摘要:

A detecting mass spectrometer has been successfully coupled to a dense gas (supercritical fluid) chromatograph to produce an instrument (DGC/MS) that may be an alternative to high performance liquid chromatograph/mass spectrometer instruments (HPLC/MS) and gas chromatograph/mass spectrometer instruments (GC/MS) for analysis of involatile and/or thermally labile compounds. The mobile phase in DGC is a gas held at temperatures above the critical temperature and at pressures sufficient to obtain nearly liquid‐like densities. DGC combines advantages of GC and HPLC: rapid separations, moderate operating temperatures, and analysis of involatile compounds. An advantage unique to DGC is the solvent power dependence upon pressure. While several groups have studied DGC, its development has been limited by the lack of a sensitive and selective detector. Hence, work has been directed towards the design and construction of a DGC/MS resulting in a trial instrument capable of chromatographic pressures of at least 300 atm and temperatures from 10° to 60°C. The DGC/MS coupling has been accomplished by the use of a supersonic molecular beam interface. This application of molecular beam formation appears to be unique in its requirements of a large pressure ratio (∼108), low flow rates, and low final pressures. The authors outline characteristics of supersonic jets and molecular beams pertinent to the design of such an instrument. The interface which uses pumping speeds of 2400 and 1200 l/s in the beam forming chambers is described in detail, while the other components—the detecting mass spectrometer, the dense gas supply, and the DGC—are briefly described. Preliminary work with this instrument has established the feasibility of DGC/MS as an analytical technique and further development is recommended.

ISSN:0034-6748    

DOI:10.1063/1.1136785

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A digital technique has been developed which allows the study of narrow structure in total electron‐atom and electron‐molecule scattering cross sections without requiring a highly monoenergetic electron beam, modulation of the electron gun, or phase sensitive detection. The electron current transmitted through a gas cell is digitized as the electron energy is stepped by &Dgr;Ethrough the energy range of interest. A transmitted electron difference signal is then obtained using a computer. As examples of this technique, the difference spectra are presented for He near 19.35 eV and for N2for the energy range from 10.3 to 15.0 eV. In the present case an instrumental resolution of 30 meV FWHM has been obtained.

ISSN:0034-6748    

DOI:10.1063/1.1136786

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An optical modulation apparatus is described for the measurements of very weak (&egr;<1 M−1 cm−1) charge transfer optical absorption bands between electron donor–acceptor complexes. Such small bands, which are of interest in understanding the mechanism of electron transfer in physical, chemical, and biological systems, are undetectable by more traditional methods of optical spectroscopy due to their small size and their superposition on much larger background sample absorptions. An additional feature of the method is that the kinetics of the electron returning to its equilibrium state following photo‐induced charge transfer can be measured.

ISSN:0034-6748    

DOI:10.1063/1.1136787

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A photoacoustic cell is described in which the sample can constitute one of the cell walls. The cell, which can be used alone or in a combined unit with a reference cell, allows nondestructive investigation of a variety of samples either in a normal reflection mode (pressure wave generated at illuminated surface) or in a thermal or elastic transmission mode (pressure wave generated at opposite surface). Simultaneous sample illumination from both sides is possible, too. The cell sensitivity, as measured for an electrically‐generated thermal wave, is about 10−2V/W at 34 Hz. Use of the cell is illustrated by the dependence of the photoacoustic signal, generated by transmitted thermal and/or elastic waves, on the thickness of copper plates, and by the photoacoustic spectrum resulting from illuminationthroughBegonia leaves.

ISSN:0034-6748    

DOI:10.1063/1.1136788

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A new circular dichroism (CD) and magnetic circular dichroism (MCD) system with narrow bandwidth and high scanning rate has been developed. The versatility of the system for the measurements of transient changes in CD and MCD has been also described. This new CD and MCD system is composed of an acoustic optical (AO) filter of TeO2as a monochromator, a stress modulator, and a photomultiplier together with their operating devices and a computer system. The linearly polarized light from the AO filter is modulated to left‐ and right‐circularly polarized light by a stress modulator operated at 50 kHz. The absorbance difference of the left‐ and right‐circularly polarized light is detected by a photomultiplier, after passing through the sample. The whole optical system and detector system are matched in time by the same computer system. A rapid scanning was performed successfully at the rate of 50 nm range/10 ms, with better spectral resolution and higher signal‐to‐noise ratio than other commercially available CD spectropolarimeters. Finally, we showed several applications of the new CD and MCD system for the observations of CD and MCD spectral changes in biological molecules.

ISSN:0034-6748    

DOI:10.1063/1.1136789

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

We present a new method of utilizing a filter–fluorescer system for measuring high fluence x‐rays above 120 keV, where there are no practical absorption edges available for conventional filter–fluorescer techniques. The absorption edge (EpkeV) of the prefilter is chosen to be less than that of the fluorescer (EfkeV), i.e., Ep<Ef, contrary to the conventional technique, Ep≳Ef. In this way the response function is rendered virtually zero between Epand Ef, while a broad peak at higher energy is enhanced for measurement above 120 keV. An application of the method for x‐ray measurement between 150 and 350 keV from laser‐driven nuclear fusion is described.

ISSN:0034-6748    

DOI:10.1063/1.1136790

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

We describe a system which compensates for variations of electron‐diffracted intensities during a measurement; these variations are due to a spatial variation of the quantity of diffracting matter, including sample and supporting grid. This system has been used in a study of polycrystalline samples deposited on amorphous carbon films supported on electron microscope grids. In this case the intensity variations correspond to the grid transparency modulation when the beam is scanned across the sample.

ISSN:0034-6748    

DOI:10.1063/1.1136791

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An ion accelerator system that successfully combines geometrical and electrostatic focusing principles is described. This accelerator system uses thin, concave, multiple‐hole, closely‐spaced graphite screen and focusing grids, coupled to single slot accelerator and decelerator grids to provide high ion extraction efficiency with good focusing. Operating characteristics for this type of accelerator system and possible applications are discussed.

ISSN:0034-6748    

DOI:10.1063/1.1136792

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A simple beam profile monitor useful for rapid optimization of low intensity, low duty cycle pulse ion beams is described. It is based on a fixed multiwire beam detector.

ISSN:0034-6748    

DOI:10.1063/1.1136766

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An experiment currently being developed on the Alcator‐Cmagnetic fusion device uses a set of poloidalB‐field pick‐up coils and unique analyzing circuits to continuously yield the amplitudes, frequencies, and rotational directions of them= 1,2,3,4, and 5 resistive MHD tearing modes. The experiment is being used to study the growth and coupling of tearing modes of different helicities due to density, temperature, and current profiles.

ISSN:0034-6748    

DOI:10.1063/1.1136767

出版商:AIP    

年代:1981

数据来源: AIP