Design and characteristics of time of flight mass spectrometer to measure or analyze gases at low pressures and time of flight of single gas molecul

Nakada, M. P.

English

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASIIINGTON, D C 70546 
REPLY TO 
ATTN OF: 
TO : 
FROM : 
SUBJECT: 
USI /Sc ien t i f ic  & Technical Information D i  * ' 
Attent ion:  M i s s  Winnie M e  Morgan 
G P / O f f i c e  of A s s i s t a n t  General 
Counsel f o r  Pa ten t  Mat te rs  
Announcement of NASA-Owned 
U.S. Pa t en t s  i n  STAR 
I n  accordance w i t h  t h e  procedures contained i n  t h e  Co 
t o  Code U S 1  memorandum on t h i s  s u b j e c t ,  da ted  June 8, 1970,  
t h e  a t t a c h e d  NASA-owned U . S ,  p a t e n t  i s  being forwarded f o r  
a b s t r a c t i n g  and announcement i n  NASA STAR. 
The fol lowing information i s  provided: 
U.S .  P a t e n t  No. : 3,340.395 
Corporate Source : califor 
Supplementary 
Corporate Source 
NASA Pa ten t  C a s e  N o , :  
Please note  t h a t  t h i s  p a t e n t  covers  an  invent ion  made by an 
employee of a NASA c o n t r a c t o r ,  Pursuant t o  Sec t ion  305(a) of 
t h e  Nat iona l  Aeronaut ics  and Space A c t ,  t h e  name of t h e  
Adminis t ra tor  of NASA appears  on t h e  f i r s t  page of the  p a t e n t ;  
however, the name of t h e  a c t u a l  i nven to r  (au thor )  appears  a t  
t h e  heading of Column No, 1 of t h e  S p e c i f i c a t i o n ,  following t h e  
words 'Io e wi th  respect t o  an  inven t ion  o f ,  . e 
Gayle Parker 
Enclosure:  
Copy of Pa ten t  
d 
https://ntrs.nasa.gov/search.jsp?R=19710013565 2020-03-17T03:01:10+00:00Z
FEEDBAG% MEANS FR3M THE DETECTOR TO 
THE LOW SOURCE AND A 
SPECIFIC COUNTER 
Filed June 22, 1964 
&jT! -.2&&+&@ +P-lo 
Y 
The invention described herein was made B the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the 
Aeronautics and Space Act of 11958, Public Law 85-568 
(72 Stat. 435; 42 W.S.C. 2457). 
This invention relates to mass spectrometers and more 
particularly to improvements in time-of-flight mass spec- 
trometers. 
A time-of-flight mass spectrometer is an instrument 
which measures the various types of proportions of 
gas in a sample. At present, this is done by ionizing por- 
tions of the gas sample, accelerating the resulting bunch 
of ions through a certain potential, and measuring the 
time required for the various types of ions to reach a 
detector. The heavier the ion, the more time it takes to 
reach the detector. Typically, the detector is an oscillo- 
scope. A pulse of current which accelerates 
wards the detector also initiates the horizontal scan of 
the oscilloscope. As the ions reach the detector, they 
produce a current which controls the vertical scan of the 
oscilloscope. The greater the number of ions in any 
instant, the greater the vertical displacement on the os- 
cilloscope. The oscilloscope screen i s  then a graph of rela- 
tive abundance versus mass number. 
When the gases under very low pressure, such as the 
pressures expected to exist in the lunar atmosphere, are 
to be analyzed, a technique of individual ion counting 
is necessary. The reason for this is that at very low 
pressures it is difficult to obtain a large number of 
ions. A large number of ions are needed to be sure of in- 
cluding the same proportion of each mass number as 
i s  counted in the entire gas sample. Therefore, an oscil- 
loscope cannot be employed in the manner described 
when gases under very low pressures are to be measured. 
An object of this invention is to provide a mass spec- 
trometer which can measure or analyze gases at low 
pressures. Another object of t h i s  invention is the pro- 
vision of a mass spectrometer that can measure the time- 
of-flight of a single gas molecule at a tine 
ject of this invention is to provide a time- 
spectrometer in which the abundance of ea 
gas may easily be determined by measuring the flight 
time of only one ion at a time. Yet another object of the 
present invention is to provide a new and improved low 
pressure mass spectrometer. 
These and other objects of the present invention may 
be achieved in an arrangement wherein the number of 
ionizing electrons which are used to create ions are con- 
The novel features that are considered characteristic 
of this invention are set forth with particularity in the 
appended claims. The invention itself both as Eo its organi- 
zation and method of operation, as well as additional 
objects and advantages thereof, w 
from the following description wh 
with the accompanying drawing w 
the number of electrons which are emitted therefrom 
are controlled by a voltage controlled heater supply . Thii may constitute any of the well-known 
voltage controlled power supplies wherein the current 
output may be determined by a voltage applied to th 
regnlating tube or transistor therein. The electrons wEch 
are emitted from the filament I6 can pass though 8 
20 grid 29 only when the potential of this grid t raised above 
the cutoff bias level. This can only occur when a pulse 
