This

' ' 2i ^spared

for fnfo n : -n ?

' ~2<r

' J O . o

purposes an: 13 not to be referenced

in any publication.

NATIONAL BUREAU OF STANDARDS REPORT

6570

rii'O report has been prepareo

for informal:::] and record

purposes and is not to be refers

in anv nubhcet'^n

A NEW MASS SPECTROMETER

by

Robert M. Mills

and

Charles C. Pugh

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the Nationa! Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards: the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems; in- vention and development of devices to serve special needs of the Government; and the development

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National Bureau of Standards ($1.25) and its Supplement ($1.50), available from the Superintendent

of Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT NBS REPORT

1002 - 11-10121 October 15, 1959 6570

A NEW MASS SPECTROMETER

by

Robert Mo Mills

and

Charles C« Pugh

NATIONAL BUREAU OF 5 intended for use yvithin th to additional evaluation an

listing of this Report, eithe the Office of the Director, however, by the Governme

to reproduce additional co,

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

progress accounting documents

formally published it is subjected

g, reproduction, or open-literature

ission is obtained in writing from Such permission is not needed,

ly prepared if that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A NEW MASS SPECTROMETER

ABSTRACT

This paper describes a time-of-flight mass spectrometer

being developed which will be used in investigations of combustion processes. The instrument is rather unique in that

its operation is similar to the phase focusing in a linear

accelerator; a defocusing effect is used rather than a focusing one. The advantages of a time-of-flight spectrometer

as well as the advantages of the particular type being developed are discussed. After the necessary equations are derived,

a description of the apparatus so far assembled is given.

I. Introduction

At the present time, many researchers are exploring the

basic mechanisms of flames. The question as to the nature of

the role played by the charged particles found in a flame

arises in their investigations, but, as yet, no explanation

has been commonly accepted. Three experimental approaches

to this problem have been microwave absorption, probe measurements, and mass spectrometry. Of these three, the last

enjoys the greatest popularity because of the high sensitivity

obtainable with mass spectrometers. Foner and Hudson (l) and

Eltenton (2) have designed an apparatus which ionizes flame

gases for mass spectrometer analysis. Knewstubb and Sugden (3)

as well as Deckers and van Tiggelen (*+) have made a more

straightforward approach, sampling the ions to be analyzed

directly from the flame. It is planned to use this sampling

technique at the National Bureau of Standards.

It is proposed to construct a flame-analyzing mass spectrometer, which will supplement the theoretical work being done

here on flame mechanisms. The results will be used to check

whether insufficient vacuum conditions inside the instruments

of previous researchers caused misleading recombinations of

the ions. Eventually, it is hoped to use the instrument to

investigate the presence of negative ions in a flame, a subject

pertinent to the current theoretical work being done at NBS,

but, as yet, largely unexplored.
.
2

After preliminary investigations concerning the construction of a mass spectrometer, it is planned to build

a time-of-flight (T.O.F.) instrument. The basic principle

of such an instrument is that heavy ions take a longer time

to reach the end of a drift tube than do light ions of

equal energy. A time-of-flight instrument has the advantage

over a magnetic mass spectrometer of relatively low cost and

simplicity of construction. The limitations on the accuracy

of a T.O.F. instrument are placed on the construction of the

electronic equipment rather than on the mechanical alignment

of the instrument. Absolute calibration is easier in T.O.F.

instruments since the three parameters involved, tube length

frequency, and magnitude of applied voltage, are easily

measured and controlled. On the other hand, hysteresis,

saturation, and fringe effects make calibration of magnetic

mass spectrometers more difficult unless a permanent magnet

is used. If the magnetic spectrometer does use a permanent

magnet, the only variable parameter is the acceleration

voltage, and care must be taken so that the ionic current

which is transmitted through the instrument is not changed

as the voltage is swept. ^ Furthermore, d-c amplifiers are

usually used with magnetic spectrometers in order to amplify

the steady stream of ions which are collected, whereas T.O.F.

instruments can use tuned a-c amplifiers which distinguish

between ion current and background noise. The major disadvantage of a T.O.F. instrument is its low resolving power

when confronted with ions of high mass. However, since it is

thought that almost all of the ions in a flame are light (of

mass less than 100), a T.O.F. machine seems likely to be useful.

2. Theoretical Discussion of Apparatus

The design of the time-of-flight instrument which is to

be constructed is somfewht different from the T.O.F. mass

spectrometers presently in existence. Essentially, it involves

a modified linear accelerator. Others have pointed out the

usefulness of accelerators as mass spectrometers (5?6). A

linear accelerator, in its simplest form, consists of a line of

coaxial metal tubes, alternate tubes being connected electrically to each side of a radio frequency source. The length of

the tubes, the frequency and the amplitude of the rf voltage

are adjusted so that an ion emerges from the field free tubes

into a gap at the proper time in order to acquire an acceleration.

1 This change could result from electrostatic focussing effects

on the ions or from the ion collisions with the walls of the

drift tube before they are collected.

