Introduction

Analysts must fulfill many requirements, they must deliver reliable results in a minimum time with more and more miniaturized and sophisticated instruments. The race to trace levels is never ending and we are currently challenging the attomole.

Chromatography and capillary electrophoresis are the best performing separation techniques.

Gas Chromatography (GC) is well suited for solutes that can readily be volatilized, whereas liquid chromatography (LC) is well suited for thermally labile solutes. However, the separation power should be transferred to a detector that would not hamper the separation and to a fast data acquisition system. Analysts have to demonstrate that the results obtained under particular application conditions are reliable and fit for the purpose. The performances of any chromatographic system are changing with time. Two desirable features of chromatographic detectors are high sensitivity and high selectivity. Typically the plate number of a column is calculated from the standard deviation of a recorded peak. However, both the column and the instrument contribute to band broadening:

""tot _ ""col T ""ext.col cr col for the chromatographic column variance, cr extcol for extra column effects contribution to the variance of the peak. It is generally agreed that the loss in column plate count based on extra column effects should not exceed 10%.1

As an example for a specific LC system the maximum acceptable variance is o-acc = 0. 10o-2 < 0. 10u212rAe2(1 + k)2/N

where N, I, and e are the plate number, the length, and the porosity of the column, respectively, and k is the retention factor. Most of the real chromatographic peaks are not symmetrical. The significant deviation of the peak shape from the symmetrical peak makes difficult the acquisition of chromatographic signal information such as the retention time, the peak area, and the peak width at half height. The asymmetry factor b/a is usually measured at 0.1 h peak height. A chromatographic column looses its performances with continuous use. To overcome this drawback and check the time of changing the column we encourage the reader to use the deferred standard.2

We can roughly distinguish concentration sensitive detectors in which the signal is proportional to the concentration of the analyte in the mobile phase inside the detector cell and mass flow detectors where the signal is proportional to the mass flow rate of analyte swept by the mobile phase into the detector cell.

We can also distinguish universal detectors and selective detectors.

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