Resolution and Sensitivity in Organic Chemical Mass Spectrography J. M. Hayes1 Chemical Ecolution Branch, Ames Research Center, NASA, MofSett Field, Calif. 94035 The capabilities of high resolution mass spectrography using llford 42 and Technical Operations evaporated AgBr photographic plates as ion detectors are discussed. The minimum line width attainable with 42 plates is found to be 2 to 3 p and the effects of instrument parameters on line width are given. For m / e > 200, the dependence of 42 plate sensitivity on ion mass can be summarized by the function D = 3.0
(‘T,:);
+
0.25 loglo where D = optical density of the blackened photoplate; p = charge density of exposing ions at surface of plate, C/mm2; and m = mass of exposing ions. The sensitivity of the evaporated AgBr plates is independent of mass, but these plates have very slight exposure latitude. It is calculated that because of mass and energy effects samples as large as 0.1 mg will be required for the photoplate recording of complete high resolution mass spectra of compounds with molecular weight -1750.
WHATARE the capabilities, present and ultimate, of high resolution mass spectrography as applied to organic chemical problems? This paper discusses that question in a preliminary way by considering the limits that the photoplate detector imposes on the technique. It must be clear that this discussion concerns only spectrography, in which the dispersed ion beam is brought to focus in a focal plane, and not spectrometry, in which the dispersed ion beam is swept past the slit of an electron multiplier or other electrical detector (any mass spectrograph can be operated as a mass spectrometer). The resolution range of interest here is (M/AM) > lo4, and two lines are considered resolved on the photoplate when their centers are separated by at least one line width, where width is measured at half-maximum optical density. Photoplate sensitivity is discussed, but considerations of ion source efficiency and instrument transmission are left aside, as are questions of precision and accuracy in the quantitative determination of relative ion abundances. The latter point leads naturally to a summary of the extant literature, since questions of precision and accuracy in ion abundance determination have been considered in some detail by spark source mass spectrographers. The outstanding work is that of Franzen, Mauer, and Schuy ( I ) , who have derived the form of the photographic emulsion characteristic response curve from first principles, studied the effect of developing conditions, and considered the effects of background fog and of grain size and density on the accuracy of quantitative results. In a later paper, Franzen and Schuy ( 2 ) have discussed the irreducible minimum error inherent in photographic quantitation and have presented an elegant digital computer program for the evaluation of characteristic curves and the determination of unknown ion abundances with the greatest possible 1 Address after January 1970, Department of Chemistry, Indiana University, Bloomington, Ind. 47401
.,
(1) J. Franzen, K. -H. Maurer, and K. D. Schuy, 2.Nuturforsch, 21a, 37 (1966). (2) J. Franzen and K. D. Schuy, ibid., p 1479. 1966
accuracy. Others have studied the distribution of silver bromide grains in the photographic plates ( 3 ) ; the variation of plate blackening with ion mass (4-9) and energy (4, 9, 10); the vacuum characteristics, emulsion uniformity, and storage and development characteristics of various photographic plates (11-13); and the formulation of special developers designed to reduce plate background fog (14, 15). There has been a constant search for a better type of photographic detector, with interest recently centering on gelatin-free plates (16, 17). Because most of this work has been carried out in the context of spark source mass spectrography, resolution has, in general, not been considered. Resolution is not the only problem of importance not well covered by the references cited above. In organic chemical work a whole different method of operation has evolved. A range of 900 mass units may be covered in a single spectrum, and ions having a relative abundance range covering three or four orders of magnitude might have to be measured. Generally, the graded series of exposures relied upon for accurate quantitation in spark source work is not available, and it is therefore necessary to develop plates to a low degree of contrast, so that bad over- and under-exposure will not result in undue loss of information. The present work duplicates the usual conditions of organic chemical investigations and considers problems of resolution, the interaction of ion abundance and resolution, and the sensitivity of photoplate detectors to ions of high mass and energies of 4 to 10 keV. Because high mass sensitivity is a characteristic of great importance, the recently introduced evaporated silver bromide plates (Technical Operations, Inc., Burlington, Mass.) are investigated along with the commonly used Q2 plates. EXPERIMENTAL
