HPLC for Food Analysis phần 7 pot

73

Derivatization

Sample Reagent

Metering

device

Sampling

unit

6-port valve

Reagent

To waste

From pump

To column

Figure 47

Automated precolumn derivatization

The robotic arm of the autosampler transports, in turn, a

sample vial and several reaction vials under the injection

needle. The needle is extended by a length of capillary at

the point at which the derivatization reaction takes place.

As discussed later in chapter 8, derivatization may be

required if the analytes lack chromophores and if detection

is not sensitive enough. In this process, a chromophore

group is added using a derivatization reagent. Derivatization

can occur either in front of or behind the analytical column

and is used to improve sensitivity and/or selectivity.

Precolumn derivatization is preferable because it requires

no additional reagent pump and because reagents can be

apportioned to each sample rather than pumped through

continuously. Automated precolumn derivatization yields

excellent precision. Moreover, it can handle volumes in the

microliter range, which is especially important when

sample volume is limited. The principles involved are

illustrated in figure 47.