Anal. Chem. 1980, 5 2 ,
LITERATURE CITED
463
463-468
(8) R. Belcher, S. Karpel, and A. Townshend, Talanta, 23, 631 (1976). (9) R. Belcher. T. A. K. Nasser, and A . Townshend, Analyst(London),102, 382 (1977). (10) J. R. Sweet. W. B. White, H. K. Henisch, and R. Roy, Phys. Left.. 33A. 195 (1970). (11) J. W. Hess, Jr., J. R. Sweet, and W. El. White, J. Electrochem. Soc., 121, 142 (1974). (12) D. M. Mason, Am. Chem. Soc.Div. Fuel Chem., Prepr.. Part 2, 11(2), 640 (1967).
(1) L. T. Minchin, Trans. Faraday Soc., 35, 163 (1939). ( 2 ) H. F. Ivey, J . Lumin., 8, 271 (1974). (3) J. Donau, Graz. Monatsh., 34, 949 (1913); Chem. Absh. 7 , 3445 (1913). (4) 0. Neunhoeffer, Fresenius' Z. Anal. Chem., 132, 9 1 (1951). (5) R. Belctrer, S. L. Bogdanski, and A. Townshend, Talanta, 19, 1049 (1972). (6) R. Belcher, K. P. Ranjtkar, and A. Townshend, Ana/yst(London). 100, 415 (1975). (7) R. Belcher, K. P. Ranjtkar, and A. Townshend, Analyst (London). 101, 666 (1976).
RECEIVED for review July 31,1979. Accepted October 30,1979.
Exact Mass Measurement by Fourier Transform Mass Spectrometry E. B. Ledford, Jr., Sahba Ghaderi, R. L. White, R. B. Spencer,' P. S. Kulkarni, C. L. Wilkins," and M. L. Gross* Department of Chemistry, University of Nebraska -Lincoln, Lincoln, Nebraska 68588
d a t a for a gaseous mixture was reported, and analytically marginal mass measurement accuracies between 7 and 180 ppm, with an average error of 77 ppm, were obtained. T h e errors were highly variable (standard deviation of the 12 measurements reported was 59 ppm as a result of resonance shifts). With such errors, elemental composition assignments are ambiguous (e.g., if only C, H, N, and 0 are considered, 29 of the 177 compositions at m / e 350 fall within 77 p p m of one another). T h e authors concluded ". . . the highest performance is to be expected from detection methods which do not eject resonant ions from the analyzer cell." This requirement is satisfied by Fourier Transform mass spectrometry (FT/MS). In recent years, high mass resolution data have been acquired in times of less than a second using F T / M S ( 5 ) . This has prompted our interest in analytical applications of F T / M S and, in particular, the possibility of real-time high resolution multiple ion monitoring across wide mass ranges in a GC/MS mode. As a first step we have made a preliminary investigation of the mass measurement capabilities of a Fourier Transform mass spectrometer. As a part of this work, we have examined previously unexplained resonance shifts with changes in the number of trapped ions, observed with all trapped ion analyzer cells. The result of our study is a practical method for rapidly obtaining exact masses of sufficient accuracy and precision for determination of elemental compositions by FT/MS. In addition, we have acquired new understanding of the factors which affect ion motion in the trapped ion analyzer cells employed in high resolution ICR spectrometers.
Reported here for the first time is a technique utilizing Fourier Transform Mass Spectrometry (FT/MS) for determining elemental compositions of gas-phase ions by exact mass measurement. In contrast to previous literature reports of mass measurement errors averaging 77 ppm via ICR measurements, accuracies averaging 3 ppm havl? been achieved in the present work. Moreover, in the course of this study, the phenomenon of shifts in ion cyclotron resonance frequency with the number of ions in the cell has been investigated. The resonance shiis are caused by electric f i M s associated with ion space charge. The consequences of this effect for mass measurement accuracy are discussed.
