John F. Holland Christie G. Enke John Allison John 1.Stults J. David Pinkston Bruce Newcome J. Throck Watson
Instrumentation
Departments of Biochemistly and Chemistry Michigan State University East Lansing, Mich. 48824
Mass Spectrometry on the ChromabgraphcTime 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 ( I ) 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, maw 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$0 1 5 0 / 0
1983 American Chemical Soclety
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 Pperialized technique is useful only for analyses in whirh the ion cur-
rents a t a few preselected masses are of interest. The example in Figure 1illustrates the critical dependence of scan rate on the capacity of a mass spectral data base to accurately represent chromatographic resolution in G U M S applications. Note that the GC peak in Figure 1is approximately 3 4 s wide. The mass chromatogram in (a) represents a chromatographic doublet, hut the apparent magnitude of the second
I 1
t
Figure 1. Comparison of true gas chromatographic profile (green line) and simulated mass chromatograms (blue line) reconstructed from acquired data bases (a) Mass chromatcgram prepared f r m mass SpecIra acquired at a rate of 1 ScBnIs. (b) Mass spectra acquired at rate of 1 scanls. but Synchrony of chromatogram and SCM cycle shined by One-third IBCmd. (cl Mass specIra acquired at rate of 3 scands
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9. AUGUST 1983
997A
component has heen attenuated. In (b), the mass chromatogram represents the true magnitude of the second component, but the first component is not apparent in this profile. In (e), agreement between the mass chromatogram and the true chromatographic profile is adequate, hut a better degree of fit would be assured from an even shorter scan cycle, Le., faster scan speed. Capillary columns that have 106 theoretical plates are now becoming availahle (7). Such columns have the capacity to present on the order of ten resolvable peaksls, especially early in the chromatogram (8).For reasonable reconstructed chromatograms under these conditions, a mass scanning (e.g., rnlz 50-500) repetition rate approaching 100 Hz (100 scansls) 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 rapidly 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 measurement (ion statistics) will diminish. Hence, the parameters of scan speed and ion statistics have a diametric relationship 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 representing transient sample concentrations in the ion source. This limit is a function of the physical principles underlying the mass selection process and the characteristics of the ion detector and recording system. These features will he analyzed in this report and a feasible solution to the increasing demand for faster scan cycle rates will be presented. A proposed solution is hased on the application of timeof-flight (TOF) mass spectrometry. Analysis of Performance Trade-offs of Present Generation GClMS Instruments
Three characteristics-scan speed, sensitivity, and useful dynamic range-affect the compatibility of a mass spectrometer with a high-resolution chromatograph. Limitations on these performance characteristics can he broken down into three categories, independent of the type of instrument. They are: limitations arising from the physical laws governing the mass selection process, bandwidth of the detector and electronic processing components, and ion statistics, that is, the number of ions observed in the signal measure998A
Magnetic Sector MS Ions are produced continuously in the source and are accelerated toward the netic field by a potential, V, such that all have the same kinetic energy = %mP = zeV. where e is magnitude of the electronic charge, z Is the number of charges on an ion, m is the MSS of the ion, and vis its velocity). The eld separates the components of the total ion beam according to By this means, the individual ion beams are separated spatiarty and alue will have a unique radius of trajectory. R. CommerclalGClMS focused optics with slits. Thus, for a given magnetic field, 8, only ions of a single m/z (m/z = B2R2/2v)will have the proper trajectq leading to the detector. By changing the magnetic field strength, ions of differing m/z values are brought to focus at the detector slit.
