Instrumentation
John F. Holland Christie G. Enke John Allison John T. Stults J. David Pinkston Bruce Newcome J. Throck Watson Departments of Biochemistry and Chemistry Michigan State University East Lansing, Mich. 48824
Mass Spectrometry on the Chromatographic Time Scale: Realistic Expectations For the analysis of complex mixtures, combined gas chromatographymass spectrometry (GC/MS) is recognized as an invaluable tool. The coupling of chromatographic separation with a two-dimensional (mass, intensity) detector that can be highly selective provides the solution to many analytical problems ranging from monitoring pollutants in wastewater (1 ) to measuring metabolite levels in body fluids (2). Continuing improvements in chromatographic resolving power, necessitated by the demand for better separations, can lead to a strain in the temporal relationship between residence time of the sample in the ion source and mass spectral scan time. In many cases, mass spectrometer scanning rates are being pushed to their limit. An examination of the scanning capabilities of modern mass spectrometers is necessary to assess the feasibility of obtaining adequate mass spectral data from increasingly narrow chromatographic peaks. As the technique of combined GC/MS has matured, there have been two compelling reasons to scan the mass spectrometer more quickly; one relates to MS, the other to GC. The first is to minimize changes in sample concentration in the ion source during the time a spectrum is acquired in order to minimize distortion of relative peak intensities for a given spectrum. With sample introduction from a typical packed column this can be accomplished by obtaining the complete spectrum in 1-2 s. For capillary column GC/MS the same problem can be minimized by collecting the spectrum in 0.2-0.5 s. The second reason for requiring high scan rates in GC/MS is to increase the frequency at which complete mass spectra are collected. High repetition rates are im0003-2700/83/0351-997A$01.50/0 © 1983 American Chemical Society
portant for those workers who wish to reconstruct the chromatogram from consecutively recorded mass spectra (3-6). The greater the frequency of spectrum acquisition, the greater the number of points available to define the chromatographic profile. Selected ion monitoring, an alternative approach, provides a large number of points per chromatographic peak, but this specialized technique is useful only for analyses in which the ion cur-
rents at a few preselected masses are of interest. The example in Figure 1 illustrates the critical dependence of scan rate on the capacity of a mass spectral data base to accurately represent chromatographic resolution in GC/MS applications. Note that the GC peak in Figure 1 is approximately 3-4 s wide. The mass chromatogram in (a) represents a chromatographic doublet, but the apparent magnitude of the second
Figure 1 . C o m p a r i s o n of t r u e gas c h r o m a t o g r a p h i c p r o f i l e ( g r e e n line) a n d s i m u l a t ed m a s s c h r o m a t o g r a m s (blue line) r e c o n s t r u c t e d f r o m a c q u i r e d data b a s e s (a) Mass chromatogram prepared from mass spectra acquired at a rate of 1 s c a n / s . (b) Mass spectra acquired at rate of 1 scan/s. but synchrony of chromatogram and scan cycle shifted by one-third second, (c) Mass spectra acquired at rate of 3 scans/s
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component has been attenuated. In (b), the mass chromatogram repre sents the true magnitude of the second component, but the first component is not apparent in this profile. In (c), agreement between the mass chro matogram and the true chromato graphic profile is adequate, but a bet ter degree of fit would be assured from an even shorter scan cycle, i.e., faster scan speed. Capillary columns that have 106 theoretical plates are now becoming available (7). Such columns have the capacity to present on the order of ten resolvable peaks/s, especially early in the chromatogram (8). For reasonable reconstructed chromatograms under these conditions, a mass scanning (e.g., m/z 50-500) repetition rate ap proaching 100 Hz (100 scans/s) will be required. It is apparent that chromatography will continue to make demands for faster and faster scan speeds in GC/MS applications. The more rapid ly a mass spectrometer is scanned, however, the less time is available for assessment of each peak in the mass spectrum (fewer ions will be detected) and the accuracy of ion current mea surement (ion statistics) will diminish. Hence, the parameters of scan speed and ion statistics have a diametric re lationship in MS. The functional characteristics of the mass spectrometer limit the extent to which one can increase the scan speed in pursuing the task of accurately rep resenting transient sample concentra tions in the ion source. This limit is a function of the physical principles underlying the mass selection process and the characteristics of the ion de tector and recording system. These features will be analyzed in this report and a feasible solution to the increas ing demand for faster scan cycle rates will be presented. A proposed solution is based on the application of timeof-flight (TOF) mass spectrometry. Analysis of Performance Trade-Offs of Present Generation G C / M S Instruments
Three characteristics—scan speed, sensitivity, and useful dynamic range—affect the compatibility of a mass spectrometer with a high-resolu tion chromatograph. Limitations on these performance characteristics can be broken down into three categories, independent of the type of instru ment. They are: • limitations arising from the physical laws governing the mass selection pro cess, • bandwidth of the detector and elec tronic processing components, and • ion statistics, that is, the number of ions observed in the signal measure-
Magnetic Sector MS Ions are produced continuously in the source and are accelerated toward the magnetic field by a potential, V, such that all have the same kinetic energy (KE = %mv2 — zeV, where e is magnitude of the electronic charge, ζ is the number of charges on an ion, m is the mass of the ion, and ν is its velocity). The magnetic field separates the components of the total ion beam according to momentum. By this means, the individual ion beams are separated spatially and each ml ζ value will have a unique radius of trajectory, R. Commercial GC/MS systems use focused optics with slits. Thus, for a given magnetic field, B, only ions of a single m/z (m/z = B2R2/2v) will have the proper trajectory leading to the detector. By changing the magnetic field strength, ions of differing m/z values are brought to focus at the detector slit. Electromagnet
