fore; and fragmentation of the ring leads to the loss of C4H7 (M-55 ion). The acyl ion at mje 111 and the cyclohexyl ion at m/e 83 both give relatively larger peaks in the fluoroalcohol ester spectrum than in the methyl ester spectrum. Diethyl succinate was treated under esterification conditions; and two products were isolated, the mixed ester and the di(fluoroalcoho1)ester. Their spectra, given in Figure 7, both show molecular ions and a large peak from a fragment at m/e 415. Being symmetrical, this is the only acyl ion that the di- ’ (fluoroalcoho1)ester shows; but the mixed ester also has an acyl ion at mje 129. The loss of C2H4from this ion leads to a very abundant fragment at m/e 101. There is no corresponding loss from the acyl ion at mje 415 because the fluorine-containing chain cannot undergo the rearrangement. The molecular ion of the mixed ester loses C2H3. to form the fragment at mle 433 in a manner analogous to the two-atom rearrangements previously shown in the spectra of butyl decanoate and dibutyl terephthalate. Figure 8 shows the mass spectra of Auoroalcohol esters of two aromatic acids, mesitoic acid, and terephthalic acid. In each case, the largest peak of the spectrum is due to the acyl ion formed by loss of a RuoroaIkoxy radical.
The last examples to be described are the two spectra shown in Figure 9. The upper is the spectrum of the methyl esters prepared from a mixture of acids ranging from C 5through CIS. The Cll methyl ester parent ion and acyl ion are marked to aid orientation. Acyloxy, alkyl, and acyl ions from the different acids overlap with the parent ions, making a molecular weight distribution very difficult to obtain. Below is the spectrum of the fluoroheptanol esters of the same mixture. As before, the cyclic six-membered rearrangement product is at mje 374 ; but above that, the only even-mass peaks are parent ions augmented a calculable amount by C13peaks from acyloxy ions of known composition. The high mass region of the lower spectrum is shown, amplified, in Figure 10. The marked similarity of the fragment-ion profile from mje 374 through mfe 443 to that shown by the dodecanoate ester (Figure 4) is evidence in favor of a straight chain structure for the components of the mixture. It is also clear that with calibration, good molecular weight distribution data could be obtained simply. RECEIVED for review July 7, 1967. Accepted September 13, 1967. Presented at the meeting of ASTM Committee E-14 on Mass Spectrometry, Denver, Colo., May 14-19, 1967.
Measurement and Interpretation of Metastable ectrometry T. W. Shannon,’ T. E. Mead,2 C. G . Warner,’ and F. W. McLafferty Department of Chemistry, Purdue University, Lafayette, Ind. 47907
A number of techniques for studying metastable ion transitions in mass spectra are compared. Advantages of the method proposed by Barber and Elliott for producing pure metastable spectra in a doublefocusing mass spectrometer include a sensitivity increase of a factor of 50 over conventional spectra and exact determination of the mass of the daughter ions. However, to identify and measure all of the metastable transitions in an unknown spectrum using electrical recording requires a separate scan of the accelerating voltage for each fragment ion in the normal spectrum. The use of the photoplate makes possible the recording of all ions simultaneously during this voltage scan, which greatly simplifies the routine collection of such pura metastable spectra. The availability of such data and its display as a metastable map aid in the elucidation of molecular structures, allow significant new correlations of mechanisms with structures, and make possible much more extensive use of the new technique of metastable ion characteristics. THEIDENTIFICATION of metastable ion decompositions is of considerable importance for structure determination by mass spectrometry. Peaks resulting from such decompositions can serve to identify particular reaction paths (1-3). (These peaks will be referred to as “metastables,” although the quotation marks formerly used to avoid ambiguity will be omitted.) (1) J. H. Beynon, “Mass Spectrometry and its Application to Organic Chemistry,” Elsevier, Amsterdam, 1960. (2) F. W. McLafferty, R. S. Gohlke. and R. C. Golesworthy, ASTM E-14 Conference on Mass Spectrometry, June 1964. (3) F. W. McLafferty, “Interpretation of Mass Spectra,” W. A. Benjamin, Inc., New York, 1966, p. 64.
