104
Anal. Chem. 1985, 57, 104-109
Impurities in Silicon Tetrafluoride Determined by Infrared Spectrometry and Fourier Transform Mass Spectrometry W. D. Reents, Jr.,* D. L. Wood, and A. M. Mujsce AT&T Bell Laboratories, Murray Hill, New Jersey 07974
Slllcon tetrafluoride, SIF,, of high purlty with respect to elther hydrogen- or oxygen-containing species Is required by the optical wavegulde telecommunlcatlons and photovoltaic Industrles, respectively. Fourier transform mass spectrometric and gas-phase Infrared spectroscoplc methods were developed to ldentlfy and quantlfy these detrlmental Impurltles. Identlflcatlon by mass spectrometry was accomplished by maklng exact mass measurements on the product Ions from chemical lonlzatlon uslng SIF, Itself as the reagent gas. Quantlflcatlon was based upon the klnetlcs of lon/molecule reactlons lnvolvlng the lmpurlty molecules wlthout the use of reference compounds. Analyses of samples from several sources showed the prlnclpal oxygen- and hydrogen-containIng lmpurltles to be hexafluorodlslloxane, SI,F,O (0.3-3 mol %), and trlfluorosllane, SIF,H (0.2-0.6 mol %). Other lmpurltles, such as octafluorotrlslloxane, SI,F,O,, pentafluorodlslloxane, SI,F,HO, dlfluorosllane, SIF,H,, and hydrogen chloride, HCI, were also detected. Extlnctlon coefficients for the Infrared bands of hexafluorodlslloxane, SI,F,O, were determlned from residues extracted elther from slllcon tetrafluoride, SIF,, or from the reactlon products of hexachlorodlslloxane, si,ct,o, with antimony trlfluorlde, SbF,, by uslng cryogenic fractionation techniques.
Silicon tetrafluoride, SiF,, is being considered for production of amorphous silicon films in the photovoltaic industry ( I ) . In the telecommunications industry it is used for fluorine doping of optical waveguide fibers (2-4). For these applications very low levels of specific impurities, e.g., oxygen-containing species for photovoltaic components ( 5 , 6 )or hydrogen-containing species for the telecommunications industry (2-4), must be maintained. Methods for analyzing silicon tetrafluoride for these impurities, however, are poorly developed. This is due to its inherent reactivity and the lack of suspected impurities commercially. SiF4also has a very low boiling point which precludes study of the liquid phase, which was very profitable for the analysis of SiC1, (7). We have found that infrared spectrophotometry and ion/molecule chemistry coupled with mass spectrometry are useful techniques to identify and quantify some of the impurities commonly found in SiFk EXPERIMENTAL SECTION Mass Spectrometry. Mass spectrometric measurements were
performed with a Nicolet FT/MS-1000 Fourier transform mass spectrometer (8-10). Typical operating conditions were as follows: magnetic field strength, 2.30 T; electron ionization energy, 70 eV; cell dimension, 2.54 cm3; trapping plate voltage, 1 V; sample pressure, 3 X lo-' torr; trapping time, 10 ms to 30 s; data points acquired, 8K; mass range examined, 40-500 amu; partial ejection of SiF3+during ion formation. For exact mass measurements, the ions from SiF, (SiF+,SiF2+, SiF3+,and SiF4+)were used to calibrate the mass axis. As other ions were identified by their mass, their calculated mass was added to the calibration table. Operating conditions were the same as previously mentioned except that 64K data points were acquired.
