justing the spectrophotometer. The data in Table I1 show higher precision for titration of hardness spectrophotometrically than by visual observation of the end point. Spectrophotometric titration curves for strontium(I1) and barium(I1) are shown in Figure 6. Special attention was given to barium(I1) which is difficult to titrate accurately by existing EDTA methods. Around the stoichiometric point the curve is nearly linear. If the concentration of Arsenazo I is increased, this linear region is further extended. To utilize this, barium may be titrated with Q.01MEDTA taking the end point as the extrapolation of two linear portions of the titration curve (Figure 7). This end point is probably slightly higher than the true end point, but this may be compensated for by standardizing the EDTA by a similar linear extrapolation method. Results for three titrations averaged 0.051 1 mmole of barium compared with 0.0510 mmole obtained by passing through a hydrogen-form cation exchanger and titrating the liberated acid with standard sodium hydroxide. 0.70 -
ACKNOWLEDGMENT
amount of indicator added must be carefully controlled, and the correct absorbance setting for the instrument before any titrant is added must be ascertained. Once this is done a large number of samples may be titrated without further ad-
We express appreciation to A. Tateda who first determined formation constants for calcium- and magnesium-Arsenazo I. The assistance of James Espenson with the stopped-flow experiment is also gratefully acknowledged. RECEIVED for review March 12, 1969. Accepted September 12, 1969. Work performed in the Ames Laboratory of the U.S. Atomic Energy Commission. Contribution No. 2516.
Mass Spectral Metastable Transitions Determined by Electric Sector Variation Robert W. Kiser, Richard E. Sullivan, and Michael S. Lupin Department of Chemistry, University of Kentucky, Lexington, K y . 40506 A technique is presented that enables unique determinations of metastable transitions to be made. The dynamic range is greater than 10,000 and the method is equally applicable to positive and negative ions. Additionally, consecutive fragmentation processes may be studied. Theoretical and experimental details are given for this method, first suggested by Major, that employs a variation of the electric sector voltage in double-focusing mass spectrometers. The application of the technique to hexacarbonylchromium(0) Is presented. Advantages and limitations of the approaches employed are evaluated.
METASTABLE TRANSITIONS observed in the mass spectrometer provide significant information about fragmentation processes and ionic mechanisms in the gas phase. In double-focusing mass spectrometers usually only a fraction of the metastable transitions that occur is recorded, because the electric sector does not transmit ions without corresponding kinetic energy and therefore the only metastable transitions observed are those that occur in the region between the electric sector and the magnetic field. Thus only a fraction of the data is commonly obtained. In 1964 Barber and Elliott ( I ) and Beynon, Saunders, and (1) M. Barber and R. M. Elliott, “Comparison of Metastable Spectra from Single- and Double-Focusing Mass Spectrometers,” 12th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Montreal, Canada, June 7-14, 1964. 1958
*
Williams (2) reported means of overcoming this loss of available metastable information for double-focusing mass spectrometers by increasing the accelerating voltage. Important theoretical and experimental implications of this technique have been discussed in subsequent work (3-15). An alter-
(2) J. H. Beynon, R. A. Saunders, and A. E. Williams, Nature, 204, 67 (1964). (3) 0. Osberghaus and Ch. Ottinger, P h y ~Letters, . 16, 121 (1965). (4) K. R. Jennings, Chem. Commun., 1966, 283. (.5 ,) M. Barber. K. R. Jennings. - . and R. Rhodes. 2.Nufurforsch., 22a, 15 (1967). (6) J. H. Beynon and A. E. Fontaine, ibid., 22a, 334 (1967) and references therein. (7) K. R. Jennings and A. F. Whiting, Chem. Commun., 1967, 820. (8) L. A. Shadoff, ANAL.CHEM.,39,1902 (1967). R. Dalv. A. McCormick, and R.E. Powell, Rev. Sci. Instr., (9) . ,39,N.1163 (1968). (10) P. Schulze and A. L. Burlingame, J. Chem. Phys., 49, 4870 ’ (1968) and references therein. (11) J. H. Beynon, J. A. Hopkinson, and G. R. Lester, Intern. J . Muss Spectry. Ion Phys., 2,291 (1969). (12) L. P. Hills, J. €3. Futrell, and A. L. Wahrhaftig, “Experimental and Calculated Metastable Peaks in Toluene,” 17th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Dallas, Tex., May 18-23,1969. (13) V. Lohle and Ch. Ottinger, “Consecutive Metastable Decompositions,” 17th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Dallas, Tex., May 19-23, 1969.
