Upper mass limit

The question of upper mass limits to stars which explode as SN II and leave neutron-star remnants is discussed by Maeder (1992,1993) and by Brown, Bruenn and Wheeler (1992) it is highly controversial. (Note that Koppen and Arimoto (1991) when referring to the Scalo IMF use the version with b T) = 1, as I have done, whereas Maeder (1993) uses the version with b (T) = 0.48, corresponding to yields that are 3 times higher )... 

Assuming the star formation rate for the Galaxy given in Table 7.9 and that all stars between 10 and 100 M explode as Type II supemovae, estimate the corresponding supernova rates for the IMFs in Table 7.8. How much difference does it make if the upper mass limit for SN is 50 M (The observed rate for SN II in galaxies like our own is of the order of 2 to 3 per century.)... 

Fig. 8.36. Abundance ratio [Mg/Fe] as a function of [Fe/H] and time (for the case rmix = 0.1 Gyr), for three different assumptions about rmjX. Yields are taken from Thielemann, Nomoto and Hashimoto (1996) up to 40 A/ the long-dashed line shows an extension to an upper mass limit of 70 A/ (with rmjX = 0.1 Gyr) SN la yields are from Model W7 (Nomoto, Thielemann Yokoi 1984). After Thomas, Greggio and Bender (1998).

R. P. Rodgers, E. N. Blumer, C. L. Hendrickson, and A. G. Marshall. Stable Isotope Incorporation Triples the Upper Mass Limit for Determination of Elemental Composition by Accurate Mass Measurement. J. Am. Soc. Mass Spectrom., 11(2000) 835-840. 

As long as we are dealing with molecular masses in a range up to some 10 u, it is possible to separate ions which differ by 1 u in mass. The upper mass limit for their separation depends on the resolution of the instrument employed. Consequently, the isotopic composition of the analyte is directly reflected in the mass spectrum. 

Every mass spectrometer has an upper mass limit for the m/z ratio it can measure. Thus the mass in Daltons corresponding to a particular tn/z will depend on the number of charges z that the ion carries. For example, with an upper m/z value of 2 000, the instrument can detect an ion of mass 80 000 Da if that ion has a charge of q = 40e. 

Figure 16.13—Resolution. The figure on the left defines the parameters used to calculate resolution. To the right a low-resolution spectrum of a sample of lead is shown. The highest mass resolutions are achieved with cyclotron resonance instruments (see Fig. 16.8). The resolution greatly depends on the compound chosen for the calculation. For instruments in which Am is a constant, the upper mass limit theoretically corresponds to the maximum resolution, i.e. this the value at which masses m and m + 1 can no longer be distinguished from one another.

The following table lists the most popular reference compounds for use under electron impact conditions in mass spectrometry. For accurate mass measurements, the reference compound is introduced and ionized concurrently with the sample and the reference peaks are resolved from sample peaks. Reference compounds should contain as few heteroatoms and isotopes as possible. This is to facilitate the assignment of reference masses and minimize the occurrence of unresolved multiplets within the reference spectrum.1 An approximate upper mass limit should assist in the selection of the appropriate reference.12... 

Ionization method Upper mass limit (amu) for MW information Advantages Disadvantages... 

Electrospray ionization >150,000 Ability to analyze mixtures limited sequence information for pure small peptides adapts easily for LC/MS (NP- and RP-HPLC) most suitable for quadrupoles multiple charging increases upper mass limit well suited to polar or ionic compounds Multiple charging may complicate interpretation of data glycoproteins may not yield useful information limited to flow rates <10 p.l/min... 

A wide variety of studies have been performed to determine the molecular weight of intact proteins. At an early stage, Peng and Konishi [34] explored the upper mass limit of ESI-MS in the analysis of antibodies and other large... 

ToF Proteins, peptides, lipids, metabolites No upper mass limit high sensitivity Low mass resolution... 

The Eddington limit is not only important in determining the upper mass limit of stars, but it is likely to induce strong mass loss once a massive star comes close to it. I.e., the Eddington limit may shape the initial-final mass relation for the most massive star, with severe consequences for nucleosynthesis of may species. 

In Figure 2 we show the theoretical evolutionary tracks of a 1 and 5M0 star on a HR diagram. In this figure, and in what follows we show model results from Karakas Lattanzio [24] and Karakas [27], These models were computed using the Monash version of the Mount Stromlo Stellar Structure code, and we refer to [24] and references therein for details. It is important to remember that the results presented here do depend on the details of the stellar models, with different codes sometimes giving different results. For example, the inclusion of core overshoot during the main sequence will lower the upper mass limit from 8Mq to 5 — 6Mq of a star that enters the AGB with a C-0 core (e.g., [28]). 

The past years have seen the development of two important ionization techniques electrospray ionization (ESI) and matrix-assisted laser desorption (MALDI), which have been used especially in conjunction with quadrupole, time of flight (TOF) and ion cyclotron (IC) analyzers. Both ionization techniques have revolutionized the essence of mass spectrometry itself, radically increasing the upper mass limit and thus the applicability of the mass spectrometric technique and indirectly affecting the endeavors for improved resolution and sensitivity. With these techniques, the analysis of peptides, proteins and oligonucleotides has become possible. The aim of this chapter is to demonstrate the capabilities of these techniques, especially electrospray mass spectrometry in conjunction with HPLC techniques, for the analysis of combinatorially generated compounds and libraries, and to exemplify and discuss both the potentials and limits of these analytical techniques. 

For libraries with an upper mass limit of approximately 500 amu, GC-MS can prove advantageous. Capillary-GC on fused silica capillaries is characterized by high separation power and the ability to analyze relatively polar substances. In many cases, the problems of tailing, thermal instability and volatility associated with excessive analyte polarity can be overcome by the use of derivatization techniques. 

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