Apparatus. The mass spectrometer used was a model 21-110 B (lot 10) manufactured by the Consolidated Electrodynamics Division of Bell and Howell, Inc., Monrovia, Calif. The instrument is equipped with the so-called “Z-stop” or “terminator,” which has proved helpful in increasing instrument sensitivity at high resolution. The basic design of the instrument follows the Mattauch-Herzog geometry (3) H. W. Werner and J. M. Nieuwenhuizen, ibid., 22a, 1035 (1967). (4) K. T. Bainbridge, J, Franklin Inst., 212, 489 (1931). (5) E. Burlefinger and H. Ewald, Z . Nuturforsch, 16a, 430 (1961). (6) E. Dornenburg and H. Hintenberger, ibid., p 676. (7) W. Rudloff, ibid., p 1263. ( 8 ) Ibid., 17a, 414 (1962). 35,1172 (1963). (9) E. B. Owens and N. A. Giardino, ANAL.CHEM., (10) J. R. Woolston, R. E. Honig, and E. M. Botnick, Reu. Sci. Instr., 38,1708 (1967). (11) J. M. McCrea, Appl. Spectry, 20,34 (1966). (12) Ibid., p 181. (13) Ibid., 21, 305 (1967). (14) P. R. Kennicott, ANAL.CHEM.,37, 313 (1965). (15) Ibid., 38, 633 (1966). (16) M. H. Hunt, ibid., p 620. (17j R. E. Honig, J. R.. Woolston, and D. A. Kramer, Reo. Sci. Instr., 38, 1703 (1967).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
(18). The electric sector radius, a, = 639.8 mm. The range
of ion orbit radii (a,) covered by the photoplate in the magnetic field is 47.2 5 a, 5 287.6 mm. At the electron multiplier/Faraday cup slit a, = 304.8 mm. No fiducial mark is put on the plate when it is in place, but the low mass end of the plate is 66.6 rt 0.5 mm from the ion beam entrance to the magnetic field, and a, for any line may thus be approximately calculated from the relationship a, =
y ~
+ 66.6 mm
(1)
4 2
where y = distance of the line from the end of the photoplate, mm. The length of the ion flight path from the ion source to the beam monitor electrodes at the entrance to the magnetic field is 1053 mm. The ion beam monitor was calibrated using the Faraday cup detector, making proper allowance for vertical ion beam spread and collector slit height. Assuming that all slits, electrodes, and resistors had dimensions at the center of their various tolerance intervals, the calibration factor (itrue/imea8,) was found to be 0.85 f 0.01 over the range of beam currents employed in these experiments. However, this assumption cannot be guaranteed valid, and absolute measurements of charge in this instrument must be regarded as accurate only to within &lox. Relative charge measurements are within =t2%, at least. The built-in integrator on the instrument drifted badly, and total exposure levels were calculated from measured beam currents and exposure times-a procedure made acceptable by the extremely steady beam currents obtained by introducing sample gases through thermostated leak valves connected to atmospheric pressure reservairs. Ion source pressures were in the range 10-7 5 P, < 5 X 10-6 torr; analyzerpressures, Pa < 10-8 torr. Both Ilford 4 2 and Technical Operations evaporated silver bromide photoplates were used in these experiments. The latter were stored in the individually sealed and desiccated polyethylene bags provided by the manufacturer, opened just prior to placement in the mass spectrometer, and developed exactly as directed by the manufacturer. The 4 2 plates (emulsion numbers S4854 and S4869, as noted in results) were stored for several months in a refrigerator and desiccated for zero to four days prior to placement in the mass spectrometer. They were developed in Kodak Microdol-X for 12 minutes at 20 “C, rinsed in 1.4% acetic acid stop bath for 30 seconds, fixed in Kodak “Ektaflo” fixer (190 ml stock/l. HzO) for 1 minute, rinsed in Kodak hypo clearing agent for 1 minute, washed in running cold tap water for 5 minutes, and rinsed with distilled water before drying. A few plates 5 parts H20) were developed in diluted (1 part developer UFG developer (Plymouth Products Go., Inc., Chicago, Ill.) for 10 minutes at 20 “C, further processing being as described above. A comparator-microphotometer (Grant Instruments, Inc., Berkeley, Calif.) was used in two different configurations, one to measure line widths, the other to measure optical densities. The objective lens normally used (magnification X49) in the automatic acquisition of high resolution data with this comparator was capable of resolving all doublets encountered, but it was found that an objective lens borrowed from a metallograph (E. Leitz type P1 80X/0.95, magnification X 140 in the comparator) provided still better resolution. Therefore, the latter was used for all line width measurements reported here. The dimensions of the measuring slit, referred to the plane of the photoplate, were 1.0 x 170 p. The photoplate was illuminated by a system that projected onto the plate the image of a slit formed from yellow filter
+
(18) For a discussion of the ion optics of this instrument, see H. E. Duckworth and S. N. Goshal in “Mass Spectrometry,” C. A. McDowell, Ed., McGraw-Hill, New York. 1963, Chap. 7.
