Instrumentation Charles L. Wilkins Department ol Chemistry University of California Riverside, Calif. 92521
Michael L. Gross Department of Chemistry University of Nebraska Lincoln, Neb, 68588
utilize Fourier transform data processing (for cross correlation of reference and spectral signals), it can be confused with the original FTMS method. However, it is different in a number of important respects. Excitation and detection are simultaneous in the Mclver technique, because a capacitance bridge detector is used.. Measurements take longer (e.g., 14.5 s for a spectrum from m/z 20 to m/z 1000). Data analysis requires cross correlation with a reference signal. In this article, the abbreviation FTMS refers only to experiments based upon the original Comisarow and Marshall technique. The primary advantage gained in Fourier spectroscopies in general is the Fellgett (or multiplex) (9) advantage, which can be realized when noise is detector- rather than source-limited (3,10,11). Experimentally, this is achieved in FTMS by the simultaneous time domain observation of signals from all excited ions, followed by Fourier transformation of the data thus obtained to yield a frequency domain (mass) spectrum. The observed frequencies are assigned mass values since the relation between frequency and mass is known. Such a time domain measurement with N data points can, in principle, provide a frequency spectrum with signal-to-noise ratio {S/N) enhanced by a factor of N in the same measurement time as the frequency domain measurement or, alternatively, provide a spectrum with the same S/N in 1/N of the time. An example of such a time-frequency domain pair is shown in Figure 1. If an entire spectrum is to be obtained in a single measurement, a frequency bandwidth of as much as 1 MHz or more may be required. This, can
Fourier Transform Mass
Spectrometry
for Analysis The evolution of ion cyclotron resomass spectrometry into a more important analytical tool is now possible because new measurement techniques and Fourier transform data analysis have been recently introduced. These methods have led to an instrument so different from the earlier conventional ion cyclotron resonance (ICR) instrument that a new name, Fourier transform mass spectrometry (FTMS), has been used in recent years to emphasize that the approach is a truly new one, basically free of the earlier limitations: restricted mass range, low mass resolution, and slow scanning speeds (1,2). In this area of spectrometry, as in so many others, the availability of capable and relatively inexpensive laboratory computers has made possible the dramatic improvements described nance
0003-2700/81/A351-1661 A$01.00/0 1981 American Chemical Society ©
here and in earlier reviews (2-4). Although a history of FTMS is discussed in a previous review (3) in some detail, with complete background references,
it is worthwhile to briefly recapitulate the milestones here. Comisarow and Marshall are responsible for the original demonstration of FTMS in 1974 (5-7). Their method involves the temporal separation of ion formation, excitation, and detection. Direct observation of induced image currents in the cell walls, rather than monitoring the power absorption of a marginal oscillator as the magnetic field is scanned, allows relatively short measurement times (tens to hundreds of milliseconds). Subsequently, another technique, called rapid scan ion cyclotron resonance spectrometry, was introduced by Mclver (8). Because this method also
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14. DECEMBER 1981
•
1661 A
970
861
752
643
534
424
315
206
97
Frequency (kHz)
Figure 1. A portion (a) of the excited ion decay (time domain signal) and the Fourier transform to the frequency domain (b) yielding the mass spectrum of 1-menthyl acetate
in turn, imposes the requirement that signal digitization be accomplished at 2 MHz or greater. Computer memory limitations may result in reduced resolution in this “direct” mode of data acquisition (12). For example, a high resolution measurement may require data acquisition for 1 s. Digitization at a rate of 2 MHz would produce two million data points, which would saturate the memory of most laboratory computer systems. Thus, for the highest resolution measurements, a suitable reference frequency is mixed with the signal, filtered, and a difference frequency extracted. This method permits observation of spectral segments at much lower frequencies and accordingly higher resolution for a fixed computer memory size (12-14). Of course, this is only true if the time domain signal persists sufficiently long to permit this “mixer” mode advantage to be realized. Detailed descriptions of experimental protocols have been published elsewhere (15-17) so they need not be repeated here. Rather, we will turn our attention to the physical requirements for practical mass spectrometry and review the recent progress of FTMS in that con-
emental composition assignments can be made if the mass measurement accuracy and precision are 3-5 ppm. The capability for chemical ionization mass spectrometry in addition to electron ionization is another requirement that should be met. Finally, rapid scanning in a GC/MS mode concurrently providing high-resolution multiple ion detection is a desired capability of most versatile analytical instruments. Conventional sector and quadruple mass spectrometers meet most of these requirements to a greater or lesser degree. For example, double-focusing mass spectrometers can be used to obtain high resolution data
either in a scanning or peak-switching mode. However, the scan cycle time for a decade of mass (40-400 amu) is at best 0.5 s at low resolution and 5 s 10 000) resolution. Quadat high (R rupole instruments can be scanned more rapidly, but the mass resolution is limited to about 2000 at best. Furthermore, it is necessary to make compromises, and one usually sacrifices speed for resolution or accuracy. Accordingly, the major efforts thus far in development of analytical FTMS have focused on evaluation of the capabilities of this new type of mass spectrometry with respect to the analytical characteristics mentioned above. =
text.
