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
o
1491
Table I. Wavelengths of Fluorescence Response, Excitation Conditions, and Limits of of Various Sources of Excitation Excitation Limit of detection wavelengths, Excitation Limit of demountable hot Element A source= detection, ppmb hollow cathode, ppm 3870 ... ... 25. Aluminum 3968 Xe Bismuth 2. 0.1 2228 Ph 0,0002 ... Cadmium 4227 450-W Xe (5) 0.1 ... Calcium 3579 ... Chromium 100. 4254 0.5 2407 450-W Xe (5) Cobalt 0.5 3248 EDT 0.04 0,001 Copper 10. 5. 4172 EDT Gallium t
Gold Indium Iron Lead Magnesium
2676 4105 2483 4058 2833 2852
Manganese Mercury Nickel Palladium
2794 2537 2320 2500
Xe EDT Xe (2) Xe
.
.
4. 10.
5. 8.
0.05 50. 5.
Detection of Elements Obtained by Use Limiting current, mA >550 200 ... , . .
Burner heightc B A ...
8 torr He
, . .
>500
C
>500
C A
400 500
Remarks
C
500
A
500
>500
B C
1.
250
A
450-W Xe (5)
0.01
(4
300
Xe (2) EDT HIHC
0.15 0.1 0.1
5.
>500
C
0.5 0.5
>500 >650
A
1
2 torr He Flame height critical d
8 torr He
Unstable (2 torr He) d
7 torr He
B
3450 -
3281 450-W Xe (5) 0.001 0.001 >500 B-C 3383 Sodium 5892 os 100. ... ... ... Thallium 3776 Ph 0.04 0.1 250 A Titanium 3895 ... ... 5. >600 A 4470 5020 3640 Zinc 2139 Ph o.ooo1 . . ... ... Zirconium 3890 ... ... 10. >600 A a EDT, electrodeless discharge lamp; Xe, 150-W xenon arc continuum; Os, Osram arc discharge lamp; Ph, Philips arc discharge lamp; HIHC, high intensity hollow cathode lamp. 6 Unless otherwise noted, limits of detection are those summarized by Winefordner ( 4 ) . c A, tip of flame 1 cm or more below bottom of monochromator slit. B, tip of flame in line with bottom of slit. C , tip of flame in line with top of slit. Extremely sensitive to burner position. Silver
marized by Winefordner (4) and subsequently supplemented by Dagnall and West (2) and Ellis and Demers (5). Also noted are the wavelengths at which the fluorescence response was determined, the limiting currents, and burner heights. The limit of detection is defined as the solution concentration giving a signal more than twice the rms noise for sample and blank. In scanning the spectra of the elements at higher amplification levels, a persistent apparent fluorescence response was obtained in the 3900-A region when water was sprayed into the flame. This effect was noted when many of the elements were excited in the hollow cathode a n d n o assignments of the specific exciting wavelengths could be made. The fluorescence response increased as the body of the flame became visible to the slit; the response was significantly lower in regions above the tip of the flame. The effect is believed to be due to scattering by water droplets which may reflect the relatively intense emission line of helium at 3888.6 A. The apparent fluorescence is not noted when argon is used in the hollow cathode. This spurious fluorescence was taken into account when the (4) J. D. Winefordner, Symposium on Trace Characterization-Chemical and Physical, National Bureau of Standards, Gaithersburg, Md., October 1966. ( 5 ) D. W. Ellis and D. R. Demers, ANAL.CHEM., 38, 1943 (1966). 1492
ANALYTICAL CHEMISTRY
fluorescence responses of titanium and zirconium were found in the 3900-A region. Unfortunately, the fluorescence quantum efficiency of these elements is too low for the monochromator slits to be narrowed sufficiently for the more exact definition of the wavelength. The fluorescence of both titanium and zirconium was measured with the tip of'the flame 2 to 3 cm below the bottom of the entrance slit and it is, therefore, believed to be valid. The limits of detection for titanium and zirconium, 5 and 10 ppm, respectively, are sufficiently low to indicate that with optimal adjustment of flame and other parameters their determination by atomic fluorescence may be analytically useful Hafnium gave no measurable fluorescence response when the vapor of a 1000-ppm solution of the element was excited by a hafnium hollow cathode operated a t 600 mA. The absence of a response for hafnium may prove useful for the determination of zirconium in mixtures of the two elements. Flame height adjustment for manganese, iron, and cobalt is unusually critical. All three elements give a fluorescence response only when the area immediately above the inner cone of the flame is visible to the slit. The fluorescence activity of these elements was missed in the preliminary phase of this study and was determined only upon more careful examination after they were reported by Dagnall and West (2). Other elements reported in this study as lacking in fluorescence response may also warrant more intensive study.