glass. Thus, while the entire viewing screen was well illuminated with yellow light, only the small area to be photometrically measured was illuminated with white light. A yellow light absorbing blue filter was placed before the photomultiplier measuring slit. The effects of scattered light on optical density measurements were thus reduced, but they could not be eliminated because the smallest practically attainable illuminating slit dimensions were 5.0 X 160 p. The comparator table was driven at a speed of 25.0 p/min and line widths were determined by measuring the width at half height of peaks on a record of optical density US. time from a potentiometric chart recorder operating at 2.50 in,/min. Clearly, the line widths found with this procedure may in fact be greater than the true line width, because of an incorrect measurement of peak optical density leading to incorrect estimation of the half-maximum or because of effects of finite slit width. However, study of enlarged and accurately scaled photomicrographs gives the impression that errors introduced in this way do not exceed 0.5 p on even the narrowest lines. For optical density measurements, both slits were greatly enlarged, typical illuminating slit dimensions (referred to the plane of the photoplate) being 25 X 700 p. Measuring slit dimensions were always set 10% larger. An objective lens with a magnification of X11.3 was used. The point of optical density = 0 was set by balancing the logarithmic amplifier after focusing the comparator on a portion of the plate from which the emulsion had been scraped. Optical density is defined as log,, I, -, I where Io,Z = light beam intensities before and after passing through the sample. Spectra Recorded. In order to determine levels of resolution practically attainable under a variety of conditions, mass spectra of the polyisotopic rare gas xenon were recorded and photoplate line widths measured over a range of ion source exit slit widths (so) 1.0 _< se 5 50 p, a range of ion orbit radii 120 5 a, _< 277 mm, and at relative exposure levels covering a range of 1 to 30,000. Additionally, the Xe test gas was diluted with representative sample gases, n-octadecane and perfluoroalkane, producing ion beams in which Xe accounted for only 1 or 10% of the current, to determine if there was some measurable effect of total beam current on line widths in the current range < ib < 10-10 A. To determine accurately photoplate sensitivity by producing well-defined areas of known exposure, mass spectra were recorded while operating the short range scan used in peakmatching. The resulting repetitive beam movement produced small blocks of constant exposure per unit area rather than sharp lines. The dimensions of these blocks were: height = 800 p (the height of the beam mask); 45 _< width _< 120 p, depending on a,. This relatively large size allowed accurate photometric measurement of the plateau optical density for each exposure block. The effect of ion energy was observed by running xenon spectra at 4, 6, 8, and 10 kV accelerating potential. The effect of ion mass was observed by running spectra of perfluoroalkane. All perfluoroalkane exposures were made at the same magnetic field strength, the ions thus covering a wide range of photoplate areas (105 _< a, 5 258 mm). Individual exposures were calculated using Equation 2.
where p = charge density, coulombs/mm*; ib = total ion beam current, A ; F , = fraction of total ion beam due to exposing ion; G = geometric factor to allow for vertical spreading and masking of the beam past the monitor electrodes; t = exposure time, sec. ; and A = exposure block area, mm2. The fraction of a beam due to a given ion was determined from Faraday cup detector measurements of the spectra in question.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14,DECEMBER 1969
e
1967
10
I
,
1
I
>-
I
800 -
.
"
u Lil
43 600 -
I
3
m
a
*
W
$ 400 W J
a
,
I
0
5
I
IO i5 20 EXPECTED LINE WIDTH, p
,
I
25
30
Figure 1. Line widths, experimental us. calculated, IIford 4 2 plates, 8 keV l%%+, charge (*%Xe+at plate) = 4.6 X IO-l4 6, 210 _< a,,, 4 277mm,1 I s , 4 5 0 p RESULTS
Resolution. The lateral magnification of the mass specThus at the photoplate, the width trograph is equal to a,&,. of the image of the ion source exit slit is s,(h/a,). Taking into account the ion beams 45' angle of incidence, and neglecting second order aberrations, line width, A y =
):(
1/?se
*
0
t
2 t 00.5 0
0 1-0 DISTANCE, p
Figure 2. Ion beam current profiles (A and B) and corresponding predicted photoplate line shapes (A' and B') The broken line plots transmittance instead of optical density. Centers of dense lines are more easily located if optical density has been recorded, and peak widths at half height are considerably greater when the height has been expressed in transmittance units
I
(3)