Measured masses were within 20 ppm of the calculated values. Double resonance experiments were performed by ejection of either SiF3+or SiF4+immediately following ion formation. The disappearance of a product ion due to ejection of SiF3+indicated that the ion was formed by halide (presumably fluoride) abstraction (11).The disappearance of a product ion due to ejection of SiF4+indicated that the ion was an adduct of SiF3+and an impurity molecule ( 2 1 ) . The event sequence differed from the normal sequence only by the injection of rf energy at the cyclotron frequency of the ion to be ejected (SiF3+or SiF4+)for a 1-ms duration at 1 ms after ion formation ended. Samples were introduced into the mass spectrometer through the gas/liquid inlet with an adjustableleak valve. The commercial materials were sampled directly from the lecture bottles via high vacuum valves and connected to the mass spectrometer through stainless steel tubing. The tubing and valve were evacuated and heated (>lo0 "C) prior to sampling the SiF, in order to remove air and moisture. The inlet was then flushed twice with SiF& Next the cylinder was opened momentarily to introduce a sample of gas between the cylinder and the high vacuum valve. The high vacuum valve was then opened to admit the sample to the mass spectrometer inlet. The quantitation standard (acetaldehyde, acetone, ethyl acetate, and 2-methyltetrahydrofuran) used commercially available chemicals without additionalpurification. The quantities added were measured using a capacitance manometer. All four chemicals were placed in a 500-mL glass bulb with an O-ring sealed stopcock. Infrared Spectrophotometry. Infrared analyses were performed with a Perkin-Elmer Model 683 infrared spectrophotometer. Our procedures, to be discussed later, for identification of fluorinated siloxanes required purification from gaseous samples containing large percentages of SiF& The necessary separations were carried out in a vacuum-tight,glass gas handling system (22). The system contained U-tube traps made from 2 cm diameter tubing, each having about 10-cm lengths immersed in low-temperature baths of ethanol contained in 7 cm diameter Dewar vessels. The infrared absorption cell, in place in the spectrophotometer and at room temperature, was attached to the vacuum manifold with Teflon barrel vacuum valves, and the electronic capacitance gauge was provided for digital pressure measurements. Commercial SiF,, which was available in lecture bottle cylinders, was admitted to the system through stainless steel fittings. For other samples, as in the case of the fluorination products of Si2C1,0, the effluent from the reaction vessel was trapped with liquid nitrogen, and the trap containing the frozen sample was connected to the manifold for purification. A valved flask was attached to the manifold to collect samples for mass spectrometry. For purification, a small amount of impure gas was frozen out in the first trap by liquid nitrogen and then warmed to a suitable temperature such as 163 K (-110 "C). At that temperature SiF4 has a large vapor pressure (160 torr) (13),but the impurities have low vapor pressure (3 torr for Si2F,0) (14). After being pumped to a low equilibrium pressure (at 163 K), the residue was warmed to room temperature to obtain the infrared spectrum of a gaseous sample of the less volatile fraction. RESULTS AND DISCUSSION Infrared Spectrophotometry. We had noticed in our
study of the infrared spectra of various samples of SiF,-containing mixed chlorofluorides of silane that the relative intensities of some of the absorption bands in SiF4 were not constant (12). This suggested the presence of impurities with varying concentrations. Some of the same absorption bands
0003-2700/85/0357-0104$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
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5
100 100-
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WAVENUMBER, CM-'
Figure 1. Infrared spectra of gaseous SiF4 samples in a 10-cm cell. (a) Commercial material 146 and 1 torr, containing about 2 % SI,F60. (b) The same material purified by cryogenic fractional volatilization, 148 and 0.8 torr, containing about 0.2% Si,F,O.
