Radiant energy – Ionic separation or analysis
Patent
1997-09-04
1999-06-08
Anderson, Bruce
Radiant energy
Ionic separation or analysis
250282, H01J 4900, B01D 5944
Patent
active
059106557
DESCRIPTION:
BRIEF SUMMARY
CROSS-REFERENCE TO RELATED APPLICATIONS
This is the national stage of International Application No. PCT/GB97/00016 filed Jan. 3, 1997.
This invention relates to mass spectrometers and methods of operation thereof. It is particularly applicable to mass spectrometers used for elemental analysis, especially those employing a plasma ion source such as an inductively coupled plasma (ICP) or a microwave-induced plasma (MIP) source. Such instruments typically use a quadrupole mass filter but magnetic-sector mass filters are also used. The invention is applicable to both types.
Mass spectrometers are used to analyse a sample by ionising the sample and separating the ions formed according to their mass-to-charge ratios. Different ionisation techniques are used depending on the different types of sample to be analysed. For elemental analysis, plasma sources are particularly valuable as they generally have low background noise and high sensitivity due in part to the very high source temperatures. However, plasma sources do have some disadvantages. In particular, interferences may occur, which may be due to isotopes of two or more different elements having approximately the same mass, from charged molecular or polyatomic species for example Ar0.sup.+, Ar.sub.2.sup.+ or oxide ions, or from doubly charged species appearing at the same mass-to-charge ratio as a singly charged ion.
One field in which this problem is particularly acute is elemental analysis, particularly of elements such as the Rare Earth Elements (REE) Coupled Plasma Mass Spectrometry" Date A. R. and Gray A. L., (1989) Pub. Blackie and Son, Glasgow, UK.! As discussed therein, many techniques have been developed for avoiding interferences, such as water-cooling the spray chamber in which the sample is nebulized, or altering the composition of the plasma gas. Furthermore, an energy- and mass- filtering method for reducing isobaric interferences in an ICP mass spectrometer has been proposed in WO94/07257.
In addition to changing the hardware parameters, it has been proposed to reduce the effect of interferences by using mathematical techniques to separate out the interference peaks. eds. Holland, Eaton, Pub Royal Soc. Chemistry, UK; pp 101-109 "Multivariate Analysis of ICP Mass Spectra: Determination of Nickel and Iron in Body Fluids"! propose a method based on Principal Components Analysis (PCA). PCA is a multivariate technique that relies on data reduction and remodelling to derive the minimum number of components necessary to account adequately for a complex system. Templeton et al postulate that the series of factors derived from the spectra by PCA gives an indication of the number of significant components in the system and implicitly the number of interfering components in the spectra. However, in actuality the spectrum may contain interferences to which PCA may be blind so that this approach is fundamentally flawed.
Various other mathematical or statistical techniques have also been proposed for data analysis in different branches of mass spectrometry. interpretation of molecular mass spectra by a simplex method, comparing Chem. (1977) Vol. 49(12) pp 1723-6! use a library-based technique, comparing the characteristic isotope patterns of Chlorine or Bromine in GC/MS spectra to analyse molecular and fragment ions. Jurasek et al technique. Although statistical in nature, these latter papers address the different problem of attempting to reconstruct a molecular formula that will fit the observed isotopic mass distribution of an unfragmented molecule. Such work is not fully probabilistic because the question of the intrinsic plausibility of different chemical formulae has not yet been rigorously addressed. In elemental analysis, by contrast, the aim is (to a first approximation) to analyse fragmented molecules in terms of separated atoms, whether or not those elements have the integer ratios that would derive from a single molecular species.
Another approach which has been applied to scientific data analysis is that of Bayesian analysis, which is known to
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Anderson Bruce
MaxEnt Solutions Ltd.
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