USE OF SUBMICROLITER-VOLUME SAMPLES FOR EXTENDING THE DYNAMIC-RANGE OF FLOW-INJECTION FLAME ATOMIC-ABSORPTION SPECTROMETRY

Three computer-controlled flow-injection manifolds for extending the d etermination range of flame atomic absorption spectrometry are discuss ed. All the manifolds are based on the metering of very small sample v olumes into a slowly moving carrier, the flow rate of which is gradual ly modified after completion of the injection. The systems use a conve ntional peristaltic pump equipped with small bore pump tubes and a cha nnel to compensate for the difference between the flow delivered by th e pump and the nebulizer uptake rate. The main difficulty which must b e overcome is the pulsation caused by the rollers of the pump. The fir st manifold uses two or more metering steps, separated by an appropria te time interval and performed on the sample which is injected into th e carrier propelled by the pump running at a low speed. The reproducib ility is poor because the pump pulsations are periodic but highly asym metric. The second system uses a single metering stage into the compen sation channel performed while the pump is delivering the carrier at a flow rate close to the nebulizer uptake rate. Relative standard devia tions ranging +/-0.6-2% are obtained but the maximum dilution degree a ttainable is about 2000. The third manifold is similar to the first, b ut a single metering stage is performed, the position of the rollers a t the beginning of the metering step being controlled by means of a si mple optical device. In this way, volumes as low as 0.02 mu l can be m etered into the system and dilution degrees of about 20,000 can be obt ained. By choosing appropriate pump rates and metering times, a wide r ange of dilution degrees can be obtained. Sampling frequencies are in the 60-80 samples h(-1) range. Calibration can be performed by using p eak height or integrated absorbance. Repeatability of the integrated a bsorbance measurements is in the +/-1.1-3.3% range.