Variations In Sample Preparation Research Articles

Liquid scintillation counting in nuclear medicine

Many of the radionuclides used in nuclear medicine can be measured by liquid scintillation (LS) counting, and the technique is the only practical approach to counting low-energy β emissions. This review is intended to be a brief exposition of the capabilities of LS counting and of some precautions that should be observed. The liquid scintillation process is basically simple: Electrons and/or photons from radioactive disintegrations are absorbed by a solvent, with the resulting energy being transferred to the π electrons of an organic scintillator. The scintillator emits fluorescence in its return to its least-excited ground state; the emitted photoelectron spectrum is proportionate to the energy of the photons emitted in the original radioactive disintegration. The photons are absorbed by the cathode of a photomultiplier tube, with the resulting pulse in voltage being amplified and resolved in nanoseconds before being recorded as a count. In modern LS counters, coincidence circuitry eliminates much of the background inherent in the counting system. Errors that must be avoided or assessed include quenching: impurity or chemical quenching that increases with the atomic number of the impurities and color quenching that distorts the photoelectron spectrum in inverse proportion to the emitted energy. Use of quench correction curves and methods used to determine the loss of efficiency or pulse height are necessary in LS counting if there is any significant variation in sample preparation. If there are two radionuclides to be counted in a sample, a quench correction must always be applied because the pulse height shifts to a lower energy region whenever quenching occurs. Thus the spillover or overlap of two isotope spectra will vary with the energy of the radionuclides and the degree and type of quenching. None of the three methods of determining the extent of quenching (internal standardization, sample channels ratios, or external standardization) is ideal for all situations or is by any means foolproof. For example, if the scintillator solution and sample are not mixed to near homogeneity, counts may be lost because of self-absorption. Excitation of the scintillation solvent is decreased and cannot adequately be comprehended by any of the quench correction techniques. Because most scintillation solvents will tolerate only minimal amounts of aqueous samples of the sort usually of interest in nuclear medicine, oxygen flask combustion (for 14 C or 3 H), hydrolysis, addition of surfactants (solubilizers), or thixotropic gels—all relatively expensive procedures—may have to be used for sample preparation. The cost of sample preparation is frequently the only significant unfavorable feature of LS counting when compared to gamma (Nalcrystal) scintillation counting.

Analysis of Gold and Platinum Group Alloys by X-Ray Emission with Corrections for Interelement Effects

The x-ray emission analysis of noble-metal alloys was investigated critically to determine optimum conditions for accurate analysis. The analytical curves for the elements Cu, Pd, Pt, and Zn were found to be linear with a deviation of less than ±0.2%. However, a nonlinear relationship existed for gold and for silver. These discrepancies could not be eliminated by variation in sample preparation. An interelement effect due to x-ray absorption and enhancement was found and was corrected to an accuracy of 0.2% by means of a mathematical treatment suggested by Lucas-Tooth and Price. The analysis by x-ray emission can be accomplished in approximately one-tenth of the time necessary for conventional wet chemical methods.