For the instrument used in this work, with Se = 10 p the theoretical A y at the low mass end of the photoplate is 1.05 p ; at the high mass end, 6.35 p . A plot of measured line widths us. predicted line widths is shown in Figure 1. The line chosen for these measurements was 128Xe+, the exposure level being held constant at total charge (lZ*Xe+ at plate) = 4.6 X C. The plate being processed as usual in Microdol-X; this is about 80 times the minimum detectable level of exposure. It is readily seen that, at least for this arbitrary definition of line width and this method of measurement, the correspondence between found and predicted line widths is not good. The deviations are easily understood, however. The photoplate places a lower limit on line width, 3.0 p being a representative average; and the instrument used in these experiments places an artificial upper limit on line width, apparently because of the formation of a virtual slit at some point in the ion path. The presence of this virtual slit is most clearly shown by the fact that, even with careful refocusing the rate of beam current increase falls off rapidly in the range 10 _< ue _< 25 p , no increase in beam current being detectable for se > 25 p. Outside the range of these effects, Equation 2 is a good guide. For example, for se = 10 p (data included in Figure l), the experimental line width varies smoothly from 2.7 p at a, = 120 mm to 5.8 p at a, = 277mm. It is a common observation that the level of exposure for a given line also affects the line width, lines due to ions of high relative abundance being noticeably broader. This is not an artifact that can be overcome through better technique. At the left of Figure 2 are shown two arbitrary profiles of ion current density us. ion beam width (where width is measured in the focal plane, not normal to the beam). Both beams have equal widths at half height, although A has a relative intensity of 200, while the relative intensity of B is 1000. The curves drawn happen to be Gaussian, but this is not necessarily 1968
-
>- 1.0
cn z 0 W
A
a5 i
Ls
54
W
53 2 I 30
100
300 1000 RELATIVE EXPOSURE
3000
10,000
Figure 3. Line width as a function of relative exposure QQ2 plates, for the conditions of this experiment, unit relative exposure = 0.15 pC/mmz)
-
Solid lines show predicted variation, and points are experimentally found. Upper line and triangles: 8 keV Xe+, a, 120 mm, sc = 18 p (nominal), width of Gaussian distribution used in model = 2.1 p. Lower line and circles: 8 keV Xe+, a , 204 mm, sB = 4 p (nominal), width of Gaussian distribution used in model = 1.4 p N
representative-depending on aberrations, these profiles may even be asymmetric. However, the approximation has served reasonably well in this case. A ' and B' are the expected profiles of optical density us. distance on the photoplate, plotted by picking relative abundance points on A and B and determining the corresponding optical densities from the experimentally determined characteristic emulsion response curve. As a base for this correspondence, an arbitrary relative abundance of 1 has been set equal to the minimum detectable level of exposure (optical density 0.04 unit greater than plate background). Thus, A' and B' (solid lines) are the predicted photoplate line shapes for peaks 200 and 1000 times the minimum detectable exposure level. The width of A and B is 2.1 p. The predicted width of A' is 4.0 p , of B', 4.6 p. The greater width of B' is due to a greater fraction of the full
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
7
1 . 5 , ,
Table I. Line Widths at High Ion Beam Currents“ Line width, ,u Total ion beam I zsxe+ 12sxe+ 1aoxet current, A Sa, P 10 3.2 4.9 3.9 0.5 X 10 3.6 4.8 3.4 0.7 X 4 2.8 3.5 3.0 0.5 x 1 0 - 1 3 4 2.7 3.5 2.5 0.1 x 10-11 a
Figure 4. Characteristic curve for Q2 emulsion (S4854) developed in Microdol-X for 12 min at 20°C. Exposing ions: 8 keV Xef, 124 _< mje 5
136 ion beam width being above the photoplate’s threshold of detectability. It is clear that heavy exposure takes a toll in line width, amounting to almost 1 p for each factor of ten in exposure. The cost of this broadening can, of course, be expressed in terms of resolution decrease and is discussed below. The agreement between this model and the experimental data is shown graphically in Figure 3, where the points indicate measured line widths and the plotted lines were derived as discussed above. The experimental points tend to be systematically high for large relative exposure, an effect for which there is no shortage of explanations. For these intense lines, space charge effects near the photoplate could lead to an artifical broadening of the beam. Alternatively, these high points may be attributed to systematic error in the choice of the half-maximum point-an error due either to microphotometer inadequacies at these high optical densities or to saturation of the emulsion at the centers of these lines. The last possibility seems the most likely. Because it is often observed that resolution levels achieved in special tests do not survive in practical application, the effect
4 2 (S4854) developed in Microdol-X, a,
=
204 mm.
