The electric quadrupole mass filter as a mass spectrometric aid in the assessment of high vacuum

The electric quadrupole mass filter as a mass spectrometric aid in the assessment of high vacuum D L Swingler,

Division of Chemical Physics, CSIRO, Chemical Research Laboratories, Melbourne,

Australia

The techniques of mass spectrometry as applied in the development and to the practice of high vacuum are well established. The electric quadrupole mass filter is particularly attractive as the mass analyzer for the reasons of compactness, performance and cost although the instrument itself presents some curious problems in electron optics, not usually encountered in the established methods of mass spectrometry. Two instruments employing separate versions of the electric quadrupole mass filter will be described. The design parameters for a mass spectrometer helium leak detector with a sensitivity of 1 x 10-l’ litre torrlsec at a port speed of 10 litrelsec will be contrasted with the requirements for an analytical residual gas analyzer with unit mass resolving power at mass 100 and a sensitivity of 1 ampltorr. The two instruments, designed and made in the CSIRO Division of Chemical Physics will be described.

Any account of the Electric Mass Filter and its applications to mass spectrometry should present, at the outset, its distinction in character from all previous methods for ionic mass determination. Whereas the magnetic analyzer selects ions of similar momentum and the time of flight ions of similar velocity, the mass filter is a truesselector.

The mass filter

In 1955, W Paul and M Raether’ showed that an arrangement of the electrostatic quadrupole had an ion transmission characteristic similar to that of the analyzer portion of a mass spectrometer. The quadrupole mass filter consists of four conducting cylinders arranged as in Figure 1 and excited by a high frequency alternating voltage superimposed on a constant dc voltage. The potential, V, at point (x,y) in the space formed by the quadrupole system is given by v,,,

=

(VII,

+

r‘4c

cos

wt)(x2

-

4e V,c

8eVnc a=mq==

and ot=Q

then jt + (a + 2q cos 21)x = 0 and j-(a+2q

cos 2r,7)y=O

The solution to these Mathieu equations is in the form of a series and is characterized by regions of stability solely determined by the values of a and q. McLachlanO has plotted the stability of the solutions in terms of the parameters a and q. The region of stability for the mass filter acting as a mass spectrometer is for q< 1. The theory of the mass filter operation has been further developed by Gunther6 and performance studies carried out by Brubaker and Tuul*. POTENTIAL

AT

V*

V. DC.+

V.

y2)/ro2

and the electric field is given by E, = ( I”nc + VAc cos wt)( - 2x/r,‘) E, = (V,,

+ V,c cos

ot)(2y/r,‘)

where VAc is the alternating potential, f-2:

V,,, is the direct potential

(the frequency of V,,) and r0 the axis-rod spacing.

The equations are

of motion for a charged particle in these fields

mi! = eE, =*Voc

+ VAC cos ot)x

. ...(l)

ro2 mj:=eE,=

V.AC.

gv,,+

V.DC.

V*c cos wt)y

...42)

u

inthe mass filter.

The solutions to equation (1) and (2) can be obtained by first transforming them into the standard form of the Mathieu equation by the substitutions. Vacuum/volume

Figure 1. The mechanical and electrical arrangement for the poles

Is/number

12.

Pergamon Press Ltd/Printed

W Brubaker3 has pointed out that the formal solution gives little insight into the mode of operation of the mass filter and in Great Britain

669

D L Swingler:

The electric quadrupole

mass filter as a mass spectrometric

has obtained approximate solutions to equations (1) and (2) making certain simplifications as to the amplitude of stable oscillations, thus replacing x and y by constants. Two equations may then be determined giving values for the acceleration of the ion in the AC field and the DC field. These are: dv dtAC = and-

1 - $$y

aid in the assessment

achieved by variation in the amplitude of the applied potentials. A linear mass scale results from this technique. The basic equations relating to the mechanical and electrical parameters for the mass filter have been experimentally determined by W Paul, H Reinhard and U von Zahn4v5. The equations are: 1. P’,,=7.22Nfzro2

’

3. The high frequency

The nett acceleration of the ion is the sum of these

a=&q2

. ...(5)

