Gas-diffusion separators system

In 1979, Baadenhuijsen and Seuren-Jacobs [2] were the first to report on a FI gas diffusion separation system with a semi-permeable dimethylsilicone rubber membrane, used for the determination of carbon dioxide in plasma. In the same year. Zagatto et al.[3] introduced an isothermal distillation FI system in which ammonia diffused from a flowing donor liquid film across an air-gap and absorbed by a flowing acceptor film on the opposite side of the gap. However, later developments on gas diffusion separations mainly followed the approach of Baadenhuijsen and Seuren-Jacobs, obviously due to its simpler design and higher versatility. The first theoretical study on an FI gas-diffusion separation system was attempted by van der Linden [4], who used a tank-in-series model for the mathematical evaluation of the separation process. 

The membranes used in FI gas-diffusion separation systems may be classified according to the mechanism of mass transfer of gaseous components across them. Microporous or heterogeneous membranes, usually made of PTFE or polypropylene, allow penetra-... 

Factors Influencing Mass Transfer in FI Gas-diffusion Separation Systems... 

Factors influencing the mass transfer and performance of FI gas-diffusion separation systems were studied in detail, and discussed by Karlberg and Pacey [22] using chlorine dioxide as the gaseous analyte. Some important observations mainly based on their results include ... 

Coupling of FI Gas-diffusion Separation Systems to Various Detectors... 

In 1981, Meyerhoff and Fraticelli [35] appear to be the first to report on the coupling of a gas-diffusion separation system to a flow-through potentiometric detector. Ammonia was isolated from the donor stream containing the sample by penetrating a microporous membrane and collection in an acceptor buffer stream, and then determined selectively using a tubular nonactin p>olymer membrane electrode. The precision (<7% r.s.d.) and sample throughput (30 h ) of this early application was rather low. 

Kunnecke and Schmid [40] introduced a gas-diffusion separation system combined with an immobilized alcohol oxidase column used for the determination of ethanol in beverages by amperometry. Ethanol vapour from the samples diffused through a silicone-modified polypropylene membrane and was collected in a potassium phosphate buffer acceptor stream before passing through the immobilized enzyme column where the ethanol was transformed into hydrogen peroxide. The peroxide was determined using an amperometric detector with excellent precision (cf. Sec. 8.4). 

FI gas-diffusion separation systems with membrane separators have been used successfully for the determination of ammonia in whole blood and plasma both with a potentiometric [13] and a spectrophotometric detector [14]. 

Zhu Z, Fang Z. 1987. Spectrophotometric determination of total cyanide in waste waters in a flow-injection system with gas-diffusion separation and pre-concentration. Anal Chim Acta 198 25-36. 

Gas-diffusion separations may be further divided into single and dual-phase separation systems. Single-phase systems are used for spectrophotometry, electrochemistry and chemiluminescence etc., with which liquid phase is used in both donor and acceptor channels. Dual-phase systems use a suitable gas as the acceptor stream, while the donor stream is liquid such systems are used with mass spectrometric (cf. Sec. 5.4.6) or electron capture detection [7J, but may also be used as a substitute for gas expansion separation in vapK>ur generation atomic spectrometric determinations (cf. Sec. 5.5.1 FI hydride generation manifolds with dual phase gas diffusion separators)... 

Fig. 2 Schematic figure of a tubular membrane gas-diffusion separator. DN, donor stream T, inner microporous membrane tubing W, waste O. outer tube of non-porous inateria) with inlet and outlet for acceptor stream. A D, to detection system.

Gas-diffusion separators have been integrated with detection cells to produce more compact systems both in optical sensing and atomic absorption spectrometry. These will be described in more detail in the sections on the coupling to individual detectors (cf. Sec. 5.4.2, 5.5.2)... 

The most frequently used detector in FI systems with gas-diffusion separation is the spectrophotometer. Quite often the gas-diffusion process offers sufficient selectivity to allow relatively non-specific chemical reactions in the acceptor stream to detect the analyte. Thus, carbon dioxide, sulfur dioxide, hydrogen sulfide, ammonia may all be determined using suitable acid-base indicators in appropriate buffer solutions used as the acceptor streams. The concentration of the buffer solutions may be adjusted to suit a certain concentration range for the analyte. In order to further enhance the selectivity and/or sensitivity more specific reagents may be introduced in the acceptor streams. In the previously mentioned example on the determination of cyanide [20] a modified pyrazolone-isonicotinic acid reaction was used for such purposes. Interferences due to Schlieren effects seem not to have been reported in gas diffusion spectrophotometric systems. This is understandable, since the matrix composition of acceptor streams is usually quite uniform, and the refractive index is little affected after absorbing the gaseous analytes. 

Tits Schematic diagram of an integrated gas-diffusion separation optosensing detection system. A. acceptor stream with reagent S, reacted sample with generated volatile species ... 

