Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
2001-03-07
2003-05-06
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
C250S287000, C250S42300F
Reexamination Certificate
active
06559444
ABSTRACT:
The invention relates to a tandem mass spectrometer and a method for scanning daughter ion spectra which uses a quadrupole mass spectrometer for selection of parent ions and another one for the measurement of the daughter ions.
The invention consists of not using a conventional third quadrupole filter as a collision cell for fragmentation of the parent ions but an ion guide system with helically coiled wires, especially in the form of a double helix, in which the ions can be completely decelerated and can be actively fed to the outlet aperture.
PRIOR ART
Quadrupole mass spectrometers can be traced back to Wolfgang Paul. In patent DE 944 900 (U.S. Pat. No. 2,939,952) by Paul and Steinwedel from the priority year 1953 both the quadrupole mass filter and the quadrupole ion trap are described. Knowledge of quadrupole mass spectrometry is assumed here.
Tandem mass spectrometry is the measurement of daughter ions in a second mass spectrometer, whereby the daughter ions are obtained from parent ions which are selected in a first mass spectrometer. Usually the daughter or fragment ions are generated in collisionally induced processes with gas molecules between the first and second mass spectrometer, but other types of fragmentation are also known for the parent ions.
Tandem mass spectrometry with quadrupole filters has been known for about 20 years (U.S. Pat. No. 4,234,791, C. G. Enke, R. A. Yost and J. D. Morrison; U.S. Pat. No. 4,329,582, J. B. French and P. H. Dawson) and normally uses a technique which is based on “triple quadrupoles” or “triple quads”. The first quadrupole serves as a mass spectrometer for selection of the parent ions, the second quadrupole serves as a fragmentation chamber with injection of the selected parent ions into a collision gas, and the third quadrupole serves as a mass analyzer for the resulting daughter or fragment ions.
The first quadrupole mass spectrometer is operated at an RF voltage with superimposed DC voltage, so that a small mass range can be selected (or more precisely: a range for the mass-to-charge ratios which can solely be determined by mass spectrometry). The second quadrupole, on the other hand, is operated only at an RF voltage without any superimposed DC voltage so it only acts as a guidance system for the ions. The ions injected at approx. 20 to 30 electron Volts diffuse very strongly in the collision gas so the guidance system for the ions (also referred to as ion guide system) prevents ion losses. The third quadrupole is again operated with superimposed DC voltage, it filters out ions of a single mass (or rather of a single mass-to-charge ratio). By changing voltages the filtered mass can be altered and in this way an entire spectrum can be scanned across all the masses.
A triple quadrupole mass spectrometer has proved particularly successful for quantitative analysis of mixtures of substances, whereby the mixtures are separated by gas chromatography or liquid chromatography and are fed to the ion source of such a spectrometer. Since the substances are known in principle, it is not necessary to measure the daughter ion spectra entirely. One can leave the mass spectrometer set so that the first quadrupole mass spectrometer admits a characteristic ion of a substance, in the second quadrupole this then produces daughter ions, of which, however, in the third quadrupole again only a characteristic daughter ion is measured. For the measurement of this substance there is therefore no scan by the third quadrupole from mass to mass but both filters remain open constantly. This produces a high transmission for the ions and a high selectivity for the substance sought.
To improve the measuring accuracy from a quantitative aspect one can add a reference substance, preferably an isotope-marked derivative of the test substance; one then measures both substances at the same retention time. By simply switching over the two admission windows of the quadrupole filters for the two substances one can determine their ratio. Here too it is not the entire mass range which is scanned, there is only a switch to and fro between the two admission states.
There are also other highly interesting methods of operation for triple quadrupole mass spectrometers but these will not be discussed individually here.
The triple quadrupole mass spectrometers known nowadays still have, despite many years of development, considerable disadvantages which are to be found in the principle of the equipment. For triple quadrupole mass spectrometers there is a fundamental problem: if one increases the collision yield of daughter ions by increasing the collision gas density in the center quadrupole, one increases the velocity inhomogeneity of the daughter ions at the output from that quadrupole, which leads to inferior transmission when passing to the third quadrupole and to an inferior mass resolution in that quadrupole mass spectrometer. The rods of this analytical quadrupole mass spectrometer must therefore be very long in order to achieve better mass filtering with a long dwell time also for faster ions in that quadrupole field; the inferior transmission on passing to that quadrupole can, however, not be improved. Long quadrupole systems are also difficult and expensive to manufacture.
To solve this fundamental problem a method has become known (Sciex Inc., Thornhill, Canada), which keeps the collision gas density relatively low in the second quadrupole and simultaneously increases the fragmentation through excitation of the ion oscillations in that quadrupole with a resonance dipole alternating field for the parent ions perpendicular to the direction of ion flight. This can be performed with an additional alternating voltage across two opposite poles of the quadrupole. Due to this additional excitation the yield of daughter ions is improved but the fundamental problem of the triple quadrupole mass spectrometer is not completely solved.
The six-dimensional space of spatial and pulse coordinates of particles is referred to as the “phase space”. In an ion beam the spatial and pulse coordinates of all the ions fill out a certain part of the phase space and this part is referred to as the “phase space volume”. The fundamental problem of any triple quadrupole mass spectrometer is that in the collision quadrupole the phase space volume of the ions is increased and the analytical quadrupole mass spectrometer can only efficiently separate ions of a small phase space volume. The mass resolution of a third quadrupole mass spectrometer therefore is quite essentially dependent on the spatial and velocity distribution of the injected ions.
According to the laws of physics a reduction in phase space volume cannot be achieved by ion-optical means but only by cooling the ion plasma of the ion beam, for example by cooling in a damping gas. Such cooling of the ions by a damping gas (at the expense of time) is, for instance, known from RF quadrupole ion traps. Cooling of the ions of the center quadrupole field fails, however, due to the fact that the ions require a residual forward velocity in order to reliably fly out of the field again.
OBJECTIVE OF THE INVENTION
It is the objective of this invention to find a device in which injected ions are not only fragmented but also cooled so that their phase space volume is reduced. It should then be possible to inject the ions as a fine beam with homogenous energy into a quadrupole mass spectrometer acting as an analyzer.
SUMMARY OF THE INVENTION
The invention consists of using—for fragmentation of the parent ions—an ion guide system with at least one helically coiled wire pair in which the motions of all the ions can be completely damped after their fragmentation due to a high gas density so that they practically come to rest in the gas and collect along the axis of the ion guide system. In such an ion guide system the ions must then be actively guided to the end of the ion guide system by an extra thrust, extracted there and be injected into the analyzing quadrupole mass spectrometer.
An ion guide system which is only comprised of one coiled pair of w
Bruker Daltonik GmbH
Hashmi Zia R.
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