sorbance curves are independent of the Ca3(P04)2 concentrations. In addition, absorbance by the bone ash or calcium phosphate is a minimum and the sensitivity to lead is a maximum. While many possitlle causes of the variation in absorptivity with the concentrations of calcium phosphate and of HCl may be proposed, such as an increase in the activity coefficient with increased ionic strength or formation of a lead phosphate complex, further work is necessary to establish the actual mechanism. The most favoratrle concentration range was determined from the plot of Figure 2 as suggested by Delahay (2). In this figure (1 - T) is plotted against log concentration for solutions containing synthetic bore [Ca3(P0&] in concentrations of both 0.10 gram/&. and 0.20 gram/&. in 9M HCl. The optimum range is 4 tc 12 pg. of Pb per ml.
this manuscript. Thanks are also due J. E. Rose and L. D. Marinelli for their interest and encouragement. LITERATURE CITED
(1) Cholak,. J., Bambach, K., J. Znd. Hyg. Tozzcol. 25, 47 (1943). (2) De,fahay, Paul, "Instrumental Analysis, 1st ed., p. 207, Macmillan, New York, 1957. (3) Ebersole, E. R., Harbertson, A,, Flygare, J. K., Jr., Sill, C. W., Internal
Report, Health and Safety Division, USAEC National Reactor Testing Station, Idaho Operations Office, Idaho Falls, Idaho, p. 11, October 5, 1959. (4) Holtzman, R. B., Argonne National Laboratory, Radiological Physics Diwsion, Semiannual Rent. ANL-6297.
p. 67 (1960). (5) Holtzman, R. B., Health Physics 9 , 385 (1983'). ( 6 ) International Commission on Radio-
Table IV. Equations for the Calibration of the Lead Absorbance"
Concn. Concn. of of Ca3(PO& acid (g./ (molarity) ml.) 4
6 9
0.00 0.10 0.20 0.00 0.10 0.20 0.00 0.10 0.20
Absorbance (z = pg. Pb/ml.)
0.043~ 0.047s 0.053 z 0.056~ 0.059 z 0.062 z 0.069s 0.069 z 0.069 z
++ 0.098 0.045 ++ 0.015 0.028 ++ 0.012 0.023
At wavelength of maximum absorptivity, 271 mp, except for first two equations which are at 268 and 270 mp, respectively. 5
\ - - - - ,
logical Protection Committee I1 Report on Permissible Dose for Internal Radiation (1959), Zbid., 3, l (1960). (7) Krause, D. P., Argonne National Laboratory Radiological Physics Division, Semiannual Rept. ANL-6398, 77 (1961).
ACKNOWLEDGMENT
(8P.Merritt, C., Jr., Hershenson, H. M., Rogers, L. B., ANAL. CHEM.25, 572
Special thanks are due D. N. Edgington for his advice in the preparation of
(1953). (9) Reilly, C. N., Crawford, C. RI., Ibid., 27, 716 (1955).
(IO) Tipton, I. H., Cook, M.J., Steiner, R. L., Boye, C. A., Perry, H. M., Jr., Schroeder, H. A,, Health Physics 9, 89 (1963).
RECEIVED for review November 13, 1963. Accepted March 2, 1964. This work was performed under the auspices of the U. S. Atomic Energy Commission.
High-Resolution Mass Spectra of Compounds Emerging from a Gas Chromatograph SIR: The tandem operation of a gas liquid chromatograph (GLC) with a mass spectrometer (MS) for the characterization of the emerging fractions is an intriguing idea of considerable potential, and this pr nciple was demonstrated as early as 1957 (9). It has since been employed in various modes, ranging from the simple monitoring of a single mass (6, 8) to the fast scanning of a preselected part (0) of the spectrum, clr all of it, using oscillograph recorders (3, 4, 10, 11) or oscilloscope photography (3, 7). -4less direct approach utilizes a manifold located between GLC and MS for intermediate trapping of the fractions (5, 1.2).