Analytical Requirements Ideally, a high-performance mass spectrometer should satisfy a number of analytical requirements. It should be capable of high mass resolution and high sensitivity, (Such requirements are in conflict in conventional sector mass spectrometers, since highly resolved ion beams are necessarily less intense as a result of the requirement to employ narrow slit openings.) High measurement accuracy is also desirable. In general, unambiguous el-
mass
Figure 2. A doublet consisting of [CSD6]+ from deuterated benzene and [C6H12] + from cyclohexane at m/z 84. Mass resolution is 220 000 (full width at half height) at a magnetic field of 1.2 T (reprinted from Reference 15) ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
•
1663 A
Table I. Qualitative Comparison of FTMS Resolution Different Masses and Fields m/z
Reference
a
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Field
28
2.0
250 000
54 000
1.2
220 000
240 000
•
760 000*
15
78
1.2
760 000
19
1166
1.9
60 000
570 500
20
166
500 000
810 000
4.7
Standard conditions, m/z
=
port of a portion of the tris(perfluoroheptyl)-s-triazine spectrum in the m/z 1166-1167 region with m/Am =
60 000 at 1.9 T [19); and the measurement of the m/z 166 ion from tetrachloroethane with an m/ Am value of 1 500 000 at 4.7 T (20). Because mass resolution is directly proportional to
magnetic field strength and inversely
mass at a constant these five measurements are best compared by normalizing to a common magnetic field and mass. The comparison (see Table I) is made for nominal m/z 78 ions at 1.2 T as a convenient choice of field and mass. The values in the table were obtained by
proportional to pressure,
application of Equation 1 to the reported values m/Am. Note that the last three measurements, which are also the most recent, are approximately equal when normalized.
Ri Actual mass , Am (calculated) 78 1,2 T m X X Actual field Am (reported) (1) They approach the theoretical maximum resolution obtainable at the pressures used (about 10“8 torr). This establishes that high resolution should be obtainable on a routine basis even for measurements above 1000 amu. It should be noted that if we had chosen to normalize the results to a 4.7 T magnetic field, singly charged ions of mass 1660 would be expected to be observed with m/Am 150 000 and ions ,
.
,,
=-
.
——
=
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
1
78, 1.2 T
=
—-—
Calculated
84
=
,
(m/Am)
18
FTMS Resolution The capabilities of Fourier transform mass spectrometry are best established with respect to obtaining high-resolution measurements. In one of Comisarow’s and Marshall’s first reports, they showed a mass spectrum of a ternary mixture of CO, N2, and ethylene with a ratio of peak frequency to peak width at half height (m/ Am.) of 250 000 at m/z 28 using a 2-T magnetic field (18). More recent results include: separation of CgDg and CgH12 molecular ions at m/z 84 with m/Am 220 000 at 1.2 T (15) (Figure 2); determination of the benzene molecular ion with m/Am 760 000 at 1.2 T (15); a Nicolet Instruments re-
-r—
(m/Am) Reported
15
CIRCLE 39 ON READER SERVICE CARD
1664 A
(tesla)
at
with m/z 16 with m/Am of 1.5 X 107. Switching from low to high resolution in FTMS is an electronic adjustment and is done by computer commands only (provided the sample pressure is low), which, in principle, can be very rapid. It is conceivable that alternate scans can be low resolution/high resolution. This should be contrasted with magnetic sector instruments for which an adjustment from low to very high resolution is a mechanical one that is usually very time-consuming.