Ion Source
ment at the peak maximum at each m/z value. The two dominant GC/MS instru ment types, magnetic sector and quadrupole, and the less common TOF and Fourier transform (FT) instru ments, will be examined with regard to these categories. Magnetic Sector Instrument (see box on Magnetic Sector MS). In most magnetic sector instruments, ions of different m/z are transmitted by using different magnetic field strengths. Thus, magnet characteristics deter mine the maximum scan speed in both single-focusing and double-focusing mass spectrometers. Magnetic mass spectrometers are constructed so that each ion experiences a constant and homogeneous magnetic field. How ever, if fast scan speeds are employed, the field can change significantly dur ing the time it takes an ion to traverse the magnet (ion transit time), causing ions to follow a distorted path to the detector; thus, design characteristics of the instrument are not met. Conse quently, when scanning toward in creasing field strength, the effective radius will be shorter than that corre sponding to the constant field value. When scanning toward decreasing field strength the opposite effect is produced. The result in either case would be a loss in both resolution and sensitivity because the exit slit is no longer at the focal point of the optical system. Electromagnets exhibit a kind of in ertia, called reluctance, that limits the rate at which a magnet can be forced to change field strength. This phe nomenon presently limits scan rates to 0.1 s/decade for modern fully laminat-
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Detector
ed magnets (a mass decade is 50-500 u for example). A time of about 0.2 s is required to reset the magnet between scans. Hence, a state-of-the-art scan repetition rate for magnetic sector in struments is 3-4 Hz. At these scan rates, the typical ion path variation would be less than 2% and subsequent defocusing not critical. Thus, the scan rate is not limited by defocusing of the ion optics, but by reluctance of pres ently available magnets. Quadrupole Instrument (see box on Quadrupole MS). The quadrupole mass spectrometer performs mass sep aration in a completely different man ner than does the magnetic sector in strument. The ions interact with an oscillating electric field in such a way that only ions of a specific m/z will be allowed to pass to the detector. For maximum transmission of the selected ions, the radio frequency (rf) and dc voltages should not change during the transit time of an ion through the filter. An ion of mass 800 u with an energy of 10 eV requires 129 μ& to pass through a 20 cm quadrupole filter. Using an intuitive approach, if one scans at a rate such that the mass fil ter changes by no more than 0.1 u dur ing the transit time of this ion, a maxi mum scan rate of 780 u/s can be reached before resolution, peak shape, and intensity are significantly sacri ficed. Under these conditions 90% of the ion current at m/z 800 is transmit ted to the detector. Somewhat faster scanning rates have been achieved without sacrificing sensitivity by in creasing the ion energy concurrently with the mass scan ramp in order to maintain a more constant ion velocity
and reduce the transit time for heavier ions. Faster scanning also can be achieved if the mass-selecting voltages are stepped rather than scanned; how ever, the resolution of the mass axis is severely degraded as fewer steps per mass unit are used. By a combination of these techniques, a useful upper scan repetition limit of 4-8 Hz may be achieved in the most recent quadrupole instruments. The ion trap detector, a mass spec trometer related to the quadrupole, has recently been introduced commer cially. This instrument uses geometry that is similar to a cross section of a quadrupole filter. Ions are created in and trapped by an rf-only field (9). The trapped ions are deflected to an electron multiplier detector by a novel scheme that separates the ions by their m/z values. The exact perfor mance characteristics of this instru ment are, as yet, unpublished. How ever, this ion-trapping mass spectrom eter has been made specifically for GC applications and appears to have the potential for very rapid scanning (10). Time-of-Flight Instrument (see box on TOF MS). The TOF mass spectrometer presently does not enjoy the popularity of the other two types of mass spectrometers for chromato graphic analysis, but because it has the potential to produce complete spectra at a rate of 10 000 spectra/s, it warrants careful consideration in sit uations where high acquisition scan rates are necessary. One of the pri mary advantages of the TOF instru ment is that a complete spectrum is produced serially at the detector for each pulse of ions from the source. The spectrum repetition rate is fun damentally limited by the transit time of the heaviest (slowest) ion in the mass spectrum. For a 2-m flight tube and ion energy of 2 keV, an ion of 800 u requires approximately 90 μβ to travel from the source to the detector. Thus, it is possible to acquire a com plete mass spectrum from 0 to 800 u every 90 μβ—a repetition rate of about 11 kHz. Commercially available TOF mass spectrometers acquire spectra by sam pling only a single arrival time window (time slice or time bin) corresponding to only one mass position in the spec trum for each pulse of the source. By collecting other time slices at different time delays after successive source pulses, a complete mass spectrum can be obtained. To maintain adequate resolution, the width of the time slice must be small, on the order of 5 ns. To acquire a single 5 ns slice from each source pulse throughout the complete range of arrival times (up to 90 μβ) would require an acquisition scan time of 1.8 s. Since ions of all masses in the spectrum are available serially at the
Quadrupole MS Ions are formed continuously in the source and are accelerated toward the quadrupole mass filter. The four quad rupole rods carry both rf and dc volt ages. Ions interact with the electric field created between the rods and are accelerated in directions perpendicular to the initial velocity vector into com plex trajectories that are a function of the ml ζ value; the rf frequency, ω; the magnitude of the rf voltage; the mag nitude of the dc voltage; and the ge ometry of the rods. By controlling the
ratio of Vdc/ Vrl, the field can be es tablished to pass ions of only one m/z value, where ml ζ oc Vrf/u>. Ions smaller or larger than this m/z will strike the rods. By simultaneously ramping the dc and rf amplitudes, ions of various m/z values are allowed to pass through this "mass filter" to the detector.