It is well known that the capability to measure exact masses of chemical substances is extremely important for both structure proofs of new compounds and for highly specific trace analysis. These measurements, which can lead to unambiguous assignments of elemental compositions, have been made traditionally using expensive and complicated doublefocusing mass spectrometers of a t least sufficient resolving power to separate isobaric calibrant and unknown peaks. One of the principal limitations of the high resolution mode is that it is extremely difficult to acquire high resolution data across a mass range of more t h a n a few amu in short times (e.g. the several second duration of a capillary column gas chromatographic peak) in either a scanning or a peak switch mode ( I , 2). Nonetheless, rapid peak switching to obtain high resolution measurements of ions widely separated on the mass scale would be invaluable in conjunction with high resolution gas chromatographic separation of mixture components. Exact mass measurements in a trapped ion ICR spectrometer have been the subject of two earlier reports. In 1974 McIver and Baranyi ( 3 ) ,reporting a single mass measurement with an accuracy of about 13 ppm, suggested that exact mass measurements in a trapped ion ICR spectrometer were possible. In 1976 Ledford a n d McIver ( 4 ) evaluated the mass measurement accuracy of a trapped ion ICR spectrometer employing electrometer detection of resonant ions. A set of
THEORY T h e fundamental quantity measured in the F T / M S experiment is the resonance frequency of ions confined in a strong and uniform magnetic field. Exact mass measurement rests on the ability to measure ion resonance frequencies precisely, and to relate frequency to mass accurately. Thus, the mathematical relationship between mass and resonance frequency is of key importance; it must be accurate to the level of about a few parts-per-million for element a1 composition assignments. T h e most elementary relation between ion resonance frequency and ion mass is given by the well known equation
Present address: Nicolet Instrument Corp., 5225 Verona Road, Madison, Wis. 53711. 0003-2700/80/0352-0463$01 .OO/O
wC =
C
1980 American Chemical Society
qB/m
(1)
464
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980 L
1
1
frequency, o,is given by ( 4 , 8). w 2 = w,2 - &),2
(3)
Both w, and wr, the radial perturbation frequency, carry mass dependence, so that a mass calibration procedure can be developed from Equation 3. Using the definitions of w, and w , and the relation w (radians s-l) = 2rf (Hz), Equation 3 may be written
mr2fi2 =a
Flgure 1. (a) Electric fields due to applied trap potentials. (b) Electric fields due to ion space charge. Radial component of space charge field reinforces radial component of trapping field. Axial component of space charge field opposes axial component of trapping field
in which q / m is charge to mass ratio, B is magnetic field strength, a n d w, is the angular frequency of orbital motion. In practice, there are electrostatic fields present in the analyzer cell which constrain ion motion parallel to the magnetic field. T h e form of the electrostatic fields is quite complicated, having vector components in directions both parallel and perpendicular to the magnetic field (see Figure 1). Electrostatic field components perpendicular to the magnetic field are known to perturb the ion resonance frequency from its nominal value, w,. T h e perturbed frequency can be calculated from the equations of ion motion if the geometric configuration of the electrostatic fields in t h e analyzer cell is known. T h a t configuration is primarily determined by analyzer cell geometry. Consequently, analyzer cells of different geometry may give rise to different mathematical relationships between mass and resonance frequency. In high resolution ion cyclotron resonance mass spectrometers, trapped ion analyzer cells rather than drift cells must be employed to obtain the long ion observation times necessary for high resolution measurements (hundreds of milliseconds, as opposed to 1-10 ms obtainable with drift cells). The most common trapped ion cell now in use has the oblong geometry introduced by McIver (6). In treatments of ion motion in this cell, it is customary to invoke the ‘‘two-dimensional quadrupole approximation” in which variation in electric field with displacement on the cell’s long axis (generally taken as the y direction) is neglected (7,8). The approximation is equivalent to assuming the cell is infivitely long on the y axis. In this case, the electrostatic field, E , is approximated near the center of the cell by
