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Anal. Chem. 1907, 5 9 , 194-197
Poly(dimethylsi1oxane) as a Reference Standard for Exact Mass Measurement in Chemical Ionization Mass Spectrometry Michel J. Bertrand,* Laurent Maltais, and Michael J. Evans
Regional Center for Mass Spectrometry, Department of Chemistry, University of Montreal, P.O. Box 6128, Station A, Montr&al,QuGbec, Canada H3C 3J7
A mixture of poly(dimethylsWoxanes) has been studled in order to evaluate Its usefulness as a mass reference standard for exact mass measurement in high-resolutlon chemlcai ionlzation. When methane and lsobutane are used as the reagent gases, the mixture produces evenly spaced ions in sequences of 30, 14, 14, and 16 from 73 to 1051 Da. The ofthe ions present In the dletnfcal lonlzetkn (CI) spectrum of the mMure has been deduced from accurate mass data and a mass reference flle has been constructed. The mlxture Is suffklently volatile to be admmed Into the Ion source by a heated reference Inlet reservoir and can be used on a routine bask for accurate mass determinatlon of common organlc compounds by peak matchlng or computer Interpolation.
A number of compounds and substances have been reported and used as mass reference standards in analytical mass spectrometers. In electron impact ionization, perfluorohydrocarbons (PFK) and perfluorotributylamine (PFTBA) are widely used and can provide evenly spaced and relatively intense ions up to m l z 850 (1,Z). Perfluorotriheptyltriazine is also utilized for calibration above mass 600 Da ( 3 ) and Fomblin-L has been reported to be useful up to 2000 Da ( 4 ) . For other ionization techniques, compounds such as phosphazenes have been reported for mass calibration in field desorption (5) and perfluorotriazines and metal halides are frequently used in desorption ionization techniques such as fast atom bombardment (6). Surprisingly, few compounds have been reported and are used as mass calibrants for chemical ionization. In electron capture chemical ionization, compounds such as phosphazenes and perfluorotriazines have been studied and it has been reported that Fomblin Oil can be used to calibrate a data system up to 3500 Da (7). For positive chemical ionization, substituted triazines have been proposed as mass markers for low-resolution experiments (8). However, for accurate mass determinations in high-resolution positive chemical ionization there is presently no reference standard that can be used routinely under a wide range of experimental conditions. Perfluorokerosene and perdeuteriated alkanes have been suggested, but they have limited applications (9). Under chemical ionization conditions, using methane as a reagent gas, perfluorokerosene produces significant background peaks at almost every mass due to hydrogen-containing species and the reference peaks below 600 Da are of weak absolute intensities. Morever, perfluorokerosene was found to be unsatisfactory with isobutane as a reagent gas (9). Perdeuteriated hydrocarbons showed good mass spectral features as a reference standard but they lack volatility and have to be introduced into the mass spectrometer by using a solid probe (9). This can create problems in the routine exact mass measurements of common organic compounds of varying volatility, since both the reference and the substance under study have to be introduced into the ion source a t the same time. 0003-2700/87/0359-0194$01.50/0
Since chemical ionization is a useful alternative for determining the exact mass and postulating the empirical formula of common organic compounds that do not yield satisfactory spectra under electron ionization, a reference standard of wide applicability for rDutine analysis in high-resolution chemical ionization is desirable. The ideal characteristics of such a reference standard should include the following: (i) be readily ionized under conditions of positive chemical ionization; (ii) be volatile enough to allow its introduction into the mass spectrometer by the simplest means; (iii) have few deleterious effects on the performance of the ion source; (iv) show evenly spaced ions that are sufficiently close together throughout the mass range of interest; (v) have ions of sufficient abundances to allow accurate determination by peak matching or computer interpolation; (vi) have ions that are mass deficient to reduce possible interferences with the ion peaks of the analyte; (vii) be chemically inert vis-&-vis the analytes so as to avoid structure modification; and (viii) be composed, if possible, of monoisotopic elements with weak minor isotopic abundances. In our search to find such a mass reference standard allowing the routine determination of the accurate mass of common organic compounds by high-resolution chemical ionization mass spectrometry, a series of poly(dimethy1siloxanes) has