Sample Introduction Device

In many instances, the limiting factor in the precision of an HPLC system lies in the reproducibility of the sample-introduction system. The sample-introduction device, also known as a 'sample injector', is used to introduce the sample into the HPLC system without depressurizing it. The most widely used method of sample injection is based upon a sample loop that can be placed in and out of the mobile phase flow path by merely switching a valve (Figure 2). When the valve is in the load position, the sample loop is filled at atmospheric pressure. Sample sizes often range from 5 to 500 |L. For best results, an excess of sample (i.e. two to five times the injection volume) is flushed through the loop to ensure that no trace of the previous sample remains. By turning the valve from the load to the inject position, the sample loop is connected to the high-pressure mobile-phase stream and the sample is then carried to the column.

From pump

To column

Vent

Sample in

Flow

Counterclockwise load

From pump

To column 3

Vent

Flow

Clockwise inject

Figure 2 Generic high-pressure injection valve shown in the load and inject positions.

The valve-based sample-introduction system is easily automated using simple robotic technology. The use of auto-injectors not only improves injection re-producibility, but they allow for the continuous processing of numerous (i.e. tens to thousands) of samples at a time. Auto-injectors have also been used for the implementation of pre-column derivatization protocols, especially for amino acid analyses. Pre-column derivatization is used to improve the chromato-graphic and/or detection properties of analytes.

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