IRA-401 resins in their hydroxide forms can be used, but not the more highly cross-linked Dowex I-X8 or Amberlite IRA400 resins, which yielded incomplete recovery of the acids. This is in direct contrast t o the work of Robb and Westbrook (3) and Hornstein, et al. (5) who used IRA-400 in their experiments. No reason for this difference can be offered; however, we recommend the less highly cross-linked resins. Robb and Westhrook suggested that any other solvent capable of dissolving the tetramethylammonium salts could be used in place of methanol. However, chlorinated solvents such as chloroform and carbon tetrachloride when used alone or when present as minor components inhibit complete methylation of the acids. Actually, chloroform when used as a (5) I. Hornstein, J. A. Alford, L. E. Elliott, and P. F. Crowe, ANAL.CHEM., 32, 540 (1960).
solvent in this procedure, provided an excellent means of preparing 50% mixtures of the dimethyl ester and the half ester of the dibasic acids. Acid samples containing as high as 50% water could be completely methylated (99.9 %) with only a trace (0.1 %) of the half ester being formed. Ethyl and butyl esters of acids can also be easily formed by use of tetraethyl or tetrabutyl ammonium hydroxide. By employing a preparative scale chromatograph, it is possible that large quantities of pure esters might be prepared. ACKNOWLEDGMENT
The author thanks W. A. Dippel for his helpful advice and suggestions in editing this article. RECEIVED for review April 11, 1967. Accepted July 17, 1967.
Demountable Hot Hollow Cathode Lamp as Excitation Source in Atomic Fluorescence Flame Spectrometry Joseph I. Dinnin and Armin W. Helz U S .Geological Surce),, Washington, D.C. 20242 IN A BASIC THEORETICAL INvEsTIGATioN of atomic fluorescence flame spectrometry ( I ) , Winefordner et al. derived the equations relating fluorescence intensity to the concentration of atoms in the vapor of a n element and to the intensity of the light source. Because fluorescence intensity is directly related to excitation intensity, improvements in sensitivity can be expected with the development of new, more intense sources of excitation. Winefordner recently reviewed atomic fluorescence flame spectrometry ( 2 ) and enumerated most of the excitation sources used in analytical investigations. These included Osram and Philips metal-vapor arc discharge lamps, electrodeless discharge tubes, 150-W xenon arc lamps, and hollow cathode discharge tubes (high intensity). More recently, Ellis and Demers (3), using a 450-W xenon arc continuum, demonstrated the inlprovenlents in sensitivity achieved by using a more intense source and using a hydrogen-entrained air flame. A demountable hot hollow cathode lamp appears to rival in intensity the most intense light sources thus far used in atomic fluorescence studies. The demountable hollow cathode lamp described here was originally designed for use as a spectrographic source ( 4 ) . It is similar in some respects to hollow cathodes used in high resolution spectrographic investigations as a source of very narrow spectral lines. The design and use of hot hollow cathodes as excitation sources in emission spectrography has recently been reviewed ( 5 ) . (1) J. D. Winefordner, M. L. Parsons, J. M. Mansfield, and W. J. McCarthy, Spectrochim. Acta, 23B, 37 (1967) (2) J. D. Winefordner, Symposium on Trace CharacterizationChemical and Physical, National Bureau of Standards, Gaithersburg, Md., October 1966. (3) D. W. Ellis and D. R. Demers, ANAL.CHEM., 38, 1943 (1966). (4) A. W. Helz, U. S. Geological Survey, Washington, D.C., unpublished studies, 1958, 1966. (5) N. P. Ivanov and V. Z. Krasilshchik, “Basic Properties and Analytical Use of a Hollow Cathode,” Melody Analiza Kliim. Reaktoc i Preparatoc, Gos. Kom. Soc. Min. S S R po Kliim, 1963, (7) 1-68.