- 3 -

The scheduling of the positions of the ion is not designed, as

one might expect, so that the ions are ejected into a gap at

the peak voltage . Instead, the ion emerges into the gaps at the

times indicated by the asterisks in Figure 1. Note that going

from one gap to the next is equivalent to a phase shift of

l80°. Thus, the two waves are drawn, and alternate waves are

used for alternate gaps. One can see that this scheduling of

the arrival of the ions results in a bunching effect. A heavy

ion which travels a bit slow arrives at the next gap late.

However, it is more strongly accelerated due to the large

electric field in the gap, which enables it to catch up.

Likewise, lighter ions experience a smaller acceleration since

they arrive at the gap early. The magnitude of this bunching

effect depends on the slope of 'the rf voltage at the time the

ion is in the gap. In fact, the effect may change to a spreading effect if the ions arrive in the gap during the negative

portion of the slope (i.e., fast ions arrive at the end of the

tube early and are made to go even faster). This latter effect

is desirable since it produces the separation of the ions

according to their masses. It is desirable to have the spreading effect occur during the largest possible portion of the

cycle, which suggests the use of a saw-tooth (S.T.) rf voltage.

The effect of going from one gap to the next is now as shown

in Figure 2. If the slope of the saw-tooth voltage is positive

at the first gap, it must be negative at the second gap.

The voltage at the second gap must have a negative slope

to obtain the spreading effect. Thus, a positive sloped sawtooth rf voltage is applied to the entrance of the tube as

shown in Figure 3° If the length of the drift tube and the

voltage used to give the ions their initial acceleration are

such that an ion of a particular mass remains in the drift tube

for a whole number of complete cycles, one can see, by referring

to Figure 2, that unlike other masses, this particular mass

has neither gained nor lost energy after it has gone through

both gaps. If an ion remains in the drift tube less than 360°

the ion gains energy, but if it stays slightly more than 360°

it loses energy. The fact that one particular mass neither

gains nor loses energy is true regardless of the portion of the

cycle through which the ion passes at the first gap, assuming

zero flyback time for the saw-tooth wave. Thus, the instrument

is able to collect a steady stream of ions instead of the pulses

2 Since the polarity of the second drift tube gap relative to

the first has been reversed, the voltage at the second gap

is the same as that found at the first, multiplied by -1.

- If -

which the usual T.O.F. instrument requires, and makes it

possible to collect a larger effective current, thereby

improving sensitivity. However, the advantage of being able

to use an a-c amplifier is retained as will be explained in

the section describing the collector system. A further

advantage of the design over the usual T.O.F. instruments is

that there is no need for a gate or pulse voltage, which is

of the same high radio frequency.

In order to write down the equations describing the

performance of the proposed spectrometer, the following definitions are used:

V - the total voltage through which an ion has fallen.

Vd-c - the d-c acceleration voltage through which an ion

passes before it emerges into the first saw-tooth

gap.

V s t

- the peak to peak saw-tooth rf voltage applied at

each end of the drift tube. 3

t - the time in seconds required for the ion to pass

through the drift tube.

T - the period of the rf applied voltage.

L - the length of the drift tube in meters.

v - velocity of the ion travelling through the drift

tube o

e - charge of an electron = 1.66 X 10“19 coulombs.

m - the mass of the ion (in kilograms, unless otherwise

stated)

n - the number of rf cycles the ion remains in

the drift tube.

amu - Atomic mass units

3 The wave form of the saw-tooth wave is actually similar to

that shown by the broken line in Figure 2. However, the

slope of the wave rather than its actual maximum value is of

importance. Thus Vs t is defined as the maximum voltage difference occurring in the wave represented by the solid line.
-

,•
- 5 -

Now:

( 1 ) = L

but since

t mv^ = eV

v f

_|/2eV

m

Using this in (1) and realizing that t = nT

( 2 ) L i/m

|/2dV~

or

(2a) T ]TT

m

L

n y~~2e

V in equation (2) is the total voltage through which an

ion has fallen after it has entered the drift tube but before

it passes into the last gap. An ion which has an atomic mass

larger or -smaller than the ion which stays in the drift tube

exactly 360° will arrive at the second saw-tooth gap ahead or

behind the 360° ion by an amount given by:

(3) dt

dm 2 y/2eVm

which can be simplified by using (2 ) to read

dt - dmt (3a) 2m

- 6 -

Since the 1.66 x 10 ^7 conversion factor cancels

out, the m in (3a) can be expressed directly in amu.

Now, if (3a) is multiplied by the rate of change of the

saw-tooth voltage at the second gap, an expression is obtained

giving the difference in potential through which an ion falls

at the second gap for a unit change in mass number.

If perfect linearity and zero flyback time are assumed,

the rate of change of the S.T. voltage is given by vst

T

Thus or

Since it has already been concluded that the total effect

on a 360°' ion after going through both .S.T. gaps is to neither

gain nor lose energy, thedV is equal to the voltage received

by an ion which is heavier or lighter than a 360° ion after it

has gone through the two S.T. gaps.

There is another effect which causes a variation in the

net potential of an ion and which, so far, has been neglected.

