Anal. Chem. 1985, 57, 1163-1 165 Fenselau, C.; Yergey, J.; Heller, D. Int. J . Mass Spectrom. Ion Phys. 1983, 53,5-20. Dell, A; Taylor, G. W. Mass Spectrom. Rev. 1984, 3 ,357-394. Barber, M.; Bordoll, R. S.; Elliot, G. J.; Horoch, N. J.; Green, B. N. Biochem. Biophys. Res. Commun. 1983, 110, 753-757. Yergey, J. A.; Cotter, R. J.; Heller, D.; Fenselau, C. Anal. Chem. 1984, 56. 2262-2263. Grotjahn, L.; Taylor, L. C. E. Abstracts of 32nd Annual Conference on Mass Spectrometry and Allled Toplcs, San Antonlo, TX, 1984; pp 359-361. COttrell, J. S.; Taylor, L. C. E.; Wllllams, D. H. Abstracts of 32nd Annual Conference on Mass Spectrometry and Allied Toplcs, San Antonlo, TX, 1984; pp 382-303. Barber, M.; Bordoli, R. S.; Elliot, G. J.; Sedgwlck, R. D.; Tyler, A. N. Anal. Chem. 1982, 54,645A-657A. Grotjahn, L.; Frank, R.; Blocker, H. Nucleic AcMs Res. 1982, 10, 467 1-4676. Heller, D. N.; Yergey, J. A.; Cotter, R. J. Anal. Chem. 1983, 55, 13 10- 13 13.
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(12) McEwen, C. N.; Layton, S. F.; Taylor, S. K. Anal. Chem. 1977, 49, 922-928.
Ronald L. Cerny* Michael L. Gross Midwest Center for Mass Spectrometry Department of Chemistry University of Nebraska-Lincoln Lincoln, Nebraska 68588
RECEIVED for review December 26,1984. Accepted February 19,1985. This research was supported by the Midwest Center for Mass Spectrometry, an NSF instrumentation facility (Grant No. CHE-8211164).
AIDS FOR ANALYTICAL CHEMISTS Circuit for Optimally Matching Instrumental Output Voltage Levels to the Input Voltage Levels of Analog-to-Digltal Converters Carl A. Kova1,*Charles J. Rutkowski, and John P. Cowan Department of Chemistry, University of 'Colorado,Boulder, Colorado 80309 An increasingly common laboratory task involves interfacing an instrument with analog output to a laboratory computer for the purpose of digital data storage and analysis. Even when the instrumental output is a voltage and the computer contains an analog-to-digital converter (ADC), the output is often not ideally matched to the input voltage range of the ADC. Furthermore, data from experiments involving small signal changes are often not represented accurately in the computer due to digitizing error. Digitizing error results when meaningful changes in the input signal are smaller than the bit resolution of the ADC. Herein, we report an analog circuit which remedies these two problems; that is, the instrument signal is matched to the requirements of the computer ADC and can be amplified to reduce digitizing error. The general application of this circuit is discussed and a specific example of its application as an interface between the photomultiplier tube (PMT) of a Dunum-Gibson stopped-flow spectrophotometer and a MINC-11 minicomputer is included. Figure 1 illustrates the digitizing error present in measuring a portion of the transmittance curve for a first-order reaction having k = 60 s-* and AT = 0.1 both with and without the described circuit.
