On-line analyte preconcentration with atomic spectrometric detection

Pre-concentration of analytes, or matrix removal to overcome interferences using mini- or micro-columns of exchange media prior to atomic spectrometric detection is becoming increasingly more common. This paper is a review of some of the more recent applications of chelating, ion exchange and other resins and gels that have been used to accomplish this.


Introduction
Many atomic spectrometric methods of analysis, including electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES), are prone to matrix interferences. Even inductively coupled plasma mass spectrometry (ICP-MS), which is generally regarded as being relatively free from interferences, suffers from polyatomic ion interferences, especially for the determination of first row transition metals. These interferences are summarized in table 1.
Ion exchange and chelation exchange are increasingly being used for pre-concentration and/or matrix removal prior to atomic spectrometric detection. They provide a relatively cheap, robust and repeatable method of pre- The first on-line pre-concentration with atomic spectrometric detection was reported by Olsen et al. [8] who used Chelex-100 to pre-concentrate metals from seawater. One problem with the use of Chelex-100 is the swelling and contracting of the resin associated with changes in its ionic form. Other resins with the same functional group, for example Metpac CC1, have been reported not to suffer this disadvantage [41]. The advantage of chelating resins is that they are fairly selective for transition metals, with alkali and alkaline earth metals being easily e!uted with an ammonium acetate buffer. The analyte is then eluted with dilute nitric acid.
Chelation may also be used to decrease interferences in ICP-mass spectrometry, for example metal oxides of titanium and molybdenum interfere with copper, zinc and cadmium determinations. By complexing the titanium and molybdenum with N-methylfurohydroxamic acid they may be retained on a Hamilton PRP-1 column, thereby removing the interference [42].     [57] Ni elutes with ethanol/HCl mobile phase. AAS detection [58] AAS detection [59] GFAAS detection [60] Chloro-complexes sorbed from sample [61] Fluorimetric detection [62] Zinc chloro complexes retained. Spectrophotmetric detection [63] GFAAS detection [64] IC-ICP [65] GFAAS detection IC-ICP [66] ISOX complex adsorbed [67] Spectrophotometric detection [68] CL detection [69] Microcolumn ICP or AAS detection [70] Removal of phosphate and Sulphate interferences [71 Cation exchange Cation exchangers have also been used for pre-concentration and matrix removal prior to atomic spectrometric detection. The most common cation exchange resins are AG 50W and Amberlite 120, both of which contain sulphonic acid functional groups. Recent applications of cation exchange resins are detailed in table 3. The eluent for cation exchange chromatography ranges from dilute acids to 8M nitric and 6M hydrochloric acids, depending upon the analyte. Further dilution may therefore be required before analysis by atomic spectrometry.

Anion exchange
Anion exchange has also been used to facilitate analyses with atomic spectrometric detection. Anion exchange resins, such as Dowex and Amberlite IRA 400, contain  [74] Na exchange with lanthanides [75] Ion pairing of Cr VI with tetrabutylammonium phosphate [76] Amphoteric resin [77] Column used to de-salt serum ICP-MS detection [78] Chelate formed with DDC, 8HQ PAR or PAN [79] quaternary ammonium functional groups. Anion exchange can be used to quantitatively retain complexes of analytes with negatively charged ligands, whilst allowing other interfering cations to elute. Others Table 5 shows some novel methods of ion exchange. Activated alumina has the advantage of being amphoteric, i.e. in its basic form it may be used to adsorb cations, such as lead [72] and chromium [73]; and in its acidic form it may be used to adsorb anions, such as sulphate [74]. Similarly, Retardion 11A8 is an amphoteric resin with both benzyltrimethylammonium and carboxylic acid exchange groups. Non-polar C-18 (octadecylsilane) columns have also been used for some applications, such as the adsorption of the diethyldithiocarbamate complexes of copper and iron [79].
In addition to the papers detailing specific applications of FI and LC, there have been a number of authoritative reviews, for example on FI-ICP [80][81][82][83] and FI-atomic spectrometry [84][85][86]. A very comprehensive review of the literature is the second edition of Ruzicka and Hansen's book [87].

Conclusions
There is a growing trend towards the use of on-line minior micro-columns of exchange resins to pre-concentrate the analyte, or remove interfering species from the matrix, prior to atomic spectrometric detection. This may, in part, be due to the simplicity of the apparatus, and to the ease of automation of such systems. The use of on-line columns also leads to a decrease in the chances of contamination associated with the sample handling ofoffline batch methods.