bm,
(4)
in which f, is the resonance frequency in Hz of ion species i, m, its mass, a = ( q B / 2 7 ~ ) and * , b = q(VT - Vu,L,E)/rz12. If the resonance frequencies of two more ions of known mass are measured, experimental values of the calibration constants a and b can be computed. Once they are known, a third and unknown mass can be computed from a measurement of its resonance frequency (9). This procedure is primarily applicable to analyzer cells having oblong geometry. In our own laboratory, following Comisarow and Marshall (IO), we are currently employing a cubic, rather than oblong, analyzer cell design. The treatment of ion motion in the cubic cell is similar to that for the oblong cell. However, owing to the symmetry of the cubic cell, the two-dimensional quadrupole approximation is not applicable. Instead, a threedimensional quadrupole approximation must be used, in which dependence of electric field upon displacement in the y direction is included. Thus, y e may write for the applied electrostatic trapping field, .Etrap,the following:
Here is the unit vector in the y direction. Equation 5 is a good approximation for the electric field near the center of the analyzer cell (8) in the absence of ions. When the electric field in Equation 5 is used in the phenomenological equation of ion motion ( I I ) , the following relationship between mass and cyclotron resonance frequency is obtained (12, 13): t
where wo2 = w: - 2wr2. It is straightforward to show that Equation 6 leads to a mass calibration equation of the form
mI4fi4= A , Here V , - VU,L,Eis the voltage difference applied to the trapping plates and th,e equibiased upper, lower, and end plates of the cell, a n d i and h are unit vectors in the x and z directions, respectively. T h e constant 1 is the plate separation of a cell (oblong in the y direction) of square cross section in the x , z plane. It is apparent from Equation 2 that part of the electric field lies parallel to the z axis (also see Figure l), the direction of the magnetic field. This axial electric field accelerates ions toward the midplane of the cell whenever they are displaced on the z axis. Ions therefore undergo axial oscillation along the z axis. T h e axial electric field component prevents ion loss parallel to the magnetic field. T h e x component of the electric field is perpendicular to the magnetic field (parallel to the plane of ion orbital motion). This “radial” component of the electrostatic field complicates ion motion in the x-y plane, and causes the resonance frequency to shift from its nominal value a t w,. In the two-dimensional quadrupole approximation, the ion resonance
-
-
Bornr
+ C,mi2
(7)
where
A, =
( q ~ / 2 T ) 4
Expanding the square root of Equation 7 in a MacLaurin series and dropping terms of less t h a n one-part-per-million significance yields the simpler form:
m12fL2 = a,
-
bornr
+ corn,’
(8)
in which a,, bo, and c, are constants which may be derived in terms of A,, Bo, and C,. Equation 8 is similar to Equation 4 except for the presence of a term in the square of the mass, c0mr2, on the right hand
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
465
Figure 2. Schematic diagram of the cubic trapped ion analyzer cell
side. This term amounts t o about a 10 ppm correction, which is small b u t significant for exact mass measurement. Over a narrow mass range, however, t h e slight curvature in a plot of ( m i , ) *vs. m, contributed by t h e term c,mlZ should be negligible, with the result t h a t a linear calibration based on Equation 4 should be valid. Over a wider mass range t h e parabolic calibration, Equation 8, should give substantially better mass measurement accuracy than the linear procedure, Equation 4.
