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Figure 1. Coupling device construction details (not to scale) tained with a variety of column flow rates and the very low air peaks in the acquired mass spectra. Further evidence of the importance of the make-up gas is the constant source pressure over a wide temperature program (150 "C) of the column which would reduce the flow rate of the column. The control of the solvent is accomplished with the vacuum pump connected to the other side of the union cross. When the needle valve is open, the vacuum pump can completely handle the helium flow rates a t the platinum crimp and compete with the ion source pump, thereby reducing the source pressure to less than 1 X Torr. When the needle valve is closed, the original flow is created at the interface and the ion source pressure is adjusted accordingly. During injection of a sample, the needle valve is wide open, allowing the by-pass pump to evacuate the union cross and keep much of the solvent from the ion source. This arrangement also protects the ion source from unusually high concentrations of any components in a sample. I t has been used successfully to obtain excellent mass spectra on minor components eluting on the tail of major peaks by controlling the amount of flow directed to the ion source until the proper time. Figure 2 shows chromatograms of a sample run using an FID of the gas chromatograph and, for comparison, a run with the column coupled to the mass spectrometer with the described interface. The mass spectra taken to examine the background with the interface in use revealed no discernible contribution from any portion of the connecting materials. These background
Figure 2. Comparison between the FID chromatogram and the GUMS chromatogram of the same mixture
spectra were compared to those using a large bore capillary column that could be threaded over the platinum capillary, thereby omitting the Teflon tubing connection. This system as described has the following advantages: 1) A high yield of sample from the column as a result of the closed system. 2) Complete control of flow to the ion source. 3) Extremely leak-free atmosphere in terms of column connections. 4)Low dead volumes and comparable GC/MS traces relative to FID runs. 5 ) Relatively simple construction and ease of column change, yet providing suitable mechanical strength and inert path to the ion source. Even in the case of column breakage, the ion source is completely protected. The system has not needed any repairs or maintenance since installation over a year and a half ago.
ACKNOWLEDGMENT We thank Courtney I. Sadler for the drawing of Figure 1, and Jackie L. White for machining the ferrules from the septum material. LITERATURE CITED (1) D. Henneberg, U. Henricks, and G. Schomburg, Chromatographia,8, (9), 449 (1975). (2) N. NeunerJehle, F. Etzweilar, and G. Zarske, Chromatographia, 6,5(1973). (3) K. Grob and G. Grob, J. Chromatogr., 62, 1 (1971). (4) P. Schultze and K. H. Kaiser, Chromatographia,4, 381 (1971). (5) K. Grob and H. Jaeggi, Anal. Chem. 45, 1788 (1973).
RECEIVEDfor review February 10, 1976. Accepted April 2, 1976.
Bipolar Averaging Circuit for Enhancing Signal-to-Noise Ratios in Recorded Spectra John A. Wehrly," J. Fenton Williams, David M. Jameson, and David A. Kolb Department of Biochemistry, University of Illinois, Urbana, 111. 6 180 1
In spectrophotometric measurements, the signal-to-noise ratio often determines the practical sensitivity of an instrument. Since the SIN ratio is proportional to the square root of the number of samples, averaging data may effect a dramatic improvement in spectral quality. A circuit capable of averaging 10 or 100 positive voltage signals was described in the Designer's Casebook section of Electronics ( I ) . We describe here a circuit designed for a ratiometric spectrofluorometer ( 2 ) which permits bipolar averaging of 100 signals a t a variable sampling rate (See Figure 1). 1424
ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
Emitted light, normally collected a t right angles to the excitation direction, provides the sample signal while a small fraction of the exciting light is directed to a quantum counter to provide a reference signal. The bipolar capability of this circuit facilitates the precise adjustment of phototube dark currents and amplifier offsets. The ADC provides a digital output corresponding to the ratio of the two analog input signals. Although designed for an Analog Devices Model ADC-17141h digit BCD output analog-to-digital converter, the circuit is easily adapted to any
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Figure 1. Circuit diagram of bipolar averager Inputs to latches L1 L3 are the BCD outputs of Counters C9 C11. Inputs to the ADC are the amplifier outputs from the signal and reference phototubes. Outputs from flip-flops FF3 and FF4 provide the switching pattern necessary to drive the stepping motor. The DAC output is monitored by a chart recorder. The components of the circuit are: ADC: Analog Devices ADC 171, DAC: Analog Devices DAC-1009, VCO: lntersil 8038, C1 C12: SN74192, C13: SN7490, C14: SN7493, G I G6, G8 C14: SN7400, G7: SN7410, MS1 MS8: SN74123, DSI DS5: SN74157, FFI FF4: SN7476, L1 L3: SN7475, DISPLAY: TIL31 1, and -SIGN: G CATHODE OF OPCOA SLA-1
