conditions such as boiling under reflux or adding some chemical to oxidize the Cr(II1) to Cr(V1) (I, 2). Manganese is best extracted at a pH of 7.0 or higher by such complexes as HTTA, DDDC, HAA, Oxine, and NaDDC. Further study on the extraction of other metals with this procedure
might be fruitful. At present it appears that antimony is also extracted under the described conditions. RECEIVEDfor review January 16, 1974. Accepted July 10, 1974. Work supported by the National Research Council of Canada.
Comparison of Continuous Wave and Pulsed Continuum Sources for Atomic Fluorescence Flame Spectrometry D. J. Johnson, F. W. Plankey, and J. D. Winefordner‘ Department of Chemistry, University of Florida, Gainesville, Fla. 326 1 1
In atomic fluorescence spectrometry, a continuum source can be used to determine many elements whlle avoiding the one source per element restriction. Previous use of continuum sources has resulted in relatively high detection limits when compared to line sources and their low output in the 200- to 250-nm spectral region has limited their use for many elements. The use of a high powered pulsed continuum source and cw source wlth large solid angle collection efficiency is described, and llmlts of detection are compared for the two source systems. Also, analog and dlgltal (photon counting) detectlon systems are used for each source, and these results are compared. Although the pulsed source was expected to give a larger signal-to-noise ratio than the cw source, the opposite results were found. This was posslbly due to the useable source flux (at the atomizer) ratlo determined largely by solid angle conslderatlons. The overall convenience of the cw source wlth elther detection system and the detection limits found for 13 elements indicate that thls source has practical value in AFS.
Although low intensity (not laser) spectral continuum sources have several obvious advantages as primary sources in atomic fluorescence spectrometry, e.g., the possibility of using only one source for exciting many elements, the possibility of wavelength scanning to compensate for any scattering interference, and the increased stability-long and short term-as compared to many line sources, such sources have been primarily a tool for physical studies and a curiosity for analytical studies. The major analytical limitations of such sources have been primarily associated with their meager output in the UV (200-350 nm) and their generally low fluxes over atomic absorption lines. Nevertheless, if the average photon flux reaching the absorption cell could be increased by increased input power to the source or by increased gathering (solid angle) efficiency of the excitation source optics and/or if the peak photon flux could be increased by pulsing the source and the detector system could be made to respond to the fluorescence signal primarily during the source “on-time”, then it would seem that a spectral continuum source could be of considerable use in atomic fluorescence spectrometry. Veillon, Mansfield, Parsons, and Winefordner ( I ) first showed that a cw continuum source (150-W Osram Lamp) Author to whom reprint requests should be sent. (1) C. Veillon, J. M. Mansfield, M. L. Parsons, and J. D. Winefordner. Anal Chem., 38, 204 (1966).