Detector
ment a t the peak maximum at each mlz value. The two dominant GCIMS instrument types, magnetic sector and quadrupole, and the less common TOF and Fourier transform (FT) instruments, 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 mlz are transmitted by using different magnetic field strengths. Thus, magnet characteristics determine 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. However, if fast scan speeds are employed, the field can change significantly during 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. Consequently, when scanning toward increasing field strength, the effective radius will he shorter than that corresponding 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 noint of the ootical system. Electromaenets exhibit a kind of inertia, called reluctance, that limits the rate a t which a magnet can be forced to change field strength. This phenomenon presently limits scan rates to 0.1 sldecade for modern fully laminatI
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9. AUGUST 1983
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 instruments 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 presently available magnets. Quadrupole Instrument (see box on Quadrupole MS). The quadrupole mass spectrometer performs mass separation in a completely different manner than does the magnetic sector instrument. The ions interact with an oscillating electric field in such a way that only ions of a specific mlz will be allowed to pass to the detector. For maximum transmission of the selected ions, the radio frequency (rf) and de 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 ps to pass through a 20 cm quadrupole filter. Using an intuitive approach, if one scans a t a rate such that the mass filter changes by no more than 0.1 u during the transit time of this ion, a maximum scan rate of 780 u/s can he reached before resolution, peak shape, and intensity are significantly sacrificed. Under these conditions 90% of the ion current a t mlz 800 is transmitted to the detector. Somewhat faster scanning rates have been achieved without sacrificing sensitivity by increasing the ion energy concurrently with the mass scan ramp in order to maintain a more constant ion velocity
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essential, dedicating a separate column to each analysis eliminates column equilibration delays, reduces interferences, and prolongs column life. Rheodynek Model 7066 Tandem Column Selector connects any ol up to five columns (or a bypass line) between the sample injector and the detector Off-linecolumns are sealed at both ends, and no column need ever be exposed to a solvent intended lor another. Ouriechnical Notes 4 tells all about column selec tion with Model 7066 and other valves.To receive a copy, along with product literature. contact Rheodyne, lnc, PO. Box 966. Cotati, California 94928, U.S.A. Phone (707) 664-9050
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and reduce the transit time for heavir ions. Faster scanning also can be achieved if the mass-selecting voltages are stepped rather than scanned however, 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 b achieved in the most recent quadrupole instruments. The ion trap detector, a mass spectrometer related to the quadrupole, has recently been introduced comme ciallv. This instrument uses geometry tha l o kHz) would permit spectrum summing. The combination of large ion beam cross section, ion source trapping, and successive spectrum summing apparently compensates for the attenuation caused by pulsing; sensitivities comparable to those from other types of mass spectrometers 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 affect 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 spectrum repetition rate in mass spectrometry may arise from the fundamental laws that govern the scanning process and the minimum time required to measure the intensity at each mass. Relief from these limitations can be found in techniques that
1 0 0 4 A * ANALYTICAL CHEMISTRY, VOL. 55,
NO. 9, AUGUST 1983
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 detected without scanning are the spatial array detector used with magnetic mass dispersion, the FT implementation (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 multipliers described by Santini (23) and developed an electro-optical ion detector (EOID) that serves as an “electronic photoplate” for a double-focusing mass spectrometer. After overcoming several difficulties in the operation of microchannel plates in strong magnetic fields (21),these workers connected several discrete sections of microchannel plates and associated photodiode arrays and fiber optic rotators to integrate ion currents in the mass range of m/z 40-700. Boerboom et al. ( 2 4 ) used a small version of the EOID in recording rapidly changing phenomena such as those in flash pyrolysis and laser pulse desorption. They used quadrupole lenses to adjust the mass range detected 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. However, they concluded that several features in state-of-the-art components of the EOID prevent the integrated unit from offering advantages over a conventional detection system consisting of a single electron multiplier (26). For example, the gains of the individual microchannel detectors in the array differed in magnitude and varied independently with time over periods as short as 30 min. These investigators 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 t o improve the dynamic range by using different integration times with rapid switching of the voltages applied to the microchannel plates were thwarted by the long recovery time of the detector. In summary, 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-