The values from the plot of the solution to the Mathieu function when combined with equation (5) lead to the operating stability criterion for the mass filter. Figure 2. 24

Y

UNSTABLE X UNSTABLE

12-

h02W2

.6

.2 4vAC

q=---Mr02W2

.8

The parameters, a and q, are directly determined by the values of VA, and V,, and the ratio of these, if kept constant, defines a certain dispersion. If this ratio is maintained as the amplitude is increased along the scan line a mass spectrum may be obtained. For a given voltage of excitation a certain mass is stable, lighter ions spiralling out of the quadrupole system and are discharged at the rods. Brubaker has shown that even for the ion in “focus” the trajectory is of a most complex form. In the x-direction, the ion is stable in the DC field while the AC field makes it unstable: the inverse is true in they direction. In the quadrupole mass spectrometer, the resolution is easily adjustable by the variation of the VAc to V,,, ratio. With this ratio fixed, mass scanning is accompanied by variation in the voltage level or the frequency, according to the relation M

2

*

f" In the exciter unit for the mass filters described the frequency has been fixed to a high order of accuracy, mass scanning being 670

C in ~LIF

power required

Where Q is the figure of merit for the quadrupole assembly and C the capacitance of the quadrupole system; fmc the frequency of excitation in megacycle/s. 4. Number

of

oscillations

required

for

a

resoiving

M power of 6~

&

= 3.5

cycles.

J The figure for resolution is calculated from the peak width at 10 per cent of the peak maximum divided by the product of peak separation and mass number. We may use equation (3) in combination with equations (1) and (2) to indicate the power requirements for a practical mass filter. This parameter is the most important consideration once a figure for r0 is selected. The CSIRO Division of Chemical Physics currently employs two versions of the Quadrupole Mass Filter in day-to-day high vacuum practice. In addition, developmental devices are under investigation.

e

Figure 2. The Stability Diagram Ion transmission in the mass filter occurs for values of the parameters which locate a and q above the scan line and inside the stability diagram.

VAC+KIVDC=K

r. in cm.

Q

The boundary between the stable and the unstable region of the stability diagram corresponds to the condition that the acceleration is zero; ie when

8v,,c e

in Mc/sec

=6.5 x 10m4CN2fsro2 watt

:=+(a-fq2)w2y

Cl=-

f

volt

2. Vnc= 1.21Nf2r02 volt

dv a, dt,,,=4°

of high vacuum

The Quadrupole Helium Leak Detector The arrangement of the vacuum system for the Leak Detector is shown in Figure 3. This consists, essentially, of a variable flow valve*, a liquid nitrogen trap common to the vapour of the gas under test and to the backstreaming vapour from the diffusion pumpt, and the mass filterl§. A Teflon annulus effectively separates the active cold surface of the cold trap so that the mass analyzer is in series with the gas under test rather than in shunt, as is the more common arrangement for an appendage leak detector analyzer. The variable flow valve allows gas sampling for a wide range of pressure in the test vessel; to atmospheric pressure if necessary. This consists of a steel disc with a groove of varying area of cross section overlying a part and sealed by three hard rubber “0” rings. Rotation of the disc provides a steady transference in pumping speed from the roughing pump to the mass analyzer pumping system.

*Variable Flow Valve: 66. tLeak Detector CSIRO fIon Source with Disc 26565161. §Collector for Charged No 26079167.

CSIRO

Australian

Patent Application

No 12002/

Australian Patent Application No 12005!66. Wall: CSIRO Australian Patent Application Particles:

CSIRO

Australian

No

Patent Application

D L Swingler:

