NATIONAL BUREAU OF STANDARDS REPORT

7537

A NEW RADIO FREQUENCY TIME-OF-FLIGHT MASS SPECTROMETER

By

R# M. Mills

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT NSS REPORT

1002-11-10421

1002-11-10121

June 26, 1962 7537

A, NEW RADIO FREQUENCY TIME-0 F-FLIGHT MASS SPECTROMETER

By

.R. M. Mills

A large portion of the funds for support of

the work reported were provided by

Bureau of Ships

Department of the Navy

Code 638

Project No. SFl50703 s Task No. 3352

IMPORTANT NOTICE

NATIONAL BUREAU OF STA

Intended for use within the G

to additional evaluation and re listing of this Report, either If the Office of the Director, Nat

however, by the Government i to reproduce additional copies

ogress accounting documents Approved for public release by the TMily published it is subjected

Director of the National Institute of e P roduc

1 on , or ° pe n -I itc rat u re on is obtained in writing from Standards and Technology (NIST) ;uch permission Is not needed, on October 9, 20 1 5. repared if that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A NEW RADIO FREQUENCY TIME- OF-FLIGHT MASS SPECTROMETER

By

R. M. Mills

ABSTRACT

A new type of" time-of-flight mass spectrometer

has been developed,, which unlike most other such instruments , utilizes a continuous, nonpulsed stream

of ions* The ions accelerate through a constant

d-c voltage and then pass into a drift tube® A

radio frequency voltage is applied at the exit and

entrance gaps of the drift tube in such a way that

the field at the entrance is the negative of that

at the exit* Ions which travel through the drift

tube in any whole number of rf cycles therefore

experience no change in energy from the rf field,

while all other ions experience some change. The

number of ions which have not had their energy

changed is measured by placing an electric barrier

in front of the collector and differentiating the

cut-off curve. The instruemnt has resolved the

isotopes of rubidium at masses 85 and 87, and has

identified homogenious ion beams ranging in mass

from helium to cesium. Due to the nature of the

instrument's output, it will be most, useful in

applications requiring the analysis of a simple

mass spectrum, that is, spectra containing only

a moderate number of different masses.

1. Introduction

This paper describes a new type of time-of-flight mass spectrometer being developed for use in combustion research. It was

designed to be capable of- using a flame as the ion source. Since

only a limited number of ions can be withdrawn from a flame, adequate sensitivity while keeping the needed resolution has been

one of the major problems of previous workers in this field [1].

The mass spectrometer has a high duty cycle and a large transmission coefficient alorg with moderate resolving power. However,

due to the nature of the instrument's output, it will be most useful in applications requiring the analysis of a simple mass spectrum, that is, spectra containing only a moderate number of differ

ent masses®
2

2® Theory of Operation

In general, time-of-flight (T.Q.F.) mass spectrometers de

pend on the fact that heavier ions take a longer time to traverse a given distance than do light ions® The velocities of

the ions vary accord^

^tns

where e is the charge of the ions, ¥ is the total voltage

through which the ions have been accelerated, and m is the mass

of the ion* If a pulse of ions of various masses enters a fieldfree drift tube, after being accelerated, the ions will travel

in bunches corresponding to their mass® The distance between

bunches will increase the further the ions travel down the drift

tube® The mass spectrum can then be measured by observing the

ion current collected at the exit end of the drift tube as a

function of time® In some instruments, electrical gates set for

the proper delay are used in front of the collector to improve

the resolution® However, all instruments of this type have to

be pulsed at the source and the time between pulses must be great

enough to clear the drift tube of ions from the previous pulse®

The pulsing greatly reduces the duty cycle of the mass spectrometer which effectively lowers its sensitivity® A T.O.F. mass

spectrometer has been developed which appears to be less subject to this defect. It operates by accumulating the ions

