1. C. O'Hoverl ond J.
D. Winefordner
University of Florida Goinesv~lle32601
I
I
An Instrument for Source intensity compensation in Atomic ~~uorescence
T h e intensity of atomic fluorescence has been shown to be directly proportional t o the intensity of the source of excitation over the absorption bandwidth of the atom in question ( 1 ) . For this reason, the source of excitation is an extremely important factor in atomic fluorescence flame spectrometry. If good limits of detection are to be obtained, the source of excitation must be both iutense and stable. A variety of types of sources of excitation have been used in atomic fluorescence flame spectrometry (2). The most generally successful source has been the electrodeless discharge lamp (5). This type of source is capable of emitting very intense and unreversed line spectra and can be made for a wide variety of metallic elements. A difficulty with many electrodeless discharge lamps, however, is that the line emission, although it may be quite intense, often exhibits considerable noise and drift (4). This would ordinarily make the lamp useless for analytical determinations. The purpose of this paper is to describe an instrumental system which compensates for variations in the source intensity, thus allowing the use of unstable electrodeless discharge lamps as sources of excitation in atomic fluorescence flame spectrometry. Methods of Source Intensity Compensation
Several methods of source intensity compensation have been used in spectrophotometric instrumentation. One method involves the operation of the source from a programable power supply controlled by feedback from a source monitor photocell. This method is not applicable to electrodeless discharge lamps because the lamp intensity is not simply related to the intensity of the microwave excitation field. Methods involving servo-controlled slit combs or other mechanical devices are unsatisfactory because of their inability to compensate for the fast and wide range intensity variations often encountered in electrodeless discharge lamps. The ratio method, in which the analytical signal is divided electrically by the source monitor signal, could be used in atomic fluorescence measurements. The main difficulty with the ratio method is its limited dynamic range; at low signal levels the contribution of electronic noise becomes significant. Furthermore, the required circuitry is elaborate and relatively expensive. The method of source intensity compensation used i n this work is similar to the technique of integration by an Taken in part from the PhD dissertation of T. C. O'Haver. This investigation was supported by AF-AFOSR 1033-6 grant. 'Present address: Department of Chemistry, University of Maryland, College Park, Maryland 20742.
added internal standard used in many commercial direct-reading emission spectrometers and is based on the analog integration of the fluorescenceand source monitor signals (see block diagram of the apparatus in Figure 1).
Figure 1. sation.
Block diagram of integration method of source intensity compen-
The fliiorescence is detected by multiplier phototube Ph21. After amplification, the signal is fed via normally closed relay RY1 into the signal integrator. The output voltage of the integrator is the time integral of the input voltage. Similarly, the intensity of the souwe is monitored by multiplier phototube PR12, and the resulting source monitor signal is amplified and fed into the monitor integrator via the normally closed relay &Y2. Under normal conditions, the outputs of the fluorescence signal integrator and the source monitor integrator increase with time at a rate directly proportional to the fluorescence intensity and source iutensity, respectively. The relay driver, which is connected to the output of the monitor integrator, energizes relays RY1 and RY2 when the monitor integrator output voltage exceeds a predetermined level, which is set by adjustment of the relay driver circuit. When the relays are energized, the input signals to the two integrators are interrupted, and the output voltages of the integrators become constant. The output voltage of the fluorescence signal integrator is taken as the analytical signal. Another measurement may be made by resetting the integrators to zero. If the source intensity were to drop to half its normal value, the output of the source monitor integrator would increase a t half the normal rate, and thus it would take twice as long for the relay driver to be activated. However, the fluorescence signal, which is directly proportional to the source intensity, would also be half the normal value; since the integration proceeds for twice the normal time, the final output of the fluorescence signal integrator is unchanged. In effect, the output of the fluorescence signal integrator after the integration has been terminated is proportional to the ratio of the fluorescence intensity to the source intensity. Volume 46, Number 7, July 1969
/
435
Table 1.
Characteristics and Functions of Principal Components in Source Monitor
Symbol
(see Fix. 2)
OAl
Type Model
3024/18
Function
Comments
Amplifier
operational
OAZ OA3
S1
amplifier Model 3021/15& operational amplifier Model 3024/15" o~eratlonal 1 poie, 6 poaitian aborting-type rotary
Integrator Comparator and rela" driver Sensitivity range switch
Positionah 1. 10 nA
Table 2. Characteristics and Functions of Principal Components in Solid State Nanoammeter Component Svn~bol (See Fig. 3)
TYPS
Function
O A l Model 301' operatmna1 amplifier OA2 Model 109 operat~onal~mplifier OA3 Model l O B operational amplifier S1 2 pole. 7 position ceramic-shortin..
