Patent GB-966531-A (GB)
Title: Apparatus and method for improving the resolving power of analytical instruments
IPC: G01J3/28
Filed: 1962-04-17
Abstract: 966,531. Photoelectric systems. CANADIAN PATENTS & DEVELOPMENT Ltd. April 17 1962 [April 25, 1961], No. 14739/62. Heading G1A. [Also in Divisions G4 and G5] An apparatus for improving the resolution of an output received from an analytical instrument in which a sharply defined input appears as a distributed output comprises, as in Fig. 1 (a) an analogue device for generating from a discrete imput signal an analogue output in the same distributed form as the analytical instrument output; (b) a comparator for comparing the magnitude of a section of the analogue output with the magnitude of a corresponding section of the instrument output for sensing a difference therebetween, the said sections being narrow in relation to the distributed width of the inputs; (c) an input signal generator connected to the comparator and sensitive to the said difference for feeding an input signal to the analogue device so as to bring the analogue output towards equality of magnitude with the instrument output so as to multiply the difference at least in magnitude; (d) an arrangement for carrying out a series of successive sweeps of both of the said outputs by the comparator; and (e) an arrangement such as a display connected to the input signal generator for sensing the input signals to the analogue device. An embodiment of the invention which may be used to improve the resolving power of a mass spectrometer 2 (Fig. 3) includes a rotary switch 3 with a series of contacts 30. A sweep generator 5 controls the sweep of the mass spectrometer so that ions having a progressively higher common mass per unit charge strike the collector 21 successively, and the sweep generator also controls the arm 31 of the rotary switch which is synchronized so that the ion current representing a given mass number (e.g. n) is fed to a given contact n and thence to one side of an associated parallel R<1> C<1> circuit, the other side of which is connected to ground. Sufficient contacts and R<1>C<1> circuits are provided to accommodate all mass numbers within the range of interest, so that the capacitor series Ca<1> -Cn<1> -Cx<1> may be considered as storing on its terminals the output spectrum of the mass spectrometer. If the arrangement had an ideal response, current representing a unique mass number would be fed to one capacitor only, but in practice some current is donated to capacitors on either side of the appropriate one as well. The large resistor associated with each capacitor allows a small amount of charge to leak away between successive rotations of the arm 31, so that the capacitors continuously represent in close approximation the correct output spectrum for the mass spectrometer. Each switch contact is also connected to one side of another parallel RC circuit consisting of capacitors Ca-Cn-Cx with resistors Ra.. Rn.. Rx, the other sides of the combinations being connected to the contacts 60 of a rotary switch 6. This switch may have three wiper arms W1, W2, W3 connected via variable resistors R1, R2, R3 to a common junction 12, and wiper arm W1 is also connected via an amplifier 9 and a diode Dl to the junction 12. The amplifier is a D.C. amplifier of large gain having reversed polarity between input and output, and with a diode D2 and series resistor RD2 connected between input and output. Considering the operation of the system for a single component input of mass number n, capacitor Cn<1> receives a charge proportional to the ion current appearing at the collector, as the spectrometer sweeps through mass number n, but also capacitors Cn-1<SP>1>, and Cn-1<SP>1>, receive some charge, a dispersion which the system seeks to'substantially reduce. The charges on the capacitors Cn-1<SP>1>, Cn<SP>1>, Cn+1<SP>1> set up corresponding potentials with respect to ground on contacts 60n-1, 60n and 60n+1. When the wiper arms W1, W2, W3 begin to sweep round the rotary switch 6, the potentials are detected by arm W1, amplified and changed in sign and returned via junction 12 to arms W1, W2, W3 to charge up capacitors Cn-1, Cn, Cn+1 to potentials tending to balance the potentials on capacitors Cn-1<SP>1>, Cn<SP>1>, Cn+i<SP>1>. A number of sweeps are required to set up an equilibrium condition, and the Specification goes through a step by step consideration of several sweeps to illustrate this, assuming that the potentials of the capacitors Cn are modified by decay through the resistors Rn-2, Rn+2, proportional to the potential difference between each contact 60 and the corresponding terminal 10 (which remains at the same potential throughout, assuming no change in input conditions). When equilibrium