quoted above do not necessarily represent the maximum practical limits for the double septum, although as the pressures and temperatures are increased, a shorter septum life is to be expected. Finally it should be mentioned that in our experience an average of 80 injections may be achieved per syringe before failure occurs.
ACKNOWLEDGMENT
Permission to publish this paper has been given by The British Petroleum Company Limited. Thanks are also due to A. F. Harding for assistance with construction of the injectionport. RECEIVED for review December 7, 1971. Accepted February 14, 1972.
Versatile Low Cost Laboratory Integrator Donald R. Kendalll Atomics International, A Division of North American Rockwell Corporation, Canoga Park, Calif. 91304
VARIOUSTECHNIQUES have been applied to the integration of electronic signals in the analytical chemistry laboratory. These techniques include ball and disk mechanical, low inertia motor, electrochemical, analog to digital conversion followed by counting, and operational amplifier. The first four types of integrators above were evaluated by Sawyer and Barr for use in gas chromatography ( I ) . These four types of integrators have certain disadvantages, such as high cost, indirect readout, insufficient accuracy and precision and/or non-applicability to all types of laboratory signals. Operational amplifier integrators for short term integrations have also been described (2, 3). A low drift, chopper-stabilized operational amplifier used for the integration of long term signals is described by Harrar and Behrin (4). The state of the art of fabrication of solid state operational amplifiers has advanced to the point where integrators of excellent precision and accuracy can be produced at a relatively low cost. Operational amplifier integrators also possess the advantage of a direct readout which can, if desired, be easily adjusted to represent concentration units.
Figure 1. Operational amplifier integrator model circuit
Figure 2. Operational amplifier integrator circuit showing bias currents
THEORY
An operational amplifier integrator model circuit schematic diagram is shown in Figure 1. The output of the circuit E,,,, is related to the input Ei,,by Equation 1 . 1
r t
,
El
By using the operational amplifier ideality assumption, = E1,it can be shown that
r t
iRedt
+ RC
lt
E,, dt (1)
The first term of Equation 1 represents the desired output while the second through fourth terms represent output error created by integration of input bias current to the inverting input ib-, current leakage through the integrating capacitor iRc, and integration of the offset voltage E,,, respectively. The two bias currents for the two input transistors iband ibt are similar in magnitude. The error caused by integration of ib- can be significantly decreased by causing ib+ to flow through a resistance. Figure 2 shows an operational amplifier integrator circuit with grounded inputs in which such a resistance, R2, is included, and in which an EOcaused only by bias currents is considered. Present address, Allied Chemical Company, P. 0. Box 2204, Idaho Falls, Idaho 83401. (1) D. T.Sawyer and J. K. Barr, ANAL.CHEM., 34, 1213 (1962). (2) J. R. Barnes and H. L. Pardue, ibid., 38, 156 (1966). (3) H. V. Malmstadt and C. G. Enke, “Electronics for Scientists,” W. A . Benjamin, New York, N. Y . , 1963, p 356. (4) J. E. Harrar and E. Behrin, ANAL.CHEM., 39, 1230 (1967).
Eo = i b f Rz
+ CR1 Rz
-
s
ib+dt
-C
s
ib- dt
(2)
By making R1 = R1 in Figure 2 and noting that the offset, = ib- - ib+, Equation 2 simplifies to Equation 3.
or difference current io.
EO = ib’R2
’s
-C
io, dt
(3)
With a value for C of 1 pF and bias current magnitudes of a few picoamperes, the first term of Equation 3 becomes negligible compared to the second term after a small portion of the integration period has passed. The equation for EO due to the integration of bias currents then becomes EO =
‘s
-C
io, dt which is significantly less than the EO =
‘S
- --C
ib- dt obtained in the absence of Rz. This holds true for both short term integrations where Rz would, for example, equal l o 3 ohms, and long term integrations where R2 would, for example, equal 106 ohms. The error caused by the integrating capacitor leakage resistance can be made negligible by employing a high quality dielectric material such as polystyrene, in the capacitor. With a 1-pF polystyrene integrating capacitor (10l2 ohms ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, M A Y 1972
1109
of air were injected into a helium carrier gas stream which flowed into a Carle Model 8000 gas chromatograph equipped with a Porapak column. The peaks from the chromatograph were recorded with a Texas Instruments Model Servo-Riter I1 recorder and simultaneously integrated with a Disc Instrument Co. Model 234A integrator and the Figure 3 operational amplifier integrator operated with 1000-ohm input resistors, The operational amplifier integral was read with a Dana Model 4430 digital voltmeter. To determine the long term drift rate, the Figure 3 circuit using 1-megohm input resistors shorted to ground, was allowed to warm up for 30 minutes, the offset nulled out with Re,and the output monitored with a Dana Model 4430 digital voltmeter. In a similar manner, the ability of the integrator to hold an integral voltage of approximately ten volts was determined by obtaining the voltage on C1,shorting the circuit input, and monitoring the output.