sensitivity range time switoh
P3 Trimmer P4 Trimmer P b Trimmer
Integrator zero bias Integrator hold bias Celibration Voitsge offset balance
Comments
Amplifier Filter-integrator Current pump
2. 100 "A 3
1 ,,A
i: id& 1 pole, 3 position shorting-type rotam
Boaking ourrent range switch
3 pole, 2 position rotary
Funation switch
2 pole, norms1ly
Reset switch
open, momentary contact, push button switch 20 Kn preaision wirewound potentiometer 10 Kn prec,sron wire-wound potentiometer 10 Kll trimpot
Fine booking ourrent
DPDT relay 2.3 mA, 8 Kn coil
3. l o p A
positions 1. integrate
2. adjust time
Push to reset
Center zero
Fine eain comparstor referenoe voitagr adjust
200 pA meter
RY1
5. 100 "A 6. 1 mA Positions" 1. 100 nA 2. 1 PA
serves as calibre..
tion oontro1 for time scale of meter M I current acs1e: 0-1 units Time soale: --5 8%
Hold relay
Normally in position 1.
'Burr-Brown Research Corp., Tucson, Arirona.
Full sealesensitivities. Ruokine currents eorrespondine to full C W or CCW rotation of fine bucking ourrent oontro1. PI.
The second stage, which includes operational amplifier OA2 switches S3 and 54, operates as an integrator (6) when switch 53 is in position 1 and as an inverting unity-gain amplifier (6) when switch S3 is in position 2. The rate of integration is determined by the 1 MO input resistor, the 0.5 pf feedback capacitor, and the input signal voltage picked off by potentiometer P2. The integrator is reset by switch 54. The third stage, which includes operational amplifier OA3 and potentiometer P3, operates as a precision comparator and relay driver (6). This circuit senses the output voltage of the integrator and compares it to a reference voltage of +10 V, picked off by potentiometer P3. When the absolute value of the integrator output voltage is less than the absolute value of the reference voltage, the feedback diode D l conducts, and the output voltage of the comparator stage is very low.. Relay RY1 is therefore not energized, and its contacts are in position 1. In this position, the output of the amplifier stage is connected to the input of the second stage via potentiometer P2. If switch 53 is in position 1 (integrate mode), the second stage integrates the signal. As soon as the absolute value of the output of the integrator exceeds the comparator reference voltage, the summing junction of OA3 is driven negative, and the feedback diode becomes reverse-biased. The output of OA3 then increases instantly to the saturation level (about 20 V), energizing relay RY1. This causes the relay contacts to switch to position 2, terminating the integration. If reset switch S4 is now pressed, the cycle repeats. The output of the comparator is brought out to the contacts of terminal board TI3 for remote connection to the hold relay in the nanoammeter (RY1
P,
~i
' 100 ,&Ameter RY1 DPDT contacts 28 VDC "oil
Hold relay'
Normally in porition 1
;Analog D?V~C?~,, 1nc.. Cambridge. Massachusetts. Full soale sensltluities. "Bucking ourrents corresponding to full C W rotation of fine bucking our rent control. PI. d Filter time oonstants. ' Intepration times. I Used for remote operation only.
in Fig. 3). Diodes are connected in series with both relays to damp out the transient oscillations which occur when the relays are energized. The second set of contacts on the reset switch 54 are also brought out to terminal board T B so that both the source monitor integrator and the nanoammeter integrator may be reset at the same time by pressing switch 54 on the source monitor. The integration time, T, i.e., the time required for the integrator voltage to increase from 0 V to the reference voltage E,, is given by
where T is the integration time in seconds, E, is the reference voltage in volts, and E2 is the input voltage to the integrator, picked off by potentiometer P2, in volts. If switch 53 is placed in position 2 (amplifier mode), meter M1 indicates voltage Ez. The scale of meter 141 is calibrated directly in integration time, in seconds, according to the above equation. With the values of the components in Figures 2 and 3, the centerscale reading of meter M1 is 10 sec. The integration time is easily adjusted by varying the coarse gain switch S1 and the fine gain potentiometer P2 until the desired integrating time is indicated by meter 111. If the source signal exhibits noise and drift, the integration time indicated by meter R'I1 will behave similarly and should be adjusted to vary over a convenient range. The time scale of meter 141 is calibrated by adjusting the reference voltage E, by means of potentiometer P3. Volume 46, Number 7, July 1969
/
437
The source monitor is connected to the nanoammeter by means of short lengths of shielded cable. Contacts 1, 2, 3, and 4 of terminal board T B in Figure 2 are connected to contacts 1, 2, 3, and 4, respectively, of the terminal board T B in Figure 3. Although the source monitor was used with a dc nanoammeter in this study, it is entirely suitable for use with ac or lock-in systems as well. An integrator with a remote control hold relay would have to be constructed to integrate the dc output of the detector in an ac or lock-in system.
