External Standard Channels Ratio Research Articles

Determination of14C in environmental materials: Combination of CO2 absorbent and a large-volume low-background liquid-scintillation counter

Commerically available CO2 absorbent and its scintillator were used to determine14C in environmental materials. Absorption of CO2 by the CO2 absorbent was quantitative, and strong and stable fixation was confirmed by no release of CO2 from the CO2 absorbent for a long period after absorption. The14C activity was measured by a large-volume low-background liquid-scintillation counter and the counts were quite stable for two weeks. Counting efficiency and background counts varied with the amounts of CO2 absorbed, indicating a change of quenching level with absorption of CO2. The different quenching levels were, however, normalized with external standard channel ratio (ESCR). Specific activity of14C in the present shell was evaluated for samples with different amounts of CO2 absorbed and were consistent with each other within the counting error, suggesting validity of the present method. Contemporary14C level in the environmental materials, 95% CO2 saturated sample, could be measured with 4% counting error (2σ) by 1000 min counting time.

The effects of nonuniform quencher distributions in quencher concentration measurements using liquid scintillation techniques

The use of liquid scintillation counting techniques for the in-situ measurement of quencher concentration in studies of transport phenomena was investigated. Quencher concentration gradients cause complex distortions of the measured scintillation pulse height spectrum. The measurement of the average quencher concentration using either sample channels ratio (SCR) or external standard channels ratio (ESCR) calibrations is unreliable if gradients are present, whereas measurement of the sample activity appears to remain unaffected by such gradients. It is suggested that case-specific test measurements be made on heterogeneous samples prior to the use of scintillation methods for in-situ measurement of quencher concentration.

Programmable calculator software for computation of the plasma binding of ligands

The computation of the extent of plasma binding of a ligand to plasma constituents using radiolabeled ligand and equilibrium dialysis is complex and tedious. A computer program for the HP-41C Handheld Computer Series (Hewlett-Packard) was developed to perform these calculations. The first segment of the program constructs a standard curve for quench correction of post-dialysis plasma and buffer samples, using either external standard ratio (ESR) or sample channels ratio (SCR) techniques. The remainder of the program uses the counts per minute, SCR or ESR, and post-dialysis volume of paired plasma and buffer samples generated from the dialysis procedure to compute the extent of binding after correction for background radiation, counting efficiency, and intradialytic shifts of fluid between plasma and buffer compartments during dialysis. This program greatly simplifies the analysis of equilibrium dialysis data and has been employed in the analysis of dexamethasone binding in normal and uremic sera.

Simultaneous determination of 241Pu, 238Pu and 239,240Pu in low activity environmental samples

Studies on plutonium in the environment are mostly directed to the alpha emitting isotopes 238Pu and 239,240Pu. However, the largest contribution to the radioactivity of plutonium is due to the beta emitting isotope 241Pu. Plutonium was separated using anion exchange after leaching the 242Pu spiked sample with concentrated HNO 3 and HCl. One aliquot of the eluate was spiked with an additional plutonium tracer, 236Pu, and electrodeposited for alpha spectrometric determination of 238Pu and 239,240Pu in the sample and for the determination of chemical recovery in the solution before electrodeposition. The activity of 241Pu was measured in the second aliquot using a calibrated liquid scintillation counter. For quench correction, the external standard channels ratio method was applied. The counting efficiency of 241Pu was about 35% and the lower limit of detection with a counting time of 120 min was 1 pCi (37 mBq). The concentration of 241Pu in lichen was about 3000 and 800 pCi/kg in 1964 and 1970, respectively and 2.5 pCi/kg in reindeer liver in 1977. The maximum activity ratio of 241Pu to 239,240Pu, about 16, in 1965 is in good agreement with that given for fresh fallout.

Effect of chemical quenching on background counting rate in tritium channel of a large volume liquid scintillation counter

Background counting rate in tritium channel of a large volume liquid scintillation counter increased with water content ranged 0 to 50% of liquid scintillator. This phenomenon can be explained as follows: The height of scintillation pulses of Compton electrons induced by background gamma radiation is lowered by chemical quenching and shifts to tritium channel. The background counting rate in tritium channel showed a linear relationship with external standard channel ratio of background samples. This relationship is applicable to determine the correct background counting rate for quenching samples and to achieve higher precision of tritium measurement.