The electric quadrupole

mass filter as a mass spectrometric

The mass filter has the following features: Quadrupole Assembly: Pole Diameter 0.50 in. Pole length 9 in Exciter: Self Excited. 6.8 MC/S. Ion Energy: + 105 eV. Electron Energy : + 150 eV. Filament Reference Potential : + 22 Volt. Ion Collector: 1 in. diameter cylinder positioned about the ion repeller electrode. Test Port Sensitivity: 1 x 1O-11Std cc at 10 l/c. 1 x lo-‘* Std cc at 4 l/s. The ion source has an open construction and has been arranged so that long term stability and sensitivity is maintained. The a-q ratio is fixed at 0.332 which provides a partially square band pass for helium ions with ample mass resolving power. Because of the oscillatory behaviour of the electrons in the semiaxial ion source, the formation of meta-stable atoms is strongly favoured. These particles traverse the mass filter without deflection but yield, on impact with a collector, a relatively large “background ion” indication. The use of the tapering cylindrical ion collector positioned about the ion repeller electrode allows for the detection of ions of the design energy while the “background ion” is discharged without detection. Photo-electric emission from the ion collector is also reduced by placing the ion collector out of line of sight from the filament. The cylindrical ion collector is essential for the operation of the mass tilter at high pressure (5 x lo-* mm). Figure 4. -

FLOW VALVE APERTURE

-TEST

PORT

ENNING

HEAD

aid in the assessment

of high vacuum

Electron Energy: 100 eV. Ion Energy: 38 eV. Filament Reference Potential: +22 Volts. In addition to the use of this quadrupole mass spectrometer for the analysis of residual gases, the assembly has been modified for use in the analysis of the molecular complexes formed in the vapour state by mixtures of molten salts at high temperatures. An examination of the molecular species in gaseous plasmas has also been carried out. A typical mass spectrum from this instrument is shown in Figure 6. The high sensitivity electric quadrupole (Developmental

HELIX

ANODE

POLE-

-MESH

ANODE

u

FILTER

.-COLLECTOR SHIELD

‘ELECTROMETER

Figure 3. The Quadrupole vacuum system.

device)

Ion transmission in the Mass Filter is strictly a function of the a-q ratio. Reducing the a-q ratio leads to increased ion transmission but a corresponding loss in resolving power occurs, such that when helium at 10-l’ mm is detectable, no useful resolution remains and the device approximates to a strong focusing lens. The total ion current in the mass filter bears a linear relation to the area of section of the entrance aperture, so that this must be large for high sensitivity. In the normal quadrupole mass filter, a large entrance aperture, =ro, causes a pronounced loss in resolution and rod surface degeneration so that no compromise appears possible in the basic device. A strong focusing lens has been used as the input section to a mass filter of r,=O.3 in. The strong focusing section is used to focus the total ion stream from an area of 12.6 mm’ into an area of 1 mm’. In this way, the resolution of the mass filter can be retained while higher sensitivity results. Initially, a diaphragm was used between the strong focusing section and the mass

Mass Spectrometer Leak Detector,

The Residual Gas Analyser The Quadrupole

Residual Gas Analyser is shown in Figure 7 and has the following features: Quadrupole Pole Diameter: 0.250 in. Pole length 6 in. Mass Range: l-200 AMU Sensitivity: 1 amp/torr using EM1 Particle Multiplier. Type 9603B. Sweep: Manual or Automatic.

ION REPELLE ELECTRODE QUADRUPOLE SCHEMATIC

ELECTROMETER

LEAK

DETECTOR

ARRANGEMENT

Figure 4. Quadrupole Arrangement.

Mass

Spectrometer Leak Detector : Schematic 671

D L Swingler:

The electric

quadrupole

mass

filter as a mass

spectrometric

aid in the assessment

of high vacuum

filter though this was later found to be unnecessary and is now omitted. The use of the strong focusing section at the input stage to the mass filter is mechanically convenient since it only requires that a short section of the quadrupole assembly be electrically isolated from the other (Figure 7). The strong focusing section is electrically excited from the quadrupole mass filter supply, a capacitor being used to remove the DC component. Care must be taken to reduce the water vapour content of the chamber to a minimum in order to eliminate the flanks of the hydrogen peak at mass 2. The intense ion current recorded at the extreme low mass end corresponds to the whole device acting as a strong focusing lens. This is caused by the diode anode-current characteristic reducing the DC component to zero at low values of excitation.

