Miniature mass spectrometer

MASS. SPECTROMETER with an unusually simple electrode structure, and with a unique method of operation is described in this paper. It is an ion storag...
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INST R UME NTAT10 N

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Jonathan W. Amy Glenn L. Boornan Robert L. Bowman

Jack W. Frazer G. Phillip Hicks Donald R. Johnson

Howard V. Malmstadt Marvin Margoshes William F. Ulrich

A Miniature Mass Spectrometer Achievements t o date hold promise for the development of a relatively inexpensive, lightweight, rugged instrument with good sensitivity, moderate resolution, and capability of easy operation from a central multistation control

Peter H.Dawson C. R .A. M ., Un ive rsite Lava I, Quebec, Canada

?IINIhTURE

MASS

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SPECTROMETER

AAivitli an unusually simple electrode structure, and viith a unique method of operation is described in this paper. It is an ion storage instrument, with the ions being mass-selectively trapped in the volume enclosed by the electrodes. The mass-selected ions of a given charge-to-mass ratio are periodically pulsed out of the trap into an electron multiplier to determine the relative nbundancc of that species. By ramping the voltages applied to the electrodes, ions of each charge-to-mass ratio are trapped in sequence, and the mass spectrum is scanned. The performance, use, and potential of this device are disciissed in the light of the development of mass spectroscopy as an analytical tool with special emphasis on recent efforts to develop simple, light-weight, low-cost instruments. Ion analyzers can be divided into two broad catcpories-static and dynamic. Static analyzers depend on differences in ion trajrctories under the influence of constant magnetic and/or electric fields. The familiar single and double focusing magnetic sector analyzers dominate this category. A%notllerexample is the cycloidal mass spectrometer in n-hich uniform magnetic and electric ficlds are applied a t right angles to each other. Dynamic sprctrometers are of more recent origin ( I ) . I n these instruments, the ion separation is based

J . W. Hedman General Electric Co., Analytical Measurements Business Section, West Lynn, Mass. 01905

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on the time dependence of one of the sThtcm parameters There are three >libgroups Energy balance spectrometers such as tlie omegatron; time-offlight spectrometers and path stability qpectrometers Our particular interest ( 2 ) has been 111 the latter category, which includes tlie quadrupole mass filter, the monopole mass spectrometrr, and the threedimen.iona1 quadrupole or ion trap mass spectrometer The mass filter and monopole tvpe of instruments have heen commerciallv available for about eight vears Our miniature spectrometer is n three-dimensional quadrupole ion trap ~

The Evolution of Mass Spectrometry

The first crude mass spectrometers were Thomson's parabola instrument of 1910, and Aston's velocity focusing device of 1918. These were used to provide information on the existence of isotopes and therefore, on atomic structure. Dempster, a t about the same time, introduced the 180' deflection magnetic spectrometer, the basic design of which is still in use. Large scale electromagnetic isotope separation has hecome technologically important since 1935. Early mass spectrometric work concentrated on improving the 180" design and achieving higher resolutions, particularly for determining the devia-

N. R. Whetten General Electric Co., Research a n d Development

II

Schenectady, N. Y. centerr 12305

tion of isotope masses from integral values. This is still an active specialized subdiscipline of mass spectrometry. -4significant instrumental advance was the double focusing spectrograph using radial electric fields before the magnetic field to sort the ions according to their energies before mass analysis. One of the best known designs was by IIattauch and Rerzog. IIass spectrometers with 90' and GO" magnetic deflection became common after the work of Kier, beginning in 1940. They have the convenience, for many applications, of removing the ion source and detector from inside the principal magnetic field and of reducing the size of the magnet. The use of mass spectrometry then began t o extend into the field of chemical analysis of multicomponent mixtures, but before 1950 remained a specialized art rather than a .qtandard technique. Progress was $purred by the interest of oil companies in analysis of complex hydrocarbon mixtiires. For several years the emphasis was on deciphering mass spectra of complex mistiires and in improving stability and reproducibility. The resolution of laboratory size instruments was pradiially improved. The development of the electron multiplier has been important, since the sensitivity obtained with the multiplier can be traded for additional resolution. The increasing demand for analysis of complex or-

ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970

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Figure 1. Schematic diagram of the quadrupole mass filter

ganic reaction products led to the construction of double focusing analytical mass spectrometers of high resolution. Spectrometers w i t h resolutions of Ip,OOO can identify the exact composition of organic molecules by an accurate measurement of mass (3). Meanwhile, in the sixties, developments in vacuum and surface physics and the increasing use of vacuum processing had brought a demand for small instruments of limited resolution hut very high sensitivity, and able to withstand the baking that is part of high vacuum processing. These instruments are often known as “partial pressure analyzers.” Space and upper atmosphere exploration also brought an interest in developing small spectrometers of limited resolution. The magnetic sector instruments were quickly adapted to this use, but the quadrupole field spectrometers soon became a more popular alternative. Quadrupole Field Spectrometers

The focusing of ions by the use of alternating quadrupole fields was introduced to mass spectrometry hy Paul and his colleagues at the University of Bonn in 1953. The first type of spectrometer to be developed was the quadrupole mass filter. A quadrupole field is formed by four rods located as shown in Figure 1. Rf and de voltages are applied between opposite pairs of rods. Ions are sorted according to their path stability as they pass through the analyzing region in the z direction. By suitably adjusting the ratio of rf-to-de voltage, the device can be made to pass only a given mass ion. Ions of all other charge-to-mass ratios are “filtered” out 104A

before reachmg the detector, giving rise to the name “mass filter.” The m a s spectrum is swept by varying either the voltage or the frequency. The maximum resolution is limited by the numher of cycles an ion spends in the field. The larger the number of cycles, the higher the maximum possible resolution. The resolution can he adjusted up to this maximum by adjusting the ratio of the rf-to-de voltage. The quadrupole requires no magnet. If the rf frequency is increased, the length of the analyzer can be reduced. Thus the mass filter has the advantages of small size and light weight. Although the original applications were to problems requiring only low resolution, high sensitivity quadrupoles are now available with a mass range up to 800 and with a resolution of 2m at any mass. Scan speeds of faster than 1 msec/amu are available, so that a quadrupole type instrument can be coupled with a gas chromatograph. The monopole has characteristics and uses that are similar to those of the mass filter (4). A sketch is shown in Figure 2,. A V-block is placed in the quadrupole ground plane so that ion trajectories are limited to one quarter of the quadrupole field. Thus the monopole utilizes both the path stability and focusing properties of the quadrupole field. One advantage is that for a given mass range, less rf power is required than for a mass filter of comparable size and frequency. The performance specifications for commercially available monopoles are very similar to those for quadrupoles. The possibility of using a three-dimensional quadrupole field to actually trap ions with a small volume was recognized by Paul. I n 1959, Fischer described an early model. It was difficult t o use this model for mass analysis owing to problems in detecting trapped ions and interactions between the various species of ions which were trapped simultaneously. The idea received little attention, We only accidentally became aware of Fiseher’s work in 1966, even though one

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Figure 2. Schematic diagram of the monopole mass spectrometer

ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970

of us was a self-confessed mass spectrometrist. We were intrigued by the principle and thought that the problems could be overcome. The principle of operation is so different from other mass spectrometers that it should give rise t o a different set of properties. In particular, the trapping of ions can occur in a small volume, and there is no longer a relatively long z direction as in the mass filter and monopole ( 5 ) . The Three.Dimensional Quadrupole

A three year program of research and development, huilding devices, improving circuitry, studying the theory of operation, and computing the trajectories of ion motion culminated in the device shown m Figure 3. This is a three electrode structure, with a ring or donut-

Figure 3. Photograph of the three-dimensional quadrupole mass spectrometer, showing the stainless steel mesh electrodes

shaped electrode and ‘two cap electrodes. The structure is rotationally symmetric about the principal axis. The electrodes are formed very approximately-in fact, by hand-to hyperbolic shapeu from a coarse stainless steel mesh. Positioning of the electrodes is not very critical. Ions are formed inside the trap (the ion source and analyzer regions are combined) by a beam of electrons from a simple electron gun. By applying an rf and dc voltage combination to the ring electrode (with the end cap electrodes a t ground potential) only ions of a corresponding mass-to-charge ratio, m/e, are trapped inside the electrodes. All other ions are ejected. The stored