were present in commercial samples of SiF& Because one prominent band occurring at 1200 cm-l could be assigned to a vibration of the Si-0-Si group on the basis of its similarity in width and frequency to that of SiZCl6Oand other chlorinated siloxanes (15),we attempted to identify the absorption bands of fluorinated siloxanes in order to compare them with those of the impurities in SiF& No pure compounds were available commercially for direct comparison with the observed spectra, so two indirect methods of identification were attempted. In the first method the impurities in SiF4 were sufficiently enriched in commercial samples by cryogenic fractional volatilization so that infrared difference spectra showed clearly the band intensities for the impurities. In the second indirect method an attempt was made to synthesize the lowest molecular weight fluorinated siloxane, SizF60,by fluorination of SiZCl6Owith SbF3 (16): The vapors of SizC160were passed through crushed SbF3 in a stream of helium with the effluent passing through an infrared absorption cell in place in the spectrometer. The effluent was then trapped with liquid nitrogen. We found major quantities of SiF, in the reaction products and very intense infrared absorption bands at the frequencies observed for the impurities in commercial SiF,, especially at the initial stage of the reaction. As the reaction progressed there was evidence that other species were being generated. However,
on purification of the early effluent by the same cryogenic means as for commercial SiF4we obtained a fraction whose spectrum very closely matched that of the impurity in the commercial product, now identified as Si2F60. Figure l a shows the spectrum of a commercial sample of SiF4, and Figure l b shows the spectrum obtained after its purification by cryogenic fractional volatilization. The most prominent impurity absorptions occur at 838 cm-l and at 1200 cm-l, although the latter is superimposed on a moderately intense band of SiF4. The impurity absorption at 1200 cm-l therefore causes a difference in the relative intensities of the components of the absorption group of SiF4 (at 1177,1192, and 1202 cm-'). The underlying impurity absorption only shows clearly when the spectrum of the purified SiF4is subtracted from that of the impure material. The difference spectrum is very similar to that of Figure 2 recorded from the residue from fractional volatilization of commercial SiF4. It is also the spectrum we obtained by purification of the reaction products of Si2C&0and SbF% Thus, it can be concluded that the infrared spectrum of Figure l b is that of hexafluorodisiloxane, Si2F60.The mass spectrometricresults (vide infra) confirm the presence of a compound whose elemental composition is SizF60. In order to analyze quantitatively the amount of SizF60in any given SiF, sample, it was necessary to obtain the absorptivities for the infrared bands. Unfortunately our purified SizF60 preparations always contained appreciable partial
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
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1800
2000
1600
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I
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800
600
400
200
Flgure 2. Infrared spectra of Si,F,O extracted from SiF, during puriflcation by cryogenic fractional voltatllization: 10-cm cell; 13- and 0.3-torr pressure; dashed cclrve, empty cell.
Table I. Absorptivities for Infrared Absorption Bands frequency, cm-' a,(torr cm)-' frequency, cm-'
Table 11. Infrared Analyses of SiF4 Samples
a, (torr cm)-'
measure-
SizFBO 2040 1962 1825 1450 1348 1263 1238 1200 1028 981
6.30 x 3.63 x 2.94 x 6.30 x 9.51 x 1.85 x 2.16 x 1.05 X 9.50 X 1.59 x
10-5 10-5 10-4 10-5 10-5 10-3 10-3
lo-' 10-3
912 838 660
3.54 x 10-4 5.66 x 10-3 1.52 x 10-4
585 432 401 388 375 304
1.47 x 4.29 x 1.19 x 5.51 X 4.41 x 4.03 x
10-4
10-3 10-2 10-3 10-4
8.98 x 3.56 x 1.w x 1.44 x 5.50 x 1.93 X
10-5 10-4 10-4 10-4
10-4 lo4
commercial
av concn no. of of Si2F60, determina% tions
21-758
2.19
commercial A "purified
14-267
commercial B
38-742
0.50 (0.150.80) 0.44
commercial
48-702
1.31
10
av dev, %
0.06
A
c4
8
3
0.02
10
0.01
"Cl detected in spectra for 700 torr at a level of about 0.05-0.1 %.
