Mass, relative molecular

Measurements of mechanical and chemical parameters (time, dimensions, force, pressure, mass, concentration, relative molecular mass) + + + + + + -H -h... 

The mass of an electron is very small compared with the total mass of the molecule. Consequently, the relative molecular mass of a molecule (M,.) is almost the same as that of the derived molecular ion (M +). For practical purposes in mass spectrometry, = M/+, and is written, M +. 

Another type of ion is formed almost uniquely by the electrospray inlet/ion source which makes this technique so valuable for examining substances such as proteins that have large relative molecular mass. Measurement of m/z ratios usually gives a direct measure of mass for most mass spectrometry because z = 1 and so m/z = m/1 = m. Values of z greater than one are unusual. However, for electrospray, values of z greater than one (often much greater), are quite coimnonplace. For example, instead of the [M + H]+ ions common in simple Cl, ions in electrospray can be [M + n-H]- where n can be anything from 1 to about 30. 

Quasi-molecular ions, [M + nH], from a protein (myoglobin) of molecnlar mass 16,951.5 Da. In this case, n ranges from 21 (giving a measured mass of 808.221) to 12 (corresponding to a measured mass of 1413.631). The peaks with measured masses in between these correspond to the other values of n between 12 and 21. By taking snccessive pairs of measnred masses, the relative molecular mass of the myoglobin can be calculated very accurately, as shown in Figure 8.4. 

Bombardment of a liquid surface by a beam of fast atoms (or fast ions) causes continuous desorption of ions that are characteristic of the liquid. Where the liquid is a solution of a sample substance dissolved in a solvent of low volatility (often referred to as a matrix), both positive and negative ions characteristic of the solvent and the sample itself leave the surface. The choice of whether to examine the positive or the negative ions is effected simply by the sign of an electrical potential applied to an extraction plate held above the surface being bombarded. Usually, few fragment ions are observed, and a sample of mass M in a solvent of mass S will give mostly [M + H] (or [M - H] ) and [S -I- H]+ (or [S - H] ) ions. Therefore, the technique is particularly good for measurement of relative molecular mass. 

M = relative molecular mass of the liquid (solvent g mof ) d = density of sample solution (g-tyn )... 

By being able to obtain an unequivocal relative molecular mass, or even a molecular formula derived from that mass, the hybrid mass spectrometer becomes a powerful tool for investigating single substances or mixtures of substances. With an APCI inlet, fragmentation can be induced to obtain structural information (see Chapter 9). 

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

A single instrument — a hybrid of a quadrupole and a TOF analyzer — can measure a full mass spectrum of ions produced in an ion source. If these are molecular ions, their relative molecular mass is obtained. Alternatively, precursor ions can be selected for MS/MS to give a fragment-ion spectrum characteristic of the precursor ions chosen, which gives structural information about the original molecule. 

Finally, note that the ions produced by the combined inlet and ion sources, such as electrospray, plasmaspray, and dynamic FAB, are normally molecular or quasi-molecular ions, and there is little or none of the fragmentation that is so useful for structural work and for identifying compounds through a library search. While production of only a single type of molecular ion may be useful for obtaining the relative molecular mass of a substance or for revealing the complexity of a mixture, it is often not useful when identification needs to be done, as with most general analyses. Therefore,... 

Integer and accurate inasses for three different gase.s, each having the same integer relative molecular mass (RMM = 28). 

A sample of the protein, horse heart myoglobin, was dissolved in acidified aqueous acetonitrile (1% formic acid in HjO/CHjCN, 1 1 v/v) at a concentration of 20 pmol/1. This sample was injected into a flow of the same solvent passing at 5 pl/min into the electrospray source to give the mass spectrum of protonated molecular ions [M + nH] shown in (a). The measured ra/z values are given in the table (b), along with the number of protons (charges n) associated with each. The mean relative molecular mass (RMM) is 16,951,09 0.3 Da. Finally, the transformed spectrum, corresponding to the true relative molecular mass, is shown in (c) the observed value is close to that calculated (16,951.4), an error of only 0.002%. 

