DISCUSSION
oE;o- ‘N
The molar absorptivity of osmium, measured as the NPD complex, is 2.4 X lo4. This compares favorably with other methods suggested for the spectrophotometric determination of osmium. There are several ions which must be absent when osmium is determined by this method. Well established methods for separating osmium based on the volatility of osmium tetroxide have been described (ZI, 12). The structural formula of NPD may be shown as
It forms colored complexes with ruthenium(II1) (purple), gold(II1) (blue), rhodium(II1) (yellow), and palladium(I1) yellow. These observations indicate a need for further investigation of the reaction of NPD with these and other ions. ACKNOWLEDGMENT
The authors thank Dr. J. H. Jefferson for valuable consultations during the investigation. ( 1 1) F. E. Beamish and W. J. Allen, ibid., 24, 1608 (1952). (12) F. E. Beamish and A. D. Westland, ibid., 26, 739 (1954).
RECEIVED for review May 14, 1969. Accepted June 23, 1969.
Application of Column Bleed Absorption in High Sensitivity Gas Chromatography and in Gas Chromatography-Mass Spectrometry R. L. Levy,’ H. Gesser, T. S. Herman,2 and F. W. Hougen Department of Chemistry and Department of Plant Science, The University of Manitoba, Winnipeg, Canada
HIGH-SENSITIVITY IONIZATION detectors and combined gas chromatography-mass spectrometry (GC-MS) have proved to be powerful tools for detection and identification of minute quantities of organic material ( I , 2). However, the evaporation of the liquid phase from a column, the so-called “column bleed,” seriously interferes in both high-sensitivity gas chromatography and GC-MS (3, 4), particularly in programmed temperature operation. The dual-column tech1 Present address, McDonnell Research Labs, St. Louis, Mo. 63166. 2 Midwest Research Institute, Kansas City, Mo. 64110.
(1) V. Svojanovsky,K. Krejci, K. Tesarik, and J. Janak, Chromatog. Rev., 8, 90 (1966). (2) W. H. McFadden, Separation Sci., 1, 723 (1966). (3) S. Dal Nogare and R. S . Juvet, “Gas-Liquid Chromatography,” Interscience, New York, 1962, p 120. (4) R. Teranishi, R. G. Buttery, W. H. McFadden, R. T. Mon, and J. Wasserman, ANAL.CHEM.,36, 1509 (1964).
1 85
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nique (5) (which is not applicable in GC-MS) was introduced t o alleviate the problem. However, it can be used only in cases of low or moderate sensitivities. For dual-column temperature programming a t high sensitivities, the two columns and all the associated components of the dual system must be perfectly matched in their behavior. This ideal condition could not be achieved in our laboratory when the sensitivity, in terms of the minimum detectable quantity, was 10-10 to lo-‘* gram/sec. In GC-MS the sensitivity is limited by the extent of column bleeding (4), and only liquid phases that d o not mask the mass spectral patterns of the sample compounds can be used. This situation greatly restricts the selection of liquid phases, and limits the maximum operating temperature of the gas (5) W. E. Harris and H. W. Habgood, “Programmed Temperature Gas Chromatography,” John Wiley & Sons, New York, 1966, p 228.
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Figure 1. Recorder base line obtained by single column temperature programming a t relatively high sensitivity A . Analytical column only B. Bleed absorbing column attached, 1st run C. Bleed absorbing column attached, 2nd run 1480
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ANALYTICAL CHEMISTRY
I
IIIIII
135'
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Figure 2. Background mass spectra of Tergitol NP-35 for different temperatures A . With analytical column alone B. With bleed absorbing column attached to analytical column
chromatograph. In temperature-programmed operation of a GC-MS system, the continually changing rate of column bleeding causes a continual increase in intensity and change of pattern of the background spectra which limits the sensitivity of the system and complicates the application of temperature programming. The temperature limitations of liquid phases in gas chromatography, including their volatility at various temperatures, were carefully studied and discussed by various workers (6-9). This report describes a new simple method for greatly reducing the effect of column bleed in gas chromatography and GC-MS. The method, in principle, consists in trapping the bleeding material, as it emerges from an analytical column, by absorption in an attached short column of high thermal stability (IO). (6) W. Gerrard, S. J. Hawkes, and E. F. Mooney, "Gas Chromatography 1960," R. P. W. Scott, Ed., Butterworths, London, 1960, p 199. (7) S.J. Hawkes and E. F. Mooney, ANAL.CHEM., 36,1473 (1964). (8) N. Petsev and C. Dimitrov, J. Chromatogr., 30, 332 (1967). (9) Ibid.,34, 310 (1968). (10) R. L. Levy, H. D. Gesser, and F. W. Hougen, ibid., 30, 198 (1967).