SiF4 2045 1823 1285 1202 1192 1177
source
ment pressure range, torr
1031 778 404 390 373 265
4.61 X 4.36 x 6.39 x 1.36 X 4.60 x 4.13 x
10-5 10-3
lo-' 10-3 10-5
pressures of SiF,. It was, therefore, necessary to subtract these pressures from the total pressure measured with the capacitance gauge. We obtained fairly consistent results by using the absorptivities of the infrared bands of SiF4to determine the partial pressure of that component and assumed the rest to be the Si,F60 partial pressure. By this means the absorptivities shown in Table I were obtained; the ab~orptivities for SiF, arg also included in the table. It is fairly obvious that the use of the bands of SiF4at 1177,1192,and 1202 cm-I and those at 373, 390, and 404 cm-' are not relialjle for the determination of the partial pressure of SiF4unless it is known that SizF60is present only in sufficiently low concentration so that it will not interfere. Using the absorptivities in Table I we used the BeerLambert law to conveft the peak infrared absorbances to concentration for some SiF, samples as shown in Table 11. It is quite clear that commercial sample B contains appreciably less SizF60than samples A or c. It is also possible to express the absorptivities of Table I in terms of the minimum detectable quantity of Si2F60in 1 atm of SiF4 in a 10-cm cell if the minimum detectable peak absorbance at the analytical frequency is 0.01. This corresponds to a minimum impurity concentration of 232 ppm (mol/mol) for the 820-cm-' band which is not subject to in-
terfering absorption. If the more intense 1028-cm-' band can be used the minimum detectable quantity under the assumed circumstances would be about 14 ppm. In the course of analyses with samples of SiF, near 1atm of pressure, several very weak absorption bands were noted but with insufficient intensity to allow certain identification or quantitative concentration estimates. Some of these occurred at 2620,2780, and 2840 cm-'. It was possible, however, to positively idehtify the presence of HC1 in commercial sample C at a level of about 0.05%-0.1% from the line spectrum near 2900 cm-l, but it was not possible to determine the concentration with accuracy. In the case of commercial sample A, we tentatively identified the Si-H stretching fundamental of SiF3H at 2315 cm-l, indicating the presence of that impurity at a level of about 0.1%. But this frequency coincides with part of the atmospheric absorption of COZ, so an accurate concentration could not be determined. These impurity absorptions require further study with absorption cells capable of sustaining greater than atmospheric pressure or having path lengths longer than 10 cm. Mass Spectrometry: Identification. Gas chromatography is an excellent method for separating components of a mixture. When coupled with a mass spectrometer, the gas chromatography introduces into the mass spectrometer the pure components for detection and identification. For reactive materials such as SiF4a gas chromatograph cannot be used since reactions may take place in the chromatograph that would increase the level of impurities. This problem can be circumvented by directly introducing the sample into a Fourier transform mass spectrometer
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Table IV. Impurities in SiFl Detected by Mass Spectrometry
Table 111. Ion Intensities in SiFdat 70-eV Ionization Energy
obsd ion SiF3(M)+
F+ Si+ SiF+ SiFzt SiF3+ SiF4+ standard spectruma spectrum after partial removal of SiF3+u
compound, M 10 74
10 100
22
100 40
5 54
SiF3H SiFzHz SiF30SiF3 SiF30SiF20SiF3 SiF30SiF2H
Ion intensities are normalized to base peak = 100. Ions due to Si isotopes or minor impurities have been deleted from table. without fractionation and exploiting its capability for low pressure ( lo-' torr) chemical ionization. Normally, high pressures (- 1torr) are required for chemical ionization, but a FTMS can trap ions in a magnetic field for operator-selected times of milliseconds to several seconds, making chemical ionization possible. Chemical ionization simplifies the mass spectrum by producing only a few ions for each molecule present. The number of ions from a complex mixture can still be unmanageable. However, the reaction time depends on the reactant molecule's concentration. Ions related to molecules