Improvements in Preparation and Operation of Electrodeless Discharge Lamps as High Intensity Sources in Atomic Fluorescence Flame Spectrometry K. E. Zacha, M. P. Bratzel, Jr., J. D. Winefordner, and J. M. Mansfield, Jr.’ Department of Chemistry, University of Florida, Gainesville, Fla. 32601 MANSFIELDet al. ( I ) have recently reviewed the historical development of electrodeless discharge lamps and have described the preparation and powering of these sources for atomic fluorescence flame spectrometry. By means of a statistical evaluation of data, they showed that source intensities were related directly t o the lamp diameter, inert gas fill pressure, and form of element (metal or metal iodide), but were independent of the weight of the metal or metal iodide introduced into the lamp. They also found that the “A” type antenna with an open quartz jacket surrounding each lamp produced the largest atomic fluorescence signals for most elements as compared t o the “C” type antenna, the 1/4-wave (Evenson) cavity, and the 3/4-wave(Broida) cavity. In this study, increases of intensity and stability of lamps of some elements result if the lamps containing metal iodides also contain excess metal. In addition, many lamps produce more intense atomic fluorescence when operated in a n evacuated jacket on the “A” antenna or when operated in a rectangular cavity. Dagnall, Thompson, and West (2-4) as well as Rains, Snelleman, and Menis ( 5 ) have also obtained more intense electrodeless discharge lamps for atomic absorption and fluorescence flame spectrometry, if excess metal is present when using metal iodide lamps. However, both groups have used the ‘/4-wave (Evenson) type cavity for all of their measurements. EXPERIMENTAL
Preparation of Electrodeless Discharge Lamps. All electrodeless discharge lamps were prepared by using the vacuum system described by Mansfield et al. ( I , 6). The important characteristics of lamps for each element are given in Table I. The amount of material in the lamps was unimportant as found by Mansfield et al. ( I ) as long as the amount was small-Le., too much material results in reduced transmittancy of the lamp walls and in diminished intensities. Only electrodeless discharge lamps which have been evaluated for atomic fluorescence flame spectrometry are listed in Table I. The characteristics of lamps of other elements will be published in future articles concerning the atomic fluorescence of selected groups of elements. 1 Present address, Columbian Carbon Co., Princeton, N. J. 08540
(1) J. M. Mansfield, M. P. Bratzel, H. 0.Norgordon, D. N. Knapp, K. E. Zacha, and J. D. Winefordner, Spectrochim. Acta, 23B, 389 (1968). (2) R. M. Dagnall, K. C. Thompson, and T. S. West, Tuluntu, 14, 551 (1967). (3) Ibid, p 557. (4)Zbid., p 1151. (5) T. C. Rains, W. Snelleman, and 0. Menis, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland Ohio, March 4, 1968, Abstract No. 77. (6) J. M. Mansfield, Ph.D. Thesis, University of Florida, Gainesville, Fla., August 1967.