of line width of total beam current was investigated. Space charge effects in the beam itself cannot be responsible for broadening at the beam current levels encountered in organic chemical work, and the absence of any other broadening effect dependent on total beam current was demonstrated by the constant line widths, which are summarized in Table I. The evaporated silver bromide plates gave much greater line widths. Within the very narrow range of correct exposure (see below), line widths were 5 to 10 p . Over-exposure resulted in intense fogging (D 1.0) for 3 to 5 p on either side of the line (D > 1.5, width > 6 p ) itself. The author has observed other Technical Operations plates exposed and processed in the same way as these in the mass spectrometry laboratory at M.I.T. Only a few of the M.I.T. plates showed the bad line widths and very sharp contrast observed in this work, and it thus seems that the data given here are not representative of the best obtainable plates. If, as production continues, plates of uniformly high quality can be made, a reinvestigation will be in order. Sensitivity. A typical emulsion characteristic curve determined in the course of this work is shown in Figure 4. Many such curves are summarized in Table 11, where the determinations are grouped according to experimental plates (the plate listed with two developers was cut lengthwise). The slope of the linear portion of the characteristic curve is given by the number y, which has dimensions of (density units/factor of ten in exposure). Values of this coefficient have been determined by the least-squares method; and the standard deviation (S,) obtained in that analysis is given along with N, the number of points in the linear range, so that the reader may judge the amount of scatter in the data. Instead of noting the exposure axis intercept of the extrapolated characteristic curve, the charge density ( p ) required to produce optical density = 0.30 (equivalent to 50% transmittance) has
-
Table 11. Variation of Emulsion Response with Ion Energy PO.30,
Plate type 4 2 (S4854)
Developer Microdol-X
pmin,
Linear range Factor Min. p pC/mmz to max. 1.4 X 400 X 480 1.2 X 130 1.3 5.5 X 66 X 1700 11.7
Exposing ions N pC/mmz pC/mmZ -Y s, 8 keV Xe+ 0.339 0.012 41 0.12 2.9 8 keV Xe+a 0.336 0.013 42 0.038 2.6 16 keV Xez+ 0.043 0.515 0.177 33 1.6 23 24 keV Xe3+ 0.663 0.053 3.0b 0.026 4 2 (S4869) Microdol-X 10 keV Xef 0.268 0.025 5.5 0.11 47 8 keV Xe+ 31 0.259 0.004 0.11 8.7 1.3 X 850 6 keV Xe+ 30 0 229 0.009 18 0.23 4.3 I X 140 4 keV Xe+ 29 0.200 0.030 35 0.30 6.0 T X 140 8 keV Xe+ Q2 (S4869) Microdol-X 0.240 0.014 9.3 37 0.11 2.2 X 450 UFG 8 keV Xe+ 0 629 0.022 26 0.52 0.033 X 38 0.52 Evap. AgBr Tech-Ops 8 keV Xe+ -1.7 E 1.o 0.50 1.o X 25 a Duplicate measurement on calibrated microphotometer. 0.30 is not in the linear range. Lowest point in linear range is (D = 0.40, p = 5.5 pC/mmZ). Fifty-three data points were obtained for this plate, but because the linear range is so extremely short and steep, no statistical treatment was possible. The tabulated y applies for D < 1.0, y N 0.5 for 1.5 5 D 5 2.2. I
I
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
1969
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m/e > 1.0
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2
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p , pc/mm2
~ h ~ r a c curves ~ e ~ for ~ ~ Q2 i ~ emulsion eveloped in ~ ~ c r o d ~ for l - X12 min at 20OC
Figure 6 . Characteristic curve points ( p < 6.3 pC/mmZ) for all masses tested on evaporated silver bromide plates developed in Tech-Ops developer for 30 sec at 20°C
How energy = 8 keV. Exposing ions, in order of increasing mass: CFs+, CaF6; @4FyT9C.jFo+, CaFiI+, C7Fia+, CsFis+, COF17'
been tabulated (~0.~0). The minimum charge density that gave detectable blackening (pmin) is also noted, though this is, of course, dependent on the interval between experimental charge density values, which can be as great as a factor of 2.13. The linear range of each characteristic curve is given by the final columns of the table. Taken together, the determinations made on the first two plates show that the effect of higher ion energy is to produce a steeper characteristic curve with, consequently, a shorter inimum detectable charge densities differ by at most a factor of ten over the range of energies investigated, but the much lower slope at the beginning of the 6- and 4-keV curves is shown by the increase in the tabulated values of p o . 8 0 . The same high charge densities that produced nonlinearity on the first plate failed to do so on the second (the linear ranges are thus at least as great as the tabulated factors indicate), and the results show that emulsion S4869 produced a longer linear range than did S4854, a finding that can be traced l o the higher contrast (greater y) of S4854. These results tend to indicate that the Q2 emulsion response is linear over a given optical density range (0.3 < D < 1.0) rather than over any fixed exposure range. The minimum detectable charge density can be improved by approximately a factor of four through the use of UFG developer. Because of the much greater y, ~ 0 . 