1898
could be used in atomic fluorescence spectrometry with a turbulent flame atomizer (H2-02 or Hs-Ar-entrained air); these authors were able to excite the resonance lines of 14 elements, including Zn and Cd but were distressed with the appreciable scatter signal. Ellis and Demers (2), shortly thereafter, described the use of a 450-W xenon arc lamp to excite eight elements in atomic fluorescence spectrometry (H2-entrained air flame). Dagnall, Thompson, and West ( 3 ) and Manning and Heneage ( 4 ) also used a 150-W xenon arc (Osram type) lamp for atomic fluorescence studies, and Bratzel, Dagnall, and Winefordner ( 5 ) described the use of a 150-W xenon arc lamp, made by Eimac, for atomic fluorescence flame spectrometry. Cresser and West (6) determined detection limits and interferences for 13 elements in atomic fluorescence spectrometry with an air-acetylene flame and a 500-W xenon arc lamp. Omenetto and Rossi (7) used a mercury vapor discharge arc lamp as a continuum source to excite several elements in atomic fluorescence flame spectrometry. In all of the above cases, the cw continuum sources resulted in detection limits from one to three orders of magnitude higher (worse) than with intense line sources (e.g., electrodeless discharge lamps), and even worse results for elements with resonance lines below about 250 nm ( e . g . ,Ni, Cd, Zn, Sb, Se, Te, Fe, Co, etc.). Certainly, the detection limits obtained in the above studies could have been improved considerably by use of better entrance optics between the source and atomizer and/or between the atomizer and spectrometer, and/or by use of a spectrometer with a larger LR product ( L = luminosity and R = resolving power), and/or by use of flames with greater atomization efficiencies assuming the changes in fluorescence quantum yields and flame background are not too great (8). The present atomic fluorescence study involves the application of a pulsed xenon lamp as well as a point source cw xenon arc lamp with a special mirror system enabling the transfer of nearly all the radiation produced by the source into the flame atomizer. In the former case, either an analog or digital boxcar detector is utilized whereas, in the latter case, either a lock-in amplifier or a synchronous photon counter is used. In both cases, flames with good atomization charac(2) D. W. Ellis and D. R. Demers, Anal. Chern., 38, 1943 (1966). (3) R. M. Dagnall, K. C. Thompson, and T. S. West, Anal. Chim. Acta, 36, 269 (1966). (4) D. C. Manning and P. Heneage, At. Absorption Newslett., 7, 60 (1968). (5) M. P. Bratzel, R. M. Dagnall, and J. D. Winefordner, Anal. Chirn. Acta, 52, 157 (1970). (6) M. S. Cresser and T. S. West, Spectrochlm. Acta, Pari& 25, 61 (1970). (7) N. Omenetto and G. Rossi, Anal. Chlm. Acta, 40, 195 (1968). (8) V. Svoboda. R. F. Browner, and J. D. Winefordner, Appl. Spectrosc., 26, 505 (1972).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
Table I. Experimental Components and Manufacturers Item
Descri tion (Model $umber)
Monochromat o r
2 18
Photomultiplier
EM1 62568
Current Pre -amplifier
427
Lock -In Amplifier
8 04
Boxcar Detector
cw-1
Photomultiplier Housing
1151
Amplifier /Dis c r i m i n a t o r
1120
Digit a1 Synchronous Com put e r
1110
Data Converter
1105
B u r n e r /Nebulizer
3 03 - 0110
Pulsed Source Flashtube
N-599
Pulsed Power Supply
457
Housing
LH 151N
CW Source A r c Illuminator Power Supply Recorder
X6163R
P-250s -2 Servo/Riter I1
Source
GCA/McPhers on, Acton, Mass. 01720 Gencom Div., Plainview, N.Y. 11803 Keithley Instruments, Cleveland, Ohio 44139 Keithley Instruments, Cleveland, Ohio 44139 PAR Corp., Princeton, N. J. 08540 SSR Instruments Santa Monica, Calif. 90404 SSR Instruments Santa Monica, Calif. 90404 SSR Instruments Santa Monica, Calif. 90404 SSR Instruments Santa Monica, Calif. 90404 Perkin E l m e r , Norwalk, Conn. 06852 Xenon Corp., Medford, Mass. 02155 Xenon Corp., Medford, Mass. 02155 Schoeffel Instruments Co., Westwood, N. J. 07675 Eimac Div. of Varian, San C a r l o s , Calif. 94070 Eimac Div. of Varian, San Carlos, Calif. 94070 Texas Instruments Inc., Houston, Texas 77006
teristics, (namely, argon separated air/CsHz or N20/C2H2 flames) are used as atom reservoirs. A high aperture monochromator is also utilized in all studies. The results enable a critical comparison of cw us. pulsed spectral continuum sources for atomic fluorescence as well as a comparison of analog us. digital detection systems when utilizing flames with good atomization characteristics. EXPERIMENTAL Apparatus (See Table I). A block digram of the experimental arrangement which was used in the present work is given in Figure 1. Source radiation was focused in the flame and then reflected back into the flame using a 140-mm diameter, 62-mm focal length mirror, coated for enhanced UV reflection. A 1:l image of the flame was formed on the entrance slit of the monochromator (0.3 m, 53 8 / m m reciprocal linear dispersion) with a lens chosen to match the monochromator's f/5.3 aperture. T o reduce the amount of stray light entering the system, this lens was enclosed within a blackened tube, and a light trap was placed on the opposite side of the flame. For the pulsed source studies, a low pressure capillary type xenon arc lamp was used. This was enclosed in a fan-cooled housing having an integral reflector and lens which focused the lamp output to form a ca. 2-cm high image of the arc. T h e lamp was operated from a high-power pulsed power supply a t 7 kV. A O.l-rF discharge capacitor was used providing an input energy of 2.5 J per flash, and a flash with a half-width of ca. 6 Wsec. T h e lamp was pulsed (no dc offset) a t a repetition rate of 100 Hz with an average power input of 250 W.
