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 spectrophotometrictechniques (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
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985 VOLTAGE FROM MASS SWEEP RAMP
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Flgure 2. OH- NCI spectra of PFK-acetone-diphenylmethane mixture using: (A) constant pole bias 2.1 V; (B) scanning pole bias 2.1 V between m/z 10 and 100 and 2.1-10.1 V between m/z 100 and 800.
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Figure 1. Circuit diagram of the pole Mas scanner.
devised (e. g., ref 3); attempts have been made to nullify the fringing field by the application of a delayed dc ramp (2);and the time that heavier ions spend in the fringing field has been shortened by increasing the pole bias voltage, Le., the electric field which accelerates the ions from the source to the quadrupoles. Care must be exercised, however, in increasing the ion velocities because higher velocities shorten the time the ions spend in the quadrupole field, which in turn might lessen the resolution. Therefore the pole bias voltage has to be continuously adjusted during the mass scan in such a way that the highest possible sensitivity is obtained without loss of resolution. Modern commercial quadrupole mass spectrometers (those sold after the late 1970s) are equipped with such automatic adjusting circuits. Our Scientific Research Instruments Biospect quadrupole mass spectrometer (early 1970s vintage) was not so equipped and suffered serious mass discrimination. We have constructed a simple device which increases the pole bias voltage in proportion to the mass transmitted, and we fitted our Biospect mass spectrometer with this device, replacing the original static pole bias voltage supply. A number of owners of old quadrupoles have expressed interest in the device, so we describe it in some detail here.
APPARATUS AND OPERATION The materials required are a f15 V power supply (ACDC Electronics Model 1201), three operational amplifiers (Type 741), and resistors, potentiometers, capacitors, and switches. The three amplifiers have to be powered from a separate source, but this f.15 V power can be supplied from the instrument itself. The circuit diagram is given in Figure 1. The device sums two voltages: a constant one selected from the power supply PS1 by potentiometer R20 and a mass dependent one which is taken from potentiometer R17. Both voltages must have polarity opposite to that of the ions to be analyzed, and that polarity is selected by the four-pole switch SW1. A voltage which varies with the mass being transmitted must be located in the mass spectrometer and fed into the
operational amplifier IC1, which is in the voltage follower configuration ( 4 ) . In the Biospect mass spectrometer this voltage is to be found at the RECORDER socket at the rear of the quadrupole control module. It is used by the recording oscillograph to establish the mass scale for the spectra recorded. The potential on this socket varies from 0 to -4 V as the m / z transmitted varies from 2 to 1000. The output of IC1 is routed by switch SW1/4 to one of the amplifiers IC2 or IC3. The gain of each amplifier is adjusted by using the 50 K trimming potentiometer feedback resistors so that the output voltage reaches a maximum of 14 V when the quadrupole is set to transmit m / z 1000. An operator-selected fraction of this mass dependent voltage is taken from potentiometer R17 and added to the constant voltage coming from power supply PS1 through potentiometer R20. The resultant total voltage is applied to the quadrupole rods as the pole bias voltage. The role of the silicon diode D1, optionally activated by the switch SW2, is to keep the input of the amplifiers IC2 and IC3 at 0.0 V until the input potential to IC1 is more negative than -0.4 V, that is, while the mass transmitted is below 100 m u . As a result, the pole bias voltage is constant below m/z 100 and increases linearly above m / z 100. If diode D1 is bypassed, the pole bias sweep starts at m / z 2. The three amplifier circuits were assembled on a 3 X 3 in. board fastened to one of the front panels of the mass spectrometer electronic unit. For easy access, the polarity switch SW1 and diode switch SW2 as well as the pole bias sweep potentiometer R17 were mounted on the same panel. The pole bias potentiometer R20 was original Biospect equipment, and it was left in its original place in the source control chassis but rewired as shown in Figure 1. The three operational amplifiers are powered by one of the power supplies available in the original equipment. The constant component of the pole bias voltage is derived from a dedicated f15 V floating power supply, i.e., one for which the “COMMON” connection is not grounded.
TUNING PROCEDURE The pole bias scanner makes the tuning of the mass spectrometer quicker and easier. With diode D1 in the circuit one adjusts a low mass ion peak (m/z < 60) for maximum intensity and proper peak shape using the appropriate controls (AM and RESOLUTION dials in the Biospect as well as the pole bias and lens voltage controlling potentiometers). At this point, the dc balance on the poles should be about 0 V and the resolution of the peaks somewhat better than unity. Then an ion between mlz 150 and 300 is displayed on the oscillo-
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Anal. Chem. 1985, 57. 1107-1168
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100 200 300 400 500 600 700 800
m/z Figure 3. Intensity enhancement In OH- NCI spectrum of PFKacetone-diphenylmethaethane mixture with scanning pole bias as a function of mass.
scope screen, and the pole bias sweep potentiometer (R17) is adjusted to give the highest peak intensity with acceptable shape. If necessary, the resolution can be decreased a t this point to about unity by increasing slightly the dc imbalance of the poles (DC BALANCE control in the Biospect). In general, this procedure is adequate to obtain a well reproducible tuned state. For scans to high mass (mlz up to goo), the above procedure is repeated with an appropriate high mass ion displayed on the oscilloscope screen.