EXPERIMENTAL Instrumentation. This work was carried out with a modified Varian ICR-9 ion cyclotron resonance mass spectrometer. A trapped ion cell with cubic geometry (plate separation 0.0254 m) was used in place of the original drift cell (see Figure 2). Primary ions are produced by electron impact and trapped in the cell by application of appropriate potentials ( V , - Vr,L,E= 1 V in most cases) to the trapping plates. Radio frequency excitation is applied to the cell transmitter plate. When the rf excitation frequency is close to the frequency of ion orbital motion, ions are coherently accelerated to orbital radii larger than the typical thermal radii of m. This process is represented by the dotted spiral path in Figure 2. T o obtain a complete spectrum, the frequency of the excitation voltage is rapidly swept across the frequency range of ions of interest (14). Typical duration and amplitude of this rf "chirp" is 0.5 ms and 7 V peak to peak. At a magnetic field of 1.2 Tesla, the mass range m / e 10 to m / e 1000 corresponds to a frequency range of 1.8MHz to 18 kHz. For narrow bandwidths (e.g., single or multiple ion monitoring), narrower frequency ranges can be swept. At the completion of the excitation, the coherently excited ions are left in "parking orbits", as a result of which they move up and down between parallel receiver plates. Coulombic attraction between the circulating ions and conduction band electrons in the receiver plates generates ion image currents in the circuit connecting the plates. This ion image current has a typical value of A and develops an rf potential across an RC load shunting the receiver plates. This load is formed by the input resistance and capacitance of a Tektronix 5A22N high-speed differential oscilloscope amplifier. If multiple ion species have been excited, a n rf signal which is a superposition of as many different sinusoidal signals as there are excited ion species is developed across the input load. The resulting composite time domain signal can be "unraveled" into a frequency spectrum by discrete fast Fourier transformation. D a t a Processing. For data acquisition and reduction, a 40-K word Nicolet 1180 minicomputer, equipped with a high-speed buffered digitizer capable of data acquisition of up to 5 MHz (9-bit resolution) or 2.5 MHz (12-bit resolution), was used. For data storage and display, the data system is equipped with a Diablo Systems Model 40 five million word disk drive, a Tektronix 4662 digital plotter, a Tektronix 4010 display terminal, and a Texas Instruments 810 impact printer. Spectral data acquisition is accomplished by passing the signal from the analyzer cell through a differential amplifier either directly to the digitizer or after mixing with a reference signal from the Rockland 5110 programmable frequency synthesizer
Figure 3. Block diagram of the Fourier transform mass spectrometer system. (Used with permission from C . I-. Wilkins, Anal. Chem., 50, 493A (1978))
Table I. Slopes and Intercepts of the Least-Squares Regression Lines through a Plot of m l ' f 1 2vs. mj derived from CCI, FT Mass Spectra emission current, nA
slope A amu H z 2 -2.28 -2.67 -2.87
20 40
60
intercept x 10-14, amu' H z 2 3.42 3 43 3.43
Table 11. Linear Mass Calibration over t h e Major Ions in the F T Mass Spectrum of 1,1,1,2-Tetrachloroethane 1in ear c a1i bra t i on
m la
errors
116.9066 118.9036
+ 0.0001
120.9007
+0.0001
4
130.9222 132.9192 134.9163
+ 0.0009
18
116.9066 118.9036 120.9007 130.9222 132.9192 134.9163
mass range 1
-0.0001
-0.0005
-0.0005
-0.0018
-0.0014 -0.0020
+0.0016 + 0.0009 -0.0026
'' All entries are in amu which is used for rf excitation. In the latter case, the mixer output of sum (beat) and difference (heterodyne) signals is filtered to reject the beat signals and pass heterodyne signals to the digitizer (see Figure 3 for the block diagram). Exponential apodization is used to enhance signal-to-noise ratio and improve peak shapes
466
ANALYTICAL CHEMISTRY, VOL. 5 2 , NO. 3, MARCH 1980
inition, its Fourier transform, H ( f ) :
Here j = +I, and f is frequency in Hz. Evaluating the integral we obtain
In general the Fourier transform is a complex function of frequency. Both the real and the imaginary parts of this complex function carry useful spectral information. Equation 11 describes the spectrum that would be obtained if ions were excited by a Dirac-delta impulse. In practice, however, ions are excited by an rf “chirp” spanning, typically, 0.5 ms of time. Because excitation of various ion species is sequential rather than simultaneous, the component oscillations making up the transient response have different phase angles. This introduces “phase distortion” terms. One method of reducing the phase distortion is to compute a magnitude spectrum, !lH(f)11, in which the real and imaginary parts of the Fourier transform are combined in quadrature (Is),thus:
L 1
1
I
158
155
152
I
I
1
117
119
121
FREOUENCY IKHzl
MASSlamul
I
1
I
141
138
136
1
1
131
133
1 35