GC/FTMS Some of the most challenging
ana-
lytical instrumentation design prob-
lems arise from the linkage of two or more instruments (often a separation system and a spectrometer) to facilitate mixture analysis. Foremost among such linked instruments is the gas chromatograph/mass spectrometer
(GC/MS) combination. Such instruare in wide use today. Unfortunately, with the development of highperformance GC technology, the capability to separate mixtures with high efficiency has surpassed the ability of conventional quadrupole and sector mass spectrometers to obtain spectra with high mass resolution for single components in the time available with high-resolution GC columns. Spectrometrists have been forced to accept the limited mass spectral resolution ments
(ca. 1000-1500) and mass range afforded by commercial quadrupole mass spectrometers in exchange for the speed (several spectra per second) these instruments allow. Alternatively, higher resolution may be obtained at the cost of either scan speed or a restriction in mass spectral range with double-focusing sector instruments. As a result, lower-performance packed GC columns or capillary columns operated at degraded resolving power are often used to better match the separation with the mass spectrometric capabilities when high mass resolution is needed. It would be desirable to sacrifice none of the high-performance separation characteristics of capillary GC while obtaining high resolution mass spectra of GC effluents.
thentic benzene and 2-propyl iodide (spectrum C, Figure 7). The isomerization was catalyzed by the metal inlet. When the experiment was repeated with a new sample of benzvalene and an all-glass inlet, no m!z 121 and only the internal chemistry of benzvalene was observed. Thus, neither neutral nor ionized benzvalene isomerize to benzene on the time scale of this experiment. The analysis of a trace amount of benzvalene to check whether it had isomeiized to benzene would have been difficult using conventional EIMS because its mass spectrum is identical to that of benzene. However, the simple expedient of using the “derivatization” reaction with 2-propyl iodide allowed us to monitor the isomerization with no difficulty. This example is a demonstration of how distinctive ion-molecule reactions can be employed to identify trace quantities of chemical substances.
MS/MS The analysis of targeted compounds be accomplished rapidly and with high specificity using mass spectrometry/mass spectrometry (MS/MS) (31) with either double- or tripie-sector magnetic mass spectrometers or triple quadrupoles (32). A similar capability exists for FTMS. An ion of interest can be excited using an rf oscillator tuned to its cyclotron frequency. Instead of detection, the translationally excited ion is allowed to undergo collisions with background gas, causing collisional activation followed by decomposition. The spectrum of the decomposition products is acquired by a second but now wide-band excitation followed by detection. Other ionized mixture components present initially in the cell are removed prior to the collisional activation by high amplitude rf pulses composed of frequencies corresponding to their cyclotron motions. The observation of collision-induced decompositions (CID) in an FTMS cell was made recently (26, 33). Although the spectra resemble those obtained for low translational energy ions as observed with triple quadrupole instruments, kilovolt translational energies can be imparted to ions stored in standard cells in the high fields of superconducting magnets (26). can
Future
Figure 7. Experimental demonstration of the isomerization of neutral benzvalene to benzene. Spectra were obtained after a delay of 100 ms between ion formation and detection, (a) Spectrum of benzvalene and 2-propyl iodide after 10 min of sample storage in the inlet system and (b), after 50 min; (c) reference spectrum of benzene and 2-propyl iodide 1672 A
.
