Mass spectrometry exchange measurements

The group of Carter and Steele [13] has developed the isotopic 02/ 02 exchange-diffusion profile (lEDP) technique, which allows estimating the surface exchange coefficient k and the diffusion coefficient D [14,15]. This technique is based on surfece thin film analysis by secondary ion mass spectrometry (SIMS) measuring a three-dimensional mapping of the distribution [16-18]. 

Sloth, J. J. and Larsen, E. H. (2000) The application of inductively coupled plasma dynamic reaction cell mass spectrometry for measurement of selenium isotopes, isotope ratios and chromatographic detection of selenoamino acids. J. Anal. At. Spectrom., 15, 669-72. Larsen, E. H., Hansen, M., Fan, T, and Vahl, M. (2001) Speciation of selenoamino acids, selenonium ions and inorganic selenium by ion exchange HPLC with mass spectrometric detection and its application to yeast and algae. J. Anal. At. Spectrom., 16, 1403-8. 

Infrared Spectrophotometry. The isotope effect on the vibrational spectmm of D2O makes infrared spectrophotometry the method of choice for deuterium analysis. It is as rapid as mass spectrometry, does not suffer from memory effects, and requites less expensive laboratory equipment. Measurement at either the O—H fundamental vibration at 2.94 p.m (O—H) or 3.82 p.m (O—D) can be used. This method is equally appticable to low concentrations of D2O in H2O, or the reverse (86,87). Absorption in the near infrared can also be used (88,89) and this procedure is particularly useful (see Infrared and raman spectroscopy Spectroscopy). The D/H ratio in the nonexchangeable positions in organic compounds can be determined by a combination of exchange and spectrophotometric methods (90). 

Powell, K.D., Fitzgerald, M.C. Accuracy and precision of a new H/D exchange- and mass spectrometry-based technique for measuring the thermodynamic properties of protein-peptide complexes. Biochemistry 2003, 42, 4962-4970. 

Powell, K.D. Fitzgerald, M.C. Measurements of protein stability by H/D exchange and matrix-assisted laser desorption/ionization mass spectrometry using picomoles of material. Ancd. Chem. 2001, 73, 3300-3304. 

In this chapter, we give an overview on how the API techniques work and which factors have an important influence on the performance. Examples are presented mostly from published work to demonstrate how LC-MS, LC-MS-MS, collision-induced dissociations (CIDs), accurate mass measurements and hydro-gen/deuterium exchange have been systematically and successfully applied in the structural elucidation of impurities, degradation products and metabolites. In addition, these also illustrate how mass spectrometry has offered a third dimension to chromatographic method development and validation. 

Deuterium may be analyzed from density measurements of waters. A confirmation method recommended here is GC mass spectrometry. Deuterium is burned in oxygen (or air) to form D2O which may be separated with helium on a GC column (of intermediate polarity) and identified from its mass spectra. The mass to charge ratio of the molecular ion is 20. Additionally, deuterated products obtained by exchange reactions with hydrogen containing substances (other than those containing C—H bonds) may be separated on a capillary GC column and identified by mass spectrometry. 

Several samarium and neodymium isotopes are isobars (Table 4.2) and cannot be separated by mass spectrometry. Thus, samarium and neodymium must be completely separated by chemical procedures prior to measuring them in a mass spectrometer. The ion-exchange chemistry necessary to separate these elements is now well developed (see Appendix). Isotope measurements can be done either by TIMS or ICPMS (inductively... 

Thermal-ionization mass spectrometers (TIMS) combine a hot-filament source with a magnetic-sector mass spectrometer. The mass spectrometers are operated at low to moderate mass-resolving power. A large number of elements can be measured with thermal ionization mass spectrometry. Special care is taken to purify the samples using ion exchange columns. Samples are loaded onto the filaments along with an emitter, and a typical run may take several hours. Modem systems have multiple collectors so that several isotopes can be measured simultaneously. High-precision measurements are done with Faraday cup detectors, but low-abundance isotopes can be measured on electron multipliers. Modem machines are capable of precisions of 0.1 to 0.01 permit. 

Other modifications to the reaction conditions of the Brust-Schiffrin method, such as a reduction temperature of — 78 °C and the use of a hyperexcess of hexanethiol, results in an Au38(thiolate)24, based on observations, LDI-TOF mass spectrometry, TGA analysis and elemental analysis [69]. The influence of preparation temperature on the size and monodispersity of dodecylthiol monolayer protected gold clusters has also been reported. Both and SAXS measurements show that higher temperatures increase polydispersity. This modification of poly-dispersity may be related to the existence of a dynamic exchange of thiols at the particle surface with thiols in the solvent [70]. 

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