Low background testing -

Special Precautions for Low-Background Testing For low-background testing, elaborate precautions must be taken to avoid background leaks. The... 

Background is, by definition, the irradiance level that remains when the source (usually blackbody) irradiance is set to zero. In low-background testing, we may use a blackbody to cause an irradiance level that simulates the background at which the detector will operate in another application. In this case, we would use the apparently contradictory expression the background due to the blackbody. ... 

Since the transmission characteristics of any optical filter can vary strongly with temperature, it is imperative that they be tested and calibrated at their operating temperature. Otherwise, unpredictable changes in the transmission characteristics can go unnoticed. The article Problems in using cold spectral filters with LWIR detectors (Stierwalt and Eisenman, 1978) should be reviewed by anyone doing low-background testing. 

Quantitation Limit. When the quantity of heavy metals is determined from the calibration curve, it is recommended to estimate the lowest value of heavy metals concentration as the quantitation limit. The methods to estimate the quantitation limit are described in JP and ICH Q2B Guidelines [2], and an appropriate method should be selected from among these methods. An estimation of the quantitation limit can be obtained from the standard deviation of measured values of the low-concentration test solution. The standard deviation of background noise will be used to estimate a value for the standard signal-to-noise ratio (10 1). 

Using this procedure, the estimated quantitation limit is obtained from the variation of actual measurement values of low-concentration test solutions instead of background noise. However, the actual value of quantitation limit needs to be verified by experiment. 

Leach rates for elements other than those listed in Table II can also be determined by this method. In fact, any element in the periodic table that is solid at room temperature and has an activation product with a half-life sufficiently long to allow leach testing can be studied with this technique. This method can also be applied to the study of the leach rates of alpha emitting actinides present in waste. In this case, standard carrier-free radiochemical procedures, coupled with low background alpha counting, would be invoked. 

If the previous tests do not suffice for a decision, further testing in mammals is indicated. If the purpose is to get mammalian germinal data on whether a substance produces mutations, the specific-locus test is suitable because of its low background rate and the extent to which the test has been performed. It should be recognized, however, that chemicals that yield inconclusive results in short-term tests are likely to yield negative or inconclusive results in mouse tests. The dominant-lethal test may be used if the expected effect is chromosomal. 

Autoradiography and staining methods are based on detection of cytoplasmic DNA (in the former case replicating cytoplasmic DNA). For this reason there will always be a low background caused by mitochondrial DNA and samples to be tested should be compared with positive and negative controls. Such prefixed controls are available from Moredun Ltd. (Appendix 3) ready for staining. 

A study is considered valid if the results obtained with positive and negative controls are consistent with the laboratory s historical data and with the literature. Statistical analysis is usually applied to compare treated and negative control groups. Both pairwise and linear trend tests can be used. Because of the low background and Poisson distribution, data transformation (e.g., log transformation) is sometimes needed before using tests applicable to normally distributed data. Otherwise, nonparametric analyses should be preferred. 

The combined low background, high detection efficiency, and 5.5-ml sample volume lead to a detection limit of 10 Bq (600 dpm) per liter for a 100-min count (95% confidence limit). These results are achieved routinely in field tests of the FDTAS at monitoring wells, surface streams, ground water remediation facilities, sewage treatment plants, and the Savannah River. Results have been confirmed by parallel sampling and laboratory analyses. 

Visibly luminescent, NIR (980 nm) excitable Er/Yb-doped nanoparticles have been used for microscopic detection in DNA microarrays [65]. The complete absence of endogenous upconversion luminescence leads to very low backgrounds and enhanced contrast. An NIR luminescent ytterbium(III) complex based on the fluorescein-isothiocyanate motif was demonstrated to be an adequate luminescent label in a sandwich-type in vitro diagnostic test [27]. 

A prospective purchaser might consider it worthwhile to obtain evidence of the measured background of the specific detector before buying. It may not be possible to reproduce the performance of the manufacturer s low-background shield in which the test was done, but such data win give a benchmark against which the measurements after installation can be evaluated. 

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