continuously and concentrating them between each pulse [2]®

However, besides adding to the complexity of the instrument,

this introduces an uncertainty about recombinations and other

ionic changes during the delay between pulses®

Another type of T.O.F. instrument which is described in the

literature [3,4-] distinguishes masses by the energies the ions

receive from a series of rf fields. A continuous beam of ions

pass through 10 or more electrodes which are perpendicular to

the beam path® The geometry of these electrodes and frequency

of the rf fields maintained between them are such that ions of

one mass receive more energy than the others. An electrical

barrier is placed in front of the ion collector. Only ions which

have received extra energy from the rf fields are able to overcome the barrier and reach the collector. This instrument gives

good resolution, but has several drawbacks® The spacing between

the electrodes is critical for proper operation which makes construction and modification difficult. The numerous electrodes

a)
- 3

~

through which the ions must travel reduce the transmission of the

Instrument. And finally, this type of instrument collects pulses

of ions even though a continuous beam of ions enters the analyzing

region* This effect is due to the fact that an ion of correct mass

must also have the correct initial phase with respect to the rf voltages as it enters the fields in order. to gain the necessary amount

of energy to reach the collector. Thus the sensitivity of that instrument is reduced in the same way as with the pulsed type of

spectrometer*

The mass spectrometer reported here resembles the last instrument described in that ions of different masses are distinguished

by their energies rather than their special positions; it differs

in that there are no initial phase requirements. Consider a drift

tube of length L which has rf electric fields applied at its ends

as shown in Figure 1. It is seen that the field applied at one

end is the negative of the field applied at the other end. The rf

waves are represented in Figure 2. Suppose an ion enters the drift

tube at some time Aq

If the frequency of the rf voltage and the

velocity of the ion are such that the ion emerges at the other

end of the drift tube after one period of the rf wave, the ion will

pass through the second rf field at time AP

. It is seen that no

matter what part of the cycle the ion enters the drift tube, the

energy which it obtains at the first field is exactly balanced out

by the energy it loses at the second. Consider now, a heavier ion

which takes a longer time to traverse the drift tube. For example,

if it enters the drift tube at time , it might pass through the

exit field at time B2

» In general, ions which stay in the drift

tube a complete cycle or any multiple, n, of complete cycles

neither gain nor lose energy. These are the ions which cause the

detection system to give output signal.

As stated before, the detection system must identify the

resonant ions by their energy. This measurement is made by retarding field method [ 5*61. An electrical barrier is placed

immediately in front of the Faraday cage collector. For a given

barrier potential, E, measured with respect to the ion sources,

only ions with energies equal to E electron volts or greater can

pass. Or

3» The Detection System

N(e) de ( 2 )
4-

where n(E) is the number of ions per unit of time that penetrate

a barrier of height E and N(e) is the number of ions approaching

the barrier per unit of time with energy e at the barrier. N(e)

is obtained from equation (2) by differentiating with respect to

jjjl/

dn(E

)

= -N(e) at the point where e = E. (3)

dE

Since the detection system must measure the number of ions which

have had their energies unchanged by the rf fields, N(0) is the

quantity sought. Figure 3 is a set of cut-off curves showing

collector current as a function of barrier height in volts, obtained with an x-y recorder. The center curve is the cut-off

curve for an ion beam homogeneous in mass which is in resonance

with the rf voltage, taken with n equal to 12.. The other curves

were taken with n equal to 11. 11. 7> 12.2, 12.4.

The differentiation represented in equation (3) is performed

by introducing a variation in E about the point E = 0 shown as

AE in figure 3» The resulting a-c component in the collector

current is proportional to dn(E). And. dn(E) is proportional to

N(e) if dE is infinitely small. The equipment seems to give best

results using a variation of 10 volts (i.e. AE = 5 volts) and this

value will be assumed in the remainder of this paper. Using a dE

of finite size means that an average value of N is actually measured over the range E - 0 ± 5 volts.