Powering of Electrodeless Discharge Lamps. A 120-watt2450-MHz medical diathermy unit (PGM-10 X 1, The Raytheon Co., Waltham, Mass.) with a reflected power meter (725-3, Microwave Devices, Farmington, Conn.) was used for powering all electrodeless discharge lamps in this study. An “A“ antenna (2254-500201, The Raytheon Co.) and a tapered rectangular cavity (7097-1001G1-5002G1, The Raytheon Co.) were used to couple microwave energy t o the electrodeless discharge lamps. To obtain stable, high intensity radiation from the lamps when using the “A” antenna, several devices t o thermally insulate the lamps were employed in several instances: an open quartz jacket with or without ~
~~
Table I. Characteristics of Electrodeless Discharge Lamps Producing Atomic Fluorescence Argon Lamp pressure length: Preparation (Torr) cm methodc Element Forma 1 4 . 3 In situ Ag 2 Au 3.5 Sublimed AuIa/Au 5 Be 4.0 BeI,/Be In situ Bi BiI,/Bi 2 3.6 Sublimed 2 Ca Ca12/Ca 3.9 Dump in 2 Cd Cd 3.5 Sublimed 2 co CoI*/Co 3.3 Dump in 1 Cr In situ Cr I,/Cr 4.1 2 2.8d In situ cu CunIz/Cu Fe 2 FeI?/Fe 3.7 In situ Ga 2 GaI,/Ga 3.9 Sublimed 5 Ge GeI,/Ge In situ 4.0 Hf HfId/Hf 2 3.8 In situ 2 3.8 Sublimed Hg Hg 2 In 3.4 Sublimed InIJIn 1 In situ 3.9 Mg MgIn/Mg 1 Mn MnIsMn 3.7 Sublimed 2 Mo MoBr3/Mo 4.2 In situ Ni 5 NiIz/Ni 3.3 In situ 2 Pb Pb 2.w Sublimed Sb SbI$/Sb 0.5 4.1 Sublimed 2 sc scc1,/sc 4.1 Dump in 2 Se Se 4.0 Sublimed 5 Sr SrIa/Sr 3.8 Dump in Te 1 Te 3.6 Sublimed 5 Ti Ti14/Ti 4.0 In situ 1 TI TI 3.3d Sublimed 1 U 3.6 In situ UIa/U 2 Zn Zn 3.6 Sublimed Zr Zr14/Zr 2 4.3 In situ a MX/M represents metal halide with excess metal (the excess metal in most cases results during the preparation procedure as a result of thermal decomposition; however, in some cases excess metal is added). * All lamps are 9 mm i.d. unless otherwise specified. In situ-metal halide prepared within the vacuum system as described by Mansfield et al. (I). Sublimed-metal species is sublimed into the lamp blank as described by Mansfield et at. (I). Dump in-metal species is placed in the lamp blank prior to connection to the vacuum system. Lamp diameter, 5 mm.
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Table 11. Experimental Operating Conditions and Limits of Detection for Several Elements in Atomic Fluorescence Flame Spectrometry” Flame conditionsc MonoPowering device Aspirant chromator Limit of H2flow Aspirant flow rate, slit Flamee detection, Power Tubeb (watts) rate, l/min type l/min width, mm height, cm Element Line (A) /rg/ml Ag 3281 A-VO 19.0 110 Air 7.0 0.5 10.5 o.Ooo1 Au 2676 14.6 R 95 0 2 6.5 2.0 0.2 7.5 Be 2349 12.2 A-0 69 Air 7.2 2.0 9.5 2. Bi 3068 11.4 70 A 5.7 0.5 A 9.0 0.7 Ca 4227 15.9 Air 7.2 110 1.o 0.02 A-V 5.0 Cd 2288 14.2 100 A-0 0 2 6.4 0.5 10.0 0.000001 15.1 6.0 co 2407 100 5.5 0.5 0.1 A A 12.2 Cr 3593 120 Air 7.1 A-VO 3.0 10. 11.0 14.6 60 A 5.6 A cu 3248 0.3 10.0 0.005 15.2 Air 6.0 A-VO 0.25 120 Fe 2483 4.0 1 .o 14.6 Air 6.6 40 R Ga 4172 7.5 1.o 1.o 12.2 10.0 70 Air 7.1 R Ge 2652 1.0 10. 11.3 Air 7.2 R 40 Hf 2866 2.5 3. 11.o 15.0 1.o R or A Hg 2537 10 A or Air 6.6 8.0 0.1 90 18.8 In 4105 A-0 A 6.1 0.1 0.5 7.5 19.2 Mg 2852 A 6.0 70 0.3 0.008 A 9.5 19.0 A 6.0 R 100 Mn 2794 0.5 7.5 0.006 80 A-VO 0 2 6.45 10.2 2.0 11.0 Mo 3798 2. 80 A-V 12.2 Ni 2320 0.04 Air 6.9 1.o 4.0 13.2 Pb 4057 2.0 90 A A 5.8 0.5 10.0 2.0 40 R Air 7.1 15.9 Sb 2311 0.4 5.5 A-VO 11.5 120 A 5.4 sc 3907 2.5 9.0 10. 10.8 2.0 100 A A 4.6 Se 1960 0.4 6.5 110 A-V A 6.15 16.5 1.o Sr 4607 0.03 4.5 8.0 100 A-0 A or Aird 3.1 Te 2143 1.0 7.0 0.5 12.2 60 A 6.15 R Ti 3949 2.0 10.0 6. 15.4 110 A A-VO 5.5 T1 3776 0.5 0.008 9.0 13.1 90 A-VO U 3812 Air 6.9 2.0 5. 