduction of fast atom bombardment (FAB) in 1981,13interest in the development of both DCI and FD sharply decreased and most researchers transferred their efforts into characterizing and applying the newer technique. Indeed, on highly polar substances FAB provided more valuable results than DCI or FD and a more stable signal. On the other hand, nonpolar substances with high molecular weight are not amenable to FAB since they are poorly ionized and also they cannot be easily dissolved in the most common FAB matrices. Thus, alternative ionization methods have to be employed with such compounds. In the last 4 years, we have been using DCI-MS for the structural characterization of a variety of slightly poiar"~5 and nonpolar16-20 substances with rather high molecular weights. In almost all cases, it was possible to choose the appropriate experimental conditionsto obtain the molecular ion as the base peak of the mass spectrum. Moreover, DCI appeared to have a series of advantages: it is extremely fast, sensitive, noise-free, cheap, easy-to-use,and widely applicable. Nevertheless, this technique is nowadays underutilized and underrated. The major disadvantage of the DCI technique is the short duration of the signal (Le. 2 s), which in the past was associated with poor reproducibility of mass spectra.' Yet this limitation has been overcome by the introduction of modern mass analyzers and data systems, which are capable of scanning and processing a large mass range in an extremely short time. (9)Hostettmann, K.;Doumas, J.; Hardy, M. Helu. Chim. Acta 1981, 64,297-303. (10)Smith, R.G. Anal. Chem. 1982,54,2006-2008. (11)Hansen, G.;Munson, B. Anal. Chern. 1978,50,1130-1134. (12)Cotter, R.J. Anal. Chem. 1980,52,1589A-1606A. (13)Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. J. Chem. Soc., Chem. Commun. 1981,325-326. (14)Guglielmetti, G.; Andriollo, N.; Cassani, G.; Vincenti, M. Biomed. Enoironm. Mass Spectrom. 1989,18, 352-354. (15)Guarini, A.; Guglielmetti, G.; Andriollo, N.; Vincenti, M. Anal. Chem. 1992,64,204-210. (16)Guglielmetti, G.; Dalcanale, E.; Bonsignore, S.; Vincenti, M. Rapid Commun. Mass Spectrom. 1989,3, 106-109. (17)Vincenti, M.; Dalcanale, E.; Soncini, P.; Guglielmetti, G. J . Am. Chem. SOC.1990,112,445-447. (18)Bryant, J. A.; Blanda, M. T.; Vincenti, M.; Cram, D. J. J. Am. Chem. SOC.1991,113,2167-2172. (19)Guarini, A.; Vincenti, M.; Guarda, P.; Marchionni, G. 12th International Mass Spectrometry Conference, Amsterdam, Aug. 26-30, 1991. (20)Vincenti, M.; Guarini, A.; Costanzi, S. 12th International Mass Spectrometry Conference, Amsterdam, Aug. 26-30, 1991. 0 1992 American Chemical Society
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The advantages of using MS techniques instead of conventional methods (e.g., gel-permeation chromatography (GPC), vapor-phase osmometry) for determining the molecular weight distributions of polymers have been recently reviewed by Hercules and co-workers.21 These advantages arise from the fact that MS measures both the mass and the abundance of each individual oligomer. However, polymers are among the most difficult classes of compounds to be studied by means of mass spectrometric methods. Polymeric samples are complex mixtures covering a wide range of molecular weights; they are pyrolyzed before being vaporized; most of them cannot be dissolved in polar solvents and hence are not amenable to solvent-assisted ionization methods such as FAB, thermospray, and electrospray. Successful analysis of polymeric materials has been achieved by FD,22-24 laser desorption (LD),2s2*and static secondary ion mass spectrometry (SIMS).21*29v30 By means of these techniques, it has been possible to obtain (at least in part) a distribution of intact molecular ions, representing the distribution of the oligomer molecular weights. With respect to the previous methods of analysis using pyrolysis-MS, these studies opened up new perspectives to the mass spectrometric characterization of polymers. Since DCI-MS also proved to be successful in producing intact molecular ions from high molecular weight substances,'6 we were stimulated to apply this ionization technique to the study of various nonpolar polymers. DCI-MS shares some similarities with LD-MS, where a short and intense ion signal results from fast vaporization of the sample. The polymers considered in the present investigation include polystyrene, poly(ethy1ene glycol), polysiloxane, and polynorbornene. A DCI-MS and DCI-MS-MS study of perfluoropolyethers is reported e1~ewhere.l~ In our study, repeatable and accurate molecular ion distributions were obtained for oligomers in the range 10008000 Da after optimization of a large set of parameters. On the basis of the experimental results, a mechanism is proposed, in which rapid vaporization of the sample is followed by thermal stabilization and subsequent ionization of the neutral intact oligomers.