The potential of the ions in the drift tube varies with time

because of the saw-tooth voltage applied at thb first gap.

Equation (2a) indicates that t will also vary (i.e., ions will

not always remain in the tube for exactly 360°) unless T is

made to change with V by frequency modulation. However, the

use of frequency modulation adds to the complexity of the

instrumentj-e specially since T does not vary linearly with V.

The variation of t will be small if V is large. This may be

calculated from the change in total potential of an ion due to

the saw-tooth voltage variation.

First, though, it will be helpful to find the change in

t due to the S.T. voltage.

dtg = . _ L m3-

dV

2 \£StT

Using (3)

/ dt _ m

/dm V

( 5 ) dt

dV

- 7 -

An ion of atomic mass number m and ions of atomic mass

numbers m t 1 all have a variation in final V due to the

S.T. voltage. Therefore, resolution of ions differing by

one mass number requires that the absolute value of the

variation be not more than one half of dV/dm.

(5a) Yst . dt . Yst <Yst . 1 . dt

* T dV , 2 T 2 dm

Which gives

<

•

In the above analysis, the fluctuation in the potential

of ions in the drift tube is given by dV. Again, m in (5b)

is expressed directly in atomic mass numbers. . There might be a second variation in the final energy

of the ion,s which would further limit the resolution' of the

instrument. The initial thermal energies of ions are usually

neglected in mass spectrometer work because, at room temperature,

the avera'ge thermal energy amounts to only O.OA ev. (7)

However, there is some question about this at the elevated

temperatures of flames. If the thermal energy appears as an

initial velocity, the final energy of the ions is a function

of the direction of the thermal velocity. It was decided to

determine the range in the thermal velocities of the ions

selected by our apparatus by experiment.

If n is to equal one, equation (A) indicates that the

resolution becomes small for increasing mass numbers. But,

equation (2b) shows that n can be increased as m is increased,

without changing the values of T, V, or L. This has two

advantages. Besides improving the resolution of high masses, it

reduces the required frequency ranges of the radio frequency

* saw-tooth generator. It is planned to vary n as follows:

(5b)

V

m st

V

- 8 -

Mass Region n

1 to 10 1

10 to 17 3

17 to 26 4

26 to 37 5

37 to 50 6

50 to 65 7

65 to 82 8

82 to 101 9

2. Harmonic Masses and Modifications Needed

for Their Elimination

Suppose L, V and T are chosen so that n=s satisfies equation (2b) for the particular mass which is to be measured*

Any other mass which satisfies (2b) by making n an integer and

which is not equal to s will pass through the two S.T. gaps

without gaining or losing energy. These unwanted masses are

called harmonic masses. The situation is similar to the case

of a car traveling at 20 mph being able to pass through a

series of traffic lights which are set for 4o mph. If n is

equal to one, only heavier harmonics can exist. Lighter

harmonics exist if n is greater than 1.

Elimination of the harmonic masses may be attained by

using two more drift tubes as shown in Figure 4. One additional

tube would be sufficient except for the fact that an even

number of gaps are needed in order that the saw-tooth voltages

at the gaps cancel out. The length of the middle tube is not

important since it cannot be used to analyze the masses. This

can be explained as follows. An ion which has fallen through

an accelerating voltage can arrive at the end of a drift tube

early for two reasons. It can be of lighter mass and thus

travel faster, or it 'could have started earlier. Thus, one

needs to schedule the ions so that they arrive at the end of

the drift tube at a known time. This could be done by pulsing

the ion beam and, thus, allowing the ions to start in bunches.

Or scheduling could be done with a saw-tooth voltage as planned.

However, the sawtooth voltages have cancelled one another

after the ion passes into the middle tube and hence they are not

scheduled. Thus, the middle tube will be made short. It was

convenient to make itA° long. The length of the first drift

tube will be such that ions remain in the tube for a time

equivalent to just under a multiple of 360° of the rf voltage

and the length of the last drift tube will be such that ions

remain in the tube for some integral number of complete cycles.

- 9 -

That this arrangement does not allow harmonics to pass,

is shown by the following pertinent equations. The following

symbols are defined:

nq - nearest number of whole cycles an ion stays in the

first drift tube.

n

2

- nearest number of whole cycles an ion stays in the

last drift tube.

n^ - nearest number of whole cycles a harmonic ion could

1 stay in the. first drift tube.

nH

- nearest number of whole cycles a harmonic ion could

^ stay in the last drift tube.

A - number of degrees less than nq 360° which an ion

spends in the first drift tube.

All of the above numbers are defined to be positive.

The time^ in degrees, an ion takes to pass through the

first tube is nq 3602e’ Av (e.g., point A in Figure 2). An ion

of another mass which is a harmonic (i.e., receives the same

net energy from the saw-tooth voltages applied at the ends

of the first drift tube) spends degrees (point B)

in the tube. If this same mass is to be a harmonic in the

last drift tube, the ratio of the times of the non-harmonic

and harmonic masses in the last drift tube must be the same

as the ratio of the times in the first drift tube:

(6) nq 360° - a, n.2 360°

nH

l 36O0- A

” nH

2

360°

In the discussion that follows, nq will be made equal to

n2 - n.