These techniques have been applied to compounds of relatively high volatility and low molecular weight [hydrocarbons up to C13and oxygenated molecules up to Cs ( ( I ) ] . While the extension to more complex, heavier molecules would be of great interest, a few experimental difficulties seem to have prevented this in the past: A
pressure-reduction system (from 1 atm. to mm. Hg) constructed of metal and/or containing metal valves may lead to thermal decomposition, if heated to prevent adsorption of compounds of low volatility. Fast recording (within a few seconds) much beyond mass 150 may result in mass spectra of decreased resolution and makes it difficult to determine reliably the m/e of peaks in the higher mass range. The change in sample concentration during the emergence of the GLC fraction may result in a change in peak height unless the spectrum is scanned very fast, In an attempt to obtain a maximum of information from mass spectra of GLC fractions representing less volatile and more complex molecules, we are using an all-glass, valveless pressure reduction system (Figure 1) to feed continuously the effluent from the packed column into a high-resolution mass spectrometer of the MattauchHerzog type (CEC 21-110). This system eliminates all the difficulties
mentioned above and, in addition, results in spectral data containing m1,ch more information than conventional mass spectra obtained with the singlefocusing instruments hitherto used, even in the sophisticated version developed by Ryhage (IS) of which we learned after this paper was submitted for publication. About 1/5 (10 ml. per minute) of the effluent from the gas chromatograph (Perkin Elmer 800, 6-foot column, I/*inch) enters the pressure reduction system (Figure l), and the required pressure drop is effected by a small capillary on either end of a frittedglass tube surrounded by a vacuum jacket which is kept a t less than 1 mm. Hg. The fritted tube acts as a stream splitter in place of a valve and has the added advantage that the faster effusion of the light carrier gas (He) results in a re!ative enrichment (at least tenfold, possibly much larger) of the compound of interest in that part which enters the spectrometer. The system can be heated t o 250' or 300' C. When VOL. 36, NO. 6, M A Y 1964
1135
d
t o ion
from
Source
Column
Figure 1.
All-glass pressure reduction system
( a ) Teflon tubing; ( b ) capillary oriflce, 0.1-mm. i.d.; (c) fritted-glass tube, 5-micron pare size; ( d ) mechanical pump; (e) constriction, 0.1 2-mm. i.d.; (f) heating mantle; ( 9 ) Teflon connection; (h) silicon rubber connection; (i) silver w:re (for heat conduction); ( j ) Teflon plug; ( k ) heated glass connection to ion source
the substance enters the ion source, the entire mass spectrum is recorded simultaneously on a photographic plate placed in the focal plane of the spectrometer. Simultaneous recording and signal integration on the photographic plate eliminate the problem of intensity variations during the emergence of the peak. Up to 30 spectra can be exposed on one plate thus permitting all cornponents of even quite complex mixtures to be recorded. The use of a high-resolution spectrometer has two more advantages : not only is mass identification very reliable but it can be done so accurately as to permit the determination of the elemental composition of the fraction and of the fragments formed in the spectrometer. In addition, it is possible to bleed continuously into the ion source a calibration compound as a mass standard. If suitably chosen, its peaks will be resolved from those of the compound of interest and are thus not interfering. A. an example of the performance of the system, the mass spectrum of