Use of the Source Monitor in Atomic Flame Fluorescence Spectrometry
A block diagram of the apparatus used for atomic flame fluorescence measurements using the source monitor to compensate for variations in the intensity of the source is shown in Figure 4. The optical arrangement is essentially that used by Mansfield, et al. (7), except for the presence of the source monitor components. Radiation from the source is focused by lens
Electrical Performance
The electrical performance of the source monitor in conjunction with the nanoammeter was evaluated by simulating the signals that would be obtained from the sample and source monitor phototubes in a typical spectrofluorometric measurement. The output of a variable voltage power supply, E,, is applied to a simple voltage divider. The two voltages picked off from the voltage divider, Em, and El,are fed via 10 Mil resistors into the inputs of the source monitor and nanoammeter, respectively; and the resulting input currents I, and I, simulate the monitor phototube current and fluorescence phototube current, respectively. The value of E , was chosen so that I, and 1,would be typical values obtained in fluorescence measurements. As the variable voltage E , is changed, the absolute values of I, and I, change, but the ratio of I, to I,, which is determined by the resistance in the voltage divider, remains constant. Thus, if the source monitor controls the integrations of I, by the nanoammeter, the integrator output signal should be independent of the value of E,, even if E , changes during the integration. The results of this procedure are shown in Table 3.
Table 3.
Electrical Performanceof Source Monitor
Relative Input Current to Nanoammeter
Integration Time, see
Relative Integrator Outout sinnal
The values of nanoammeter input :current, I,, and integrator output signal have been normalized to unity. The relative error in the integrator output signal does not exceed 3% over two decades of nanoammeter input current. Various step and periodic waveforms were also tried in an attempt to simulate the type of instabilities sometimes encountered in electrodeless discharge tubes. The resulting relative errors did not exceed 1%. It is interesting to note that turning the variable voltage E , down to zero during an integration results in little error; this simulates the effect of the source going out during a fluorescence determination, a possible occurrence with certain electrodeless discharge tubes. Integrator drifts are quite low, and E , can remain at zero for as long as 20 sec before an appreciable error is introduced. 438
/
Journal of Chemical Education
NANOAMMETER
MONOCHRWATMI 1
Figure 4. Apparatus used for atomic flvorercence measurements with the source monitor.
1 onto the flame. Fluorescence radiation from the flame is focused by lens 2 onto the entrance slit of monochromator 1, and the intensity of the spectral line selected by the monochromator is measured by multiplier phototube PM1. A baffle box surrounding the optical components reduces interference from room light. The source monitor components are positioned perpendicular to the source-flame axis. Lens 3 focuses the source radiation onto the entrance slit of monochromator 2, and photomultiplier PA12 measures the intensity of the spectral line selected by the monochromator.' The purpose of monochromator 2 is to ensure that the fluorescence signal is compensated only for the intensity of the spectral line which is responsible for the excitation of atoms in the flame. In general, of course, both monochromators will be set to the wavelength of the fluorescence excitation line. An alternate optical arrangement, in which the source radiation was sampled by a small quartz plate placed behind lens 1 at a 45' angle to the optical axis, was also investigated. This arrangement, however, was found to be unsatisfactory because of the difficulty involved in adjusting the optics to obtain a representative sample of the source intensity. Furthermore, the quartz plate reduces slightly the excitation intensity a t the flame. The arrangement shown in Figure 4 avoids this drawb k k and is much easier to adjust for optimum results. It is important to note that the distribution of glowing plasma in an electrodeless discharge lamp is almost always quite symmetrical and that it usually remains symmetrical during an instability. This condition is necessary if the system in Figure 4 is to be effective. In any case, this system was found to be more satisfactory in practice than the alternate arrangement. In order to evaluate the performance of the source monitor, it is desirable to eliminate all causes of drift and noise in the fluorescence signal which are not caused by variations in the source intensity. These include, for instance, flame noise, changes in the gas or sample flow rates, and source scatter noise due to particles in
