double wave could not be made. I n Figure 1 the ratio of the two waves is the same for both the 0.1M and 0.2M solutions of potassium thiocyanate. This indicates that the waves are not due to two different divalent cobalt complexes. If two complexes were present, the wave height ratio would most likely change with changing thiocyanate concentrations. When concentrations of thiocyanate greater than about 0.3M are used the first wave shows a rounded maximum and the second becomes so irreversible (broadens) that the waves are difficult to define, With thiocyanate concentrations of less than O.liM, the two waves still form but they tend to merge and may be mistaken for one wave.
These comments apply to solutions containing cobaltous ions in concentraM. tions of not greater than 5 X Irregular waves and pronounced maxima are obtained with higher cobalt concentrations. This difficulty cannot be overcome by using higher thiocyanate concentrations. The maxima cannot be suppressed by gelatin or Triton X-100, and the second wave broadens so much as t o be unrecognizable. Furthermore, when concentrations greater than 0.005% of the above suppressors are used, the waves are distorted in various ways. With cobalt concentrations less than 5 x 10-4M and thiocyanate between 0.1J.1 and 0.2M the limiting currents of both waves are proportional to cobalt
concentration, and might be used for analytical purposes. They are probably diffusion-controlled because they conform to the relation:
id
= kpllz
where p is the mercury column height. LITERAlURE CITED
(1) Lingane, J. J., Kerlinger, H., IND. ENG.CHEM.ANAL.ED. 13, 77 (1941).
DEMETRIOS KYRIACOU The Dow Chemical Co. Pittsburg, Calif. RECEIVEDfor review March 22, 1965. Accepted April 26, 1965.
Fast Scanning of High Resolution Mass Spectra SIR: There are a number of advantages to recording mass spectra in the minimum time consistent with the production of spectra of adequate resolution and signal t o noise ratio. When sample size is limited, long recording times may result in significant decreases in sample pressure in the spectrometer ion source before the entire spectrum is recorded. Ryhage ( 1 4 ) has demonstrated the advantage of high speed magnetic scanning for small quantities of sample at moderate resolving power ( M / A X = 500). The use of a rapid scanning mass spectrometer for monitoring gas chromatographic effluents is a valuable technique and has been used by many investigators in recent years (3, 5-7, 9, 12-14, 16, 1 7 ) . High resolution mass spectra provide a great deal more structural information than those recorded with only integral mass separation. The recording of high resolution spectra of gas chromatographic effluents on a double focusing mass spectrometer using photographic plate detection has been demonstrated by Watson and Biemann ( 1 7 ) . Biemann ( I , 2 ) has shown that structural information may be presented in the form of “element maps” by computer analysis of exact mass and intensity measurements. For full utilization of all the information present in high resolution mass spectra, it is necessary t o make exact mass measurements on each ion peak in the mass spectrum. Photographic plate detection permits simultaneous recording and integration of all ion peaks, but the spectra recorded in this manner do not lend themselves readily t o rapid automatic digitizing and computation necessary prior t o interpretation. Intermediate steps of development and conversion of the
optical image t o a distance on the plate are required. The plates provide a permanent record of the spectrum, but t o extract information from them requires the measurement of distances with an optical comparator. The digitizing of the output from a n optical comparator may be done automatically. The process, however, is rather slow compared t o the time required for spectrum recording and machine computation of mass number. I n the analysis of complex multicomponent mixtures by mass spectrometric analysis of gas chromatographic effluents, it is often necessary t o record large numbers of mass spectra. Since some components are generally present in much larger quantities than some others in such mixtures, the greatest possible dyanamic range is required of the mass spectrometer. Large differences in peak intensities in t h e spectra of single compounds have necessitated the use of the familiar multichannel recording system in mass spectrometry. In