Flgure 4. Time domain transient and FT mass spectrum of 1, 1,1,2tetrachloroethane
in FT mass spectra. Typically 4K transient data points are digitized. Zero-filling to 16K is performed prior to discrete Fourier transformation to increase the number of points on the peaks. Typical FT Mass Spectra Conditions. Sample pressures Torr were used in the experof between 5 X Torr and iments reported here. Data presented in Tables I and I1 represent typical determinations on a single FT mass spectrum resulting from ensemble averaging 200 time domain transients over a 10.2-s time period. Typical conditions were: ionizing electron beam duration 5 ms, 20 nA instantaneous emission current, magnetic field 1.2 Tesla, rf excitation amplitude 7 V peak to peak, excitation bandwidth 25 kHz centered a t 147.5 kHz, A/D conversion rate 50 kHz, and excitation rf sweep rate 5 X lo7 Hz/s. All experiments were done in mixer mode. Furthermore, the “quench pulse” generally used in ICR experiments to remove ions from the cell between successive electron beam pulses was turned off. Thus, the cell was operated in a quiescent ion storage mode for these experiments. We estimate approximately IO4 ions are trapped in the cell during an experiment. P e a k Fitting. A prerequisite for accurate mass measurement is accurate determination of the center frequencies of peaks in Fourier transform spectra. A peak fitting routine employing an analytical lineshape expression was developed for this purpose. The time domain transient signal generated by a single ion species was assumed to have the form
in which A is the amplitude of one sinusoidal component of a composite transient signal, fo is the frequency of that component, and a is a damping constant. (Note that a phase angle is not present in this expression. Omission of phase angle is justified later). Ion transient signals are damped (see Figure 4) because of processes (e.g., ion molecule collisions) which degrade, or randomize, the unison motion of coherently accelerated ions. The spectral representation of this component oscillation is, by def-
This procedure gives, for frequencies near fo, the following lineshape expression:
Here riH(f)/Imay be considered the height of a spectral peak a t frequency f . It is clear from Equation 13 that the peak maximizes and is symmetric about fo Equation 13 is the lineshape at f = io, adopted for this study. It is apparent from Equation 13 that the inverse square of a spectral peak should lie on a parabola in f:
or, in which c1 = 1 6 ~ ~ / Ac2 ’ ,= -32f07r2/A2,and cg = (48’+ l67r2fO2)/A2. is given by f o = -cp/2c1, It follows that the center frequency, io, the value o f f a t which the parabola is extremal. Experimentally, the best parabola through the inverse squares of the points near the top of a sampled peak was computed using a least-squares method, and center frequencies were then calculated using the empirically determined values of c1 and c2 for each peak in the spectrum. Frequency spectra are stored on disk and accessed by a BASIC program which automatically searches the spectra for peaks and computes center frequencies. The same BASIC program uses the center frequencies so obtained together with manually input values of the exact masses of calibrant peaks to test mass calibration functions. General Protocol. The experiments proceeded in two stages. The first stage of the study involved derivation and testing of mass calibration procedures for a cubic cell. The second stage was a qualitative investigation of resonance shifts induced by changing the number of ions in the analyzer cell, an effect which has been observed for many years in ICR work, and which may have important consequences for the mass measurement capabilities of FT mass spectrometers. The derived mass calibration procedure was tested against the six major ions in the spectrum of 1,1,1,2-tetrachloroethane, which provides six intense calibration peaks in a useful region of the mass spectrum (see Figure 4). Mass accuracy was evaluated over various mass ranges using both linear and quadratic mass calibration formulas. The investigation of resonance shifts was designed as follows. Carbon tetrachloride was introduced to the spectrometer and resonance frequencies of ions a t known m / e values of 116.9066, 118.9036, and 120.9007 were measured using the peak fitting procedure described above. Since the linear calibration using
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
Table 111. Typical Results of a Parabolic Mass Calibration over the Major Ions in the Mass Spectrum of 1,1,1,2-Tetrachloroethane error?
m1
116.9066 118.9036 120.9007 130.9222 132.9192 134.9163
calculated massesU 116.9064 i- 0.0003 118.9041 I0.0003 120.8999 * 0.0008 130.9226 * 0.0004 132.9188 i 0.0004 134.9164 i 0.0006
amu -0.0002 +0.0005 -0.0008 +0.0004
-0.0004 +0.0001
PPm (1.7) (4.2) (6.6) (3.1) (3.0) (0.7)
a Average of ten determinations made on ten successive mass spectra. Tolerances shown represent single standard deviations over ten trials.
