Measurements atomic mass units

In Laser Ionization Mass Spectrometry (LIMS, also LAMMA, LAMMS, and LIMA), a vacuum-compatible solid sample is irradiated with short pulses ("10 ns) of ultraviolet laser light. The laser pulse vaporizes a microvolume of material, and a fraction of the vaporized species are ionized and accelerated into a time-of-flight mass spectrometer which measures the signal intensity of the mass-separated ions. The instrument acquires a complete mass spectrum, typically covering the range 0— 250 atomic mass units (amu), with each laser pulse. A survey analysis of the material is performed in this way. The relative intensities of the signals can be converted to concentrations with the use of appropriate standards, and quantitative or semi-quantitative analyses are possible with the use of such standards. 

The weight of atoms and their constituents can be given in kilograms. A proton, for example, weighs 1.67 x 10 kilograms, but its weight or mass can be expressed more conveniently in a measure called the atomic mass unit (amu). One amu is defined as 1/12 the mass of a carbon atom that consists of six protons. 

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. 

Continuous monitoring of the carbon monoxide and carbon dioxide evolved during thermal decomposition of brominated polyester resin samples, has been carried out using a simultaneous thermal analysis-mass spectrometry technique. In order to allow measurement of the carbon monoxide evolved, the atmosphere chosen for these runs was 21% oxygen in argon, since the peak at 28 atomic mass units (amu)... 

Relative isotopic, atomic and molecular masses are measured on a scale in which the mass of an atom of carbon-12 is exactly 12 atomic mass units (a.m.u.). 

MS equipment is evaluated on several performance metrics. Mass accuracy, mass resolution, and mass range are standard parameters frequently assessed to determine the suitability of an instrument. Mass accuracy is defined as the extent to which a mass analyzer reflects true m/z values and is measured in atomic mass units (amu), parts per million (ppm), or percent accuracy. 

Accurate Mass An experimentally determined mass of an ion that is used to determine an elemental formula. For ions containing combinations of the elements C, H, N, O, P, S, and the halogens, with mass less than 200 Da, a measurement with 5 ppm uncertainty is sufficient to uniquely determine the elemental composition. See also related entries on average mass dalton molar mass monoisotopic mass nominal mass unified atomic mass unit. 

Unified Atomic Mass Unit (u) A non-SI unit of mass defined as one twelfth of the mass of one atom of 12C in its ground state and 1.66 x 10-27 kg. The term atomic mass unit (amu) is not recommended to use since it is ambiguous. It has been used to denote atomic masses measured relative to a single atom of 160, or to the isotope-averaged mass of an oxygen atom, or to a single atom of 12C. 

In the graphic form the abscissa represents the mass of ions (to be more precise, the mass-to-charge ratio, m/z), while the ordinate represents the relative intensity of these ions peaks. Atomic mass units (unified atomic mass unit) or daltons are used as units to measure masses of ions, while intensity is represented in percent relative to the base peak in the spectmm or to the total abundance of all the ions in the spectra. The atomic mass unit (dalton) is equal to the mass of one-twelvth of the mass of a 12C atom (1,661 x 10-27 g) (see Chapter 1). 

Problem 7 Listed in the following chart are the atomic weights (measured in atomic mass units) for natural silver and its two isotopes. Use this data to calculate the percentage of silver-109 in the natural mixture. 

Often an expression such as unit resolution with 15% valley is used. This means that the bottom of the valley between two adjacent peaks of identical height comes to 15 % of the height of the peak or, put another way, at 7.5 % of its peak height the line width DM measured across an individual peak equals 1 amu (atomic mass unit) see in this context the schematic drawing in Fig. 4.10. 

By means of mass spectrometry, the mass of atoms and molecules, via mass-separated charged atomic or polyatomic ions, can be determined by measuring the mass-to-charge ratio m/z), whereby the mass of an atom or a molecule is not measured in g or kg, but in a multiple of the atomic mass constant mu (atomic mass unit). The atomic mass unit mu is defined as one-twelfth the mass of a neutral 12C atom, ma (12C), in its ground state ... 

