Ultrahigh Resolution Isotope Distribution Calculations

Abstract

A modification of a previously described Fourier-transform-based algorithm for calculating isotope distributions is presented. It allows one to calculate ultrahigh resolution isotope distributions on computers having limited memory resources and incudes a peak-shape function as an inherent part of the calculation. The method zoomsin on a limited mass range and uses digital filtering to reduce the required array size for the calculation. Alternative methods typidy have very unfavorable scaling properties as molecular weight increases, making them unsuitable for this application. At present, the method is probably the only practicable way to calculate ultrahigh resolution isotope distributions of very large resources.Rockwood et al. recently introduced a new method to calculate isotope distributions.1v2 The new algorithm uses Fourier transform methods and is generally fast, very accurate, and applicable to a wide range of compounds. One of the properties of the new method is that it produces peak profiles that can much more readily simulate instrumental peak widths and shapes than previous approaches, particularly for high molecular weight molecules. Using a relatively small number of sample points, e.g. 2048 complex double precision data points, most calculations on molecules of molecular weight > lo' produce 'high resolution' isotope distributions with resolutions typically ranging from lo" to well over 10' and complete in about one second on a typical personal computer such as a 50 MHz 486dx2-based microcomputer. The method is, in principle, extendable to "ultrahigh resolution" by increasing the array size. This would allow the details of the distibution to be investigated at the millidalton level or better, allowing one to resolve 'micropeaks' arising from isotopic combinations having the same nominal mass but slightly different exact masses. As a rule of thumb, millidalton peak widths are sufficient to resolve some or all of the micropeaks comprising a nominal isotope peak, and this can serve as a practical definition of 'ultrahigh resolution'. If computer memory is limited, simply scaling up the array size to achieve ultrahigh resolution may not be practicable. Here, we describe a method to mom-in on an individual isotope peak for an ultrahigh resolution view, without the need for a large computer memory.About a decade ago, Yergey et al. discussed some aspects of ultrahigh resolution mass spectrometry and showed results of ultra high resolution calculations of isotope peaks from compounds as heavy as 104Da?-' (Isotope distributions for compounds as high as 10s Da were calculated, but the ultrahigh resolution aspect was not discussed for these higher molecular weight systems.) As a rule, microscopic structure of an isotope peak is revealed at about the millidalton level. At this level, a single nominal isotope peak is split into a complex pattern due to slight differences in mass defects of different isotopes of different elements. Although instrumental performance at that time was not sufficient to resolve th...

Spectral accuracy of molecular ions in an LTQ/Orbitrap mass spectrometer and implications for elemental composition determination

“…Indeed, our observation that the fluorophosphazine ion [C 30 H 19 O 6 N 3 P 3 F 48 ] ϩ (m/z 1521.9715) from the Agilent tune mix, which has limited fine structure due to its low oxygen and high fluorine content, does not display a large difference in spectral accuracy between low and high resolving powers as does thiostrepton and the other natural products with significant fine structure that we studied with similarly high mass (see Table 2), is consistent with these considerations although additional effort may be needed to more rigorously explain these phenomena. Although in practice we were not able to resolve the fine structure of the compounds studied here, as this would require resolving power of about at least 400 K, the ability of high-resolution mass spectrometry to resolve isotopic fine structure and the resulting impact on mass spectra has been investigated by earlier researchers [19,29]. However, isotope fine structures contain important elemental composition information regardless of the resolving power, i.e., whether the isotope fine structure is visually resolved or not.…”

“…There are several published papers on the efficient calculation of theoretical isotope distributions, including both discrete distributions and the superimposition or convolution of a peak shape function for continuum distributions, notably by Yergey [17] and most recently by Rockwood et al [18,19]. However, these programs do not provide a numerical assessment comparing the theoretical spectrum with the experimentally observed spectrum as does the program self-Calibrated Lineshape Isotope Profile Search (sCLIPS) described here.…”

A hierarchical algorithm for calculating the isotopic fine structures of molecules

“…On the other hand, based on the convolution theorem, Rockwood and colleagues developed Fourier transform-based methods [21][22][23][24] to directly calculate the theoretic isotopic envelope. These Fourier transformbased methods are conceptually independent of polynomialbased methods, although they can be connected with each other [22].…”

Efficient calculation of exact mass isotopic distributions

“…A number of methods [1][2][3][4][5][6][7][8][9] have been employed to elucidate the fine isotopic structure. The majority are polynomial based methods that rely on pruning to reduce the complexity to a more manageable size.…”

“…Rockwood et al [8,9] use a Fourier transform method to zoom in and achieve ultrahigh resolution around a single mass peak. As with any Fourier analysis method, care must be taken to choose an appropriate window function.…”