1748
a
ANALYTICAL CHEMISTRY
Although such evidence is often cited in mechanistic studies. it does not appear to be generally recognized that reaction pathway identification can also indicate a particular arrangement of atoms. For example, in a hypothetical spectrum the presence of ions corresponding in mass to AB and ABC could indicate either of the molecular structure possibilities ABCBA or ACBBA. However, a metastable decomposition of ABC -+ AB would be possible, barring rearrangements, only for the structure ABCBA (3). In addition, recent work indicates that metastable ions can be used as a characteristic property of a particular ion structure (4), and applications to a variety of systems indicate this to be a valuable new tool for the elucidation of ion decomposition reactions (5-10). These potentialities of metastables prompted our search for a more sensitive, unambiguous, and convenient method for their identification. Narrowing the recorded width of the ordinary ion peaks using high resolution greatly facilitates recognition and measurement of metastable peaks, and Postdoctoral Research Fellow. Visiting Scientist, 1965; permanent address, Research Laboratories, American Cyanamid Co., Stamford, Conn. 1
2
(4) T. W. Shannon and F. W. McLafferty, J. Am. Chem. SOC.,88, 5021 (1966). (5) F. W. McLafferty and W .T. Pike, Ibid..89, 5951 (1967). (6) F. W. McLafferty, M. M. Bursey, and S. M. Kimball, Ibid..88,
5022 (1966). (7) M.M. Bursey and F. W. McLafferty, Ibid.,p. 5023. (8) Peter Brown and Carl Djerassi, Ibid.,89,2711 (1967). (9) F. W. McLafferty and W. T. Pike, Ibid.,p. 5953. (IO) W. T. Pike and F. W. McLafferty, Ibid.,p. 5954.
Rhodes, Barber, and Anderson (11) and Mancuso, Tsunakawa, and Biemann (12) have recently shown that computer techniques can be applied for convenient reduction of these improved data. The first authors (11) employ a mass spectrometer of Nier-Johnson geometry (AEI MS-9). Previous observations of their laboratory (13) had indicated this instrument's superiority for the detection of metastable peaks arising from decompositions between the electrostatic and magnetic analyzers in comparison with an instrument of Mattauch-Herzog geometry (AEI MS-7). Our studies with another instrument of the latter geometry (CEC 21-llOB) confirm that such metastable peaks are of somewhat reduced intensity and are often distorted by broadening (14) or noise which makes accurate mass measurement more difficult: a number of the metastables reported (11) for n-decane are unobservable or of doubtful assignment in the scanned spectrum (multiplier detection) from our Mattauch-Herzog instrument. This reduced performance is mainly due to the shorter drift region between the electrostatic and magnetic sectors and to the lack of an intermediate focus of the ion beam in this region (14). Pure Metastable Spectra. Metastable transitions can also be observed which occur in the longer drift region between the ion source object slit and the electrostatic analyzer (13, 15). For the daughter ion m2 to have sufficient kinetic energy to pass through the electrostatic analyzer the ion accelerating voltage is raised to VI, where VI = (ml/mz)Vo. The daughter ion will then appear at the position in the spectrum of a normal ion of mass m2, and the mass of the precursor ion ml can be calculated from V,/V,. Note that the mass, and thus the elemental composition, of the daughter ion of each metastable transition is now uniquely determined, in contrast to the metastables that have undergone only magnetic analysis (a variety of solutions are possible in the latter for m* = m22/ml). Note also that all normal ions are removed from the spectrum because they have too much energy to pass through the electrostatic sector. For this reason such spectra have been referred to as pure metastable spectra (13). Surprisingly, this method has previously been applied only to propane (15), benzene (16), and a few peaks in n-hexadecane (13). To test the general applicability of this technique, portions of the spectrum of 2,3-dihydroxy-2,3-diphenylindanone (direct ion-source introduction, sample temperature 130' C, are recorded in Figure 1 at increasing accelerating potentials relative to a constant electrostatic analyzer voltage. Figure 1A shows the spectrum recorded in a normal manner (but with wide slit settings) with a broad metastable peak at a mass of approximately 281. In Figure l B , the metastable peak has moved up in mass while the normal peaks are much reduced in intensity. Figure 1C shows large well-resolved peaks in the 298 and 281 mass regions which are the ions produced by metastable transitions between the ion source and the electrostatic analyzer. Using these daughter masses and the voltage ratios required to focus these ions, the precursor (parent) ion of the transition can be calculated to within 0.1 mass unit. (11) R. E. Rhodes, M. Barber, and R. L. Anderson, ANAL.C H m . , 38, 48 (1966). (12) N. R . Mancuso, S. Tsunakawa, and K. Biemann, Ibid.,p. 1775. (13) M. Barber and R. M. Elliott, ASTM E-14 Conference on Mass Spectrometry, Montreal, June 1964. (14) T. W. Shannon, F. W. McLafferty, and C. R. McKiimey, Chem. Cornrnun. (London), 478 (1966). (15) J. €3. Futtrell, K. R. Ryan, and L. W. Sieck, J . Chern. Phys., 43, 1832 (1965). (16) K. R. Jennings,J. Chern. Phys., 43,4176 (1965).
