Bisegmented flow system for determination of low concentrations of gaseous constituents in gaseous samples
Introduction
The use of flow systems for determination of gaseous analytes in gaseous samples is very seldom described in the literature, despite the necessity for procedures that are rapid, robust, less reagent demanding, less prone to interference and with lower cost per analysis. Gaseous constituents can range, on a volume basis, from 50% or more to a few nl l−1 for those analytes of interest for pollution studies, such as nitrogen dioxide and ozone.
The flow injection (FI) approach [1]has been employed in the past aiming for the determination of SO2, Cl2, Br2 and CO22, 3, 4, 5. In some cases the flow system was employed only for sample transportation [2]while in others an in line reaction was promoted between the gaseous analyte and a reagent stream [3]. Low sensitivity and lack of versatility were, perhaps, the most unfavourable characteristics of such proposals. Recently an interesting flow cell has been developed for the spectrophotometric determination of low contents of NO2 in gaseous samples employing permeation through a PTFE membrane and a static reagent acceptor containing Saltzman reagent [6]. The sensitivity attained was very good and demonstrates that the chemical approach to gas determination still has many aspects to be exploited. However, the analyte transportation to the reagent solution is restricted by the membrane and, at suitable flow rates, only about 2% of the analyte reaches the reagent solution.
Even more recently the monosegmented approach to flow analysis [7]has been used for gas analysis. The monosegmented system has intrinsically the characteristic that gas and liquid segments are simultaneously generated in the system by a single movement of an injection device [8]. Aiming at the determination of gaseous analytes, one of the gaseous segments is constituted by the sample and various strategies can be carried out in order to provide an analytical response dependent on the analyte concentration. The first approach employs the volume contraction of the gas segment, occurring after selective absorption of a given analyte, as the analytical parameter. O2 and CO2 have been determined using this approach in samples containing 2–80% (v/v) and 2–50% (v/v) of the gaseous analytes, respectively [9]. Further development of the monosegmented system allowed the determination of CO2 in atmospheric samples containing a low concentration of this substance (100–800 μl l−1) using conductimetric detection of the ions formed after CO2 dissolution in the deionized water layer of the reactor tube[10]. These ions were collected in a liquid monosegment, also constituted of deionized water. Therefore, no reagent is necessary to perform CO2 determination.
On the other hand, many chemical procedures for spectrophotometric determination of gaseous analytes are based on the absorption of the analyte in a suitable reagent that can produce a detectable product or simply retain the analyte in a solid or liquid substrate. Liquid absorbing reagents have the advantage of being renewed from one determination to other and are attractive for use within flow systems. Unfortunately, the monosegmented system, as originally conceived, is not capable of realising the two basic operations necessary for gas determination, namely, the gas absorption/reaction and product transportation to the detector. Furthermore, the determination of low concentration samples (for instance, in the range of nl l−1) would require a pre-concentration of the analyte/product which can not be supplied by the simpler monosegmented systems described so far.
In view of the drawbacks indicated above, this paper proposes, a new flow system that produces a bisegmented flow pattern which can ensure in full the requirements for determination of low content gaseous samples. As the system produces a pattern containing two sequentially arranged liquid segments bordered by three gas bubbles it was named as `bisegmented flow system'. The use and performance of this system for determination of O2 in the atmosphere of food packages and of NO2 in synthetic atmospheric air are described below.
Section snippets
The bisegmented system
Fig. 1 depicts the arrangement of the injection device in the injection (A) and sampling (B) positions used to produce the bisegmented pattern employed in this work (C). The injection device is made of acrylic and the manifold tubing was made of 1.0 mm id PTFE.
The overall idea is that of producing a liquid segment (ls#1) which already contains a reagent or to which a reagent will be added on the path to the detector. This segment is employed to produce an absorbing or reactant layer in the glass
Determination of oxygen
The flow method for determination of O2 was developed for the determination of this analyte in the atmosphere of food packages where typical values are in the range 0.25–2.0% (v/v). Frequently, the limiting factor for such determinations is the size of the sample, as some food packages have a very small `free' inner space. The determination is based on the reaction between oxygen and potassium pyrogallate (KP) in a strongly alkaline medium. The product formed can vary from purple to yellow in
Conclusions
The bisegmented flow system described in this work showed very good performance for the determination of gaseous constituents in gaseous samples. The ability of simultaneous management of the necessary reagent solutions for analyte absorption and collection/reaction is a unique characteristic that increases the versatility of the system allowing it to be used for the determination of many kinds of analytes over a wide concentration range. The spectrophotometric reaction can supply the
Acknowledgement
The authors are grateful to C.I.G.L. Sarantópoulos for chromatographic determinations and to Dr. C.H. Collins for manuscript revision. M.C.H da Silva thanks FAPESP (Proc. 95/06756-0) for a fellowship.
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