I nstrument for Time-Resolved Phosphorimetry Using an Electronically Gated Photomultiplier T. D. S. Hamilton, Atomic and Molecular Physics Group, Department of Physics, The University, Manchester, M73 9PL, U . K
K. Razi Naqvi’ Department of Chemistry, The University, Sheffield, S3 7 H f , U.K.
Phosphorescence spectra are almost invariably (and delayed fluorescence spectra inevitably) recorded with intermittent excitation of the sample. In order to discriminate between prompt and delayed emissions, the exciting light is periodically interrupted by a mechanical shutter (or altogether extinguished by electronically pulsing the excitation source), and the emitted light is viewed only during the dark period. Fisher and Winefordner ( 1 ) have recently given a succinct account of the experimental techniques employed in phosphorimetry; we therefore refrain from reviewing the subject here. All mechanical phosphorimeters entail certain disadvantages. Those using two co-axially mounted chopper disks or a slotted cylinder are easy (and inexpensive) to construct but difficult to use: they impose severe restrictions on the physical size and the position of the sample compartment. Moreover, geometrical rearrangements may have to be made if one wishes to switch over from measuring delayed emission to studying prompt emission. Choppers driven separately by two synchronous motors overcome the aforementioned difficulties, but add a great deal to the cost of the instrument. Finally, it is extremely inconvenient, if not altogether impossible, to vary, in a mechanical phosphorimeter, the dead time t d and/or the viewing time t,; thus time-resolved phosphorescence spectra cannot be easily recorded with mechanical phosphorimeters. O’Haver and Winefordner ( 2 ) proposed the use of a spectrophosphorimeter employing an electronic gate which switches on the photomultiplier only during a part of the dark period. Later Fisher and Winefordner ( 1 ) mentioned that this method was difficult to achieve instrumentally and adopted instead two other approaches to time-resolved phosphorimetry. We have been able to overcome the difficulty experienced by Fisher and Winefordner ( I ) , and have constructed a spectrophosphorimeter that uses a pulsed photomultiplier and obviates all t,he disadvantages of mechanical instruments. By using the electronic circuits described in thi’s and a previous paper ( 3 ) ,any existing spectrofluorimeter can be easily modified, a t very little extra cost, and converted into a versatile and easy-to-use phosphorimeter. Prompt or delayed emissions can be seen without disturbing the geometrical arrangement; time-resolved spectra and lifetimes can also be easily measured. Operation of the instrument is best understood by referring to the block diagram in Figure 1. A (xenon) lampchopper combination is used as the intermittent excitation source. The chopper disk also serves to generate a square-wave signal from an optical pick-off circuit. The signal from the latter is used to produce two sharp pulses after variable delays to, and t , (Figure 2 ) which open and close the electronic gate. In the “on” state, the dynode lPresent address, Department of Textile Industries, The Polytechnic, Huddersfield HD13DH, U.K. (1) R. P . Fisher and J. D.Winefordner, Ana/. Chem., 44, 948 (1972). (2) T. C. O’Haver and J. D. Winefordner, Anal. Chem., 38, 1258 (1966) (3) T. D. S. Hamilton, J. Phys. E , Sci. Insfrum., 4 , 326 (1971).