It is apparent that the broad, asymmetric metastable peak observed in the 281 mass region is due to the loss of water from the three isotopic parent ions; these are clearly separated in Figure 1C. Also, a transition involving a loss of OH from the M-H20 ion occurs. This metastable transition was hardly detectable when operating in the normal manner (m* = 265.0), but is a fairly intense peak when observed in the first. drift region. Note that this is a sensitivity increase of a factor of approximately 50. The obvious value of such a sensitivity enhancement prompted this investigation of experimental techniques to observe these ions. The method originally used by Barber and Elliott (13) is to make repeated magnetic scans (such as Figure 1) at increasing accelerating voltages; at various voltages metastable peaks will be recorded. This is a tedious procedure, and establishing the mass scale in the individual spectra is difficult because of the absence of normal peaks, The surprising absence of utilization of this valuable technique appears to be largely due to the time and inconvenience of measurement. Accelerating-Potential Scan. These difficulties can be partially overcome by another technique. The magnet is set so that normal ions of a particular mje will be collected at the final exit slit; ions of the mass resulting from metastable transitions can be recorded by scanning the accelerating potential (V). As this potential is raised, the normal peak will disappear, and at the voltage VI = (ml/m2)Vothe daughter ion from the metastable decomposition ml + m2 will appear. The accelerating potential V can be changed manually, preferably with observation of the ion current on an oscilloscope which scans the spectrum over a few mass units; or V can be scanned automatically with display of ion current on a conventional recorder. The latter is illustrated in Figure 2, in which the magnetic analyzer was set so as to focus normal mje 43 ions from n-decane on the collector using an ionaccelerating potential of 3590 V ; the signal shown was recorded while scanning the accelerating potential from 5800 to 6100 V. The large peak corresponds to the transition 71+ -+ 43+ 28; the shoulder illustrates the resolution possible to distinguish precursor ions of adjacent mass which yield daughter ions of the same mass. It should be expected that the Nier-Johnson geometry will give superior resolution in such cases. This procedure, while giving an accurate determination of mass and accelerating voltage, is laborious and sample-consuming, as it requires an individual study of every peak in the normal mass spectrum. Photoplate Recording. The availability of photoplaterecording makes it possible to acquire these data for all of the daughter ions of the spectrum with only one scan of the ionaccelerating potential. Figure 3 shows the m/e 281-316 portion of a photoplate exposed in this manner using 2,3dihydroxy-2,3-diphenylindanone. The top line is a 1-minute exposure (1 X 10-9 coulomb) at an accelerating potential of 3609 V which allows optimum transmission of normal ions. (Many of the peaks of Figure 1A do not appear because of the lower dynamic range of the photoplate and the limits of reproduction in print). The remaining 3 exposures on the photoplate represent ions recorded in 8 minutes each with increases of the accelerating potential by 62-V steps. Thus the ion lines of the bottom exposure correspond to the 17 and 316+ -+ 298+ metastable transitions 298" -r 281* 18 which are also shown in Figure 1C. Figure 4 shows the mass 37-44 portion of the photoplate recording of the pure metastable spectrum of n-decane. The top line is the normal mass spectrum, a 1-minute exposure of
+
+
+
VOL. 39, NO. 14, DECEMBER 1967
0
1749
---_.-
nL
i
281
1A
-A-
h
--
28 I
298
IB 1750
0
ANALYTICAL CHEMISTRY
285
h
-A h .
d h
281
298
IC Figure 1. Partial mass spectra of 2,3-dihydroxy-2,3-diphenylindanone
(Electron multiplier output with magnetic scan): A , normal spectrum, ion accelerating potential VO = 7138 V ; B, VI = 7355 V (electric sector potential held constant); C, V I = 7570 V (298 amu X 7570/ 7138 = 316.0 amu; 281 amu X 7570/7138 = 298.0 amu)
1
x
10-9 coulomb at an accelerating potential of 3620 V.
