Abstract: The advantages of zone-trapping flow injection analysis over stopped-flow and intermittent-flow methods are discussed. In this method, the central portion of a processed sample is removed from the analytical path, set aside for a specified time under certain conditions, and later re-introduced into the same carrier stream. The technique was studied for a model system without chemical reaction, and then applied to the determination of NH4+ in natural waters via a modified Berthelot reaction. The main portion of the reacting sample zone is retained in a water bath at 38°C so that ~80% complete reaction is achieved without affecting the sampling rate. About 90 to 100 measurements can be made per hour; the coefficient of variation is 0.5%. Beer's law is obeyed up to 1 mg l-1, and the detection limit is 5 µg l-1. Interfering metal ions are masked by EDTA.

Abstract: A review is presented, with 43 references. A solution of 0.1% bromocresol green in 0.01 M Na2B4O7, with the latter as the carrier stream and absorbance measurement at 617 nm, is used to illustrate loop- and time-based, hydrodynamic, sequential and nested injection procedures.

Abstract: A flow system based on multicommutation to implement a sequential zone trapping approach to retain multiple sample zone into the anal. path during a previous preset time interval is proposed. This permits an increase of the sample residence time to improve sensitivity of spectrophotometric methods based on a relatively slow reaction. The flow network comprises a set of solenoid valves assembled in order to form three anal. pathways with equal length. With this configuration three sample zones could be sequentially loaded and trapped into the reaction coils to allow the chemical reaction to reach completeness. The flow system was controlled by means of a 486 microcomputer equipped with an electronic interface running software written in Quick BASIC 4.5. Feasibility of the proposed system was ascertained by the spectrophotometric determination of boron in a mineralized plant material using Azomethine-H as chromogenic reagent. The proposed system showed a linear response from 0.25 to 6.00 mg B L-1 and an anal. throughput of 35 determinations per h. Other favorable features such as a low relative standard deviation of 2.5% (n=9) and a consumption of only 0.5 mg Azomethine-H per determination were also obtained.

Abstract: Combinations of the injector commutator and solenoid valves provided a simple way of achieving different flow injection configurations for standard additions and dilutions. Results for Ca, Mn and Zn in three vegetable samples agreed well with reported values. A flexible flow injection system to perform multipurpose functions related to sample handling is proposed. The flow network consists of a sliding injector commutator, solenoid valves, and other minor components. A dedicated microcomputer controls these devices, facilitating the definition of different flow configurations. Merging zones, zone sampling, intermittent flow, sequential injections, zone trapping, and stopped-flow are possible. Online programmed dilutions and standard additions for simultaneous determinations on plant digests by inductively coupled plasma atomic emission spectrometry were chosen to demonstrate some features of the system. Sample dilutions were accomplished by confluence at different points along the analysis path through a proper programming of the solenoid valves. Standard additions were performed by merging the sample zone with an aliquot delivered from a trapped standard zone in a modified version of the zone sampling process. To minimize matrix effects from the nitric-perchloric acid digests, a sequential dilution procedure is proposed for the major elements and standard additions for the minor constituents. No other standardization procedure was necessary and results were calculated by using the least squares regression method. Results obtained with the proposed procedure compared well with the reported values for plant reference materials.