Electrospray ionization has recently emerged as a powerful technique for producing intact ions in vacuo from large and complex species in solution. To an extent greater than has previously been possible with the more familiar "soft" ionization methods, this technique makes the power and elegance of mass spectrometric analysis applicable to the large and fragile polar molecules that play such vital roles in biological systems. The distinguishing features of electrospray spectra for large molecules are coherent sequences of peaks whose component ions are multiply charged, the ions of each peak differing by one charge from those of adjacent neighbors in the sequence. Spectra have been obtained for biopolymers including oligonucleotides and proteins, the latter having molecular weights up to 130,000, with as yet no evidence of an upper limit.

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Ions were produced in an electrospray ionization (ESPI) source from solutions of poly(ethy1ene glycol) (PEG) samples with average molecular weights ranging from 200 to 17 500. Mass analysis of these ions provided evidence that up to at least 23 sodium cations could be deposited on the largest oligomers. A simple model based on equating the affinitiy of an ion for an oxygen site to the electrostatic potential energy of the centermost ion indicates that the capacity of these molecules to hold charge has not yet been reached. Even so, the extent of multiple charging that has already been achieved suggests that ESPI can substantially extend the mass range of an analyzer. In the reported experiments a quadrupole analyzer with an upper limit for m / z of 1500 daltons per charge, with good signal/noise but not-so-good resolution, was able to detect PEG ions whose actual masses were in the range from 15 000 to 20000 daltons! IntroductionPoly(ethy1ene glycols) (PEGs) are interesting model species for characterizing ionization methods used in mass spectrometry. They are commercially available with nominal molecular weights from 200 to 17 500 (e.g., Carbowaxes from Union Carbide). The "nominal molecular weight" refers to the most abundant oligomer in a mixture containing a rather broad distribution of sizes. These commercial products are useful for spectrometer calibration because a wide range of masses can be spanned by a small number of samples. Moreover, each prominent peak in a spectrum is separated from its neighbors by the well-defined and convenient interval of 44 daltons corresponding to the mass of the ethylene oxide building block. This interval is larger, and the relative abundance of each mass less disparate, than is the case for isotopic variants of a single large molecule. On the other hand, the interval is not as large as is the case for singly and doubly charged ions of the same parent molecule. When that parent molecule is large, the spectrometer mass scale must be accurately calibrated over a wide range if one is to be sure that two widely separated peaks indeed correspond to singly and doubly charged species. It will emerge, however, that the broad distribution of oligomer sizes in each sample leads to difficulties with overlapping peaks when extensive multiple charging occurs.Mass spectra for PEGs have previously been obtained with several other soft ionization methods including fast atom bombardment (FAB), thermospray (TS), electrohydrodynamic (EH), and field desorption (FD).1-4 We have found reported results indicating up to four charges per molecule with E H but none showing appreciable multiple charging with TS, FAB, or FD, although up to three charges per molecule have occasionally been observed for other species with these methods as well as with secondary ionization mass spectrometry (SIMS), laser desorption (LD), and plasma desorption (PD) ionization methods. Here we present experimental results which show that electrospray ionization (ESPI) can deposit as many as 23 Na+ ions on PEG

We report the study of temperature-dependent electrical resistivity, Seebeck coefficient, and thermal conductivity of the Sr1−xCaxSi2 and Sr1−xBaxSi2 alloys to elucidate the chemical pressure effect on the thermoelectric performance, characterized by the figure-of-merit, ZT. It is found that the room-temperature electrical resistivity and thermal conductivity are slightly reduced through the substitution of Ca and Ba (x < 0.10) onto the Sr sites of SrSi2. Moreover, the room-temperature Seebeck coefficients of these samples are substantially enhanced upon substitution. These promising results lead to a significant enhancement in the ZT value of the substituted SrSi2 alloys as compared to the parent compound SrSi2. Namely, the highest room-temperature ZT values of ∼0.17 and ∼0.11 were obtained for the Sr0.92Ca0.08Si2 and Sr0.93Ba0.07Si2 alloys, much larger than that of pure SrSi2.

The specific volumes of Pd40Ni40P20 were measured in the liquid, glass, and crystallized states. It was found that in the molten state, for temperatures below ∼1000 K, the specific volume measurement was successful only if the specimens were properly purified. Such purification could be achieved by fluxing the molten specimen in dehydrated boron oxide. Relaxation effects are observed and they manifested themselves in the variation of the thermal expansion coefficient with the quenching rates of the glassy specimens. The intersection points of the specific volume curves of the liquid (both stable and metastable), glass and crystallized are 573±80 and 492±80 K (extrapolated values), respectively. The former agrees very well with the glass transition temperature 577 K, obtained by the calorimetry method, while the latter implies that the melt, in accord with free volume theory, will have collapsed to the glass before that temperature is reached.