Although intense fluorescence has been reported for calcium when excited by a 450-W xenon arc (3,no response was found in this study when 1000-ppm solutions were excited by a cathode containing calcium metal. High thermal emission of the flame in the region 4220 and 5540 A precluded intensive study of these regions. The use of a hydrogen-entrained air flame ( 5 ) may provide a preferable flame environment. The hot hollow cathode does not appear t o be the most satisfactory source of excitation for magnesium. The cathode source, containing magnesium metal, is operable only a t a helium pressure of 2 torr and is too unstable for routine use. Barium, with barium oxide in the cathode source, gave n o measurable fluorescence. Strontium was not investigated. The demountable cathode excited satisfactory fluorescence responses for copper, silver, gold, lead, bismuth, thallium, nickel, and gallium and yielded limits of detection equal t o those that have thus far been reported for these elements. Elements that were tested and yielded no fluorescence response were barium, beryllium, cerium, hafnium, lanthanum, molybdenum, niobium, platinum, rhenium, tantalum, and tin in addition to those noted earlier. However, hollow cathodes prepared from metals of these elements appear to be intense excitation sources and should be investigated more fully in various flames and under varied flame conditions. The sensitive fluorescence response of palladium (limit of
detection 0.5 ppm) was surprising inasmuch as it had been sought and not found in an earlier study using a 150-W xenon arc (2). The increased intensity offered by the hot hollow cathode may have been a factor in eliciting a satisfactory response. The fluorescence intensity of the vapors of the transition elements warrants much more intensive study than was possible in this preliminary investigation. It seems likely that the limit has not as yet been reached in the range of application of this new method of analysis and that more intense sources of excitation and different flame conditions may reveal additional elements that yield a significant fluorescence response and may provide still lower detection limits for the elements known to be amenable to the technique. ACKNOWLEDGMENT
The aid of several colleagues at the U S . Geological Survey is gratefully acknowledged: Armin W. Helz gave advice and assistance in the optical and electronic assembly of the spectrometer; Fred Rosenbaum constructed components for the monochromator; and Ralston V. Fones constructed many of the electronic components and gave valuable electronic assistance. Karel Stefan, University of Leyden, Netherlands, gave valuable assistance in the study of palladium. RECEIVED for review May 25,1967.
Accepted August 8,1967.
Determination of Trace Quantities of 2,6-Di-tert-butyl-4-methylPhenol in Polyethylene Using Electron Capture Gas Chromatography R. E. Long and G . Christian Guvernator I11 Gulf Research & Development Co., Kansas C i t y Laboratory, 9009 W . 67th St., Merriam, Kan. 66202
BHT (2,6-di-tert-butyl-4-methyl phenol) is an antioxidant which inhibits the thermal oxidation of polyethylene. Small amounts are usually added t o polyethylene t o protect the polymer against deterioration during processing and t o improve its aging characteristics. The determination of low concentrations of BHT in polyethylene is usually accomplished by colorimetric or spectrophotometric methods (1-4). G a s chromatography has been used by several workers with satisfactory results (5-8). Present column technology (8)t o perfect separation of the major antioxidants is now satisfactory so that elaborate separation schemes are no longer necessary. (1) C. Stafford, ANAL.CHEW,34, 794 (1962). (2) D. F. Slonaker and D. C. Sievers, Ibid.,36, 1130 (1964).
(3) R. H. Campbell and K.W. Wise, J . Chromatog., 12, 178 (1963). (4) M. R. Sahasrabudhe, J . Assoc. Offic. Agr. Chemists, 40, 880
(1964). ( 5 ) Y . Norikov (1 965).
and V. N. Vetchinkina, Neftekhimiya, 5 (2), 284
(6) C. B. Roberts and J. D. Swank, ANAL.CHEW,36, 271 (1964). (7) W. G. Schwien, B. J. Miller, and W. H. Conroy, J . Assoc. Ofic. Agr. Chemists, 40, 888 (1964). (8) T. K. Choy, J. J. Quattrone, Jr., and N. J. Alicino, J . Chromalog., 12, 171 (1963).
EXPERIMENTAL
Apparatus. A Varian Aerograph Model 204-1B chromatograph with a single column, a 1 :1 splitter, and dual detectors was used. One of the detectors was a hydrogen flame ionization detector. Hydrogen and oxygen were supplied to this detector by a n Aerograph Model 650 hydrogen generator. The other detector was electron capture of concentric tube design. The cell voltage of the detector was fixed a t 90 V by a resistor built into the connecting cable. The concentric tube detector was selected because of its simplicity of design which enables easy cleaning and replacement of the foil. Also, the concentric tube establishes equilibrium rapidly after cleaning and when septums or columns are changed. The column used (a 2-meter, 0.25-inch copper tube) was packed by vibrating with 2 0 x SE-30 on 60- t o 80-mesh Chromosorb W-HMDS and was preconditioned with nitrogen flow a t 225" C overnight. A glass column containing the same phase and support was used with an all-glass system and offered no noticeable advantages. The injector and detectors temperatures were maintained a t 225" C and the column was operated isothermally a t 200" C throughout the entire study. The nitrogen carrier gas flow was set a t 70 cc/min for the study. A Wiley Mill (intermediate model) equipped with a No. 10 mesh screen (Arthur H. Thomas Co.) was used t o grind the VOL. 39, NO. 12, OCTOBER 1967
1493