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ly. Pulsed ionization followed by rapid (generally on the order of milliseconds) simultaneous detection eliminates the problem of a changing sample concentration in the ion source during acquisition of the mass spectrum, which can occur in a conventional sector or mass filter instrument (27). It appears that the sensitivity is approacbing levels snitahle for GCNS; 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 lo4,may he difficult (28). Available dynamic range depends on the number of transients averaged to reduce the uncertainty (noise) of the measurement and upon the resolution of the ADC. Time Array Detection for T O F MS.As pointed out in the initial discussion, every ion source pulse in the TOF MS contains information on the complete mass spectrum. I t 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 “ w i d t h of 0.1 ps. 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 0.1-ps-wide portion of the complete spectrum presented to the detector is collected for each pulse of the ion source. If the “opened” time bin is removed 0.1 ps with each successive pulse, ions a t rnlz 27 will he collected from the 69th pulse because ions of rnlz 27 have a flight time of 6.9 ps under these conditions. Ions a t rnlz 28 are collected from the 70th pulse, and so on, until ions of mlz 58 appear a t the detector in synchrony with the open time bin following the lOlst 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
ime Slice Detection
._ E m
m/z 27
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Arrival Time (~s) Figure 2. Comparison of time slice and time array detection A simulated TOF mass spectrum is shown for +butane. using a flight tube length of 100 cm a d an acceleratingvoltage Of 3000 V. In time slice detection. only one time bin is measured lor 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
1006A
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
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 continuous sequence. Thus, no information is lost. In several ways, the conceptual representation 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 ps portrayed in Figure 2 would not he practical for compounds with molecular weights much larger than 60. Since the arrival time of an ion in TOF is proportional to (mlz)’/2,the interval between arrival times of ions of adjacent mlz decreases as rnlz increases. The difference in arrival times of mlz 12 and mlz 13 is 0.19 ps; however, this value shrinks to 0.05 ps for the ions of mlz 200 and 201. Thus, the width of each time bin must he small enough so that, over the entire mass range, ions of only one rnlz value will be d e w in a given bin. As the width of the time bin is decreased in order to improve resolution on the mass axis, the sampled fraction of the spectrum in time slice detection is decreased. Given the same analysis time, time array detection will he more sensitive than time slice detection by the inverse of this fraction. Although improvements in SIN, sensitivity, and dynamic range over normal TOF MS can be predicted by time array detection (TAD), comparisons with other types of GCIMS instruments have not yet been made. The statistical fidelity ultimately rests on the number of ions available for measurement. Using the assumption of lo6 ionsIcm3 as a concentration approaching the space charge limit (29),the number of ions available for measurement by TOF TAD falls within an order of magnitude of those common to the other types of mass spectrometers. The real limitations for all mass spectrometers may he the ionization efficiency of the source and the number of ions that can he used for measurement purposes. The only presently implemented technique with 100%collection efficiency for time array detection is the multichannel scalar approach in which arrival of an individual ion a t the detector terminates a time interval measurement (30).This approach is not useful for G U M S applications hecause 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 he sampled for each ion source pulse (31-33), hut the amount of memory of commercial units limits the
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Flgure 3. Integrating transient recorder This device enables time away detection in real time. Reduction 01 the data throughput is aCcompliShed by a two-step process s m i n g with sequential spectral summing. and foliowed by background eliminb tian. ion peak delineation. and data packing %hemes that result in the storase 01 averanad mass spectra containing only informative data
mass range that can be observed. A multichannel digitizer has been developed that samples only preselected time bins in which ions are known to be present (34). This system allows an extended mass range but requires calibration with each compound to be analyzed. As with other transient recorders, once a transient has been digitized substantial time is required to transfer the information to other memory or mass storage-time during which information 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. T o overcome this limitation without loss of information, it will be necessary to develop an acquisition system that can record 10 000 full 100-ps 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 instrument. Integrating Transient Recorder for TOF MS
The principal problems with recording 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 digitized is determined by the maximum frequency component of information in the transient. For typical ion detectors, this is on the order of 100 MHz. Applying the Nyquist criterion, a sampling rate of 200 megasamples (Mspl) per second is required. For a typical TOF MS, this sampling rate requires conversion and storage of an ion inI008 A
tensity measurement every 5 ns with approximately 20 OOO measurements obtained for each transient. If an 8-bit ADC is used, 20 kbytes will be generated from each transient. At a repetition rate of 10 OOO/s, an overall data rate of 200 Mbytesh will he reached. A GCITOF MS experiment that takes 1h would result in 7.2 terabytes (7.2 X 10” bytes) of data. This is presently an unmanageable amount of data; however, a solution to this problem is possible since the chemical information (component concentration in the sample) changes much more slowly than the pulse rate of the source. Thus, hy integrating (summing) successive transients, the total data volume and memory speed required are reduced, the S/N is improved, 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 assembled using commercially available microelectronic components. The main components of the integrating transient recorder are shown in Figure 3. The first functional block is the ADC, which digitizes the transient signal a t a rate of 200 Mspl/s. An ADC of 8-bit resolution is sufficient for this application since the arrival a t the detector of more than 256 ions in 5 ns (corresponding to an isobaric ion A) is unlikely to current of 8.2 X occur. This operation can produce 10 000 digitized spectrah. The sum and store function sums 10-1023 of these spectra with high-speed digital 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 averaged spectrum per millisecond could be achieved.