The feasibility of FI mass spectrometry was demonstrated by Canham and Pacey [41] using a gas-diffusion separator as an interface between the FI system and a quadrupole mass spectrometer. 

The same woiiters [42] used the dual phase gas diffusion FI system with a mass spectrometer to study the formation of different species of volatile hydrides of selenium and arsenic. The combined technique offered extremely high selectivity and sensitivity through the synergistic combination of the selectivity of FI gas diffusion separation and... 

Gas-liquid separation in FI hydride generation have also been achieved using dualphase gas-diffusion separators both in a sandwich and tubular configuration. Yamamoto et al.[47] were the first to report on such a system using a tubular PTFE microporous separator (cf. Sec. 5.2.2) in the hydride generation AAS determination of arsenic, achieving a characteristic concentration of 0.06 The system is shown schematically in Fig. 

Fig. lO Schematic figure of a FI hydride generation AAS system with segmented carrier stream and tubular membrane dual phase gas diffusion separator reponed in ref. 48. S. sample At, aigon flow T, microporous PTFE tubing G, dual-phase gas-diffusion separator, BH, borohydride reductant W, waste and AAS, quartz atomizer cell. 

Sullivan et al.(4S] used an FI gas-diffusion spectrophotometric system to determine sulfite in food products including fruits, vegetables, shrimps and wines. Sulphur dioxide is separated from the sample using a gas-diffusion membrane separator, and determined by decolorization of Malachite Green. The method shows good selectivity and sensitivity, and sulfite may be determined with a detection limit of 0.1 mg 1 in the food extracts, corresponding to 1-10 mg kg in the food product, and with a precision of 1-2% r.s.d.. 

Linares et al.[49] proposed a FI system with on-line gas-diffusion for the simultaneous determination of carbon dioxide and sulphur dioxide in wines. The two gaseous constituents were separated from the acidified sample in a sandwich-type membrane gas diffusion separator, and collected in an acceptor stream. Two detectors, one potentiome-tric, responsive to both analytes, and the other photometric, responsive only to sulphur dioxide (after reaction with a p-rosaniline-formaldehyde solution) were connected in series to determine the two constituents in the acceptor. The method was applied to the determination of carbon dioxide and sulphur dioxide in different types of fruity wines and the analytical results were in good agreement with those obtained by standard methods. 

J. Junsomboon, J. Jakmunee, Flow injection conductometric system with gas diffusion separation for the determination of Kjeldahl nitrogen in milk and chicken meat. Anal. Chim. Acta 627 (2008) 232—238. 

Flow systems with gas diffusion separation and spectrofluorimetric detection have been proposed for the determination of cyanide anion in aqueous samples (Figure 7.14). The cyanide transferred... 

E. Miralles, R. Compano, M. Granados, M.D. Prat, Photodissociation/gas-diffusion separation and fluorimetric detection for the analysis of total and labile cyanide in a flow system, Fresenius J. Anal. Chem. 365 (1999) 516-520. 

A FI system with gas diffusion separation and spectrofluorimetric detection has been recommended for the determination of acid dissociable cyanide in waters [36]. Cyanide diffuses through a microporous PTFE membrane from an acidic donor stream into a sodium hydroxide acceptor stream. The cyanide transferred reacts with o-phthalaldehyde and glycine to form a highly fluorescent isoindole derivative. Complete recovery of cyanide was found for Zn(CN) , Cu(CN), Cd(CN) , Hg(CN) -, Hg(CN)2, and Ag(CN)2 complexes and low recovery from Ni(CN)4. No recovery was obtained from the species that are considered as nonfree cyanide producing, viz., Fe(CN)g, Fe(CN)g, and Co(CN)g. The sampling frequency was 10 Hz h and the detection limit was 0.5 pg L . ... 

Automated systems for the determination of total and labile cyanide in water and waste-water samples have been developed using a photodissociation/gas diffusion/chromatography system [63]. The stable metal-cyanide complexes such as Fe(CN)/ are photodissociated in an acidic medium with an online Pyrex glass reaction coil irradiated by an intense Hg lamp. The released cyanide (HCN) is separated from most interferences in the sample matrix and is collected in a dilute NaOH solution by gas diffusion using a hydrophobic porous membrane separator. The cyanide ion is then separated from remaining interferences such as sulfide by ion-exchange chromatography and is detected by an amperometric detector. 

A FIA gas diffusion (FIGD) system has also been used for determination of nitrites in food products and aqueous solutions (Haghighi and Tavassoli, 2002). The sample solution is injected into a stream of water which then reacts with a stream of hydrochloric acid. The gaseous products (NO, NO2, HNO2, and NOCl) are separated from the liquid stream by the home-made gas-liquid separator and are swept by the carrier O2 gas into a home-made flow-through cell that has been positioned in the cell compartment of a UV-Vis spectrophotometer. The transient absorbance of the gaseous phase is measured... 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

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