9.5 14.8 Air 6.2 95 A-0 8.0 1.0 0.OOO04 Zn 2139 11.5 69 2.0 R Zr 3520 Air 7.2 11.0 4 Some of the results are taken from ref. 10. The limit of detection was defined as that concentration resulting in a signal-to-noise ratio of 2 for 3 combined measurements of blank and sample using a I-second time constant and 20 seconds for each measurement (7, 8). b A: “A” antenna with no jacket. A-0: “A” antenna with open quartz jacket. A-V: “A” antenna with quartz vacuum jacket with no glow in vacuum jacket surrounding lamp. A-VO: “A” antenna with quartz vacuum jacket with glow in jacket surrounding lamp. R : Rectangular cavity. c The Zeiss total-consumption nebulizer burner was used for all studies except where indicated. d The Beckman total-consumption nebulizer burner was used. 8 The flame height is the height of the fluorescence measurement above the burner tip. 0
quartz wool surrounding the lamp; and a quartz vacuum jacket surrounding the lamp and attached to any mechanical vacuum pump capable of producing a pressure of 1 Torr or less. The open quartz jacket device which merely rests on the “A” antenna (about 2.5 cm 0.d. and 10 cm long) is illustrated in Figure 1. The quartz vacuum jacket device (about 2.5 cm 0.d. and 10 cm long with a side arm for connecting to the vacuum pump and with a 24/40 standard taper female joint) is illustrated in Figure 2. In the latter device, the electrodeless discharge lamp is held in place in the vacuum jacket by means of a Teflon gland (ASCO Teflon thermometer seal for 6-mm shaft, Arthur F. Smith Co., Pompano Beach, Fla.). The Teflon gland consists of a center hole with an O-ring seal to hold the lamp in place and with an outer O-ring to hold the gland in place in the vacuum jacket. The use of the tapered rectangular cavity has been described by Mansfield et al. (I). All electrodeless discharge lamps were “broken-in”-Le., operated until a constant, high intensity resulted-by overdriving the lamps by operating them at high powers, e.g., 100% of the power, for a time period of approximately 15 minutes. After the breaking-in period, most lamps came to a constant, reproducible (from one operating time to the next), stable intensity within a time period of 5 minutes or less when operated at the powers specified in Table 11. Experimental Conditions. The instrumental system, except for the sources of excitation and the powering devices, used 1734
ANALYTICAL CHEMISTRY
for measuring the atomic fluorescence was identical to the one used by Mansfield et al. (I). The experimental conditions for the atomic fluorescence flame spectrometric measurements are listed in Table 11. RESULTS AND DISCUSSION
The limits of detection for elements using atomic fluorescence flame spectrometry, obtained using electrodeless discharge lamps prepared and powered as described in the previous section, are given in Table 11. For completeness, some of the results obtained by Bratzel and Winefordner (9) are also listed. Except for Cd and Zn, no exhaustive effort was made t o obtain optimum experimental conditions. Even so, the limits of detection obtained for many elements using atomic fluorescence flame spectrometry compare favorably with the limits of detection for the same elements measured by atomic absorption flame spectrometry using the Perkin-Elmer Model 303 atomic absorption flame spectrom(7) P. A. St. John, W. J. McCarthy, and J. D. Winefordner, ANAL. CHEM.,39, 1495 (1967). (8) J. D. Winefordner, W. J. McCarthy, and P. A. St. John, J. Chem. Educ., 44,80 (1967). (9) M. P. Bratzel, and J. D. Winefordner, Anal. Leffers, 1, 43 (1967).