of a specific concentration are enhanced relative to ions from molecules with much lower or much higher concentrations at a specific trapping time. This not only reduces the number of ions observed but also provides a mean for quantifying the various compounds present as impurities. This latter aspect will be described in detail later. Although a smaller subset of ions is present when the reaction time is varied, the identification of the molecules which form the observed ions is still formidable. Our previous paper on the ion/molecule reactions of SiF, (11)indicated that two types of reactions are possible. They are adduct formation (reaction 2), which involves SiF3+transfer to a molecule, or halide abstraction (reaction 31, which involves removal of a fluoride ion from a molecule. These reactions will continue N
+ SiF3++ SiF3H
--
SiF3+(SiF4) SiF3H
SiF3+(SiF3H)+SiF, SiF2H+
+ SiF,
107
(2) (3)
with compounds of lower abundance to form their respective ions. A limitation is that if a compound of lower abundance is less reactive than a compound of greater abundance, then no ions related to the less reactive compound will be observed. From reactions 2 and 3 one can see that ions of 85 amu greater than the molecular weight of the compound and ions of 19 m u less than its molecular weight will be observed. The mass difference from the molecular weight may be determined for each ion by double resonance (17), a procedure which identifies the reactant ion forming the ion of interest and thereby indicates the reaction type (adduct formation or halide abstraction). Note that if the compound has no abstractable fluorides, then only reaction 2 occurs. Although both sets of reactions are present, the first type of reaction is available to more types of compounds. Once a suspect molecular weight is calculated the molecular formula may be determined. Assistance is available by assuming that the impurities present in SiF, are the fluorinated analogues to impurities in SiC1, (7). Thus, first we check as possibilities partially fluorinated silanes and fluorinated siloxanes. A high-accuracy mass measurement will then eliminate the incorrect prospective molecular formulas. The mass spectrum of SiFl is presented in Table 111. Analytical data were obtained with partial ejection of SiF3+ during ion formation (cf. Table 111). This has no effect on the analytical results; it simply provided a means to have comparable intensities for the two reaction pathways. Table IV presents a listing of molecules identified by this procedure and the associated ions observed. The most reasonable
..
(M - F)+
SiF2Ht Si2F60+ Si3F702+ Si2F4HO+
Si2F6H+ Si2F6Hz+ Si3F90t Si4FIIO2' Si3F8HOC
- - -.Si F4+
12----
-si$;
si
F
O+ +
Figure 3. Time variation of select ions produced from impure SiF, in Fourier transform mass spectrometer: P(SiF,) = 2 X IO-' torr.
structure for each formula is presented. Figure 3 illustrates a typical time variation of intensities for selected ions. Mass Spectrometry: Quantification. Our method for quantification by mass spectrometry is kinetic; we relate the formation rate of an ion to the concentration of the corresponding neutral molecule. For the case where small amounts of Si,F60 are present in SiF,, we observe the reactions
+ SiF4 -% Si2F7++ F SizF7++ SizF60A SiF3(Si2F60)++ SiF4 SiF3+ + SizF60 Si2F50++ SiF, SiF4+
-
(4)
(5)
k6
(6)
Quantification of SizF60in SiF4 may be based either upon SiF3+adduct formation (reaction 5 ) or fluoride abstraction (reaction 6). The relevant equations for quantification based upon SiF3+adduct formation will now be derived. The rate of formation of SizF7+and SiF3(Si2F60)+are d[SizF7+]/dt = k4[SiF4+][SiF,]
(7)
d[SiF3(Si2F60)+] / d t = k6[SizF7+][Si2F60]
(8)
Dividing eq 8 by eq 7 gives d[SiF3(SizF60)+]/ a t d[SizF7+]/ a t
k5 [SizF7+][Si,F@] = --k4 [SiF4+] [SiF41
(9)
We assume that the ions are trapped efficiently in the mass spectrometer,that all SiF3+adducts will react with a molecule of lower abundance to form another adduct, and that the total concentration of SiF3+adducts is a constant. Therefore [SiF,+] = [Si,F,+] (10) In eq 10,we do not refer to an instantaneous relation. Instead, we equate the total amount of SiF3+adducts a t a given time to the total amount at another time. Equation 10 is a specific application of this.