Detector
Quadrupole Mass Filter
Source
detector after each source pulse, mul tiple slices could be acquired from each pulse so that the overall data ac quisition time could be shortened and the scan rate increased or the signalto-noise ratio (S/N) improved. Data systems for TOF instruments now allow acquisition of full spectra at
Source
8-10 Hz. The maximal advantages in sensitivity and S/N that can be real ized by successive summing of individ ual time slices must be traded off against the time required for total spectrum scanning. In reality, the us able scan speed of T O F is presently limited by the method of data collec
Flight Tube
Detector
TOF M S This type of mass spectrometer operates in a pulsed rather than a continuous mode. The ions are formed in the source and a discrete ion packet—initially consisting of ions of a variety of m/z values—is pulsed into the field-free region of the flight tube (see a). All of the ions are accelerated to the same kinetic en ergy, hence each ion has a characteristic velocity that depends on its mass (v = (2zeV/m)1/2). As a result, ions of different m/z values spatially separate as they travel down the flight tube (see b). Ions of high velocity (low m/z) arrive at the detector before ions of lower velocity (high m/z). Thus, the TOF of ions can be correlated with their mass.
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FTMS Ion formation and mass analysis occur in a cubic cell roughly two centimeters on an edge, which lies between the pole caps of an electromagnet, or in a superconducting solenoid magnet. The instrument is operated in a pulsed mode. In (a), a burst of electrons forms an assortment of ions in the cell. In the presence of a constant and uniform magnetic field, all ions move in circular orbits with a characteristic cyclotron frequency, (w c ), depending only on the ion's m/z value (a>c = ζθΒ/2πιτι). In (b), a frequency-swept " c h i r p " signal is applied to the cell. Ions that experience oscillating fields equal in frequency to their cyclotron frequencies absorb energy from the circuit generating the "chirps." These ions move to orbits of larger radius. In (c), the translationally "excited" ions move coherently between the receiver plates and establish image currents in an external conducting net work attached to these plates. Thus, the ions transmit a complex rf signal that contains frequency components characteristic of the ions present. The time-dependent image current is subjected to Fourier transformation to yield a mass spectrum.
tion and storage, not by fundamental principles of mass analysis. Fourier Transform Instrument (see box on FTMS). First demon strated by Comisarow and Marshall (11-13), the exciting new field of FTMS was reviewed recently (14). Fourier transformation of the time de pendent transients from coherently excited ions in an ion cyclotron reso nance mass spectrometer produces a frequency (related to mass) spectrum of the ions. One of the major features of this instrument type is its capacity for high-resolution mass measure ments. High-resolution selected ion monitoring GC/FTMS as well as chemical ionization (CI) and mixed Cl/electron ionization GC/FTMS have been described (15). High vacuum (low sample pressure) is necessary for high resolution, thus requiring high pumping speeds or specially designed chromatographic interfaces (16). However, for low-resolution GC/MS, the pressure requirements are compa rable to those of other common mass spectrometer types. The accuracy and
precision of the spectra produced in FTMS depend on the integrity of the experimental parameters and on the quality of the Fourier transformation. Modern microelectronics enable the transformation to be made on single transient decay waveforms in the mil lisecond time domain, a time scale that will accommodate scanning speeds compatible with chromatog raphy. Reasonable retention of chromato graphic resolution can be achieved in GC/FTMS as demonstrated in mass chromatograms reconstructed from mass spectra obtained every 0.3 s from a number of averaged transients (17). Since the ionization, excitation, and data collection each can occur on a millisecond time scale, scan speeds ap proaching 10 Hz are attainable. As with other measurement sys tems, the number of transient signals to be averaged (observation time) for transformation determines the accura cy of the corresponding spectra. How ever, FTMS is unique in that in creased measurement time increases
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both sensitivity and resolution, which presents an interesting potential for application in time domains that are less restrictive than those of GC/MS. Signal-Processing Components. Detector response and electronic com ponent bandwidth are major problems whenever older instrument systems are used for GC/MS. Magnetic sector instruments in particular were origi nally designed with high-speed re cording galvanometers; bandwidths from 3 to 10 kHz were sufficient. The attachment of spectrometers to com puters, and the subsequent utility of these systems as chromatographic de tectors, have increased the bandpass demands on the analog sections of the ion detection and measurement cir cuits. Modern circuit design and construc tion can produce bandpasses suffi ciently wide so as not to be limiting at scan speeds anticipated for GC/MS. A special case—the FTMS instrument— is a direct outgrowth of advances in modern microelectronics. This instru ment resolves periodic components
