Flow-injection techniques

Flow injection techniques can be used to inject sample volumes as small as 10 jiL into a flowing stream of water with little degradation of detection limits. Frit nebulizers have efficiencies as high as 94% and can be operated with as litde as 2 jiL of sample solution. 

The flow injection technique is based on three main principles sample injection, reproducible timing, and controlled dispersion [128]. The dispersion can be described as limited, medium, or large in a colorimetric system based on a reaction between the sample and a suitable reagent, a medium dispersion is preferred. Thus in the flow injection determination of nitrate, the reductor column should not excessively increase the dispersion. In a copperised cadmium reductor, more than 90% of the total nitrate is reduced within 1 - 2 s with minimum risk of further reduction of nitrite [167]. Consequently, the reductor can be made very small, which results in a minimal increase of dispersion. 

In HPLC, the mobile phase is constantly passing through the column at a finite rate. The sample injections are made rapidly in the dynamic state in the form of a narrow band or plug (see Fig. 15.1), which provides a significant advantage over older stop-flow injection techniques in HPLC. After a sample has been injected as a narrow band, the separation can then be envisioned as a three-step process shown in the figure ... 

Solvent extraction can be automated in continuous-flow analysis. For both conventional AutoAnalyzer and flow-injection techniques, analytical methods have been devised incorporating a solvent extraction step. In these methods, a peristaltic pump dehvers the hquid streams, and these are mixed in a mixing coil, often filled with glass ballotini the phases are subsequently separated in a simple separator which allows the aqueous and organic phases to stratify. One or both of these phases can then be resampled into the analyser manifold for further reaction and/or measurement. The sample-to-extractant ratio can be varied within the limits normally applying to such operations, but the maximum concentration factor consistent with good operation is normally about 3 1. 

A number of applications of flow-injection techniques have been made to flame atomic absorption spectrometry [22]. Although manifolds can be connected directly to the nebuhzer, the response of the spectrometer is dependent on the flow rate of the sample into the nebuhzer [23], and some adjustment to the manifold may be required. The optimum flow rate for maximum response when the sample enters the nebuhzer as a discrete sample plug can be different from that found for analysis of a continuous sample stream. 

Various methods ofachieving preconcentration have been applied, including Hquid -hquid extraction, precipitation, immobihzation and electrodeposition. Most of these have been adapted to a flow-injection format for which retention on an immobihzed reagent appears attractive. Sohd, sihca-based preconcentration media are easily handled [30-37], whereas resin-based materials tend to swell and may break up. Resins can be modified [38] by adsorption of a chelating agent to prevent this. Sohds are easily incorporated into flow-injection manifolds as small columns [33, 34, 36, 39, 40] 8-quinolinol immobilized on porous glass has often been used [33, 34, 36]. The flow-injection technique provides reproducible and easy sample handhng, and the manifolds are easily interfaced with flame atomic absorption spectrometers. 

CONTENTS Preface, Joseph Sneddon. Analyte Excitation Mechanisms in the Inductively Coupled Plasma, Kuang-Pang Li and J.D. Winefordner. Laser-Induced Ionization Spectrometry, Robert B. Green and Michael D. Seltzer. Sample Introduction in Atomic Spectroscopy, Joseph Sneddon. Background Correction Techniques in Atomic Absorption Spectrometry, G. Delude. Flow Injection Techniques for Atomic Spectrometry, Julian F. Tyson. 

A rapid potentiometric flow inject technique for the simultaneous determination of oxychlorine species (e g., CIO2 ) was developed by Ohura et al. (1999). The analytical method is based on the detection of a large transient potential change of the redox electrode due to chlorine generated via the reaction of the oxychlorine species (e.g., CIO2 ). The detection limit for C102 is 3.4 ppb. 

Calibration and quantification procedures are easier in LA-ICP-MS compared to other solid-state mass spectrometric techniques because the laser ablation and the ICP ion source operate at normal pressure and the laser ablation of solid samples and ionization of analytes are separated in space and time. Therefore the advantage of solution calibration in ICP-MS can be applied in this solid-state analytical technique. The introduction of solution based calibration, which is only possible in LA-ICP-MS, was an innovative step in the development of this sensitive mass spectrometric technique. A number of different calibration approaches using aqueous standard solutions in the dual gas flow technique have been discussed by various authors.74 75 In the dual gas flow injection technique , the nebulized standard solution and the laser ablated sample material are mixed in the -piece and the two gas flows from the nebulizer (e.g. ultrasonic nebulizer) and laser ablation chamber are added. Using solution based calibration with the addition of a standard solution, Leach et alP determined minor elements in steel reference materials with a relative accuracy of a few %. In comparison to the so-called dual gas flow technique proposed in the literature, where the argon flow rates through the nebulizer and ablation cell add up to 11 min-1 (e.g. 0.451 min-1 and... 

Transient signals are typically obtained in atomic spectrometry when samples are introduced by flow injection techniques or when the spectrometer is used as an element-specific detector in hyphenated techniques. Inductively coupled plasma mass spectrometry has nowadays become the detection technique of choice for multielement-specific detection in speciation as it allows multielemental... 