the flame. I n addition, the half-width of spectral lines from the source may increase (broadening) as the source line intensity (source temperature) increases, resulting in a nonlinear relationship between the intensity of atomic fluorescence in flames and the intensity of the exciting source line. Also at high source intensities, the exciting source line may become self reversed which could result in a decrease in fluorescence intensities of atoms in flames with increase in source intensity. Of course, the source monitor system described in this paper or any other source intensity compensation method does not correct for the above noises, changes, and nonlinearity problems. I n the initial work, the flame in Figure 4 was replaced with a small piece of white card stock positioned so that it would scatter some of the incident radiation from the source onto lens 2. The resulting signal was directly proportional to the source intensity, as is actual fluorescence, but was free of other causes of noise and drift. Furthermore, the scattered source radiation was sufficientlyintense so that the shot noise of photomultiplier tube PM1 would not interfere with the measurements. Care was taken, however, to adjust the slits of both monochromators so that the light intensities would not be high enough to cause fatigue of the multiplier phototuhes. The source which was selected to demonstrate the source intensity compensation ability of the system was a useless beryllium electrodeless discharge lamp. The construction and operating conditions of this lamp are described by Zacha et al. (8). This particular lamp was chosen because it exhibits both types of instability commonly observed in electrodeless discharge tubes (4) : a long-term drift and a slow, periodic oscillation. When the lamp is first started, the emitted radiation is due primaely to the argon fill gas, and the intensity of the 2349 A resonance line is very low. During the first hour or so of operation, the intensity of the resonance line of beryllium gradually increases, finally reaching a temporary steady state at a value two or three orders of magnitude greater than the initial intensity. Eventually, however, the discharge within the lamp begins to shrink and grow periodically, with the result that the intensity of the lamp oscillates with a slow, almost sinusoidal waveform. The oscillation continues indefinitely and is surprisingly constant in both period and intensity. Needless to say, the drift and periodic oscillations make the beryllium lamp useless for analytical work. The periodic oscillation of the beryllium lamp is illustrated by the dotted line in Figure 5. The frequency of oscillation is approximately 0.021 Hz. The ratio of the peak-to-peak deviation to the average value (about 6.7units on the relative scale of Figure 5) is 1.1. The solid lines show the integrations performed with the source monitor system in operation. Note that the rate of integration, i.e., the slope of the leading edge of each integration, is proportional to the source intensity at each instant. The integration times vary from 5 to 25 see. The ratio of the peak-to-peak deviation to the average value of all the integrals is 0.043, which is an improvement by a factor of 25 compared to the uncompensated system. The relative standard deviation of the integrals is 1.6%, which is certainly adequate for analytical work.
Figure 5. Source moniior cornpenrotion of a periodic instability in o beryllium electrodelerr d i r c h w g c tube.
The source monitor compensation of the long-term drift of the beryllium lamp over a period of 9 min is illustrated by the data in Table 4. The uncompensated source intensity increases by a factor of 42, whereas the relative peak-to-peak variation in the values of the integrals when the source monitor is used is only 8.3%. The relative standard deviation of the integrals is only 2.5'%. Table 4.
Source Monitor Compensation of the Drift of o Beryllium Electrodeless Discharge Tube
The atomic -fluorescenceof beryllium was also measured with the source monitor system. The flame conditions, burner height, and monochromator slit width were the same as those used by Zacha, et al. (8). The fluorescence signal of a 244 ppm solution was considerably less intense and more noisy than the signal obtained with a scattering card in place of the flame; and, as expected, the reproducibility of the source monitor integrations was poorer. I n one measurement, a sixfold increase in the source intensity produced a relative standard deviation of 14y0 in the source monitor integrations. Although this is relatively poor compared to the data of Table 4, it nevertheless represents a significant increase in the stability of the measured signal. The poorer precision is most likely a result of flame background noise, source scatter noise, fluorescence noise, fluctuations due to variation in gas or sample solution flow rates, and a change in half-width of the resonance line from the source as a function of source intensity (source temperature). Literature Cited (1) WINEFORDNER, J. D., PARSONS,M. L., MANSPIBLD, J. M., AND MCCARTHY, W. J., Spectroehim. Ada, 23B,37 (1967). (2) VEILLON, C., Ph.D. Dissertation, University of Florida, August, 196.5. (3) MANSFIELD, J. M., JR., Ph.D. Dissertation. University of Florida, August, 1967. (4) MANSFIELD, J. M., JR., BRATZEL, M. P., JR., NORGORDON, H. 0.. KNIPP. D. 0.. ZACHA. K. E.. AND 1 WINRFORDNISR.
Volume 46, Number 7, July 1969
/
439
( 5 ) O'H~v~irr, T.C., AND WINEFORDNER, J. D., J. CHEM.EDUC.,
to appear later this year. (6) Applicatiom Manual for Computing Amplifiers, Philbriek Researches, Inc.
440 / Journal o f Chemical Education
'(7) MANSFIELD, J. M., JR., WINEFORDNER, J. D., AND VICILION, C., Anal. Chem., 37, 1049 (1965). (8) ZACHA, K. E.,BRATZEL, M. P., MANSFIELD, J. M., JR., AND WINEFORDNER, J. D., Anal. Chem., submitted.
