Mass direction

The ions so produced are separated by their mass-to-charge (m/z) ratios. For peptides and proteins, the intact molecules become protonated with a number (n) of protons (H+). Thus, instead of the true molecular mass (M), molecular ions have a mass of [M + uH]. More importantly, the ion has n positive charges resulting from addition of the n protons [M + uH]". Since the mass spectrometer does not measure mass directly but, rather, mass-to-charge (m/z) ratio, the measured m/z value is [M + uH]/u. This last value is less than the true molecular mass, depending on the value of n. If the ion of true mass 20,000 Da carries 10 protons, for example, then the m/z value measured would be (20,000 + 10)/10 = 2001. 

Except for the moon and the planet Mars, no extraterrestrial body has had pieces of its mass directly examined by scientists in an earthly laboratory. This means that there has been no laboratory sample preparation scheme performed on samples of solid matter from any of the other planets, their moons, comets, or asteroids. And yet we read repeatedly about how scientists have been able to surmise the... 

Hai Zao, Kun Bu and Hai Dai are the chief substances. They eliminate phlegm and soften the masses directly. 

DMAP (40 xL, 8.0 tmol, 0.1 eq, 0.2 M in anhydrous DCE), and methanesulfonyl chloride (320 xL, 160 xmol, 2.0 eq, 0.5 M in anhydrous DCM) were added to a 10x95 mm test tube. The test tube was sealed with a test tube cap and stirred in glove box for 3 h at ambient temperature. The solvent was evaporated (SpeedVac or GeneVac, vacuum, medium heat, 16 h) and the residue was dissolved in anhydrous DMF (400 p,L). TEA (80 xL, 80.0 xmol, 1.0 eq, 1 M in anhydrous DMF) and the amine (480 xL, 240.0 xmol, 3.0 eq, 0.5 M in anhydrous DMF) were added. The test tube was sealed with a test tube cap. The reaction was heated and stirred at 80°C for 5 h. The solvent was evaporated (SpeedVac or GeneVac, vacuum, medium heat, 16 h), and the residue was dissolved in DMSO (1.340 mL). The reaction mixtures were analyzed by LCMS and the products isolated by automated mass-directed HPLC. 

As an atom is extremely small, it is not possible to establish its weight (or mass) directly. The atomic mass or weight of an element is established indirectly. Dalton selected hydrogen... 

Dear GJ, Plumb RS, Sweatman BC et al. (2000) Mass directed peak selection, an efficient method of drug metabolite identification using directly coupled liquid chromatography-mass spectrometry-nuclear magnetic resonance spectroscopy. J Chromatogr B Biomed Sci Appl 748 281-293... 

Figure 11-7. A five-component fluorous split-mix crude reaction mixture was injected onto a 20-mm x 50-mm-i.d. reversed-phase column. (A) UV chromatogram and (B) Total ion current chromatogram. Compounds were purified using mass-directed fraction collection (peaks highlighted).

Figure 13.5. Mass-directed purification of a combinatorial library. Chromatographic separation was carried out using gradient elution of 10-90%acetonitrile in water for 7 min after an initial hold at 10%acetonitrilefor 1 min. (a)Total ion chromatogram showingdesired product and impurities, (b) Computer-reconstructed ion chromatogram (RIC) corresponding to the expected product, (c) Postpurification analysis of the isolated component with a purity >90%. (Reproduced from Ref 15 by permission of Elsevier Science.)...

To enhance the efficiently of this purification procedure, the steps of HPLC purification and mass spectrometric analysis may be combined into automated mass-directed fraction-... 

MASS DIRECTED AUTOPREP 8.7.1 The addition of a mass spectrometer... 

To deal with this problem we set about coupling an autoprep system to a mass spectrometer [14]. The addition of a mass spectrometer enabled the system to become far more specific and by inputting the molecular weight, the system would collect only the compound(s) of interest. In the majority of cases, only one desired component is required to be collected so this means that one sample will prcxluce only one purified fraction, two samples will produce only two and similarly, a plate of 80 samples will produce only 80 purified fractions. A mass-directed preparative HPLC (MS-prep) instrument would therefore eliminate the re-analysis and recombination steps from the purification process. 

Given that soils only contain a few percent OM by mass, direct measurement of SOM mass loss is much more difficult than for litter studies. The four most common techniques for measuring soil organic carbon (SOC) turnover are (1) laboratory incubations (2) in situ soil respiration measurements (3) stable isotope measurements and (4) radiocarbon methods. We will introduce the basic methodology and highlight some of the strengths and weaknesses of each of these four techniques. For much more detailed discussions of these methods, see the references cited within. 

Various tools have been developed to enhance the potential of the techniqne, snch as mass-directed peak selection [55], on-line SPE for on-line preconcentration [56] or post-column peak trapping and stop-flow NMR [57]. 

G.J. Dear, R.S. Plumb, B.C. Sweatman, J. Ayrton, J.C. Lindon, J.K. Nicholson, I.M. Ismail, Mass directed peak selection, an efficient method of drug metabolite... 

He was one of the first to conclude in 1912 that some elements can exist in forms that are chemically identical, and save only as regards the relatively few physical properties which depend on atomic mass directly, physically identical also. He called them isotopes. Later he promoted their use in determining geologic age. He is credited (with others) with the discovery of the element protactinium in 1917. 

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