2.4. HS-dynamic in-syringe LPME with in situ derivatization procedure A 20-mL sample or standard solution (after adjusting pH to neu- tral) was taken in the sample container, and closed loosely with a lid to pass nitrogen for about 5 min through the septum to remove the headspace air (to avoid the possibility of ammonia from headspace air). The lid was closed tightly prior to stop the nitrogen passage. Then the sample container was placed over the preheated water bath (70 ◦ C) and 1 mL of 1 M NaOH was immediately added and a 4 min reaction was allowed, in order to evolve ammonia from sam- ple solution into the headspace under 380 rpm stirring of aqueous sample. After the reaction, 1-mL gastight syringe (Hamilton, USA) con- taining 100 L of OPA working reagent (as derivitazation cum extraction reagent) was inserted through septum for headspace sampling. The derivatization process consisted of the following steps: (1) withdrawn 100 L of OPA (derivatization reagent) into the syringe. (2) Withdrawn the plunger position to 0.3 mL mark. (3) Inserted the syringe needle through the septum on sample container and the needle kept above the liquid sample (in the headspace). (4) Allowed the headspace gaseous sample to 1 mL mark by withdrawing the plunger position, held for 3 s and then depressed the plunger back to the original mark of 0.3 mL using a programmable syringe pump (SK scientific Inc., MA). The recipro- cated movement of plunger was repeated 64 times (8 min at the rate of 8 extraction cycles per min). Derivatization that occurred inside the syringe was described as follows: when the syringe plunger accompanying the gaseous sample was withdrawn, a very thin OPA working reagent was generated on the inner syringe wall to absorb ammonia and achieve the in situ derivatization. When the syringe plunger was depressed, the derivatized product of ammonia was transferred into the bulk of OPA working reagent. This operation could considerably increase the surface area of the interface by using 100 L of OPA working reagent, which led to increase in the derivatization/extraction efficiency. Finally, the syringe needle was removed from the sample container and the ammonia derivative solution (acceptor phase) was manually injected into HPLC–FLD for analysis. The conceptual representation of ammonia derivatization has been shown in Fig. 1(b).