EXPERIMENTAL SECTION Chemicals. Polystyrene (average MW 1250,2700, and 4000 Da) and poly(ethy1eneglycol) (averageMW 2000,3400, and 5000 Da) samples included in this study were purchased from Aldrich (Steinheim, Germany). Polysiloxane and polynorbornene samples were synthesized in our laboratories. Stock solutions of the polymers were prepared by dissolving about 50 mg of solid sample in 10 mL of chloroform. These solutionswere diluted further in chloroform to yield the analytical solutions, containing 5-200 mg/L of polymer (concentrations in M, as calculated from the average the range 2 X 104-5 X molecular weight). Each experiment was run by loading the DCI wire with 1 p L of analytical solution (5-200 ng of polymer). (21)Bletsos, I. V.;Hercules, D. M.; van Leyen, D.; Hagenhoff, B.; Niehuis, E.; Benninghoven, A. Anal. Chem. 1991, 63, 1953-1960. (22) van Breemen, R. B.; Huang, C.-H.; Bumgardner, C. L. Anal. Chem. 1991, 63, 2577-2580. (23)Rollins, K.;Scrivens, J. H.; Taylor, M. J.; Major, H. Rapid Commun. Mass Spectrom. 1990,4, 355-359. (24) Rollins, K.; Scrivens, J. H.; Taylor, M. J. 12th International Mass Spectrometry Conference, Amsterdam, Aug. 26-30, 1991. (25)Brown, R. S.;Weil, D. A.; Wilkins, C. L. Macromolecules 1986, 19, 1255-1260. (26)Nuwaysir, L. M.; Wilkins, C. L. Anal. Chem. 1988,60, 279-282. (27)Nuwaysir, L. M.; Wilkins, C. L.; Simonsick, W. J., Jr. J.Am. SOC. Mass Spectrom. 1990, 1 , 66-71. (28) Liang,Z.; Marshal1,A. G.; Westmoreland, D. G. Anal. Chem. 1991, 63, 815-818. (29) Bletsos, I. V.; Hercules, D. M.; van Leyen, D.; Benninghoven, A. Macromolecules 1987,20, 407-413. (30)Hagenhoff, B.; Benninghoven, A,; Barthel, H.; Zoller, W. Anal. Chem. 1991,63, 2466-2469.