ANALYTICAL CHEMISTRY, VOL. 55. NO. 9. AUGUST 1983
When the desired number of spectra have been summed, the resulting averaged spectrum is transferred to the data reduction function, the second stage in the data rate reduction process. This section converts the
substantial reduction in the data volume and rate. The mass-intensity data pairs appear a t a rate that is slow enough for storage onto a large disk for subsequent analysis. T o illustrate the data volume efficiency, the ITR could be set to sum spectra from 1000 pulses, which would result in 10 averaged spectrals. If each averaged spectrum contains 100 peaks, only 1000 masshntensity pairs per second need be stored. At 4 bytes of storage per pair, the required storage rate is 4 khytesh or 150 Mbytesh. 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-density, high-speed disk storage devices used with modern computers. Dualport access will allow G U M S data processing and output with data systems now in conventional use. Potential for MSIMS on the Chromatographic Time Scale
In mass spectrometryhass spectrometry (MS/MS) techniques, two mass analyzers normally are employed in tandem (36-40).The first mass analyzer allows only ions of a particular m h 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 neutral atoms or molecules in the collision chamber. This process is generally called collision-induced dissociation (CID) for high-energy ions (sector instruments) and collisionally activated decomposition (CAD) for low energy ions (quadrupole instruments). The second mass analyzer, placed between the collision chamber and the detector, allows only ions of a selected mlz to he detected. This extension of normal mass spectrometric techniques can be used to provide greatly enhanced chemical selectivity when analyzing a complex mixture or to provide a substantially increased amount of mass spectral information when identifying an unknown 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 continuously for from tens of seconds to several minutes. Important as these applications are, there are many situations in which neither GC/MS nor MSIMS is adequate. In these cases, the possibilities of GC-MS/MS are extremely attractive, either using MSIMS as an enhanced selectivity detector for the GC or using the GC as a prepurifier to ensure acquisition of proper MSIMS spectral information on each component in the sample. The most readily usable mode of application of current GC-MSIMS instrumentation is single reaction monitoring (SRM). In this mode, which is analogous to selected ion montoring in GCIMS, the masses of parent and daughter ions, ml and m2, are preset and fixed throughout the chromatographic run. The mass spectrometer serves only as a sensitive and extremely selective detector, responding, ideally, 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 ml, mz values, completing the entire sequence several times during a chromatographic peak. Even in the quadrupole implementation of MSIMS, 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 characterization, is presently not possible on this time scale. A new technique for the achievement of MSIMS data has been proposed and is under development. This technique, called time-resolved magnetic dispersion MS ( 4 1 ) ,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 parent can be determined. Since all arrival times can be measured for each value of the magnetic field, all the information on parent and daughter masses can be obtained in a single sweep of the magnetic field. This technique has the potential to apply the full power of MS/MS characterization to sample components separated by packed column GC. Use of the integrating transient recorder is essential in achieving this capability. Rapid ac-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