The operation of the hollow cathode was tested on five elements (Ag, Au, Bi, Ni, and Pb) whose vapors were known ( 2 ) to give a fluorescence response. The elements were arbitrarily selected because of analytical interest and because they cover the lower temperature range a t which operation of the cathode was thought to be most questionable. EXPERIMENTAL
Apparatus. A simplified exploded view of the hollow cathode is shown in Figure 1. The cathode is a graphite rod having a cavity at one end t o hold a small amount of the element. It is contained in a water-cooled brass housing and is operated under reduced pressures in the presence of an inert gas at operating currents of 300 to 600 mA and at potentials of as much as 1100 V. The replaceable cathode is made of a 2-inch section of 0.375-inch graphite rod with a cavity, 0.25 X 0.75 inch, machined in one end to contain the element. The graphite cathode, mounted on an insulated holder, Kovar seal No. 95. 2115, is inserted into the brass housing. A quartz window and cathode holder are joined to the housing by atmospheric pressure. Relieving the vacuum permits the housing to be disassembled in a few minutes so that the cathode material can be changed. Pressure regulation in the hollow cathode is accomplished by means of the valve and piping system shown in Figure 2, which incorporates a GHD-100 diaphragm pressure gauge (Consolidated Vacuum Corp.) in the system. DC power is supplied to the cathode holder by a high voltage Model F-6002, Industrial Service Power Supply (Beckman Instruments, Inc., Cedar Grove Operations) capable of providing as much as 1.2 A at voltages as high as 2 kV. A ballast resistance of 1250 0 is used to control the current. High intensity gold and silver hollow cathode lamps (Atomic Spectral Lamps Pty., Ltd.), also used in this work, are powered by a MLS-1A multiple hollow cathode lamp power supply (Techtron Pty., Ltd.) with auxiliary current supplied by a high brightness lamp source power supply 303-0169 (Perkin-Elmer Corp.). The atomic fluorescence flame spectrometer is similar to that used by Veillon et al. (6) and is described more fully in the following paper (7). VOL. 39, NO. 12, OCTOBER 1967
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4 Figure 1. Diagram of hot hollow cathode
From Hollow Colhode
1
li
To Hollow Colhode
t
I
Procedure. Approximately 50 mg of metal cuttings o r powder is inserted into the graphite cavity, the graphite is mounted on the cathode holder and inserted into the brass housing. Water is circulated through the outer envelope of the housing. All valves are closed and a vacuum pump is used to evacuate the system to a pressure of 10-1 torr or less. Helium or argon is bled into the system to a designated pressure. Direct current to the cathode is gradually increased in increments of 50 mA until a designated operating current is reached.
Needle VOlV*
t Helium TO V O C Y Y m PumQ
RESULTS AND DISCUSSION
The operating currents and limits of detection obtained for gold, silver, nickel, bismuth, and lead are summarized elsewhere (7) with data for other elements. The hot hollow cathode lamp was a satisfactory excitation source for gold, silver, and nickel, making possible sensitivities 10 or more times lower than those attainable with a 150-W xenon lamp and approximately the sensitivity attained for silver with a 450-W xenon lamp (3) and for nickel using a high intensity hollow cathode lamp (8). The intensity of the hot cathode lamp was compared to the intensity of two commercially available high intensity hollow cathodes, gold and silver, at the same instrumental settings, burner height, and flame conditions. Auxiliary current was set a t the maximum. The fluorescence response excited by the commercial lamps was approximately the same as that excited by the hot cathode. Although the fluorescence of lead and bismuth could be satisfactorily excited, operation of the cathode with these elements was not as convenient as its use with the more refractory metals. The relatively low boiling points of lead and bismuth limit the current at which their cathodes can be operated. Prolonged operation a t elevated current levels resulted in deposition of an opaque metallic deposit o n the cathode window. Onset of deposition could be readily noted by a relatively rapid decrease in the fluorescence quantum efficiency of a solution of the element. In such instances, operation of the cathode was stopped, the window removed, and the deposit dissolved with dilute acid. The use of a spare quartz window reduced the down time for the cathode to the
(6) Claude Veillon, J. M. Mansfield, M. L. Parsons, and J. D. Winefordner, ANAL.CHEM. 38, 204 (1966) (7) J. I. Dinnin, Zbid.,39, 1491 (1967). (8) D. N. Arrnentrout, ANAL.CHEM. 38, 1235 (1966).
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Figure 2. Schematic diagram of pressure regulation system.
i i80--
E
$ E W
5
60-
-
I
IO0
I
I
200 300 SOURCE CURRENT, MILLIAMPERES
400
Figure 3. Fluorecence intensity as a function of source current Gold hollow cathode operated at 4 torr He; wavelength
3428 A; solution, 10 ppm Au
10-1 5 minutes required for reassembly, evacuation, and gradual increase of the operating current. The hot cathodes were also operated with argon atmospheres and required different gas pressures and current levels for optimum performance. Although some minor improvement in intensity or performance could be accomplished by change in gaseous atmosphere o r pressure, no detailed investigation of these operational variables was made. The applied current was the most significant variable affecting the cathode source intensity as measured on a microphotometer, As expected, the fluorescence intensity varied
directly with the intensity of the excitation source, expressed in milliamperes of applied current, as shown for a gold cathode in Figure 3. Cathodes of gold and silver have been operated for more than 100 hours in increments of 6-8 hours of continuous use. Although some thinning of the wall of the cathode, due to volatilization of the graphite, was apparent after prolonged operation, no diminution of the light intensity was noticed. The demountable hot hollow cathode is a simple, practical, and intense excitation source for atomic fluorescence flame spectrometry and may prove useful in other flame spectrometric work.