Apparatus and Procedure. Experiments were run on two different mass spectrometers: a Finnigan-MAT 8400 doublefocusing reverse geometry instrument, and a Finnigan-MAT 95 Q hybrid mass spectrometer with BEQ geometry (the Q section was not utilized in the present experimenta and ions were collected at the first detector, after the electrostatic analyzer). The maximum mass range for the 8400 instrument is 2000 amu at full acceleration voltage (3 kV) and 8000amu at reduced acceleration voltage; for the 95 Q mass spectrometer it is 3500 amu at full acceleration voltage (5 kV). Experiments with polystyrene and poly(ethy1ene glycol) were run on both instruments in order to compare the results. In a typical experiment the ion source was kept near ambient temperature. In order to avoid heating from the electron-emitting filament,the latter was switched on just a few seconds before the experiment start and switched off immediately after the end of it. Using this procedure the ion source temperature was maintained at 40-50 OC. In some experiments the source was deliberately heated (140 and 200 "C), in order to check the temperature effect. The DCI rhenium wire (standardfrom Finnigan)formsa singlecoil ring, where the droplet of sample solution can be accommodated. After evaporation of the solvent a thin film of polymer remained on the coil. The wire was loaded on a probe and positioned in the center of the ion source. The wire heating current was programmed as follows: from 0 to 1 A at a rate of 40-80 mA/s (about 40-80 "C/s, up to about 1000 "C), corresponding to 12.5-25 s of total experiment time. The CI reagent gas mainly used was isobutane, at a pressure of 0.3-0.7 mBar. The electron energy was set to 100-150 eV (for positive ions), and the emission current was 0.2 mA. The resolution was normally set at a value such that unit mass resolution was achieved at masses above the average molecular weight of the polymer under study when the 95 Q instrument was employed. When the 8400 mass spectrometer was used, the resolution was kept at m/6m = 1000 (10% valley)throughout all experiments, otherwise its data system could not process all signals (see the Results and Discussion). Upward exponential (95Q and 8400)or quadratic (8400)scans of the magnetic analyzer was set up at full or reduced accelerating voltage, depending on the instrument used and the mass range investigated;accelerating voltages were 750-3000 V for the 8400 mass spectrometer and 2500-5000 V for the 95 Q. A 20-kV postacceleration and conversion dynode is present on both instruments, which minimizes the mass-discrimination effects of the detector. Cycle scan times of 0.5-1 s were adopted for the magnetic analyzer: shorter scan times gave minimal spectrum distortion but limited the mass range which could be investigated, whereas 1-8 scans yielded large mass range spectra, which in some cases could not be reproduced perfectly. During the current ramp of the DCI wire, some form of signal is produced only for 3-5 scans, provided that sufficientlydiluted solutionswere sampled. Under such experimental conditions, the scan showingthe highest total ion current gave the mass spectrum which exhibited the most accurate molecular weight distribution; later spectra usually showed partial or complete thermal degradation of the polymer. Gel-permeation chromatography experiments were run on a Waters 600 chromatograph, using four Waters Microspiragel columns (103-106 Da); tetrahydrofurane was employed as a solvent.
RESULTS AND DISCUSSION Polymer Spectra. Figure 1 represents the positive ion DCI mass spectrum of a branched polysiloxane, which is industrially used as an additive for polymers and whose structure is given in the figure. This spectrum shows a sequence of protonated molecular ions of the oligomers from the 6th to the 31st member. Since this polysiloxane was a raw synthetic product, some impurities were present and can be distinguished as protonated molecular ions between the major peaks in the low mass range (upper part of Figure 1); in the high mass range, represented in the lower part of Figure 1,the corresponding signals are indistinct and contribute to the chemical background. Although this sample is not a
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
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loo;
so0
1600
2400 m/z
'
3000
Figure 2. Positive ion DCI mass spectrum of a poiynorbornene, synthesized in laboratory. 2000
3000
4000
5000 miz
so00
M25
lo] 50
provides, in an extremely short time, all main information desired, including molecular weight distribution, purity, and exact molecular weight of oligomers, from which conclusions regarding the nature of the terminal groups can be inferred. Some commercialpolymers,with average molecular weights provided by the manufacturers, were analyzed with the aim to compare the distribution of peaks obtained from the DCIMS experiment with known molecular weight distributions. There are two common methods to calculate the average molecular weight of a di~tribution,2~ namely the number-average molecular weight (M,) and the weight-average molecular weight (A&,), respectively. They are defined by eqs 1 and 2
CN,M,ICN, M,,, = CN,M,"ICN~M~ M, =
6000
7000
'
Flgurr 1. Positive ion DCI mass spectrum of a branched polysiioxane, synthesized in our laboratorles: (a) mass range from 1350 to 0100 Da; (b) mass range from 5800 to 8100 Da.