The remaining exposures (6 minutes each) on the photoplate represent ions recorded with a stepwise increase of accelerating potential by 127-V increments. Note that ions resulting from metastable transitions are now easily discerned by inspection, and that the mass of the daughter ion of the transition is determined directly by vertical alignment with the normal spectrum. The position along the Y axis represents the accelerating voltage, and thus indicates the precursor ion of the transition. Greater sensitivity or shorter exposure times are possible by the detection of the ion lines with a microphotometer. The stepwise change of the accelerating voltage gives less sensitivity for transitions whose optimum-accelerating voltage lies between two exposures, and this also leads to inaccuracies in determining the mass of the precursor ion. The ion lines appear to be unsymmetrical when the accelerating potential is not quite the calculated value, but this did not seem to be a reliable factor for judging the direction of offset; apparently other factors also affect ion line symmetry. Thus with the stepwise method of photoplate recording, it is difficult to distinguish between transitions yielding the same ion from ions which differ in mass by a relatively small amount. In Figure 4 the m/e 43 ions in the 18th and 19th exposures after the normal spectrum arise from both mje 71 and 72 as precursor ions; the scanning technique of Figure 2 resolves these. We plan to reduce this ambiguity by moving the photoplate continuously instead of stepwise
,7l
9
72
+
*
-
6019 V
5936v
Figure 2. Scan of ion accelerating potential to detect ions of m/e 43 formed from n-decane by metastable transitions in the drift region between the ion source and the electrostatic analyzer
Mass 43 ions formed in the ion source are transmitted by the electrostatic analyzer using an ion accelerating potential of 3590 V VOL. 39, NO. 14, DECEMBER 1967
0
1751
MASS 281
316
298
i
I1
Table I. Metastable Transitions of n-Decane
+
m2+
114 113 113 112 112 100 99 99 98 97 86 85 84 84 83 83 82 72 72 71 71 71 71 71 70 70 70 70 70 70 69 69 65 58 58 58 57 57 57 57 57 57 57 56 56 56 56 56 56
1 c
Figure 3. Portion of a photoplate-recorded pure metastable spectrum of 2,3-dihydroxy-2,3-diphenylindanone First line is an exposure of 1 minute of the normal spectrum. Each of the successive three spectra were 8-min. exposures at 62-V increases in the accelerating potential starting at 3609 V. No further ion lines appear at these masses at higher potentials
"MASS 37 1
I
I
I
I .
44
I
I
'
1
I
I
1
I
1
55 55
ml+ 143 143 142 143 142 143 143 142 142 112 143 142 142 112 112 98 112 143 114 143 142 127 114 113 142 113 112 99 98 85 98 84 67 114 100 86 142 114 113 100 99 98 85 113c 112 99 98 84 72 98 84
mi+ -+ m2+ m33 Intensitya m+ mlT w 55 83 VWb Sb
vw Sb
w Wb
Sb Sb
vw vw Sb Sb
vwb Wb VWb vwb
vw Wb VW W
vw VWb VSb
vw VWb VWb
vw vw Wb
mb Sb
W
W
mb w VU' Wb Sb
mb VSb
mb Sb
vw vw W
mb Sh
vw vw
55 55 53 52 44 44 43 43 43 43 43 43 43 42 42 42 42 42 42 41 41 41 41 41 41 39 39 39 39 38 37 30 30 29 29 29 29 29 29 29 28 28 27 27 27 27 26 26 25
71 70 55 54 86 72 142 113 99 86 85 72 71 112c 71 70 58 57 44 70 69 58 57 56 43 65 55 54 41 39 39 58 56 99 85 71 58 57
54 55 56 30 54 53 43 29 28 27 27
Intensitya vw S VWb
m VWb Wb
mb vw VW
vw Wb Sb
mb VSb
vw vw Sb
w W rnb
m m w VSb
m Sb
w mb mb mb W
vw vw vw vw vw VW W Sb S Sb
vw vw m m m Sb
S
vw vw
W