Equation 4 was expected to be valid over such a narrow mass range, the least squares regression line through a plot of (mi,)’ vs. rn, was computed. The experiment was repeated at various values of electron beam emission current in order to determine whether changing the number of ions in the cell exerted its primary effect on the intercept, a, or the slope, b, of the plot. From the definitions of a and 6, it is clear that the intercept depends solely on the strength of the magnetic field, while the slope depends solely on the strength of the electrostatic field (more specifically, the radial component of the electrostatic field). By determining the effect of changing the number of ions on the slope and intercept of the (rnf,)2 vs. m, plot, the observed resonance shifts could be ascribed to changes in the strengths of either the electric or magnetic field, or both.
RESULTS AND DISCUSSION Mass Measurement Accuracy. T h e results of t h e mass calibration studies are shown in Tables I1 and 111. Both linear (Equation 4) and parabolic (Equation 8) calibration equations were tested. Over a 4-amu range, the linear procedure produces submillimass unit errors (see Table 11). Over a wider mass range, the linear procedure is not as accurate as the parabolic, as expected from theory. W e find t h a t t h e parabolic calibration procedure of Equation 8 produces mass measurements of sufficient accuracy a n d precision to permit elemental composition assignments over a mass range of at least 18 amu, centered a t m l e 126. Frequency measurement precision for the six major ions in the spectrum of 1,1,1,2-tetrachloroethane, (standard deviation over ten successive Fourier Transform mass spectra) was typically 0.3 Hz and in no case exceeded 1.5 Hz. T h e quantity ( mj J 2was fitted to the parabolic form in Equation 8 and the masses then back-calculated (see Table 111) using the empirically determined values of a,, bo, and co. Over an 18-amu range, the parabolic calibration procedure gives mass errors of less t h a n 1 mmass unit. I n practice, we have found the mass accuracy shown in Table I11 is routinely obtained over a broad range of operating conditions including various values of electron beam emission currents. Computerized d a t a reduction of successive mass spectra makes calculation of masses and standard deviations fast and reliable. At present, calibration studies over a wider mass range are in progress. Radial Space Charge Field. T h e mass calibration equations derived above (Equations 4 and 8) do not take into account the presence of ions. However, a phenomenon observed with all trapped ion analyzer cells is a decrease in ion resonance frequency when the number of trapped ions is increased. Resonance shifts of -200 Hz have been observed by making the number of trapped ions large (13). An important objective of this study was to uncover the physical basis of this effect and to evaluate the consequences for mass calibration. T h e results of our study of resonance shifts are presented
467
in Table I, which shows the effects of changing the electron beam emission current (number of ions) upon a least squares regression line calculated through a plot of the quantity of vs. m,. I t is found that the slope of the plot varies with emission current, while the intercept remains constant. This effect is qualitatively indistinguishable f r o m small changes in the trapping voltage difference, VT - VuL,E. Changing the t r a p voltage also changes the slope of the ( mj,)’ vs. m, plot, a n d leaves the intercept unchanged, in accordance with the definitions of the calibration constants a and b in Equation 4. From the qualitative similarity between changes in trap voltage and changes in t h e number of trapped ions we conclude t h a t there is associated with any ensemble of trapped ions an electric field having radial components which reinforce the radial components of the applied electrostatic field. Thus t h e resonance shift observed with changing number of ions is a space charge effect. (An earlier paper attributed the resonance shifts to a magnetic field perturbation ( 4 ) . This resulted from miscalculation of the weak magnetic field generated by circulating ions, and is incorrect.) The resonance shifts are related to the total number of ions in the cell. T h e resonance frequencies of all ion species are shifted by the radial space charge field. Accuracy and Precision in Mass Measurement. Other workers, principally McIver a n d his students, have pointed out the potential for exact mass measurements using ion cyclotron resonance methods ( 3 , 4 ) . However, as mentioned earlier, to be analytically useful, both the accuracy a n d t h e precision of the mass measurement must be sufficient to rule out incorrect elemental compositions. T h e nature of this requirement can be illustrated by