10

Harmonic masses may be found by considering nH1

to be

the independent variable in equation (6) and nir to be the n

2

dependent variable, using the appropriate values of n as discussed earlier. 4 Strictly speaking, a mass can be considered

to be a harmonic of both tubes only if n^ is a whole number.

But it must be known how close to a whole number njj can be

and still have the corresponding mass considered to be a

harmonic >by instrument. Which of the masses to be considered

harmonics were determined by the following criteria:

The average particle has a thermal energy fluctuation

of i i KT = _ 0.0604< ev at flame temperature. It is

probably safe, then, to assume that the instrument will

be able to distinguish ions which have energy differences of . 5 ev or more. If V s

-^ is to equal 30 volts (so

that (5b) holds up to m = 100), the corresponding difference in arrival time is 6 degrees. This requires

that

:

(6a) ^

jnj^ - n^j^ 0.0167

Since n^ was defined to be a whole number, nu

^ was used

Xl'2 to denote the value of the dependent variable in (6) which

might not be a whole number.

4 Since masses above 100 will not be considered, the^ following

program was used to look for harmonics:

Mass in this Range

Values of njj to be investigated. In case number is

not a whole number, next

lower number is used.

n=l 1 to 10

3 10 to 17

4 17 to 26

• » • » n (n2' + 1 )to (n+1 ) 2 + 1

• • 9 62 to 100

2 to 10

1,2,4, . .3 vTo

1 ? 2 5 3 7 6,. * .4 \ 100

. 17

1,2,... (n-1 y

n+1 ) . . . i^j2+

1 , 2 ,... 8 .

-11

A suitable value of h has not yet been established. It

should be small since the first degrees of the saw-tooth

voltage cycle cannot be' used. This can be seen by referring

to Figure 2.

Ions of the correct mass starting through the first drift

tube during the first /A degrees receive a relatively large net

loss in energy after passing into the middle drift tube; the

rest of the ions (i.e., correct ions leaving after the first

A.'

J

) gain a small amount of energy. Thus, since the instrument is made to accept only the small energy gain, ions

entering the first tube during the first A ' are discriminated

against by the instrument and are not collected. However,

this does not reduce the sensitivity as much as might be expected,

since ions starting at this time are lost anyway due to the

fefbt that S.T. oscillators cannot be made to have zero return

time and perfect linearity at the beginning and end of the cycle.

To find how small .A can be and still have the instrument

distinguish harmonics, it is required that there be at least

a difference of 6° between the time-of-flight in the last tube

of an ion which is a harmonic in the first tube and time-offlight of another mass which could be a harmonic in the last

tube

(6b) n

Hl X 360° - A

n x 360° - A

n 360° - nH360° n

2 y 6

°

The first term on the left represents the time taken in the

last tube by a mass which was a harmonic in the first tube;

the second term corresponds to the time taken by a possible

harmonic in the last drift tube. It was determined that the

absolute value of njj - np^ is the smallest when the values of

n and n

Hl in equation (6) are themselves small. Thus, when

n = 1 and n-g = 2 in (6b) and _l\ is chosen to equal 7°> this

value of A in (6a) gives the minimum value of njq^ - ng^ equal to

0.0196 and this was taken to be an acceptable guard against

harmonics, at least until proven by experiment.

The ions which have been discussed above represent masses

which are able to pass as harmonics through both the first and

last drift tubes. There is a second type of harmonic mass for

the instrument as a whole which does not pass through either

one of the tubes as such. Instead, the mass is able to gain

and then lose (or vice versa) enough energy to be collected

with the wanted ions. This could possibly occur if the ion

12

starts during the first ^ degrees. But the value of the

mass which is able to do this is not independent of the

starting time, resulting in an additional ion current

which will be the same for all instrument settings. An

ion could pass as the second type of harmonic if the first

tube is A degrees less than 360° and the last is ^ degrees

longer than 360°, since the same amount is gained in the

first drift tube that is later lost in the last. One would

not be able to distinguish between different values of n. By making the first drift tube 7° less than 360° and the

last just equal to 360°, there is an increase of about 0.5

electron volts per unit increase of n in the final energy

of the ions to be collected.

Using more than one drift tube to get rid of the harmonic masses also improves the resolution of the instrument.

Equations may be derived corresponding to (4) when three

drift tubes are used. From (2a) and (3a):

(7) dt - i t

dV • 2 V

(7a) ft = £ i

dm m

Assuming that V^_

c is large enough so that, effectively,

there is no variation in the final energy of the ion from the

saw-tooth voltage (i.e., that equation (5b) holds), then:

The first term in brackets is due to the change in energy

imparted to the ion by the first tube. The second term, as

with the single drift tube case, is due to the slowness of a heavy ion or to the extra speed of a light ion in reaching the

end of the last. .tube, dt'is negative; dV is negative because

dV dm

of the negative slope at the end of the first drift tube.