vs, very strong; s, strong; m, medium; w, weak; vw, very weak. Intensities are only qualitative. b Also observed in conventional spectra from instrument of Nier-Johnson geometry (11). c Mass identification not positive. a
I Figure 4. Portion of photoplate-recorded pure metastable spectrum of n-decane Top line is a 1-minute exposure of a normal spectrum. Each successive line is a 6-minute exposure at stepwise increases of 127 V of accelerating voltage starting at 3620 V; the last exposure on which ions (m/e 43) are recorded is at 7176 V
along its Y axis with a continuous scan of the accelerating voltage; a continuous plate-scan for normal mass spectra has been proposed for effluents emerging from a gas chromatograph (17). Using a microdensitometer to scan a particular daughter mass (I' axis) on such a photoplate should then 1752
ANALYTICAL CHEMISTRY
produce an output similar to Figure 2. Programming of a sufficiently mechanized microdensitometer-comparator, such as the Grant-Datex instrument ( l a ) , should make possible the automatic measurement of the pure metastable spectrum and subsequent computer calculation of the transitions along with the automatic determination of the element map from the normal high-resolution spectrum (18). Similar computercontrolled scanning of the ion accelerating voltage of a double(17) Dieter Henneberg, ANAL.CHEM., 38, 495 (1966). (18) R. Venkataraghavan, F. W. McLafferty, and J. W. Amy, Zbid.,39, 178 (1967).
NORMAL MASS SPECTRUM RELATIVE 50: ABUNDANCE
-
I 1
I
I
Figure 5. Metastable map of n-decane illustrating the monoisotopic transitions of Table I Relative intensity indications are: very weak,
A;
focusing mass spectrometer at the magnet setting corresponding to each ion of the normal spectrum should yield the same information, but require much more spectrometer time and sample size.
weak, D; medium,
0;
strong,
0;
This technique was used to determine the metastable transitions in n-decane using V2/Vl = 1.0-4.0; thus daughter ions down to 25 of the mass of the precursor ion could be detected, These are listed in Table I; the transitions occurring between the electric and magnetic sectors as observed on the Nier-Johnson spectrometer (11) are also noted for comparison. Four transitions reported for the Nier-Johnson instrument (11) are not found in this pure metastable spectrum. One of these, 72-1. + 42+ 30, was assigned (11) to the observed m* = 24.39, but this metastable peak may actually correspond to the transition 69+ -+ 41+ 28 (calculated m* = 24.36) of Table I. The former assignment is not consistent with our present knowledge of such reaction mechanisms (3) and with correlations of these transitions discussed later in the paper, and it cannot be due to a heavy-isotope species of any other transition of Table I. Similar arguments appear applicable 15, 86+ -+ for the other three transitions (loo+ -+ 85+ 56+ f 30, and 99f +. 55+ + 44) which were not found in the pure metastable spectrum. On the other hand, 53 of the 101 transitions of Table I were not reported (11) from the conventional scan of the Nier-Johnson instrument. For some of
+
+
+
+
+
55+ -+53+ 2, m* = 51.07; 67+ -+ 65+ 2, 63.05; 3 0 f - t 28+ f 2, m* = 26.13; 98+ 70+ 28, m* = 50.00; and 86+ 58+ 28, m* = 39.11-the
these-e.g.,
m*
=
-+
+
-+
+
calculated m* values correspond closely to the values of other transitions which are present-respectively, 142+ -+ 85+ 57, m* = 50.96; 112+-+ 84+ 28, m* = 63.13; 71+2, m* = 50.05; 43+ 421- + 28, in* = 26.06; 54+ -+ 52+
+
RESULTS
and very strong, 0
+ 41+
+ 2, m* = 39.16.
