Mass differences, exact

Table 6.6 presents a list of some of the most commonly encountered atoms in polymer/additive analysis, together with their monoisotopic and average masses. For the same nominal mass, different exact masses (elemental compositions) do exist. Knowledge of the exact mass of an unknown substance allows its atomic composition to be established. The exact mass of an ion proves the presence of a particular species (compound in a mixture). 

Exact Mass Differences. If the exact mass of the parent or fragment ions are ascertained with a high-resolution mass spectrometer, this relationship is often useful for combinations of C, H, N, and O (Table 1.15b) ... 

Exact mass difference from nearest integral mass -I- 0.005 Iz — 0.003 ly 0.0078... 

High mass resolution techniques are used to separate peaks at the same nominal mass by the very small mass differences between them. As an example, a combination of Si and H to form the molecular ion Si H , severely degrades the detection limit of phosphorous ( P) in a silicon sample. The exact mass of phosphorous ( P) is 31.9738 amu while the real masses of the interfering Si H and Si H2 molecules are 31.9816 amu and 31.9921 amu, respectively. Figure 8 shows a mass... 

This strategy is used for the synthesis of three different exact-mass telechelic oligomers. GPC, NMR, and GC/MS evidence indicates that clean depolymerization chemistry occurs for all three samples. Poly( 1,4-butadiene) (38) is broken down into oligomeric units with two, three, and four repeat units using catalyst 23. Catalyst 14 is more efficient and produces even lower molecular weight oligomers, primarily one and two repeat units. When allylchlorodimethylsilane is used instead of ethylene with 14, telechelic dimers are the only product. 

Table A. 1 comprises the stable elements from hydrogen to bismuth with the radioactive elements technetium and promethium omitted. Natural variations in isotopic composition of some elements such as carbon or lead do not allow for more accurate values, a fact also reflected in the accuracy of their relative atomic mass. However, exact masses of the isotopes are not affected by varying abundances. The isotopic masses listed may differ up to some 10 u in other publications.

It is usually simple to find an orientation in the mass spectra. For example, in the mass spectrum of SSMS, LIMS, GDMS or LA-ICP-MS of a graphite or graphite mixture sample, carbon cluster ions (C + with n = 1-24) occur with an exactly constant mass difference of 12u. As described in... 

As shown in Fig. 4.8, the mass difference between the two metabolites is 36.4 mDa and a mass spectrometer with a resolving power of at least 9500 (M/AM = 344/ 0.0364 = 9450) is required to separate between these two metabolites. MS/MS fragmentation combined with accurate mass measurement is the preferred method for structural elucidation of metabolites, especially when it can help to correlate the elemental composition determined in the MS mode. In the example shown in Fig. 4.8, exact mass measurements of the precursor ions are used as lock mass to measure the exact mass of each fragment ion. Exact mass measurements of the fragment ion at m/z 226 help to narrow down the sites of modifications and allows one to distinguish between rabeprazole-sulphide and rabeprazole-aldehyde. 

Consider a molecular ion with a mass of 44. This approximate molecular weight might correspond to C3H8 (propane), C2H4O (acetaldehyde), C02, or CN2H4. Each of these molecular formulas corresponds to a different exact mass ... 

The reason that exact masses are not integers lies in the slight mass difference between a proton (1.67262 x 1CT27 kg) and a neutron (1.67493 x 1CT27 kg) and in the fact that electrons have mass (9.10956 x 10-31 kg). 

Since carbon has 6 neutrons and 6 protons, you might assume that protons and neutrons have masses of exactly 1 u each. In fact, protons and neutrons have masses that are close to, but slightly different from, 1 u. As well, the mass of electrons, while much smaller than the masses of protons and neutrons, must still be taken into account, v /... 

Trypsin digests of both wild type HRV virus and the mutant were analyzed using MALDI-TOF and MALDI Fourier transform mass spectrometry (FTMS). For HRV, the mass spectra for both wild-type and mutant were identical except for one peptide occurring at mlz 4700. This corresponds to residues 187-227 in the wild type sequence. The corresponding peak in the mutant mass spectrum occurs at 4783.5 (Fig. 4, inset). This mass difference of 83 Da corresponds exactly to a mutation of a Cys to Trp residue and there are no other possible mutations that would be separated by 83 Da. Since there is only one Cys in the peptide 187-227 at position 199, the mutant can be localized as HRV14-Cysl99Trp, which contains a Trp at position 199 instead of Cys in the wild type. 

It is customary to describe the decrease in mass by means of a quantity called the "packing fraction." This is the difference in mass, per fundamental particle (proton or neutron) in the nucleus, relative to as standard. An isotope which has atomic mass equal exactly to its mass number on the scale is said to have zero packing fraction. 

Isobars Isobars have different elemental compositions. Isobars have identical nominal mass, but (slightly) different exact mass. 

Recall from Table 4-1 that the masses of both protons and neutrons are approximately 1.67 x 10 g. While this is a very small mass, the mass of an electron is even smaller—only about that of a proton or neutron. Because these extremely small masses expressed in scientific notation are difficult to work with, chemists have developed a method of measuring the mass of an atom relative to the mass of a specifically chosen atomic standard. That standard is the carbon-12 atom. Scientists assigned the carbon-12 atom a mass of exactly 12 atomic mass units. Thus, one atomic mass unit (amu) is defined as the mass of a carbon-12 atom. Although a mass of 1 amu is very nearly equal to the mass of a single proton or a single neutron, it is important to realize that the values are slightly different. As a result, the mass of silicon-30, for example, is 29.974 amu, and not 30 amu. Table 4-2 gives the masses of the subatomic particles in terms of amu. 

An alternative method for the easy and exact determination of total porosity can be used for small-scale columns. As long as a column can be weighed exactly, the mass difference of the same column filled with two solvents of different densities can be used to determine the porosity. The column is first completely flushed with one solvent and then weighed, afterwards the first solvent is completely displaced by a second solvent of different density. For normal phase systems methanol and dichloro-methane can be used, for reversed phase systems water and methanol are quite commonly employed. The volume of the solvent, representing the sum of the interstitial volume and the pore volume, is determined by Eq. 2.15 ... 

Mass accuracy is the difference (AAT) between the measured accurate mass M and the calculated exact mass. It can be stated as absolute units of mass (differences of so many millimass units, mmu, 10-3 u) or as a relative mass accuracy in parts per million. 

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