.QUARTZ J A C K E T
, T O VACUUM PUMP
REFLECTOR
COAXIAL CONNECTOR
>
Figure 1. Open quartz device for operating electrodeless discharge tubes
Table 111. Comparison of Limits of Detection for Several Elements Measured by Atomic Fluorescence, Atomic Absorption, and Atomic Emission Flame Spectrometry Limit of detection (nom) Flame Atomic Atomic emissionc absorptionb fluorescencea Element Ag
Au Be Bi Ca Cd co Cr
cu Fe Ga Ge Hf Hg In Mg
Mn Mo Ni Pb Sb sc Se Sr Te Ti TI
0.0001 0.2
2. 0.7 0.02 0. 000001 0.1 10.
0.005 0.25 1.0 10. 3. 0.1 0.1 0.008
0.006 2.
0.04 0.5 0.4 10. 0.4 0.03 0.5
6.
0.005 0.2 0.003 0.05 0.002 0.005 0.005 0.005 0.005 0.005
0.07 1.o
15. 0.5 0.05 0.0003 0.002 0.03
0.005 0.03 0.1 0.1 0.5 0.01 0.3 0.1 0.025 30.
0.02 0.5 0.2
9. o.ooo1
0.9 0.03
0.005 0.01 0.03 0.01 0.5
...
20. 0.005
0.004 0.005 0.09 0.03 0.3
1.5 0.03
... 0.0002 30. 0.2
0.002 30. 5. 0.002 100. Zn O.ooOo4 5. 3. Zr 4. a Atomic fluorescence values were taken from Table 11. b Atomic absorption values were taken from commercial literature on Perkin-Elmer Model 303 atomic absorption flame spectrometer. c Flame emission values were taken from S. R. Koirtyohann U
(10).
0.008
Figure 2. Quartz vacuum jacket device for operating electrodeless discharge tubes
eter (see Perkin-Elmer Brochure on Model 303 atomic absorption spectrophotometer) and by flame emission spectrometry reported by Koirtyohann (IO). For a comparison of limits of detection for several elements obtained using the three flame methods, see Table 111. The limits of detection reported for the atomic fluorescence flame spectrometric measurement of Ag and T1 are considerably lower than limits of detection previously reported by Bratzel and Winefordner (9) using electrodeless discharge lamps. This is a result of the use of the vacuum jacket in the present study, whereas Bratzel and Winefordner (9) used only the open quartz jacket to thermally insulate the lamps for Ag and T1. Also, this is the first time that atomic fluorescence in flames for Be, Ca, Cr, Fe, Ge, Hf, Mo, Ni, Sb, Sc, Sr, Ti, U, and Zr excited by electrodeless discharge lamps has been reported. Some other interesting observations include : a variation of +0.5 l/min in the fuel flow rate or 50.1 l/min in the aspirant flow rate usually results in little change in the signal-to-noise ratio; a variation of =tl cm in the height of measurement generally results in little change in the signalto-noise ratio ; the principal source of systematic error in most cases was source intensity drift, whereas flame flicker noise was negligible in all cases; and the discharges in all lamps were one of four types-namely, a diffuse arc which illuminates the entire lamp, a uniform glow which illuminates
(10) S. R. Koirtyohann, Atomic Absorption Newsletter, 6,77 (1967). VOL 40, NO. 1 1 , SEPTEMBER 1968
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the entire lamp, an intense glow along the lamp inner surface only, or a combination of the above three. By proper choice of flame to achieve minimal quenching and maximal atomization, of source conditions t o achieve maximal stability and intensity over the absorption line(s) producing the atomic fluorescence, of height of measurement in the flame, of slit width, and of measurement electronics to achieve maximal signal-to-noise ratio, it is not unreasonable to expect the limits of detection for most elements with simple spectra, such as Cu, Ag, Au, etc., to approach the value for Cd, and to expect the limits of detection for elements with
complex spectra to approach a value of 0.001 ppm. Because electrodeless discharge lamps generally produce much more intense resonance radiation than hollow cathode discharge lamps (8) and also emit narrow resonance lines, such sources should also find considerable use in atomic absorption flame spectrometry as has already been demonstrated by Dagnall, Thompson, and West (2-4). RECEIVED for review April 4,1968. Accepted June 18, 1968. Work supported in part by AFOSR(SRC)-OAR, U.S.A.F. Grant No. AF-AFOSR-1033-68.
