Post column detection, process chromatography

In the second case monitoring is performed on the column outlet with the main objective of identifying when the individual fractions elute. Post column detection is a very importemt part of analytical chromatographs where sophisticated diode array detectors may be utilized, and base line correction and peak area calculations are used for concentration determination of individual fractions. In contrast, in process chromatography, post column detection tends to be used purely qualitatively and may even be absent. 

QqQ in MRM mode and enables use of shorter chromatography columns and shorter run times (and thus increased throughput), while the additional selectivity provided by MRM detection can permit simplification of sample preparation procedures (Chapter 3). In practice, usually for each target analyte just one reaction channel (sometimes referred to as an MRM transition) is used to provide the quantitative data while one or two others are sometimes monitored simultaneously in order to provide confirmation of analyte identity via the relative responses (essentially a check on the selectivity of the analytical method. Section 9.4.3b). In contrast with the QqQ, the 3D ion trap has a poor duty cycle in MRM mode and the full scan product ion scan method is the method of choice if this analyzer is used for quantitation, since it is often possible to acquire 10 such scans across a chromatographic peak with adequate S/B values. Additional post-acquisition data processing is required to obtain quantitation data from such full scan MS/MS experiments. 

As noted in the previous chapters, when an EC detector is used in a flowing system, such as HPLC, it is simply one type of post-separation reaction detector. As with all post-colunm reaction detectors some knowledge of the chemistry involved in the detection process is essential in order to be able to use such detectors successfully. For maximum sensitivity with an EC detector, it is necessary to optimise conditions, such as the pH and composition of the reaction medium, the energy input, the time allowed for the reaction and the nature of the catalytic surface. However, when ED is used in conjunction with chromatography, the EC reaction conditions usually have to be a compromise with the chromatographic conditions necessary to achieve optimum resolution of the analytes, in particular the pH and the composition of the chromatographic eluent, as well as the need to maintain column stability. 

The most widely used method for ion chromatography detection is to treat or choose the eluent prior to detection to make the eluent ions less detectable and/or make the sample ions more detectable. The most common example of this is chemical suppression used in conductometric detection. The suppressor is really a chemical reactor that reacts with the post-column eluent stream and changes the ionic counterion for the eluent and for the sample peaks. In its most common form, sodium or potassium ions are removed from the stream and hydronium ions are added in an exchange process. This makes the background signal less... 

Many chemicals are not detected using a fluorescence detection scheme unless they are chemically derivatized with a fluorescent label. A reaction may be done before a separation is performed, but there are advantages if it is done after the separation. In chromatography this is known as post-column reaction. This is a chemical reaction performed "on-the-fly" in the sample stream as it moves towards a detector, and it must create a fluorescent product only when sample is present. One of the most common reagent used for post-column derivatization is o-phthaldialdehyde (OPA), which reacts with primary amines to create a fluorescent product. In electrophoresis this process might be called an in-capillary reaction, since due to the electric field the separation... 

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