I AIDS FOR ANALYTICAL CHEMISTS On the Use of a Power Divider for Thermostated Electrodeless Discharge Lamps in Atomic Fluorescence Spectrometry D. 0 . Knapp, C . J. Molnar, and J. D. Winefordnerl Department of Chemistry, University of Florida, Gainesville, Fla. 3267 1
In recent years, electrodeless discharge lamps (EDLs) have been p u t forth as intense sources for atomic spectrometry. However, it has not been until recently that work involving temperature-controlled (thermostated) EDLs has resulted in stable, intense, reliable EDL sources for atomic spectrometry. It has previously been demonstrated that single- and multielement EDLs are very sensitive t o temperature changes when operated with a thermostated antenna ( I , 2). The insensitivity of spectral outp u t of EDLs operated with the thermostated "A"-antenna to changes in microwave power has been discussed previously (1-3). Some of the reasons given for using thermostated EDLs have been: microwave generators are difficult t o stabilize adequately, whereas air temperatures can be regulated with good precision; and changes in the coupling efficiency between the microwave field and the EDL discharge can, in the absence of temperature control, lead to drastic changes in lamp spectral output. Until recently ( 4 ) , one microwave generator per antenna had been used when operating EDLs. This is readily understood when one considers the myriad of papers on EDLs and the a r t and skill needed to utilize EDLs for analytical studies. However. with the insensitivity of thermostated EDLs to changes in microwave power, it is readily conceivable t h a t more than one thermostated EDL could be operated simultaneously from one microwave generator utilizing a microwave power divider and t h a t such a system would be very advantageous for multielement analysis. Norris and West ( 4 ) have reported the use of a two-port power divider for two dual-element EDLs and mentioned t h a t it was a n economical means of illuminating a flame cell with radiation from more t h a n one EDL simultaneously. In this study. thermostated EDLs operated a t their optimal temperatures and powered by one microwave generator using a power divider are evaluated. Detection limits for atomic fluorescence spectrometric measurement of several elements with these EDLs were obtained and compared with other published results.
EXPERIMENTAL A block diagram of the optical system is shown in Figure 1. T h e microwave power from a 20O-W, 2450-MHz generator with a separate reflected power meter (Electromedical Supplies Ltd., Wantage, Berkshire, U.K.) was divided (D2-2TN, 2-Port-3 d b Power Divider, Microlah/FXR. Livingston, N.J.) and used t o power two "A" antennas (Model 2254-5002G1; the Raytheon Co., Microwave 1
Author to whom reprint requests should he sent. R. F. Browner, B . M . Patel, T. H. Glenn, M. E. Rietta. and J. D. Winefordner, Spectrosc Lett.. 5 , 31 1 (1972) B. M . Patel, R . F. Browner. and J. D. Winefordner, Anal. Chem.. 44, 2272 (1972) R F. Browner and J. D Winefordner, Spectrochim. Acta. 288, 263 (1973). J. D. Norris and T S. West, Ana/. Chem.. 45, 226 (1973). A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 4 , A P R I L 1974
Devices, Farmington, Conn.). The EDL temperature control assemblies used have been described before ( I ) . The spectral radiation from the sources was mechanically modulated with a laboratory-constructed mechanical chopper operating a t 248 Hz and focused onto a n air/hydrogen flame used with a pre-mix laminar flow burner chamber (Model 303-0110; Perkin-Elmer Corp.. Norwalk, Conn.) and a capillary burner ( 5 ) . Atomic fluorescence measurements were performed with a 0.5-m Ebert monochromator (Type 82-000 with grating blazed a t 3000 A and 1180 grooves/ m m ; Jarrell-Ash Corp., 590 Lincoln St., Waltham, Mass.), and R.C.A. 1P28A photomultiplier a n d a lock-in amplifier (Model 353, Ithaco, Inc., Ithaca, N.Y.). A potentiometric recorder (Sargent Model SR, Sargent-Welch Scientific Co., Birmingham, Ala.) was used t o record data. Optical grade biconvex quartz lenses (SlUV, Esco Products. Oak Ridge, N.J.) were utilized throughout for forcusing. The single element Ag, Mg, Pb, S b , Sn, Te, and TI EDLs contained the iodide form of each element, and the Cu, Cr, and Ni EDL, t h e chloride; while the metals were used for the Hg and Cd, Zn, and Se EDLs.