voltages are of the correct polarity. When an “off” pulse arrives, it reverses the voltage(s) between one(two) pair(s) of dynodes; secondary electrons are, therefore, repelled and the photomultiplier is cut off. In luminescence spectrometers employing mechanical shutters, the photomultiplier contributes its noise during the entire cycle time t,, but views the emitted light only for a time t , < tc. In our instrument, on the other hand, the photomultiplier is switched off when it is not detecting the luminescence of the sample, and a large fraction of the undesirable noise is automatically suppressed. Thus, besides being convenient to use, it affords a better signalto-noise ratio than other instruments; when used in conjunction with a phase-sensitive detector, our instrument becomes akin to, but remains less expensive than, a boxcar detector. DETAILS OF ELECTRONIC CIRCUITS Square-Wave Generator and Trigger Generator. The electronic gating circuit described in this paper requires reference signals in-phase and 180 degrees out-of-phase with the signal produced by chopping the light beam from the exciting lamp. A small incandescent bulb and a phototransistor are generally used for this purpose, but stray light from the bulb often enters the detecting system and creates additional problems for the experimenter. We, therefore, used a commercial unit (Texas Instruments SDA20) comprising an infrared emitter and detector in our pick-off circuit. The output from the latter is fed to a squaring circuit which gives two complementary outputs I and V (Figures 2 and 3). Each of the sharp pulses which trigger the photomultiplier gate is produced by a combination of two integrated circuits-a IVAND gate and a monostable multivibrator (MS). Depending on whether prompt or delayed emission is to be studied (uide infra), output I or V is fed into a NASD gate. The output of the NAND gate is an exponentially rising voltage (whose rise time is determined by the RC time constant a t the output) and is applied to the input of an MS. The latter gives a sharp (30 nsec) negative pulse (the “on” trigger) when the voltage a t its input exceeds the threshold voltage (for “High” input state). The delay between the trailing edge of the square pulse and the sharp pulse from the MS is WYSRn’Cn ( n = 1,2), where R,‘ denotes the parallel combination of Rn and 4 KR and Rn(minimum) = 330 R. The sharp pulse is also fed to a flip-flop made by combining two NAKD gates. One of the outputs of the flip-flop is connected to a second delay-MS combination to generate the “off” trigger. Most phase-sensitive detectors are gated a t half-cycle intervals and require the reference waveform to be symmetrical. Output I or V can be applied to the reference channel of such instruments. However, the reference channel of some phase-sensitive detectors will accept any waveform which crosses its mean value twice per period; when an instrument of this type is available or when boxcar integration is employed, one of the outputs of the flipflop, IV, should be connected to the reference channel. A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
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Xenon Sample
Photomultiplier
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11
Figure 1. Block diagram of t h e i n s t r u m e m Light 'on'
I
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"-t
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:
IV
II 111
I
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I
n
; n.
I I
IV
Figure 3. Square-wave generator and trigger-generator A:
'/* SN7413N; B: '14 SN7401N; MS: SN74121 N; C: SDA2O (Texas Instruments); All IC's: Pin 7 to 0 V; pin 14 to + 5 V
prompt fluorescence signal measured with the photomultiplier gate held “off” was 10-3 times the signal measured with the ungated (i. e., permanently ‘‘on’’) photomultiplier. To improve the on/off ratio, we inserted a second gate between the seventh and the eighth dynode (d, and ds). With the two gates working simultaneously, the on/off ratio was found to be 106 which equals that obtainable by the most efficient mechanical shutters ( 4 ) . Gate-Removing Switches. I t is desirable to have an instrument where the photomultiplier gate(s) can be removed and re-inserted easily. We therefore incorporated two toggle switches which enable us to by-pass the gates. Figure 5 gives the wiring diagram for the photomultiplier bleeder chain, the gates, and the switches; the symbols A, B, C, D, E, and F have the meaning assigned to them in reference ( 3 ) .To illustrate the function of the switches, it is enough to indicate only the zener diodes in the gating circuits; when a switch is “up,” the corresponding gate is by-passed and the two dynodes remain permanently on.
u
I PROCEDURE
Figure 4. Waveforms of the photomultiplier signal during adjustment of the instrument
When a double beam or a dual trace oscilloscope is available, the experimental procedure is simple and self-evident. The following account is intended for those with access only to a single beam oscilloscope. Measurement of Phosphorescence or Delayed Fluorescence during the Dark Period. ( a ) Connect the output of the pulseshaper (I) to the input of Delay t d . ( b ) P u t SIand SZ in position “up” (z.e., work with an ungated photomultiplier).
See text for expianations
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d
LJ
6 LJ
~ d g d g LJ LJ LJ LJ
43k
60k
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Figure 5. Wiring diagram for the photomultiplier,gate-removing switches, and the gating circuits
Photomultiplier: 9558 QB ( E M I ) ; zener diodes: 2 , = ZY150 (ITT), Zr = Z Y l O O (ITT): switches SIand load resistors are: 100 51. 1 KO. and 10 KC1: inter-dynode resistors are 43 K12, unless otherwise specified
Photomultiplier Gate. The circuit for gating the photomultiplier has been published elsewhere ( 3 ) . At first only one gate was used to switch the photomultiplier off by reversing the voltage between the second dynode (d2) and the third dynode ( d 3 ) . When the photomultiplier gate was opened during the dark Period (Figure 4b)$ Prompt fluerescence (in the “light on” period) of a test sample could still be detected (during the “photomultiplier Off” period) by enhancing the sensitivity of the detection system. The
S2:
two-way two-pole toggle switches: anode
(c) Monitor the fluorescence signal from a highly fluorescent sample on the oscilloscope (Figure 4a). Set the wavelength dial of the emission monochromator a t one of the peaks of the fluoresCencesPectrum. (d) Put S Z in position “down” and increase the sensitivity setting on the oscilloscope by 103. Adjust RI and Rz until a pattern like that shown in Figure 4b appears on the screen, Put SI also in position “down.” The instrument is now set u p for measuring delayed emission, and will produce signals like that of Figure 4c. (4) J.
