Counting with High Efficiency

There are two reasons for the continuum underl5dng peaks in a spectrum  

Variation of the Common continuum due to the sample, with detector size 

we can expect that the larger detector would provide a slightly lower MDA. In order to improve the MDA significantly, we would need a much bigger detector. For example, the 114% detector referred to in Table 11.3 (Chapter 11) would provide an improvement of 0.65, albeit at considerable extra cost. 

Variation of the environmental background, with detector size 

While this seems a very worthwhile strategy, the author, being notoriously cynical, wonld not care to place all his eggs in this particular basket The following counterindications should be considered  

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First, solutes with larger electrophoretic mobilities (in the same direction as the electroosmotic flow) have greater efficiencies thus, smaller, more highly charged solutes are not only the first solutes to elute, but do so with greater efficiency. Second, efficiency in capillary electrophoresis is independent of the capillary s length. Typical theoretical plate counts are approximately 100,000-200,000 for capillary electrophoresis. 

By number count the great majority of particles in the outside air are likely to be less than 1 pm. By weight, these small particles will account for a very small proportion of the sample. A filter with a high efficiency measured by weight of particles trapped may be almost transparent to the small ones. Very high counts can be found in rural areas from pollen or agricultural activities. 

Liquid scintillation counters are highly efficient for low CL intensities and consist of two photon-counting channels provided with a variable discriminator. The sample is placed between these two detectors to ensure a high optical efficiency. The discriminator is adjusted to allow photon impacts to be transmitted and small background noise pulses to be rejected. As disadvantages they suffer from saturation errors and provide nonlinear relationships between the CL intensity and the total counts. 

In more demanding separations that require higher plate counts, specially designed rapid analysis columns packed with very high efficiency 2 to 3 /.an porous particles are available from several manufacturers. In addition, monolithic columns with improved flow-through characteristics are also commercially available. Figure 13.4 depicts a comparison of inlet pressure and flow rate for 4.6 mm inner diameter x 50, 100, and 150 mm columns packed with 5 /an particles. 

The fact that neutrons can be detected with reasonably high efficiency and with minimal interferences from other radiations permits the practical determination of fissionable species such as isotopes of uranium and thorium by delayed neutron counting. The known delayed neutron emitter precursors are all short lived and the irradiated samples are counted with 10BF3-filled proportional counters immediately after irradiation without any separation chemistry. 

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