Limit and Sensitivity

Detection Limit It may be defined as the concentration (meg ml-1) of an element that gives rise in the shifting of absorbance signal to an amount which equals to the peak-to-peak noise of the base-line. 

Sensitivity It may be defined as the concentration of element present in the sample solution that produces 1% absorption. 

From the above definition it is quite evident that the sensitivity takes no cognizance of the noise-level of the base-line, therefore, it is more or less of no use as a definite guide to the least quantity of an element which may be estimated. However, the sensitivity of a 1% absorption-is a pure theoretical number only that would undergo a change solely depending on the efficiency of the lamp (hollow-cathode-lamp), atomizer, flame-system employed, monochromator (prism, grating used), and finally the photomultiplier used. 

The sensitivity for 1% absorbance is determined by the help of the expression given below  

It is an usual practice to perform an actual-test-run over a sufficiently large range by employing the necessary prevailing expansion facility so as to ascertain fully whether or not the atomic absorption technique is reasonably applicable to a specific low-level estimation. Such a data may ultimately reveal the exact and true detection limit which is normally equals to twice the noise level. 

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Differentiate between detection limit and sensitivity. Compare flame AA, graphite furnace AA,... 

The magnitude of the dissociation constant A plays an important role in the response characteristics of the sensor. For a weakly dissociated gas (e.g., CO2, K = 4.4 x 10-7), the sensor can reach its equilibrium value in less than 100 s and no accumulation of CO2 takes place in the interior layer. On the other hand, SO2, which is a much stronger acid (K = 1.3 x 10-2), accumulates inside the sensor and its rep-sonse time is in minutes. The detection limit and sensitivity of the conductometric gas sensors also depend on the value of the dissociation constant, on the solubility of the gas in the internal filling solution, and, to some extent, on the equivalent ionic conductances of the ions involved. Although an aqueous filling solution has been used in all conductometric gas sensors described to date, it is possible, in principle, to use any liquid for that purpose. The choice of the dielectric constant and solubility would then provide additional experimental parameters that could be optimized in order to obtain higher selectivity and/or a lower detection limit. 

Fig. 23 Calibration experiment for the determination of cadmium. Detection limit and sensitivity are graphically evaluated...

Multielement determination (sequential or simultaneous) faster analysis time minimal chemical interaction detection limits and sensitivity fall in between that of flame and graphite furnace measurements. 

Ionization efficiency of a compound defines the conversion of a neutral molecule to a charged particle (ion). It directly affects the detection limit and sensitivity of a mass spectrometer in LC/MS system. There are several instrumental parameters that play important roles in the ionization efficiency of a compound. These parameters (and their names) are usually vendor-specific. 

Recording the applied av potential and the resulting ac current on a twin-beam oscilloscope produces Lissajous curves (in this case an ellipse), which may be used to the determine the impedances. Because of its frequency limitations and sensitivity to noise, this technique is not currently used in electrochemical measurements. 

Table 4.3 Atomic absorption spectrometry indication of detection limits and sensitivity obtainable by flame and by furnace techniques (by permission of Varian Analytical Instruments)...

For the determination of trace elements in the soil, atomic absorption spectrophotometry is also very suitable. A solution should be obtained from the sample to be analysed, most frequently by a decomposition with hydrofluoric acid extracts can also be prepared from the samples to be analysed. The detection limits and sensitivities for this technique are shown in Table 7.7. 

Table 7.7. Atomic absorption detection limits and sensitivities with a conventional flame atomizer...

An inherent drawback of the system depicted in Fig. 63a is that with a sample volume of 30 jxL, the maximum sensitivity reached is only about 5 ppm P-PO4. Yet as derived from Rule 1 in Chapter 2, the sensitivity of measurements is, within a certain range, proportional to the injected sample volume 5,. Thus by increasing Sr, above 30 xL, the detection limit and sensitivity of the assay will be further increased. This will be demonstrated in the next exercise. 

Polycyclic hydrocarbons, pesticides (dieldrin, ODD, DDT) and sulpha drugs were investigated. No detection limits and sensitivities were stated. 

Multiple-layer scatterers, for example consisting of a thin layer of HOPG glued on top of an AI2O3 substrate, in combination with a Rh tube are useful to determine a wide range of elements simultaneously with good detection limits and sensitivities. 

Since atomic absorption spectroscopy utilizes the ground state atom population for its measurements, it would appear that atomic absorption has a great advantage over flame emission in terms of detection limits and sensitivities of detection. An inspection of Appendix VIII, where detection limits are given for a number of elements for flame emission and atomic absorption, indicates this is not true. The reason for this apparent discrepancy lies in the relative stabilities of ground state and excited state atoms. An excited atom has a lifetime of the order of 10 -10 sec, and thus emits its energy very quickly after being excited. The usual flame emission source has an upward velocity of from 1 to 10 m/sec, so the excited atom will move only about 10 -10 m between the time of excitation and emission. 

The experiments started with equilibration of a solution containing n/O.SS moles C02/mole DIPA at 25°C. The amount of CO2 charged was determined by the detection limits and sensitivity of the infrared spectrophotometer and the expected CO2 gas-phase concentration increase. Equilibration of the solution was checked by the infrared spectrophotometer and took some 2 hours. 

Solvent-solute interactions are important in fluorescence analyses. These interactions have large effects on the reproducibility, detection limits, and sensitivity of a method. First, for n transition compounds (e.g., ketones) in nonpolar and aptotic solvents (e.g., alkanes and ethers), little or no fluorescence occurs. However, in protic solvents (e.g., methanol) these cmnpounds fluoresce. Conversely, n- - n transition compounds (e.g., polycyclic aromatic hydrocarbons) fluoresce readily in nonpolar solvehts and less in protic solvents. 

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