RESULTS In the well-tuned state, one can easily observe the important role of the pole bias scanner by setting R17 (the pole bias sweep potentiometer) to zero. While the peak intensities below m/z 100 do not show any change, those around m / z 300
decrease by about a factor of 2 and those for m / z 600 fall nearly to zero. An example of the spectra obtained with the different conditions is shown in Figure 2 parts A (pole bias scanner off) and 2B (pole bias scanner on). The enhancement in intensity at higher mlz values produced by the pole bias scanner is obvious. A quantitative measure of the enhancement is given in Figure 3, which shows the ratio of intensities with and without the pole bias scanner at selected mass ions in the OH- NCI spectrum of a PFK-acetone-diphenylmethane mixture. These intensities were measured with the oscilloscope of the mass spectrometer. The intensity enhancement varies from a factor of 1.5 a t m / z 231 to 6.5 a t m / z 849.
ACKNOWLEDGMENT We wish to thank Lawrence Eisenberg, Rockefeller University Electronics Laboratory, and Marvin Vestal, University of Houston, for helpful discussions and advice. (1)
LITERATURE CITED Dawson, P. H. In "QuadrupoleMass Spectrometry";Dawson, P. Ed.; Elsevier: Amsterdam, 1976; Chapter V.
D.,
(2) Brubaker, W. M. Adw. Mass Spectrom. 1968, 4 , 293.
(3) Carter, M. H. "Techniques for Optimizing a Quadrupole"; EPA-6001476-004, March 1976. (4) Malmstadt, H. V.; Enke, C. G.; Horlick, G. "Electronic Measurements for Scientists";W. A. Benjamin: Menlo Park, CA, 1974; p 94.
RECEIVED for review October 24, 1984. Accepted December 20,1984. This work was one of the activities of the Rockefeller University Extended Range Mass Spectrometric Research Resource, which is supported by the Division of Research Resource, NIH Grant RR00862.
Preparation of Unlform Films from Latex for Infrared Analysis G. C. N. Cheesman* and L. J. Gaskin Analytical Laboratory, Doverstrand, Ltd., Harlow, Essex, England The determination of the monomer ratio of copolymers is conveniently carried out by infrared analysis (I), but in the case of synthetic rubber latex there are difficulties due to the fact that the latex often contains a considerable amount of gel (2) polymer, i.e., polymer which is cross linked and insoluble in all solvents unless it is degraded. It is therefore not possible to use the conventional solution techniques for IR analysis. A technique commonly used is to spread the latex by hand direct onto a silver chloride window. Because the latex or polymer emulsion is nearly always pseudoplastic in its rheology, it is impossible to prepare a film of uniform thickness in this way because it will not flow out as a polymer solution would. This leads to inaccurate results.
EXPERIMENTAL SECTION More uniform films can be formed by spinning the window. A spinner made for preparing paint films was used (Sheen spinner, IC1 pattern, Sheen Instruments, Ltd., 15/16 Sheendale Road, Richmond, Surrey, England). The window was fixed about 4 cm from the axis, excess latex was poured on the window, and the window spun for 30 s at 650 rpm. Neither distance, time, nor speed is critical. The particular latex used was Revinex 98F, a carboxylated styrene/butadiene latex made by Doverstrand, Ltd., and used as a paper coating binder. The viscosity was 150 mPa.s (Brookfield LVT, Spindle 2, Speed 60). Latices with much higher viscosity need diluting with water and those with much lower viscosity may need thickening, but the exact viscosity is not critical. Infrared measurements were carried out on a Perkin-Elmer 681 ratio recording spectrophotometer connected to a 3600 Data 0003-2700/85/0357-1167$01.50/0
Station. For this sample, styrene was determined using the 765-cm-' band and butadiene using the 967-cm-' band of the trans isomer. In this case, the method is valid only if the trans/cis/vinyl butadiene ratios are constant, i.e., for polymers prepared at the same temperature. However, the method of film preparation is suitable for any latex, and any suitable internal reference band can be used.
RESULTS AND DISCUSSION Nonuniform films introduce both random and bias errors into infrared determinations. For example, consider a uniform film which has two measured infrared bands giving absorbances of 0.6 and 0.2, respectively, then the ratio of the bands is 3.0. For a film of the same mean thickness, but which has half the film a t 1.5 X the thickness, and half the film a t 0.5 X the thickness, then the calculated absorbances would be 0.504 and 0.189, respectively, giving a ratio of 2.67. Since this error will always make the ratio nearer to 1than the true value, no amount of replication will remove the bias error introduced by uneven films. Twelve determinations of the butadiene/ styrene absorbance ratio using windows prepared by hand spreading gave a mean of 1.22, standard deviation of 0.0606, and hence a relative standard deviation of 5.0%. Twenty-three determinations of the same absorbance ratio using windows prepared by spinning gave a mean of 1.43, standard deviation of 0.0198 and hence a relative standard deviation of 1.38%. Both the increase in the mean and the decrease in the standard deviation are highly significant, with a probability of