The first term, then, is positive. The term Vs^ is negative

because the voltage decreases with time at the end of the last

tube.

r Subscripts refer to the number of tubes used to separate

the ions

- 13 -

Using (7), (7a), and (4):

or, since t = nT

:

(7c)

Equation (7c) gives only the change in energy of the ions

resulting at end of last drift tube. But there has already

been a change at the end of the first tube, and this change,

equation (V), should be added to (7c). Thus:

It is planned to make V s £ - 30 volts and V about 3>000 volts.

Also, if reference is made to the discussion of the values

n to be used with the various masses, it is found that

§ 7 1. Thus, the first term is small. But, the remaining

term makes (7d) twice as large as the resolution for the

single drift tube case. Equation (7d) has replaced only (4).

Equation (5b) is still valid for the 3 drift-tube instrument.

Equation (2b) is valid if L is taken to be the length of each

drift tube and n is the number of degrees spent in this partic

ular tube divided by 300°

.

dm j2^-m V

n vst

m

- lb ~

3. Design of Apparatus

The values of the terms in equation (2a) will be as

follows

V st

Vd-c

L

l> L 2’ l

3

n

1

T

30 volts peak to peak

3000 volts

55.1, 1.1, 56.2cm

increase with m as discussed earlier

from 0.45131 to 1.35319 Mc/sec

The change in the length of the second and third drift

tubes needed to compensate for the energy change at the end

of the first tube is found from equation (2a).

(8) dL - u. nT /2e - n. L

dV 2 |/mV~

2 V

This amounts to 0.0037 mm/volt for the second tube and

0.0955 mm/volt for the third. The largest value of is

7° x 9 (i.e., when n = 9), which corresponds to:

3^0°

^ volts 5 volts

Thus the second and third drift tubes should be increased, at

most, by 0.02 mm and 0.5 mm, respectively. These corrections

were neglected in computing L 2 and L3.

The collector system must be able to distinguish between

masses by their energies. If the final energy of the ion to

be measured is E electron volts, it is planned to apply a potential barrier in front of the collector which is first

equal to E + c< , then is changed to E - • The value of o

is such that the resulting change in collector current is due

entirely to the on-off switching of the mass to be measured.
'
- 15 -

Figure 5 is a sketch of the electrical connections.. The

connections were made so that the velocity of the ions stays

high until just before they enter the Faraday cage., A

retarding field, which is needed for a secondary electron

repeller, could not be applied at the entrance of the Faraday

cage because the velocity of the ion has already been reduced

by the potential barrier » Therefore, a long slender Faraday

cage was built and depended on to prevent secondary electrons

from escaping. An electronic lens is used to ensure that the

ions travel to the back end of the cage before colliding with

the walls. The cut-off control is adjusted until the ionic

current is cut off when the and S.T. generators are not

operating. The cut-off control is, also used to select different

values of n. The 3000 volt d-c supply is regulated and has a fine voltage control. The 110 volt supply consists of dry

cell batteries which are also used with the vacuum indicating

ionization gauge.

Figure 6 shows the burner, the flame chamber assembly,

and the intermediate vacuum chamber. The burner consists of

two concentric brass tubes. The outside diameter of the

larger tube is 3/A of an inch. The inner tube feeds gas to

the end of burner where it mixes with oxygen and burns. A

ring which' fits loos-ely around. the burner compresses a teflon

ring against the burner and flame chamber body to form a vacuum seal. An observation window on the top side of the

assembly makes it possible to check on flame size and shape.

The assembly is cooled by a water jacket which surrounds the

vacuum chamber where the flame burns. Pressures below one mm

of Hg can be obtained by using Cenco Rotovac Pump when the

flame is not burning. Pressures of 2 to 7 cm are obtained

with the flame burning. A closed end mercury monometer is

used to determine the flanle chamber pressure. The flame

chamber is connected to the pump through a water trap and a four liter flask both of which are used as ballasts. Glass

wool is stuffed into both tubes of the burner to stabilize

the flame at low pressure.

Throughout the design of the entire instrument, special

efforts were made to permit its easy • disassembly for alterations or cleaning. For this ‘reason, bolts were used to

connect the intermediate vacuum section with the burner section.

A lead foil gasket ai'ds in making a vacuum seal. A compressed

"0" ring makes a vacuum seal agdinst the glass "T", which

forms the intermediate’ region proper. An oil diffusion pump

which has a pumping 3peed of about 25 liters/sec at 10- 3mm

- 16

is connected to the bottom of the glass "T" with a short

piece of rubber hose. The hose fits firmly over the butt

joint formed by the ends of the diffusion pump and"T',' and

then is covered with several coats of shellac. Two vacuum

gauges, a thermocouple and an ionization gauge (not shown

in figures), are sealed directly to the glass "T". The

first orifice is made by drilling a hole six thousandths of

an inch in diameter through . 0043n.pl at inum foil. Platinum

was used to withstand the heat from the flame and so that

the hole would not cause the foil to rip when atmospheric

pressure is applied. The foil is sandwiched between a ring

and the burner assembly. By placing the ring on the burner

side, the first orifice can be aligned with the second

orifice before the rest of the burner assembly is bolted in

place. The ring is held in place with six screws which

allow easy replacement of the foil. The second orifice is

0. 010 in. diam. It is similar in design except that it is

located on the end of a tube which extends the length of

the intermediate region as shown in the figure. This allows

the distance between orifices to be small without impairing

the pumping speed of the diffusion pump. It is desirable to

have the orifices close together to minimize ion-air collisions

and to facilitate the passage of the ions through both- orifices.