If the ion beam is not homogeneous in mass, the resulting

a-c signal is the sum of the contributions from each mass. The

variation in the ion current is amplified by a-c amplifiers and

then fed to a pha.se detect or , AE supplying the reference phase used

in the detector. An x-y recorder is connected to the output of

the detector. The masses are swept by changing the frequency of

the rf voltage. A potentiometer is attached to the tuning knob

of the rf generator. The voltage from the potentiometer is thus

directly related to the rf frequency and is connected to the other

input of the x-y recorder. The potentiometer and tuning knob are

driven by a one rpm motor. No development work has been done to

make this instrument a fast-sweep instrument, capable of displaying its output on an oscilliscope , but this seems feasible.

i7~~~

For the rules in differentiating under the integral sign, see

any text such as Hllderbrand ' s "Advanced Calcuies for Engineers,"

p. 352.
4. Resolution

An indication of the resolution which can be expected- with

this instrument can be obtained by writing equation (1) with

v = L/t:

_ \nr t L

V 2eV (b)

where t is the time-of-f light of the ion through the drift tube

and L is the length of the drift tube. Differentiating:

dt = Or dm + dV = 1/2 4” dm - 1/2 -4“ dV £m 3 V m V (5)

Assume for the present that Vpf is small enough compared to the

d-c accelerating voltage so that the second term in equation (5)

can be neglected. Then equation (5) can be written as:

dt _ t

dm 2m (6)

This expression gives the difference in flight time between ions

of mass m and m + dm. An expression for the net change in voltage or energy, dX, which an ion receives from the rf voltage

is obtained by multiplying equation (6) by the time rate of

change (or slope) of the rf voltage:

dX = dt_ dVrf

dm dm dt

Since the rf voltage is a sine wave, the magnitude and sign of

the slope is a function of time. The resolution depends on the

magnitude of the slope, but is independent of the sign. Therefore, equation (6) is multiplied by the rms value of the slope,

equal to trVr f ¥ Vr f is defined as the' peak to peak voltage of the

TZ T

rf wave and T as its period: dX = irt V ^ dm ^2m T (7)

If m is the resonant mass, the above expression gives the energy

in electron volts, which an ion different in mass from m by dm

receives from the rf field,

written as: dX

Since t = mn VrfnT, equation (7) can be

( 8 )
6 ~

It is

tooth wave

to V

r£ and

have’ teen

interesting to note that had the rf voltage been a sawwith constant slope (that is, rising linearly with time

then falling quickly back to zero) equation (6) would

multiplied by V -/T, making equation (8) equal to nVrf . X X Therefore, better resolution should be obtained using a sine wave

rather than a saw-tooth wave, and this was verified experimentally*

Equation (8) indicates that the resolution increases with Vr£.

However, if Vrf is too large compared to the d-c voltage through'

which the ions have been accelerated, the last term in equation (5)

becomes significant. This means that the rf voltage begins to modulate the energy and thus the flight time of the ions through the

drift tube. It is desirable, however, that the variation in flight

time expressed in equation (5) be due mostly to mass difference

rather than voltage fluctuations. Experiments indicate that best

results are obtained in separating masses m and m +• dm when

V

rf ~ 2 dm

Vdc M (9)

A second condition for good resolution is that V r£ be at least

several times as great as the variation in AH . Equation (8) also shows that the resolution increases with

n. Figure b shows the peaks of k;39 and Ed1 for values of n equal

to 8 through 20 (i.e., the 8th through 20th harmonics). It can

be seen that the separation of the isotopes does increase for

higher values of n. Two conditions place an upper limit on the

value of n. Since for a given mass, n is proportional to the

frequency, the usual engineering problems of high frequency can

become troublesome. The other limitation results from a decrease

in spacing between harmonics for higher values of n. When optium

values of n and Vrf are used, the maximum resolution that can be

obtained from this Instrument depends on how small a change in ion

energy, dX, can be detected. This, in turn, depends on how monoenergetic the ions are to begin with at the source, how monoenergetic they remain as they enter and travel down the drift tube,

how well focused the beam is at the barrier [ 71 s and how sharply

the barrier and collector can cut off ions with insufficient

energy. 2/

17

“For sharp cut off, the barrier must be such that no fie.].d can

penetrate through. The collector must be able to collect ions

of .large and small energy with equal efficiency and be able to prevent ions which have experienced elastic reflections from escaping

the collector.
7

-

5. Harmonics

As has been mentioned before, ions may stay in the drift

tube any whole number, n, of cycles in order to be detected.

