External standard ratio

Picer et al. [49] described a method for measuring the radioactivity of labelled DDT contaminated sediments in relatively large volumes of water, using a liquid scintillation spectrometer. Various marine sediments, limestone and quartz in sea water were investigated. External standard ratios and counting efficiencies of the systems investigated were obtained, as was the relation of efficiency factor to external standard ratios for each system studied. 

The colour quenching effect of carotenoids in liquid scintillation spectrometry has been studied in detail. The most efficient quenchers were lycopene [i/>, -carotene (225)] and echinenone [j8,/3-caroten-4-one (226)]. Use of the external standards ratio method to correct for colour quenching in the radioassay of carotenoids is justified.84... 

The tritium measurement protocol Is relatively simplistic and consists of neutralization of the sample, single plate distillation, removal of an appropriate size aliquot (usually eight mL.) via reproducible automatic pipets and the addition of 15 mL. of dark adapted scintillation cocktail under Incandescent lighting conditions. After shaking to ensure a uniform gel, the sample is allowed to settle and dark adapt for up to twenty but no more than sixty minutes. The liquid scintillation unit efficiency Is determined dally on a previously prepared standard and background measurements are determined at least dally. Quench corrections are not applied to the system due to the lack of an external standards ratio capability and an effort to minimize the amount of hazardous waste which would be generated if an Internal standards approach were adopted. 

The samples were counted in a liquid scintillator spectrometer. Quenching was monitored by employing automatic external standard ratio. The counting efficiency for carbon-14 was measured by using toluene- C as a reference standard. The efficiency factor was used to convert the collected data in counts per minute (cpm) to disintegration per minute (dpm). 

An internal standard method gives more reliable results when elaborate sample preparation is required, as in extraction of a drug substance from biological fluids, or extraction of pesticides and herbicides from soil and plant matter. The addition of internal standard (IS) to the sample and standard acts as a marker to give accurate values of the recovery of the desired compound(s). Since the determination of wt% involves the ratio of the detector responses in the two chromatograms, the injection volume is not critical as in an external standard method. 

Compound 6 contains seven iron-based units [ 12], of which the six peripheral ones are chemically and topologically equivalent, whereas that constituting the core (Fe(Cp)(C6Me6)+) has a different chemical nature. Accordingly, two redox processes are observed, i.e., oxidation of the peripheral ferrocene moieties and reduction of the core, whose cyclic voltammetric waves have current intensities in the 6 1 ratio. Clearly, the one-electron process of the core is a convenient internal standard to calibrate the number of electron exchanged in the multi-electron process. In the absence of an internal standard, the number of exchanged electrons has to be obtained by coulometry measurements, or by comparison with the intensity of the wave of an external standard after correction for the different diffusion coefficients [15]. 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

In solution, vitamin D (both D2 and D3) isomerizes to previtamin D and forms a temperature-dependent equilibrium mixture [520], which leads to quantification problems. Previtamin D is difficult to quantify because of interference from co-eluted contaminants. The reversibility of the isomerization is very slow, therefore the percentage of previtamin can be considered constant during the entire analysis. The quantification of the potential vitamin D can be performed using an external standard that has undergone saponification procedure as the sample [521]. Vitamin D2 and D3 can be used as an internal standard to quantify the other one. Indeed, the isomerization rates of vitamins D2 and D3 are virtually the same thereby the previtamin D/vitamin D ratio will be the same for both vitamers at any temperature. The isomerization problem can be resolved by... 

Quantitation by Internal Standard. Quantitation by internal standard provides the highest precision because uncertainties introduced by sample injection are avoided. In this quantitation technique, a known quantity of internal standard is introduced into each sample and standard solutions. As in the external standard quantitation, chromatograms of the standard and sample solutions are integrated to determine peak heights or peak areas. The ratio of the peak height or area of the analyte to an internal standard is determined. The ratios of the standards... 

GC-MS SIM analysis for prenatal diagnosis requires a different calibration standard from that used for urine from children and adults. Table 5.3.12 shows the steroids present in our external standard and the monitored ions. For serum analysis we employ the highly sensitive ion-trap MS/MS technique described at the beginning of this chapter. Many steroid ratios have been calculated that can be used to distinguish causes of low-estriol, a few that we consider useful are given below, values considered positive for particular disorders are in parenthesis. 