reference material, its mass spectrum is interesting because high intensity signals are detected over the entire mass range from mlz 1500 to mlz 8000 (actually the mass limit of the 8400 magnet). This represents by far the highest mass molecular ion ever produced by DCI-MS. It is also remarkable that the spectra were obtained at greatly reduced acceleration voltage (750 V), with limited effect upon the sensitivity. No evidence of molecular ion fragmentation is present in the spectra reported in Figure 1. For example, no peaks can be attributed to the cleavage of the monomer side chain. This observation contrasts with the conclusions drawn in early reports on DCI,11ggJ2 affirming that this technique provides structurally informative fragmentations. The different behavior from literature reports is reasonably due to the low temperature of the ion source, which virtually suppresses the fragmentation, while fragment ions are produced as the source temperature is increased. A clear evidence of this ion source temperature effect in DCI experiments was already reported in previous papers, both for synthetic macrocycles16 and biological substances.14 Of course,also the molecular structure has a large influence on the extent of fragmentation: for example, cleavage of glycosidic bonds in polysaccharide residues is partially observed even at low ion source temperature.14 Figure 2 shows the DCI mass spectrum of a polynorbornene. As in the previous example, a sequence of equally spaced peaks is observed, representing the protonated molecular ions of the oligomers, from the 6th to the 24th member. A second sequence of minor peaks in the spectrum corresponds to the adduct ions formed by attachment of tert-butyl ions to the neutral oligomers ([M + 57]+). Again in this spectrum, fragment ions are not present. For preliminary characterization of synthetic mixtures of this sort, the DCI spectrum
where Ni is the number of moles of molecules (the abundance factor) with molecular weight Mi. The ratio of M,IM,, called polydispersity factor, measures the width of the distribution, and for narrow distributions (as it is common in polymer standards) should not be far from unity. Values of these three parameters, obtained experimentally by DCI-MS for various standard polymers, are reported in Table I. For a poly(ethy1ene glycol) (PEG) sample with declared molecular weight of 2000, the DCI mass spectrum reported in Figure 3a represents a reasonable description of the molecular weight distribution. In fact, the experimental average molecular weight is close to that declared by the manufacturer and the polydispersity factor is close to one, as expected. The main peaks correspond to the protonated molecular ions, but a small amount of fragmentation is evident (e.g.loss of water). A further sequence of minor peaks could be either due to the formation of adduct ions between neutral oligomers and tertbutyl ions, or to a fragmentation process in which the C-C bond is cleaved. Extensive discussion on the fragmentation modes of polyglycolsinduced by LD,31 FAB,32 and 252Cf plasma desorption33 has been reported. Despite the success obtained in the characterization of the PEG 2000 sample, DCI-MS is not the technique of choice for the study of polyglycols. Our attempts to obtain molecular weight distributions from heavier PEGS were not successful. The DCI mass spectra of PEG 3400 and PEG 5000 showed extensive fragmentation and, for the latter sample, some evidence of thermal decomposition. These processes are probably due to the rather polar character of polyglycols, requiring high temperature for the vaporization. Difficulties in obtaining molecular weight distributions for polyglycols, due to extensive fragmentation, was noted in experiments (31) Mattern, D. E.; Hercules, D. M. Anal. Chern. 1986,57,2041-2046. (32) Lattimer, R. P. Int. J.Muss Spectrorn. Ion Processes 1983/1984, 55, 221-232. (33) Chait, B. T.;Shpungin, J.; Field, F. H. Int. J . Mass Spectrorn. Ion Processes 1984, 58, 121-137.