Another convenient unit for describing the mass of a single atom or molecule is the atomic mass unit (formerly amu, now commonly denoted u). One atomic mass unit (1 u) is defined as one-twelfth the mass of an atom of carbon-12. Since the experimentally measured mass of an atom of carbon-12 is 1.9926 X l(T23g, 1 u = 1.6606 X 10 24g. The atomic mass unit is convenient for describing the mass of a peak observed by mass spectrometry (see Box 3-2). 

An unknown substance, X, was isolated from rabbit muscle. Its structure was determined from the following observations and experiments. Qualitative analysis showed that X was composed entirely of C, H, and 0. A weighed sample of X was completely oxidized, and the H20 and C02 produced were measured this quantitative analysis revealed that X contained 40.00% C, 6.71% H, and 53.29% O by weight. The molecular mass of X, determined by mass spectrometry, was 90.00 u (atomic mass units see Box 1-1). Infrared spectroscopy showed that X contained one double bond. X dissolved readily in water to give an acidic solution the solution demonstrated optical activity when tested in a polarimeter. 

Because osmotic pressures can be routinely measured down to the sub-torr (about 10 3 atm) level, the above equation permits MW determinations ranging up to Modaltons (kDa, 103 atomic mass units) and beyond, as shown in the following problem. 

Was this your answer Both terms include the word mass and so are easily confused. Focus your attention on the second word of each term, however, and you ll get it right every time. Mass number is a count of the number of nucleons in an isotope. An atom s mass number requires no units because it is simply a count. Atomic mass is a measure of the total mass of an atom, which is given in atomic mass units. If necessary, atomic mass units can be converted to grams using the relationship i atomic mass unit = 1.661 x io 24 gram. 

Every held has its own special units of measure and nuclear chemistry is no different. The unit of length is the femtometer (10-15 m), which is called a fermi. The unit of mass is the atomic mass unit (amu or u), which has a numerical value of 1.66 x 10 24 g or expressed in units of MeV/c2, it is 931.5 MeV/c2. The unit of energy is MeV (106eV), which is 1.602 x 10 13 J, the energy gained when a proton is... 

One of the most important nuclear properties that can be measured is the mass. Nuclear or atomic masses are usually given in atomic mass units (amu or u) or their energy equivalent. The mass unit u is defined so that the mass of one atom of 12C is equal to 12.0000. .. u. Note we said atom. For convenience, the masses of atoms rather than nuclei are used in all calculations. When needed, the nuclear mass mllucl can be calculated from the relationship... 

Elements differ from one another according to how many protons their atoms contain, a value called the atomic number (Z) of the element. The sum of an atom s protons and neutrons is its mass number (A). Although all atoms of a specific element have the same atomic number, different atoms of an element can have different mass numbers, depending on how many neutrons they have. Atoms with identical atomic numbers but different mass numbers are called isotopes. Atomic masses are measured using the atomic mass unit (amu), defined as 1/12 the mass of a 12C atom. Because both protons and neutrons have a mass of approximately 1 amu, the mass of an atom in atomic mass units (the isotopic mass) is numerically close to the atom s mass number. The element s atomic mass is a weighted mass average for naturally occurring isotope mixtures. 

About 1837 electrons are equal in mass to the mass of one proton or one neutron. A summary of each type of particle, its mass and relative charge is shown in Table 3.1. You will notice that the masses of all these particles are measured in atomic mass units (amu). This is because they are so light that their masses cannot be measured usefully in grams. 

Resolving Power (RP) A measurement of how effectively a mass analyzer can distinguish between two peaks at different, but similar m/z. Mathematically, the formula M/ AM is used, where M is the m/z value for one of the peaks and AM is the spacing, in unified atomic mass units, between the peaks. Most commonly, AM is the mass resolution, either via the 10% valley or FWHM definitions (see below). (Note that the definition used will affect the resolving power calculated.) Resolving power of 500-1000 approximately corresponds to unit resolution (e.g., at m/z 700 and FWHM resolution of 0.7, RP = 1000). 

The measured value is equal to (Z — 1 )m/gM, where m is the electron mass, and M is the nuclear mass. The latter is usually known accurately in atomic mass units. If we determine the bound electron g factor from the theory, then the experiment leads to a new precision value of the electron mass in atomic units. The accuracy of the electron mass value is compatible with the one from a direct measurement [3]. The uncertainty of the theory of the g factor in the carbon ion is on about the same level as the experimental one. The theoretical precision can... 

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