8 is 0 an order of magnitude below the p 0 . 3 0 value obtained with Microdol-X developer. Line widths obtained with UFG are no greater than those obtained with Microdol-X (though they appear to be, probably because of the different grain structure produced by UFO) if the comparison is made between lines of equal maximum optical density. However, the high contrast produced by UFG results not only in the short linear range shown in Table 11, but also in a more rapid increase of line width relative to ion abundance, an effect that becomes quite severe for lines about 200 times more abundant than the minimum detectable. Under the conditions employed here, the evaporated silver bromide plate exhibits a contrast so great as to make its use impractical, A set of characteristic curves revealing the effect of ion mass on emulsion response is shown in Figure 5, and the results of duplicate experiments are summarized in Table PIT. The ion charge density required to produce D = 0.30 increases consistently with mass for each plate, the overall increase exceeding a factor of 20 in each case. Changes in y beyond m/e 231 1970
a
Ion energy = 8 keV. Exposing ions, in order of increasing mass: CFa+, C3F6+, C4F7+, C&+, CaFII+, C7F18'9 C8F15A,C9F17' are small, and the decrease in sensitivity comes about mainly through a shift of the entire curve toward higher required exposure levels. As the linear ranges indicate, the mass range (very approximately) 100 < m/e < 250 may be regarded as "best behaved": at lower masses, the characteristic curve tends to be steep and to exhibit nonlinearity at its high end, mls 69 even showing solarization at heavy exposure; while at higher masses the high end of the curve tends to bend over at lower optical density (that is, lower optical density at emulsion saturation, an effect better seen on plate 2). For mle 69 and m/e 131, the two plates show a considerable variation in y, and on plate 2, p 0 . 3 0 for m/e 181 is less than p 0 . 3 0 for m/e 131-effects that can be attributed only to variations in the emulsion, since the two plates were run on successive days and were handled identically. Two kinds of nonuniformity-between two plates and between two areas on the same plate-are required, and McCrea (13) has previously observed both. Figure 6 stands in striking contrast to Figure 5. The evaporated silver bromide plate appears to be equally sensitive to ions of all masses. Scatter is great, but it seems there is also no readily apparent trend in y values. The only massdependent parameter found was the optical density at saturation, which varied as follows (in order of increasing mass): 2.00, 1.65, 1.75, 1.60, 1.55, 1.50, 1.40, 1.25. Some systematic summary of the mass and energy effects will aid in their discussion. However, a quantitative treatment soundly based on first principles is not possible because the mechanism of the ion-emulsion interaction is so poorly understood. Franzen, Maurer, and Schuy ( I ) , for example, acknowledge this problem and restrict their comments to a very general level, chiefly noting their agreement with the long-extant (4) idea that mass and energy effects are primarily due to the influence of these parameters on ion penetration of the emulsion. Werner and Nieuwenhuizen ( 3 ) have shown that although most of the silver bromide grains in improved 4 2 plates are near the surface of the emulsion, even those right at the surface are covered with a gelatin layer about 250 A thick, a thickness probably sufficient to cause significant mass and energy discrimination. If penetration depth varies with the third power of ion velocity, then it should be proportional to (energy)a/2 and m-3i2.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
Table 111. Variation of Emulsion Response with Ion hlass
Plate type 4 2 (S4854)
4 2 (S4854)
a
Developer Microdol-Xa
Microdol-X
Exposing ions (all 8 keV) CFa+, m/e 69 C3F;+,m/e 131 C4F7+, m/e 181 CsFg+,mle 231 CeFll+,m/e 281 c7F18+, m/e 331 CsF16+,m/e 381 C9FI7+,mle 431 CFa+, m/e 69 CaFh+, m/e 131 C4F7+, m/e 181 C6F9+,m/e 231 C6Fllf, m/e 281 C7F13+, m/e 331 C8Fljf, m/e 381 c9F17+, m/e 431
po.ao,
Y
s,
0.43 0.45
0.05 0.04 0.02 0.02 0.02 0.03 0.02
0.40
0.31 0.27 0.28 0.25 0.22 0.74 0.73 0.38 0.25 0.26 0.26 0.25
0.25
0.01 0.06
0.04 0.02
0.03 0.04 0.02 0.02 0.02
N 13 14 10 12 14 12 12 10 10 10 12 13 14 10
8 8
Linear range Min. p / Factor pC/mmB pC/mma to max. 1.5 x 300 0.46 2.1 x 1100 2.4 X 380 0.85 1.0 3.1 X 340 2.5 X 360 1.6 2.7 x 340 0.81 2.4 >x 260 2.4 3.5 3.5 >x loo 4.3 x 340 0.38 5.6 X 130 0.56 3.8 X 760 3.6 X 800 0.78 0.85 1.7 X 230 0.97 1.9 x 140 1.7 3.4 x 55 2.1 4.2 > X 52 pmin,
pC/mmZ 1.7 4.4 6.9 9.8 13 15 25 35 1.7 4.9 3.8 9.7 12 17 37 56
Same as first plate in Table 11.
