Corrected mass determination

Nevertheless, correct mass determinations require some effort. Electrostatic charges, magnetic forces, adsorption or desorption especially of water, other contaminations, air drafts caused by temperature differences between the sample and the balance, and so on must be excluded. In mass determinations, a buoyancy correction is necessary when the density of the sample differs from that of the calibration weights. For further information, see, for example, Debler (2000). 

Other limitations involve both the mass absorption coefficient of soil components and secondary and tertiary excitation. The mass absorption coefficient can be calculated and used to correct fluorescence determinations if the exact composition of the material being analyzed is known. This is not possible in soil. Secondary and tertiary excitations occur when X-rays emitted by an element other than the one of interest may cause emission or fluorescence of the element of interest. These potential sources of error are possible in any soil analysis using XRF. 

The reported amino acid composition cannot be correct. The minimum molecular mass calculated from it is about 120 u higher than the molecular mass determined by mass spectrometry. Also the amino acids acting as ligands for Fe " are missing 2D-ser, 2L-Ser... 

For routine analysis, it is suggested that mass determination of an HPLC purified product is sufficient to be relatively assured that the correct product has been made. Of course, this approach will neither determine if the correct amino acids have been incorporated into an incorrect sequence nor determine if a substitution of amino acids of identical mass has occurred. However, the numerous checkpoints for automated peptide synthesis (bar codes, printouts, etc.) should greatly reduce the probability of this occurring. Should access to mass spectrometric analysis not be available, amino acid analysis is preferable. 

Support Protocol Density measurement The required time depends strongly on the equipment being used. Modern analytical equipment can correctly measure density within minutes. If the density measurement is conducted using the specific gravity bottle method, then an initial calibration is required that can take up to several hours (due to the fact that the volume of the bottle has to be determined as a function of temperature). After the initial calibration curve has been obtained, tests can be conducted within 15 to 20 min since only a precise mass determination is necessary. 

Correct mass of purified proteins is determined using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF). N-terminal sequencing and/ or LC-ES-MS can verify correct amino acid sequence. Folding of the recombinant protein should be compared to the natural counterpart by measuring CD-spectra. Furthermore, nuclear magnetic resonance (NMR) analysis can be performed to ensure the presence of tertiary structure, important for IgE binding activity (Neudecker et al. 2003). 

Accurate determination of the molecular mass of a protein is useful for its identification and the determination of its purity. Owing to the possibility of post-translational modifications that are not contained in the database, the correct mass often is not sufficient for identification of a protein but it does facilitate the search. 

The molecular mass of the analyte can be determined from the m/z of the molecular ion in El, if it is observed in the mass spectmm. With a soft-ionization method, next to, or even instead of, the protonated molecule in the positive-ion mode or the deprotonated molecule in the negative-ion mode, a variety of adducts ions may occur in the mass spectrum (Table 2.2). The presence of adduct peaks can be often helpful in assigning the correct molecular mass. The use of both the positive-ion mode, resulting in m/z of the [M+H] ion, and the negative-ion mode, resulting in m/z of the [M-H] ion, if applicable, also leads to an unambiguously molecular-mass determination for an unknown compound. 

Lewis SA, Hardison NW, Veillon C. 1986. Comparison of isotope dilution mass spectrometry and graphite furnace atomic absorption spectrometry with Zeeman background correction for determination of plasma selenium. Anal Chem 58 1272-1273. 

When the mass Af of a body is determined in air, a correction is necessary for the buoyancy of the air. The corrected mass is given by Af + AAf/1000, where A is a function of the material used for the weights, given by... 

A rough value of the molar mass is usually sufficient to determine the molecular formula of a substance. For example, if chemical analysis of a gas yields an empirical formula (CH2) , then the molar mass must be some multiple of 14 g/mol the possibilities are 28,42, 56,70, and so on. If a molar mass determination using Eq. (2.20) yields a value of 54 g/mol, then we may conclude that n = 4 and that the material is one of the butenes. The fact that the gas is not strictly ideal does not hinder us in this conclusion at all. In this example the possible values of M are well enough separated so that even if the ideal gas law were wrong by 5 %, we would still have no difficulty in assigning the correct molecular formula to the gas. In this example it is unlikely that the ideal gas law would be in error by as much as 2 % for a convenient choice of experimental conditions. 

The other approach is the terminal velocity-slip velocity theory . It is postulated that the slip velocity will be of the order of the particle terminal velocity uj when the agitator speed is Nj. Subsituting, in equation (17.13c), a mass transfer coefncient k can be calculated and if the postulate is correct kn determined experimentally should be constant and of the order of ky. As already indicated, is found to be constant and in addition kjs /kj is found to... 

We can determine how successful the synthesis was by comparing the observed products to the expected products. Usually the expected product is observed as the major component otherwise, if another compound in the correct mass range is observed, an identifiable (and generally correctable)... 

Figure 1.67 shows a schematic of a simple, but effective set-up for cryoscopy, the method for the measurement of the freezing point lowering. Cryoscopy is perhaps the easiest of the molar mass determinations. The main prerequisites are a good temperature control and uniformity, corrections for the common supercooling observed on crystallization, and the usual extrapolation to infinite dilution. The thermodynamic equations are derived in Sect. 2.2.5, together with the equations needed for the ebulliometry. 

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