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ADC with binary or BCD output. A DAC (Analog Devices Model 1009) supplies an analog output of the averaged ratio which is monitored by a strip chart recorder (Honeywell Electronik 19). All I.C.’s used (except the VCO) are in the Texas Instruments SN7400 series. The ADC sampling rate is controlled by a VCO (Intersil8038); the strobe rate may thus be varied to collect 100 samples in a few seconds (or faster with a successive approximation converter) or as slowly as deemed
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pratical. The strobe pulse initiates the ADC conversion. Completion of conversion sends the STATUS bit high, triggering a monostable (MS1) which sends a load pulse to counters C1 to C5. The ADC output is loaded into the counters’ preset inputs. The counters then count down from the preset condition to zero, at which point the borrow output from the most significant bit goes low, disabling the clock through flip-flop FF1 and gate G7. The next strobe pulse resets the STATUS bit initiating the next conversion. Counters ANALYTICAL CHEMISTRY, VOL.48, NO. 9, AUGUST 1976
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Figure 2. Fluorescence emission spectra of in carbonate buffer
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C13 and C14 tally the STATUS bit and stop data collection after 100 conversions. In addition to toggling counters C1 to C5, clock pulses also enter the up/down counters C6 to C12. The contents of the five most significant counters (C8 to C12) are displayed, showing the average of 100 voltage signals. The flip-flop (FF2) and NAND gates G1 to G4 control the count direction of counters C6 to C12 as required by the voltage polarity. For example, if the first signal is positive, the count direction is up. If the next signal is negative and of greater absolute value than the first signal, count direction is down until the counters reach zero. At this point, the borrow output of counter C12 toggles FF2 which, acting through gates G1 to G4 and the data selectors (DS1 and DS2) reverses the count direction. The output of FF2 always follows the sign of the averaged signal. One may now manually reset the counters to collect another 100 average, In the chart mode, however, monostable MS6
provides reset pulses after the completion of each 100 average. T h e reset mode is determined by data selectors DS3 to DS5. In the average mode, the display latch is always low, allowing the display to continually monitor the contents of counters C8 to C12. In the chart mode, the display changes only when a new 100 average is completed. Following each 100 average, monostable MS7 sends a load pulse to latches L1 to L3 which then load the new average from counters C9 to C11 into the DAC. The DAC's output then supplies the signal for the chart recorder. The outputs of monostables MS3 and/or MS5 supply pulses which, acting through flip-flops FF3 and FF4 provide the switching pattern necessary to drive the Slo-Syn Model SS50-1009 DC bifilar stepping motor. This motor features 200 steps per revolution which permit steps of 1nm when coupled to the 200 nm/turn wavelength selector on the Bausch & Lomb monochromator. Steps of 2 nm each are accomplished by allowing a second pulse, delayed approximately 10 ms from the first pulse, to enter the switching circuit through gate G13. The improvement in signal-to-noise is readily apparent in the spectra shown in Figure 2. Fluorescein a t M in p H 8.2 carbonate buffer was excited a t 480 nm and scanned from 490 to 580 nm. High voltage (-2000 V) on the photomultiplier tube (EM1 62568) and high gain on the amplifiers were necessary to detect the very weak emission. Under these conditions, a direct scan gave the somewhat noisy result shown in spectrum A. The scan was repeated with the same instrument settings but using the averaging circuit to yield spectrum B which, because of the considerable noise reduction, enabled more accurate determination of the peak area and the wavelength maximum.
LITERATURE CITED (1) G. Mitchell and R. D. Spencer, Electronics, 46, (9), 103 (1973). (2) G. Weber and L. B. Young, J. Blol. Chem., 239, 1424 (1964).
RECEIVEDfor review January 14, 1976. Accepted April 1, 1976. This work was supported by Grant GM 11223 from the National Institutes of Health.
Active, Modular, Digital Data-Collection System Clive Way-Jones* and Leslie Glasser Departments of Physics and Chemistry, Rhodes University, Grahamstown 6 140, South Africa
Available digital data-collection equipment may be classified into a few general types. The simplest type is the entirely passive, point-by-point digitizer triggered a t fixed time intervals or by some event within the instrument to which it is slave; a rudimentary example is the digital printer attached to a mass balance, while a much more complex example is the transient recorder with continuously circulating memory and pre-trigger. At the other end of the scale of versatility and sophistication is the on-line computer system with programmable operation and control feedback. The system described in this paper lies between these levels of instrumental capability, having a small repertoire of hard-wired programs capable of actively driving an instrument, and with a circulating memory which acts as a transient recorder, and providing output in the form of punched paper tape; but without the expense, interfacing, and programming problems of a fullscale computer system. While the system was built with a specific purpose in mind (time-domain spectrometry, see 1426
ANALYTICAL CHEMISTRY, VOL. 48,
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