methyl stearate will be discussed, obtained from a GLC fraction from a 9component mixture of esters (methyl undecanoate through methyl nonadecanoate). The spectrum which was recorded (within 15 seconds) in its entirety in the form of lines on the plate can be converted into a densitometer trace, part of which is shown in Figure 2 (middle trace). The spectrum of the calibration compound (perfluorodimethylhexane), an exposure a t a time when no fraction emerged, is shown directly below t o facilitate recognizing the peaks due to methyl stearate. Many of the multiplets can be seen even in this representation and it is obvious that because of the wide spacing of the peaks there is no ambiguity in mass identification and that the peaks of the calibration compound are well resolved and can be easily
distinquished from the peaks of the substance emerging from the chromatograph. An even better indication of the resolving power is given in the top of Figure 2, which shows a rescan of the region of m/e 83-85 a t a slower speed of the reccrding densitometer (CF&1 is due to an impurity in commercial CgFls). Furthermore, accurate measurement of the line positions permits determinaOf the exact mass Of each ion and thus of its ~?lementalcomposition (1). Some of these results, including the molecular weight, are listed in Table I. The error in the mass determination, (expressed as the difference-in millimass units-between the mass found and that calculated for the combination listed in column 2) in column 3 of the table, is only about a factor of two larger as compared with the values we generally obtain with pure, stationary samples that were admitted in the conventional way via an inlet system with reservoir or directly into the ion source (for some representative examples see reference 2). It should be noted that Figure 2 and Table I represent only part of the total information permanently recorded on the photographic plate within 15 seconds during the emergence of a gas chromatographic peak representing about 90 pg. of methyl stearate. For example, the top trace in Figure 2 represents a 2.5-mm. section of the 325-mm.-long plate containing the entire spectrum. Obviou~ly, the high quality of the
Table 1. Masses of Selected Ions in Spectrum of Methyl Stearate
Mass
found 298.2855 143.1086 129.0942 115.0777 111.1192 111.0815 100.1032 101.0608 97.1012 97.0664 95.0862 95.0127 87.0454 85.1028 85.0674 85 0308 84 0939 84.0591 83.0868 83.0499 74.0369 71.0862 71.0512
1 136
Elemental
composition
Error (m.m.u.)
C18H8802
- 1.6
CsHisOz C7Hi302 C6Hi102 CsHis C7H110 c&13
CsHeOz C7Hl3
CJ3eO CiHii C~HSOZ C1H702 CeHis CsHsO C4HsOz C6Hi2 CbHsO CsHii CsH7O C3HeOz CsHn C4H70
1.4 2.6 1.8 1.8 0.5 1.5 0.5 -0.6 1.1 0.1 -0.6 0.8 1.1 2.1 1.8 0.0 1.6 0.5 0.2 0 1
ANALYTICAL CHEMISTRY
0 1
1.5
74 Figure 2. in text
77
81
85 87
91 93 95
Densitometer recordings of portions of spectra discussed ( a ) Perfluorodimethylhexane (K.& K.) ( b ) Same with methyl stearate fraction superimposed (c) Part of ( b ) a t slower densitometer speed
mass spectral data clbtained with the technique described above will aid significantly in the identification or determination of the structure of compounds separated b:7 gas chromatography. ACKNOWLEDGMENT
R e are indebted to the R. & H. Filter Co., P.O. 5048, Wilmington, Del., for constructing the pressure reduction system for us. LITERATURE CITED
(1) Biemann, K., 11th Annual Conference
on Mass Spectrorietry and Allied Topics, San Francisco, May 1963.
(2) Biemann, K., Bommer, P., Burlingame, A. L., McMurray, W. J., Tetrahedron Letters, No. 28, 1969 (1963). (3) Brunnee, C., Jenckel, L., Kronenberger, K., 2. Anal. Chem. 189, 50 (1962); 197,42 (1963). (4) Dorsey, J. A., Hunt, R. H., O'Neal, M. J., ANAL.CHEM.35, 511 (1963). (5) Ebert, A. A., Ibid., 33,1865 (1961). (6) Gohlke, R. S., Ibid., 34,1332 (1962). (7) Ibid., 31, 535 (1959). (8) Henneberg, D., 2. Anal. Chem. 183,12 (19611. ( 9 j Holmes, J. C., Morrell, F. A., Appl. Spectry. 1 1 , 86 (1957). (101 Lindeman. L. P.. Annis. J. L.. ANAL. ' &EM. 32,1742 (1960). ' (11) McFadden, W. H., Teranishi, R., Black, D. R., Day, J. C., J. Food Science 28,316 (1963).