Thus, as the frequency is varied, the mass peaks will repeat

corresponding to various values of n as in Figure 4, The two

rf frequencies at which two adjacent harmonics of a particular

ion appear must be measured to determine the ion's mass. The

ratio of these two frequencies is equal to the ratio of their

harmonic numbers. That is*.

This equation can be solved for n, or n can be found conveniently on a slide rule. The procedure is to look for two consecutive integers which have the same ratio as f /fn+q. Knowing

n and the corresponding frequency, fn (equal to S/Tn ) , t is found

from t = nTn , which in turn gives m in equation (4).

The determination of m becomes difficult if the ion beam contains ions of many masses. The harmonics of the mass peaks become too cluttered to pick out two adjacent harmonics. In applications requiring the study of complex beams, it would be desirable to eliminate all but one of the harmonics for each mass.

Partial elimination of the harmonics has been achieved by applying

two rf frequencies referred to as fq and f^ as in Figure Frequency f^

is generated from fu by frequency division so that the ratio of

f^ to fq is known whole number, say r. With the present equipment, r can be chosen to be equal to 2, 4, 8, 16 or 32. In order

for the ions to escape a change in energy, they must pass through

two drift tubes, emerging from each tube at the same rf phase that

they entered. Let nq be the number of rf cycles an ion takes to

travel through the first drift tube, and n^ the number of rf cycles

an ion takes to travel through the other tube. Since both tubes

are of equal length, the transit times for both tubes must be equal.

That is, nqTq = n^T^. By definition, fq = 1/Tq and f^ = 1/T^.

Thus, if nq is equal to one, n, must be equal to r. An ion which

would pass through the second urift tube with npftr does not pass

through the first drift tube in a whole number of rf cycles. The

voltage at frequency fq discriminates against the unwanted harmonics while the voltage at frequency f^ resolves the masses as

before. As was the case in using a single drift tube, the rf

n+1

n

( 10 )
- 8 -

voltages can be increased until the improvement in resolution is

balanced out by the adverse effects of velocity modulation. The

value for the optimum rf is again expressed in equation (9)» Had

the two voltages been applied together to one long drift tube, the

sum of their magnitudes would have to be less than 2VC| C dm . Applying them separately, each rf voltage must be less than

2V^

c dm, resulting in larger permissible magnitudes.

Figure 6 was obtained by using this principle of harmonic

elimination. It shows the frequency range where the 10th through

28th harmonics of potassium would normally appear. The largest

peak at the center is the l6th harmonic of k39. The two small

peaks on either side are the 15th and 17th harmonics of K39.

Figure 6 was made using the maximum low frequency voltage which

the equipment could generate. It is felt that better results

could be obtained by increasing this voltage, but up to now, no

further developmental work has been done along this line.

6. Results

The following positive ions have been detected with this

instrument: 39 *+l 23 ^ + ++ 35 37 85

K , K , Na , He , NP , N , G1 , Cl , Rb , 87 133

Rb and Cs . All except the helium and nitrogen ions were

obtained by placing salts on a hot tungsten wire filament [8].

Helium and nitrogen were ionized by heating a bare tungsten filament in atmospheres of helium and nitrogen at reduced pressures.

