was not used. With a different lot of 27 samples, the SPADNS mean was 0.38 and the electrode 0.28. Without TISAB, the standard deviation was 0.14 ppm.
We that the approach make possible direct concentration readings for low levels of fluoride in a wide variety of aqueous systems. Aside from the examples cited here, we believe that the same approach can probably be extended in fluoride determinations in urine, or from scrubbers which monitor for air pollution.
ACKNOWLEDGMENT
The generous cooperation of Robert C. Kroner of the Federal Water Pollution Control Administration and Robert M. Scott of the Illinois Department of Health is gratefully acknowledged, RECEIVED for review December 27, 1967. Accepted March 8, 1968.
Logarithmic Attenuator for Use with Flame Ionization Detector Gas Chromatographs Karl A. Chen E . I. du Pont de Nemours and Co., Organic Chemicals Dept., Jackson Laboratory, Wilmington, Del. POWER SUPPLY
THEHIGH VOLUME of work, performed in this laboratory with a gas chromatographic-time-of-flight mass spectrometric technique, clearly established the need for automatically attenuating the flame ionization detector signals. Proper selection of column and chromatographic conditions for the meaningful resolution of components in wide range mixtures had to be established before identifications could be attempted. The proposed attenuator allows the operator t o perform more immediate work on the mass spectrometer while a chromatogram, reflecting the choice of column and conditions, is being automatically recorded. Furthermore, such an attenuator is also quite useful when a computer or integrator is being used. I n such cases, the recorded profile provides a check on the proper resolution and operation of the gas chromatograph. The most common type of automatic attenuators are electronic or motor-driven stepping switches used to switch voltage divider circuits automatically ( I , 2 ) . Such devices are very reliable; and, although they are used extensively with thermal conductivity detectors, they cannot be used with the electrometer amplifiers which are used in conjunction with flame ionization detectors. On the Perkin-Elmer Model 800 gas chromatographs, attenuation is accomplished by two mechanically coupled switches which are manually operated as a single switch on the front panel of the instrument. One switch selects the feedback resistors to the amplifier. The other activates the relays for selecting the proper high input grid resistances for the electrometer. This arrangement creates a problem recognized as quite common to many flame-ionization detectors. T o cover the wide dynamic range of these detectors, the high input grid resistances must be changed or the electrometer will “saturate” or self limit. This occurs when the detector produces more output than the electrometer can handle; the voltage is then unable to respond to excitation in a proportional manner, and the electrometer output would not represent the total input. However, switching of the high impedance input circuit creates switch transients which are transmitted to the amplifier by the feedback loop and which would be considerably increased if the attenuator were external to the (1) D. J. Darling, F. D. Miller, R. C. Bartsch, and F. M. Trent, ANAL.CHEM., 32, 144 (1960). (2) J. F. Johnson, R. F. Klaver, F. Bauman, and J. Y. Beach, Jourke’s Inter. Etudes Separation Immediate Chromatogr., 1961, p 136 (published 1962).
r-l ELECTROMETER
NULLING VOLTAGE
f
Figure 1. Functional block diagram of modified electrometer system
electrometer amplifier. Thus, it became quite evident that a different type of attenuator was required. One device which provides a satisfactory solution to this problem is an automatic knob turner (3). An alternative solution is t o bypass completely the existing electrometer amplifier unit. The latter solution was chosen in this laboratory. A functional block diagram is shown in Figure 1. The detector signal is fed into either the existing electrometer amplifier (below the dotted line) or the electrometer-logarithmic amplifier system developed. A detailed diagram of the circuitry is shown in Figure 2. The electrometer is essentially a current-to-voltage transducer, or current amplifier, whose output voltage is a direct function of the input current. Because the feedback holds the input voltage a t a virtual ground, the output voltage is developed a t a low impedance and high energy level. To accommodate the wide dynamic range of the flame ionization detector, an “FET” operational amplifier, Melcor 1619, is used because of its high input impedance and its low current (3) F. Bauman, F. A. White, and J. F. Johnson, ANAL.CHEM., 34, 1351 (1962). VOL. 40, NO. 7 , JUNE 1968
1171
Figure 2.