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ions have complicated trajectories but remain within the t r a p unless they are scattered out by multiple collisions with neutral gas molecules or other ions. Following a suitable storage or sorting time, ions of the particular m / e ratio are detected by applying a negative pulse to one of the end caps. The ions are pulled through the mesh electrode into an electron multiplier. At 10-6 torr a typical sequence is to have the electron beam on for 1 msec, switch it off for 50 psec to complete the ion sorting process, and then to apply a 3-psec pulse to one end cap electrode to empty the trap and draw the ions through the mesh electrode into the electron multiplier. The cycle is then repeated. The mass spectrum can be scanned by varying the rf and dc voltages, but with a fixed ratio between them. The resolution may be electronically controlled by varying this voltage ratio. The voltages needed are given approaimately by

Vrf = 1.23 mz02w2/2e U,, = 0.67 mzo2d/4e where o is the angular frequency of the applied rf voltage and zo is half the distance between the two end caps. A 1 5 c m long device (end cap spacing) can operate u p to about mass 200 with an rf supply of 1800 volts a t 500 kHz. Higher masses could be scanned with a larger voltage, a smaller device, or a lower frequency. Each mass peak is observed a t the electron multiplier out-

put, as a train of pulses. These are converted to a conventional display (Figure 4) by a peak-height reading circuit. The circuitry to operate the device has been constructed in a single module. The effectiveness of the ion storage property of the ion trap is shown in Figure 5 . Here, the relative number of ions remaining in the trap (after the ionizing filament was turned off) are given in a log-log plot as a function of time. These data were taken a t a pressure of 3 x 10-10 torr and in a low resolution mode. I t is shown by the shape of the decay curve that a t these low pressures, ion loss results primarily from ion-ion scattering events. That is, ions are eventually scattered out of the trap by other trapped ions. At higher pressures, neutral gas scattering is important, although the ions may undergo many scattering events before being lost from the trap. There are, in principle, two unique features of the three-dimensional quadrupole. First,, it is an ion storage device. The storage of the ions allows the ion current to be integrated with time. If the ion formation rate is low, one can store for longer times and still obtain an easily mensurable signal. The second feature is the long path length in the field even though the field is small, -4 third feature, which was not predicted theoretically, but has been found experimentally, is that a simple electrode structure with a noncritical geometry works quite well for a moderate resolution up to about 150.

The achievements to date hold promise for the development of a relatively inexpensive, rugged, miniature mass spectrometer with good sensitivity and moderate resolution, and capability of easy operation from a central multistation control. Some potentialities for this kind of device and the achievements to date are as follows : Sensitivity. The ion storage feature should be valuable at low ion formation rates, that is, for very low pressures or for very low partial pressures or for trace analysis. This has still to be demonstrated a t very low pressures. The lowest partial pressures measured to date have been 10-13 torr and the trace analysis capability has been about one part in 104. Experiments are in progress to extend these limits. Resolution. The long pat,h length of the ions in the field should permit a good resolution. Resolution can be traded for sensitivity, as with t,he quadrupole mass filter. Here the results have been good. Devices like that of Figure 3 have had half-height resolutions up t o 150. The resolution may be limited by the field perfect,ion. I n larger devices with carefully machined electrodes, w have observed resolutions up to 1000 over a limited mass range. Size. Adequate performance in small simple devices has been demonstrated. Tube construction costs should be low. P o u r . -4limiting factor in rf quadrupole instruments is often the rf power. I n all quadrupole instruments,

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Figure 5. Ion loss from the three-dimensional quadrupole mass spectrometer operated in a low resolution mode. The relative number of ions remaining in the trap is given on the ordinate for the corresponding storage time on the abscissa

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Figure 4. Mass spectrum taken with a three-dimensional quadrupole

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ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970

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ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970