is applied to the grid 2 
emitted electrons can p 
into which there is admitted gas from 
26 the source 14. At the side of the shielded region, whk% 
is opposite to the electron input Side, there is position . This collecting mode is biased pus& 
tively to attract thereto electrons which have passed 
through the shielded region. This collecting anode h also 
30 shaped to eliminate or prevent soft X-rays produced by 
electron impact from electron multiplier 32. 
The pulse generato a pulse to the grid 20 
whereby electrons are permitted htQ the shielded region 
23. "bere they co any molecules in the gas 
Thus, they provide ions of 
the gas. The output pulse the generator 22 is also 
applied to an acceleration 6 which forms a wall of 
the shielded region 3, nearest the outside of the tube 
10. This is a positive pulse which is applied through a 
40 delay circuit 30 which delays the application of this 
pulse to the acceleration grid for the time required b 
the control grid 20 to subside. 
when it is pulsed, accelerates any 
ed i n t ~  the drift region of the tube 
SI admitted from the 
46 ,XO. These ions travel down the tube toward the 
end wherein an electron multiplier 32 is placed 
an ion hits the electron multiplier 32, it is given 
in energy by acceleration to the first dynode of the elec- 
tron multiplier tube by the effect of the 3 kilovolts bias 
To accurately determine the mass number of an ion, 
it is important that no stray fields impart a velocity to 
the ion before or during its flight period. 
up by the control grid 2@ and the anode 2 
ath of the ion by a 
field grid 36 is connected to an 
center portion 38 of the tube 3BP 
50 that the center volume of the tube is almost equipo- 
tential and there is little field to accelerate an ion during 
the ion hits the electron multiuHer 3% the 
50 voltage applied to the first dynode 34 of the tube 31. 
BO r periods of its flight. 
multiplier generates a current pulse which-it Peeds-to a 
low jitter discriminator and pulse forming circuit. Tbk 
circuit provides a pulse output to a gate circuit 42. An 
plied to the gate through a ich delays 
this pulse for a predetermined time i d e r  the 
application of the accelerating pulse. 
m e  output of t lh~ gate circuit 42 is applied to a scala 
46 which indicates by the number of osciIlations counted, 
the time of flight of the ion. Since the ocmrrence of the: 
turn-off pulse is fixed, the greater the number 041 0sci.I- 
lations which are counted, the shorter the time of flight 
of the ion and the shorter the number of oscillatioas which 
are counted, the greater the time of flight of the ion. 
The scaler output iP recorded by any suitable memory 
device Sa. 
Since the apparatus described and shown i s  to measure 
the time d %light of one ion, it i 5  important that only 
one ion be created and accelerated at a time. Otherwise, 
the fastest ion would turn on the oscillator while the 
slower ions would have no effect and the instrument would 
show a higkr proportion of faster (lower mass) ions. TO 
prevent this, the number of electrons made available for 
Ehe purpose of ionization is adjusted so that, on the 
average, only one ion is created for every five ionizing 
pulses from the pulse source 22. As a mult, two ions are 
seldom created at the same time. 
Hn order to effectuate the above-described operation, 
the current pulse provided by the receipt of an ion on the 
electron multiplier tube 32 is applied to a count rate 
meter 52. The count rate meter is a commercially purchas- 
able item of electronic equipment which counts pulses 
over a predetermined interval from the output d the 
and provides an output voltage indicative 
of the average count which it attains. This voltage is ap- 
plied to a comparator 5 
age from a constant vo 
or error signal, is applied to the voltage controlled heater 
supply source 18. 1 will be appreciated that by setting 
the value of the refesence voltage source 
determine how much current i s  applied to the filament 
from the supply iource 88 and thereby can determine 
how many electrons are available for creating ions. This 
voltage setting, is established to provide 
trons so that on the averagep only one 
every five ionizing pulses. 
Though the embodiment of the invention shown uses 
a grid control stream of electrons to ionize gas for acceler- 
ation, any other suitable controllable ionization means, 
such as radioactive mrce with a shutter or 
impinging on an electron em 
ionizing electrons, can be used. 
turned on in the presence of 
useless operation of &e circuit occurs. 
W e  a specific embodiment of the inventtiox bas been 
selected for detailed description, the invention is not, of 
course, limited in its application to the de- 
scribed. The embodiment which has been uld 
be taken as illustrative rather than res in- 
&vidual circuits represented by rectangles i the drawings 
are well known in the Reld and both their operation and 
interconnections am easily performed by 
in the art. Therefore a detailed &~crigpth of 
is omitted. 
m a t  is claimed is: 
I. A timed-ffight mass spectrometer 
