Quench Correction Curves Research Articles

Sources of error in the channels ratio method for efficiency determination in liquid scintillation counting

The reliability of the channels ratio method for determining counting efficiency in liquid scintillation counting was investigated. It was found that the efficiency of counting gels, cloudy samples, two-phase samples, samples in which the radioactive material was precipitating, and samples on solid supports could not be reliably determined from a normal quench correction curve. A curve constructed from external standard channels ratios was unreliable when mixing different vial sizes and sample volumes, but one constructed from sample channels ratios was not. It was also found that variation in instrument performance can result in large errors unless samples and standards are counted together. Statistical error changed relatively little within the range of ratios 0.3 to 0.8.

Liquid scintillation counting of aqueous samples using Triton-containing scintillants

Markedly unstable count rates were observed using a toluene-Triton (2:1, v v ) scintillant during counting of water-soluble radioactive compounds when < 5% ( v v ) water was present, because of the separation of phases. Efficiency correction in these instances could not be made by using 3H 2O as internal standard, because under the same conditions count rates with tritiated water were stable. Increasing water to ≥6% stabilized the count rates. With toluene-Triton (2:1, v v ) scintillant, the water level should preferably be maintained between either 6 and 12 or 18 and 24% for 14C- and 3H-labeled compounds for counting at 6°C or at ambient temperature (but only between 6 and 12% for 3H counting at room temperature). With a “Tritosol” ( Anal. Biochem. 63, 555 (1975) modified to contain 35 ml of ethylene glycol, 140 ml of ethanol, 250 ml of Triton X-100, 575 ml of xylene, 3 g of PPO, and ±200 mg of POPOP, water levels of up to 23% were acceptable for 14C and 3H for counting at room temperature or at 6°C. Within these limitations, with the toluene-Triton or with the modified Tritosol as scintillant, both polar and apolar radioactive compounds exhibited similar efficiencies and gave quench-correction curves, based on the external standard ratio, that were linear for both 14C and 3H-labeled compounds.

The surfactant Bio-Solv-BBS-3 as a scintillator in liquid scintillation counting

When the surfactant mixture Bio-Solv-BBS-3 is added to a scintillation solvent it acts as a primary scintillator in response to β emissions (and Compton electrons from γs). The fluorescence excitation threshold is higher and fluorescence yield is lower than those of the primary scintillators usually employed in scintillation counting. Presence of a surfactant in a sample containing 14C or more energetic βs will be counted at higher efficiency than would be indicated by a quench correction curve (efficiency vs sample channels ratios or external standard channels ratios) derived from standards not containing surfactant.

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.

Comptage direct des echantillons Plasmatiques en Scintillation Liquide par Etalonnage Interne-Externe

A direct incorporation method for liquid scintillation counting of plasma samples is described. after dilution in saline, the plasma samples are mixed with a new scintillator emulsifiying mixture (“Scintingel”). Counting efficiency is obtained by a pseudo-internal method. Results show great significance of plasma quenching particularly in the field of clinical studies. Quench correction curves for tritium, sulphur-35 and calcium-45 are plotted using counting efficiency versus external standard channel ratio. The sample activity in dpm is given with a 5 per cent precision.

A comparison of methods for the preparation of 14C-labelled plant tissues for liquid scintillation counting

Four methods for the preparation of 14C-labelled plant tissues for liquid scintillation counting have been compared for accuracy and precision. The wet oxidation method of Mahin and Lofberg, although easy and rapid, proved to be unsuitable for obtaining a reliable estimate of 14C present in lyophilized plant material, showing non-quantitative recovery and poor reproducibility. Solubilization in Hyamine hydroxide provided accurate and reproducible results, if experimental conditions were strictly adhered to; however, especially when larger samples were employed, severe—and variable—color quenching resulted in very low counting efficiency, which necessitated construction of quench correction curves specific for each source of plant material. Van Slyke wet combustion, in a device slightly modified after Jeffay and Alvarez, on the other hand, always resulted in reproducible and reliable data, irrespective of the carbon compound administered and the nature of the plant material. With samples of up to about 10 mg, Schöniger dry combustion in the Micro-Mat BF 5010 oxidation system even outrivalled Van Slyke wet combustion, because of speed and ease of manipulation. The latter methods are, therefore, evidently superior to the wet oxidation method of Mahin and Lofberg and solubilization in Hyamine hydroxide in preparing plant tissue samples for liquid scintillation counting.

Quench correction in Čerenkov counting: Channels ratio and external source channels ratio methods

Channels ratio and external source ratio methods which may be used for accurate quench correction in Čerenkov counting systems have been developed. In general, the channels ratio method will be more accurate for quench correction in high count rate samples where good ratio statistics are available. For low level samples, the external source channels ratio method will provide more accurate quench correction if efficiencies are greater that 10%. In either method, quench correction curves (efficency vs ratio) will be the same for a given solvent regardless of quencher. However, the solvent must be the same for all standards and samples since quench curves vary somewhat with solvent. Both channels ratio and external source channels ratio calibration methods are volume dependent. In either case, however, the results show that the effect of sample volume is sufficiently small that it will be negligible if volumes are approximately constant (approximately ±10%). An exception to this is present in the high quench region when using external source ratio. In this case, low volume samples are shown to deviate significantly. For this reason, it is recommended that samples be made to a volume of greater than 10 ml when this technique is utilized. The development of simple and accurate quench correction techniques for Čerenkov counting make the technique far more useful for the determination of beta emitters which are sufficiently energetic to generate Čerenkov radiation.