Mass spectroscopy molecular weight determinations

Analysis and mass spectral molecular weight determination established the empirical formula, C21H26N2O3, for stemmadenine (8, 116, 117). Its UV-spectrum was characteristic of an indole (cf. ref. 55), while the IR-spectrum indicated the presence of a normal ester grouping (1718 cm-1) and the absence of any substituent in the indole aromatic ring (116). These findings were fully borne out by NMR-spectroscopy which showed the presence of an indole NH (9.3 S), four aromatic protons, and a carbo-methoxyl methyl singlet (3.79 S). A single vinyl proton quartet (5.4 S)... 

Molecular weight determination of carbosilane dendrimers is best accomplished with electrospray or MALDI-TOF mass spectroscopy, but is also possible with vapor pressure osmometry measurements. In many cases, MALDI-TOF data is sufficiently accurate to indicate slight structural imperfections in dendrimers due to incomplete reactions. Gel permeation chromatography is generally not an effective means of measuring molecular weights of carbosilane dendrimers due to the substantial differences in hydrodynamic volume between spherical dendrimers and linear polystyrene standards. 

The elucidation and confirmation of structure should include physical and chemical information derived from applicable analyses, such as (a) elemental analysis (b) functional group analysis using spectroscopic methods (i.e., mass spectrometry, nuclear magnetic resonance) (c) molecular weight determinations (d) degradation studies (e) complex formation determinations (f) chromatographic studies methods using HPLC, GC, TLC, GLC (h) infrared spectroscopy (j) ultraviolet spectroscopy (k) stereochemistry and (1) others, such as optical rotatory dispersion (ORD) or X-ray diffraction. 

Various techniques have been used depending on the polymer and the nature of the chemical transformations infrared, ultraviolet and electron spin resonance spectroscopy, vapour phase chromatography, mass spectrometry, molecular weight and gel fraction determination, luminescence measurements, etc. These techniques have recently been discussed in a well-documented review on analytical methods applied to the study of the photodegradation of polymers [19]. 

Molecular weight determination of PEG and PPG by laser mass spectroscopy [98]. 

The usual chemical and spectroscopic techniques such as elemental analyses, molecular weight determinations, and infrared, NMR, and electronic spectroscopy, are frequently useful for the characterization of transition metal cluster compounds. However, x-ray crystallography and, to a lesser extent, mass spectrometry, are almost indispensable for the unambiguous identification of new metal cluster systems. Indeed, much of the rapid progress in metal cluster chemistry since the appearance of Cotton s review article in 1965 (98) can be attributed to the widespread availability of automated x-ray-diffraction equipment. This increases significantly the number and accuracy of crystal structures that can be determined with a given amount of manpower, and thus the analyses of a great many more new metal cluster crystal structures are completed each year. 

Due to the lack of X-ray data the structure of Ln-Al alkoxide has been proposed on the basis of indirect information. The molecular weight determination, the mass-spectra, the NMR and IR spectroscopy data suggest that lanthanoid alumoalkoxides have a structure similar to those of tetrameric aluminium alkoxides [53, 109, 112, 115]. 

In most published techniques MALDI mass spectroscopy and SEC are not directly coupled off line. For further discussion see Chapter 5. An exception to this is the work of Esser and co-workers [71] in which the two units are interfaced via a robotic interface. This technique was applied to PS, PMMA and butyl (methacrylate-methyl methacrylate) copolymer. Mehl and co-workers [88] combined SEC with MALDI-MS to provide accurate molecular weight determinations on polyether and PU soft blocks. 

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

Previous authors have taught the principles of solving organic structures from spectra by using a combination of methods NMR, infrared spectroscopy (IR), ultraviolet spectroscopy (UV) and mass spectrometry (MS). However, the information available from UV and MS is limited in its predictive capability, and IR is useful mainly for determining the presence of functional groups, many of which are also visible in carbon-13 NMR spectra. Additional information such as elemental analysis values or molecular weights is also often presented. 

---