conventional moderate resolution instruments, using attentuations of 1, 10, and 100, it is possible t o achieve a dynamic range of IO4 to 1 if the smallest significant peak is assumed t o be 1% of full scale on the most sensitive range. I n the fast recording of high resolution mass spectra, some concession on precision of measurement of peak intensities must be made. For example, the signal t o noise ratio of the high band width amplifiers required is less than that of the amplifiers used under normal scanning conditions. Also, statistical variations in the weak ion beams necessitate the minimum peak signal t o electronic noise of about 5 t o 1 in order that accurate mass measurements can be made. A maximum useful
peak intensity ratio of about 500: 1 can be obtained in practice. This is not a serious limitation since the additional information provided by unambiguous assignment of atomic compositions t o fragment ions often permits interpretation of spectra without exact knowledge of peak intensity ratios. The primary advantage of electrical detection is that data are extracted from the spectrometer in the form most suitable for digitizing either on-line or after recording on magnetic tape (8, 1 2 ) . Spectra may be presented on an oscilloscope screen for continuous visual monitoring or recorded by galvanometers of suitable frequency response. McFadden and Day (10) have demonstrated the relationship between scan time, sensitivity, and resolution for an instrument of moderate resolving power ( k f / A M = 400-10% valley definition). These authors extrapolated their results t o resolving power of 10,000 and concluded that fast scanning and electrical recording of spectra a t this resolving power was impractical because of statistical considerations, and that photographic plate detection is necessary t o record high resolution mass spectra in the shortest possible time, A basic reason for recording spectra in the minimum time is for detecting changes in composition of chromatographic peaks during the course of elution. It is not uncommon t o record hundreds of spectra during a single gas chromatographic separation of a very complex mixture. At present, only electrical detection could permit on-line digitizing of mass spectral data. Certain computation steps, such as location of peak maxima prior t o recording would greatly reduce the quantity of information which must be recorded VOL. 37, NO. 8, JULY 1965
1037
Table 1. Standard Deviation, in p.p.m., to Be Expected from Mass Measurement of Peaks in a 10,000-Resolving Power Spectrum Where the Peaks Contain Only N Ions 300 30 100 3 4 10 1 2 N 1.18 3.72 2.04 11.8 10.2 6.45 20.4 14.4 Std. dev. (p.p.m.)
and processed and could be incorporated in the data acquisition system. Such systems are presently being investigated. We examined the statistical considerations which McFadden and Day (IO) suggest make fast electrical recording of high resolution mass spectra impractical. Our objective was the recording of mass spectra over a ten to one mass range a t a resolving power of 10,000 in ten seconds for samples in the microgram range. In a preliminary experiment, one microgram of ibogaine was placed on the M S 9 direct evaporation probe and completely evaporated. During the evaporation, the ion current of the base peak (m/e 310) was continuously monitored and a curve of ion current us. time was plotted. The resolving power of the spectrometer when making these measurements was 2500 (loyo valley definition). From calibration of the electron multiplier and amplifier gains, the total charge falling on the multiplier from this one microgram of sample was 1.6 X lo-" coulomb, or lo8 ions. The number of ions that will be present in this base peak under fast scanning conditions can now be directly calculated from known changes in the sensitivity of the MS9 when the resolving power is changed. In going from a static resolving power of 2500 to a static resolving power of 10,000 the sensitivity is reduced by a factor of
Table II. Accuracy and Precision of Mass Measurement from 13-Second Scans a t Resolving Power 9000
11. Thus the total charge which would be collected a t a resolving power of 10,000 is 9 x lo6 ions. If a high band width amplifier and recording system are used for fast scanning there is only a small loss in resolving power from the static value to that actually found when scanning a decade in the fast conditions already described. For an exponential scan with decreasing magnetic field,