108
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Table V. Evaluation of Mass Spectrometric Quantification by Kinetics
compound
re1 molar concn
measured re1 molar concn
acetaldehyde acetone ethyl acetate 2-methyl-THF
100 2.5 0.23 0.064
100 2.2 0.19 0.043
We also assume that the rate constants for adduct formation are equal. Although this is not strictly true, it has been shown that such rate constants do not differ greatly (11). When the assumptions previously discussed are applied, eq 9 may be simplified d[SiF3(Si2F60)+] /at
=-
d[Si2F,+]/et
[Si,F,O] [SiFd
(11)
The slopes on the left are initial slopes, Le., before these product ions start to react away. So, the ratio of the initial slopes is proportional to the concentrations of the reacting molecules. This analysis may be continued for any of the observed impurities to give a result similar to that in eq 11. It should be noted that this analysis requires a kinetically controlled condition far from equilibrium. This is attained within the FTMS by using low pressure (-lo-' torr). The assumption that SiF,+ adducts will react with molecules of lower abundance to form another adduct is not strictly correct. The reaction will proceed only if the molecule of lower abundance has a significantly greater affinity for SiF3+. This is true for systems where the main constituent is the first member of a homologous series. The larger members, which are frequently impurities, typically have higher affinities for cations. Compounds of a different chemical class may or may not have a higher affinity. For SiF4,a potential impurity, HF, will not be detected because it has a lower affinity for SiF3+ than does SiF, (11). The subset of fluoride abstraction reactions may also be used for quantification. An equation analogous to eq 11 can be derived in a similar manner. However, no fluoride abstraction from SiF4is observable because no new ion is produced: *SiF3+ SiF, *SiF4 SiF3+ (12)
-
+
+
Therefore, a different compound which undergoes the fluoride abstraction reaction must be used as a secondary reference in the denominator. If SiF3His used as a secondary reference, then the corresponding equation would be d(Si2F50+)/et [Si2F6O] =(13) 8(SiF2H+)/dt [SiF3H] Of course concentrations determined in this manner must be multiplied by the concentration of SiF3H,determined via the SiF3+adduct formation method, to obtain the concentration
relative to SiFb Not all compounds undergo fluoride abstraction reactions. This will, however, provide an internal check on the concentrations determined by the SiF3+adduct formation method for those compounds that do undergo the fluoride abstraction reaction. The detection limit by the mass spectrometric method is adversely affected in three ways. First, a large number of different impurities at low levels strains the assumption regarding a constant concentration of SiF3+adducts (or fluoride abstracting agents). One or more of these ions formed may be more reactive than other ions which are formed simultaneously. The more reactive ions would react away faster to form the less reactive ions. This would cause an underestimation of the concentration of the more reactive species and an overestimation for the less reactive species. Second, hydroxy-containing compounds, e.g., SiF30SiF20Hor H20, which form an SiF3+adduct do not transfer SiF3+to a silicon-containing molecule. Instead they transfer a proton, typically to H20,which has a very high proton affinity relative to other materials present. Thus, the level of these hydroxy-containingcompounds forms our detection limit. This, of course, requires an extremely clean background especially in regard to water. Third, ion loss from the cell (18) limits the trapping time and, therefore, the detection limit. With our system, this limits the trapping time to -100 s which translates into a detection limit of -300 ppm. The validity of this method is demonstrated for a gas mixture whose composition is shown in Table V. The reaction involved was simple proton transfer; the protonated molecules were monitored. When the kinetic method is used, the accuracies decrease with decreasing concentration of impurity. The measured value is still within a factor of 2 at 600 ppm. Although the error may appear larger, remember that no calibration curve for the system was prepared. The impurity levels of SiF4determined by mass spectrometry are presented in Table VI. The concentrationof SizF4H0 is larger relative to Si2Fs0than SiF3H is relative to SiF,. If one assumes that both are formed by reaction with H 2 0
+ H20 5 SizF60 + 2HF SiF, + SiF3H + H20 -kSi,F,HO + 2HF 2SiF4
(14)
(15)
then, kinetically, the concentrations of Si2F60and SizF5H0 should be proportional to their rate constants times the concentration of SiF, and SiF3H,respectively. The concentration ratios of Si2F5H0to SizF60 may be expressed as [SizF5HO] k15 [SiF3H] =-(16) [SiZF60] k14 [SiF,I It follows that the rate constant for formation of SizF,HO, k15, must be significantly larger than the rate constant for formation of SizF60,k14. This has been observed for SiC1, where SiC13Hreacts more readily with HzO than does SiC1, (19, 20).