Quench correction in liquid scintillation counting by a combined internal standard-samples channels ratio technique

A well-known problem in liquid scintillation counting (LSC) is that radioactivity cannot be measured with 100% efficiency, e.g., due to ''quenching'', which thus needs be corrected for. Three methods (viz., those of internal standard (IS), samples channels ratio (SCR), and external standard channels ratio (ESCR) are in common use to accomplish quench correction. None of these methods is ideal. This paper shows that a combination of the IS and SCR methods (IS-SCR) ameliorates the major disadvantages of both techniques and the disadvantage of the SCR technique at low count rates have been eliminated in the IS-SCR method, which also has a low volume dependence compared to the IS and ESCR methods. The IS-SCR method is not affected by time-dependent diffusion of solutes and solvents into the walls of plastic counting vials, which is a major drawback of the ESCR technique. Used with a simple linear regression technique, the IS-SCR quench curves may be linearized over wide ranges of efficiencies. In view of the wide-spread application of LSC, the IS-SCR technique is therefore likely to be useful to many investigators. 2 figures, 2 tables.

Evaluation of counting efficiency in liquid scintillation counting of pure β-ray emitters

A practical procedure is described for the calculation of the counting efficiency curve as a funtion of the external standard channel ratio in β-ray emitters. Using tritium as a standard we obtain the efficiency curves for 14C, 99Tc, 185W, 36Cl, 89Sr and 32P in different liquid scintillators. It is shown that the use of tritium as a standard reduces the uncertainties in the counting efficiency for higher energy β-ray emitters. The agreement betweeen calculated and experimental results is better than ± 4% in al cases.

44] Radioimmunoassay of the major plasma metabolite of PGF2α, 15-keto-13,14-dihydro-PGF2α

Publisher Summary This chapter discusses radioimmunoassay of the major plasma metabolite of PGF2α, 15-Keto-13,14-dihydro-PGF2α. In this particular assay, plasma volumes of as much as 0.5 ml may be assayed without disturbing interferences from other plasma constituents. After the addition of the labeled ligand, the tubes are left for incubation. The next step is the separation of the free and the antibody-bound fractions. Of all methods tried, 3 the polyethylene glycol method gave the most reliable results. The tubes are vigorously vortexed, the formed grayish-white pellet contains the precipitated γ-globulins including the antibody-bound fraction of the prostaglandin metabolite, whereas the free fraction is found in the supernatant. The yellow color of unextracted plasma gives about 10% reduction in counting efficiency in the scintillation counter as compared to the standard vials without plasma. It is important that the counted values for counts per minute are corrected for quenching. This can easily be done if the scintillation counter is equipped with automatic quench correction facilities, such as automatic external standard channels ratio. Before setting up routine radioimmunoassay runs, the optimal titer of the antibody must be established. This should always be done also with the commercial preparations, as experience shows that the recommended procedure from the manufacturer is often far from optimal.

Determination of phosphorus-32 and calcium-45 in biological samples by Čerenkov and liquid scintillation counting

A practical procedure is described for the determination of 32P and 45Ca in various biological samples. Samples were wet-ashed with 70% HClO 4 and 30% H 2O 2. The activity of 32P was estimated by Čerenkov counting and sum of the activities of both the nuclides were measured by liquid scintillation technique after addition of the “Aquasol”-solgel scintillator. Quench correction was accomplished by the sample channels ratio and by the automatic external standard channel ratio in the <Cerenkov and scintillation technique respectively. The recoveries of both nuclides obtained in biological samples of various origin (0.995 for 32P and 0.960 for 45Ca) indicate the suitability of this method for the simultaneous determination of these nuclides.

A single-step method for determination of the specific radioactivity of lipids.

The use of a liquid scintillation counter to measure both the mass and the radioactivity content of charred 14C-labeled lipid bands from thin layer chromatoplates has been evaluated. Following lipid mass determination from a measurement of the external standard channels ratio, a suitable choice of counting parameters enabled a reproducible and efficient 14C count to be obtained over virtually the whole range of lipid concentrations tested. Although the charring procedure resulted in some loss of radioactivity, the efficiency of counting remained high enough for accurate dpm measurements.

