Flow injection (FI) analysis, the first generation of this technique, became in the 1990s supplemented by its second generation, sequential injection (SI), and most recently by the third generation (i. e., Lab-on-Valve). The dominant role played by FI in automatic, on-line, sample pretreatments in recent decades is amply demonstrated by the large number of publications to which it has given rise. Among these, its hyphenation with electrothermal atomic absorption spectrometry (ETAAS) is one of the most attractive sub-branches because of the high sensitivity of ETAAS instruments for metal species. After a decade of development, it is apparent from the literature that coupling FI sample pretreatment with ETAAS remains most dynamic in the new millenium; meanwhile, it is exciting to note that some novel trends associated with this subject have also emerged. The aim of this mini-review is thus to illustrate the state-of-the-art progress in implementing miniaturized FI/SI systems for on-line separation and preconcentration of trace metals with detection by ETAAS since 2000. We also discuss future perspectives in this field.

A sequential injection (SI) on-line matrix removal and trace metal preconcentration procedure by using a novel microcolumn packed with PTFE beads is described, and demonstrated for trace cadmium analysis with detection by electrothermal atomic absorption spectrometry (ETAAS). The analyte is initially complexed with diethyldithiophosphate (DDPA) and adsorbed onto the column, which is afterwards eluted with 50ml of ethanol and subsequently introduced via air segmentation into the graphite tube for quantification. The ETAAS determination is synchronized with sample pre-treatment in the SI system. No flow resistance is encountered at a flow rate of 9.0ml min21 through the column. The preconcentration efficiency was improved significantly by using the packed column as compared with a knotted reactor. With a 40 s sample loading time at 4.5ml min21 along with a sampling frequency of 16 h21, quantitative adsorption of cadmium (99% retention efficiency) and an enrichment factor of 59.4 were obtained, as compared with only 46.7% and 28.0 by using a knotted reactor of similar internal surface area as the packed column. The detection limits and precision (RSD, 0.1mg l21 Cd) are at the same levels, i.e., 1.3ng l21 (LOD), 1.3% (RSD) for the packed column, and 1.2ng l21 (LOD), 1.5% (RSD) for the knotted reactor. The practical applicability of the procedure is demonstrated by the determination of trace levels of cadmium in three certified reference materials.

Termed the third generation of flow-injection analysis, sequential injection (SI)-lab-on-valve (LOV) has specific advantages and allows novel, unique applications-not least as a versatile front end to a variety of detection techniques. This review presents and discusses progress to date of the SI-LOV approach as well as its applications in the automation and the micro-miniaturization of on-line sample pre-treatment. Special emphasis is placed on using SI-LOV in conjunction with bead injection (BI) for on-line separation and pre-concentration of ultra-trace levels of metals by exploiting the renewable micro-column approach. With detection by ETAAS and ICP-MS, it is shown, as illustrated by recent results in the authors' laboratory, that this methodology eliminates the problems encountered in conventional on-line column pre-concentration systems, improves the overall operational efficiency and yields the robustness necessary for routine assays. Also discussed is the future potential of the SI-LOV approach as a front end to various analytical protocols.

Within the last decade, the first generation of flow injection (FI) has been supplemented by sequential injection (SI), also termed the second generation, and, recently, by the third generation, i.e., SI-Lab-on-Valve (SILOV). As apparent from the literature, FI and/or SI have become dominant as substitutes for labor-intensive, manual, sample-pre-treatment and/or solution-handling procedures prior to analyte detection by inductively coupled plasma mass spectrometry (ICP-MS). The present review presents and discusses the progress of the state of the art in implementing miniaturized FI/SI systems for on-line matrix separation and pre-concentration of trace levels of metals with detection by ICP-MS. It highlights some of the frequently applied on-line, sample-pre-treatment schemes, including solid phase extraction (SPE), on-wall molecular sorption and precipitate/(co)-precipitate retention using a polytetrafluoroethylene (PTFE) knotted reactor (KR), solvent extraction-back extraction and hydride/vapor generation. It also addresses a novel, robust approach, whereby the protocol of SI-LOV-bead injection (BI) on-line separation and pre-concentration of ultra-trace levels of metals by a renewable microcolumn is interfaced to ICP-MS, as conducted in the present authors' group. It discusses the future outlook in this field.