Gas-Oil Relative Permeability Ratio Correlation From Laboratory Data

Abstract Gas-oil relative permeability ratio is an important relationship in oil reservoir predictive calculations. A correlation has been developed from 107 gas-flood krg/kro tests on Venezuelan core samples. The correlating parameter is based on restored-state water saturation tests and is applicable to both consolidated and poorly consolidated sandstone reservoirs. Data of the correlation show that there are no distinguishable differences between the mass-data groupings for the two classifications. A procedure is recommended for running sufficient relative permeability analyses to compute a geometric mean of the sample group. The geometric mean is more representative of the total core, and probably the entire reservoir. For example, while only one in four of the krg/Kro test curves agreed closely with the resultant correlation of this report, the geometric mean curves of the 16 suites (three samples or more) showed good agreement in three cases out of four. Introduction The gas-oil relative permeability, ratio is an important, fundamental relationship in most oil reservoir predictive calculations. Predictive calculations are made to estimate future reservoir production characteristics and ultimate oil recovery. The Krg/Kro relationship is specifically needed to relate the surface gas-oil ratio to the reservoir oil and gas saturation, and to calculate the relative movement of these phases within the reservoir whenever some of the more complex driving mechanisms are present. Laboratory Krg/Kro tests are not generally run as a routine analysis. Consequently, Krg/Kro data often are not available when needed because the cost of laboratory work could not be justified or the need for such data had not been properly anticipated. When laboratory Krg/Kro data are available, they are often very difficult to interpret. For example, wide divergence is sometimes shown in a family of Krg/Kro tests representative of the producing horizon in a single well. With these considerations in mind, a study was made to determine if a relationship might exist between the Krg/Kro curve and some other simple laboratory test criteria. The most probable Krg/Kro curve correlation for Venezuela described in this paper is the result of the investigation. The presented correlation defines the most probable gas-flood Krg/Kro curve through the medium of air-water capillary displacement and centrifuge water saturation tests. The laboratory procedures of these tests are relatively simple and inexpensive: test data should be widely available from routine analysis. DATA AVAILABLE, LABORATORY METHODS The report correlation utilized 107 gas-flood Krg/Kro tests run on sandstone cores of Venezuelan reservoirs. Table 1 is a general tabulation of data pertinent to the tests, while Table 2 summarizes the data. The tests include 96 from Western Venezuela and 11 from Eastern Venezuela. Eighty-two of the 107 test samples were sandstones that varied from poorly consolidated to unconsolidated; 25 were consolidated. The average sample porosity was 26.7 per cent and the average permeability was 1,121 md; these values typify the better sandstone reservoirs of Venezuela. The Welge gas-flood technique. based on fundamental Buckley-Leverett frontal displacement theory, was introducted in about 1952 and is widely accepted in the industry. The laboratory procedure is relatively simple, rapid, and can be performed on small core samples. While there have been some minor variations in sample preparation and laboratory procedure in the tests used for the correlation, these tests can be generally summarized as follows. The core sample was first solvent-extracted and dried. Connate-water saturation was restored by the oil-flushing or evaporation-blowdown methods. At the beginning of flood the hydrocarbon pore volume was completely saturated with the test oil phase. Unsteady-state gas-ail displacement then began with the injection of nitrogen or helium, while the displaced oil and gas phases were incrementally metered at the out-flow face. From the test data. the Krg/Kro curve was calculated by the Welge method. The individual oil and gas relative permeabilities were also calculated. CORRELATING PROCEDURES In attempting to establish a basis of correlation, we found that broad mid-range sections of 105 of the 107 Krg/Kro test curves could be closely duplicated by a straight line. Only two curves did not show a degree of linearity in this region. Correlation-curve definition parameters were subsequently developed from this observation of consistent mid-range linearity. Possible correlating variables were limited to the physical properties measured on core samples that were widely available as common test data and could be easily and cheaply obtained through future laboratory work. JPT P. 1111ˆ

Evidence for retention of the crystalline‐amorphous ratio of cellulose during heterogeneous acid hydrolysis

AbstractThe literature reports that hydrolysis of cellulose under heterogeneous conditions proceeds by an initially rapid attack on the amorphous regions and then a much slower hydrolysis of the ordered regions or “crystallites.” During such hydrolysis, the degree of polymerization (D.P.) of cellulose initially decreases quite sharply and then levels off and declines at a much slower rate. This leveling off of the D.P. has been interpreted as being due to the slow hydrolysis of the “crystallites.” Some results obtained in our laboratories suggested that heterogeneous hydrolysis does not generate “crystallites” as has been assumed. Hence, a study of the crystallinity of the hydrolysis products was undertaken. The x‐ray crystallinity of cellulose was evaluated by comparing the integrated intensities of the main diffraction peaks before and after hydrolysis. The effects of variations in sample preparation and orientation of cellulose samples as well as fluctuations in the operation of the diffractometer were assessed and were found to introduce no artifacts. The x‐ray crystallinity of slash pine sulfate pulp did not increase upon hydrolysis as would be expected if hydrolysis had liberated “crystallites.” Cotton linters and slash pine sulfite pump exhibited only small increases in crystallinity upon hydrolysis. A decrease in x‐ray crystallinity was observed for gum sulfate pulp which cannot be explained at this time. Thus it appears that hydrolysis does not convert cellulose to a wholly crystalline or ordered state. It might be well to review past work in which complete crystallinity of cellulose “crystallites” has been assumed.