considering a hypothetical measurement giving an exact mass of 200.0477 f 77 partsper-million, which is the average error range of previous measurements ( 4 ) . If i t is assumed that the sample contains only C, H , N, 0,there are eight elemental compositions within this error range (16). However, if the measurement is now made with an error of 1 ppm, only one elemental composition is possible: CI2Hl0NO2. “Unambiguous” elemental composition assignments demand both accuracy and precision of about 3 parts-per-million or less. Our present results establish that ICR measurements can be used for rapid a n d unambiguous elemental composition assignments to organic ions u p to m l e 135. Mass Resolution. I t is well known that high mass resolution is not necessarily needed for exact mass measurements, so long as mass spectral peaks are not composed of multiplets of isobaric ion species (17-19). T h e measurements reported here were made a t mass resolution of ca. 1000 (full width a t half height definition). For analytical purposes, it should be possible to perform exact mass measurements on wideband, low resolution ICR mass spectra, then switch to a high resolution multiple ion monitoring mode to resolve potential multiplet peaks as a check. Since this operation would involve only frequency switching and pulse width adjustments, it could be performed rapidly and automatically under computer control. It should be noted that FT’/MS is capable of much higher mass resolution than 1000. T h e use of high resolution mass spectra is expected to further increase the precision, and possibly the accuracy, of mass measurements by F T / M S . Absolute vs. Relative Mass Measurement. T h e calibration methods described here facilitate relative mass measurements, in which an unknown mass is measured relative to calibrant masses simultaneously present in the analyzer cell. T h e calibration equations presented here suffice for relative mass measurements regardless of the number of ions trapped in the cell. If a quantitative model of the space charge electric field can be developed, it will be possible to calculate the resonance shifts caused by a given number of ions in the cell. In that
468
Anal. Chem. 1980, 52, 468-472
case, the radial electric field strength could be corrected for the total peak intensity (number of ions) in a mass spectrum. T h a t would raise the exciting possibility of absolute, as opposed to relative mass measurements. In an "absolute" mass measurement scheme, the sample would be run in the absence of calibration compound. Such a scheme would involve calibrating the instrument using methods similar to those described in this report, then pumping out the calibrant compound, introducing a sample, and measuring its exact mass using results of the previous calibration. Exact mass measurement in the absence of calibrant would signficantly expand the analytical capabilities of FT/MS. CONCLUSIONS Determination of elemental compositions by Fourier transform mass spectrometry is possible. Although spacecharge effects shift resonance frequencies, mass measurement accuracy is not appreciably degraded. Finally, it may be possible to develop absolute mass measurement schemes for F T / M S , in which sample and calibrant are run separately. LITERATURE CITED (1) Lewis, S.; Kenyon, C. N.; Meili, J.; Burlingame, A. L. Anal. Chem., 1979, 57, 1275. (2) Kimble, B. J. "High Performance Mass Spectrometry: Chemical Applications"; Gross, M. L., Ed.; American Chemical Society: Wash-
ington D.C., 1978; pp 120-149. McIver. R. T., Jr.; Baranyi, A. D. Int. J. Mass Spectrom. Ion phys. 1974, 14, 449. Ledford, E. B., Jr.; McIver, R. T., Jr. fnt. J . Mass Spectrom. Ion Phys. 1976, 22, 399. Comisarow, M.; Marshall, A. G. Proceedings of the 23rd Conference on Mass Spectrometry and Allied Topics, Houston, Texas, 1975; No. R-5. McIver, R. T., Jr. Rev. Sci. Instrum. 1970, 4 1 , 555. Beauchamp, J. L.; Armstrong, J. T. Rev. Sci. Inshum. 1969, 4 0 , 123. Sharp, T. E.; Eyler, J. R.; Li, E. Int. J , Mass Spectrom. Ion Phys. 1972, 9 , 421. Beauchamp, J. L. J . Chem. Phys. 1967, 4 6 , 1231. Comisarow, M. 8.; Marshall, A. G. J . Chem. Phys. 1976, 6 4 , 110. McIver, R. T., Jr.; Ledford, E. B., Jr.; Hunter, R. L. J . Chem. Phys ,, in press. Hildebrand, F. B., "Advanced Calculus for Applications"; Prentice-Hall: Englewood Cliffs, N.J., 1962. Ledford, E. B.. Jr. Ph.D. Thesis. University of California at Irvine, 1979. Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 26, 489. Comisarow, M. 6.; Marshall, A. G. Can. J . Chem. 1974, 52, 1997. Beynon, J. H. "Mass and Abundance Tables for Use in Mass Spectrometry"; Elsevier: Amsterdam, 1963. Karasek, F. W. Res. Dev. 1970, 25, 30. Weinkam, R. J.; D'Angona, J. L. Anal. Chem. 1979, 57, 1074. Hunt, D. F.; Stafford, G. C.; Shabanowitz, J.; Crow, F. W. Anal. Chem. 1977, 4 9 , 1884.