*&, SUPPlY
:/,
Figure 1. Block diagram of experimental system
For the cw source studies, an Eimac 150-W illuminator similar to that previously described by Bratzel, Dagnall, and Winefordner (5)was employed. This was operated a t the manufacturer's recommended settings (12 V, 12.5 A) using an Eimac power supply. The
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1899
Figure 2. Timing diagram for synchronous photon counting with pulsed and chopped cw lamps ( a )Synchronization signal. Pulsed lamp 100 Hz, chopped cw lamp, 50 Hz. ( b ) Lamp intensity incident on flame. (c) DATA gate opening sequence (high state = gate open) (d)BACKGROUND gate opening sequence (high state = gate open). Note: ( b ) ,( c ) ,and (d)(Pulsed Lamp) time scale distorted for clarity
prefocused light was mechanically chopped a t 50 Hz and then focused in the flame, producing a circular image ca. 6 mm in diameter. A Perkin-Elmer burnerhebulizer assembly was employed, after modification to accept a capillary-type burner head (9) with a sheathing gas provision. Both argon-separated air/CzHz and argon-separated NzO/CzHz flames were used in the present studies. For all atomic fluorescence measurements, an EM1 62568 photomultiplier tube, operated a t -1450 V, was employed. The detector was enclosed within a magnetically- and electrostatically-shielded housing to reduce RF and other sources of interference. The (digital) photon counting system used in part of the studies, consisted of an amplifier/discriminator, the output of which was fed to a digital synchronous computer. This instrument has two separate scalers, a variable sampling period, and was capable of synchronous operation when provided with a suitable trigger waveform. The output of the two scalers (DATA and BACKGROUND) can be presented individually, as their sum, or as their difference. The synchronization of the two scalers with the pulsed and chopped cw lamps is illustrated in Figure 2. During the period in which the DATA gate is open, the signal measured is equal to the fluorescence signal plus flame background signal plus the photomultiplier dark signal. When the BACKGROUND gate is open, only the latter two are measured and hence the quantity DATA minus BACKGROC‘ND gives the fluorescence signal directly. The gate width when using the pulsed source was chosen as 5 psec (opening 3 psec after the start of the pulse-see Discussion). A 9-msec gate width was chosen when using the Eimac cw source, to avoid possible overlap of the DATA and BACKGROUND sampling periods. For both sources, a count period of 10 sec was employed. The analog system utilized with the pulsed source system was a boxcar detector. As with the photon counter, a 5-psec gate width, delayed by 3 psec, was used. The time constant was adjusted so that a steady signal was obtained after ca. 10 sec. The analog system employed with the Eimac source was a lockin amplifier together with a current pre-amplifier. The sensitivity of the lock-in was set constant a t 1 mV, and the overall gain was adjusted by varying the gain of the pre-amplifier. For all measurements, a time constant of 3 sec was used, so that a steady signal was obtained after a ca. 10-sec period. Reagents. Reagent grade chemicals were used to prepare stock solutions of all elements. Serial dilutions were performed to prepare standard solutions for determination of the detection limits and analytical curves. Procedure. The source, burner-nebulizer, and electronic measurement systems were operated as described in the manufacturers’ manuals. Experimental conditions for pertinent experimental components are given in Table 11. No attempt was made to optimize the monochromator slit width, the flame conditions, or the observation height in the flame for each individual element. Instead, these parameters were opti(9) K. M. Aldous, R . F. Browner, R. M. Dagnall, and T. S. West, Anal. Chem., 42, 939 (1970).