ANALYTICAL CHEMISTRY, VQL. 41, NO. 14, DECEMBER 1969
Figure 1. Electric sector voltage supply for determining metastable transitions by electric sector variation
SIto Sd. Ganged (position 1 = fixed voltage mode; 2 = decay mode; 3 = charge mode) Si,Se. Ganged (position A = total voltage across sector; B = one side of sector and C = other side of sector, with respect to ground) native means suggested recently by Major (16, 17) to accomplish the same objective employs a variation of the electric sector voltage. Subsequent reports (18-21) have discussed several aspects of this technique. We report here a detailed investigation of the variation of the electric sector voltage for observation and specific identification of metastable transitions. A dynamic range of 10,000 in intensities is easily accommodated, and the number of metastable transitions observed in the test compound has been increased over that usually found by approximately an order of magnitude. EXPERIMENTAL
A Hitachi Perkin-Elmer Model RMU-7 double-focusing mass spectrometer was modified by replacing the normal electric sector supply with a new sector voltage supply, shown schematically in Figure 1. All studies were conducted with a 70-ev electron beam; however, some of the negative ion metastable processes were studied also in the region of their electron capture maxima. The ion source and inlet (14) S. Jones and K. R. Jennings, “Structure in Broad Metastable Peaks,” 17th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Dallas, Tex., May 18-23, 1969. (15) R. M. Elliott, S. Evans, M. Barber, and W. A. Wolstenholme, “Applications of High Resolution and Ultra High Resolution Mass Spectrometry in Organic Chemistry,” the International Conference on Mass Spectroscopy, Kyoto, Japan, Sept. 8-12, 1969. (16) H. W. Major, Jr., private communication to R. W. Kiser. (17) M. L. Gross, R. B. Fairweather, W. F. Haddon, F. W. McLafferty, and H. W. Major, “Metastable Ions in Mass Spectra,” 16th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry,Pittsburgh, Pa., May 12-17,1968, (18) A. H. Struck and H. W. Major, Jr., “Enhanced Metastable Ion Measurement by Defocussed Operation of Double Focussing Mass Spectrometers,” 17th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Dallas, Tex., May 18-23, 1969. (19) R. Venkataraghavan, R. J. Klimowski, J. E. Coutant, and F. W. McLafferty, “Applications of a Small Digital Computer in the Mass Spectrometer Laboratory,” 17th Annual Meeting of ASTM Committee E-14 on Mass Spectrometry, Dallas, Tex., May 18-23, 1969. (20) F. W-.McLafferty, J. Okamoto, H. Tsuyama, Y. Nakajima, T. Noda, and H. W. Major, Org. Mass Spectry., 2, 751 (1969). (21) J. H. Beynon, J. W. Amy, and W. E. Baitinger, Chem. Commuu., 1969, 723.
temperatures were maintained at (ql/q2>(V/r,)
(3)
since the velocities of the mother and daughter ions of the metastable transition are the same because of the conservation of mass and momentum. Thus, the ratio of voltages required to transmit the daughter and mother ions is related to their masses and charges by
P
=
(EdEd = (m2/ml)(ql/qd
(4)
If the accelerating voltage rather than the electric sector voltage is varied, a similar relationship is obtained ( I , 2, 5, 8-10), However: in neither case does an experimental value of p only provide a unique definition of m2 and ml. In metastable studies made by variation of accelerating voltage, mz is usually known by prior selection and ml is determined. Investigations made by variation of the electric sector voltage combine Equation 4 and those equations of the cases already described to permit singular solutions for ml and m2 and, hence, a definitive identification of the fragmentation that occurs. For cases I and I11
mp
=
m*/p
(5)
whereas for case I1
m2 = 2m*/p
(6)
and, once m2 is defined, ml is determined for cases I and I1 from
ml = malp
(7)
(22) J. A. Nipple, R. E. Fox, and E. U. Condon, Phys. Reo., 69, 347 (1946).
1960
0
ml = 2mz/p (8) Experimentally, it is required only to determine EDand E, accurately to obtain p, and to determine m* to identify completely the fragmentation (metastable transition). This is accomplished by plotting the metastable ion intensity as a function of E p to obtain the value of Ep at maximum ion intensity and by using the corresponding m* for this Ep, From Equation 3 and for cases I and I11 m* = m2r,Ep/2V
(9)
whereas for case I1
Wipple, Fox, and Condon (22) have shown that the apparent mass, m*, of the daughter ion is given by
m* = m22q1e/mlq22
and for case I11 from
m* = m2r,Ep/4V (10) It is necessary to utilize m* corresponding to p at the peak intensity, p*, in the calculations. The variation of m* as a function of Ep is dependent upon m2 and instrumental parameters-ie., u = dm*/dE, = mzr,/2V
(11)
for cases I and 111; similarly for case 11, u = m2re/4V (12) The slope of c as a function of m2 is simply the inverse of Es for cases I and I11 and the inverse of 2E8 for case 11.