++
These transitions probably produce unresolved doublets in the conventional determination of metastable transitions. Thus Table I illustrates the more definitive nature of the information of the pure metastable spectrum as well as its greater sensitivity. Occasionally anomalous metastable ions are observed for abundant daughter ions which appear to originate from a broad range of masses. For example, in Figure 4 some of the ion lines of product mass 43 appear to arise from precursors of mje 44-55. These signals may be caused by reflections of the ion beam within the instrument. METASTABLE MAP
Studies of unimolecular ion dissociations have dealt mainly with the initial decompositions of the molecular ion, and, to a lesser extent, the decompositions of the primary fragment ions. Postulations of mechanisms for the formation and dissociation of secondary fragment ions are generally of a tenuous nature because multipie reaction paths and rearrangeVOL. 39; NO. 14, DECEMBER 1967
e
1753
ments become much more probable. We find that the abundance of the more specific data in a pure metastable spectrum makes possible many interesting correlations of these metastable reactions with the types of precursor ions and products involved. To illustrate this the n-decane data have been plotted in Figure 5 according to the mfe of the precursor and of the daughter ion of each of the monoisotopic transitions of Table I ; this display will be referred to as a metastable map, The ion abundances of the normal mass spectrum are shown on the axes, and the masses of the neutral products are indicated by the diagonals. Automatic preparation of a metastable map should be feasible using a computer-controlled plotter; the relative abundances of the ions could be indicated in a 3dimensional display similar to the topographical element map used for high resolution mass spectra (19). The odd-electron molecular ion, CnH2,+*+,decomposes to form the saturated alkyl ion, Cn-xH2n-sz+1A, or the C, - , H ~ , - Z ~ ~ ion by losses of, respectively, an alkyl radical (C,H2,-1.) or an alkane molecule (C,H2,+2). These transitions are abundant for the formation of the larger daughter ions. The alkyl ions are the most abundant type in the normal spectrum. These are involved as precursors in 18 of the metastable transitions, primarily cleaving to form another alkyl ion and a C,Hz, molecule. The alkyl ion tends to cleave in a nearly symmetrical fashion; the number of carbon atoms in the ion product is usually equal to or slightly larger than the number in the neutral molecule. The only other decompositions of the alkyl ions observed involve losses of Ht or CHI from relatively small alkyl ions. Most of the metastable transitions leading to daughter ions of higher masses (C4Hs’; or larger) involve the decomposition of odd-electron ions, except for the alkyl ions noted above. The most common precurso’r of this type is the C,Hz,t ion (olefin or cycloalkyl), decomposing by the loss of CH3. or CzHB to produce Cn-zH2n-2z--1+ ions, or by the loss of C2H4or C3Hs to produce a smaller Cn-,Hzn-2,T ion. AIthough nearly one third of the transitions observed are of these types, in general they do not produce metastable peaks in high abundance. The more abundant metastable transitions which produce the smaller fragment ions generally involve the loss of a small molecule, especially CPHI, CH4, and €32. This is not too surprising in view of the stability of these products, and the fact that in a cleavage producing a larger complementary (19) R. Venkataraghavan and F. W, McLafferty, ANAL.CHEM.,39, 278 (1967).
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ANALYTICAL CHEMISTRY
product, the charge retention would be less probable on the smaller fragment. As expected (20) almost all of the 31 transitions involving the decomposition of an even-electron ion yield another evenelectron ion and a molecule. The exceptions, C3H3+ C3H2’ and C2HSL--+ C2H2t,although of weak intensity, are surprising from our present knowledge of such reaction mechanisms. The advantages of high sensitivity, low ambiguity of identification, and amenability to automatic data-handling techniques suggest photoplate-recorded pure metastable spectra as a general method for the identification of metastable ion transitions.
-
ACKNOWLEDGMENT
The authors are grateful to W. A. Henderson, American Cyanamid Co., for the sample of 2,3-dihydroxy-2,3-diphenylindanorie and to R. E. Peterson for photography. RECEIVED for review January 9, 1967. Accepted August 25, 1967. Work supported by National Institutes of Health (GM 12755). (20) F. W. McLafferty, “Mass Spectrometry of Organic Ions” Academic Press, New York, 1963, p. 309.
Correct ion Determination of Sodium in Ultrapure Silicon and Silicon Dioxide Films by Activation An a lysis In this article by James F. Osborne, Graydon B. Larrabee, and Victor Harrap [ANAL.CHEhI., 39, 1144 (1967)], on page 1148 the Acknowledgment and credit for financial support were inadvertently omitted. The authors express sincere thanks to H. G. Carlson and C. R. Fuller for many helpful suggestions and discussions. Thanks are also due to C. E. Jones for the ellipsometer measurements, The financial support of the Rome Air Development Center under Contracts AF 30(602)-3723, 3727 for part of this work is gratefully acknowledged.