present in an array of frequencies. For this application, the higher the digi tization rate, the greater the certainty in the measurements. Transformation of single transients to produce signifi cant scan ranges requires digitization rates in excess of 2 MHz and large high-speed memory arrays. These de vices are now available for this appli cation. For the other types of mass spec trometry, the events being measured are discrete ion collisions on the detec tor. With this type of detection, the higher the bandpass, the greater the noise. For other than ion counting techniques, the rule of thumb has been to use maximum filtering com patible with the desired scan speed. For example, for scan repetition rates approaching 10 Hz, a bandpass of ap proximately 100 kHz is required. For integrating or rate measurement elec tronics, this presents a problem since fewer ions will be involved in the mea surement and a greater noise results from the random distribution of ion arrivals. At rapid scanning speeds, the slow-response amplifiers that normal ly provide a filter for this noise cannot be used. In these measurement tech niques, the accuracy cannot exceed that of classical ion counting ap proaches, in which the accuracy of the measurement is equal to 1 part in ΛΠ/2, where Ν is the number of ions detected at a given mass value. Either the ion current measurement or the ion counting approach can be used to achieve scan times of 0.1 s/decade, but a price must be paid in precision and accuracy as well as in sensitivity. Thus, for conventional GC/MS, ion statistics become the limiting factor, not circuit design or component selec tion. Ion Statistics. The accuracy with which ion current measurements can be made depends on the total number of ions involved in the measurement. In order to present a useful analytical dynamic range, reasonable levels of S/N must be achieved. Again, this pre sents a situation in which the ultimate speed of scanning may be compro mised. For magnetic and quadrupole instruments, the relationship of ion statistics to scan speed is straightfor ward. Assuming unit resolution at m/z 800, the total number of ions available for detection across one mass unit will be linearly related to the observation time. For example, in a magnetic in strument using a scan rate of 2 s/dec ade and a typical ion current of 10~ 12 A, approximately 3390 ions will strike the detector during the observation time for that ion. Assuming a resolu tion of 0.1 u and a triangular distribu tion, approximately 640 ions establish the peak current value. The relative error of the measurement will be
(640) _ 1 / 2 or ±4%. At a scan rate of 0.1 s/decade this relative error will in crease to ±18%. Clearly, a compromise between ion observation time and the rate of spectrum generation must be made with reference to accuracy and dynamic range objectives of the anal ysis. Ion currents available in TOF MS are directly dependent upon sample concentration, ionizing efficiency, and ion transmission efficiency. The ne cessity for resolution in the time (mass) axis requires that the time width for each extracted ion packet be extremely short—on the order of a few nanoseconds. This pulsed extraction significantly limits the number of ions available for analysis. For a 10-ns pulse width followed by a 100-μβ anal ysis time, the attenuation would be 104. Various schemes for ion trapping and storage have been developed (18) in an effort to reduce these losses. On the other hand, tight radial ion focus ing is not needed, permitting increases in the aperture of ion transmission and collection efficiency above that available with other mass spectrome ters. Additionally, the high scan repe tition rates (>10 kHz) would permit spectrum summing. The combination of large ion beam cross section, ion source trapping, and successive spec trum summing apparently compen sates for the attenuation caused by pulsing; sensitivities comparable to those from other types of mass spec trometers have been observed (19). The TOF instrument has the capacity to provide GC/MS with a different and possibly more favorable set of limitations in scan rate optimization than the scanning mass filter MS techniques. In FTMS, the amplitudes and frequencies of the cyclotron motion of ions are being detected. In this case, the number of cycles observed for any specific ion will be the limiting factor in the accuracy and sensitivity of the mass and intensity measurements. The digitization rate and the precision of the analog-to-digital conversion af fect the quality of the transformation. The dynamic range for the intensity measurements is determined by the accuracy and resolution (number of bits) of the analog-to-digital converter (ADC). Array Detection in Mass Spectrometry It is clear from the above discussion that the ultimate limitations in spec trum repetition rate in mass spec trometry may arise from the funda mental laws that govern the scanning process and the minimum time re quired to measure the intensity at each mass. Relief from these limita tions can be found in techniques that
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avoid the scanning process altogether by detecting substantial portions of the mass range at essentially the same time. Three very different techniques by which an array of masses can be de tected without scanning are the spa tial array detector used with magnetic mass dispersion, the FT implementa tion (frequency array detection) of ion cyclotron resonance MS, and time array detection in TOF MS. The present capabilities of these three "array detector" techniques and their potential for GC/MS application are discussed briefly in this section. Spatial array detection. In early applications, Boettger and Giffin (20-22) extended the spectroscopic utility of microchannel electron multi pliers described by Santini (23) and developed an electro-optical ion detec tor (EOID) that serves as an "elec tronic photoplate" for a double-focus ing mass spectrometer. After overcom ing several difficulties in the operation of microchannel plates in strong mag netic fields (21), these workers con nected several discrete sections of mi crochannel plates and associated pho todiode arrays and fiber optic rotators to integrate ion currents in the mass range of m/z 40-700. Boerboom et al. (24) used a small version of the EOID in recording rap idly changing phenomena such as those in flash pyrolysis and laser pulse desorption. They used quadrupole lenses to adjust the mass range detect ed by the EOID. Hedjfall and Ryhage (25) also adapted a small version of the EOID for evaluation in a single-focusing mass spectrometer with the view toward GC/MS applications. How ever, they concluded that several fea tures in state-of-the-art components of the EOID prevent the integrated unit from offering advantages over a conventional detection system consist ing of a single electron multiplier (26). For example, the gains of the individ ual microchannel detectors in the array differed in magnitude and var ied independently with time over peri ods as short as 30 min. These investi gators were not able to demonstrate a dynamic range greater than 100 even with cooling of the photodiode array to —35 °C to reduce the dark current. Attempts to improve the dynamic range by using different integration times with rapid switching of the volt ages applied to the microchannel plates were thwarted by the long re covery time of the detector. In sum mary, the EOID does not yet appear to have evolved sufficiently for general purpose GC/MS applications. Frequency Array Detection. FTMS provides an excellent example of an array detection scheme since all ions are truly detected simultaneous-