quisition of MSIMS data in this manner provides lower signal levels than those found in other MSIMS techniques when covering the same range of parent and daughter masses. Again, spectral summing can be used to improve sensitivity, where desired. As with TOF MS using time array detection, the scan speed limitation has essentially 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 analysis establish limitations in instrument performance at high scan speeds. By the nature of its operating principle, the TOF MS can produce in excess of 10 000 complete mass spectra 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 problems will be encountered as the number of spectra produed per second (scan speed) is increased. Array detectors that permit the simultaneous detection of ions of more than one mass give the mass spectrometer a greater range of useful scan speeds before ion statistics become limiting. Recent developments in microelectronics have made it possible to design a device for TAD a t high time resolution and pulse repetition rates, thereby permitting the measurement of all ions accelerated from the TOF ion source. TAD has the potential to enable TOF instruments to be used in conventional GC/MS at scan repetition rates significantly greater than those now possible, 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 momentum dispersion may permit all relevant MSIMS data to be acquired on the time scale of a GC peak. Acknowledgment
This work was supported by a grant from the Biotechnology Research Program of the Division of Research Resources a t NIH, RR-00480. The assistance of Brian Musselman, manager of the NIH/MSU Mass Spectrometry Facility, is gratefully acknowledged. J. D. Pinkston gratefully acknowledges 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, B. E. Clin. Chem.
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Shimadzu’s full family oi accurate balances. i
A ECS Series Eleclronlc Counllng Scale The ECS Series is available in three weight capacities ... 140, 1,400 and 5.600 grams, covering most counting for small parts. It is equally suitable for statistical and math functions of count/ weight values. The series offers high sensitivity as is demonstrated by one model’sabilityto count partsweighing as little as lmupiece. Inherent error caused by uncertainties in sample weight iseasyto predetermine. The ECS Series offers wide-ranging complementing advantages, too. It features auto-ranging for easier operation and greater versatility. Weight and count values are totalled simultaneously with the push of a button. And the series offers digital outputsforquick-connection toacomputer.You can classify weights in a maximum of 5 arouos usina the ~ ~ - 4 0 1 . printer-in o the series. And usinq the Shimadzu’s economical printer have a printed record for reference. CIRCLE 205
Series Electronic Reading Balance The EDK Series is designed for large weighing. A high-pefformance electronic reading balance, it is available in four models offering full scale ranges from 28 kilograms to56 kilograms. Justas with Shimadzu’s other balances, it offers a host of outstanding functions including dual range, microcomputercontrol and multifunctions to make costly options unnecessary. Applications flexibility through conversion functions and statistical calculations allow wideranging utilization. T EDK
A EB Series Electronic Reading Balance This series of standard electronic reading balances is available in standard, super and dedicated models. Standard models are available in two dual range and three wide range types offering many features such as averaging, automatic stability and detection functions, onetouch taring to the full capacity and a unique multicontrol system. Onetouch zeroing and easy conversion are also key Shimadzu features. Super models come in two dual range types and three wide single range types featuring automatic statistical calculation in addition to the features of the standard models. Dedicated balances in the EB Series include animal balances, counting balances. carat balance and moisture balance.
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Write today lor more hfonnation on these and other Shimadzu instruments.
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SHIMADW SCIENTIFIC INSTRUMENTS. INC. 7102 RiYerwDOd Drir-. -.dmbia, Maryland 21046, U.S.A. PhOne:(M1)987-1227 SHlYADW (EUROPA) GMBH Acker Slrassse 111.4WO DUsSeldOrl. F.R. Germany Phone: (M11)6€6371 Telex: 08586839 SHIMADZUCORPORCTIONINTERNATIONAL MARUETINO DIV. Shinjuku-MitSUi Building. 1-1. Nishishinjuku 2+hme. Shinjuku-hu. Tokyo 180. Japn. 10n0:Tokyo G-36?&41 Telex:CQ32-3291 SHMDT J. ~
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