EXPERIMENTAL
An Aerograph gas chromatograph Model 660 was used with a 37-ft X l/s-inch 0.d. column packed with 1 % wt/wt Tergitol NP-35 (Nonyl phenol polyethylene glycol ether) on 60-80 mesh DMCS Chromosorb G. The column effluent (30 ml/min) was split in approximately 2: 1 ratio and fed to a flame ionization detector (FID) and to the inlet of a HitachiPerkin Elmer mass spectrometer Model RMU-6D. The temperature of the column was manually programmed from 50 to 175 "C at an average rate of 4 "C/min without injecting any sample. The outputs of the FID and the total ion monitor (TIM) systems were simultaneously recorded at the highest operable sensitivity (attenuation 1 X 10 for the FID system, estimated equivalent to a minimum detectable quantity of approximately gram/sec). In a subsequent experiment, an 8-inch X 'js-inch o.d., column packed with 15% wt/wt Carbowax 20M-TPA (polyethylene glycol-terephthalic acid ester) on Anakrom ABS, 60-80 mesh, conditioned at 245 "C for 2 hr was connected between the outlet of the analytical column and the stream splitter to serve as a bleed-absorbing column. The temperature of the analytical column and the bleed-absorbing column was programmed from 50 to 197 "C at an average rate of 4 "C/ min. The oven was kept at 197 "C for 4 min, and then cooled to 100 "C. Starting from 100 "C the programming of the VOL. 41, NO. 11, SEPTEMBER 1969
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Figure 3. Thermogravimetric curves for Tergitol NP-35 obtained at vacuum and at atmospheric pressure ( A ) Vacuum (0.01 Torr) ( B ) Atmospheric pressure (argon)
oven temperature was repeated at an average rate of 15 "C/ min up to 192 "C. In order to check the effect of the bleed absorbing column on the retention times of compounds to be analyzed, a mixture of CIO,CII, and CIZnormal paraffins was run under identical conditions with and without the bleed-absorbing column attached to the analytical column. Thermogravimetric analyses of Tergitol NP-35, Squalane, and other liquid phases were carried out at vacuum (0.01 Torr) and in an argon atmosphere. RESULTS AND DISCUSSION
The recorder base lines obtained with the FID system for temperature-programmed runs with and without the bleedabsorbing column (BAC) attached to the analytical column are shown in Figure 1 . The temperature-programmed run with the analytical column alone showed noticeable bleeding at about 105 "C (Figure 1 , A ) . The first run after attachment of the bleed-absorbing column shows sudden bleeding only at about 193 "C (Figure 1, B). The second run with the BAC attached showed comparable bleeding at 187 "C (Figure 1, C). This occurs as a result of the fact that the Tergitol vapors had saturated the BAC at 193-5 "C during the first run and reached the detector as soon as the temperature is high enough to allow migration-Le., about 187 "C. The total amount of the column bleed retained or trapped on the BAC after cooling affects the temperature at which the column bleed reaches the detector in the subsequent run. This amount depends on the highest temperature to which the system has been exposed. From separate experiments, the liquid phase of the BAC was found to produce significant bleeding only at 245 "C. These results showed that the vapors of the liquid phase from the analytical column were retained in the bleed-absorbing column up to a temperature of approximately 190 "C. The bleed-absorbing column effectively extended the useful range of the analytical column from 105 1482
ANALYTICAL CHEMISTRY
to 190 "C. In this temperature range, the column prevented the column bleed from entering the mass spectrometer or the GC detector. With the GC-MS system, the background mass spectra were recorded at different temperatures of the programmed runs before and after attachment of the bleed-absorbing column. Temperature programming of the analytical column alone resulted in a continual increase in the intensity of the background spectra (Figure 2 , A ) , indicating how the sensitivity of the GC-MS system decreased as the temperature of the column increased ( 4 ) . Temperature programming of the analytical column with the bleed-absorbing column attached resulted in nearly constant background spectra of low intensity throughout the tmperature range of 50 to 175 "C (Figure 2, B), indicating that the column bleed had been effectively absorbed and its interference eliminated in this wide temperature range. The bleed-absorbing column caused only a slight increase, about IO%, in the retention times of the Cl0, Cll, and C1z normal paraffins at 125 "C, as compared with their retention times obtained with the analytical column alone. The additional retention caused by the bleed-absorbing column could probably be reduced by adjusting its length, amount of liquid phase, and temperature, without adversely affecting the bleed absorption. To avoid loss of resolution in the bleed-absorbing column, its liquid phase should be selected to match the properties of the liquid phase of the analytical column with respect to the order of elution of the substances to be analyzed. The bleed-absorbing column could presumably be regenerated at intervals by flushing it with carrier gas at a higher temperature. It might thus be of advantage to contain the bleed-absorbing column in a separate compartment with independent heat control. Such a compartment could be installed within the main column oven. As an alternative to
regeneration in situ, it should be feasible to prepare a number of bleed-absorbing columns, to be regenerated or discarded after use. The useful life of a BAC without regeneration depends mainly on its length or the total amount of liquid phase it contains; the nature of its liquid phase and the phase it absorbs; the highest temperature to which the analytical column and the BAC are brought to; the flow rate of the carrier gas; and the rate of temperature programming or the period of time the BAC is exposed to high bleeding rates. Thermogravimetry of liquid phases at atmospheric pressure (Figure 3, B ) is a common method for determining their upper temperature limit ( I I ) . However, the data obtained (11) R. W. McKinney, J. F. Light, and R. L. Jordan, J. Gas Chromatogr., 6, 97 (1968).