ACKNOWLEDGMENT
We are indebted to Camillo Massoni, US.Geological Survey, for construction of the hot hollow cathode.
RECEIVED for review May 25, 1967. Accepted August 8, 1967. Presented a t the Pittsburgh Conference o n Analytical Chemistry and Applied Spectroscopy, Analytical Chemistry Group, Pittsburgh Section, ACS, Pittsburgh, Pa., March 1967.
Atomic Fluorescence Flame Spectrometric Detection of Palladium, Titanium, Zirconium, Chromium, and Aluminum Using a Hot Hollow Cathode Lamp Joseph I. Dinnin U. S. Geological Suruey, Washington, D.C. 20242
A DEMOUNTABLE HOT HOLLOW CATHODE LAMP ( I ) proposed earlier as a n excitation source in atomic fluorescence flame spectrometry has proved useful for exciting the fluorescence of a t least 14 elements. The fluorescence of palladium, titanium, zirconium, chromium, and aluminum has been observed for what is believed to be the first time. I n addition, the fluorescence response of manganese, cobalt, and iron ( 2 ) has been verified. The hot hollow cathode possesses numerous advantages as a light source and supplements other sources of excitation in atomic fluorescence studies. EXPERIMENTAL
The flame spectrometer was assembled from components similar to those described by Veillon et al. (3) except that a Model BZ-1 mechanical light chopper (Princeton Applied Research Corp.) was used t o modulate the light beam and provide a reference signal for operating the Model JB-5 lock-in amplifier (Princeton Applied Research Corp.). The hot hollow cathode lamp, chopper, condensing lens, and burner were mounted on an optical rail to facilitate optical alignment. The burner was attached to a standard pin accessory which can be inserted into an optical rail carrier for rotation or vertical adjustment of the burner height. A photomultiplier tube housing C 319-61041 (American Instrument Co.) was mounted directly to the exit slit of the monochromator. A hinged black-coated aluminum box, open at the bottom, enclosed the burner and entrance slit; a chimney cut into the roof of the box was attached with flexible ducting to an air-blower which vents into a n exhaust hood. A Model 10-213 microphotometer (American Instrument Co.) was used to monitor the emission spectrum of the hollow cathode. For this purpose the burner is replaced by a mirror mounted on a standard pin which can be inserted into the optical rail carrier. Monochromator slits were 0.05 mm or less during spectrum scanning operations. During ( l j J. I. Dinnin and A. W. Helz, ANAL. CHEM.,39, 1489 (1967). (2) R . M. Dagnall and T. S. West. Anal. Chim. A m , 36, 269
(1966).
( 3 ) Claude Veillon, J. M. Mansfield, M. L. Parsons, and J. D. Winefordner, ANAL. CHEM., 38, 204 (1966).
atomic fluorescence studies an entrance and a n exit slit of 2.0 mm were used unless otherwise specified. Helium at a pressure of 4 torr was used for all elements except as otherwise specified. A Model 4020 medium bore total consumption burner (Beckman Instruments, Inc.) was used. A thin polyethylene tube attached to the aspirator tube gave convenient access to a sample solution positioned outside of the housing and in front of the optical rail. An air-hydrogen flame was used for most of the studies. Gas pressures were controlled with a Model 9220 regulator unit (Beckman Instruments, Inc.). Unless otherwise specified, air flow rate was approximately 3.5 liters per minute; hydrogen, 9.5 liters per minute. The metals excited in the hollow cathode were Johnson Matthey spectrographically certified materials (Jarrell Ash Co.) or ACS reagent grade. Solutions of the elements were diluted from aqueous acidified stock solutions containing 1000 ppm of the elements prepared from the metals or soluble salts. Procedure. The hollow cathode tube containing the element of interest was excited to give maximum intensity and, using a mirror and microphotometer, the emitted light was examined for identifying lines of the element. The burner was then inserted in the optical path to replace the mirror and a 1000-ppm solution of the element was aspirated into the flame. The fluorescence response of the element a t various heights in the flame was then scanned over the full available wavelength range and measured o n the microphotometer. Where significant fluorescence responses were obtained, they were then measured with the chopper-lockin amplifier assembly and the detection limits were determined on diluted solutions of the element. Fluorescence responses were continually checked against water blanks to eliminate spurious scattering signals. RESULTS AND DISCUSSION
The detection limits obtained using the hot hollow cathode as excitation source are compared in Table I with the detection limits obtained from other excitation sources as sumVOL. 39, NO. 12, OCTOBER 1967
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