RECEIVED for review August 2, 1979. Accepted December 3, 1979. Support of this research by the National Science Foundation (Grant No. CHE-77-03964)and a grant from GLlf Research Foundation are gratefully acknowledged.
Mass Spectrometry of Nickel Carbonyl for Modeling of Automobile Catalysts J. E. Campana' and T. H. Risby"' Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802
The charge exchange mass spectrum of nickel carbonyl has been reported using a typical gasoline engine exhaust gas as the reactant gas. Various posilive ions which can be attributed to nickel carbonyl and its fragments were observed in this mass spectrum. The effect of pressure on the intensity of both the reactant gas ions and the nickel carbonyl was studied in an attempt to find the optimum condition for the quantification of nickel carbonyl. Using these data, the minimum detectable limit was found to be 10 ppb for nickel carbonyl. This methodology was used to monitor nickel carbonyl in the effluent from a model reactor for a catalytic controlled automobile. Based on these studies, no measureable quantities of nickel carbonyl can be expected from catalytic controlled automobiles.
Nickel carbonyl [nickel tetracarbonyl, Ni(CO)4]is one of the most dangerous chemicals known. It exhibits acute toxicity, carcinogenicity, teratogenicity, and can be produced spontaneously in unsuspecting environments whenever carbon monoxide contacts an active form of nickel. M e t h o d s of Analysis. 'l'he toxicological and carcinorenic
' Present addrei 11: Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins University School of Medicine, Baltimore, Md. 21205. 2Author to whom all correspondence should be addressed; present address: Division of Environmental Chemistry, Department of Environmental Health Sciences, The Johns Hopkins University. School of Hygiene and Public Health, Baltimore, Md. 21205. 0003-2700/80/0352-0468$01 O O / O
d a t a on nickel carbonyl has led to the lowest eight-hour average allowable exposure of any chemical. T h e Occupational Safety and Health Administration has set this level a t 0.007 mg/m3 or 1 ppb (I),which has dictated a need for ultratrace methods of analysis for nickel carbonyl in ambient air. Recently the instrumental methods, infrared spectrophotometry (IR) (21, Fourier transform IR (FTIR) ( 3 , 4 ) ,plasma chromatography ( 4 ) , and chemiluminescence ( 5 ) ,have been used to analyze for nickel carbonyl. All of these methods have the advantage of direct measurement with detection limits of about 1 p p b and are also adaptable to process stream monitoring. The methods based on the FTIR and the plasma chromatography methods have been compared in real sample analyses and agree within a few percent. T h e chemiluminescent analysis for nickel carbonyl demonstrates a detection limit of 0.01 ppb with a linearity over four orders of magnitude. Therefore, it is suitable for industrial applications from both the practical and economic standpoint. M a s s S p e c t r o m e t r i c Studies. T h e first detailed mass spectrometric study of metal carbonyls was published in 1951 (6), and in 1961, Vilesov and Kurbatov (7) measured the ionization potentials of five metal carbonyls using photoionization techniques. Later, Winters and Kiser (8)reported the 70-eV electron impact mass spectra of nickel carbonyl and iron pentacarbonyl. They also reported the ionization a n d appearance potentials for these compounds and their fragment ions, and from these data the heats of formation of the fragment ions were calculated. The clastograms they observed for the molecular and fragment ions were found to "peak-out" and then decrease rapidly (pseudo-exponential) with inC 1980 American Chemical Society
