Joseph M. Jaklevic

An approach to rapid protein crystallization using nanodroplets

An approach that enables up to a two order of magnitude reduction in the amount of protein required and a tenfold reduction in the amount of time required for vapor-diffusion protein crystallization is reported. A prototype high-throughput automated system was used for the production of diffraction-quality crystals for a variety of proteins from a screen of 480 conditions using drop volumes as small as 20 nL. This approach results in a significant reduction in the time and cost of protein structure determination, and allows for larger and more efficient screens of crystallization parameter space. The ability to produce diffraction-quality crystals rapidly with minimal quantities of protein enables high-throughput efforts in structural genomics and structure-based drug discovery.

A .beta.-gage method applied to aerosol samples.

An instrument for the routine measurement of aerosol mass using the beta-particle attenuation method is described. Factors affecting the precision and accuracy of the measurement are discussed in detail. Results of intercomparison studies between the beta gauge method and conventional gravimetric are presented. The design of the present instrument is particularly well suited for the automatic analysis of membrane filter obtained from modern dichotomous samplers.

XRF ANALYSIS - SOME SENSITIVITY COMPARISONS BETWEEN CHARGED-PARTICLE AND PHOTON EXCITATION

A comparison is made between the limits of detection for trace elements when charged-particle and photon excited X-ray fluorescence analysis are performed on a specific type of sample (5 mg/cm/sup 2/ organic based). Large-scale analysis (approximately 30,000 samples per year) at levels of 1 ppM or lower is shown to be practical with either technique when well executed. Determining the physical reason for unexplained detector background is shown to be very important particularly for the potential improvement that might be realized in photon-excited analysis applications.

Detection of Low Energy X Rays with Si(Li) Detectors

The continuing improvement in energy resolution of semiconductor detector X-ray spectrometers has led to interest in the use of these devices at energies less than 2 keV. This is an energy region of potential analytical interest since the K X rays of several elements of biological and chemical importance occur at these energies. However, the low X-ray fluorescence yield in low-Z elements, combined with the absorption of low-energy X rays due to the entry window of the vacuum chanber and that of the detector, have made work in this energy region impossible with the standard semiconductor detector X-ray spectrometers. In the present work, the X-ray absorption path has been reduced to a minimum to permit low-energy X ray studies of excitation, entry window thickness, detector linearity and resolution. Using electron beam excitation on low-Z targets, we have performed measurements of characteristic K X rays of elements down to and including carbon (277 eV).

High Rate X-Ray Fluorescence Analysis by Pulsed Excitation

We describe the application of pulsed X-ray excitation to X-ray fluorescence spectrometry as a method for increasing the output counting rate of the system by a substantial factor. Using a pulsed X-ray tube that is immediately turned off when a signal is detected, and held off during the pulse processing time, it is possible to eliminate the need for a pulse pile-up rejection. We have achieved output counting rates significantly greater than with conventional operation for equivalent shaping networks. No significant degradation in spectrometer resolution was observed at the increased counting rates.

Background reduction in x-ray fluorescence spectra using polarization

Abstract X-ray fluorescence spectra are observed by exciting a sample with both polarized and unpolarized radiation. A significant reduction in the magnitude of the scattering peaks and background results from the use of polarization.

High‐temperature annealing characteristics of tungsten and tungsten nitride Schottky contacts to GaAs under different annealing conditions

We have systematically investigated the structural and electrical characteristics of thin‐film tungsten and reactively sputtered tungsten nitride (WNx) Schottky contacts to GaAs under high‐temperature annealing conditions (with annealing temperatures ranging from 700 to 850 °C) in an arsenic‐overpressure and flowing nitrogen ambient with and without a silicon dioxide capping layer. Compositions of the WNx films measured by Rutherford backscattering spectrometry and proton resonant scattering techniques indicate a linear relationship between x and the nitrogen partial pressure during sputtering. Glancing angle x‐ray diffraction studies revealed that for nonzero nitrogen partial pressure, the as‐deposited films were amorphous, and after annealing these films converted to polycrystalline W2 N and W phases. A surface layer of W2 As3 phase was also observed after As‐overpressure capless annealing and was believed to be the result of reactions between W and the ambient As gas. Electrical measurements showed tha...

A high rate, low noise, X-ray silicon strip detector system

An X-ray detector system, based on a silicon strip detector wire-bonded to a low noise charge-sensitive amplifier integrated circuit, has been developed for synchrotron radiation experiments which require very high count rates and good energy resolution. Noise measurements and X-ray spectra were taken using a 6 mm long, 55 /spl mu/m pitch strip detector in conjunction with a prototype 16-channel charge-sensitive preamplifier, both fabricated using standard 1.2 /spl mu/m CMOS technology. The detector system currently achieves an energy resolution of 350 eV FWHM at 5.9 keV, 2 /spl mu/s peaking time, when cooled to -5/spl deg/C. >

Lattice location of diffused Zn atoms in GaAs and InP single crystals

We have investigated the saturation phenomenon of the free carrier concentration in p‐type GaAs and InP single crystals doped by zinc diffusion. The free hole saturation occurs at 1020 cm−3 for GaAs, but the maximum concentration for InP appears at mid 1018 cm−3. The difference in the saturation hole concentrations for these materials is investigated by studying the incorporation and the lattice location of the impurity zinc, an acceptor when located on a group III atom site. Zinc is diffused into the III‐V wafers in a sealed quartz ampoule. Particle‐induced x‐ray emission with ion‐channeling techniques are employed to determine the exact lattice location of the zinc atoms. We have found that over 90% of all zinc atoms occupy Ga sites in the diffused GaAs samples, while for the InP case, the zinc substitutionality is dependent on the cooling rate of the sample after high‐temperature diffusion. For the slowly cooled sample, a large fraction (∼90%) of the zinc atoms form random precipitates of Zn3P2 and ele...

Large-scale measurement of airborne particulate sulfur☆

Abstract We describe an aerosol sampling and analysis system which represents an integral approach to large-scale monitoring of airborne particulate matter. During our two-year participation in the St. Louis, Missouri, Regional Air Pollution Study (RAPS), 34,000 size-fractionated samples were collected by automated dichotomous samplers characterized by a particle size cutpoint of 2.4 μm. The total mass of the particulate matter was measured by beta-particle attenuation and the elemental composition, including sulfur, was determined by photon-excited X-ray fluorescence. The long-term performance of the system will be reported. Potential systematic effects related to the sampling and analysis of sulfur particles are treated here in detail. Both the accuracy and precision of sulfur measurement are estimated to be 2%. While the X-ray attenuation correction required is typically only a few per cent, a larger correction is required for a small fraction of the samples due to the migration of the sulfur into the filter. This correction is derived from the ratio of sulfur determinations made on the front and back surfaces of the membrane filter. Laboratory and field experiments have shown insignificant gaseous SO2 conversion on the type of filters employed in the study. Preliminary data on the composition and the temporal and spatial distribution of the St. Louis aerosol are presented. About 90% of the sulfur was found in the fine-particle fraction. Sulfur variations were significantly slower than those of the trace elements. Sulfates usually constitute about 35% of the total fine-particle mass, but may rise to 41% during an ‘episode’. The long-term (4 month average) sulfur data indicate that the background air masses arriving at St. Louis from the west and north were about 30% lower in particulate sulfur than those from the east and south. Also, an urban station may experience local increases in sulfur level up to a factor of two greater than the general background. Short-term (6 h average) data indicate that the effects of stationary SO2 sources extend for long distances, (at least 40 km) and are highly directional in character.