Table VI. Relative Molar Concentrations of Impurities in SiF, Determined by Mass Spectroscopy
compound SiFl SizFBO Si3F802 SizFSHO SiF3H SiFzHz
commercial sample A adducta av 100
3.4 0.42 0.28 0.36 0.05
C
(3.4)d
0.20 0.10 0.31
f
100 3.4 0.31 0.19 0.34 0.05
commercial sample B adduct" av 100
C
100
0.31
0.30
0.30
ND'
ND'
ND'
0.04 0.78
0.01
0.02 0.78 0.04
(0.78)d
commercial sample C adducta av 100 3.0 0.56 0.26 0.17 0.01
C
(3.0)d 0.28 0.08 0.15
100 3.0 0.42 0.17 0.16 0.01
f f Quantification based upon formation of SiF,+ adduct. Quantification based upon fluoride abstraction. No fluoride abstraction reaction for SiF4. dValues in parentheses are taken from the adduct formation measurements. They are the references for the fluoride abstraction reactions. eNot detected. f No halide abstraction product has been observed. 0.04
109
Anal. Chem. 1985, 57, 109-114
LITERATURE CITED
CONCLUSIONS Agreement between the infrared and mass spectrometric measurements for levels of Si2F60are good despite the lack of pure Si2F60for reference. The detection limits by mass spectrometry are 5600 ppm; accuracies decrease with decreasing concentration to -50% at 600 ppm. For infrared spectroscopy the detection limit for Si2Fs0is estimated to be -200 ppm. The levels of hydrogen- and oxygen-containing impurities in SiF4vary greatly among the samples. Thus, it is important to check the quality of SiF4 if either of these types of impurities pose a problem. Infrared spectroscopy is well suited for quantification of Si2F60,the major oxygen-containing species in SiF& The major hydrogen-containing species, SiF3H, is not conveniently detectable at these levels by infrared spectroscopy, presumably due to low absorptivity. Conventional electron impact mass spectrometry can detect SiF3H by its most abundant ion, SiF2H+. However, it must be resolved from %SiF2+ which is of equal or greater intensity. The mass difference requires a resolution of 18000 to separate these peaks. When a calibration table for quantitation cannot be made due to unavailability of standards, the mass spectrometric method provides a rapid means to estimate the impurity levels in gases. Registry No. SiF,, 7783-61-1;. Si,FnO, 14515-39-0; SiFqH, 13465-71-9; Si3F802,26121-10-8; Si2F6H0,92985-12-1;.SiF2H2;
RECEIVED for review April 16,1984. Resubmitted September
13824-36-7.
24, 1984. Accepted September 27, 1984.
(1) Madan, A.; Ovshinsky, S. R. J. Non-Ctyst. Sollds 1980, 3 5 / 3 6 , 171-181. (2) Muhlich, A.; Rau, K.; Simmat, F.; Treber, N. I€€ Conf. Pub/. 1975, 132, postdeadllne paper. (3) Abe, K. Eur. Conf. Opt. Fibre Commun., 2nd 1976, 133, 59. (4) Kuppers, D.; Koenings, J.; Wilson, H. J. €lectrochem. SOC. 1978, 125, 1298-1302. (5) Otsuka, Toyozo; Kltsugi, Naomichi; Fukinaga, Teruo British Patent 2079282, 1981. (6) Kltsugi, Naomichi; Fujinaga, Teruo; Otsuka, Toyozo British Patent 2 103 198, 1982. (7) Rand, Myron J. Anal. Chem. 1963, 3 5 , 2126-2131. ( 8 ) Comisarow, M. Adv. Mass Spectrom. 1078, 7 6 , 1042-1046. (9) Wilklns, Charles L. Anal. Chem. 1878, 5 0 , 493A-500A. (10) McIver, Robert T., Jr. Am. Lab. (Fairfield, Conn.) 1980, 18-30. (11) Reents, W. D., Jr.; MuJsce, A. M. Int. J. Mass. Spectrom. Ion Proc. 1984, 59, 65-75. (12) Wood, D. L.; Walker, K. L.; Csencsits, R., private communication. (13) "Gmelins Handbuch der Anorganische Chemie", 8th ed.; Verlag Chemie: Weinheim, 1959; Part B, p 624. (14) "Gmeiins Handbuch der Anorganische Chemie", 8th ed.; Verlag Chemie: Weinheim, 1959; Part B, p 654. (15) Wood, D. L.; Mac Chesney, J. B.; Luongo, J. P. J. Mater Scl. 1978, 13, 1761-1768. (16) Booth, Harold Simmons; Osten, Reuben Alexander J. Am. Chem. SOC. 1945, 67, 1092-1096. (17) Comisarow, Melvln B.; Grassi, Valerio; Parisod, Gerald Chem. Phys. Letts. 1978, 5 7 , 413-416. (18) Franci, Thomas J.; Fukuda, Elaine K.; McIver, Robert T., Jr. Int. J. Mass Spectrom. Ion Proc. 1983, 5 0 , 151-167. (19) Kometani, Thomas Y.; Wood, Darwln L., U.S. Patent 4282 198, 1981. (20) Kometani, T. Y.; Wood, D. L., AT&T Beii Laboratories, private communication, 1980.