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Table I. Polymer Number- and Weight-Average Molecular Weights Determined by DCI-MS from Experimental Relative Peak Intensities and by GPC
DCI-MS sample polysiloxane polynorbornene poly(ethy1ene glycol) (PEG 2000) polystyrene (PS 1250) polystyrene (PS 2700) polystyrene (PS4000) polystyrene (PS1250) negative ions polystyrene (PS2700)negative ions a
MIl 3414 1824 2110 1322 2644 3906 1217 2760
GPC MwIMn 1.14 1.09 1.02 1.07 1.02 1.05 1.06 1.03
M W
3908 1990 2144 1416 2703 4090 1292 2847
M"
Mw
MwIMn
2339"
2426a
1.04
2570 3744
2805 4142
1.09 1.10
Calibration was performed using polystyrene standards. 1001
"30
-Y
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50
loo]
2000
Y 4 0
,"
4
50
2000
1200
I
4600
Flgurr 4. Positive ion DCI mass spectrum of with average molecular weight of 4000 Da.
miz
6000
polystyrene standard,
I
I
b
,
50
1200
2000
1200
2000
50
2800
mlz
spectrum of poly(ethylene glycol) standard, with average molecular weight of 2000 De: (a) Ion source temperature T, = 50 OC; (b) T, - 140 OC; (c) T, = 200 OC. Flgurr 3. Positive ion DCI mass
employing a variety of ionization m e t h ~ d s ~even l - ~with ~ low molecular weight PEGS. The declining relative abundance of molecular ions with increasing molecular weight of PEG oligomers was alsoreported earlier.33 However, the Occurrence of a different ionization mechanism makes LD-FTMS25 and
electrospray i0nization3~more suitable techniques for the study of polyglycols. Further problems arose when our PEG samples were analyzed by the 8400 instrument: when fast scanning of the magnet was required, the data system was not fast enough to process the whole data arising from peaks very close to each other, so that the spectra were often truncated. This problem was not encountered with the 95 Q instrument. Most of our experimental work, including method setup, was done with three polystyrene (PS) samples,for which many comparisons with literature data could be made. A demonstration of the effectiveness of DCI-MS for the determination of molecular weight distributions in PS samples is given in Figure 4. This mass spectrum shows a sequence of protonated molecular ions of PS oligomers. Sincethe distribution is quite wide, peaks are present up to 6000 Da. This experiment was executed on both of the instruments available and the distributions obtained thereby were in good agreement ( M , = 3906 and 3987 Da, respectively). The average molecular weights calculated from these distributions were also in good agreement with the declared value and with M , and M , values measured by GPC (see Table I). For both PS 2700 and PS 4000, DCI-MS measures higher M , and lower Mw values than GPC, i.e. the polydispersity factor M,/M, is slightly lower in DCI-MS determinations than in GPC. The agreement between DCI-MS and GPC data is within f 4 % , but the limited set of comparisons available do not allow one to consider this figure surely established. A test was run with the PS 2700 sample, aimed at establishing the minimum amount of sample necessary for the analysis, although polymers are generally available in large amount for the characterization. Anyway, this test was considered useful for better understanding the ionization prdcess itself (see below). A total amount (sample loaded) (34) Musselman, B. D.; Tamura, J.; Cody, R. B.; Kassel, D. B. 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 19-24, 1991.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, lQQ2 1883
Table 11. Number- and Weight-Average Molecular Weights Determined Experimentally for PS 2700 Sample (Nominal Average Molecular Weight 2700 Da) as a Function of the Analyzer Scan Speed. cycle time (8) 0.575 0.711 0.849 1.052
Mn= M,= M, = M,= M, = M,= Mn= M, =
1 2651 2714 2609 2114 2451 2553 2409 2501
scan cycle 2 2620 2679 2506 2625 2615 2715 2394 2522
3 2656 2115 2540 2643 2500 2615 2508 2606
av 2644 2703 2552 2661 2522 2628 2437 2545
loo]
U
21 20 52 41 84 82 62 53 800
Measures repeated 3 times and averaged.