For general purpose tracer work, however, and particularly in polymer chemistry, the liquid scintillation counter surpasses all other instruments in its sensitivity and adaptability. There is no question on the author s mind that at the present time such an instrument would be the first choice, particularly where tritium, carbon-14 or sulphur-35 were involved. Samples for assay are dissolved in a phosphor whose major solvent usually consists of toluene, toluene-alcohol, or dioxan. Many polymers and low molecular weight compounds are readily soluble in these solvents. Prospective users should not be deterred by alleged complications due to "variable quench effects" as these effects are readily corrected for via internal or external standards or the channels ratio method (7, 46, 91). Dilution quench corrections, though valid, are tedious and unnecessary. Where samples are insoluble in phosphor they may be suspended (e.g. as gels or as paper cut from chromatograms, etc.) or they can be burnt and the combustion products absorbed in a suitable phosphor solution. A modification of the Schoniger flask combustion technique is particularly suitable for this purpose (43—45). 

Taking the crystallinity parameter QA (external standard [6]) as a measure for content of crystallized zeolite. Starting Si02/Al203-ratio in the reaction mixture was 69. The QAi-values of about 1 are representing a tranformation of 100% into the zeolitic material. 

The internal standard ratio method for quench correction is tedious and time-consuming and it destroys the sample, so it is not an ideal method. Scintillation counters are equipped with a standard radiation source inside the instrument but outside the scintillation solution. The radiation source, usually a gamma emitter, is mechanically moved into a position next to the vial containing the sample, and the combined system of standard and sample is counted. Gamma rays from the standard excite solvent molecules in the sample, and the scintillation process occurs as previously described. However, the instrument is adjusted to register only scintillations due to y particle collisions with solvent molecules. This method for quench correction, called the external standard method, is fast and precise. 

OF THE RADIOACTIVITY CONCENTRATION Radioactivity measurements are carried out by the liquid scintillation counting procedure in -spectrometers using an external standard device which permitted the counting efficiency to be determined by the channel ratio method (explained for instance by Dyer (1980)). 

The channels ratio method makes use of existing counts within the sample vial. This method is suitable when large numbers of counts are present, but it becomes very time consuming with samples containing few counts, because a long time is required to accumulate sufficient counts for statistical accuracy. Most modern scintillation counters therefore employ an automatic external standardization system of quench analysis to avoid the time required for the internal channels ratio method. This method utilizes a specially selected external y radiation source carried in a lead-shielded chamber that is buried in the instrument. Before the regular counting of the sample, the external standard is... 

This experiment demonstrates two methods for analysis of quenching within samples, namely, the channels ratio method and the automatic external standardization method. Either or both of the methods may be demonstrated with the quench series of bottles described in the following protocol, depending on the capabilities of your particular scintillation counter. 

If the channels ratio method is used, proceed with Steps 3 to 8. If the automatic external standardization method is used, proceed with Steps 9 to 13. 

Isotope ratio, based on a comparison of the ratio between the intensity of signals generated by selected isotopes of a particular element in the sample to the intensity of the added standard. An external standard is routinely added in a stream of auxiliary liquid. 

Many of these difficulties can be monitored and overcome with the use of standards, either internal or external (Zevin and Kimmel 1995). For the internal standard method, a known quantity of standard material is added to an unknown mixture, and the ratio of the intensity of the standard component is compared to a previously determined calibration curve to determine the mass fraction of the unknown (e.g. one or more of the polymorphic components). In the external standard method, the entire composition of the unknown sample is determined simultaneously by comparing the measured intensities and respective calibration constants of reference intensity ratios (determined beforehand), which must all be with reference to the same reference standard. 

Three methods have evolved to ascertain the degree of efficiency loss both within the instrument and as a result of quenching. These techniques are termed (1) internal standardization, (2) channels ratio quench correction, and (3) external standard channels ratio quench correction. Determination of counting efficiency by internal standardization may be performed in two steps. The sample is first accurately counted followed by the addition of a precisely known quantity of radioactivity to the vial (50,000-80,000 dpm C or 100,000-150,000 cpm H). It is important for the amount of added radioactivity to be considerably larger than that originally present in the vial. The sample is then counted a second time. The first count is the sample cpm and the second count is the sample cpm + (efficiency)(standard dpm). That is. 

Dividing equation 3-22 by 3-21 gives the channels ratio for the external standard ... 

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