APPLICATION TO Cr(CO)6
To illustrate the application of this new technique to singly and multiply charged positive and singly charged negative ion metastable transitions, we have chosen hexacarbonylchromium(0), a compound that has been studied rather extensively (23-30). The 70-ev monoisotopic positive and negative ion mass spectra of Cr(CO)6 are shown in Figure 2. The total negative ion current is 8% of the total positive ion current. Several metastable transition peaks of low (60,000 and 41 have been observed in other systems. This demonstrates a dynamic range of greater than four orders of magnitude. The widths of the peaks at half-height, rlj2, are dependent upon the kinetic energy involved in the metastable transition. 4000
3000
5h
5
2000
c
5 .9 c
1.5~.
2
2
1000
p* = 0.5898 I
0.73
0.74
0
0.58
0.59
I
PFigure 5. Typical plots of metastable transition intensities as a function of p p*
1962
determined from center of peak
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
Table I. Positive Ion Metastable Transitions for Hexacarbonylchromium. Calculated miq Relative Mother ion Daughter ion my Transitionb intensity P" 0.6420 77.82 32.07 49.96 [CrCOf + Cr+] 120 0.65073 79.82 33.79 51.93 CrCO+ 4 Cr+ 2240 0.6553 80.75 52.91 34.67 [CrCO+-+ Cr+] 260 0.6596 81.73 35.56 53.91 [CrCO+-+ Cr+] 80 0.7364 105.8 77.95 57.40 190 [Cr(CO),+ -+ CrCOf] 107.8 0.74141 79.92 59.25 Cr(CO)2+-+ CrCOf] 3590 0.7436 108.7 60.11 80.83 [Cr(CO),+ -+ CrCO+] 490 0.7461 109.8 81.91 61.11 130 [Cr(C0)2+-+ CrCO+] 0.7909 134.0 106.0 83.82 75 [Cr(CO),+ + Cr(CO)2+] 0,79403 135.9 85.70 107.9 Cr(C0)3+-+ Cr(CO)2+ 1500 0.7958 136.9 86.72 109.0 220 [Cr(CO),+ Cr(CO)z+] 0.7973 138.0 87.70 [CI(CO)~++ Cr(CO),+ 110.0 60 112.69 0.82932 163.9 Cr(CO),' -+ Cr(CO)3+ 135.9 240 113.64 0.8299 165.0 136.9 [Cr(CO),+ Cr(CO)3+] 50 0.85403 140.00 192.0 163.9 Cr(CO)5++ Cr(C0)4+ 388 140.87 0.8547 192.8 164.8 [Cr(CO),+ + Cr(CO),+] 75 165.49 217.9 0.8714 189.9 [Cr(CO),+ Cr(CO),+] 75 0.87273 167.54 220.0 192.0 Cr(CO)6+ Cr(CO):+ 1075 168.42 0.8733 220.8 192.9 [Cr(CO)6+ Cr(CO),+] 190 0.8737 169.46 222.0 194.0 [Cr(CO),+ Cr(CO),+] 62 0.6965 44.55 91.82 63.96 CrCOC+ + CrC+ 9 0.65163 16.94 39 90 26.00 CrC02++ Cr2+ 55 53,83 0.74203 29.64 39.94 Cr(C0)22++. CrCOZf 142 0,79503 67.93 42.93 54.00 Cr(CO)a*+-+ Cr(C0)22+ 690 0,82923 82.04 56.41 Cr(CO)4zf-+ Cr(C0)32+ 68.03 255 0.85473 95.95 70.09 Cr(CO)a*++ Cr(C0)42+ 82.01 1000 0.8731 109.9 83.81 95.99 Cr(CO),z+ + Cr(C0),2+ 10 0.8110 74.05 48.71 60.06 Cr(CO)aCz++ Cr(CO)zC2+ 2 0.2395 214.5 12.30 51.36 Cr(CO),+ +. Cr+ 4 0.3657 217.1 29.04 79.40 Cr(CO)6+-+ CrCO+ 10 0.4928 218.9 53.17 107.9 Cr(CO)6T+ Cr(C0)2+ 12 219.1 0.6194 84.08 135.7 Cr(CO),+ Cr(CO)a+ 35 0.74563 122 23 219.8 163.9 Cr(CO)e+-+ Cr(CO),+ 87 0.4186 33.28 189.9 79.51 Cr(C0)6+-+ Cr(CO)+