More will be said about this later. A tight vacuum seal is

made at the intermediate-drift tube region junction with hard

black sealing wax.

*> A second mechanical vacuum pump is on order which is

similar to the one being used as a roughing pump for the

diffusion pumps. The new pump will permit reducing the pressure

in the flame chamber still further and thereby lowering the pressure in the rest of the apparatus. This increase in pumping

capacity will be necessary if the equipment is modified to work

with flames at atmospheric pressure at some future time.

The electrical equipment needed for the vacuum gauges and

the controls for the diffusion pumps has been constructed and

tested. The design of this equipment follows fairly closely

the suggestions of the manufacturer.

-7

Ion currents as large as 10 ' amps have been measured

coming through the first orifice, but further construction of

equipment is necessary before measurements can be made on the

current passing through the second orifice and reaching the

collector. It has been shown, however, that because of ionic

losses, the ion current can be a function of the accelerating
.
- 17 -

voltage which means that the masses to be measured should

not be swept by changing the voltage. The shape of the

electric field in the intermediate vacuum region where the

ions receive their first acceleration causes electrostatic

focusing of the ions. The focal length of this lens will

be controlled by the potentiometer shown in Figure 5«

Figure 7 shows the drift tube and collector assembly.

The diffusion pump connection and another set of vacuum

gauges are not shown.. The diffusion pump used has three

stages and is capable of reducing the pressure in the drift

tube region to below 10“5mm 0 f Hg with the flame burning.

The drift tubes themselves are cut from copper tubing and

have copper end plates soldered at each end to center the

tubes. The plates are square to offer a minimum of impedance to the pumping speed. They are bolted together with

nonconducting nylon screws. Sandwiched between the plates

are teflon insulating washers and fine tungsten screens

which have an ion absorption of about 1 per cent. The

screens are stretched across the ends of the tubes to accurately define the electric fields in the gaps. Standard

tapered glass joints are used to connect the diffusion pump

and the end glass piece to the long glass tube which

surrounds the drift tubes. Rubber bands hold both joints

together. A spring is compressed between the drift tube

assembly and Faraday cage to connect :i.-n . The Faraday cage and drift tube assembly are kept from

separating by a constriction in collector end of the drift

tube and two O.Q32in> diam tungsten wires at the other end.

Electrical connections are made internally with teflon

covered wire and brought out at the collector end of the

apparatus through 6 electrical vacuum seals.

Three pieces of electronic equipment remain to be constructed. The current detection instrument should be able

to go down to 10 amps. Since the ion beam is modulated

by the generator, an a-c amplifier can be used; its design

will probably be similar to that used in the amplifying part

of a vibrating reed electrometer. The generator frequency

will be timed to the a-c amplifier and will be a relatively

low value (i.e., 30 cps). Its peak to peak voltage should not

be any larger than twice the energy separation of masses corresponding to nearly the same T, but different n's (i.e., one

volt). The frequency of the saw-tooth generator must be

variable from 1.35 to 0.4-50 Mc/sec in steps as small as 6 kc/sec.

It must be as linear as possible between In and 360°- n

'

,

but

there are no linearity requirements during the rest of the cycle.

Its peak to peak volthge needs to be somewhat above 30 volts.

- 18 -

4. Future Improvements

There are several possibilities for increasing the resolution of the mass spectrometer. The most obvious way is to

add more drift tubes as was done to eliminate harmonics.

This can be done by increasing the length of the entire instrument, by increasing the frequency of the saw-tooth wave

or by decreasing However one must compensate for

phase changes along the instrument if its length is comparable

with the wavelength of light at frequency of the S.T. generator.

Also, equation (5b) limits the amount by which V^_ c can be

decreased. Overcoming this limitation is the second possibility

of increasing the resolution.

The change in voltage per atomic mass difference for an

instrument having (2r-l) drift tubes can be shown to be:

(&l \ =-vst

dmyr ~

dt x 7dV

d?

X

Id r-1 dm

+ PdV\

|dmj r _i

Using equations (7) and (7a),

=

" Vst —T“ i_ t

8 f dm/ r _ 1 m dm/ r-1

(9) [dv»=-

Idm,

*st r i /m) +

1 1

ImI

I V (dnjk.i m

J ^dm/ r_ lt

As an example, consider the case of r = 3:

(9a) n

2 V 2 st + 3 n

v

s t

m V 2 m

The first two terms are small enough to be neglected

because of the relative sizes of V st and Vd_ c (2rV).

