Compound molecular mass

Compound Molecular Mass MH+ Daughter Spectrum Comments Peak Intensity Bile Urine ... 

Compound Molecular mass (Da) Precursor ion mass (Da) Product ion mass (Da) Ionization mode... 

Compound Molecular mass (monoisotopic) Found mass (M + H)+ Found mass M+... 

There are two limitations of electron impact ionization, which in other ways is nearly an ideal ion source for identification of organic compounds. The sample must be stable as neutral molecules in the gas phase prior to ionization. There are many compounds, however, that are simply not stable at the temperatures required for vaporization. Radical molecular ions are formed with excess energy that may be sufficient for complete elimination of molecular mass information from the mass spectrum, even at low electron energies (e.g. 15-20 eV). Molecular mass information is almost always required for structure elucidation of unknown compounds. A softer ionization technique, such as chemical ionization, may be used in these circumstances to establish the compound molecular mass. 

Compound Molecular mass (g/mol) Moles Mass (g) Volume (ml)... 

For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

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,... 

Naturally occurring isotopes of any element are present in unequal amounts. For example, chlorine exists in two isotopic forms, one with 17 protons and 18 neutrons ( Cl) and the other with 17 protons and 20 neutrons ( Cl). The isotopes are not radioactive, and they occur, respectively, in a ratio of nearly 3 1. In a mass spectrum, any compound containing one chlorine atom will have two different molecular masses (m/z values). For example, methyl chloride (CH3CI) has masses of 15 (for the CH3) plus 35 (total = 50) for one isotope of chlorine and 15 plus 37 (total = 52) for the other isotope. Since the isotopes occur in the ratio of 3 1, molecular ions of methyl chloride will show two molecular-mass peaks at m/z values of 50 and 52, with the heights of the peaks in the ratio of 3 1 (Figure 46.4). 

Ideally, a mass spectmm contains a molecular ion, corresponding to the molecular mass of the analyte, as well as stmcturaHy significant fragment ions which allow either the direct deterrnination of stmcture or a comparison to Hbraries of spectra of known compounds. Mass spectrometry (ms) is unique in its abiUty to determine direcdy the molecular mass of a sample. Other techniques such as nuclear magnetic resonance (nmr) and infrared spectroscopy give stmctural information from which the molecular mass may be inferred (see Infrared technology and raman spectroscopy Magnetic spin resonance). 

Another interesting application of LC-LC is the determination of low-molecular-mass carbonyl compounds in air. Carbonyl compounds, such as aldehydes and ketones, are now being given more and more attention, both as pollutants and as... 

Figure 13,12 Illusti ation of the clean-up method, showing the analysis of an air sample (a) with and (b) without column switching. Details of the analytical conditions are given in the text. Reprinted from Journal of Chromatography, A 697, R R. Kootsti a and H. A. Herbold, Automated solid-phase exti action and coupled-column reversed-phase liquid cltromatogra-phy for the trace-level determination of low-molecular-mass carbonyl compounds in ak , pp. 203-211, copyright 1995, with permission from Elsevier Science.

P. R. Kootstr-a and H. A. Herbold, Automated solid-phase extraction and coupled-column reversed-phase liquid cliromatogr aphy for the trace-level determination of low-molecular-mass carbonyl compounds in ak , 7. Chromatogr. 697 203-211 (1995). 

For compounds of high molecular mass, however, the formation of derivatives does not help to solve the problem of in volatility. This difficulty may be overcome by breaking the large molecules up into smaller and more volatile fragments which may then be analysed by gas-liquid chromatography, i.e. by using the technique known as pyrolysis gas chromatography (PGC). 

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. 

An electrical output for a polarographic detector is produced according the above reactions. Again, various electrolytes can be used for this type of detector, but they are usually based on KC1 or AgCl with additional high molecular mass compounds, which are added to prevent the loss of electrolyte during sterilisation. 

On the basis of the nucleophilicity parameters B, NBs, and fi (see Table 8-2) one expects less of the homolytic product in water than in methanol. This is, however, not the case. It has been known for many decades that a very complex mixture of products is formed in the decomposition of diazonium ions, including polymeric products, the so-called diazo tars. In alcohols this is quite different. The number of products exceeds three or four only in exceptional cases, diazo tars are hardly formed. For dediazoniation in weakly alkaline aqueous solutions, there has, to the best of our knowledge, been only one detailed study (Besse et al., 1981) on the products of decomposition of 4-chlorobenzenediazonium fluoroborate in aqueous HCOf/ CO]- buffers at pH 9.00-10.30. Depending on reaction conditions, up to ten compounds of low molecular mass were identified besides the diazo tar. 

Polymers generally do not have definite molecular masses because there is no fixed point at which the chainlengthening process will cease. There is no fixed molar mass, only an average molar mass. Because there is no one unique compound, there is no one unique melting point, rather a range of melting points. 

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