other end, mans at said other end for generating a volt- 
age pulse responsive to the receipt of an ion, oscillator 
from said willator 
counted, and means 
tive at a predeterm 
means for grreratina 
tion af wciUatioaS 
m l y  proportional to &e 
tromefer as recited in 
generating at predeter- 
d Bight is to be mew- 
iWg external gases to said one elad ob 
said tube; 
a pulse $enerator; 
means for ionizing molecules of said admitted external 
gas responsive to a pulse €rom said wise genemtctr; 
said means at said other end of said tube for generatiw 
a voltage pulse responsive to the receipt of an ion 
includes electron multiplier means for generating m 
outtpu#t signal responsive to meiving an ion. 
3. Apparatus as recited in claim 1% wherein there i s  d -  
8 
lo 
15 
ditionallv included: 
means responsive to the output signals generated by 
said electron multiplier means for generating a con- 
trol voltage representative d the number of a t p u t  
signals produced by said electron multiplier anew 
w e t  a predetermined interval of time; and 
means responsive to said control voltage €or OontmlGng 
the number of ions produced by said means fob i&- 
ing the molecules of said external gas. 
4. Apparatus as recited in claim 1 wherein the center 
region of said tube through which said ions are acceler- 
ated contains: 
means for substantially isolating said region fmm stray 
field including: 
a conductive inner lining for said region; and 
a perforate grid at the end of said conductive inner 
lining nearest the end of said tube containhg said 
means for generating a voltage pulse responsive to 
the receipt of an ion. 
5. A time-of-Kit mass spectrometer comprising an 
elongated vessel having an opening at one end for ad- 
sired to $e ionized, an endosure adjacent 
said opening to receive gas therefrom, all acmeleratimg 
grid constituting a first wall of said enclosure which is 
positioned opposite the opening for admitting gas to be 
ionized, a second wall of said enclosunz ad jmst  said SIC- 
celerating grid having an electron exit opening, an anode 
electrode positioned adjacent said electron exit opening 
outside of said enclosure, a third wall of said endosure 
opposite said second wall having an electron entry open- 
ing therein, a control grid pmitioned outside of said elec- 
won entry openin& a cathode structure adjacent said con- 
trol grid, a controllahle current source connected to said 
cathode structure, pulse generating means connected to 
said control grid for enabling said control grid to parss 
electrons from said cathode structure in response to a 
pulse from said pulse generating means, delay means con- 
nected between said pulse generating means and said 
accelerating grid for applying a pulse to said accelerating 
grid at a predetermined interval after the application of 
a pulse to said control grid, electron multiplier means 
positioned at the end of said elongated vessel opposite 
said one ead for generating a signal voltage responsive 
to an ion being received thereby, mans  for producing 
oscillations, gate means to which said oscillations are a p  
plied, said gate means being rendered operative respon- 
sive to said signal voltage, means for applying a pulse 
from wid pulse generator means at 8 predetermined 
interval after the application of a pulse to said accelerating 
grid to said gate means to render it inoperative, means 
fa r  counting the number of oscillations in the output d 
said gate means over the internal during which it is o p  
erative to provide an indication representative of tbe ion 
% _ I -  
6. A time-ofdight mas spectrometer 8s recited in 
claim 5 wherein said means responsive to the number 
d signal voltages occurring over a predetermined interval 
of time for controlling said ControllabIe: means for apply- 
ing a current to said cathode structure co 
for establishing a voltage representative d tmC: number 
of signal vcvltages occurring of time, ta rderence voltage, 
means for comparing said generated voltages with said 
reference voltage to product an error signal, and means 
for applying said error signal to said controllable means 
for applying a current to said cathode structure. 
7. A time-of-flight mass spectrometer as recited in 
claim 5 wherein said elongated vessel has a center region 
between said one end and the end opposite said one end 
comprising a flight region, said flight region containing 
means for substantially isolating said region from stray 
fields inchding a wnductive b e r  lining for said region, 
10 
16 
and a perforate grid at the 
lining nearest the end of staid enclosure contraining s 
d of said conductive i 
2,798,162 7/1957 Hendee _ _ _ _ _ _ _ _ _ _ _  25U1.911 
2,913,582 1111959 Collins et al. _ _ _ _ _ _ _  250-95 X 
3,247,373 4/1966 Herzog et al. _ _ _ _ _ _  250-41.9 
RALPH 0. NILSON, Primary Exminer. 
ARCH.E R. BORCHELT, Exminer. 
Assistant Exnmher. 


Time of flight mass spectrometer with feedback means from the detector to the low source and a specific counter Patent

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