1900
1
1 0.001
0.01
0.1 Anaiyte C m m l r a l i o n I p g h f l
10
100
Figure 3. Growth curves for seven elements using chopped cw source and synchronous photon counting (0 indicates limit of detection) Air/C2H2flame
*7
G I
IO
100
Analyfe Conc~llrafronfpg/miJ
Figure 4. Growth curves for 5 elements using chopped cw source and synchronous photon counting (0 indicates limit of detection) N20/C2H2flame mized initially for silver in the air/CnHz flame and aluminum in the NzO/CzHz flame, and these conditions were used throughout the study.
RESULTS AND DISCUSSION In Table 111, atomic fluorescence detection limits are listed for eight elements (Ag, Cd, Co, Cr, Mg, Mn, Se, and Zn) in a separated air/CzH2 flame and for five elements (Al, Be, Ti, Mo, and V) in a separated N20/C2H2 flame for various combinations of xenon source (pulsed us. cw) and detection system (analog us. digital). The detection limit is defined as that concentration (in kg/ml) of the element in pure aqueous solution resulting in a signal-to-rms (background) noise of 2. (With the photon counting system, the rms noise was taken as the square root of the total number of counts accumulated during the counting time when a blank solution was aspirated into the flame.) Previously measured detection limits for these same elements excited by cw continuum sources as well as line sources in atomic fluorescence spectrometry are also listed in Table 111.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
Table 11. Experimental Conditions for Present Atomic Fluorescence Studies (0.3-m, Czerny-Turner, f/5.3 600 g/mm grating blazed at 3000 Monochromator Slit width 125 pm Slit height 10 m m Band p a s s 0.6 nm Pulsed Source Voltage 7 kV P u l s e energy 2.5 J P u l s e width (FNHM) 6 psec Repetition r a t e 100 Hz CW Source Voltage 1 2 VDC Current 12.5 A Chopper duty factor 0.5 Chopper frequency 50 Hz Observation Height in F l a m e , 2-3 c m above b u r n e r top Flame A i r /C2Hz Air 9.7 l./min CZHZ 1.5 l./min Argon 15.5 l./min N,O/C,H, NZO 4.4 l./min CZHZ 1.2 l./min Argon 13.4 1./min
A)
Table 111. Comparison of Detection Limits Resulting from Continuum (Pulsed us. CW) Sources in Atomic Fluorescence Spectrometry with Previous Atomic Fluorescence Flame Spectrometric Results Detection limits, u g / m l Present work
Element
Line, nm
Flame typea
Ag Cd co Cr Mg Mn Se Zn Be A1 V Mo Ti
328.1 228.8 240.7 357.9 285.2 279.9 203.9 213.9 234.9 309.2 318.4 313.3 320.0
CzHz-Air CzHz-Air CzHz-Air CzHz-Air CzHz-Air CzHz-Air C2Hz-Air CzHz-Air CZHZ -N20 CZHZ -N20 CzHz -NzO CzH2 -NzO CZH, -N,O
a
Continuum -pulsed digital/analog
0.02/0.08 0.08/0.03
0.2/0.1
-/0.007/0.004 0.06/0.03
-/0.2/0.1 0.4/0.2 7./-
-/-/-/-
Literature
Continuum - cw digital/analog
0.004/0.006 0.01/0.01 0.02/0.02 0.01/0.01 0.0003/0.0003 0.004/0.004 43. 0.006/0.01 0.06/0.07
0.2/0.4 0.1/0.2/0.9/-
Continuum-cw
Line-cw
0.001 ( 2 ) 0.08 ( 1 ) 1. (5)
0.0001 ( 1 0 ) 0.000001 ( 1 0 ) 0.01 ( 1 1 ) 0.005 (12)
0.01 (2) 0.1 (2) 103 (fi) 0.01 ( 1 6 )
0.0001 ( 1 3 ) 0.001 (14) 0.04 ( 1 5 ) 0.00001 ( 1 7 ) 0.01 ( 1 8 ) 0.1 ( 2 0 ) 0.07 ( 2 0 ) 0.5 ( 2 0 ) 4. ( 1 9 )
... ...