Chemical structures for BFB and DFTPP and their relative molecular masses (RMM molecular weight) (a) 4-bromoflu-orobenzene (BFB) RMM = 174 and 176 (b) decafluorotriphenylphosphine (DFTPP) RMM = 442. 

This inlet/ion source is a simple system with no moving parts and yields many ions from the original dissolved sample. Even more attractive is the tendency for electrospray to produce multicharged ions, a benefit that makes accurate measurement of large relative molecular masses much easier. 

Additional ionization occurs by collision between the ions and other neutral species (ion/molecule collision see Chapter 1). Unless special steps are taken (see Chapters 8 and 11 ), the ions formed do not fragment, so little or no structural information is obtained. However, the lack of fragmentation does mean that good relative molecular mass data can be obtained. The assembly of ions formed by ion... 

The observation of molecular size or polydispersity and the subsequent determination of relative molecular mass, (MJ or molecular mass (weight) distribution (MWD), is the most common analytical application of SEC. The goal of these types of experiments is to either observe the solvated size of one or more molecular species or to observe the distribution of sizes present in a mixture... 

The molecular weight (mean relative molecular mass) was obtained by determination of density but, in order to determine that the gas was monatomic and its atomic and molecular weights identical, it was necessary to measure the velocity of sound in the gas and to derive from this the ratio of its specific heats kinetic theory predicts that Cp/C = 1.67 for a monatomic and 1.40 for a diatomic gas. 

ASTM, Standard Test Method for Estimation of Molecular Weight (Relative Molecular Mass) of Petroleum Oils from Viscosity Measurements, ASTM Standard D-2502-92, 1992. 

The relative molecular mass of magnesium hydroxide is 58.3. Each mole of magnesium hydroxide, when dissolved, yields 1 mole of magnesium ions and 2 moles of hydroxyl ions. If the solubility is xmolL-1, [Mg2 + ] = x and [OH-] = 2x. Substituting these values in the solubility product expression ... 

The term titrimetric analysis refers to quantitative chemical analysis carried out by determining the volume of a solution of accurately known concentration which is required to react quantitatively with a measured volume of a solution of the substance to be determined. The solution of accurately known strength is called the standard solution, see Section 10.3. The weight of the substance to be determined is calculated from the volume of the standard solution used and the chemical equation and relative molecular masses of the reacting compounds. 

As the term mole refers to an amount of substance with reference to the specified mass of carbon-12, it is possible to express the relative molecular mass (the basis for the mole) for any substance as the additive sum of the relative atomic masses (R.A.M.s) of its component elements, for example ... 

The relative molecular mass for sulphuric acid, H2S04, is calculated from the relative atomic masses as follows ... 

It should have a high relative molecular mass so that the weighing errors may be negligible. (The precision in weighing is ordinarily 0.1-0.2mg for an accuracy of 1 part in 1000, it is necessary to employ samples weighing at least about 0.2 g.)... 

B. Standardisation against sodium tetraborate. The advantages of sodium tetraborate decahydrate (borax) are (i) it has a large relative molecular mass, 381.44 (that of anhydrous sodium carbonate is 106.00) (ii) it is easily and economically purified by recrystallisation (iii) heating to constant weight is not required (iv) it is practically non-hygroscopic and (v) a sharp end point can be obtained with methyl red at room temperatures, since this indicator is not affected by the very weak boric acid. 

This solution is widely employed, particularly for the titration of organic adds. Barium carbonate is insoluble, so that a clear solution is a carbonate-free strong alkali. The relative molecular mass of Ba(0H)2,8H20 is 315.50, but a standard solution cannot be prepared by direct weighing owing to the uncertainty of the... 

DETERMINATION DF THE RELATIVE MOLECULAR MASS OF AN ORGANIC ACID 10.39... 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

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