M
Measured Mass at peak 508 507 506 505 503 502
f8.6 -13.2
-8.7 +9.4
at 507,
at 15%
+6.9
+7.4
-6.3 -15.4 -4.0
-3.5 -21.4 -6.7
points
Reference
points
Reference STANDARD DEVIATION OF MASS MEASUREMENTS (IN P . P . M . ) ~
508 507 506 505 503 502
21.9 13.9 13.2 17.0
19.3
22.3
2.9 11.4 15.1
6.1 11.6 14.0
Reference
Reference Computed from seven separate 13second per decade scans. a
1038
ANALYTICAL CHEMISTRY
(1)
M = mass being collected M o = starting mass
T = time constant of scan For a decade scan in time tlo, M / M o = 0.1 and therefore, tlo = 2.303T. By differentiating Equation 1,
EXPERIMENTAL
d_ M - dt M T
(2)
M Setting - equal to the resolving dM power, then dt is the time per peak, t,, and is constant for constant T, or t, = tI0/2.202 X resolving power. Therefore, for a decade mass scan in tlo = 10 seconds, t , = 435 pseconds. -4reasonable approximation to the shape of the ion peak is a triangle in which the time across the base is 435 pseconds. The number of ions, N,, to be expected in the base peak (m/e 310) of the ibogaine spectrum when a decade is scanned a t a resolving power of 10,000 in 10 seconds is given by:
N,
=
2
(L) (total number of scan time ions arriving a t multiplier during scan time)
1
Measured Measured
Mo exp ( - t / T )
where
MEAN ERRORO F MASS MEASUREMENT ( I N P.P.M.)a
=
made with a dynamic resolving power of 10,000 and a uniform sample consumption of lo-' gram per second during a decade scan in 10 seconds. In these experiments the samples were introduced through the standard hot inlet system. Values of N , of about 1000 for normal hexadecane, C1&4, and about 4000 for heptacosafluorotributylamine, (C4F9)3N,were obtained. Accuracy of mass measurement is as important as sensitivity in high resolution mass spectrometry. If it is assumed that an ion peak has a fixed base time, it is possible for ions to arrive a t any time during this interval. For a limited number of ions of a given mass, accurate estimation of the mean time of arrival and, therefore, true mass is limited by statistical factors. Table I shows the standard deviations to be expected from a triangular spectral peak at resolving power 10,000 containing N ions, (4, 25). As long as 10 or more ions are present during the time in which a peak is scanned, it is theoretically possible to attain mass measurement precision greater than 10 p.p.m.
= 2
435
(
x
10-6
)
(9
x
106) =
200 ions assuming 1 pg of sample being uniformly consumed during 10 seconds, or a sample flow rate of lO-jgram per second. Since the noise levels of electron multipliers are orders of magnitude less than the current resulting from 200 ions in 435 pseconds, it is theoretically possible to measure the current resulting from an ion peak of 57, the intensity of the base peak, or 10 ions, with an extremely high probability of finding a number of ions sufficient to distinguish them from noise or random pulses. Direct measurements of the number of ions in the base peak have since been
The feasibility of recording high resolution mass spectra a t high scan speeds was tested using an MS9 mass spectrometer with the slits adjusted for a static resolving power of 10,000. The output amplifier was modified t o provide a d.c. to 10-kc. bandwidth. Spectra of a mixture of hexatriacontane and heptacosafluorotributylamine were recorded on a Honeywell 8100 magnetic tape recorder, operating in the F M mode at a tape speed of 30 i.p s. The recorder bandwidth was d.c. to 10 kc. A 5-kc. timing signal was simultaneously recorded on a separate tape channel to minimize the effects of inaccuracy of tape speed, flutter, and variations of speed in the chart drive of the recorder used for final display of the spectrum on time measurements. A complete mass decade was scanned in 13 seconds. A portion of the spectrum, played back a t a tape speed of 17/8i.p.s. into the standard h'IS9 recorder is shown in Figure 1. The recorded resolving power, computed from the ratio of peak spacing to peak width is about 9000. Ion currents indicate that only 10 to 15 ions contributed to the peaks a t mle 505 and 508. ilccuracy and precision of mass measurement were estimated from seven separate 13-second scans. Peak centers were defined in three ways: the point a t which maximum amplitude occurred; the center of the 50y0 height points; and the center of the 15% height points. Chart distances between peaks of the 5-kc. timing signal were measured with a traveling microscope. The precision of the time measurement was about 5 2 pseconds or 0.5 p.p.m. The scan rate was assumed to be exponential and the masses of the peaks a t m/e 503, 505, 506, and 508 were computed using the 502 and 507 peaks as reference