... ...
Line-pulsed (9 )
0.004 0.004 0.007 0.004 0.001 0.002 1. 0.003 0.07 0.02 0.06
See Table I1 for conditions
It is apparent from the results in Table I11 that with the Eimac cw source, detection limits approximately 1OX lower than with the present pulsed source are obtained in atomic fluorescence flame spectrometry. It is also quite interesting that detection limits obtained with , t h e cw continuum source are within approximately an order of magnitude of the best detection limits previously obtained with electrodeless discharge lamp line sources. A comparison of the detection limits obtained with either source and with the analog or digital (photon counting) detection systems indicates that the digital detection system has little advantage over the analog detection system; this is to be expected, since only in low background systems would photon counting be advantageous. The atomic fluorescence flame spectrometric analytical curves, determined for several elements excited with the cw
source, are given in Figures 3 and 4. Figure 5 depicts the growth curve for magnesium; this curve shows the shape characteristic of atomic fluorescence using a continuum (10) K. E. Zacha. M. P. Bratzel, J. M. Mansfield. and J. D. Winefordner, Anal. Chem., 40, 1733 (1968). (1 1) J. Matousek and V. Sychra, Anal. Chem., 41, 518 (1969). (12) J. D. Norris and T. S. West, Anal. Cbim. Acta, 59, 355, (1972). (13) P. L. Larkins, Spectrochim. Acta, Part B, 26, 477 (1971). (14) L. Ebdon, G. F. Kirkbright, and T. S. West, Talanta, 17, 965 (1970). (15) R. M. Dagnall, M. R. G. Taylor, and T. S.West, Spectrosc. Lett., 1, 397, (1968). (16) R. M. Dagnall, K. C. Thompson, and T. S. West, Anal. Chim. Acta, 36, 269 (1966). (17) M. P. Bratzel and J. D. Winefordner, Anal. Lett., 1, 43 (1967). (18) D. N. Hingie. G. F. Kirkbright, and T. S. West, Analyst(London),93, 522, (1968). (19) P. L. Larkins and J. B. Willis. Spectrochim. Acta, Part B. 26, 491 (1971). (20) R. M. Dagnall, G. F. Kirkbright. T. S. West, and R . Wood, Anal. Chem., 42, 1029 (1970).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1901
,dl -20,
01
I CD?CC"lTdllO"
-.