Search for New Alkaloids in Pachycereus weberj by Tandem Mass Spectrometry Robin A. Roush a n d R. Graham Cooks* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Stephanie A. Sweetana and J e r r y L. McLaughlin Department of Medicinal Chemistry and Pharmacognosy, School of Pharmacy and Pharmacal Sciences, Purdue University, West Lafayette, Indiana 47907 Twenty new alkaloids, of whlch 13 are new natural products, have been discovered in Pachycereus weber1 by tandem mass spectrometry using a mass-analyzed Ion klnetlc energy spectrometer. These observations have been confirmed by chromatography and, in many cases, by simple Synthetic interconverslons from known alkaloids. Particularly slgnlficant is the discovery of several alkaloids having the same molecular formula. Isomer dlstinctions such as these, which are dlfflcult to make on pure compounds by mass spectrometry, were made by utilizing daughter spectra recorded successively during the evaporatlon of the material from the probe and/or from spectra recorded on different types of plant extracts. This study suggests that many natural materials may contain trace amounts of compounds of potential biomedical or synthetic Interest and, in particular, that the number of known alkaloids might be greatly Increased by experlments of this type.
This study takes as its aim the discovery of new natural products in a complex matrix. Pachycereus weberi, commonly called candelabro and the largest of all columnar cacti, is native to Puebla and Oxaca provinces, Mexico (1-3). Prior to studies 0003-2700/85/0357-0109$01.50/0
by tandem mass spectrometry (MS/MS), nine simple isoquinoline alkaloids were known to occur in the plant ( 4 , 5 ) . Anhalonidine (16) (Table I), the first alkaloid reported to be isolated from the candelabro cactus, was identified by Djerassi et al. in 1954 (3). Other identifications followed, first by chromatographic and spectroscopic techniques (2) and later by mass-analyzed ion kinetic energy spectrometry (MIKES) (5). The importance of the mass spectral techniques used in , the current study has been established in the natural products field and in other instances of complex mixture analysis (6-11). Tandem mass spectrometry has been applied to a number of problems involving the analysis of complex mixtures (12-14). A strength of this approach is its high sensitivity illustrated early in ita development by the 1-ng detection limit obtained in the analysis of plant tissues for cocaine and cinnamoylcocaine (15). In a more recent study of the cactus species Backebergia militaris using MIKES (12), seven alkaloids previously unknown to the species were tentatively identified and confirmed by more established techniques. Nevertheless, there have been only a few reports (12,16,17) of the use of tandem mass spectrometry to identify unknown mixture constituents. The present study seeks to evaluate the capabilities of tandem mass spectrometry for new compound discovery and to inquire into the nature and number 0 1984 American Chemical Society