(12) Miller, D. O., ANAL.CHEM.35, 2033 (1963). (13) Ryhage, R., Ibid., 36, 759 (1964).
J. THROCK WATSON~ K. BIEMANN
Department of Chemistry Massachusetts Institute of Technology Cambridge, Mass. l
64.
M.I.T. DuPont Teaching Fellow 1963-
RECEIVED for review December 23, 1963. Accepted March 3, 1964. Work supported by research grants from the National Science Foundation (G-21037) and the National Institutes of Health, Public Health Service (RG-9352).
Ligand Bridging in the Oxidation of Chromium(II) at Mercury Electrodes SIR: Preliminary ~ r o r kon the effect of halide on the rate of electrolytic oxidation of Cr(I1) a t mercury electrodes has been done by Abubaker and Malik ( I ) , and more extensive citudies have been made by hikens and Ross (2) and Kemula and RakoMska ( 5 ) . These experiments suggested that the electrode reaction mechanism in the presence of chloride involves t k e adsorption of chloride on the mercury electrode followed by electron transfer through the adsorbed chloride to give CrC1+2as the product. However, the observed rate enhancement in the presence of halide does not by itself imply a bridged activated complex. 'Te have studied the reaction by controlled potential electrolysis to obtain direct evidence for this mechanism.
was carried out substantially to completion (98-99%) and the integral of the current-time curve read from the integrator. Cr(I1) solutions were prepared and stored by a modification of the method suggested by Stone (6'). Analysis of the Cr(I1) solutions was accomplished by oxidation with E'e(II1) followed by titration of the resulting Fe(I1) with a standard KRIIn04solution. Chloride and iodide analyses were done by potentiometric titration of the product solution with a standard solution of AgClO4 in 2F HC104using a silver wire indicator electrode and a glass reference electrode. When the ligand is chloride, the system is especially amenable to study because the product, CrC1+2, while thermodynamically unstable, is highly substitution-inert. This makes it powible to titrate a solution containing CrCl+Z and C1- with Agf
without appreciable decomposition of the complex, so that the concentration of free chloride ion in the solution can be readily determined. When iodide is present, any CrI+2 formed is readily decomposed by silver ion, so that the iodide titration does not distinguish between bound and free iodide ( 7 ) . RESULTS
With chloride as the complexing ligand, a series of electro-oxidations was carried out at various initial ratios of chloride to chromium(I1). The results are presented in Figure 1. (C1-)a and [Cr(II)Iorefer to initial concentrations, and (CrC1+2) refers to the final concentration of this species. These data demonstrate that a t least one chloride ion is present in the transition state for each Cr(I1) ion that is oxidized, as long
EXPERIME '4TAL
Controlled potential oxidations of e out a t Cr(I1) in 2F HClO, w ~ carried -0.012 volt vs. SCE in the presence and absence of chloride a t :t stirred-mercurypool electrode with area 10 sq. cm., in a three-compartment cell designed to provide complete exclusion of oxygen during the course of the reaction. When the effect of iodide was being studied the oxidationf, were carried out a t -0.192 volt. An .halytical Instruments, Inc., potentios tat and integrator were employed. A solution (40 ml.) of the complexing ligand a t the desired concentration in 2F HC104 was placed in the main compartment and Cr(1I) solution placed in the separate reference and auxiliary electrode compartments to prevent diffusion of oxygen into the main compartment through the fritted disk connections. The main compartment was thoroughly deaerated with purified Nz, and then 10 ml. of 0.1F Cr(I1) solution in 2F HC104 was added under anaerobic conditions. The electrolysis
0
0
predicted experimental
100
A h
la
Y
L.
2
80
h + 2 0
60
Y
8 40 20
20 Figure 1.
40
60
100 120 140 160 % (Ct-lO/(Cr( ~ 1 ) ~
80
180
200
Results of oxidation or Cr(ll) at mercury pool electrodes VOL. 36, NO. 6, MAY 1964
0
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