Emulsion “sensitivity” (defined in a number of different ways, here it is taken as p0.30-’) is generally found to be dependent upon ion energy in a way which can be represented as a straight line on a log-log plot of sensitivity us. ion energy over a limited energy range (IO). The slope of a double logarithmic plot of p0.30--1 us. E for the data in this paper is 2.0, a value in good agreement with the slope found by Woolston, Honig, and Botnick (IO) for 4 2 plates in this energy range. Plots of optical density (for a given value of p ) us. Ea12 are also linear and have AD/AE3I2 0.012 density units.eV-3/2in the energy range studied. The effect of ion mass is generally considered by plotting “relative sensitivity” us. ion mass and observing to what power ion mass (m) must be raised in order to obtain a function approximating the relative sensitivity curve (5, 9). A common result is that the sensitivity is directly proportional to m-0.6. Taking mje 181 as the point of normalization, ( p 0 . 3 0 l ) , j ( p 0 . 3 0 - ~ ) l ~ l may be plotted us. ion mass. The function (181/m)1.6 then approximates the experimental data of this paper very closely. Thus, while the dependence of sensitivity on ion energy for these polyatomic ions appears to be the same as that found in previous work using monoatomic species, the dependence of sensitivity on ion mass differs markedly, being directly proportional to r n - 1 - 5 instead of m--0.5. That this difference is not due to a difference in sensitivity definitions has been confirmed by adapting the method of Owens and Giardino (9) to recalculate sensitivities. In this procedure, optical densities corresponding to p = 10 pC/mmz for the range of masses discussed above were taken from the data of Figure 5. The data of Figure 4 provided a calibration curve allowing calculation of “apparent yields” (see ref. 9) which could be normalized at mje 181 and plotted us. mass. The same dependence on was found. The effect of mass may be further summarized. The straight line portion of each of the curves shown in Figure 5 can be represented by an equation having the form
-
r n - I e 5
D = Do
+ Y log PP,
(4)
where D = optical density, Do = a constant, y = the slope of the linear portion of the characteristic curve, p = a constant, andp = charge density expressed in C/mm2. Changes in y are small for m/e > 200, and all the data in that mass range may be
Table IV.
Variations of a with Ion Mass a
mle
231 281 331 381 431 * See Table 111.
Plate 1* 1.14 1.16
Plate 2* 1.14 1.15 1.17 1.27 1.31
1.15
1.21 1.24
summarized by a single expression if the factor /3 is made dependent on mass and given the form
p = - 1000 ma
where m = ion mass, and a = an empirical constant. For Do = 3.0 and y = 0.252, a = f ( m ) is given in Table IV. Thus the results of this work for ions of 8 keV energy and mje > 200 can be summarized by Equation 6 :
D = 3.0 f 0.252 log
1000 p
m1.2
where all the symbols have been defined above. This is an adaptation of the “reduced blackening function” developed by Dornenburg and Hintenberger for higher energy carbon atoms on unimproved 4 2 plates (6). In that work the exposing ions all had the formula C,+ (n 26), and the parameter n was the mass dependent variable. The experimentally found value of a was 1.25 i 0.25. Such reduced blackening functions have no physical meaning and are simply empirical fits to experimental data. The agreement between the experimentally determined a values is, nonetheless, significant.
100 if se < l o p . If se = 4 p, the predicted line width at m/e = 500 is 2.4 p and the minimum photoplate line width thus determines the maximum attainable resolution over the full mass range. In Figure 7, the line labelled “3 p between line centers” depicts this case (3 p is not the absolute minimum, as is shown by Figures 1 and 3, but it is a realistic practical minimum). The lower lines show resolution attainable for greater separation of the line centers, and may be used in conjunction with Figure 3 to learn the effect of ion abundance on resolution. For example, if both lines in a doublet at m/e = 300 give an exposure slightly greater than 100 times the minimum detectable, the maximum resolution will be about 3.8 x lo4instead of the 5.1 X l o 4that could be achieved with optimum exposure. The doublets shown in Figure 8 are practical demonstrations of these considerations. For the doublet M/AM = 3.18 X
Table V. Calculated Charge Densities Required at High Mass and Low Energy. p0.80,pC/mmab mle 10 keV 8 keV 6 keV 4 keV 500 21 33 60 130 750 54 98 220 1000 140 310 1250 1500
1750
180
400 480 610
2 keV 580 960
1300 1800 2100
2700
2000
3000 4 2 plates developed in Microdol-X. More energetic development could increase sensitivity by at least an order of magnitude at the cost of lower resolution. Q
* The values given here are probably underestimates, see text.