conditions are established, the decay between each sweep is proportional to the difference between the amplitude of the corresponding section of the analogue output and a reference level. Furthermore the current passed by the centre wiper arm W1 approaches zero for all positions except when this arm is on contact 60n, in which position it tends to a value proportional to the voltage on capacitor Cn1. If therefore this is indicated on the oscilloscope, the display approaches an indication of a single peak of mass n. The Specification considers the operation of the system for a complex input and shows that adjacent mass numbers may be substantially resolved. In the arrangement of Fig. 3, the charges on capacitors Ca<1>, Cn &c. represent the instrument output spectrum S of Fig. 1, the strip S1 being represented by the charge on that capacitor associated with the mass number corresponding to a single component. The analogue output is represented by the charges on the capacitors Ca, Cn &c. The amplifier 9 and diodes D1, D2 provide the input signal generator, while the amplifier 9 also functions with the centre wiper arm W1 as the comparator. The analogue device is represented by the resistances R1 to R3, the rotary switch 6 and the storage capacitors Ca, Cn &c. The inclusion of more wiper arms will give closer approximations to the required result, and thus greater resolving powers. It may be necessary to alter the resistors R1, R2 &c. (depending on the number of wiper arms) through a scanning cycle if the ordinal extent and formation characteristic of the instrument input alter. This may be achieved by including further stepping switches with resistor banks. An alternative arrangement is described (Fig. 3 (a) not shown) which dispenses with the necessity to allow the analogue storage capacitors to undergo some voltage decay during each sweep by switching to them during each sweep a voltage proportional to the voltages on capacitors Ca<1>, Cn<1> &c. through servo amplifiers. In another embodiment of the invention the information from the analytical instrument is stored on one tape 71 (Fig. 3 (b)) which is driven in a loop synchronously with two others 72 and 73. Tape 72 stores the analogue output spectrum, and read out heads 75, 76 feed corresponding sections to the comparator 77, the result of this comparison being fed to the input signal generator 78, from which is obtained an output representing the required resolved output spectrum. This is fed to a read in head 79 and stored on the tape 73 and also to a standard peak shape storage 82, where it is fed to a read-in head 83. This head is arranged to oscillate back and forth along the tape to correspond to the distributed width of a spectral peak, while according to the position of the oscillation the charge fed to the head may be varied. A negative rejection circuit 86 is included in this system to reduce any tendency to instability. A further system (Fig. 4) for use with an optical spectrometer arranges for the instrument output to be displayed as a luminous intensity variation along a trace representing wavelength by moving an intensity modulated spot linearly over a persistent surface. The optical spectrometer feeds its output to a light source 106, whence a record is made on the photo persistent surface A of a rotating drum rotating in synchronism with the spectrometer sweep. This is sensed by photocell 110, and this sensed output compared in amplifier 109 with the output from photo-cell 111, sensing the reference analogue on surface B of the drum. The result of the comparison is used to control the source 112 through diode D, which produces the reference trace on the surface B through a transparency 115. If the ordinal extent and formation characteristic of the instrument output varies through a sweep, provisions for changing this transparency will be necessary. Another optical system (Fig. 5), applicable to systems where the instrument output spectrum is essentially two dimensional, utilizes a method of indirect comparison. The instrument output is applied in negative form i1 to a screen 200 bearing a translucent phosphor coating, while the analogue output is applied in positive form by means of light 212 acting through transparency 215 and is adjusted so as to maintain the photo persistent surface at a uniform standard level of illumination. This is performed by sensing a part of the image i, using a photo-cell and comparing the sensed voltage with a reference voltage RREF, which corresponds to the background brightness of the projected negative image. Any difference is fed back to the light source 212 so as to produce the necessary change in illumination. This system also includes an arrangement for producing a display i2 also in two dimensional form.