The technique of automated discontinuous programmed flow analysis Is described, together with a working prototype Instrument employing syringe pumps driven by Interchangeable cams. This technique relies on the cyclic generation of flow profiles for both a suction and a discharge pump, such that at various times sample only, reagent only, or a predefined combination of the two Is conveyed via a mixing chamber to a detector. Different cam profiles allow a number of direct reading or reagent addition analysis methods, as well as both single and dual flow titrations. Results are presented for argent ometrlc titrations of chloride In water using as detectors anodized silver microelectrodes contained In two alternative mixer/sensor assemblies. In the range of 60-160 mg/L, precision for analysis times of 30 s or less is typically better than 1 %, with sample consumption of 0.9 mL. Accuracy for chloride determinations In city water samples Is limited by the characteristics of the electrodes to ca. 2-3 %. Cycle times for this Instrument are selectable from 6 to 90 s. Carryover from sample to sample Is negligible because of the incorporation of a sample flush step to commence each cycle. The operating characteristics of this analyzer were studied by using a conductivity detector, and pump displacement precision Is better than 0.1%.
Discontinuous Flow Analysis (DFA) was applied to the determination of chloride in water in the range 60-180 mgh, using a three step, four plateau cam profile, chloride ISE sensor and single internal standard. Duplicate measurements generated an eight row matrix of calculated sample concentrations, system slopes and Nernst parameters. Control charts were used to identify trends during the measurement, and to estimate a tolerance specific for each sample. The potential of DFA for the real time verification of the analytical measurement is discussed. Key wordsDiscontinuous flow analysis, incremental methods, chloride in water, quality assurance.The international trend towards quality assurance (QA) has led to a growing realization of the importance of QA in laboratory and on-Line analysis [1]. As the reliability of the measurement is of critical importance in QA, verification tests are necessary. For batch analysis, standard additions and recoveries are widely used, whereas flow based instrumental techniques require regular recalibration with standards, and RSD's on samples. In all periodic recalibrations, a compromise exists. Very frequent recalibration improves reliability, but at the expense of sample throughput Infrequent recalibration increases the risk of an out of tolerance condition between recalibrations, requiring the measurements to be "reworked". This is the very problem QA systems are designed to minimize. Ideally, every sample measurement should be subjected to a real time, reliable, operator independent verification procedure which provides sufficient information to resolve all relevant system parameters so that a tolerance specific f or each measurement can be calculated Discontinuous Flow Analysis (DFA) [2] is a flow technique characterised by highly reproducible programmed flow ratios of sample to reagent or diluent. Initial work [2,3] described gradient titrations for flow titrations. As well, the flow profile can be programmed to produce a stepped or incremental profile of fixed, defined flow rate ratios, giving plateau sensor responses analogous to the standard additions procedure of batch analysis. However, the advantages of flow analysis, namely high speed, small sample volumes, ease of automation and suitability for on-line analysis are preserved.This paper presents the results of the determination of chloride in water by DFA using a stepped flow profile and a chloride ISE sensor to demonstrate the concept of inbuilt QA for flow based chemical analysis. EXPERIMENTALApparatus The instrument used was a single cam, four pump planar configuration (Fig 1) with pumps P3 and P4 reversed to the constant flow suction pump P2 so that they operate in constant flow discharge modes during the measurement half-cycle. Pump P3 delivers ionic strength adjustor (ISA) and pump P4 delivers a reference bleed to isolate the reference electrode from the sample. Pump P1 adds a chloride standard to the aspirated sample in a series of defined ratios determined by the cam profile. At the end of the measurement...
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