Quantification of lipid mass by a liquid scintillation counting procedure following charring on thin-layer plates

A novel method for the densitometric measurement of charred lipid bands from thin-layer chromatograms using a liquid scintillation counter is described. Following suspension in a scintillator gel, the reduction in the external standard channels ratio was used as a measure of the quenching effect, and therefore density, of the charred material. The relationship between the external standard channels ratio and mass of the lipid was linear for all the common lipid classes up to about 100 μg although accurate interpolations of the calibration curves were possible up to at least 1 mg. The extent of charring of the neutral lipids did not appear to depend on the degree of unsaturation of the constituent fatty acids although in the case of the phospholipids significant differences in the extent of charring were observed between naturally occurring and completely saturated lecithins. Analysis of total lipid extracts by this method yielded results closely comparable with those obtained by other well-established quantification procedures. The sbility of this method to provide a quick and accurate lipid quantifieation procedure is discussed.

Solving the problems of counting 14CO 2 produced in biological experiments

14 CO 2 can be trapped efficiently as Na 2 14 CO 3 in aqueous solutions of NaOH. In this state it can be dispensed and counted with great accuracy and stability, provided the correct conditions are observed. In this paper the various separate causes of inaccuracy in handling and counting 14 CO 2 are explained and demonstrated. These include the rapid loss of 14 CO 2 from aqueous solutions of mid-range pH, the effects of varying the constitution of the mixture being counted, the use of high specific activity Na 2 14 CO 3 , adsorption on the surface of the vial, the misuse of the “merit value” concept, and the effect of the dimensions of the counting vials on the rate of change of counting efficiency with volume of scintillant. The relevance of these conclusions to the work of other authors in this field is discussed. An account is given of some satisfactory procedures for counting 14 CO 2 using both commercial liquid scintillation fluids and “home made” mixtures. The applicability of the sample channels ratio and external standard channels ratio techniques to these mixtures is discussed. A method is described for the separation of 14 CO 2 from larger amounts of 14 C labelled volatile fatty acids and alcohols, which may have been either evolved or used as substrates in biological experiments.

An inexpensive method for the acquisition of external standard quench correction capability

A simple and economical method of acquiring external standard quench correction capability for liquid scintillation counters is described. The technique utilises a commercially available, weak (<1 μCi) 137Cs source, sealed in glass and used in conjunction with a flat-bottomed, 55 × 15 mm glass tube positioned in the centre of the conventional 20 ml scintillation vial. Each sample is counted with and without the 137Cs source in place and the counts added by the external standard are analysed in two channels. The external standard channels ratio and external standard gross counts so obtained have been found to be linearly related to 14C counting efficiency for both chemical and colour quenched samples.

Differential determination of203Hg and14C or35S in double labelled biological samples

The differential determination of203Hg and14C or35S in double labelled biological samples is presented. The biological samples were mineralized with 70% HClO4 and 30% H2O2 in minivals MILLI-6. The γ-activity of203Hg was measured on a well scintillation counter. The total activity, due to203Hg and14C or35S, was measured by the liquid scintillation technique after addition of Aquasol into the same vials. The method of external standard channel ratio was used for standardization. Very good recoveries were obtained: 100±0.7% for203Hg and 94.6–101.0% for14C and35S. This method could be used for other β, γ and β-active nuclides with similar β-spectra.

The effect of volume and quenching on estimation of counting efficiencies in liquid scintillation counting

The effect of volume on the liquid scintillation counting performance of 14C-samples has been investigated. A decrease in counting efficiency was observed for samples with volumes below about 6 ml and those above about 18 ml when unquenched samples were assayed. Two quench-correction methods, sample channels ratio and external standard channels ratio, and three different liquid scintillation counters, were used in an investigation to determine the magnitude of the error in predicting counting efficiencies when small volume samples (2 ml) with different levels of quenching were assayed. The 2 ml samples exhibited slightly greater standard deviations of the difference between predicted and determined counting efficiencies than did 15 ml samples. Nevertheless, the magnitude of the errors indicate that if the sample channels ratio method of quench correction is employed, 2 ml samples may be counted in conventional counting vials with little loss in counting precision.