182
184
183
Figure 2. Portion of mass spectrum of hexatriacontane recorded directly on an oscillograph
Figure 1. Portion of spectrum of a mixture of hexatriacontane and heptacosafluorotributylamine recorded on magnetic tape and replayed on an oscillograph recorder. Ion currents indica e 10 to 15 ions contribute to m/e 505 and m/e 508. The 5-kc timing signal was recorded on a separate tape channel
Scantime, 5 seconds per decade Resolving power, ca. 9000 Galvanometer response, 8000 c.p.s. Chart speed, 120 i.p.s. The 5-kc. timing signal appears at the top of the trace
Tape recorder, Honeywell 81 00 Record speed, 30 i.p.5. Reproduce speed, 1 7 / 8 i.p.s. Bandwidth, d.c. to 10 kc. Scantime, 13 seconds per decade Resolving power, ca. 9000
masses. Results of these measurements are presented in Table IT. Higher recording speeds have also been attained. Figure 2 shows a portion of a 5 second per decade scan of hexatriacontane recorded directly by a n 8000 C.P.S. galvanometer a t a chart speed of 120 i.p.s. The spectrometer was set for a static resolving power of 11,000. The recorded resolution is approximately 9000. The 5-kc. timing signal appears a t the top of the trace. RESULTS AND DISCUSSION
The results presented here suggest that fast scanning and electrical recording of high resolution mass spectra is feasible. The sensitivity and mass measurement accuracy attainable appear adequate to permit the use of a double focusing magnetic scanning mass spectrometer for the analysis of gas chromatographic effluents and other samples if the quantity of material available is on the order of lo-’ gram per second. Orders of magnitude higher in sensitivity can be achieved at lower resolving powers (3, 7 ) or by the use of the integrating property of photographic plates. I n general, the practical sen-
sitivitv limits of mass sDectrometers are determined by sample contamination, chromatographic column bleed, and instrument background rather than by the detection system. The system described provides the spectrometer output signal in a form suitable for either direct digitizing or recording in analog form on magnetic tape for subsequent conversion to a form suitable for computer processing. Methods for the improvement of precision are presently being studied. Improved tape recording systems as well as on-line digitizing of spectra for subsequent machine computation of exact masses are being investigated. LITERATURE CITED
(1) Biemann, K., J. Pure Appl. Chem. 9 , 95 (1964). (2) Biemann, K., Bommer, P., Desiderio, D. M., Tetrahedron Letters ?io. 26,
1725 (1964). (3) Brunee, C., Jenkel, L., Kronenberger. K.. 2. Anal. Chem. 197. 42 (1983): ‘ (4) Cramer, H., “Mathematical Methods of Statistics,” Almquist and Wicksell, Stockholm, 1945. (5) Dorsey, J. A., Hunt, R . H., O’Seal, M. J., ANAL.CHEM.35, 511 (1963).
(6) Gohlke, R. S.,Ibid., 31, 535 (1959). (7) Ibid., 34, 1332 (1962). (8) Issenberg, P., Baainet, ?VI. L., Merritt, C. Jr., Ibid., 37, 1074 (1965). (9)Lindemann, L. P., Annis, J. L., Ibid., 32, 1742 (1960). (10) McFadden, W. H., Day, E . A,, Ibid., 36, 2362 (1964). (11) Merritt, C. Jr., Paper presented before 3rd Annual Meeting ASTM Committee E-19, Houston, Texas, October 1964. (121 Merritt. C. Jr.. Walsh. J. T.. Forss. . D. A., Angelini,’ P., Swift, S . M.: ANAL.CHEM.36, 1502 (1964). (13) Ryhage, R., Ibid., 36, 759 (1964). (14) Ryhage, R., Arkiv Kemi 20, 185 (1962). (15) Sarham, A. E.. Ann. Math. Stat. 25. 317 (1954). (16) ’Teranishi, ‘ R., Buttery, R. G., McFadden, W. H., hlon, T. R., Wasserman, J., ANAL. CHEM. 36, 1509 (1964). (17) Watson, J. T., Biemann, K., Ibid., 36, 1135 (1964). CHARLES MERRITT,JR. PHILLIP ISSENBERG’
hl. L. BAZINET Pioneering Research Division U. S. Army Satick Laboratories Natick, Mass.
B. K.GREEN T. 0. MERRON J. G. MURRAY Associated Electrical Industries, Ltd. Instrumentation Division Scientific Apparatus Dept. Barton Dock Road, Urmston Manchester, England Present address, Department of Xutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Mass. RECEIVED for review March 22, 1965. Accepted May 6, 1965.
VOL. 37, N O . 8, JULY 1965
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