with TGA at atmospheric pressure cannot be used to determine the applicability of a liquid phase in high sensitivity G C or in GC-MS. This is due to the fact that ionization detectors and mass spectrometers are many times more sensitive than thermogravimetry. Thermogravimetric curves of Tergitol NP-35 obtained at vacuum, however, (Figure 3, A ) were found to be similar to the base line of the Tergitol NP-35 column shown in Figure 1, A , which indicates the possible use of vacuum therrnogravimetry as a rapid method for estimation of the maximum permissible temperature of a liquid phase when used at high sensitivities or in GC-MS. RECEIVED for review December 16, 1968. Accepted July 7, 1969. Presented at the 155th ACS Meeting, San Francisco, Calif., April 1968. Support of the National Research Council of Canada and Distillers Corporation Limited is gratefully acknowledged.
Coulometric Titration of Ytterbium(l1) and Tungsten(l1)Tungsten(0) Potential in Lithium Chloride-Potassium Chloride Eutectic Melt K. E. Johnson Diuision of Natural Sciences, Unimrsity of Saskatchewan, Regina, Saskatchewan, Canada
J. R. Mackenzie’ Department of Chemistry, Sir John Cass College, London, E.C.3, Englana THEFORMAL POTENTIALS in LiCl-KC1 eutectic melt at 450 “C of the redox systems Yb(II1)-Yb(I1) and Eu(II1)-Eu(I1) were found to be -1.375 V and -0.554 V, respectively, us. a 1.0m Pt(I1)-Pt(0) reference ( I ) . The divalent species were generated coulometrically from solutions of the trivalent chlorides. When attempts were made to use tungsten as an indicator electrode for the coulometric reduction of Eu(II1) ( I ) , the tungsten corroded. Consequently, the magnitude of equilibrium potentials of tungsten ion-tungsten metal couples in LiCl/KCl eutectic melts was of immediate concern to this investigation. However, efforts to introduce tungsten into solution by anodization of bright tungsten electrodes merely resulted in the evolution of chlorine. Various attempts were made to remove the surface film from anodically polished tungsten electrodes prior to anodization in the eutectic and methods included mechanical polishing with fine emergy cloth, heating in hydrogen, and short term ( ‘/z to 1 minute) cathodic polarization. Subsequent chlorine evolution still occurred, however, and no appreciable solution of tungsten could be detected. It seemed unlikely, in view of the foregoing, that passivity could be attributed to oxide film formation, but the possibility remained of anodic polarization producing adherent films of insoluble polymeric chlorides. The hypothesis that such films were responsible for passivity appeared to receive support from the observation of similar behavior with electrodes 1 Present address, Department of Chemistry, The University, Southampton, England.
(1) K. E. Johnson and J. R. Mackenzie, J. Electrochem. SOC.,in
press.
prepared from the neighboring heavy transition metals, tantalum and rhenium, the lower chlorides of which are known to be polymeric and to form molecules of the metal-metal bond ‘cluster’ type (2). In the case of rhenium, Re&& was shown to disproportionate in LiCI-KC1 eutectic melt (3). Tungsten counter-electrodes employed as anodes during coulometric cathodic reduction of rare earth species were corroded in use and the contents of counter electrode compartments became colored an intense, but indeterminate, dark blue. Such electrodes had of course been cathodically polarized previously during anodization of platinum foil electrodes, and it was reasonable to suppose that any surface film would be reduced by this procedure, either directly or by lithium generated in the compartment. It was first thought that the dark color of the compartment contents arose solely as a result of the presence of lithium and its subsequent reaction products with glass. However, the presence of substantial quantities of tungsten in the solidified contents was demonstrated by qualitative analysis (tungsten blue with sodium silicate) and spectroscopically. It was conceivable that the function of lithium in preventing the development of passivity on anodization of tungsten electrodes was that of a depolarizer, permitting a smooth transition from cathodic to anodic conditions at the electrode surface. Such near equilibrium conditions would be more favorable to the formation of free complex chloro-anions, e.g., WCl,(2-n)0r(3-n), the only likely labile ionic species. For quantitative study the use of lithium as depolarizer was precluded by its extreme reactivity with glass of the electrode (2) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry,” Interscience, New York, 1966, pp 926-67. (3) R. A. Bailey and J. A. McIntyre, Znorg. Chem., 5, 1940 (1966). VOL. 41, NO. 11, SEPTEMBER 1969
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