EXPERIMENTAL SECTION Circuit Description. The circuit diagram for the interface is shown in Figure 2. The Durrum-Gibson stopped-flow spectrophotometer normally uses the input impedance on a Tektronix RM564 storage oscilloscope to convert the current from the PMT to a voltage. Since our computer software provides extensive graphics capabilities for observing the digitized data, the oscilloscope was removed from the circuit. The option of observing the kinetic data on an oscilloscope for testing purposes was retained. The PMT current flows through precision resistor R1 in the interface to provide the necessary current-to-voltage conversion and op amp OAl serves as a unity gain high impedance buffer. It is possible that a precision resistor would not be the best method of current-to-voltage conversion and that it could introduce external noise. In certain circumstances it would be better to replace R1 with an operational amplifier current-tovoltage converter stage. We chose to keep this part of the circuit
the same as the commercial stopped-flow instrument in order to reduce the number of op amp stages required to implement the more important aspect of the problem which is the voltage range matching provided by the remaining portion of the circuit. R1 is also easily removed for applications in which the instrument provides a voltage output rather than a current output. The overall function of the remaining circuit is to take this voltage from OAl, Vh, and give an output, Vout,according to the equation Vout = [(Vi, - Vinmin+ Voff) X GAIN] + Voutmin (1) This function is shown schematically in Figure 3. The addition and subtraction of the appropriate signals which are then multiplied by a variable gain is implemented by basic operational amplifier analog circuitry (1-3). The minimum input voltage, Vinmin= -1.0 V, corresponds to the 100% T level set with blank in the observation cell or, more generally,to the minimum signal from the instrument. Vinminis provided to the inverting input of OA, either from a simple voltage divider or through a digital-to-analog (DAC) output from the computer (not shown in Figure 2). Voutminis provided to the inverting input of OA6and corresponds to the minimum voltage level accepted by the computer ADC. The signal corresponding to Voutmt"would be obtained in the same way as described above for VhmLn.Since Vhmh and Voutmlnare the same in our particular application, both of these voltages were obtained from the offset voltage divider circuit at the point labeled -1 V SOURCE. The initial adjustment was simplified by this modification of the general circuit. The voltage for the divider and the supply voltages for the op amps are provided by the power supply output (115 V) on the Durrum D-150 temperature jump system through connector J1. The range of input signals is related to the output of the circuit by the gain control, switch S1 RANGE = Vi,' - Vi,- = (Voutmax- Vo,,tmi")/GAIN (2) The quantities VoutmsIand Voutmhfrom the circuit correspond to the maximum and minimum input voltages to the ADC, fl V in this application. With Vi,' = Vinmax(Vinmmis the maximum output from the instrument, 0 V corresponding to 0% Tin this case) and Vi,,- = Vinminthe lowest GAIN for any application is calculated from eq 2. Other values of GAIN are obtained from
0003-2700/85/0357-1163$01.50/00 1985 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985 0.500 T
? ;,
PMT S I G N A L
M MAX VOUT
OUTPUT t o
?
I
W
V
z
2 t 5,Z 2
k-
0 = 4000 s t e p s / 0.1 T
0.502 T
0
,
70
-*-*O
l
I
I
80 90
I
I
100 110 TIME (rnsec)
,
l
-0.5
120
Flgure 1. Error due to digitization of transmittance data from final portion of fiist-order reaction (k= 60 s-', A, = 0.2218, A m = 0.3010): solid line, analog data; filled circles, digital data from simple 1: 1 current-to-voltage converter; open circles, digital data from interface described herein, GAIN = 20, VOlf = -0.4 V.
-0.4
20
S2. This divider is designed to give ten discrete values of offset between Vh" and Vh-. Minor modifications would allow other input levels to be used; however, the MINC DAC is the most general discrete input into OA., that is easily controlled. Once a discrete value of Voffis chosen, the GAIN can be selected by equating the largest input signal to Vh+ and calculating a nominal gain using eq 2. The value of the GAIN from OA3 must be less than this nominal gain. Relating the input voltage region that is to be amplified to the ADC full scale range by linear interpolation gives rise to the equation vi,
- vi,- -
vin+- vi,
Flgure 2. Circuit diagram for instrument-computer interface: OAl, OA,, OAa, OAB, OAT = OP-27EP; OA,, OAS = LM-11CN; D1, D2 = IN4148; R1 = 1 MR; R2, R7, R20 = 10 kR; R3 = 1 kR, 5 % , ' I 4W; R4, R31 = 100 0,5 % , ' I 4W; R5, R6, R13, R22, R23, I325430 = 51.1 kR; R8 = 20 kR; R9 = 40.2 kR; R10 = 66.5 kR; R11 = 100 kR; R12 = 200 kR; R14-Rl8 = 1.1 kS2; R19 = 2.7 kR; R32-R41 = 100 R; R43 = 13.0 kS2; R21, R24 = 100 kR multiturn trlmpot; R42 = 2 kR multiturn trimpot; fixed resistors are 1%, ' I , W unless otherwise noted above; Cl-C6 = 33 pF; C7, C8 = 47 pF at 35 V tantalum capacitors; S1 = DP-6Pos rotary switch; S2 = SP-12Pos
rotary switch. the output of OA, and are the ratio of one of the feedback resiston, R8-Rl2, to the input resistor R7. The particular GAIN values are easily changed to accommodate different instrument-tocomputer matching requirements. The highest GAIN for this single stage should be 1100. An additional op amp could easily be added to provide more gain. When the range of input signals is smaller than Vhm - V.mmint the GAIN is increased from the lowest value using S1. The lower voltage, Vh-, of the range to be amplified is controlled by the offset, Voff,away from the minimum input voltage and is given by the equation v .in- = Vinmin - v,,
-0.6
Function of circuit showing two different instrumental output ranges expanded to full scale input range of computer ADC.
