Sensitivity, measurement instruments

Random errors arise in all measurements and are inevitable, no matter what the experiment, the quality of the instrument, or of the analyst. They are a consequence of the limitations of experimental, observations. For example, an instrument reading can only be taken within the limits of accuracy of the scale, as read by a particular observer. The position of the pointer between two division marks may be estimated to one fifth of a division by a skilled experimenter, but only to one half a division by another. Such skill may be improved with practice, but will never be totally perfected. Random errors cannot be eliminated, but can be reduced by using more sensitive measuring instruments or an experienced experimenter. The magnitude of random errors can be estimated by repeating the experiment. 

He went on to calculate from the expansion of air a further set of values for the mechanical equivalent of heat which broadly agreed with the electromagnetic experiments. These experiments and, in particular. Joule s concern for very sensitive measuring instruments suggest that he was, to some extent at least, using experiments to prove his theory of the nature of heat. This experimental style contrasts with his earlier, more open-ended approach. 

Cavity pressure sensors are a contradiction in themselves on the one hand, they are highly sensitive measuring instruments on the other, they are in practice often treated no more respectfully than bolts, pins, or other mold standards, although a significant investment is made in them. 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

Figure 6. Schematic outline of the first commercially available multiple collector ICPMS, the Plasma 54, after Halhday et al. (1995). This instrument uses Nier-Johnson double-focusing and is equipped with eight independently adjustable Faraday collectors. The axial collector can be wound down to provide access to a Daly detector equipped with ion counting capabilities and a second-stage energy filter for high abundance sensitivity measurements. The sample may be introduced to the plasma source by either solution aspiration or laser ablation.

If a small amount of gramicidin A is dissolved in a BLM (this substance is completely insoluble in water) and the conductivity of the membrane is measured by a sensitive, fast instrument, the dependence depicted in Fig. 6.15 is obtained. The conductivity exhibits step-like fluctuations, with a roughly identical height of individual steps. Each step apparently corresponds to one channel in the BLM, open for only a short time interval (the opening and closing mechanism is not known) and permits transport of many ions across the membrane under the influence of the electric field in the case of the experiment shown in Fig. 6.15 it is about 107 Na+ per second at 0.1 V imposed on the BLM. Analysis of the power spectrum of these... 

Sensitivity is a significant characteristic in all scientific disciplines which have to do with measurements. Sensitivity is defined from the viewpoint of instrumental measuring as the change in the response of a measuring instrument divided by the corresponding change in the stimulus (ISO... 

Sensitivity The change in the response of a measuring instrument divided by the corresponding change in the stimulus. 

The sensitivity of a method is a measure of its ability to distinguish one concentration from another. If a particular instrument could be used to determine the concentration of a heavy metal such as lead and could reliably distinguish a 25 ppb solution from a 30 ppb solution, it would be more sensitive than an instrument that could barely tell the difference between a 25 ppb solution and a 50 ppb solution. The best quantitative measure of the sensitivity of an instrument and/or an analytical method is to determine the slope of the calibration curve. The greater the slope, the more sensitive the instrument and/or method. 

The sensitivity of instruments using low resistance circuits is determined primarily by the sensitivity of the galvanometer (Figure 4.5). Electrode systems that have a high resistance, e.g. glass electrodes, require a high impedance voltmeter, which converts the potential generated into current which can be amplified and measured. Such instruments are commonly known as pH meters but may be used for many potentiometric measurements other than pH. 

Figure 3. Optical system of instrument of spectral sensitivity measurement (L) Xe arc lamp (Ml) concave mirror (M2, M3) mirror (S) shutter (SI) slit (G) concave reflective grating...

It has already been stated that a suitable quantitative assay technique must be available to measure the reaction of interest and it is assumed that the experimenter has determined optimal reaction conditions for the enzyme of interest. All kinetic assay techniques assume that v is a variable and that [S] is known as such, preparation of substrate must be meticulous in terms of ensuring that concentrations are correct, and this in turn will rely upon factors such as good weighing and pipetting techniques with calibrated instruments capable of precise, accurate, and sufficiently sensitive measurement. 

Analytical-scale SFE can be divided into off-line and on-line techniques. Off-line SFE refers to any method where the analytes are extracted using SFE and collected in a device independent of the chromatograph or other measurement instrument. On-line SF techniques use direct transfer of the extracted analytes to the analytical instrument, most frequently a chromatograph. While the development of such on-line SFE methods of analysis has great potential for eventual automation and for enhancing method sensitivities [159-161], the great majority of analytical SFE systems described use some form of off-line SFE followed by conventional chromatographic or spectroscopic analysis. 

Schiff, H. I G. I. Mackay, C. Castledine, G. W. Harris, and Q. Tran, Atmospheric Measurements of Nitrogen Dioxide with a Sensitive Luminol Instrument, Water, Air Soil Pollution, 30, 105-114 (1986). 

The sensitivity of a method (not to be confused with selectivity or limit of detection) is how much the indication of the measuring instrument increases... 

With the maturation of tandem mass spectrometry (MS/MS) instruments and methodologies, many commercial laboratories are switching to this technique, and high-sensitivity measurements on high volumes of samples are becoming routinely achieved. Since the future of steroid hormone analysis is MS/MS, only MS techniques will be described in this chapter. Immunoassay procedures have been well described and reviewed over the years and will cease to be mainstream methodologies within the next decade. 

Measurements also commonly involve random errors. These are errors whose size and direction differ from measurement to measurement that is, they are unpredictable and unreproducible. They are commonly associated with the limited sensitivity of instruments, the quality of the scales being read, the degree of control over the environment (temperature, vibration, humidity, and so on), or human frailties (limitations of eyesight, hearing, judgment, and so on). We shall say much more about random error later in this chapter. 

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