Table I. Measured Resistance Resistor value. Resistance Range setting mega displayed, megn (S7-1, me@ 10.0 i 1% 10.7 i 0 10-100 10-100 26.5 i 0 25.0 1 1% 100i 1% 96.5 i 0 10-100 IWIOOO 500 i 2% 480 1 1 lOOoi:l% 952 i: 3 1w1000 1ooo- 10,ooo 3040 1 10 2950 =t1% 1000-10, OOO 5OOOi 2 % 4830 i 30 1m10,000 9200 i 200 10,OOO i 4% corresponds to 10 V or nearly full output at A4) can be automatically blanked by connecting the logic output for the fourth digit to the external blank input of the meter through deck E of S1. The latter permits blanking only when the instrument is in the resistance measurement mode. As stated previously, the lower limit of reading for a given resistance range should be 10.0 on the display of the DPM in order to minimize errors associated with the divider amplifier. The requirements for amplifiers A1 and A2 are stringent; they must have low bias current and high input impedance. Although such units are available commercially at reasonable prices, we have constructed our own by interfacing a National Semiconductor LM3OlA opamp with a Siliconix U251A FET pair. The latter is dual junction FET with typical gate current of well under 1 PA. The circuits for the voltage and current followers are presented in Figure 2. Amplifier A3 is a n LM301A and the analog divider A4 is a Hybrid Systems Corporation Model 107C. The gain adjust terminal on A4 was not utilized since adjustment of P2 has essentially the same effect. The balance adjustment on the divider can be utilized to provide slightly higher accuracy. Feedback resistors for A2 should be of good quality; Victoreen 2 units were used. The capacitor, C1, must be a low leakage type (polystyrene for example). There are two routes which one may take with regard to the switch, S1. Insofar as high impedances are being switched, leakage must be kept to a n absolute minimum. The first method which was employed, one which has provided several months of steady, trouble free service, utilized a Centralab rotary switch (Steatite) with three decks, each two-pole six-position. Before assembling the switch, all surfaces of each wafer (excepting switch contacts, of course) were coated with Dow Corning No. 630 protective coating to minimize leakage due to moisture absorption. A second, more reliable technique, is to use reed relay switches activated via S1 to switch the high impedance points. This is the method diagrammed in Figure 1. All exposed terminals were coated with Dow Corning No. 630 after connections had been completed. Switch NO. 2 can either be a Centralab rotary switch or Grayhill miniature rotary switch. The high impedance gates of the FET's were tied directly to Teflon (Du Pont) stand off terminals to minimize leakage. In addition, the underside of each FET was cleaned and coated with the Dow Corning No. 630. Common sense precautions must be taken to minimize ac pickup by the high impedance areas of the circuit. The convenience of commercial encapsulated power supplies prompted us to use the Analog Model 902 ( 1 1 5 V at 100 mA). The operating procedure for the instrument is quite simple After allowing five minutes for warmup, the electrode to be tested is plugged into the instrument. Potential measure190
ments in the desired pH buffers are made with Switch S1 in position 1. The resistance measurement is obtained by switching S1 from position 1 to position 3 after the potential reading has become nearly steady. S2 should initially be set at position 1, the 10-100 megQ range. If the display blanks, S2 can be switched to the next higher range until a reading is obtained. It is not necessary to reset S1 from position 3 to position 1 after adjusting S2. Calibration of the instrument is straightforward: With the input shorted, the voltage follower, A l , can be zeroed via P3. At the same time the output of A3 can be set to 100 mV by means of P1. With S1 in position 1, the output of A2, the current amplifier can be zeroed by means of P4. The adjustment of P2 for the divider unit is best made by using two precision resistors, such as 10 mega and 90 megn, byadjusting P2 for the best compromise. The accuracy of the potential measurement portion of the test instrument was checked using a recently calibrated Analogic Model AN3100 voltage reference standard. Its output was rechecked using a recently calibrated Data Technology Model 350 Digital Voltmeter having a millivolt preamplifier plug-in permitting resolution of 100 pV. The deviation between input voltage and DPM display on the test instrument did not exceed 0.1 mV over the fullscale range of i19F' 9 mV. In addition, no deviation in excess of 0.1 mV was noted upon introducing series resistance up to 3000 mega. The accuracy of the resistance measurement was checked in the following way: 1 and 2 x standard resistors ranging from 10 mega to 10,000 mega were used to stimulate the electrode. The measured resistance has been tabulated in Table I. The readings were unaffected by introducing voltages of up to 500 mV in series with the resistors. ACKNOWLEDGMENT
The author wishes to thank William Maxwell for many helpful discussions. RECEIVED for review April 5 , 1971. Accepted August 6, 1971.
Corrections Gas Chromatography of Monoolefins with Stationary Phases Containing Rhod ium Coordinat ion Compounds In this article by E.Gil-Av and V. Schurig [ANAL.CHEM., 43, 2030 (1971)l on p 2030, column 2, paragraph 3, line 5 should read ". . . and 2.84 grams of Chromosorb (85z).. . " The equation in footnote d of Table 11, p 2031, should be - ro X K = r__._ The subcaption to Figure 1 , p 2032, in rg 0 OS line 2 should read ". . . emerged at 1.35, 3.45, and 9.8 min. . . " In the Acknowledgment, p 2033, the correct spelling in line 2 is Mr. FrantiEek Mike:. ~
Any specific article cited in the December 1971 issue which has a page number higher than 2037 will appear in the January 1972 issue.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