The Ion Source In both the medium resolution and high sensitivity mass filters ions are produced by electron impact in the cylindrical anode of an axial ion source assembly, Figure 4. The anode and filament shield assembly is mounted over the quadrupole entrance and aperture and located by ruby balls. The anode consists of five nickel annuli, spaced 0.1 in. apart or, more recently, a helix of 0.008 in. nichrome wire. Ions are extracted from the anode by a mechanically elevated ion injection electrode which penetrates a distance of I cm into the quadrupole field system. This is done in order to overcome the influence of the fringing fields at the end of the quadrupole Figure 5. The Residual Gas Analyzer.

Figure 6. A Mass Spectrum of a chamber

background

taken with the Residual Gas Analyzer.

D L Swingler:

The electric quadrupole

mass filter as a mass spectrometric

assembly. The ion extraction electrode carries a potential equal and opposite to that on the anode. This ensures a good clearing of ions from the anode, though at this energy, the velocity of the ion is in excess of the design velocity for the mass filter. The ion velocity is reduced by raising the level of the potential of the quadrupole system, irrespective of VAc and Vno, to a high positive value. Thus on entering the mass filter, the ions meet a retarding field and lose velocity. This technique gives the optimum beam current. The velocity of the ions entering the quadrupole entrance aperture of the helium detector is a maximum since the ion extraction electrode potential is at earth potential. The arrangement of the potentials is shown on the assembly diagram (Figure 4 and 8). A tantalum filament,

aid in the assessment

of high vacuum

the equipotential surfaces in such a way that ions are gathered into the aperture. A mesh of 0.0005 in. tungsten, 98 per cent transmission, stretches across the entrance to the anode and is held in place by a spot welded retainer. The mesh plate is bent to include an angle of 145”: this allows the use of two filaments

/6X

Figure 8. The circuit diagram

for the exciter

used in the Leak

Detector.

in the case of the leak detector and helps to prevent the migration of positive ions in the direction of the filament.

The exciter

Figure 7. The experimental rupole input section

Mass Filter showing the isolated quad-

0.002 x 0.020 in, is used to produce an ionizing electron current of 500 PA. The filament is mounted in a shield assembly which is biased below the potential of the filament centre tap; this has a stabilizing influence on the electron and ion currents. Higher ion currents were obtained by tapering the walls of the extraction electrode in the region of the anode. The taper shapes

The radio-frequency exciter consists of a self-excited oscillator operating at 6.8 MC/S for the leak detector or 4.5 MC/S for the RGA. The quadrupole assembly is fed via the coupling condenser, “C”. The direct potential is obtained by diode rectification and after a proportional value is tapped off, the DC is added to the AC feed, through radio frequency chokes. Variation of the DC potential in the symmetrical way required by each pair of poles is achieved by an accurately ganged potentiometer. Mass scanning is accomplished either by manual variation in the potential of the amplifier screen grid or by an automatically reset Miller sweep sawtooth generator. Mass spectra up to a frequency of ten scans per second have been G73

D L Swingler:

The electric

quadrupole

mass filter as a mass spectrometric

clearly viewed when the sawtooth scan potential is applied to the X-deflection system of a cathode ray oscilloscope.

Acknowledgment

The author wishes to thank Dr A L G Rees, Chief of the CSIRO Division of Chemical Physics, for his advice and encouragement during the development of these instruments.

References

1 W Paul and M Raether, Zeitschrift fiir Physik, 140, 1955, 8262-273. pW M Brubaker and J Tuul, Performance Studies of a Quadrupole Mass Filter. The Review of Scientific Instruments, Vol35, No 8, August 1964. 3 W M Brubaker, “The Quadrupole Mass Filter”. Paper presented at Congress Znternational Des. Techniques Et Applications Du Vide. Paris 20-24 Juin 1961: M&on de la Chimie. 4 W Paul, H P Reinhard and U van Zahn, Zeitschrift fiir Physik Bd. 152S, (1958), 143-182. 5 K G Giinther, Vacuum, 10 (4), Sept 1960, 293-309. 6 N W McLachlan, Theory and Applications of Mathieu Functions, Oxford lJni”ersity Press, 1947.

674

aid in the assessment

of high vacuum