Therefore, if it were possible to overcome the limitation

of (5b), V could be reduced to make the addition of more

stages practical without changing T or the overall length

of the instrument. Also, if V s

^- can be increased and/or

V decreased, the first terms in (9a) could become significant

and the resolution increased greatly. This is equally true

for other values of r. There is another limitation on how

small V^_ c can be made which will be discussed in the appendix,

but the magnitude of Vs -^ is limited only by the engineering

problems in the design of the S.T. generator, if it were not

for equation (5b).
.

.
- 19 -

A second type of bunching has been considered, which

has been used in other types of T.O.F. mass spectrometers (8)

and which will allow an increase in the magnitude of Vs t

It is a somewhat academic idea so far , since it has not been

possible to apply it to an instrument having more than one

drift tube.

Consider the case of a single tube. Equation (5b) can

be interpreted to require that the relative positions of

the ions in the drift tube should not change to a significant

degree since this would upset their schedule. If the magnitude of V st j is too large, ions starting early would not

have as large a velocity as those starting later, which

results in a continuous change of the relative positions of

the ions. However, this can be allowed if the ion schedule

has been changed in a particular way. A positive sloped sawtooth wave can be applied to the gap through which the ions

pass befofe entering the drift tube. Ions which start early

should pass through this gap when the S.T. voltage is low,

whilb ions leaving later should be accelerated more by the

larger value of the S. T. wave. Somewhere in the drift tube,

then, it may be possible for the faster ions to catch the

slower ones. In other words, at one point along the drift

tube, the ions may be bunched. After the ions pass this point,

the fast ones will have overtaken and passed the slower ones,

so that there will be only one spot, for any given mass, at

which the ions are bunched. However, ions starting at

different points in the cycle have different energies as they

pass through the drift tube. The collector system planned

should distinguish between different masses by their energy.

This idea cannot be employed unless the variation in energy

of a particular mass which is imparted to it at the first sawtooth gap can be cancelled out. This could be done by requiring that the ionic bunchings take place exactly at the center

of the tube. By the time the ions reached the far end of

the drift tube, the fast ions would be ahead of the slow ones

by the same amount that the fast ones were behind the slow

ions at the starting end of the drift tube. If another positive sloped wave is used at the far end of the drift tube,

the variation in energy received at the first S.T gap would

be cancelled out. The point at which the ions bunch is a function of the mass; thus this system does not pass harmonic

masses (since the ions bunch only once). Thus, this system

differentiates between ions of different masses. If the ions

are too heavy, they bunch at a point further down the drift
. *
20

tube and have not spread out as much by the time they reach

the far end of the tube. No ion of this heavier mass reaches

the end of the drift tube in time to receive the voltage

needed in order to cancel out that which it received at the

first S.T. gap. The opposite is true of ions which are too

light

.

Derivation of new equations requires the definition of

^ , as the time at which an ion enters the drift tube. The symbol

Jf - the point at which the ions bun^h in the drift tube. In

the proposed system, $ = L/2. Let t

/ be the time an ion takes

to reach point j/ if it' passed the first S.T. gap when the

voltage is zero. Then,, requiring that all ions of a certain

cycle arrive at f at the same time means that:

t

7 - r = -1—

v J ^m

or

V

d-c

m

This can be expanded:

vd - c + Xst r

m

• oo
-

.

>

.
21

( 10 )

If is small:

Vd - C + V

m

jLL.

(

1 + 2 "Tpl

In order for (6) to hold for all values of X , it is

required that:

(10a) Vd-c = m 2et TT

and

UOb) Vgt = Jh

m et / ^

In effect, this results in discarding the condition that

required equation (5b) to hold in introducing the effect to

eliminate harmonics. A thorough investigation of the resolution expected of this type of system has not been made or of the

values of the voltages and frequency which would be required.

But as for the latter, it is probable that a higher V s t and

lower V

d _ c would be required.

5 . Summary

The characteristics of the mass spectrometer which has

been described are summarized as follows:

By using the saw-tooth wave scheduling, it is possible to

utilize a large per cent of the ionic beam emerging from the

first orifice instead of pulses of ions. The value of the

mass which is being collected is known if the frequency of the

saw-tooth wave being used and the other parameters of the instrument are known. Neighboring masses are separated by at

least 2.7 electron volts on the energy scale (for mass 100).

This resolution is achieved by the "light ions travel faster"

property; equation (5b) makes the "light ions are made to

travel even faster" effect small. Harmonic masses are separated

from the mass to be collected by at least 0.5 electron volt.

22

It is not known whether this 0.5 ev separation is large

enough. The deciding factors are |iow monoenergetic the

initial ionic beam is and the quantum mechanical probability

of an ion passing through a 0.5 electron volt barrier. The

range of energies of the ions is determined by their thermal

velocities together with the direction of these velocities and

by any kinetic energy variation which the ions receive as they

pass from a high to low pressure region. The former has been

shown to be much less than 0.5 volts. And the transmission of

ions through a 0.5 volt barrier 2 centimeters thick was calculated to be negligibly small (9).