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Instrumentation

the ions must stay in the field sufficient cycles of the rf field to be adequately sorted. For mass filters and monopoles of moderate length the frequency is high, typically 2-6 MHz. This limitation does not apply to the ion trap, and the long ion path length inside the trap allows reduction of the rf frequency to the hundreds of kilohertz range. The rf power depends on the fifth power of the rf frequency. A lower frequency also facilitates the remote use of the analyzer and multistation operation from a single control center. Scan Speed. Due to its pulsed mode of operation, the quadrupole ion t r a p is not a rapid scanning device. The scan speed is determined by the number of ion pulses required in each mass peak. At high pressures where storage time can be reduced, the scan speed can be increased. Conversely, a t low pressures where longer storage times are required to build up sufficient ions in the trap, the scan speed must be reduced. Use of Miniature Mass Spectrometers

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The demand for miniature spectrometers has led to several approaches by different workers. One solution has been a resurrection of the 180” magnetic deflection design with radius of only about 1 cm which has the disadvanrages of low resolution and the inconvenience of a magnet (6). A novel approach has been the use of 90” magnetic deflection with multiple passages through the same field ( 7 ) . Some of the potential uses of miniature mass spectrometers are: Standard Vacuum Applications. I n research and production involving vacuum technology, a knowledge of the nature and quantity of the gases present is increasingly necessary. For many processes such as vacuum decarburization, outgassing studies, semiconductor processing, vacuum heat treating, and tube manufacture, the requirements on mass range and resolution are not severe. However, the spectrometer should be rugged, resistant to contamination, small and low in cost. Process Control Applications. Mass spectrometers are a possible method of monitoring the composition of process lines in the petrochemical and chemical industries. Low cost, reliability, and ruggedness are essential. The speed of response of a mass spectrometer is a desirable feature. Space Uses Miniature mass spectrometers are well suited for space applications due to their size and weight and low power requirements. Ruggedness is another prime consideration. Jledzcal C‘ses. Mass spectrometers have medical application for blood gas, 42, NO. 12, OCTOBER 1970

body fluid, and respiratory gas analysis (8). Continuous monitoring of patients is desirable during operations and in intensive care units. For blood and respiratory gas analysis, only low resolution is required. However, the instrument must be extremely reliable and easy to operate. Low cost and small size are other important needs. Environmental Monitoring. Monitoring the environment to detect pollutants is another potential application. At present, miniature mass spectrometers do not have sufficient trace analysis capability for broad area monitoring. However, they can be used for monitoring emissions a t the source such as a t smoke stacks. Monitoring of the cabin atmosphere in airplanes and space stations is also possible. I n many cases, the mass range need not be large, but the sensitivity should be high. I n addition, the instrument should be extremely reliable. Geological Uses. Two potential applications are the detection of helium anomalies near uranium deposits and the detection of mercury near certain mineral deposits. Ruggedness, reliability, and portability are essential. Reaction Studies. Miniature mass spectrometers can be used to follow the rate of chemical reactions, the lifetime of metastable species, and the rate of ion-molecule reactions. For some of these applications, the trapping ability of the three-dimensional quadrupole should be useful. These diversified applications suggest future interesting developments in instrumentation.

References

(1) E. W. Blauth, “Dynamic Mass Spectrometers,” Elsevier, Amsterdam, 1966. (2)( P. H. Dawson and N . R . Whetten, Mass Spectroscopy Using Radio Frequency Quadrupole Fields,” in “Advances in Electronics and Electron Physics,” L. Marton, Ed., Vol. 27, p. 58, Academic Press, N. Y;: 1969. (3) F. W. McLafferty, Interpretation of Mass Spectra,” W. A. Benjamin. Inc., K. Y . ,1966. (4) U. von Zahn, Rev. Sei. Instrum., 34, 1 (1963). ( 5 ) P. H. Dawson and PIT. R. Whetten,

J.

Vue. Sei. Technol., 5, 1 (1968); 5, 11 (1968). P. H . Dawson, J. W,. Hedman, and N . R. Whetten, Rev. Scz. Instrum., 40, 1444

(1969).

(6) D . Allenden, R. D. Craig, and R. G. Johnson, 16th Annual Conf. on Mass

Spectrometry and Allied Topics, unpublished, Pittsburgh, May, 1968. (7) M. B a d , Int’l. Conf. on Mass Spectroscopy, unpublished, Kyoto, Japan, September, 1969. (8) S. Woldring, D. C. Woolford, and G. Owens, Science, 153, 885 (1966), also M. Mosharrafa, Res.lDevelop., 21 ( 3 ) , 24, (1970).