been studied. This paper reports on the feasibility of utilizing these compounds as mass reference standards for exact mass determination up to m / z 800. EXPERIMENTAL SECTION The mixture of poly(dimethyLsiloxs) (average MW 770) used in this study was obtained from Polysciences (Warrington, PA). Its composition was measured by gas chromatography-mass spectrometry and gave relative a bundances of 18,8.5, 25,25, 10, 8, 2.5, 2, 0.5, 0.2, and 0.2% for oligomers with n = 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14. The gas chromatograph interfaced to the mass spectrometer was a Carlo Erba Fractovap 4160 and the analysis was performed isothermally at 270 "C on a DB-1 capillary column 15 m X 0.25 mm (J & W Scientific) inserted directly into the ion source. All mass spectral analyses were conducted on a Kratos MS-5OTCTA mass spectrometer arranged in an EBE geometry (IO)and equipped with a postacceleration detector (11). The instrument was operated at 8 kV of accelerating voltage and the ion source was maintained at 220 "C. For accurate mass determinations, the reference compounds were introduced in the Kratos Mark IV E l / C l source through a standard heated inlet reservoir maintained at 200 "C and the compounds to be analyzed were introduced on the direct insertion probe. The reagent gas (CH4 or i-C4HI0)was admitted into the ion source by a standard reagent gas inlet and also through the heated reservoir in order to carry the reference compound and eliminate any pressure drop in between the reservoir and the ion source. Accurate mass determinations were done at 10 000 resolving power by using a seven-place peak matching unit or by computer interpolation with a Kratos DS-55 data system. RESULTS AND DISCUSSION A series of compounds that appears to possess most of the required properties of a mass reeference standard are poly(dimethylsiloxanes). As can be seen from their chemical structure, (CH3)3Si(-OSi(CH3)z)n-OSi(CH3)3, these compounds exhibit periodicity and corresponding ion sequences have been observed in their low-resolution electron ionization spectra 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
195
21
35
47
la) 250
350
300
400
m/z Figure 1. Chemical ionization spectrum of the mixture of poly(dimethylsi1oxanes) with CH, as a reagent gas.
(12,13).Although the (CHJ2Si-0 units of the backbone are not mass deficient (74.0188 Da) the mass excess is small and the ion peaks should be easily separated at moderate resolutions from those of common organic compounds with a high hydrogen content. Poly(dimethylsi1oxanes) are readily available because of their extensive use in chemistry, and their volatility is compatible with conditions usually encountered in a mass spectrometer. The volatility of the small oligomers (mlz I 1OOO) is such that they can be used as a mixture and its volatility can be adjusted to operating conditions by varying the relative concentrations of the constituents. Similar to other silicon-containing compounds (14-1 7), poly(dimethy1siloxanes) should be readily ionized under conditions of chemical ionization with methane or isobutane as a reagent gas. It is expected that the reactions they can undergo would be similar to those of tetramethylsilane, which is known to form tetramethylsiliconium ions under chemical ionization conditions (17). The mixture of poly(dimethylsi1oxanes) was first analyzed by electron ionization, and the spectrum showed a major ion sequence at mlz 73,147,221,295, and every 74 Da up to 500 Da, above which the ion intensities were weak. Other overlapping sequences were observed in the spectrum of the mixture and they are identified as S1to S4in Table I. The elemental composition of these ions was determined by accurate mass measurements using PFK (in electron ionization) as a reference compound. The sequences S1and Sz are generated by simple bond cleavage whereas Sz and S4result from internal cyclization with the'elimination of the terminal silicon atom as tetramethylsilane. This elimination reaction as been reported for similar compounds (18)and is supported by the mass analyzed ion kinetic energy spectroscopy (MIKES) spectrum of the ion a t m / z 295 in sequence SI, which shows that it decomposes in the metastable region to give rise to an ion at m/z 207. The overlapping of the four sequences produces a repetitive pattern in the spectrum which is seen by the presence of ions at every 16, 30, 14,and 14 Da. In the electron ionization mode, ions above a mass of 500 Da are of weak relative intensities due to the extent of fragmentation.
Table I. Ion Sequences in the E1 Spectrum of Poly(Dimethylsi1oxanes) m / z : 73, 147,221,295,369,443, 517...
Ch3-i?1(0-1?'
I c43
c43i.
m/Z:
89, 163,237,311,385,459,533...
S3 CZn-SHBn-BSinOn+l m/z: 119,193,267,341,415,489... Czn-1Hsn-sSinOn mlz: 133,207,281,335,429,503...