Apparatus and Materials for Hyperpressure Gas Chromatography of Nonvolatile Compounds Nicholas M. Karayannis,’ Alsoph H. Corwin, Earl W. Baker,2 Ernst Klesper,* and Joseph A. Walter Department of Chemistry, The Johns Hopkins University, Baltimore, Md. INTRODUCTION of the method of gas-liquid chromatography (1) literally revolutionized the art of separation of materials possessing sufficient vapor pressure to permit its use. In practice, however, a wide variety of materials of limited volatility have resisted efforts to purify them by this means. In particular, organic compounds of moderate molecular weights with polar groups attached, most inorganic compounds (2), and ionic organic compounds, such as amino acids (3),etc., have resisted efforts at separation without functional group modification. In these laboratories, especial interest has attached to the purification of porphyrins. The potential advantages of the use of gas chromatography in this application are obvious but the limited volatility of the materials made the method impractical. Efforts were made to bring about the volatilization of porphyrins by raising the column temperature and by operating under vacuum. Partial success was achieved with etioporphyrin I1 at temperatures in excess of 250 “C, using the lowest pressure available with an oil pump. Under these conditions, however, more porphyrin was decomposed than was volatilized. After numerous unsuccessful efforts to volatilize porphyrins for gas chromatographic purification, our search for a new approach to the problem of gas chromatography of materials of low volatility was successful. A preliminary publication by Klesper, Corwin, and Turner (4) reported the separation of a metalloporphyrin mixture by high pressure gas chromatography above the critical temperature of the carrier gas, which was dichlorodifluoromethane. The method was suggested by the fact that liquids, for thermodynamic reasons, show higher vapor tension when under pressure from insoluble gases (5). It had been observed further that inorganic and organic solids (6, 7), including a derivative of
Drexel Institute of Technology, Philadelphia, Pa. Mellon Institute, Pittsburgh, Pa. 3 Institut fur Makromolekulare Chemie, Freiburg, W. Germany 1
(1) A. T. James and A. J. P. Martin, Biochem. J., 50, 679 (1952). (2) R. S. Juvet, Jr., and F. Zado, in “Advances in Chromatography,” J. C. Giddings and R. A. Keller, Eds., Vol. 1, Marcel Dekker, New York, 1965, pp 249-307. (3) C. W. Kehrke and F. Shahrokhi, Anal. Biochern., 15,97 (1966). (4) E. Klesper, A. H. Corwin, and D. A. Turner, J . Org. Chem.,27, 700 (1962). (5) F. Pollitzer and E. Strebel, 2. Physik. Chem., 110, 768 (1924). (6) H. S. Booth and R. M. Bidwell, Chem. Reus., 44, 477 (1949). (7) M. Centnerszwer, 2.Physik. Chem., 46, 427 (1903). 1736
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chlorophyll (8), are soluble in various solvents-e.g., carbon dioxide, sulfur dioxide, carbon disulfide, ammonia, methanol, ethanol, and ether-under pressure and above the critical temperature of the solvent. It had also been found that the quantity of solute in the gas phase is frequently far greater than can be accounted for by normal volatility (9, IO), while the effect of pressure on the amount of solid dispersed is much larger than that expected from the normal increase of vapor pressure because of external pressure. Klesper, Corwin, and Turner reported that dichlorodifluoromethane, c.t. 111.5 “C ( I I ) , is a solvent, under high pressure, for porphyrins. It is superior to other less chlorinated Freons and nitrogen (4). Dichlorodifluoromethane has the additional advantages of noninflammability and favorable corrosion characteristics with many metals and alloys (12). The pressures applied during the preliminary explorations ( 4 ) were in the range of 800-2300 psi. Since then work on high pressure gas chromatography has been reported, but with one exception (13, 14), in quite different directions. Thus, Kobayashi et al. (15-17) have applied pressures up to 2000 psi for the evaluation of physical constants. Myers and Giddings (18, 19) have made a combined high pressure-small particle (