ly. Pulsed ionization followed by rapid (generally on the order of millisec onds) simultaneous detection elimi nates the problem of a changing sam ple concentration in the ion source during acquisition of the mass spec trum, which can occur in a conven tional sector or mass filter instrument (27). It appears that the sensitivity is ap proaching levels suitable for GC/MS; however, results to date indicate the dynamic range of FTMS is such that the analysis of complex mixtures, where component concentrations may differ by as much as 104, may be diffi cult (28). Available dynamic range de pends on the number of transients av eraged to reduce the uncertainty (noise) of the measurement and upon the resolution of the ADC. Time Array Detection for TOF MS. As pointed out in the initial dis cussion, every ion source pulse in the TOF MS contains information on the complete mass spectrum. It is the scanning of the sampled time slice in current instrumentation that produces the present scanning rate limitation for data collection. Figure 2 depicts the time slice detection process. For
simplicity, the figure shows these time slices as detection windows or time bins having a "width" of 0.1 μβ. The location of the opened time bin is scanned along the time axis, from low values to higher values of arrival time. In this mode, only a O.l^s-wide por tion of the complete spectrum pre sented to the detector is collected for each pulse of the ion source. If the "opened" time bin is removed 0.1 μβ with each successive pulse, ions at m/z 27 will be collected from the 69th pulse because ions of m/z 27 have a flight time of 6.9 μβ under these condi tions. Ions at m/z 28 are collected from the 70th pulse, and so on, until ions of m/z 58 appear at the detector in synchrony with the open time bin following the 101st pulse. Thus, using the conditions described, 101 pulses are required to produce a complete spectrum because 99% of the available information in each pulse is ignored in this example of time slice detection. If all the information in each pulse were used, the spectrum acquisition rate could be increased 100-fold or more. Time array detection provides complete utilization of the available
F i g u r e 2 . C o m p a r i s o n of t i m e s l i c e and t i m e a r r a y d e t e c t i o n A simulated TOF mass spectrum is shown for η-butane, using a flight tube length of 100 c m and an ac celerating voltage of 3000 V. in time slice detection, only one time bin is measured for each pulse of the ion source, necessitating multiple pulses to acquire the entire spectrum. In time array detection, the entire spectrum is acquired from each pulse of the ion source
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information, as shown in the bottom panel of Figure 2. Following each pulse the time bins are opened rapidly (on a nanosecond time scale) in a con tinuous sequence. Thus, no informa tion is lost. In several ways, the conceptual rep resentation in Figure 2 of the TOF spectrum and its acquisition are oversimplified. In reality, TOF mass spectral peaks approximate Gaussian curves with finite widths. Also note that the time bin width of 0.1 μβ por trayed in Figure 2 would not be practi cal for compounds with molecular weights much larger than 60. Since the arrival time of an ion in TOF is pro portional to (m/z) 1 / 2 , the interval be tween arrival times of ions of adjacent m/z decreases as m/z increases. The difference in arrival times of m/z 12 and m/z 13 is 0.19 μβ; however, this value shrinks to 0.05 μβ for the ions of m/z 200 and 201. Thus, the width of each time bin must be small enough so that, over the entire mass range, ions of only one m/z value will be detected in a given bin. As the width of the time bin is decreased in order to improve resolution on the mass axis, the sam pled fraction of the spectrum in time slice detection is decreased. Given the same analysis time, time array detec tion will be more sensitive than time slice detection by the inverse of this fraction. Although improvements in S/N, sensitivity, and dynamic range over normal TOF MS can be predicted by time array detection (TAD), com parisons with other types of GC/MS instruments have not yet been made. The statistical fidelity ultimately rests on the number of ions available for measurement. Using the assumption of 106 ions/cm 3 as a concentration approaching the space charge limit (29), the number of ions available for measurement by TOF TAD falls with in an order of magnitude of those common to the other types of mass spectrometers. The real limitations for all mass spectrometers may be the ionization efficiency of the source and the number of ions that can be used for measurement purposes. The only presently implemented technique with 100% collection effi ciency for time array detection is the multichannel scalar approach in which arrival of an individual ion at the de tector terminates a time interval mea surement (30). This approach is not useful for GC/MS applications be cause it is limited to only a few ions per pulse. Some improvements over simple time slice detection have recently been made. Employment of high-speed transient recorders allows many time bins to be sampled for each ion source pulse (31-33), but the amount of memory of commercial units limits the
Figure 3. Integrating transient recorder This device enables time array detection in real time. Reduction of the data throughput is accomplished by a two-step process starting with sequential spectral summing, and followed by background elimina tion, ion peak delineation, and data packing schemes that result in the storage of averaged mass spec tra containing only informative data