S4
In order to study the behavior of these siloxanes under conditions of chemical ionization, the mixture was introduced into the heated inlet reservoir at 200 "C and analyzed with methane and isobutane as reagent gases. The chemical ionization spectrum obtained with methane is shown, in Figure 1. The ionic distribution in the spectrum is similar to that observed in electron ionization, but under conditions of chemical ionization the ions at higher masses are significantly more abundant. The sequences SIto S4are again observed and, although the relative intensities of ions above m / z 800 are weak, ion peaks are present up to m / z 1051. This is attributed partly to the lower concentrations of the oligomers with n > 10 ( m / z > 800) in the mixture (see Experimental Section). Experiments made with isobutane gave similar results, but the absolute intensities of the ions were weaker than those obtained with methane. A similar spectrum could be obtained with ammonia, but it showed additional ions due to the addition of ammonia. The effect of source temperature was also studied, and the pattern of the spectrum was found to be constant up to 300 OC, after which ions at higher masses were found to decrease slowly. A list of the ions present in the methane chemical ionization (CI/CHJ spectrum of the mixture, along with their theoretical masses, empirical formulas, and relative intensities, is given in Table 11. Accurate masses have been determined by
IS6
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
Table 11. Exact Masses of Ions in the CI/CH4 Spectrum of the Mixture mass
formula
73.0473 89.0423 118.9621 133.0141 147.0661 163.0610 192.9809 207.0329 221.0849 237.0798 266.9997 281.0517 295.1037 31 1.0986 341.0184 355.0705 369.1225 385.1174 415.0372 429.0893 443.14 13 459.1362 489.0560 503.1080 5 17.1600 533.1550 536.0748
% base peak
mass
77 0.1 0.7 5.3 77.3 0.7 2.3 55.2 100" 1.4 15.2 66.4 76.6 1.0 23.7 59.9 65.8 0.1 5.6 22.4 55.4 0.2 0.1 7.3 24.2 0.4 0.1
577.1268 591.1789 607.1738 637.0936 651.1456 665.1977 681.1926 711.1124 725.1644 739.2165 755.2114 785.1312 799.1832 813.2353 829.2302 859.1500 873.2020 887.2540 903.2490 933.1688 947.2208 961.2728 977.2677 1007.1876 1021.2396 1035.2916 1051.2865
formula
% base peak
1.4 6.4 0.2 0.1 0.2 2.9 0.4 0.1 0.1 1.5 0.2 0.1 0.1 0.3 0.1 0.1 0.1 0.1 0.1b 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
a This peak is usually saturated under the experimental conditions used. Although the relative intensities of these peaks are 0.1 %, their absolute intensities are high enough to be used. "R) denotes values that have been used to construct a calibration file for computer interpolation.
Table 111. Examples of Exact Mass Determination of (M H)' with Poly(dcmethylsi1oxanes)as a Mass Reference Standard compound C19H3802
C12H1502s1Br C18H2106N3 CzzH3&3iN C414S a
theoretical
measured'
error, ppm
299.2950 273.0315 376.1508 342.2617 587.5904
299.2945 273.0315 376.1509 342.2634 587.5921
1.5 0 0.2 5 2.8
+
Mean of three measurements.
calibration of the peaks at m/z 295 and 281 against an external reference (the (M H)+of methyl stearate) and by subsequent repetitive peak matching. For manual peak-matching experiments, the peaks given in Table I1 have sufficient absolute intensities to be used up to m / z lo00 and those indicated by an (R) have been used to construct a calibration file for computer interpolation. With this file and a data system, the mixture allows calibration up to m/z 739, which is a reasonable value for the routine analysis of common organic compounds by chemical ionization. Other polar materials of molecular weight above this mass have low vapor pressures or are thermally unstable and a desorption ionization method such as fast atom bombardment is more suitable for their analysis. In order to verify the usefulness of the mixture of poly(dimethylsiloxanes) as a mass reference standard for the routine exact mass determinations of common organic compounds, several samples have been analyzed, and a few examples are given in Table 111. In these experiments, the reference was admitted through the inlet reservoir kept at 200 "C and the sample was analyzed at a resolution of 10OO0, using the direct insertion probe. Comparison of the values obtained with the theoretical masses of the compounds indicates that the deviations under those experimental conditions are all within the usual error for these measurements. The presence of hetero atoms such as Br and Si on the molecules or the low H / C ratio did not create any interference with peaks of the
+
calibrant. This is not surprising since the mass excess of the reference peaks is much lower than that of n-alkanes. For example, the mass excess of the reference peak at m / z 207 is 0.03 and it is 0.25 at m / z 665 as compared at 0.25 and 0.75 for alkanes of corresponding masses. In most cases a reference peak can be found within 15% of any mass value except at the lower end of the mass scale where the ion abundance between m/z 73 and 147 are rather weak. When a data system is used, the isotopic peaks of silicon can be entered in a mass exception f i e in order to eliminate them from the report. The A + 1 and A 2 peaks are composed of several individual isotopic components, but a t resolutions
