Knotted reactors

The advantages of the knotted reactor precipitate collector may be summarized as follows  

The foregoing list included almost all the required features of an ideal precipitate collector mentioned in Sec. 7.2.1, except at the moment it may not be applicable to all forms of precipitate, particularly those with a hydrophylic nature. However, much remains to explored in the material of the reactor, and in the types of precipitate which are applicable with the present design. Even during the preparation of this manuscript, dithizone and APDC were added to the list of complexing agents which produced precipitates suitable to be processed in coprecipitation procedures using the knotted reactor 

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Cd precipitation in a knotted reactor was studied with a 3-variable full design... 

The wall of a knotted reactor was covered with a chelating agent which retains Cd. A factorial design was employed to ascertain the main factors influencing the system and then the final experimental conditions were determined using a Box-Behnken design... 

S. Cerutti, S. L. C. Ferreira, J. A. Gasquez, R. A. Olsina and L. D. Martinez, Optimisation of the preconcentration system of cadmium with 1 -(2-thiazolylazo)-7 -cresol using a knotted reactor and flame atomic absorption spectrometric detection, J. Hazard. Mater., 112(3), 2004, 279-283. 

A. S. Souza, W. N dos Santos and S. L. C. Ferreira, Applieation of Box Behnken design in the optimisation of an on-line pre-eoneentration system using knotted reactor for cadmium determination by flame atomic absorption spectrometry, Spectrochim. Acta, Part B, 60(5), 2005, 737-742. 

E. Ivanova, F. Adams, Flow injection on-line sorption preconcentration of platinum in a knotted reactor coupled with electothermal atomic absorption spectrometry, Fre-senius J. Anal. Chem., 361 (1998), 445 D450. 

Ultrasonic nebulizers have also been employed in continuous flow systems as interfaces between sample preparation steps in the analytical process and detection by virtue of their suitability for operating in a continuous mode. Thus, preconcentration devices have commonly been coupled to atomic spectrometers in order to increase the sensitivity of some analytical methods. An enhancement factor of 100 (10 due to USNn and 10 due to preconcentration) was obtained in the determination of platinum in water using a column packed with polyurethane foam loaded with thiocyanate to form a platinum-thiocyanate complex [51]. An enhancement factor of 216 (12 with USNn and 18 with preconcentration) was obtained in the determination of low cadmium concentrations in wine by sorption of metallic complexes with pyridylazo reagents on the inner walls of a PTFE knotted reactor [52]. One special example is the sequential determination of As(lll) and As(V) in water by coupling a preconcentration system to an ICP-AES instrument equipped with a USN. For this purpose, two columns packed with two different resins selective for each arsenic species were connected via a 16-port valve in order to concentrate them for their subsequent sequential elution to the spectrometer [53]. 

Flow injection procedures are very useful for performing trace analyses in highly concentrated salt solutions. Fang and Welz [270] showed that the flow rate of the carrier solution can be significantly lower than the aspiration rate of the nebulizer. This allows even higher sensitivities than with normal sample delivery can be obtained. Despite the small volumes of sample solution, the precision and the detection limits are practically identical with the values obtained with continuous sample nebulization. The volume, the form of the loop (single loop, knotted reactor, etc.) and the type and length of the transfer line between the flow injection system and the nebulizer considerably influence the precision and detection limits that are attainable. 

In-line filtration without a filtering element is also feasible. To this end, a three-dimensional reactor [299], also called a knitted or knotted reactor (see 6.2.3.4), can be used, as emphasised in the landmark article reporting the flow injection determination of lead in blood and bovine liver by flame atomic absorption spectrometry [300]. The analyte was co-precipitated the complex formed was retained on the inner walls of a knitted reactor and then released by isobutyl methyl ketone and transported to the detector. Interference from iron(III) at high concentrations was circumvented, sensitivity was markedly improved and precise results were obtained. This innovation was recently exploited to remove organic selenium and determine the speciation of inorganic selenium in a flow-injection system with atomic fluorescence spectrometric detection [301]. 

Chen H, Xu S and Fang Z (1994) Determination of copper in water and rice samples with flow-injection on-line adsorption preconcentration using a knotted reactor. Anal Chim Acta 298 167-173. 

Ivanova E, Benkhedda K and Adams F (1998) Determination of copper, manganese and nickel in biological samples and sea-water by flow injection on-line sorption preconcentration in a knotted reactor coupled with electrothermal atomic absorption spectrometry. J Anal Atom Spectrom 13 527—531. 

The so-called single bead pearl string reactor first described by Reijn [10], and other packed reactors using inert packing materials are also shown to be effective as mixing reactors [4]. However they are more difficult to produce and less flexible than the knitted or knotted reactors, and, perhaps for this reason, are not used as broadly. 

During the extraction sequence, the loop filling time should also be sufficient to produce an extract volume at least 30% larger than the loop volume (usually l(X)-200 /il) in order to prevent carryover between samples in the loop. The implementation of a knotted reactor loop may be helpful in reducing dispersion and hence the minimum washing volume required for avoiding carryover. 

Recently an efficient on-line coprecipitation-dissolution system has been developed by Fang et al.[21] using a knotted reactor collector without filters, which has triggered a series of related publications, using flame and graphite furnace AAS as detectors [22,23]. 

The Author s own experience of using a 25 mm membrane filter for on-line coprecipitation was discouraging, the sensitivity in the determination of lead being only about 20% of that obtained using a knotted reactor (cf. Sec. 7.2.5). 

Fig.7.6 FI manifold and main operation sequences for on-line coprecipitation ETAAS determination of Cd with the Fe(U)-HMDTC/IBMK system, a, delivery of stored concentrate (from previous sample) b, precipitation of sample with HMA-HMDTC c, washing of precipitate with aqueous solution of HMA-HMDTC and d, precipitate dissolution by IBMK. Pi, P2, peristaltic pumps KR, knotted reactor, B, solvent displacement bottle V, 4 5 channel injector valve C, PTFE concentrate collector tube (60 /xl) GF, graphite furnace and W, waste [23].

Welz et al.[l] determined cadmium in whole blood digests by flame AAS following on-line coprecipitation using a modified procedure of that used for the determination of lead. Cadmium is coprecipitated with the carrier Fe(II)-HMDTC which is collected in a knotted reactor and dissolved by IBMK. A 52-fold signal enhancement was obtained with a sampling frequency of 72 h (for details cf. Sec. 9.5.3). 

Lead, cadmium and other trace metals are coprecipitated from blood and urine matrices and collected on-line on the tube walls of a knotted reactor. Iron(Il), formed by reduction of iron(IIl) using ascorbic acid, is used as a coprecipitation carrier to form a black precipitate with HMDTC. Therefore, 200 mg 1 iron are added to all sample digests to provide a minimum iron concentration. The collected precipitate is subsequently dissolved by IBMK and transported to the flame atomizer of an AAS system. The method features high sensitivity and sample throughput with high tolerance to the principle matrix interferents, including iron and copper. 

Knotted reactor precipiiaie collector made by tying 5-mm-diameier interlaced knots in 150 cm long, 0.5 mm i.d., 1.8 mm o.d. Micro-Line (Thermoplastics Scientifics Inc.) or PTFE tubing (cf. Fig. 2.9). 

Fig.9.4 FI manifold for the FAAS determination of lead and cadmium in blood and urine by on-line coprecipitation, a. sample loading (precipitate collection) sequence b. precipitate dissolution sequence. P, P2, peristaltic pumps V, 4 5 channel injector valve S. sample KR. knotted reactor precipitate collector and W. waste 1.2).

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