It should be noted that (5b) is a limitation only on the

larger masses. A 3 kilovolt supply is being used because it

was readily obtainable. If it becomes necessary, Vd_ c can be

increased or the limitations of (5b) can be overcome by the

bunching effect described earlier or possibly by frequency

modulation. By increasing the value of V st,both the resolution and the harmonic separation should be improved.

-23-

APPENDIX

The problem here dealt with is how much acceleration

v’bltage between orifice A and B is needed in order that the

average ion leaving A reaches the end of the drift tube

without colliding with the plate of orifice B or with the

walls of the drift tube.

0 = the voltage at orifice B when orifice A is grounded.

_0 is. negative # x is the distance from plate A, measured toward B.

Voltage = V(x) = where b is the distance between

orifice A and orifice B (in meters).

- 17 V(x) = field = - &

v b

a = acceleration - field x charge and assuming charge = e mass

0e

a = - —

r— , a constant. In a field of uniform acceleration bm 7

x = f at 2 and v = at.

Thus, the time necessary for a particle with zero initial

velocity in the longitudinal direction to get from A to B is

t = = b j 2-iC

a f

- 0e

The longitudinal velocity at B = v = at =

- 2b -

The average particle:

1. Starts at the center of orifice A

2. Has a radial energy of -§-KT (=-§-mv ) due to the heat

of the flame from which it is emitted.

KT = mv2 or v =

where T = flame temperature = l4-00°K

The radial displacement of the average particle is

b

Thus

:

/0/

= mb2

(r . displ . ) 2

The average particle can travel any radial distance less than

the radius of orifice B and still get through. The problem is

(a) what voltage is required between the two orifices such that

that the average ion has radial displacement equal to r« (the

radius of orifice B)? In the instrument being constructed,

b = 8. 5mm and rB

= .127mm. Thus, /0/ - 77^ volts, (b) when

the average ion has fallen through 77^ volts and has barely

missed the lip of orifice B, will it get to the end of the

drift tubes without colliding with the tubes' walls?

The critical inside dimensions of the system are as follows

The longitudinal distance from orifice B to :

1. point C (in Figure 6) 1 25/32 in.

2. point D (in Figure 6) h 3/8 in.

3. point E (in Figure 7) 48 in.

The radial distance from the center of orifice B to:

1. point C 3/32 in.

2. point D 7/16 in.

3. point E 9/16 in.

- 25 -

Since ^ ’ anci s ^ nce both longitudinal and radial velocities are essentially constant, it is

only necessary to consider ions hitting the exit lip of the

drift tube. That is, if an ion's radial displacement is less

than 9/16 in. during the time necessary for it to get to the

end of the drift tube, it will not collide with any part of

the inside of the tube. The time that it takes a particle

to go E-B distance is:

(e-b) ~ 20.e . m

Since the radial velocity isyj-jj- , the radial displacement

at the end of travel from B to E equals

(E-B) * ]/:

m

22fe m

KT ~ E-B - .0108 meter: 9/16 in.

.01^3 meters.

Thus an average particle getting through orifice B at 77*+ volts

also gets through the drift tube.

References

1. S. N. Foner and R. L. Hudson, J. Chem. Phys. , 2, No. 8,

1374-1382 (1953).

2. G. C. Eltenton, J. Chem. Phys., 1%, 455 (1947)

3. P. F. Knewstubb and T. M. Sugden, Seventh Symposium

(International) on Combustion.

4. J. Deckers and A. van Tiggelen, "Combustion and Flame'} 1

281 (1957)

5. Paul B. Weisz, Phys. Rev. 22? 91 (1946)

6. W. Donner, Appl, Spectroscopy, 8, 157 (1954)

7. G. P. Barnard, "Modern Mass Spectrometry" Inst, of Phys

London, 42.

8. W. E. Glenn, Jr., "A Time of Flight Mass Spectrograph

(thesis)" U. S. Atomic Energy Commission, AECD3337,.

9» David Bohm, "Quantum Theory" p. 240.

USC OMM-NBS-DC

VOLTAGE

—

>-

VOLTAGE

FIG. 2

AVI

/l/W \N\K

FIG. 3

vn/ SfsN /W \N\f>

, T

. r

sJ

SAW TOOTH GENERATOR

FIG.4

SAW TOOTH GENERATOR COLLECTOR ASSEMBLY

FIG. 5

OBSERVATION

WINDOW

t—UJCO<o

_l5o<

LlJ

trUJNlb<<

LO

CO<

Li_ o

COOl

o

Ld

COUJI

<Q

UJ

01

LlJ h-Q

21

<LCC

LlJ z:

o:

Z>CO

CDCD

Ll_

CM CO LJ

CD3 2

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h- (-

U- (O 5Q o

Q.

u. 2 O LlJ $ t—< LU _l > a

(— o 2 2 O LlJ cc

Ll_

FIG.

7-

DRIFT

TUBE

AND

COLLECTOR

SECTIONS

U.S. DEPARTMENT OF COMMERCE

Frederick II. Mueller, Secretary NATIONAL 1MJHLAU OF STANDARDS

A. V. Amin, Director THE NATIONAL BUREAU OF STANDARDS

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