Data Acquisition and Calculation of Results

But of all of the many choices for detector, the conductivity detector remains the most popular. Conductivity detection is universal, rugged and sensitive. Adding a suppressor (discussed in Chapter 4 and elsewhere) dramatically improves the sensitivity for the majority of anions. The only exception to this is weak base anions which become low conducting in a suppressor. But the gain in detection for most anions and cations is so impressive that suppressed conductivity is the most popular form of detection, which means that a suppressor is placed after the exit of the separation column and before the conductivity cell. Membrane suppressors are marketed by Dionex. As described above, several column suppressor patents have expired and there are several companies offering this type of suppressor. Nonsuppressed conductivity, where the exit end of the column is connected directly to the conductivity cell, is effective for measuring many types of cations and weak acid anions. 

The most useful types of information are the peak retention times and the peak areas or peak heights. Retention times are used to confirm the identity of the various peaks, and peak area or peak height is a measure of concentration. Two general methods can be used to calculate the concentration of a given peak use of a calibration curve or use of a standard-to-unknown ratio. 

Generally, it is usually better to prepare a calibration curve by a plot of peak area (or height) against the concentrations of known standards. Such a plot will normally be a straight line, but a perfectly valid calibration plot may deviate from linearity, especially at lower concentrations. 

If the calibration curve is linear and passes through zero, a fast method is to compare an analytical peak of interest to a single standard of known concentration. This calculation can be performed by use of a simple ratio  

If the calibration curve is not linear or does not pass through zero, then it best to use the calibration curve to calculate unknown concentrations. 

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