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 determination range of flame atomic absorption spectrometry are discussed. All the manifolds are based on the metering of very small sample volumes into a slowly moving carrier, the flow rate of which is gradually modified after completion of the injection. The systems use a conventional peristaltic pump equipped with small bore pump tubes and a channel to compensate for the difference between the flow delivered by the pump and the nebulizer uptake rate. The main difficulty which must be overcome is the pulsation caused by the rollers of the pump. The first manifold uses two or more metering steps, separated by an appropriate time interval and performed on the sample which is injected into the carrier propelled by the pump running at a low speed. The reproducibility is poor because the pump pulsations are periodic but highly asymmetric. The second system uses a single metering stage into the compensation channel performed while the pump is delivering the carrier at a flow rate close to the nebulizer uptake rate. Relative standard deviations ranging ± 0.6–2% are obtained but the maximum dilution degree attainable is about 2000. The third manifold is similar to the first, but a single metering stage is performed, the position of the rollers at the beginning of the metering step being controlled by means of a simple optical device. In this way, volumes as low as 0.02 μl can be metered into the system and dilution degrees of about 20,000 can be obtained. By choosing appropriate pump rates and metering times, a wide range of dilution degrees can be obtained. Sampling frequencies are in the 60–80 samples h−1 range. Calibration can be performed by using peak height or integrated absorbance. Repeatability of the integrated absorbance measurements is in the ±1.1–3.3% range.