ANALYTICAL CHEMISTRY, VOL 53, NO
14, DECEMBER 1981
It is anticipated that methods for doing MS/MS experiments using FTMS will develop rapidly. It should be noted that spectroscopic methods of ion activation in addition to collisions with background gas molecules have been demonstrated already with ICR. Specifically, acquisition of UV/
range produced the results summarized in Table II (25). These results are encouraging and certainly suggest that sufficiently accurate mass measurements are possible with FTMS. A three-parameter procedure is necessary for FTMS measurements with a cubic cell because the trapping field affects the cyclotron resonance frequency. This effect should be reduced by using elongated cells (having greater distance between trapping plates) such as can be accommodated in a superconducting magnet system. This was demonstrated recently at the University of California—Irvine (26) using a cell with a width of 15 cm. Frequency shifts corresponding to less than two millimass units were observed for trapping voltage variations over a range of 1-7 V. Another calibration procedure has been recently reported for data acquired with a system equipped with a superconducting magnet (27). Accurate (± 1.5 ppm) masses were determined by measuring the frequency shifts of low intensity (ca. 1%) side bands adjoining the main resonance frequency as a function of trapping field. It was suggested that these side bands originate because of the anisotropic rf field produced during an ion excitation using only one rather than both transmitter plates. In any event, an important observation was that the accuracy remained very high (± 2 ppm) even after five days without recalibration. This lack of a need for recalibration over extended periods (i.e., perhaps a single calibration per day) derives from the stability of the superconducting magnetic field. Similar stability of instrument calibration has been observed with the lower field superconducting magnets at the University of California—Riverside and the University of California—Irvine. mass
Figure 5. GC/FTMS spectra of methyl formate obtained in an isobutane chemical ionization mode. The time delay is not sufficient to allow formation of m/z 57 (reprinted from Reference 16) tion gas chromatography and to make measurements of selected ions over wide mass ranges. Since FTMS normally employs a magnetic field fixed at some convenient value, there is, in principle, no instrumental limit on the number or identity of ions monitored at high resolution. Switching from observation of one ion to another is accomplished within a few microseconds without limitations due to magnet hysteresis, if magnet switching is employed, or slow settling times of high voltage power supplies, if accelerating voltage switching is used. Another type of analytical measurement easily performed with FTMS is low pressure chemical ionization. By taking advantage of the sample cell’s ability to efficiently trap ions, as suggested by Mclver in 1975 (22), it is possible to make chemical ionization measurements at reagent gas pressures of 10-6 torr or less. This is accomplished by operating the FTMS with a suitable time delay between ion formation and excitation (17). Typical time delays needed to establish a high reagent ion concentration (e.g., CHS+ and C2H5+ from methane) range from 50-250 ms. The time scale is such that
this also is compatible with GC experiments. One such example is the chemical ionization spectrum of methyl formate (see Figure 5) obtained under low pressure conditions using the isobutane m/z 43 peak as the reagent (16). In this example, a constant background of isobutane was maintained via an inlet system as methyl formate eluted from the GC column. The elution was signaled by both the appearance of protonated methyl formate (m/z 61) and the disappearance of C3H7+ (m/z 43). The selected time delay was not sufficient to allow for conversion of m/z 43 to m/z 57. FTMS Mass Measurement
Accuracy As mentioned earlier, the capability measure exact masses is important for unambiguous determination of elemental composition of gas phase ions. Recently this question has been addressed with respect to FTMS (23). Using a cubic cell (24), it was found
to
