Trace Analysis
The FIAlab 2500 instrument is compatible with the flow schemes
shown here for hydride generation and trace analyte
preconcentration.
Atomic Absorption (AA), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) are powerful techniques for analysis of traces of elements. They also share a significant drawback, a serious interference caused by matrix components such as high salt content, which makes them unsuitable for analysis of sea water, brine and similar samples, without sample pretreatment.
AA Flame
ICP Torch
This is why Flow Injection has been directly coupled as a “front end” to all these techniques in order to automate either hydride generation or solid phase extraction for matrix removal and analyte preconcentration.
Automated Spectroscopies
A single line FI system provides automated calibration and unattended serial analysis of multiple samples. Additional benefits include preservation of sample material, and no clogging of the nebulizer when analyzing samples with high salt content or serum, since carrier solution provides washout after each sample injection.
Hydride Generation
Hydride generation is routinely used for trace analysis of As, Bi, Ge, Hg, Pb, Se, Sn and Te. In addition, assay of volatile compounds of Ag, Co, Cu, Ni, and Zn has been reported.
Separation of the trace metals from complex matrices, analyte enrichment, fast reaction speed, and ease of automation were first demonstrated by Astrom (1982) in his pioneering work on the FI-based hydride AA assay of bismuth. By combining an acidified sample stream with a strong reducing agent (sodium borohydride), hydrogen and metal hydride is rapidly released, and the gaseous phase is separated with the aid of purging gas (air or argon) and swept into the detector.
Solid Phase Extraction
SPE, (or sorbent extraction), can be automated online, as discussed in this section or offline as discussed in Sorbent Extraction page of this website. The advantage of the on-line approach is that analyte is released from the column directly into a detector, which preserves sample identity and prevents contamination from outside sources. The drawback is that a long preconcentration/elution cycle (needed when large sample volumes are processed) yields a low sampling frequency, an undesirable situation, since AA and ICP use large volumes of expensive gases.
Single Line FI System
Simplicity is the chief advantage of this manual approach. It is best suited for study of column performance and for processing a small number of samples.
Column–in-Valve FI System
The advantage of this most popular configuration is the ability to process large sample volumes and to automate
assay of large numbers of samples.
In step one target analytes are captured on the column while the sample volume is selected by the flow rate and time interval during which the sample tube is connected to the sample source. Note that during this time the line to the AAS and the nebulizer is being rinsed.
In step two the valve is switched, eluant passes through the column in the opposite direction, while sample solution and subsequent wash are directed to waste.
Columns
For capture of tri- and tetravalent metal ions, use of cation exchangers or chelating materials is most common. Chelator 8-hydroxyquinoline or derivatives are most frequently used, since an 8-HQ column will capture a wide range of target cations, which are easily eluted by diluted acid. Low flow resistance and small internal volumes are the main characteristics of the columns, which typically are 1 cm long and have a 5mm internal diameter. Although they are usually “home made” it is possible that in the future they will be gradually replaced by short monolithic columns loaded by the user with desired ion exchange or chelating groups.
Resources
Hydride generation and on-column separations of trace metals are covered in detail in a monograph by Zhaolung Fang “Flow Injection Separations and Preconcentration” (VCH Publishers New York 1993). This comprehensive work includes not only material aspects of this technique but also its agricultural, environmental, clinical and pharmaceutical applications.
More recently published chapters “On line Sample Treatment, Extraction and Preconcentration” by Shoji Motomizu and Tadao Sakai, and “Atomic Spectroscopic Detection” by Elo H. Hansen and Manuel Miro in the monograph “Advances in Flow Injection Analysis and Related Techniques” by Spas. D. Kolev and Ian D. McKelvie ( Elsevier, New York 2008) are the primary sources, discussing the present state of this field, highlighting FI and also Sequential Injection and Bead Injection techniques.