A calibration factor defined as:

CF = 1 n/jL = N/2eV (11)

was measured for each of the ions listed above. If m is in

atomic mass units ( oA-8 - 16 . 00) and 1/Tn in megacycles, the

calibration factor was found to be:

CF = O .678 ± 0.003 (= ± 0.h%)

This figure was obtained by using known values of mass for the

above ions. The frequencies at which the various harmonics appeared

were measured with a timer-counter. The d-c accelerating voltage

USC OMM-NBS -DC
- 9

-

was approximately 3,000 volts, and the drift tube length was

roughly 1.12 meters long (0.56 meters long when used with the

harmonic eliminator). With the present source, it is necessary

to apply a focusing field in the region where the filament is

located. The proper focusing voltage changes each time the filament is removed to replace the salt. It has been found that

this change in focusing voltage produces a slight change in the

calibration factor, accounting for a portion of the 0.4# variation. For a fixed value of the focusing voltage, the calibration

factor is found to vary about 0.1#. Figure 7a and 7b show the

30th through 48th harmonics of Rb^5 and Rb°'(a), and the 15th

through 37th harmonics of 03^33 (b). Figure 8 is a mass spectrum

showing , Cl35 and C1 J/ . Some variation in the peak

heights of the harmonics can be observed in figures 7b and 8.

Duplicating these graphs shows that this fluctuation is due to

the instability of the source. Little care has been taken, up

to date, to ensure linearity in peak heights. In Figure 9? the

ion source at the right, the tube containing the drift tube, the

collector shielding box, and some of the electronic equipment are

visible.

7. Acknowledgments

The author wishes to thank Mr. Carroll E. Greitz for his

help and interest, Mr. William H. Bailey for his shop work,

Mr. Louis A. Marzetta and Mr. Malcom S. Morse for their work

in developing the rf generators, and others who have made helpful suggestions.
References

]~L F„ Calcote, “Ions In Flames” Aero Chem Research

Laboratoreis, Inc., Princeton, N» J.

W. C. Wiley and I. H® McLaren., "Time-o.f-Flight Mass

Spectrometer with Improved Resolution” Rev® Sci.

Instru. 26 9 1150) (1955)

P. A® Redhead, “A Linear Radio Frequency Mass Spectrometer”) Canadian Journal of Physics 1, (1952)®

P« A® Redhead and C„ R. Crowell, “Analysis of the Linear

R. F® Mass Spectrometer" Journal of Applied Physics 2k- ? 331, (1953).

Fox, Hickam, Grove, Kjeldaas, Rev® Sei® Instr. 26, 1101

(1955)

Leder and Simpson, Rev. Sci. Instr. 29 ®, 571 (1958).

j* A. Simpson, Rev. Sci. Instr. 1283 (1961).

Mark G. Inghram and Richard J. Hayden, “Mass Spectroscopy

Nuclear Science Series Report No. lh9 National Academy

of Sciences-National Research Council,, Washington. D.C.

(1955-).
TO

AE

VOLTAGE

GENERATOR
FIELD

AT

EXIT

Figure 2 0 Radio frequency fields at entrance and exit of a drift

tubeo A resonant ion entering at time A, , and leaving

at time A

2 experiences no change in energy <= A typical

nonresonant ion, entering at time B, , and leaving at

time experiences a net gain in 1

energy 0
COLLECTOR

CURRENT

2

Figure 3° Cut-off curves for n " 12.0 (central curve) n = 11.5 (1),

n = 11.7 (2), n = 12.2 (3)« and n = 12.4 (4) showing the

difference in output signals for resonant and nonresonant

-Ions o'
Eighth

through

twentieth

harmonics

ofK
Figure

5

°

-

Drift

tube

arrangement

used

to

eliminate

harmonics
Figure

6°

Mass

spectrogram

of K^

jislng

the

harmonic^elimination

equipment

Figure

7fc°

Harmonics

of

Cesium.
Figure

8.

Harmonics

of

k39^

Cl3^

9

and

Cl37

0

U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL. BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BIJREAll OF STANDARDS

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its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

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