Background, absorption fluorescence

Quantitative elemental analysis Raw data corrected for background, absorption, fluorescence and atomic number effects to produce quantitative results with accuracy of 2-3% relative to the amount present. 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

Optical properties of the material are less critical for microchips hyphenated with MS than for devices with on-chip optical detection where low background absorption or fluorescence is mandatory. Thus, completely opaque polymers like glassy carbon or polyimide " can be used as microfabrication substrates. Furthermore, polymer microchips are of great interest because their potentially low manufacturing costs may allow them to be disposable. Methods used for the fabrication of plastic chips include laser ablation and molding methods. 

RI detectors measure this deflection, and are sensitive to all analytes that have a different R1 than the mobile phase. There are two major limitations First, Rl detectors are very sensitive to changes in the temperature, pressure, and flow rate of the mobile phase, and so these measurement conditions must be kept stable in order to obtain low background levels. Second, Rl detectors are incompatible with chromatographic separations using gradient elution. Furthermore, because Rl detectors are nonselective, they must be used in conjunction with other detection methods if specificity is required. Nevertheless, they have found wide application in isocratic chromatographic analysis for analytes that do not have absorptive, fluorescent, or ionic properties, such as polymers and carbohydrates. 

A second factor that contributes to the baseline variation is the difference in the background signal (absorption fluorescence) between the two solvents. This effect causes the difference in the baseline level between the left and the centre in figure 6.6b. A more extensive discussion on baseline variations in programmed solvent LC can be found in ref. [607]. 

Similarly, bilirubin and methotrexate can be determined in serum with a Hber-optic system terminated in a 19-gauge hypodermic needle and a reflective cap at its end so to produce a small absorbance cell [50]. Although these methods usually do not yield absolute analyte concentrations owing to background absorption or fluorescence of serum, they do reflect relative concentration changes sufflciently correctly. This kind of sensor was expected to be applicable also to the photometric determination of other important clinical analytes such as drugs, toxins, and biomolecules, but the limited selectivity and sensitivity of spectrophotometry will possibly also limit the scope of the method when applied directly to serum or whole blood. 

All data obtained from the proportional counters were corrected for background, absorption, counter dead time and nonlinearity, fluorescence, and atomic number. In this case the counter dead time, fluorescence, and atomic number corrections are negligible and the background can be accounted for by a simple subtraction. The absorption, however, requires more careful consideration. Detailed procedure and mass absorption coefficients were taken from Smith ( ). 

The detection limits in the table correspond generally to the concentration of an element required to give a net signal equal to three times the standard deviation of the noise (background) in accordance with lUPAC recommendations. Detection limits can be confusing when steady-state techniques such as flame atomic emission or absorption, and plasma atomic emission or fluorescence, which... 

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