mass range that can be observed. A multichannel digitizer has been devel oped that samples only preselected time bins in which ions are known to be present (34). This system allows an extended mass range but requires cali bration with each compound to be an alyzed. As with other transient record ers, once a transient has been digitized substantial time is required to transfer the information to other memory or mass storage—time during which in formation from many other source pulses is lost. Another approach uses a 100-MHz ADC with 16 000 time bins, allowing sequential spectra to be scanned without loss of information (35). The accumulated data in the time bins are processed off-line after each sample has been analyzed. To overcome this limitation without loss of information, it will be necessary to develop an acquisition system that can record 10 000 full 100-μβ transients every second with a resolution of 5 ns. Modern microcircuitry now makes such capabilities possible. The next section describes a practical approach to the development of such an instru ment. Integrating Transient Recorder for TOF MS The principal problems with record ing all information available in the transient resulting from each ion source pulse are the rate and volume at which data are produced. The rate at which the transient must be digi tized is determined by the maximum frequency component of information in the transient. For typical ion detec tors, this is on the order of 100 MHz. Applying the Nyquist criterion, a sam pling rate of 200 megasamples (Mspl) per second is required. For a typical TOF MS, this sampling rate requires conversion and storage of an ion in
tensity measurement every 5 ns with approximately 20 000 measurements obtained for each transient. If an 8-bit ADC is used, 20 kbytes will be gener ated from each transient. At a repeti tion rate of 10 000/s, an overall data rate of 200 Mbytes/s will be reached. A GC/TOF MS experiment that takes 1 h would result in 7.2 terabytes (7.2 Χ 1012 bytes) of data. This is pres ently an unmanageable amount of data; however, a solution to this prob lem is possible since the chemical in formation (component concentration in the sample) changes much more slowly than the pulse rate of the source. Thus, by integrating (sum ming) successive transients, the total data volume and memory speed re quired are reduced, the S/N is im proved, and no useful information is lost. The device envisioned for this purpose will be called an integrating transient recorder (ITR). This device is presently being designed and assem bled using commercially available mi croelectronic components. The main components of the inte grating transient recorder are shown in Figure 3. The first functional block is the ADC, which digitizes the tran sient signal at a rate of 200 Mspl/s. An ADC of 8-bit resolution is sufficient for this application since the arrival at the detector of more than 256 ions in 5 ns (corresponding to an isobaric ion current of 8.2 Χ 10" 9 A) is unlikely to occur. This operation can produce 10 000 digitized spectra/s. The sum and store function sums 10-1023 of these spectra with high-speed digi tal logic and stores the result in fast memory. The selection of the number of spectra to average is based on the trade-off of sensitivity, precision, and the dynamics of the sample in the source. A maximum rate of one aver aged spectrum per millisecond could be achieved.
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When the desired number of spec tra have been summed, the resulting averaged spectrum is transferred to the data reduction function, the sec ond stage in the data rate reduction process. This section converts the 10-1000 complete, averaged spectra per second into a mass-intensity pair format from which only those intensi ties above threshold are stored for each spectrum. This results in another substantial reduction in the data vol ume and rate. The mass-intensity data pairs appear at a rate that is slow enough for storage onto a large disk for subsequent analysis. To illustrate the data volume effi ciency, the ITR could be set to sum spectra from 1000 pulses, which would result in 10 averaged spectra/s. If each averaged spectrum contains 100 peaks, only 1000 mass/intensity pairs per second need be stored. At 4 bytes of storage per pair, the required stor age rate is 4 kbytes/s or 150 Mbytes/h. Normally, much fewer data will be stored since most of the spectra will not have 100 peaks. Clearly, these rates and volume of data production are amenable to the typical high-den sity, high-speed disk storage devices used with modern computers. Dualport access will allow GC/MS data processing and output with data sys tems now in conventional use. Potential for MS/MS on the Chromatographic Time Scale In mass spectrometry/mass spec trometry (MS/MS) techniques, two mass analyzers normally are employed in tandem (36-40). The first mass an alyzer allows only ions of a particular m/z to pass from the ion source to an ion-molecule collision chamber. These selected "parent" ions can fragment to produce lower-massed "daughter" ions as a result of collision with neu tral atoms or molecules in the collision chamber. This process is generally called collision-induced dissociation (CID) for high-energy ions (sector in struments) and collisionally activated decomposition (CAD) for low energy ions (quadrupole instruments). The second mass analyzer, placed between the collision chamber and the detec tor, allows only ions of a selected m/z to be detected. This extension of normal mass spec trometry techniques can be used to provide greatly enhanced chemical se lectivity when analyzing a complex mixture or to provide a substantially increased amount of mass spectral in formation when identifying an un known compound. These capabilities sometimes allow MS/MS to replace GC/MS, often with greatly reduced chemical workup, when only a few prespecified components in a mixture are to be detected or when the sample