1200
1600
2000 mi2
100
of polymer equivalent to 5 ng was sufficient to obtain a mass spectrum showing an excellent signal-to-noise ratio (about 1:20)at 1000resolution (5% ' valley). A t a resolution of 3300, the uncertainty of oligomer peak intensities was still low enough to allow a correct measurement of the average molecular weight. Under such conditions, individual oligomers were present in the tens of femtomole range. The repeatability of distribution determinations was systematically checked by repeatedly (three times each) recording the DCI mass spectrum of PS 2700 at different analyzer scan speeds and mass ranges. The results are summarized in Table 11. It is evident that, using a scan time of 0.575 s, the M,and M, values are very close to the nominal value and that these determinations are highly repeatable with a small absolute standard deviation (u). From Table I1 it is also possible to note that a continuous negative shift of M,and M, from the nominal value is produced by increasing the analyzer scan cycle time. Given the short duration of the DCI signal, the repeatability of M,,and M, determinations also decreases as the scan time increases. This effect is quite similar with that observed in capillary column GC-MS experiments, where the distortion of mass spectra decreases as the analyzer scan rate increases since the concentration of eluted analytes varies during the scan time. However,in DCIMS experiments a second element should be taken into account, namely the simultaneity of the scan time with the vaporization time of oligomers with different molecular weights. Although the two contributions (vaporization time and residence time of oligomers in the ion source) cannot be separated, the experimental results point out that decreasing the scan time is beneficial. Evidence of reproducibility was obtained by comparing the molecular weight distributions derived by experiments run on the two different instruments, which gave nearly identical results. Unlike the other polymers studied, the PSs contain an aromatic ring, which, in principle, can capture the nearthermal energy electrons present in the CI plasma. Therefore, we recorded the negative ion DCI mass spectrum of PS samples (two spectra are reported in Figure 5), in order to establish whether these spectra are representative of the polymer molecular weight distribution. As evident from Figure 5,sequencesof peaks due to molecular anions produced by electron capture are obtained. The distributions are centered around the expected averages (declared M, values are 1250 and 2700, experimental M, are 1217 and 2760, respectively). The spectra show high intensity, good S/N ratios, and excellent repeatability. Thus, for the characterization of PS samples, the negative ion DCI mass spectra can be utilized as effectively as the spectra of positive ions. From the comparison of results shown in the present paper with literature data, it turns out that DCI is approximately
50
Figure5. (a)Negathre ion DCI mass spectrumof polystyrenestandard, with average molecular welght of 1250 De; (b) negative ion DCI mass spectrum of polystyrene standard, wlth average molecular welght of 2700 Da.
as efficient as LD25-27and SIMS20729in producing intact molecular ions from polymeric samples and in measuring molecular weight distributions as well as average molecular weights. In some respects, FD has recently proved to be the most powerful technique for the analysis of PS samples,*3 yielding impressively clean mass spectra up to 16 m a , although some other polymers might not give rise to such high-quality spectra as PS.24 On the other hand, the characteristic advantages of DCI are its minimal need of sample manipulation (the sample should not be spiked with salts), its general availability, and its ease of execution. Critical Parameters. As discussed briefly above, in the analysis of high molecular weight compounds by DCI-MS it is extremely important to keep the ion source cold. Heating from the electron-emitting filament should be avoided by minimizing the time it is turned on. A high source temperature promotes both fragmentation and thermal decomposition. DCI-MS experiments run at 140 "C on PS 2700 and PEG 2000 samples showed greatly increased fragmentation (Figure 3b), and even some thermal degradation became evident as the ion source was heated to 200 "C (Figure 3c). Thus, the CI reagent gas must be as cold as possible in order to extract efficiently and rapidly the excess thermal energy from the molecules of polymer. Another important experimental parameter is the amount of sample loaded on the DCI wire, which should be rather small. Experiments run by loading 5 pg of PS 2700 on the wire again showed evidence of thermal decomposition. Optimal sample loading is in the 50-100-ngrange, typically correspondingto about 1 ngof each individualoligomer. When the layer of sample on the DCI wire is too thick, it is reasonable to assume that overheating of the inner molecules takes place before they can interact with the reagent gas.
