Pulse introduction

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

MESI operation requires processing of the whole sample to be extracted and has to reach steady-state permeation, which usually takes a long time. Thus, a new technical modification of MESI, called pulse introduction (flow injection-type) membrane extraction (PIME), has been developed, in which the sample is introduced to the membrane as a pulse pushed by a stream of eluent (usually water).55 This means that attaining a steady state is no longer crucial. PIME therefore provides not only a faster response and higher sensitivity, but also allows extraction of individual samples via discrete injections in addition to continuous on-line monitoring by sequential injection of a series of samples. Guo et al.56 described a mathematical model for the PIME permeation process, which showed that (a) there was a trade-off between the sensitivity and the time lag (the time taken to complete the permeation process) and (b) a large sample volume and a low flow rate enhance the sensitivity but also increase the time lag. 

Juan, A.S., X. Guo, and S. Mitra. 2001. On-site and on-line analysis of chlorinated solvents in ground water using pulse introduction membrane extraction gas chromatography (PIME-GC). J. Sep. Sci. 24 599-605. 

Guo, X. and S. Mitra. 1999. Theoretical analysis of non-steady-state, pulse introduction membrane extraction with a sorbent trap interface for gas chromatographic detection. Anal. Chem. 71 4587 1593. 

Collection of analytes from a stream of gas on a sorbent bed and their release by thermal decomposition priori to the final determination stage, Membrane Extraction with Sorbent ) Interface - MESI), (Hollow Fiber Sampling Analysis -HFSA), (On-line Membrane Extraction Microtrap -OLME), (Membrane Purge and Trap - MPT), (Pulse Introduction Membrane Extraction - PIME), (Semi Permeable Membrane Devices - SPMD)... 

FiG. 90. Response curve to pulse introduction of ZnO into a tank furnace Joosen, 1973). 

Guo, X. M. and Mitra, S., Development of pulse introduction membrane extraction for analysis of volatile organic compounds in individual aqueous samples, and for continuous online monitoring, J. Chromatogr. A, 826(1), 39-47, 1998. 

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