Mass Spectrometer

Abstract

A mass spectrometer having means operatively connecting a circuit which provides a time lag between each ionizing electron pulse and each ion accelerating pulse to a delay circuit in an output scanner comprising the spectrometer so that each mass in the spectrum which is under observation will always be in best focus.

Description

BACKGROUND OF THE INVENTION

Attempts have been made heretofore to improve the performance of a time-of-flight (TOF) mass spectrometer by introducing a time lag between each ion forming electron pulse and ion accelerating pulse. However, this technique has the disadvantage of being mass dependent, i.e., the time lag required for optimum focus is related to the mass which is in best focus while other masses in the spectrum experience defocusing, whereby there is a basic problem of providing optimum focusing for a plurality of masses in the spectrum under observation.

SUMMARY

This invention provides an improved mass spectrometer having means operatively connecting a circuit which provides a time lag between each ionizing electron pulse and each ion accelerating pulse to a delay circuit operatively connected to electrical gate means associated with an output scanner comprising the spectrometer so that one or more masses in a particular spectrum which is under observation will always be in best focus.

Other details, uses, and advantages of this invention will become apparent as the following description of the exemplary embodiment thereof presented in the accompanying drawing proceeds.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing illustrates one exemplary embodiment of a mass spectrometer of this invention with certain parts in cross section, other parts broken away, and still others shown schematically.

DESCRIPTION OF ILLUSTRATED EMBODIMENT

Reference is now made to the drawing which illustrates an exemplary time-of-flight (TOF) mass spectrometer 10 which is made in accordance with this invention. The spectrometer 10 has an ionization chamber and drift tube assembly 11 which has an ionization chamber 12 suitably defined at one end thereof and electrical gate means provided at its opposite end and shown in this example as a plurality of three electrical gates.

The anodes 13, 14, 15 of the electrical gates are suitably operatively connected to associated measuring means which are utilized to determine and indicate the character of each mass being observed by the spectrometer 10. In this example of the invention a two channel recorder 16 is provided and has its channel 1 operatively connected to anode 13 and its channel 2 operatively connected to gate 14 and oscilloscope 17 is provided and operatively connected to gate 15.

The spectrometer 10 has an inlet 18 which receives molecules of a fluid such as a gas "G" which is to be analyzed and conveys such gas molecules into the ionization chamber 12. The molecules of gas "G" in the ionization chamber 12 are converted to ions, in a known manner, by electron bombardment in the form of electrical pulses which will be referred to as ionizing electron pulses provided in this example by an electron pulse generator 20.

The ions thus formed by electron bombardment are periodically moved out of the ionization chamber 12 by the application of what will be referred to as ion pulses which are provided by an ion pulse generator 21 which provides suitable electrical pulses to a grid 22 arranged adjacent the downstream end of the ionization chamber 12. The ion pulses provided by the ion pulse generator 21 are effective in accelerating the ions toward the electrical gates, and initially such ions move through a field free region 23 of the ionization and drift tube assembly 11. The assembly 11 may comprise suitable deflector plates 24 and lens 25 for purposes which are well known in the spectrometer art.

The ions move through the field free region 23 of the TOF mass spectrometer 10 as a function of their mass-to-charge ratios, and in a manner well known in the art, whereby they are separated into groups or bunches with the lightest group reaching an electron multiplier 26 comprising the spectrometer 10 first followed by successively heavier groups. The electron multiplier 26 may be supplied with power from any suitable source and an ion cathode 30 of the multiplier 26 is kept at several thousand volts negative with respect to ground. In this example of the invention the cathode 30 is shown at -3 Kv negative as indicated at 31.

Each mass group passes through the electron multiplier 26 and then through selected ones of the gates 13-15 as controlled by an electrical charge supplied to a grid 32. The grid 32 is controlled by a suitable electrical device 33 which is utilized to provide electrical pulses and introduce a suitable time delay between the ion pulses and what will be referred to as the gate pulses operating the electrical gates 13-14 and in essence such pulses comprise electrical pulses to the grid 32. The device 33 may also be referred to as a mass selector 33 in this specification.

Having presented in general terms a description of the operation of a TOF mass spectrometer the following description will highlight the manner in which various additional components cooperate with components previously presented whereby this invention makes possible continuous high speed analysis of each mass of a plurality of masses in a spectrum which is under observation so that each and substantially every mass will always be in best focus. As previously explained, the ions travel, in groups, through the field free region 23 in accordance with their mass-to-charge ratios. Expressed in basic terms this invention assures that as each group passes through the field free region, through the electron multiplier 26, and toward the electrical gates the mass selector 33 operates to introduce a suitable automatically variable time delay so that the lightest mass will be brought into focus first then successively heavier masses until all masses in the spectrum have been analyzed.

The spectrometer 10 is provided with trigger pulses as indicated at 34 and such trigger pulses pass through what will be referred to as a voltage ramp generator indicated at 35 which is used to control the voltage level of the trigger pulses and the increased voltage level trigger pulses are provided to the mass selector 33. Trigger pulses are also provided from the generator 35 to an amplifier 36 connected in parallel with the mass selector 33. The amplifier 36 amplifies the signal, i.e., electrical pulses, and passes such signal to a switch assembly 40.

The switch assembly 40 has three positions which will be referred to as an off position 40A, a programmed position 40B, and a manual position 40C. The switch assembly 40 also has a switch arm 41 which may be moved to any one of the three positions 40A - 40C either manually or using automatic switching means.