Langelaar, G.
A.
de Vries, and D. Bebelaar, J . Phys. E,
Sci. In-
strum.. 2 , 149 (1969).
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(e) Use IV or V as the reference signal for the phase-sensitive detector or the boxcar detector, if available. Measurement of Prompt Spectra. (a) Attach the output of the inverter (V) to the input of the Delay t d . Repeat steps (b) and (c) outlined above. (b) Place SI and Sz in position “down.” Adjust RI and R2 until a pattern like that in Figure 4d is seen on the screen. (c) If a phase-sensitive or boxcar detector is being used, connect I or IV to its reference channel.
ACKNOWLEDGMENT One of US (KRN) would like to thank G. J. HoYtink for laboratory facilities and A. Wilson and D. K. Sharma for technical assistance. Received for review December 19, 1972. Accepted FebruarY 16, 1973. This work was supported by grants from the Science Research Council of U.K.
Solid Injection System for Gas Chromatography E. C. Pease Gulf Oil Canada Limited, Research & Development Centre, Sheridan Park. Ontario
During a study of the chemistry of a petroleum process, gas chromatographic analysis of the reaction product was required., The product was a liquid which was intimately mixed with the process catalyst. The liquid product contained components boiling as low as ethane and had a final boiling point of approximately 900 O F . The extraction of the product from the catalyst without loss of the lowboiling components was not possible. Therefore, to examine the product by GC without loss of light ends required that the mixture of liquid plus catalyst be placed in the chromatograph. With the sample precooled to dry ice or liquid nitrogen temperatures, loss of volatiles could be reduced still further. To meet these requirements, a solid injection system was constructed and is described here.
bers are isolated from each other by a ball valve, Hoke valve, Model 7115G4B. The procedure for injecting the reaction product was as follows. The sample, cooled in dry ice, was packed into the sample holder, also cooled in dry ice. The steel rod carrying the holder was inserted through the “0” ring seal on the outer chamber and the retaining nut made finger tight. The ball valve was opened and the rod advanced rapidly until it encountered the column inlet. By suitably adjusting the length of the sample holder, the sample could be placed a t any point within the heated zone of the column inlet, as shown in Figure 1. Engraved marks on the steel rod were used to indicate the position of the holder tip in relation to the ball valve and injection port.
STANDARD INJECTION
CARRIER GAS I N
Figure 1. Solid sample injection
PORT
A SSEM BLY
system
The construction of the injection system is shown in Figure 1. The system provides for on-column injection of solids without interruption of column gas flow and the sample may be precooled as required. The assembly was fitted to a Hewlett-Packard 7620A gas chromatograph, but may be readily adapted to other gas chromatographs. The sample holder is made from a length of Yle-in. diameter stainless steel capillary tubing machined a t one end to give a shallow cup in which the sample is placed. The end of the Ys-in. diameter stainless steel rod is drilled out so that the Yla-in. diameter sample holder is a tight push fit in the hole. This allows for the ready interchange of sample holders of different configurations and capacities. The injection port consists of two chambers; one connected to the chromatograph injection block and open to the column, the other open to atmosphere. The two cham-
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The rod and holder may be left in the injection port during the run, or can be withdrawn without interrupting the analysis. Since its construction, the injection system has been used, without modification, for such solid samples as wax, pitch, contaminated molecular sieves, and coke extracted from cracking catalysts. Other possible applications include the GC analysis of contaminated soil and sand and marine oil spill samples which frequently contain a variety of solid material. The injection system is simple, yet effective. The system is inexpensive, being fabricated from standard parts, and can be attached to most gas chromatographs. Received for review January 31, 1973. Accepted April 18, 1973.
