Anal. Chem. 1989, 61 749-754
749
Multichannel Heterodyne Detection for Accurate Mass Selected Ion Monitoring in Fourier Transform Mass Spectrometry: Implementation and Comparison with Undersampling D. L. Rempel, E. B. Ledford, Jr.,l T. M. Sack,and M. L. Gross* Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588 A multlchannel recelver was bullt to monltor simultaneously and at high mass resolutlon Ion specles of three different and wldely spaced masses In a Fourler transform mass spectrometer. The method of detectlon, whlch Is slmllar In Intent to peak matchlng In sector mass spectrometry, was used to test the feastbillty of petformlng accurate mass measurements wlth wlde mass separation between test and reference Ions. Sub-parlper-mllllon mass measurement errors were obtalned under condltlons of low space charge by using reference Ions separated from the test Ions by approxlmately 50 % Thls capablltty Is unique to Fourler transform mass spectrometry (FT-MS). An alternate FT-MS approach for slmultaneous detectlon of peaks that are widely separated In mass Is undersampllng or “foldover”. The two approaches are compared by treailng the nolse for each, and It Is found that undersampllng Is more susceptlble to nolse than are heterodyne techniques.
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INTRODUCTION In ion cyclotron resonance (ICR) and Fourier transform mass spectrometry (FT-MS) (I),two methods have been used to extract the cyclotron frequency spectrum of ions stored in the spectrometer trap. In the so-called “narrow-band” mode of operation, various electronic means [reference electrometer detection (2),marginal oscillator (31, heterodyne receivers (411 permit a narrow region of the cyclotron frequency spectrum to be analyzed. A complete spectrum can be obtained by sequentially examining a multiplicity of narrow bands. Because each narrow-band data acquisition requires a length of time on the order of a cyclotron transient response, very long periods of time (ca. 10 min),however, are required to construct a full spectrum with this method. In the “broad-band” mode of operation, data sufficient to construct a full spectrum are acquired during a single ICR event sequence. Broad-band detection permits rapid location of spectral intensity, but at the expense of resolution, which is limited by practical interrelationships between analog-to-digital conversion rate, buffer memory size, and the time required to perform large (>256K) discrete Fourier transforms with present-day digital computers (5). To illustrate the last point, consider the time required to measure directly the mass range of m/z 78 to 156 by using Torr and 3 T. That time is an FT-MS ion trap a t 3 x 20.7 s if data are acquired for two times the time constant for decay (calculated from a collision frequency of 9.66 X as was recommended (6). The number of data points needed for this time while sampling at the Nyquist rate is 23.6 X lo6. A 32768K transform requires 768 s given that a commercially available FT-MS array processor performing lo’ multiplication/s requires 1.9 s to compute a 64K transform. In this paper, a method for observing a t high resolution widely separated peaks in ion cyclotron frequency spectra is demonstrated in which the cyclotron interferogram is passed Present address: ICR Research Associates, Inc., Lincoln, NE.
to a parallel array of heterodyne receiver circuits. Each of these circuits functions as a “window” on the frequency spectrum. The position and width of each “window” are governed by the reference and low-pass cutoff frequencies, respectively, of each heterodyne receiver channel. By simultaneously observing a multiplicity of narrow to moderately wide frequency bands, important regions of the frequency spectrum can be analyzed a t high resolution with no loss of multichannel advantage over those regions. The array of heterodyne receivers was used to evaluate the mass calibration law for the cubic cell over a wider mass range than was previously reported (7). Another means of observing at high resolution widely separated peaks in ion cyclotron frequency spectra involves permitting the higher frequencies of low mass ions to fold back into the higher mass region of the spectrum as was introduced by Cody and Kinsinger (8). This approach, referred to as undersampling or “foldover”, was known to the analytical chemistry community in 1970, when Horlick and Malmstadt used it for FT-IR (9). Since its introduction to FT-MS, Marshall et al. (IO) combined undersampling and tailored excitation to permit the use of large numbers of “foldovers”. A proposed important application of multiple “foldover“ is selected ion monitoring. High signal-to-noise ratios (low detection limits) are often an essential requirement of selected ion monitoring. The response of “foldover”to noise is treated in this paper. It is found that the “foldover”technique is more susceptible to noise when compared to heterodyne techniques especially under conditions of large numbers of foldovers. EXPERIMENTAL SECTION A prototype array of heterodyne circuits, called here the “multichannelreceiver” (MCR),was designed and built to operate in conjunction with the hardware and software of the Nicolet FTMS-1000 console (see Figure 1 for a schematic drawing). The present configuration of the MCR is capable of monitoring three 2-kHz windows with the provision for adding five more in the future. In the acquisition of a single cyclotron interferogram,the interferogram was presented to all three heterodyne channels. The three resultant audio frequency time-domain signals were analog-multiplexed to the analog-to-digital (A/D) converter of the FTMS-1000 data system. This was done in a manner so that each time domain signal was sampled at the Nyquist rate. The sampling of the time domain signals was simultaneousin the sense that the multiplexer sequenced through all channels before the next sample of the first channel was taken. As a result of this procedure, the samples for each signal were interleaved with the samples of other signals in the computer memory. Multiple interferogramswere coadded in memory before further processing. Time domain data were sorted in place to reconstruct in computer memory three contiguous digitized transients, which were submitted to fast Fourier transformation (FFT). The use of FORTRAN modules and user-defined procedures (macro-command files) provided access to any of the three high-resolution spectra. A useful presentation of MCR data was constructed by positioning three narrow-band high-resolution spectra above a broad-band low-resolution FT-MS spectrum (Figure 2). The mixers employed were Analog Devices Model 429 A, followed by passive LRC low pass filters with 2-kHz cutoff frequencies. Each mixer board derived its reference signal from a
0003-2700/89/0361-0749$01.50/0@ 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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digital frequency synthesizer internal to the MCR chassis, or external via a relay board. The internal frequency source was the Proteon Associates, Inc., Pro80 frequency synthesizer. Low pass fiter outputs were routed to a sample/hold (Analog Devices AD583) through an analog multiplexer circuit. Sample/hold and multiplex address generation signals were tapped from the A/D converter board of the Nicolet FTMS lo00 data system. All boards were of Multibus configuration and were controlled remotely by transmitting ASCII sequences to a single board (Multibus) microcomputer (Intel 8086) via a RS232C serial link or by means of a front panel control module (Burr-brownTM-77). A variety of functions (e.g., phase or frequency scanning,externally triggered synthesizer start, remote or local control, and synchronous stepping of all channel reference frequencies) was provided by the software written for the 8086. The multichannel receiver (Figure 1)was assembled by Detrek Engineering, Inc., of Kansas City, KS. The FT mass spectrometer was built around a Varian electromagnet (1.2 T) and interfaced to a Nicolet FTMS 1000 data system. The combined system was described previously (7). The experiments were conducted at total pressurea in the low lo4 Torr range and with an ionizing energy of 70 eV. The trapping voltage was 1 V. All chemicals were from commercial sources and were used without further purification. RESULTS AND DISCUSSION The first application of the MCR was to perform new and more rigorous tests of the mass calibration law introduced by Ledford, Rempel, and Gross (7). Functional dependences of mass measurement accuracy upon the mass difference between calibrant and analyte peaks, the number of stored ions, excitation waveform, and ion orbit size were studied. Effect of Mass Separation between Reference and Unknown. Because the mass calibration law is a closed-form solution to a characteristic equation (7), mass measurement accuracy is theoretically independent of the mass differences between calibrant and analyte ions. To test this, the accurate
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