that an easily derived three-parameter calibration procedure permitted mass measurement accuracy averaging 3 ppm over an 18 amu range, centered at m/z 126. Later work over a greater
Table II. Mass Measurement Accuracy for Ions from Benzene and Dibromobenzene Ion
composition
Measured
mass
Calculated
mass
Error
(ppm)
78.0469
78.0470
1.3
C6H4 79Br
154.9488
154.9497
5.8
C6H4 s1Br
154.9486
156.9477
5.7
CsH4 alBr 79Br
235.8660
235.8661
0.4
c6H6
1668 A
•
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
Chemical Ionization The capability to conduct chemical ionization measurements using ion cyclotron resonance (28) and FTMS (17) is now well established. The ionization process is conducted at low partial pressure (ca. 10~6 torr) of the reagent gas (e.g., methane, isobutane, water, ammonia, etc.) and lower partial pressure of the analyte (ca. 10-8 torr). This should be contrasted with chemical ionization using conventional mass spectrometers for which the reagent gas pressure must be 0.1-1.0 torr. The lower pressures in FTMS are counterbalanced by large ion trapping times that permit the Cl reactions (Equations 2 and 3 illustrated for water Cl) to occur to produce a protonated sample (SH+). (2) HsO+ + OH H20+ + HaO —
H30+ + S
—
SH+ + H20
(3)
Figure 6. Comparison of (a) El and (b) "self Cl” spectra of the insect pheromone Z-7-hexadecenyl acetate
A number of advantages accrue to Cl in an FTMS system. First, switching from El to Cl is very rapid, requiring only a computer command to extend the time between ion production and ion detection (from 0 s for El to approximately 200 ms for Cl). Switching from positive ion to negative ion Cl is also fast and requires only a reversal of the polarity of the trapping field. Higher sensitivity is obtained in the Cl mode. If the sample partial pressure were 5 X 10-8 torr and the rate constant for proton transfer (Equation
were large (ca. 3 X 10-9 cms molecule-1 s-1), 80% of the reagent ions would be converted to product ions within 330 ms. Because the reagent gas is present at higher pressure than the sample, considerably more reagent ions (and ultimately more SH+) are produced than could be by direct electron ionization of the sample. The speed of data acquisition can be increased for slower Cl reactions (such as when the sample partial pressure is less than 5 X 10-8 torr) by acquiring spectra in a “quench-ofF’ mode (17). Ions accumulate in the cell in this mode and are observed over and over as a result of foregoing a quench pulse to drive them from the cell. “Self Cl” or “Cl without a reagent gas” is easily done using FTMS. This experiment is particularly appropriate for compounds that produce abundant hydrocarbon or even electron ion series (C2Hs+, C3H7+,... etc.) by electron ionization-promoted decomposition. The closed shell ions are particularly good gas phase acids and will transfer a proton to the neutral sample molecules if a suitable delay is inserted between ion formation and detection. We illustrate application of this method to the analysis of the insect pheromone Z-7-hexadecenyl acetate (Figure 6). Note the significant improvement in S/N of the Cl spectrum. This is due to the fact that all of the fragment ion current has been
3)
1670 A
•
concentrated in the [M + H]+ ion at m/z 283. Cl measurements can be adapted easily to other reagent ions such as radical cations and metal ions because clustering and polymerization doesn’t occur at the low pressures in the cell. For example, the double bond in the hexadecenyl acetate can be located at the 7-position by reacting the neutral acetate with the methyl vinyl ether radical cation. The observation of a product ion at m/z 170 (Equation 4) indicates the presence of a C8Hi7 substituent on the double bond of the pheromone. CsHn
^^TCH2)(£>Ac
+
170
^"(CH^OAc
(4)
Although this chemistry can be utilized with a high pressure Cl source and a conventional mass spectrometer (29), the experimental conditions are simpler and more rapidly implemented using FTMS. Moreover, application of double resonance techniques in the FTMS mode serves to prove the reaction sequence in Equation 4 and adds confidence to the double bond location assignment. Finally, the mechanisms of Cl reactions can be investigated using the capability to make time-resolved measurements with FTMS. Double resonance schemes and ion ejection allow assignment of reaction pathways.