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The third important parameter is the temporal gradient of the heating current, which should be high. The time required to vaporize the sequence of oligomers, having different molecular weights and inherently different volatility, should be small compared to the scan time of the analyzer, otherwise a significant distortion of molecular weight distributions arise. Morevoer, with some polymers, too low a current gradient was found to induce thermal degradation processes since the molecules were heated for a long time on the wire before the vaporization temperature was reached. The beneficial effect of rapid heating has been known for a long time (see ref 2 and references cited therein). A high scan rate of the analyzer is also essential since the duration of the signal in a DCI experiment is short. Each oligomeric peak lasts for about 2 s from its appearance to disappearance. This period encompasses both the vaporization time and the residence time of the neutral and charged oligomer inside the ion source. Cycle scan times above 1 s produce a significant distortion of molecular ion distributions and incorrect evaluation of the average molecular weight of the polymer. From the data reported in Table 11,it is evident that cycle scan times of about 0.8 s are not perfectly safe, whereas 0.5-0.6-sscans led to measurements of the average molecular weight which are accurate and to repeatable distributions. The modern magnetic mass analyzer are generally capable of scanning one decade in less than 0.5 s and resetting in 0.2 s, but older instruments might not be able to perform in this way. On the other hand, some of the most powerful magnets with an extremely large mass range exhibit significant hystheresis, which precludes fast scanning. Thus, when a large mass range must be investigated, the use of analyzers such as time-of-flight, ion cyclotron resonance, or ion trap, which are commonly associated to pulsed ionization techniques, might be beneficial. Further parameters briefly studied were the type and the pressure of the reagent gas. The use of methane, as an alternative to isobutane, produced a slight modification of positive ion mass spectra, with evidence of increased fragmentation, as expected from this smaller proton affinity. Variations of the isobutane pressure seemed to affect mainly the sensitivity. Seeing that an improvement of the signal cannot be obtained by increasing the amount of sample analyzed, the choice of a correct gas pressure is important in order to get reliable distributions. Mechanism. In the early applications of DCI the mechanism of vaporization and ion formation was frequently argued.4 The discussions tried to establish whether rapid vaporization of the sample preceded a gas-phase chemical
ionization process12 or the ions were produced at the solid surface of the probe and then thermally desorbed.11 The conclusions reached heavily depended on the experimental conditions chosen and, in particular, on the material where the sample was deposited: Teflon,'J1 ~ e s p e lor , ~metal.8 For example, Munson,ll using a glass/Teflon sample support, observed no sample ions when the source temperature was low, while Hostettmanng recommended that the ion source temperature be kept as low as possible. Therefore, it is possible that the different laboratories were observing different processes. In the present study, the ability of DCI to produce intact molecular ions from compounds with very high molecular weights has been demonstrated. This observation raises again a question about the mechanism of the whole process. One wonders how such involatile compounds can be sublimated without undergoing thermal degradation. As a matter of fact, the present experiments indicate a kinetic control of the vaporizationlionization process. Apparently, the thin layer of molecules covering the DCI wire are readily vaporized as soon as the wire is heated to their sublimation temperature; the overheated molecules do not decompose in the gas phase if the excess of thermal energy is immediately removed by multiple collisions with the reagent gas. In other words, decomposition does not take place when the kinetics of thermal stabilization is faster than that of degradation. The ionization process can occur subsequently in the gas phase. If the reagent gas is cold and its pressure is rather high, the deexcitation process is faster and more efficient. If the wire is overloaded with the sample, the vaporization process takes more time to occur and the inner layers are decomposed before they interact with the reagent gas. Also in this case, the observation of a longer signal of pyrolized products is justified. A specific investigation on the ionization mechanism in DCI, employing a variety of compounds and reagent gases, is in progress.
ACKNOWLEDGMENT We wish to acknowledgeG. P. Chiusoli for kindlyproviding the polynorbornene sample, R. G. Cooks and G. Guglielmetti for helpful discussions, and E. Tampellini for executing the GPC analyses.
RECEIVEDfor review March 2, 1992. Accepted May 20, 1992. Registry No. Polystyrene, 9003-53-6;poly(ethy1ene glycol), 25322-68-3; polynorbornene, 25038-76-0.