c
In ut
L -
1/10 amp
Power
RESULTS AND DISCUSSION
-15 V
Figure 3. Operational amplifier laboratory signal integrator Table I. Operational Amplifier Integrator Parts List Burr Brown Model 3420K Operational Amplifier 1-pF, 200-V polystyrene capacitor 0 . 1-pF capacitor 1-K, 1 % metal film resistor 10-K, 1 metal film resistor 100-K, 1% metal film resistor 1-Meg, 1 metal film resistor 9.1-K, 1% metal film resistor 0-5 K, 10-turn potentiometer double-pole, four-position switch push button SPST switch SPST switch
leakage resistance) ( 5 ) with, for example, 10 volts on the capacitor, an error of only 0.006z/minute would occur. The problem of eliminating errors caused by the offset current and offset voltage is complicated by the fact that both vary with changes in temperature, time, and power supply voltage. These variations in io, and E,, are collectively called drift. When using an operational amplifier for long term integration, it is necessary, in addition to providing offset voltage nulling, that the initial io, and drift be very low. Recently, improvements in the technology of manufacturing FET input operational amplifiers have produced devices whose offset current and drift are so low that they may be directly used in long term integrators (6). EXPERIMENTAL Circuit. A schematic diagram of the operational amplifier signal integrator is shown in Figure 3 while Table I is a parts list. The power supply was a Semiconductor Circuits Inc. Model 2.15.50K. The two sets of resistors R1 through R 4 and R7 through Rioprovide a range of time constant. The resistors R s and R gprovide a means of obtaining the initial offset voltage null. Evaluation. To determine the precision of the Figure 3 circuit for short term integration and compare it to the precision of a ball and disk mechanical integrator, twenty 50-111 samples ( 5 ) Southern Electronics Corp., Burbank, Calif., Data Sheet P-68 (1968). (6) Burr-Brown Research Corp., Tucson, Ariz., Data Sheet PDS-255 (April 1971). 1110
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
The relative standard deviation of the Figure 3 laboratory signal integrator for short term integrations was 0 . 8 3 z while that obtained with the mechanical type of integrator was 2.1 %. A portion of these deviations was caused by the uncertainty in the volume of air injected. As can be seen from Equation 1, a small R has the effect both of increasing the sensitivity (value of Eo obtained by integration of E,,J of the integration and of increasing the error due to integration of the E,, present because of drift. Thus, when using an operational amplifier to integrate signals, as small an R should be used as is possible without introducing an undesirable amount of offset voltage integration error. By switching in various input resistors, it is possible to use the operational amplifier integrator for various analytical chemistry signal integration requirements. Thus, for example, while using the Figure 3 operational amplifier circuit with 1000-ohm input resistors for the integration of the gas chromatographic signals above (1-2 millivolt peaks of several secEo( volts) _ _ _ -onds duration) an excellent sensitivity, E,,t (volt-seconds) 1
- (sec) = 1000, with better than 1 precision was obtained. RC The size and expense of low leakage integrating capacitors precludes varying C instead of R to obtain different integration time constants. Care must be taken when using the operational amplifier integrator to use an input resistance R, which is sufficiently large compared to the signal source’s output resistance that an undesirable amount of “loading” does not occur. This prevention of signal degradation by loading is particularly important in integrating the output signal from a controlled potential coulometer where the integral is to be directly and quantitatively related to the number of coulombs passed in the coulometer cell. If it is desired to integrate a signal from a very high output impedance transducer, it would be necessary to preface the operational amplifier integrator with a low drift type of voltage follower. For the integration of long-term signals, 1-megohm input resistors would be utilized to reduce the error due to integration of offset voltage generated during any long-term drift. The long term error due to integration of the offset current could of course be reduced by use of a larger integrating capacitor. However, low leakage integrating capacitors larger than 1 pF are quite expensive and the amount of error incurred with a 1-pF capacitor would better be made acceptable, if possible, by using an operational amplifier with a sub picoampere offset current.