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can be supplied to the source contin uously for from tens of seconds to sev eral minutes. Important asithese applications are, there are many situations in which neither GC/MS nor MS/MS is ade quate. In these cases, the possibilities of GC-MS/MS are extremely attrac tive, either using MS/MS as an en hanced selectivity detector for the GC or using the GC as a prepurifier to en sure acquisition of proper MS/MS spectral information on each compo nent in the sample. The most readily usable mode of application of current GC-MS/MS in strumentation is single reaction moni toring (SRM). In this mode, which is analogous to selected ion montoring in GC/MS, the masses of parent and daughter ions, mi and m2, are preset and fixed throughout the chromato graphic run. The mass spectrometer serves only as a sensitive and extreme ly selective detector, responding, ide ally, to only a single component in the sample. To achieve multiple reaction monitoring (MRM), so that several components can be monitored, the mass analyzers must be synchronously stepped through a sequence of pairs of mi, m 2 values, completing the entire sequence several times during a chro matographic peak. Even in the quadrupole implementation of MS/MS, in which peak skipping is most readily achieved, only a few parent-daughter mass pairs can be monitored on the time scale of a GC peak. Collection of a matrix of all parent and daughter ions, often useful for compound char acterization, is presently not possible on this time scale. A new technique for the achieve ment of MS/MS data has been pro posed and is under development. This technique, called time-resolved mag netic dispersion MS (41), uses the multiplex advantage of TOF MS by which a complete ion arrival-time spectrum is produced after each source pulse. The flight time of a daughter ion produced in the first field-free region of a magnetic sector spectrometer is longer than that of stable ions of the same momentum, but the same as that of its parent. From the knowledge of flight time and magnetic field strength, both the mass of the fragment ion and that of its par ent can be determined. Since all arriv al times can be measured for each value of the magnetic field, all the in formation on parent and daughter masses can be obtained in a single sweep of the magnetic field. This tech nique has the potential to apply the full power of MS/MS characterization to sample components separated by packed column GC. Use of the inte grating transient recorder is essential in achieving this capability. Rapid ac
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quisition of MS/MS data in this man ner provides lower signal levels than those found in other MS/MS tech niques when covering the same range of parent and daughter masses. Again, spectral summing can be used to im prove sensitivity, where desired. As with TOF MS using time array detec tion, the scan speed limitation has es sentially been removed. Summary Few of the ever-increasing demands placed on MS by advances in GC can be met merely by increasing the scan speed. In mass filter spectrometers, the operating principles of mass anal ysis establish limitations in instru ment performance at high scan speeds. By the nature of its operating principle, the TOF MS can produce in excess of 10 000 complete mass spec tra per second without degradation of performance. However, in all cases in which a mass spectrometer detects a signal at one mass to the exclusion of all others, severe ion statistical prob lems will be encountered as the num ber of spectra produed per second (scan speed) is increased. Array detec tors that permit the simultaneous de tection of ions of more than one mass give the mass spectrometer a greater range of useful scan speeds before ion statistics become limiting. Recent de velopments in microelectronics have made it possible to design a device for TAD at high time resolution and pulse repetition rates, thereby permitting the measurement of all ions accelerat ed from the TOF ion source. TAD has the potential to enable TOF instru ments to be used in conventional GC/MS at scan repetition rates signif icantly greater than those now possi ble. This development should remove scan speed as the limiting factor in the optimization of sensitivity, accuracy, and dynamic range for specific GC/MS applications. In addition, time array detection coupled with momen tum dispersion may permit all rele vant MS/MS data to be acquired on the time scale of a GC peak. Acknowledgment This work was supported by a grant from the Biotechnology Research Pro gram of the Division of Research Re sources at NIH, RR-00480. The assis tance of Brian Musselman, manager of the NIH/MSU Mass Spectrometry Facility, is gratefully acknowledged. J. D. Pinkston gratefully acknowl edges a fellowship from Dow Chemical Corporation. References (1) Hites, R. Anal. Chem. 1979,51, 1452 A. (2) Gates, S. C; Sweeley, C. C; Krivit, W.; Dewitt, D.; Blaisdell, Β. Ε. Clin. Chem. (continued on p. 1112 A)
197S, 24, 1980. Ci) Hites. H.; Biemann, K. Anal. Chem. 1970,42,855. (4) dates, S. C ; Smisko, M.J.; Ashendel, C. L.; Young, Ν. D.; Holland, J. F.; Sweelev, C. C. Anal. Chem. 1978, 50, 433. (5) Sweeley, C. C ; Vrbanac, -I.; Pinkston, I).; Issachar, I). Biomcd. Mass Spec. 1981,8,436. (6) Vrbanac, -I. -I.; Sweelev, C. C ; Pink ston, J. D. Blamed. Mass Spec. 1983, 10, 155. (7) Kttre, L. S. In "Applications in Class Capillary (las Chromatography"; .Jen nings. VV. C„ Ed.; Marcel Dekker: New York, 1981; Chromatographic Science Series, Vol. 15, Chapter 1. (8) Horvath, C. C ; Lipskv, S. R. Anal. Chem. 1967,39, 1893. (9) Dawson, P. H. In "Quadrupole Mass Spectrometry and Its Applications"; I*. H. Dawson, Ed.; Elsevier: New York. 1976; Chapter 2. (10) Stafford, C. C ; Kellv, P. E.; Bradford, D. C. Am. Lab. 1983, 15, 51-57, and as summarized by Borman, S. Anal. Chem. 1983, .5.5,726 A. (11) Comisarow, M. B.; Marshall, A. G. Chem. Phvs. Lett. 1974,2.5,282. (12) Comisarow, M. B.; Marshall, A. G. Can. J. Chem. 1974, .52, 1997. (13) Comisarow, M. B.; Marshall, A. 0 . Chem. Phvs. Lett. 1974, 26, 489. (14) Wilkins, C. L.; Cross, M. L. Anal. Chem. 1981,53, 1661 A. (15) Ledford, E. B., -Jr.; White, R. L.; Chaderi, S.; Wilkins, C. 1..; Cross, M. L. Anal. Chem. 1980, .52, 2450. (16) White, R. L.; Wilkins, C. L. Anal.