The handling and disposal problems associated with the use of liquid solvent extractors have resulted in increased attention to the separation and preconcentration of organic compounds in water by collection in synthetic polymers followed by elution with an organic solvent (2). For example, selective collection and concentration of organic bases on methylacrylic ester resin from dilute water samples have been reported (3). Such collection techniques are especially well-suited to flow-injection measurement techniques. In this study, ionizable organic analytes such as salicylic acid and 8-hydroxyquinoline (oxine) were extracted into a polymer and then back extracted by an aqueous solution. Amperometric measurement using a flow-injection technique was employed to monitor the process. 

The detection step involves electrochemical oxidation at a nickel electrode. This electrode has been applied to measurements of glucose (4), ethanol (5), amines, and amino acids (6,7). The reaction mechanism involves a catalytic higher oxide of nickel. The electrolyte solution consists of 0.1 M sodium hydroxide containing 10-4 M nickel as suspended nickel hydroxide to ensure stability of the electrode process. The flow-injection technique offers the advantages of convenience and speed in solution handling and ready maintenance of the active electrode surface. 

Benkhedda, K., Epov, V. N., and Evans, R. D., Flow-injection technique for determination of uranium and thorium isotopes in urine by inductively coupled plasma mass spectrometry, Anal. Bioanal. Chem., 381, 1596-1603, 2005. 

Egorov, O., Grate, J. W., and Ruzicka, J., Automation of radiochemical analysis by flow injection techniques Am-Pu separation using TRU-resin sorbent extraction column, J. Radioanal. Nucl. Chem., 234, 231-235, 1998. 

Kuwabara, J., Tolmachyov, S., and Noguchi, H., The development of flow injection technique for rapid uranium determination in urine, J. Nucl. Sci. Technol., Suppl. 3, 556-559, 2002. 

Sample dispersion is another cause of the long lag time in flow injection techniques where an aqueous carrier fluid is used [63,64]. Dispersion is caused by axial mixing of the sample with the carrier stream. This increases the sample volume, resulting in longer residence time in the membrane. Dilution reduces the concentration gradient across the membrane, which is the driving force for diffusion. The overall effects are broadened sample band and slow permeation. 

The most common introduction of the samples in this source consists of a pneumatic nebulizer which is driven by the same flow of argon which carries the resulting droplets in the plasma. An ultrasonic nebulizer and heated desolvation tube are also used because they allow a better droplet size distribution which increases the load of sample into the plasma. Generally, the sample solutions are continuously introduced in the nebulizer at the rate of about 1 ml min-1 with the help of a peristaltic pump. However, this is not acceptable with small-sample solutions. Therefore an alternative method using the flow injection technique is employed to introduce a small sample of about 100 pi. The sample solution is injected into a reference blank flow so that the sample is transported in the nebulizer and a transitory signal is observed. 

The most developed areas for imprinted polymers are for sequestration and separation. Several reported imprinted polymer sensors have in actuality been pretreatments by separation to allow a selective determination using a general detection method. The adaptation of this to an FIA system would be relatively simple. For example, Kriz et al. [31] report a sensor for morphine (see Chapter 18). The method of morphine detection involved two steps. The first step was to immobilise the morphine by loading it on the imprinted polymer. In the detection step, the morphine was released from the column by elution of an electro-inactive competitor (codeine) and the released morphine was detected by an amperometric method. The polymer was tested after exposure to extremes of heat and chemicals and proved resilient. This method would probably be suitable for automation as a flow injection technique. 

Quantitative Analysis Using Flow Injection Technique [13]... 

Flow injection analysis is a continuous flow method in which highly precise sample volumes are introduced into a stream using segmented or unsegmented flow. The method must be accurate, precise and reproducible before it can be considered as a useful technique and the following test proves that this technique does meet all the requirements. Tyson [3], carried out several studies involving flow injection techniques and atomic spectroscopy with considerable success. 

Automation is especially advantageous if a large number of samples need to be analysed on a routine basis. There is little doubt that sample introduction using flow injection is superior to other solvent delivery techniques. Analysis using flow injection techniques with ICP-OES offers a number of advantages ... 

The methods involving Chrome Azurol S and surfactants were used for determining aluminium in water [27,89-93], steel [94,95], copper alloys [26], magnesium alloys [21], chromium alloys [96], and titanium [3]. Trace amounts of aluminium were determined in tap water by means of CAS and CP (pH 5.7 30% ethanol) using the flow-injection technique (FIA) [91]. 

The dithizone method has been applied in determining cadmium in food products [12], natural waters [19], organic materials [76], zinc sulphide [23], beryllium [17], zirconium alloys [8], uranium compounds [77], Cd-Se and Cd-Te thin films [78]. The flow-injection technique (FIA) has also been applied in determining Cd with dithizone [79,80]. 

Chlorophenazo III has been used for determination of calcium in waters [29,30,72-74], snow [74], soil [75], steel [76], molybdenum alloys [77], cobalt [78], aluminium alloys [79], and boric acid [29]. The flow injection technique has also been applied [73,80]. 

Calcium has been determined with o-cresolphthalein in proteins [59], waters [60-62], and soil extracts [62]. The flow-injection technique has been applied [60,62]. Thymolphthalein has been used for determining Ca in blood semm [64]. Calcium has been determined with metalphthalein in water, urea, and pharmaceutical samples [50]. 

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