Chemistry of Gas Phase Ions Ion cyclotron
spectromeextremely powerful tool for studies of the rates and equilibria of gas phase ionic reactions and for investigations of new resonance
try has proved to
be
an
ANALYTICAL CHEMISTRY, VOL. 53. NO. 14, DECEMBER 1981
also posed and solved. We have been interested in the chemistry of ionized benzvalene (I), a valence isomer of benzene, as one part in a long-term investigation of CgHg radical cations. was
0 1
m/z +
chemistry of ions. This research should be promoted by using FTMS because full spectra can be taken in a time-resolved manner in lieu of monitoring a single ion as is done using trapped-cell ICR. We illustrate this capability by presenting one example of an ion chemistry study during which a problem in chemical analysis
Specifically, we wished to know whether ionized benzvalene isomerizes to the more stable benzene radical cation. The strategy adopted to answer this question was to make use of a distinctive reaction of ionized benzene, the reaction with 2-propyl iodide to displace I and form C9Hi3+ (m/z 121) (30). A synthetic sample of benzvalene along with 2-propyl iodide was admitted to the FTMS cell via a gas inlet system constructed of stainless steel. After residing in the all-metal inlet for 10 min, a sample was admitted and spectrum A of Figure 7 was taken, using a 100-ms delay between ionization and mass analysis. The m/z 121 signals either the isomerization of neutral or ionized benzvalene to benzene. The unmarked peaks are attributed to internal reactions of ionized benzvalene and its neutral precursor. The signal at m/z 213 is (C8H7)2l+ and m/z 170 is (CsH7I]t. After the benzvalene had resided in the metal inlet for 50 min, spectrum B (Figure 7) was taken. The internal chemistry of benzvalene could no longer be observed, which we interpreted to indicate complete isomerization of neutral benzvalene to benzene. In fact, the spectrum is nearly identical to that of au-
thentic benzene and 2-propyl iodide (spectrum C, Figure 7). The isomerization was catalyzed by the metal inlet. When the experiment was repeated with a new sample of benzvalene and an all-glass inlet, no m!z 121 and only the internal chemistry of benzvalene was observed. Thus, neither neutral nor ionized benzvalene isomerize to benzene on the time scale of this experiment. The analysis of a trace amount of benzvalene to check whether it had isomeiized to benzene would have been difficult using conventional EIMS because its mass spectrum is identical to that of benzene. However, the simple expedient of using the “derivatization” reaction with 2-propyl iodide allowed us to monitor the isomerization with no difficulty. This example is a demonstration of how distinctive ion-molecule reactions can be employed to identify trace quantities of chemical substances.
MS/MS The analysis of targeted compounds be accomplished rapidly and with high specificity using mass spectrometry/mass spectrometry (MS/MS) (31) with either double- or tripie-sector magnetic mass spectrometers or triple quadrupoles (32). A similar capability exists for FTMS. An ion of interest can be excited using an rf oscillator tuned to its cyclotron frequency. Instead of detection, the translationally excited ion is allowed to undergo collisions with background gas, causing collisional activation followed by decomposition. The spectrum of the decomposition products is acquired by a second but now wide-band excitation followed by detection. Other ionized mixture components present initially in the cell are removed prior to the collisional activation by high amplitude rf pulses composed of frequencies corresponding to their cyclotron motions. The observation of collision-induced decompositions (CID) in an FTMS cell was made recently (26, 33). Although the spectra resemble those obtained for low translational energy ions as observed with triple quadrupole instruments, kilovolt translational energies can be imparted to ions stored in standard cells in the high fields of superconducting magnets (26). can
Future
Figure 7. Experimental demonstration of the isomerization of neutral benzvalene to benzene. Spectra were obtained after a delay of 100 ms between ion formation and detection, (a) Spectrum of benzvalene and 2-propyl iodide after 10 min of sample storage in the inlet system and (b), after 50 min; (c) reference spectrum of benzene and 2-propyl iodide 1672 A
.
ANALYTICAL CHEMISTRY, VOL 53, NO
14, DECEMBER 1981
It is anticipated that methods for doing MS/MS experiments using FTMS will develop rapidly. It should be noted that spectroscopic methods of ion activation in addition to collisions with background gas molecules have been demonstrated already with ICR. Specifically, acquisition of UV/
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VIS spectra of ions in the form of photodissociation spectra is now well established (34). More recently, acquisition of partial infrared spectra of ions has been accomplished using multiphoton absorption from an IR (CO2) laser (35). These methods should be adaptable to mixture analysis in an MS/MS mode simply by adding suitable high-amplitude pulses to eject unwanted ions from the cell prior to the activation. The exceptional mass resolution of FTMS may make it the method of choice for analysis of high molecular weight materials. There is no mass discrimination inherent in the FTMS detection scheme. The ultrahigh resolution at low mass extrapolates to reasonable resolution at high mass. For example, a resolution of 800 000 at m/z 78 and 1.2 T becomes 8000 at m/z 7800. Increasing the magnetic field by a factor of 10 should permit m/z 7800 to be observed with a resolution of 80 000. Of course, this can only be accomplished if high molecular weight materials can be volatilized and ionized. Pulsed laser desorption, which is compatible with the pulsed FTMS detection, may be appropriate. Fast atom bombardment is another possibility for volatilization of materials that are ionic in the solid state. Advances in FTMS are tied directly to progress in electronics and computers, both of which have been highgrowth technologies. For example, we should expect the addition of sensitive broad-band amplifiers for improving S/N ratios and array processors for doing real-time Fourier transforms in the near future. In conclusion, we point out that the experiments done to date are of a demonstration nature and represent efforts to develop methodology. The real test for FTMS will come when analytical chemists have access to these instruments and apply them to real analytical problems.