IO
IO0
i+g,mo
Flgure 5. Growth curve for magnesium using pulsed lamp and synchronous photon counting
source, as described by Zeegers and Winefordner (21). I n no instance was scattering of the exciting radiation from particles in the flame (both air/C2H2 and N20IC2H2) detected, when using the chamber nebulizer, pre-mixed burner with capillary head. In those instances (e.g., real samples with high solids content) where scatter may be appreciable, a wavelength scan of the fluorescence line region would result in a simple means of correcting for the scatter. In Figure 6, a wavelength scan of the spectral region from 274290 nm is given; this figure for Mn and Mg depicts the excellent resolution of the present system, and the opportunity of correcting for background scatter (although in this instance, no scatter was evident). The detection limits obtained for all elements studied with the pulsed source were poorer than with the cw source (see Table 111) despite the authors expectations to the contrary (22); from a simplistic consideration of source powers and solely shot noise contribution for the two sources in atomic fluorescence measurements, one would expect a signal-to-noise ratio gain for the pulsed source AFS system. The reasons for these unexpected results are probably due to: (i) the noises for the 2 sources being quite different; (ii) the source areas and the solid angle of collection of radiation for the 2 sources being much different; (iii) power losses occurring in electrical components with the pulsed source to a greater extent than with the cw source. No detailed study was performed to evaluate the exact sources of noise for the two sources and corresponding atomic fluorescence systems, and so the relative effect of items (i) and (iii) is not known. However, it is interesting to compare the useful solid angle and area factors (item ii) for the two sources. The Eimac cw xenon source is essentially a point source with an elliptical reflector which enables virtually all radiation from the point source to reach the flame cell and be utilized in producing atomic fluorescence within the slit height of the spectrometer. On the other hand, the pulsed source is a linear flash tube with a mirror reflector behind it. By focusing an image of the linear source into the flame, it is possible to obtain useful fluorescence over only about 1 cm (the slit height) as opposed to the 2-cm image height, but more important the solid angle of collection is
276
278
MO 282 284 286 Wavelengrh inml
Figure 6. Fluorescence Wavelength Scan of 0.15 ,ug/ml Mg plus 2.5 pglml Mn, showing the Mg 285.2-nm line and the Mn 2 7 9 5 , 279.8-, 280.1-nm triplet. Chopped cw source, lock-in detector. Slit width 25 pm, time constant 1 sec, scan rate 2 nm/min
no more than about 0.15 sr, and so the gain factor for the cw source is (4a/0.15) X (2/1) = 170. Furthermore, the radiative efficiency for a long capillary flashtube is low compared to a short-arc lamp (23) so that an overall S/N advantage for the cw lamp can be partly accounted for. The pulsed source system has several other practical disadvantages in addition to the poorer detection limitsnamely, the high voltage (-25 kV) pulses to start the pulsed arc cause considerable RF interference with gated detectors, and so a delay in gate opening was necessitated until the starter pulse had subsided which resulted in another factor of -2 loss in fluorescent signals. In addition, with the pulsed lamp, the noise level was highly dependent upon the placement of the cables leading from the pulsed power supply to the flash lamp. Finally, the lifetime of the pulsed lamp was quite limited (about lo6 flashes before the need to replace, and so operation a t 10' Hz gives a lifetime of IO4 sec). In contrast, the cw Eimac source proved to be convenient to use, and the lifetime is quite long (over 1000 hr). However, it should be stressed that the mirror behiud the flame and in line with the Eimac source should not focus the exciting beam directly back into the Eimac source because of the danger of overheating the source and destroying the lamp. A procedure involving slight misalignment of the mirror source was adopted after the explosive destruction of an illuminator. This misalignment reduced the fluorescent signal by only 10-20%. In summary, the cw Eimac xenon arc lamp proved to be an excellent source of excitation in atomic fluorescence flame spectrometry with either analog or digital detection. The present system would appear to have considerable use in multielement analysis, particularly if a high powered source with a reflector of enhanced UV output were used. Such studies are currently in progress.
RECEIVEDfor review March 14, 1974. Accepted July 17, (21)
P. J. Th. Zeegers and J. D. Winefordner, Spectrochim. Acta, Part 6, 26,
181 (1971). (22) N. Omenetto, L. M. Fraser, and J. D. Winefordner, in "Applied Spectroscopy Reviews" Vol. 7, E. G. Brame, Ed., Marcel Dekker, New York, N.Y., 1973, p 147.
1902
1974. This work was supported by AF-AFOSR-74-2574. (23) J. D. Winefordner, S G. Shulman, and T. C. O'Haver, "Luminescence Spectrometry in Analytical Chemistry," Wiley, New York, N.Y, 1973, p 149.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