The mean long term drift of the Figure 3 operational amplifier integrator starting from a null was 0.4 mV/15 minutes while the mean long term drift starting with 10.000 volts on the integrating capacitor was 0.001 volt/l5 minutes. CONCLUSIONS
Operational amplifiers with Field Effect input transistors are now available for performing reliable integrations of chemical laboratory signals without requiring chopper stabilization. Although the integrator described here, which uses a Burr-Brown Model 3420K operational amplifier, possesses drift characteristics which are acceptable for most
applications even less drift may be obtained if desired by use, at a slightly greater cost, of a Burr-Brown 3420L operational amplifier. The total material cost of the laboratory integrator including power supply and cabinet is approximately one hundred and fifty dollars. Because of the small number of components and simplicity of the circuit, construction of the integrator is a relatively easy task. Care should, however, be taken during construction to provide proper shielding, particularly of the summing point circuitry from noise pickup. RECEIVED for review September 24, 1971. Accepted December 14, 1971.
Inexpensive Mercury-Specific Gas Chromatographic Detector James E. Longbottom Enuironmental Protection Agency, National Encironmental Research Center, Analytical Quality Control Laborator),, Cincinnati, Ohio 45268
MERCURY in some form or another has been found present in most phases of our environment. After Jenson and Jernelov ( I ) established that methylation of the inorganic form of the compound t o the toxic methyl mercury can occur in natural waters, the heretofore unrestricted dumping of elemental mercury into our streams was sharply curtailed. Massive surveys are currently under way to assess the extent of mercury pollution and t o further define the complex interrelationships between inorganic and organic forms of mercury. The gas chromatograph has been used t o identify specific organomercurials. Westoo ( 2 , 3 ) and later Newsome ( 4 ) reported the separation and detection of methyl mercury salts as the chloride using an electron capture detector (ECD). Nishi and Horimoto ( 5 , 6 )and Sumino (7,8) used the ECD for detecting a wide variety of alkyl and aryl mercury salts, while Tatton and Wagstaffe ( 9 ) used the ECD after first forming the dithizonate derivatives of organomercurials salts. The direct determination of dimethyl mercury and similar non-salts was virtually ignored, however, until Bache and Lisk ( I O ) reported the chromatograph of dimethyl mercury and the detection of many organomercurials with the mercury-specific emission spectrometer. This detector gave interference-free chromatograms of dimethyl mercury and the organomercury salts without risk of the detector poisoning reported with use of the electron capture (9). Our intention a t the National Environmental Research Center, Cincinnati, was to achieve the sensitivity and selectivity of the emission spectrometer for considerably less cost by adapting mercury meter t o a gas chromatograph. The meter used as the detector was a Coleman Model 50 Mercury Analyzer System, designed specifically for use with ( 1 ) S. Jensen and A . Jernelov, Nature: 223,753 (1969). ( 2 ) G. Westoo. Acto. C ~ J WS cZn. d . , 20, 2137 (1966). ( 3 ) [bid., 22.2277 (1968). (4) W . H. News0me.J. Agr. FootiC/rcni..19,567:(1971). ( 5 ) S. Nishi and Y . Horimoto. J o p . A I I N I 17, . . 75 (1968). (6) [bill.. p 1247. (7; K.Sumino. KobeJ. ,ck~~tl.Sci..14,115(1968). (8) [bid., p 1 3 1. (9) J. O'G. Tarton and P. J. Wagstaffe, J . Cl?ron?atogr.,44, 284 (1 969). ( I O ) C. A . Bache and D. J. Lisk. ASAL.CHEM..43,950(1971).
FROM
FID
7
e+=-
>
e
-
MAGNESlbM PERCHLORATE
iANH 1
u i ' Figure 1. Condensor assembly used for drying flame effluent before it enters the detector E X H A U S T ( P U M P OFF1 AIR I N T A K E ( P U M P O N )
-
L
TO C O N D E N S E R (PUMP ON1
Figure 2. Tee arrangement for exhaust dilution or vent the Hatch and Ott (11) wet chemical method for determining total mercury. The instrument consists of a pump that draws vapor through a 15-cm cell where UV absorbance is continuously monitored at 254 nm. The system is very sensitive for elemental mercury but requires the prior reduction of all mercury to the elemental state. This was accomplished after (1 1) W . R. Hatch and W . L. Ott, ANAL.CHEM.; 40,2085 (1968). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
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