Figure 3.
(3)
Voffmust be chosen so that Vh- is less than or equal to the lowest value of input signal. V,, is provided to the noninverting input of OA4and is obtained from one of the discrete resistors of the calibrated voltage divider or from the computer DAC using switch
vou,- voutmin voutm=
- voutmin
(4)
Substituting eq 2 and 3 into eq 4 and rearranging results in eq 1 which includes the adjustable offset and gain parameters in the relationship between the output and input voltages. Circuit Adjustment and Use. By use of an external voltmeter, the circuit is adjusted as described below. First, R42 is set by trimming the offset voltage divider using the -1 V test point. With switch 52 set to zero offset, R21 is adjusted until the offset obtained at the SENSE OFFSET BNC is also reading zero. The adjustment of the voltage divider also serves to set both Vinmin and VOuth in our application, but in the general case these would be adjusted independently of each other. Op amp OA, is trimmed by varying R24 until the inverse of Vinminis obtained at the test point on the output side of this op amp. After these initial adjustments, the maximum and minimum voltage levels from the instrument are provided sequentially to OAz. In the case of a spectrophotometer, this corresponds to the dark current (0 V, 0% T )and the blank (-1 V, 100% r). With the GAIN at its lowest setting, V,, (OA,) should read Voutmaxand Voutmin,respectively. Data from an experiment can then be recorded using the combination of VoEand GAIN which minimizes the digitizing error in the analog-to-digital conversion step. The ADC measured voltages, Vout,can be related to the input voltages, Vi,, by using eq 1.
RESULTS AND DISCUSSION Application to Stopped Flow. The circuit has been used in our laboratory as an interface between a Durrum-Gibson stopped-flow spectrophotometer and a MINC-11 minicomputer. The circuit takes the linear 0% to 100% transmittance scale which corresponds to the 0 V to -1 V output from the stopped-flow spectrophotometer and matches it to the +1 V to -1 V ADC input on the preamplifier to the computer. The development of the appropriate equation for any particular instrument voltage levels that are to be optimally matched to the available ADC voltage levels is analogous to developing the equations relating different temperature scales by linear interpolation. The additional feature provided by the circuit is the expansion of a portion of one scale onto the other. The
Anal. Chem. 1985, 57, 1165-1167
GAIN and Voffobtained from the outputs of OA3 and OA4, respectively, provide this expansion. The simplifications in the general circuit that were used for this specific interface are noted in the Experimental Section. The gain and offset functions of our circuit could also be obtained by modification of commercial devices to place the choice of these parameters under computer control. It should be noted that application of this alternative approach would require implementation of the logic in Figure 3 and eq 1 in order to provide for electronic ultimate precision spectrophotometry as explained below. Without this circuit, half of the available resolution of the computers 12-bit ADC would not have been used as a result of the mismatch in the output voltage levels on the stopped flow with the nearest input levels on the ADC. More importantly, the kinetics of reactions involving small transmittance changes on the order of 0.1 T can be monitored more accurately regardless of the absolute values of the transmittances as shown in Figure 1. In the absence of this interface, reactions with AT = 0.1 could be divided into 200 measurable steps (0.5 mV/bit) which resulted in appreciable digitization error. With this circuit, 4000 steps (0.025 mV/bit) between the Voutminand Vout- are used to measure the transmittances for such reactions. The 12-bit ADC on the MINC-11 provides 4096 steps between -1.024 V and +1.024 V which results in 4000 steps between the -1 V and +1 V used for the output range in the interface. An extensive software package was written to perform the voltage conversion and kinetic calculations, to simplify the control of the interface by the operator, and to provide graphical representations of the data. The SENSE OFFSET and SENSE GAIN signals are used by the program for calculations and to direct the operator’s use of the interface. From a preliminary kinetic experiment with the offset set to zero and the GAIN set to ita lowest value of 2, the computer calculates the optimum settings for the expansion and prompts the operator to change the settings. Prior to collecting the next set of data, the computer senses the offset and gain switch settings and warns the operator if either the settings are different from the currently recommended values or the measured offset does not