Acknowledgment We are particularly indebted to
our
co-workers Sahba Ghaderi, E.B. Ledford, R. L. White, and R. B. Spencer for the progress we’ve made in analytical FTMS. We also thank R. T. Mclver, Jr., and M. M. Bursey for helpful comments on this manuscript.
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References (1) Henis, J. M. S. Anal. Chem. 1969,41 (10), 22-32 A. (2) Gross, M. L.; Wilkins, C. L. Anal. Chem. 1971, 43 (14), 65-68 A. (3) Wilkins, C. L. Anal. Chem. 1978,50, 493-500 A. (4) Mclver, R. T., Jr. Am. Lab. 1980,12, 18-30. (5) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974,25, 282-83. (6) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974,26,489-90. (7) Comisarow, M. B.; Marshall, A. G. Can. J. Chem. 1974, 52,1997-99.
(continued
on
page 1676 A)
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(8) Hunter, R. L.; Mclver, R. T., Jr. Am. Lab. 1977, 9, 13-19. (9) Fellgett, P. J. Phys. Radium 1958, 19, 187-91. (10) Marshall, A. G.; Comisarow, M. B. Anal. Chem. 1975,47, 491-504 A. (11) Bell, R. J. “Introductory Fourier Transform Spectroscopy”; Academic Press: New York, 1972; pp 23-25. (12) Comisarow, M. B.; Marshall, A. G. J. Chem. Phys. 1976,64, 110-19. (13) Ledforcl, E. B. Jr.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Ada. Mass Spectrom. 1980,8A, 1707-24. (14) Ledford, E. B. Jr.; White, R. L.; Ghaderi, S.; Gross, M. L.; Wilkins, C. L. Anal. Chem. 1980,52,1090-94. (15) White, R. L.; Ledford, E. B„ Jr.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980,52, 1525-27. (16) Ledford, E. B., Jr.; White, R. L.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L.; Anal. Chem. 1980,52,2450-51. (17) Ghaderi, S.; Kulkarni, P. S.; Ledford, E. B„ Jr.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1981,55,428-37. (18) Comisarow, M.; Marshall, A. G. “Abstracts of Papers,” 23rd Annual Conference on Mass Spectrometry and Allied Topics, Houston, Tex., 1975; paper R5, 453-55. (19) Nicolet Analytical Instruments. “Pre-
liminary Bulletin for 1981 AMS Conference,” Nicolet FT-MS 1000, 1981, 5.
(20) Allemann, M.; Kellerhals, H.P.; Wanczek, K. P. Chem. Phys. Lett. 1980, (21) White, Robert L.; Ledford, E. B., Jr.; Wilkins, C. L.; Gross, M. L. “Abstracts of Papers,” 29th Annual Conference on Mass Spectrometry and Allied Topics,
Minneapolis, Minn., 1981; MAMOA2. (22) Mclver, R.T., Jr.; Ledford, E. B„ Jr.; Miller, J. S. Anal. Chem. 1975, 47, 69297.
(23) Ledford, E. B., Jr.; Ghaderi, S.; White, R. L.; Spencer, R. B.; Kulkarni,
American Chemical Society
Attn: Circulation Development 1155 16th St., N.W. Washington, D.C. 20036
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i would like to begin AIR FREIGHT service immediately on my ANALYTICAL CHEMISTRY subscription. I understand you will bill me for the appropriate amount. I have attached a recent mailing label of ANALYTICAL CHEMISTRY.
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