R1 = 108R R* = 104 R Pj, P2,P3,Pq
=
Electrometer-logarithmic amplifier
10 K
A I = "FET" operational amplifier, Melcor 1619, open loop gain of 50,000; V/day; input impedance of 1010R drift of 30 X V/"C, 100 X Az = Nexus SA-3 operational amplifier, open loop gain of 30,000; drift of 15 X 10-8 V/'C, 30 X 10V V/day; input impedance of 1011 n L = Nexus LGP-4 logarithmic module, input current range 10-8-10-3 A, drift of 180 X 10P V/"C
characteristics. This stage is mounted in a shielded box adjacent to the existing electrometer with the necessary switching to allow regular use of the instrument. A coaxial cable connects the electrometer stage with the main chassis. Output voltage, V,,, of the electrometer for a given input current, If, is related to the feedback resistance, Ri, by V, = Zi, x R1. This produces a potential 10"l dynamic signal range through its ability to handle input signals from 5 X lO-'3 A to 5 X 10W A. The upper input level develops an output voltage equal to (5 X 10+ A) X ( 2 X lo8 Q) = 10 V, the maximum output of the Melcor 1619 before saturation, and limits the sample sizes to those which would produce signals less than 5 X lo-* A. The logarithmic amplifier consists primarily of a Nexus LGP-4, a miniature encapsulated logarithmic module, and a Nexus SA-3 operational amplifier. A 10-K input impedance converts the electrometer output voltages to suitable current inputs to the LGP-4 logarithmic module. The resulting output is a voltage proportional to the logarithm of the independently variable input current. Additional potentiometers provide external trimming to achieve a logarithmic slope of 1 V per decade change of signal current and adjustment of the reference level or base line around a design center of zero output a t 0.1 pA signal level. A 15-V power supply, Philbrick SP-300, located in the amplifier casing, supplies the power to both the electrometer and the logarithmic amplifier. For the LGP-4 log module t o function properly, the input current must always be greater than A. To maintain this current, the offset voltage of the Melcor 1619 "FET" operaV. This biasing tional amplifier is set at a maximum of distorts the smaller peaks somewhat. However, for qualitative purposes, they are still recognizable. Calibration of the output voltage at a nominal transfer characteristic of 1 V per decade of input signal current exhibited approximately deviation from linearity over five decades of input signal. A variable 10-V recorder, with matching impedance, is used to record the output of the attenuator. Calibration of the recorder voltage in terms of chromatographic attenuations allows the expansion of the largest peak to almost full scale by presetting the voltage range of the recorder. A chromatogram of a test solution, using the electrometer-logarithmic amplifier, is illustrated in A, Figure 3. A smooth continuous curve is obtained in comparison to the preattenuated chromatogram B, where the attenuations had been determined through the preliminary run C .
2z
1172
ANALYTICAL CHEMISTRY
100
n""
80
::I
lOOr
70
I
30 20 10
TIME, MINUTES
Figure 3.
Gas chromatograms of test solution A. Logarithmic attenuation B. Pre-set attenuation C. Manual attenuation
Because the instrument is presently being used only as an exploratory tool in developing chromatographic conditions prior to gas chromatographic-time-of-flight mass spectrometric analysis, the quantitative results of the chromatograms were not investigated. However, replicate runs of a test solution produced chromatographic peaks with a precision of + 5 % . Noise and drift were negligible as both were logarithmically reduced. In fact, both noise and drift are still dependent on the characteristics of the detector, column bleed, and pressure changes. Consequently, the application of expensive, low noise techniques to reduce the amplifier noise was unnectssary. Parts for the attenuator cost about $400, and about two days are required for construction. ACKNOWLEDGMEhT
The author wishes to thank Stephen J. Hitzfelder for his assistance in the design and construction of the attenuator.
RECEIVED for review December 18, 1967. Accepted February 26, 1968.
