Molecular mass number

Mass number Molecular species Polyethylene Polystyrene ... 

Mass number Molecular species Tefzel Polyimide... 

Amines have odd numbered molecular weights which helps identify them by mass spectrometry Fragmentation tends to be controlled by the formation of a nitrogen stabilized cation... 

Molecular formula Name Mass numbers (and intensities) of ... 

The ions so produced are separated by their mass-to-charge (m/z) ratios. For peptides and proteins, the intact molecules become protonated with a number (n) of protons (H+). Thus, instead of the true molecular mass (M), molecular ions have a mass of [M + uH]. More importantly, the ion has n positive charges resulting from addition of the n protons [M + uH]". Since the mass spectrometer does not measure mass directly but, rather, mass-to-charge (m/z) ratio, the measured m/z value is [M + uH]/u. This last value is less than the true molecular mass, depending on the value of n. If the ion of true mass 20,000 Da carries 10 protons, for example, then the m/z value measured would be (20,000 + 10)/10 = 2001. 

A SSIMS spectrum, like any other mass spectrum, consists of a series of peaks of dif ferent intensity (i. e. ion current) occurring at certain mass numbers. The masses can be allocated on the basis of atomic or molecular mass-to-charge ratio. Many of the more prominent secondary ions from metal and semiconductor surfaces are singly charged atomic ions, which makes allocation of mass numbers slightly easier. Masses can be identified as arising either from the substrate material itself from deliberately introduced molecular or other species on the surface, or from contaminations and impurities on the surface. Complications in allocation often arise from isotopic effects. Although some elements have only one principal isotope, for many others the natural isotopic abundance can make identification difficult. 

The nitrogen rule of mass spectrometry says that a compound containing an odd number of nitrogens has an odd-numbered molecular ion. Conversely, a compound containing an even number of nitrogens has an even-numbered M+ peak. Explain. 

Summary If the molecular weight is odd, then the compound contains an odd number of nitrogens. Fragment ions observed at even-mass numbers suggests the presence of nitrogen. The loss of ammonia is fairly common in nitrogen compounds and may not indicate exclusively that an amine is present. Chemical derivatization will easily determine if the unknown is a tertiary amine. 

Calculation of Thermodynamic Properties We note that the translational contributions to the thermodynamic properties depend on the mass or molecular weight of the molecule, the rotational contributions on the moments of inertia, the vibrational contributions on the fundamental vibrational frequencies, and the electronic contributions on the energies and statistical weight factors for the electronic states. With the aid of this information, as summarized in Tables 10.1 to 10.3 for a number of molecules, and the thermodynamic relationships summarized in Table 10.4, we can calculate a... 

A hydrocarbon of empirical formula C,H, takes 349 s to effuse through a porous plug under the same conditions of temperature and pressure, it took 210. s for the same number of molecules of argon to effuse. What is the molar mass and molecular formula of the hydrocarbon ... 

In POLYM the output data of KINREL are used with compositional information to calculate the number and mass average molecular masses (Rn and Rm, respectively) and number and end-group average functionalities (fp and fg> respectively) in the pre-gel region in all stages. In addition, the network characteristics such as sol fraction, mj, and the number of elastically active network chains per monomer (5), Ng, are calculated in the post-gel regime of stage 3. 

Figure 1.2. A number molecular mass distribution N (M) of an ideal chain polymer. N (M) is defined for integer multiples of Mm, the monomer mass. The integer factor, P, is called the degree of polymerization... 

Now the function displays the number fraction of molecules with a certain molecular mass. Its integral is 1 by definition. Nevertheless, we still call it the number molecular weight distribution because the factor /N (A/) dM is nothing but a constant. 

The concentration of the synthase or the number of enzyme copies has been assumed to have an influence on the molecular mass and molecular mass distribution of the synthesized polymer [33],but this has not been confirmed. The only variables found so far to control the molecular mass of the polymer are the initial ratio of substrate to enzyme levels, and the concentration of inducing factors in the culture medium [34-36] cf. also Chap. 9 of this book. 

An example is shown in figure 1 of the molecular interferences which must be dealt with around mass 87 if one wishes to use a mass spectrometer for rubidium/strontium measurements in a geological sample [22]. The major elements in this lunar sample all have mass numbers less than 48. Thus, the mass 87 region should be completely free of atomic peaks except for the minor components such as rubidium and strontium. This is clearly not the case and at most mass numbers in the rubidium region there are major interferences from molecules. 

Until recently the only satisfactory way to separate these molecular interferences has been on the basis of nuclear mass defects, i.e., the mass of molecules having the same mass number differs from that of the atoms of the same mass number. Figure 2 shows the resolution that is needed to resolve the molecular impurities present in the previous example. Clearly, an unambiguous identification can be made, and all molecular fragments can only be eliminated for an instrument with resolution M/AM approximately 20,000. Once again, the need for high resolution will cause the transmission efficiency to be low. 

Table I. Molecular structure and mass numbers of selected products by photoetching of PMMA...

The mole is the most important concept in this chapter. Nearly every problem associated with this material requires moles in at least one of the steps. You should get into the habit of automatically looking for moles. There are several ways of finding the moles of a substance. You may determine the moles of a substance from a balanced chemical equation. You may determine moles from the mass and molecular weight of a substance. You may determine moles from the number of particles and Avogadro s number. You may find moles from the moles of another substance and a mole ratio. Later in this book, you will find even more ways to determine moles. In some cases, you will be finished when you find moles, in other cases, finding moles is only one of the steps in a longer problem. 

Restricting to the more common elements in organic mass spectrometry (H, B, C, N, O, Si, S, P, F, Cl, Br, I, etc.), a simple rule holds valid With the exception of N, all of the above elements having an odd number of valences also possess an odd mass number and those having an even number of valences have even mass numbers. This adds up to molecular masses fulfilling the nitrogen rule (Tab. 6.7). 

A closer look at the spectrum of K-decane also reveals fragment ions at m/z 84, 98, and 112, i.e., rearrangement ions at even mass number. The origin by loss of H from the accompanying carbenium ions at m/z 85, 99, and 113, respectively, can be excluded by application of the even-electron rule. Instead, alkane molecular ions may undergo alkane loss, [76] e.g.,... 

Example At molecular weights of some 10 u the isotopic distribution of organic ions becomes several masses wide (Fig. 11.20). The minimum resolution for its full separation is always equal to the ion s mass number (Chaps. 3.3.4 and 3.4.3). Lower resolution can only provide an envelope over the distribution. At insufficient resolution, the resulting peak may even be wider than the envelope. [98]... 

To complete the calculation we must assume values for Cgi and the total number of molecules migrating over the surface, not related to the number of active sites. For our calculation we assume Cg, = c 2 = 10 molecules cm . Using these values and converting masses to molecular weights, Eq. (39) becomes... 

---