Weights, reference masses

Compilations of Reference Spectra There are several compilations of reference mass spectra available of which the oldest is the American Petroleum Institute (Ref 82) collection of spectra obtained mostiy on the older type instruments. Recent collections index spectra variously, eg, under reference number (Ref 19). molecular weight (Refs 12 19), molecular formula (Ref 19), fragment ion values (Ref 19), and base peak (Refs 12 19). A quarterly journal, Archives of Mass Spectral Data ... 

Principles and Characteristics Mass spectrometry can provide the accurate mass determination in a direct measurement mode. For a properly calibrated mass spectrometer the mass accuracy should be expected to be good to at least 0.1 Da. Accurate mass measurements can be made at any resolution (resolution matters only when separating masses). For polymer/additive deformulation the nominal molecular weight of an analyte, as determined with an accuracy of 0.1 Da from the mass spectrum, is generally insufficient to characterise the sample, in view of the small mass differences in commercial additives. With the thousands of additives, it is obvious that the same nominal mass often corresponds to quite a number of possible additive types, e.g. NPG dibenzoate, Tinuvin 312, Uvistat 247, Flexricin P-1, isobutylpalmitate and fumaric acid for m = 312 Da see also Table 6.7 for m = 268 Da. Accurate mass measurements are most often made in El mode, since the sensitivity is high, and reference mass peaks are readily available (using various fluorinated reference materials). Accurate mass measurements can also be made in Cl... 

Ans. (a) The nucleus is a distinct part of the atom. Neutrons are subatomic particles which, along with protons, are located in the nucleus. (b) Mass number refers to individual isotopes. It is the sum of the numbers of protons and neutrons. Atomic weight refers to the naturally occurring mixture of isotopes, and is the relative mass of the average atom compared to l2C. (/) Atomic mass is the same as atomic weight [see (b)]. Atomic mass unit is the unit of atomic weight. 

Strictly speaking, the terms molar mass and molecular weight refer to different quantities. Molar mass is the mass of one mole of polymer molecules and measured in g/mol, whereas molecular weight is dimensionless and refers to the weight (mass) of a single polymer molecule measured as multiples of the atomic mass unit u. The numerical values of both quantities are identical. Usually, no such strict distinction is made in the literature, and also in this contribution both terms will be used as synonyms for the strict term molar mass. 

Parenthetical weights refer to radioactive elements the mass number, not the atomic weight, of the isotope with the longest half-life is given. 

A particular element is defined by its atomic number - the number of protons in the nucleus (which will equal the number of electrons surrounding the nucleus in a neutral atom). For example, iron is the element of atomic number 25, meaning that every iron atom will have 25 protons in its nucleus. Chemists use a one or two-letter symbol for each element to simplify communication iron is given the symbol Fe, from the old Latin word for iron, ferrum. The sum of the protons plus neutrons found in a nucleus is called the mass number. For some elements only one mass number is found in nature. Fluorine (atomic number 9, mass number 19) is an example of such an element. Other elements are found in nature in more than one mass number. Iron is found as mass number 55 (91.52%), 54 (5.90%), 57 (2.245%), and 58 (0.33%). These different mass numbers of the same element are called isotopes, and vary in the number of neutrons found in the nucleus. Atomic weight refers to the average mass found in nature of all the atoms of a particular element the atomic weight of iron is 55.847. For calculation purposes, these... 

The reference mass is assumed to be the typical weight of all oil standards. 

Equivalent The amount of a substance that will react with one mole of H+ or OH-. The related term equivalent mass ( weight ) refers to a molar mass of a substance divided by the absolute value of its valence state. For anion exchange capacity and cation exchange capacity measurements, the results are usually reported in milliequivalents (1/1000 of an equivalent) per 100 g of material. 

Note The number in parentheses indicates the uncertainty in the last digit of the atomic weight. Monoisotopic mass (relative atomic mass) refers here to the mass of a specific nuclide (isotope). Atomic weight from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C. 

In an early attempt to calculate the phase fractions in an approximate implicit volume fraction-velocity-pressure correction procedure, Spalding [176, 177, 178, 180] calculated the phase fractions from the respective phase continuity equations. However, experience did show that it was difficult to conserve mass simultaneously for both phases when the algorithm mentioned above was used. For this reason, Spalding [179] suggested that the volume fraction of the dispersed phase may rather be calculated from a discrete equation that is derived from a combination of the two continuity equations. An alternative form of the latter volume fraction equation, particularly designed for fluids with large density differences, was later proposed by Carver [26]. In this method the continuity equations for each phase were normalized by a reference mass density to balance the weight of the error for each phase. 

The subscripts in the formula of a compound give the ratio of the number of atoms of each element to the number of atoms of each other element in the formula. The collection of atoms written to represent the compound is defined as one formula imit. That is, the formula unit of ammonium sulfide, (NH4)2S, contains two atoms of nitrogen, eight atoms of hydrogen, and one atom of sulfur. The term formula mass (sometimes called formula weight) refers to the sum of the atomic masses of every atom (not merely every element) in a formula unit. There are several names for formula masses corresponding to different kinds of formulas. For uncombined atoms, the formula mass is the atomic mass. For covalent compounds, which consist of molecules, the formula mass can be called the molecular mass. For ionic compounds, there is no special name for formula mass. These terms are summarized in Table 4-1. 

Atomic weight (A - The ratio of the average mass per atom of an element to 1/12 of the mass of nuclide C. An atomic weight can be defined for a sample of any given isotopic composition. The standard atomic weight refers to a sample of normal terrestrial isotopic composition. The term relative atomic mass is synonymous with atomic weight. [2]... 

Most commercial plastics, also known as resins in North America, are based on the element carbon and are synthesized, or made, from simple, oil-based raw materials. These starting materials are called monomers and these simple, low molecular weight materials are put together, by a process known as polymerization (polymerisation), so as to form polymers. This term means that the final product consists of many identical, repeat units. Because the final molecular weight, or mass, is so large the material may also be referred to as a high polymer or, as a macromolecule . 

One mole of a compound contains Avogadro s number of formula units of that compound. The terms molecular weight, molecular mass, formula weight, and formula mass have been used in the past to refer to the mass of 1 mol of a compound. However, the term molar mass is more inclusive, because it can be used for all types of compounds. 

All weight and mass measurements are relative and cannot be known as absolute values. The definition of standard weights and measures provides a reference framework in which units can be treated as though they are absolute measurements. 

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