With the switch 40 arranged with its switch arm 41 at the programmed position 40B a signal from the amplifier 36 passes through an adjustable electrical load means which is shown in this example as a variable electrical resistor 42 to a device which will be referred to as a time lag focus device 43 which is operatively connected in series between the variable resistor 42 and the ion pulse generator 21. The operation of the device 43 will be described in more detail subsequently.

As trigger pulses are supplied to the voltage ramp generator 35 corresponding pulses are also supplied through a line 44 to the electron pulse generator 20. The electron pulse generator 20 changes the wave form and energy level of the pulses enabling efficient electron bombardment of the molecules in the ionization chamber 12. Through the action of the time lag focus device 43 a time lag is introduced between the time a particular ionizing electron pulse is provided by the generator 20 and an ion pulse is provided by the ion pulse generator 21. Thus, electron bombardment causes ionization in a known manner and a controlled time interval thereafter the ion pulse generator 21 accelerates the mass groups through the ionization chamber and drift tube assembly 11 whereby the groups or bunches of ion particles move through the field free region in accordance with their mass-to-charge ratios.

The trigger pulses provided to the generator 35 are also provided to the mass selector 33 and the mass selector 33 operates to provide an appropriate charge to the grid 32. The grid 32 is charged in accordance with a predetermined time sequence which may be such that as each mass group of the plurality of masses in the entire spectrum of a particular fluid being analyzed reaches the end of the assembly 11 the mass selector 33 provides a suitable charge to the grid 32 which allows observation of each particular mass at the time when it is in best focus. The charging of grid 32 in this example causes flow through gates 13 and 15 to channel 1 of the recorder 16 and to the oscilloscope 17 respectively.

Thus, it will be appreciated that with this technique it is possible to precisely observe each and every mass of the entire spectrum and this is accomplished in a simple manner by the cooperation of the time lag focus device 43 and the mass selector device 33.

As previously mentioned, the switch assembly 40 also has an off position 40A and a manual position 40C. With the arm 41 in the manual position 40C the spectrometer 10 may be operated independently of the mass selector 33. Also, in the manual position electrical power is provided through connection 45 of the switch 40 and may be of any suitable voltage level such as plus 15 volts for example.

With the cooperating arrangement of components presented herein the variable restrictor 42 may be used to adjust the time lag focus at any particular mass in either the manual or programmed position while the amplifier 36 is used to amplify the signal to the time lag focus device 43. It will also be appreciated that the slope of the voltage ramp for the time device or circuit 43 can be changed in order to obtain best focus for all masses in the spectrum. In particular the resistor 42 may be used to change the high end of the slope while the bias voltage on the amplifier 36 is used to change the low end whereby different slopes may be obtained for various voltage levels.

The particular electrical components comprising the mass selector 33, the voltage ramp generator 35, the amplifier 36, and the time lag focus device 43 may be of any suitable known construction provided that they perform the function essentially as described herein. It will also be appreciated that the electron pulse generator 20 and the ion pulse generator 21 may also provide electrical signals in a known manner.

The mass selector 33 may also be used to control the voltage magnitude and polarity of each gate pulse. In addition, it will also be appreciated that the mass selector may be provided with suitable external adjusting means, shown as a manual adjusting knob 47 which may be used to control a suitable electrical load means such as a variable resistor to more precisely control the delay between each ion pulse and each gate pulse for a particular mass.

Thus it is seen that this invention synchronizes the time delay between each ionizing electron pulse, each ion pulse, and each gate pulse so that each mass of a plurality of masses under observation is always kept in best focus.

In this example of the invention the time lag focus device 43 and mass selector 33 in their simplest forms may be considered as time delay means. Further, the mass selector 33 may be considered as either defining or comprising what may be commonly referred to as the output scanner for the spectrometer 10, and such output scanner has the measuring means defined by the recorder 16 and oscilloscope 17 associated therewith.

While present exemplary embodiments of this invention, and methods of practicing the same, have been illustrated and described, it will be recognized that this invention may be otherwise variously embodied and practiced by those skilled in the art.

Claims

1. In combination with a time-of-flight mass spectrometer of the type having an ionization chamber for receiving molecules of material to be analyzed, means for providing electron pulses to convert the molecules into ions, means for periodically accelerating the ions to provide ion pulses, said accelerating means including a grid disposed in said ionization chamber and means for applying a plurality of electrical pulses to said grid to accelerate ions out of said ionization chamber, a tube for receiving the ion pulses, means for measuring the time-of-flight through said tube of ions in said ion pulses, said measuring means including an electron multiplier having a plurality of channels for measuring electron current and means for gating electrons, said gating means including means for applying voltage pulses thereto to gate electrons associated with ions of a particular mass into a predetermined channel of said measuring means, the improvement comprising:

said accelerating means further comprising an electrical circuit having means for establishing a first variable time delay between the end of each electron pulse and the beginning of each ion acceleration pulse applied to said grid to selectively vary the mass of the ion in best focus; means electrically connected to said gating means for establishing a second variable time delay between the beginning of each ion accelerating pulse applied to said grid and the beginning of each gate pulse to selectively vary the detection of electrons associated with different ion masses; means for applying said gate pulses to one of said electron multiplier channels in accordance with a predetermined time sequence to gate electrons associated with different masses into a predetermined channel for measurement; and means electrically connected to said first time delay circuit and said second time delay circuit to synchronously vary said first and second time delays so that as said gate pulses are sequentially applied to one of said electron multiplier channels each particular mass at the time is in best focus.