Chem. 1982, 51. 2443. (17) White, R. L.; Ledford, E. B., Jr.; Wilkins, C. L.; Cross, M. L. Presented at 29th Annual Conference on Mass Spec trometry and Allied Topics, Minneapo lis, Minn., June, 1981; p. 5 of "Extended Abstracts." (18) Studier, M. H. Rev. Sci. Instr. 1963, 34, 1367. (19) Lehman, J. P.; Younginger, E. J. Int. J. Mass Spec. Ion Phvs. 1980, 33, 95. (20) Ciffin, K.; Boettger, C. E.; Norris, I). D. Int. J. Mass Spec. Ian Phvs. 1974, 7.5,437. (21 ) Boettger, H. C ; Ciffin, C. E.; Norris, D. D. In "Multichannel Image Detec tors," ACS Symposium Series—102; Talmi, Y„ Ed.; American Chemical Soci ety: Washington, D.C., 1979; pp. 291-318. (22) Ciffin, C. E.; Britten, R. Α.; Johansen, R. A. Presented at 28th Annual Confer ence on Mass Spectrometry and Allied Topics, New York, N.Y., May 25-30, 1980; p. 115 of "Extended Abstracts." (23) Santini, R. E.; Milano, M. J.; Pardue, H. L. Anal. Chem. 1973, 45, 915 A. (24) Louter, G. J.; Boerboom, A.J.H.; Trithof, H. H.; Kistermaker, J. Int. J. Mass Spectrom. Ion Phvs. 1980, 33, 335. (25) Hedfjall, B.; Rvhage, R. Anal. Chem. 1979, 51, 1687. (26) Hedfjall, B.; Rvhage, R. Anal. Chem. 1981,.53, 1641. (27) Kinsinger, J.; Chaderi, S.; Hein, R.; Hanna Α.; Marra, J. Presented at 30th Annual Conference on Mass Spectrome try- and Allied Topics, Honolulu, Hawaii, June, 1982; p. 280 of "Extended Ab stracts."
Front, left to right: J. Pinkston, J. Stults, C. Enke. Rear, left to right: J. Watson, J. Holland, J. Allison, B. Newcome John Holland received his BA in chemistry from Macalester College in 1951 and his PhD in chemistry from Michigan State University in 1973. He is presently professor of biochem istry at MSLI, where he also serves as codirector of the NIH/MSU mass spectrometry facility. His research interests include general instrumen tation, spectroscopy, electrochemis try, and computer-assisted measure ments on biological systems. C. G. Enke obtained his BS in chemistry from Principia College (1955) and his PhD in chemistry from the University of Illinois (1955). He is currently a professor of chemistry at MSU. His research interests include analytical instrumentation, mass spectroscopy, electroanalylical chem
istry, applications of spectroscopic instrumentation, and mini- and mi crocomputers in chemical research. John Allison earned his BS in chemistry at Widener University in 1973 and his PhD in chemistry at the University of Delaware (1977). He is currently an assistant professor of chemistry at MSU, where his inter ests include metal ion chemical ion ization, gas phase ion-molecule reac tions, electron impact-induced fluo rescence, and analytical aspects of thermionic emission from solids. John T. Stults received his BA in chemistry from the College of Wooster in 1980. He is presently in the an alytical chemistry PhD program at MSU under the supervision of C. G. Enke and J. T. Watson. He is the re
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(28) Ledford, E. B„ Jr.; Chaderi, S.; Wilk ins, C. L.; Gross, M. L. Adv. Mass Spec trom. 1980, HB, 1715. (29) Walls, F. L.; Dunn, G. H. Phys. Today 1974,27,30. (30) Bonner, R. P.; Bowen, D. V.; Chait, B. T.; Lipton, A. B.; Field, F. H.; Sippach, W. F. Anal. Chem. 1980,52, 1923. (31) Dunbar, R. C ; Armentrout, P. Int. J. Mass Spectrom. Ion Phvs. 1977, 216, 465. (32) Lincoln, K. A. Dyn. Mass Spectrom. 1981,6', 111. (33) Denover, E.; Van Crieken, R.; Adams, F.; Natusch, D.F.S. Anal. Chem. 1982, .54,26 A. (34) Ettinger, D. E.; Winiecki, A. L.; Bourne, S. of the Chemical Engineering Division, Argonne National Laboratory, personal communication, and as summa rized by Thomas, H. L. Ind. Res. l)ev. February 1983, p. 160. (35) Steffens, A. P.; Niehuis, E.; Friese, T.; Benninghoven, A. Presented at 1983 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, N.J., March 7-12, 1983; abstract No. 88. (36) McLafferty, F. W. Ace. Chem. Res. 1980, 73,33. (37) McLafferty, F. W. Science 1981,27-7, 280. (38) Yost, R. Α.; Enke, C. G. Anal. Chem. 1979,57, 1251 A. (39) Cooks, R. C ; Clish, G. L. CTiem. Eng. News 1981,59, 40. (40) Yost, R. Α.; Fetterolf, D. D. Mass Spectrom. Rev. 1983,2, 1. (41) Stults, J. T.; Holland, J. F.; Enke. C. G. Anal. Chem.. in press. cipient of a Summer Research Fellow ship from Union Carbide and an ACS Division of Analytical Chemistry fel lowship supported by the Procter & Gamble Company. J. David Pinkston obtained a BS in chemistry and mathematics in 1979 from Ouachita Baptist University in Arkadelphia, Ark. At present he is a Dow Fellow working toward his PhD in analytical chemistry under the su pervision of J. Allison and J. T. Wat son. In addition to new applications of TOP MS, his research interests in clude analytical applications of thermionic emission in MS and meta bolic profiling. Bruce Newcome obtained a BS in chemistry (1978) from the University of Illinois. At present he is in the an alytical chemistry PhD program at MSU, working under the supervision of C. G. Enke. His research interests are in high-speed electronics and computer systems, chemical instru mentation, and MS. Jack Throck Watson earned his BS in chemistry from Iowa State Univer sity in 1961 and his PhD in analytical chemistry from MIT in 1965. He is presently professor of biochemistry and chemistry at MSU, where he also serves as director of the NIH/MSU mass spectrometry facility. Professor Watson's research interests center on development of quantitative method ology and instrumentation with em phasis on biomedical applications.