RESULTS A N D DISCUSSION Thirteen elements were evaluated and detection limits, defined as the concentration resulting in a signal-to-rms noise ratio equal to 2, are given in Table I and are compared to other reported best detection limits. The optimum temperature reported for each element in a particular EDL was, in general, reproducible from day to day. There were, however, several notable exceptionsi.e., for two M g single element EDLs, the optimum temperature increased from 340 "C in initial work to 400 "C in later work with a loss in radiant output of 15X for one EDL, whereas for the other M g EDL actually used for the M g AF measurements, the optimum temperature was a reproducible 470 "C. Three Cd/Zn EDLs were evaluated; two dual-element EDLs containing just Zn and Cd metal had optimum temperatures of 280 and 250 "C, respectively, while for a third tube containing Cd, Zn, and Se as metals, the optimum temperatures were 400 and 420 "C for Zn and Cd, respectively. There was no observable Se emission from this (which was also the case with other Cd/Zn/Se EDLs) EDL. It was previously assumed t h a t the temperature dependence of the radiant output for ( 5 ) L. M. Fraser and J. D. Winefordner. Ana/. Chem., 43, 1693 (1971). (6) K . E. Zacha, M . P. Bratzel, J. M. Mansfield, and J. D. Winefordner, Anal. Chem.. 40, 1733 (1968). (7) J. D. Norris and T. S. West, Ana/. Chim. Acta. 59, 355 (1972). (8) H. G. C. Human, Spectrochim. Acta, 2 7 8 , 301 (1972). (9) P. L. Larkins. Spectrochim. Acta, 2 6 8 , 477 (1971). ( l o ) J. Matousek and V. Sychra, Anal. Chem., 41, 518 (1969) (11) D. L. Manning and P. Heneage. At. Absorp. Newslett., 6, 124 (1967). (12) R. M . Dagnall, K . C. Thompson, and T. S. West, Talanta, 1 4 , 1151 (1967). (13) R. F. Browner, R. M . Dagnali, and T. S. West, Anal. Chim. Acta, 46, 207 (1 969) (14) A. Hell and S. Ricchio, Pittsburgh Conference on Analytical Chernistry and Applied Spectroscopy, Cleveland, Ohio, 1970. (15) M. P. Bratzel and J . D.Winefordner. Anal. Lett., 1 , 43 (1967).
R EDL Temperature
Controlled System #I
EDL
Temperature
C o n t r o l l e d System +2
Figure 1. Block diagram of optical system containing power divider for thermostated electrodeless discharge lamps in atomic fluorescence spectrometry
Table 1. D e t e c t i o n L i m i t s of Several E l e m e n t s in A t o m i c F l u o r e s c e n c e Flame (Air/H2) S p e c t r o m e t r y Using Electrodeless Discharge L a m p s O p e r a t e d w i t h T w o “A” A n t e n n a s Joined ‘viaT w o - P o r t P o w e r Divider EDL Wavelength, temperature, Element
Ag Cd
Cr cu Hg Mg Ni Pb Sb Sn Te T1 Zn
nm
328.1 228.8 357.9 324.8 253.7 285.2 232 .O 405.7 231.1 303.4 214.3 377.6 213.8
“CQ
580 420 475 430 50 470 65 255 150 110
Detection limits, en/ml This study
0.0008 0,001 0.01
0.002 0.01
0.00005 0.07 0.03 0.01
0.5
Best reported (Reference)
0.0001 (6) 0.000001 (6) 0.005 (7)
0.0005 (8) 0 . 0 2 (2) 0.00015 (9) 0.003 (10) 0.02 (21) 0.05 (12) 0 . 1 (13)
410
0.07
0.006 (14)
310
0.002 0.005
0.008 (6) 0.00004 (15)
400
a All EDLs except for Hg and P b were operated at 120-W microwave power or 60 W per E D L because of use of the power divider. The Hg and Ph EDLs were operated at SO W or 40 W per EDL.
each element (or compound) present in a lamp is largely uninfluenced by the presence of other elements (2). T h e T l and t h e multielement C d / Z n / S e EDL were operated simultaneously with the power divider; switching the two lamps in the thermostated systems resulted in similar atomic fluorescence signals for a solution containing 1 Fg/ml of each element (signals within 8% for Cd, Zn. and T1 resulted for the two orientations). The P b and Hg EDL were operated simultaneously a t 40-W incident microwave power under both thermostated and nonthermostated conditions. Without thermostating, because of the low radia n t output from the P b EDL, there was no observable AF signal, while for a 100 Fg/ml solution of Hg, without thermostating, there was a decrease of 10x in AF signal. There were not any noticeable problems or difficulties associated with using two thermostated EDLs with the power divider and a single microwave generator. The most obvious advantage with the use of thermostated EDL,s is the ease with which maximum spectral output from a n EDL can be obtained cia variation of the EDL temperature. Future work is planned for the use of a multiport power divider and a 800-W microwave power supply for the simultaneous use of several thermostated EDLs. Received for review August 22, 1973, Accepted Novenlber 19, 1973. The research was supported by AF-AFOSR1880-701.
High Pressure Gradient Chamber for Liquid Chromatography E. H. Pfadenhauer, T.
E. Lynes, and T. V. Updyke
Ne wport Pharmaceuticais Internationai, lnc.. 7590 Monrovia Bouievard. Ne wport Beach. Calif. 92660
Gradient elution is a useful technique in many applications of chromatography, and is especially important to high pressure liquid chromatography where speed and resolution of analysis are major factors in justifying the expense of the necessary equipment. In our work, we were interested in doing rapid, high resolution determinations of oxypunnes in blood plasma, and wished to explore the possibility of using gradient elution in high pressure liquid chromatography. In cases where t h e gradient can be made u p at atmospheric pressure, the problem is trivial. However. if a screw-driven syringe type high pressure p u m p is used, the gradient must necessarily be made a t high pressures.
Commercial gradient accessories include a second high pressure p u m p (Sester-Faust 1200, Varian Aerograph 4200) or a complex arrangement of 5 high pressure valves, a holding coil. and a mixing chamber (DuPont 820). The device described here suffers from the limitation of being able t o deliver only a single convex gradient shape which is not usually the most desirable chromatographically, but is considerably less expensive than a second pump. entailing approximately only one-tenth the cost. The DuPont arrangement is not available as a separate unit. As seen in Figures l a and l b , the gradient device consists of a removable mixing chamber. and a cap through which is drilled a n inlet from the high pressure p u m p and A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 4 , A P R I L 1974
623