Evaluation of the reproducibility in radioactivity measurements with liquid scintillation counters--effect of anomalous count rate-dependent photomultiplier gain variations (author's transl)

Time variations in count rate were measured for prolonged period of time using a set of 3H samples and a few other specially prepared samples of 14C, 32P, and 137Cs. Besides, beta-ray spectra of a particular 3H sample were measured as a function of time with a multichannel pulse height analyzer. These measurements lead to the following conclusions: (1) Anomalous count rate variations are attributed to strong dependence of photomultiplier gain on count rate. (2) Different steady state count rates can be observed for 3H samples having activities of approximately 10(5) dps when they are measured immediately after the measurements of background and 3H samples of higher activities. (3) The count rate-dependent effect on photomultiplier gain can be described properly by the current flowing through the tube rather than the count rate. (4) The measurement of the external standard channel ratio using the built-in 137Cs source can affect seriously the successive measurements on 3H samples, while the channels ratio itself exhibits strong time dependence.

Time-dependent change of external standard channels ratio values measured with polyethylene vials in different counting conditions.

Time-dependent change of external standard channels ratio values measured with polyethylene vials in different counting conditions.

Some empirical observations concerning the liquid scintillation counting of coloured solutions

The colour quenching capabilities of nine dyes have been examined, and compared with the visible absorption spectra of the dyes in toluene solution. From these data an empirical relationship is derived, from which the liquid scintillation counting efficiency is estimated simply from the λmax (wavelength of maximum absorption) of the dye and its absorption, A, at that wavelength. For the particular instrument, window settings, and scintillator solutions used in this work the relationship was[Formula: see text]Colour quenching agents are classified as severe (λmax 380–480 nm), moderate (λmax 480–520 nm), light (λmax 520–560 nm), or zero (λmax &gt; 560 nm).The normal external standard channels ratio method for obtaining counting efficiency of chemically quenched samples has been applied to samples that contain colour quenchers and various combinations of colour and chemical quench. The results are in excellent agreement with the actual counting efficiencies; this result is in direct contrast to current views on the matter. It appears, therefore, that the counting of samples quenched both chemically and by colour may not always be impossible and the user is well advised to investigate his own System to evaluate whether results within acceptable error may be obtained.

Determination of the Liquid Scintillation Counting Efficiency of 3H and/or 14C Labelled Samples Independently of the Degree of Colour and/or Chemical Quenching

Abstract A value R is obtained by the ratio of the summed, coincident pulses of both photomultipliers to the coincident pulses of one photomultiplier in the same selected range of the external standard puls height spectrum. This value R together with the external standard channel ratio (ESCR) show the extent of chemical and/or colour quenching in a field described by a set of calibration samples with increasing chemical or colour quenching (Fig. 3). The counting efficiency of samples with different and unknown proportion of chemical and/or colour quenching can be determined much more accurate than by any calibration approach using the ESCR only (Table VI). The method can be used for 3H samples with a counting efficiency ≧10% and 14C samples of ≧5%, respectively. The new correction procedure is also applicable for 3H and 14C double-labelled compounds.

Cryptic “photon” quenching in dioxane-based liquid scintillation mixtures

Dioxane-based scintillation mixtures containing β-emitting radioisotopes show a decreased efficiency under the following conditions: low water concentration in the sample, the presence of some metal ions, e.g., Ca2+, Mg2+, and certain3H- and 14C-labeled compounds, e.g., nucleotides. If only one of the preconditions was not fulfilled, the effect disappeared. There is no visible sign of any form of sample inhomogeneity, and the composition of the scintillation sample gives no reason to suppose that a special sort of quenching would occur. In addition, no commonly used standardization method, such as external standard channels ratio, internal standard, or sample channels ratio could be used to detect this sort of quenching, to say nothing of correcting for it. A further analysis shows that the decreased count rate has to be explained by a special sort of “photon” quenching. The so-called “double ratio plot” proposed by Bush ((1968) Int. J. Appl. Radiat. Isotop.19, 447–452) to provide an indication of sample heterogeneity failed to indicate this sort of photon quenching.