appear to be within the tolerance limits for a particular switch position. The former would allow the operator to correct any mistake that may have been made in setting the switches and the latter would inform the operator that a calibration check of the interface should be made before continuing. The digitized output voltages that represent the spectrophotometric data are obtained and converted to absorbance readings for graphic presentation on the computer terminal along with the results of the regression analysis (4-6)used to calculate the rate constants. Electronic Ultimate Precision Method. The circuit, when used as an interface between the stopped-flow spectrophotometer and MINC-11 minicomputer, electronically mimics the method of ultimate precision (7). The ultimate
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precision method normally requires that two solutions of known absorbance, which bracket the expected absorbance of the unknown, replace an occluded light beam and the blank for setting the 0% T and 100% T levels. In this way, the real transmittance of the unknown can be linearly interpolated from the more precise reading made on the expanded scale. It should be noted that the use of isomation with ultimate precision spectrophotometry requires the use of only a single chemical standard and is the most precise method, but this technique is also time-consuming (8). The circuit shown in Figure 2 requires only that the light beam be blocked for setting the 0% T level and an appropriate blank be used for the 100% T level as is done in the normal nonexpanded transmittance scale. The expansion of any portion of the input voltage scale is made electronically with an offset and a gain control which obviates the need for additional standards. Ingle gives a complete error analysis comparing different instrumental factors that affect the relative concentration standard deviation in the normal and various precision spectrophotometric techniques (9). The relative improvement that can be obtained in using the precision methods in place of the normal method under various limiting conditions was also discussed. Both the chemical and the instrumental ultimate precision methods in Ingle’s classification scheme should have comparable precision. The accuracy of the instrumental ultimate precision method might suffer some uncompensated systematic error if the circuit is not calibrated properly. The chemical ultimate precision method also suffers systematic errors in accuracy if the reference solutions are not prepared carefully or if there are unaccounted differences in the cell path length. With proper calibration of the circuit and the use of high precision components in its construction, the instrumental ultimate precision method may even be superior to the chemical method in that comparable precision and sufficient accuracy can be obtained at a reduced expenditure of time and effort.
LITERATURE CITED (1) Vastos, Basil H. Ewing, Galen W. “Analog and Digital Electronics for Scientists”; Wiley: New York, 1972; Chapters 5-7. (2) Malmstadt, Howard V.; Enke, Christie G.; Crouch, Stanley R. “Electronics and Instrumentation for Scientists”; BenjaminKummings Publishing Co.: Reading, MA, 1981; Chapter 5. (3) Diefenderfer, A. James “Princlples of Electronic Instrumentation”, 2nd ed.; W. B. Saunders: Philadelphia, PA, 1979; Chapter 9. (4) Willis, Barry G.; Bittlkofer, John A.; Pardue, Harry L.; Margerum, Dale W. Anal. Chem. 1970, 42, 1340-1349. (5) Mieling, Glen E.; Pardue, Harry L. Anal. Chem. 1978, 50,1611-1618. (6) Koval, Carl A.; Pravata, Robin L. A. Reidsema, Cindy M. Inorg. Chem. 1984, 23, 545-553. (7) Willard, Hobart H.; Merritt, Lynne L., Jr.; Dean, John A.; Settle, Frank A., Jr. “Instrumental Methods of Analysis”, 6th ed.; Van Nostrand: New York, 1981; Chapter 3. (8) Ramaley, Louis; Enke, C. G. Anal. Chem. 1965, 3 7 , 1073-1074. (9) Ingle, J. D., Jr. Anal. Chem. 1973, 4 5 , 861-868.
RECEIVED for review October 22,1984. Accepted January 17, 1985. This research was supported by the National Science Foundation under Grant 8204000.
Pole Bias Scanner Circuit for Quadrupole Mass Spectrometers of Early Design F. Aladar Bencsath,* Yiu Ting Ng, and Frank H. Field
The Rockefeller University, New York, New York 10021 As a rule, the resolution of the quadrupole mass spectrometer is increased for higher masses during the mass scan, which decreases the sensitivity and gives rise to high-mass discrimination. This is further aggravated by the dispersing action 0003-2700185/0357-1165$01.50/0
of the fringing field between source and quadrupole rods ( l ) , which acts for a longer time and hence more effectively on the slower heavy ions (2). In order to lessen this major drawback of quadrupoles, careful tuning procedures have been 0 1985 American Chemical Society