lo4(Figure Sa), the distance between line centers is 5.06 p, and the lines are very clearly resolved even though they are exposed at levels 200 and 90 times greater than the minimum detectable. Figure 86 shows a doublet with line center separation near the minimum, 3.52 p, M/AM = 5.04 X lo4. Relative Abundance Determinations. The continuous decrease in emulsion sensitivity observed for increasing ion mass must obviously be taken into account in the accurate calculation of relative ion abundances from optical density us. distance data. However, there is one other effect that is generally not considered in organic chemical practice. The spreading of the ion beam on its vertical axis is an elementary phenomenon, easily taken into account. Let h = the height of the beam at a distance L from the ion source, and h Ah = the height of the beam at a distance L AL, then:
+
+
For example, in the instrument used in this work, at the low mass end of the plate, h = 2.44 mm, and at the high mass end, h = 3.27 mm. If a mask of constant height = 1.0 mm is placed in front of the photoplate, from 41.0% to 30.6% of the beam is collected. In any case the ion beam current densityand therefore, the apparent relative abundance-decreases by 34% over the a, range covered by the photoplate. For the instrument used in this work, the required correction factor is F = 1.000 9.61 X y , where y is the distance (in mm) of the line from the end of the plate. Correction for mass discrimination by the emulsion is much more difficult. If, in the automatic acquisition of high resolution data, optical density values are recorded at intervals along the photographic plate and are used in the determination of relative abundances, then some determination of approximate mass will allow choice or construction of the proper characteristic curve to be used in scaling the individual optical densities and calculating integrated peak intensities. On the other hand, if all integrated peak intensities are calculated using a single characteristic curve during data condensation, it is difficult to imagine the successful use of some correction factor that would accurately reassign relative abundances after peak masses had been found. In particular, it can be seen that, since under this practice ions at high mass having large true relative abundances may be assigned intensities smaller than those assigned to less abundant peaks at low mass, any correction such as those based on xenon or hexachloropropene
1972 * ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
+
multiplets and currently used in some laboratories (19,20)will not remedy errors due to mass discrimination. In fact, the value of these corrections seems marginal, since similar errors can arise whenever an incorrect characteristic curve is used in data reduction. The peak integration method of relative abundance determination is soundly based and in wide use. But, given a wellestablished reduced blackening function and proper corrections for decreased current density due to vertical beam spreading and variable line width, it seems that the old approach of determining relative abundance through use of only the peak maximum optical density might be at least as successful in the high mass range. Sensitivity. Little elaboration on the results is needed here, though some consideration of emulsion sensitivity at very high mass is of interest. Progress in derivitization techniques has been such that mass spectra can be obtained from organic molecules having molecular weights approaching 2000, the mass taken as the upper limit in Table V, which tabulates ~ 0 . 8 0as a function of ion mass and energy. Consideration of energy is very important here, since in a practical instrument the mass range at a given energy is limited. These limitations are reflected in the range of entries in each column of Table V. The tabulated charge densities required at high mass and low energy are very high indeed. It appears that where 10 pg of a compound with molecular weight = 750 are needed for a spectrum, at least 0.1 mg of a compound with molecular weight = -1750 will be required. Of course, similar amounts of mass standardization compound would be needed, and it must be concluded that the outlook for use of emulsion photoplate ion detectors for m/e 2 1250 is bleak. Improvements in the evaporated silver bromide plate may
lower its contrast and improve its minimum line width to the point where a good alternative is available. Table V was constructed by solving Equation 6 for p and calculating p = f(m>,given D = 0.30. Values of a given in Table IV trend toward a = 1.25 for m/e > 350. Therefore, a = 1.20 may well be too low a value, particularly at very high mass. Since an increase in a requires an increase in log p 0 . 3 0 , the values given in Table V are probably underestimates. The extrapolation to lower energies was made on the basis of the variation shown in Table 11, and it is not definitely known that the same dependence of sensitivity on energy will prevail at these high masses. More energetic development can increase sensitivity by at least an order of magnitude and offset the effects of mass discrimination to some extent. The discussion of resolution above, however, has indicated the problems inherent in that approach. Because the reduced blackening function represents simply an empirical fit to data obtained using fluorocarbon ions, its applicability to organic ions in general can be questioned. Caution is required particularly because of the divergence in mass dependence between these results and those previously obtained using monoatomic ions. On the other hand, the value of a found here is in good agreement with that found in the single previous study using polyatomic ions (6). Thus, it is reasonable to attribute the different effect of mass to the very different physics involved in the impact of large polyatomic ions os. smaller monoatomic species. Secondary effects of unknown magnitude must lead to different sensitivities for various polyatomic species. Evaluation of those effects will necessitate careful studies of molecules of both varying size and varying chemical composition. ACKNOWLEDGMEKT
(19) R. Venkataraghaven et al., Proc. 15th Annual Conf. on Mass Spec. and Allied Topics, 93 (May 14-19,1967). (20) K. Biemann, P. V. Fennessey, and J. M. Hayes, Proc. of the Society of Photo-optical Instrumentation Engineers, Filmed Data and Computers, 11-1 (1966); D. M. Desiderio, Ph.D. Thesis, Dept. Chem., Massachusetts Institute of Technology, Cambridge, Mass., 1965.
I am grateful to Donald Ramey for extremely skillful alignment and maintenance of the mass spectrograph and to C.R. McKinney for helpful discussions. RECEIVED for review May 9,1969. Accepted August 29,1969.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
1973
