Reference mass

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

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... 

Nomenclature K0 reference mass transfer coefficient (length/time)... 

MCLAFFERTY, F.W., STAUFFER, D.A., LOH, S.Y., WESDEMIOTIS, C., Unknown identification using reference mass spectra. Quality evaluation of databases, J. Am. Soc. Mass Spectrom., 1999,10, 1229-1240. 

On the LC/MS side, the offerings are limited. Other than certain prototype instrumentation26 only Waters offered with its MUX technology a type of parallel mass spectrometer ion interface. The Waters MUX technology does not truly operate in parallel, but multiplexes among combinations of four or eight liquid streams. Combined with Waters LockSpray technology even an additional fifth or ninth channel to introduce a reference mass was used. 

Today, GC-MS (see Section 4.1.1) is a golden standard for detection and quantification of drugs and poisons volatile under GC conditions, whereas nonvolatile compounds require LC-MS (see Section 4.1.2). The GC-MS technique is much more popular for identification purposes than LC-MS, because of the easy availability of the reference mass spectra for many xenobiotics and their derivatives, either in printed or computer form. The most popular libraries are the NIST library, which contains the mass spectra of 130,000 compounds, the Wiley Registry of Mass Spectral Data, which contains 390,000 reference spectra, and the Pfleger-Maurer-Weber library, with 6,300 mass spectra and other data, such as chromatographic retention indexes. 

The linear law. This law (Fig. 1) is probably the most intuitive of all the mass-fractionation laws. Let us expand the transmission T(M) of isotopic beams at mass M as a function of the mass difference with a reference mass M, ... 

Once the mass spectmm of a calibration standard is known and the elemental composition of the ions that are to be included in the mass reference list are established by an independent measurement, a reference list may be compiled. For this purpose, the listed reference masses should be calculated using six decimal places. Otherwise, there is a risk of erroneous reference values, especially when masses of ion series are calculated by multiplication of a subunit. Such a tasks are well suited for spreadsheet applications on a personal computer. 

In Figure 6.22, the product ion scan of (M—H) mIz 352, C17H10N1O2F3CI, showed three major product ions at miz 286 (loss of F2CO) with the fluorine migrating to the indole ring, miz 283 (loss of CF3 radical) and m z 266 (loss of CF3OH). Using the (M—H) ion at miz 352 as the reference mass the measured exact masses were all within 5.8 ppm of the theoretical value for the proposed formula. 

In the past, PTRC screening was mainly based on gas chromatography-mass spectrometry (GC-MS) [116]. The choice of GC-MS was based on a number of good reasons (separation power of GC, selectivity of detection offered by MS, inherent simplicity of information contained in a mass spectrum, availability of a well established and standardized ionization technique, electron ionization, which allowed the construction of large databases of reference mass spectra, fast and reliable computer aided identification based on library search) that largely counterbalanced the pitfalls of GC separation, i.e., the need to isolate analytes from the aqueous substrate and to derivatize polar compounds [117]. 

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

In equations 5-8, the variables and symbols are defined as follows p0 is reference mass density, v is dimensional velocity field vector, p is dimensional pressure field vector, x is Newtonian viscosity of the melt, g is acceleration due to gravity, T is dimensional temperature, tT is the reference temperature, c is dimensional concentration, c0 is far-field level of concentration, e, is a unit vector in the direction of the z axis, Fb is a dimensional applied body force field, V is the gradient operator, v(x, t) is the velocity vector field, p(x, t) is the pressure field, jl is the fluid viscosity, am is the thermal diffiisivity of the melt, and D is the solute diffiisivity in the melt. The vector Fb is a body force imposed on the melt in addition to gravity. The body force caused by an imposed magnetic field B(x, t) is the Lorentz force, Fb = ac(v X v X B). The effect of this field on convection and segregation is discussed in a later section. 

This procedure was repeated, first evacuating at 350°C, and finally, at U00°C. The results are expressed in Table I as residual masses of (presumed) water present after activation at 300 and 350°C presuming none to he present after activation at 1 00°C. They show that 1) retention of water by these zeolites at high temperature increases in the sequence CaA NaYwell defined reference masses in every case except that of NaX, where the indefiniteness is of the order of experimental uncertainties. 

The values of a and b were determined empirically by measuring the frequencies of six reference masses and fitting mass-frequency data to the form of the equation by means of a least-squares procedure. By using empirical values of a and b and the measured ion frequencies, the masses of the six ions were calculated. A statistical evaluation between the known masses and the experimentally determined masses was then made to determine the accuracy of the calibration law. Systematic errors were observed as is described in (40). 

Reference Compounds under Electron Impact Conditions in Mass Spectrometry Major Reference Masses in the Spectrum of Heptacosafluorotributylamine (Perfluorotributylamine)... 

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... 

MAJOR REFERENCE MASSES IN THE SPECTRUM OF HEPTACOSAFLUOROTRIBUTYLAMINE (PERFLUOROTRIBUTYLAMINE)... 

The following list tabulates the major reference masses (with their relative intensities and formulas) of the mass spectrum of heptacosafluorotributylamine.1 This is one of the most widely used reference compounds in mass spectrometry. 

Figure 7. Molecular ion envelope of glycosylated lysozyme Amadori products (top) and cesium iodide reference mass cluster ions (bottom). The centroided masses of the tiro labeled peaks are 16,265 and 16,427 corresponding to lysozyme Amadori adducts containing 12 (G 12) and 13 (G=13) glucose residues respectively.

Figure 2 indicates how the initial mass, M0, is fractionally distributed through a trace detection portal system. The mass and location of M0 on a person is a complex parameter to determine. Considerable research has been performed to understand the amount and distribution of trace explosives material on the body [9-11], Even with a better understanding of M0, there is still a wide variability in M0 in a real-world environment. The mass and location of explosive on the person is never exactly known. Establishing a mass for M0 as a standard is convenient for comparative testing between portal systems, but the convenience comes at some risk [12], The risk in setting a standard for M0 is that it sets a reference that may not be a realistic representation of the explosives on people. If the reference mass for M0 is set to a particular amount and placed in particular locations on the person, then a certain number of people with... 

Hindawi, R. K., Gaskell, S. J., Read, G. F., and Riad-Fahmy, D. (1980). A simple, direct, solid-phase enzyme immunoassay for plasma cortisol validated by a reference mass spec-trometric procedure. J. Endocrinol. 85, 18P. 

Reproducibility of the relative intensities in El mass spectra is a concern, despite the fact that spectra are recorded under standardized conditions. Different mass spectrometers produce spectra with, sometimes, large spreads in the relative intensities. An impression of the possible variation is given in Table 2. In this table, the relative intensities of major fragments in the El mass spectra of three nerve agents, taken from three different sources, are presented. It is clear that there exist no reference mass spectra of these Schedule 1 chemicals, which can be considered as true physical constants. A compilation of more than one spectrum of the same compound in the OPCW Analytical Database gives an indication of the possible spread. 

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