Preconcentration derivatization

Morales-Munoz, S., J.L. Luque-Garcia, and M.D. Luque de Castro. 2004. Screening method for linear alkylbenzene sulfonates in sediments based on water Soxhlet extraction assisted by focused microwaves with on-line preconcentration/derivatization/detection. J. Chromatogr. A 1026 41 —46. 

Morales-Munoz, S., Luque-Garcia, J. L., and Luque de Castro, M. D. (2004). A continuous approach for the determination of Cr(VI) in sediment and soil based on the coupling of microwave-assisted water extraction, preconcentration, derivatization and photometric detection. Anal. Chim. Acta 515, 343-348. 

The solvent technique for concentrating surfactants is not discussed here. See Ref. [40] for details. Morales-Munoz et al. discuss a screening method for LAS in sediments. It is based on water Soxhlet extraction assisted by focused microwaves. The extractor is coupled with an online preconcentrator/derivatization/detection manifold through a flow injection interface [107]. 

Ion chromatography using a weak cation-exchange column with direct conductivity detection was applied in the determination of aqueous tributyltin ions, with short elution times LOD 0.01 ppm, without preconcentration or derivatization. No organotin remains adsorbed on the column94. 

Oximes, hydroxamic acids and related species are often used as reagents in inorganic analytical chemistry for precipitation, gravimetric and volumetric determinations as well as for preconcentration, extraction, derivatizations and subsequent determination of analyte using instrumental techniques. A brief review of analytical chemistry in general and of these species in particular follows. 

Analytes can be separated from complex matrices by sample preparation techniques that include liquid extraction, supercritical fluid extraction, and solid-phase extraction. Dilute ionic analytes can be preconcentrated by adsorption onto an ion-exchange resin. Nonionic analytes can be concentrated by solid-phase extraction. Derivatization transforms the analyte into a more easily detected or separated form. 

A simpler procedure consisting of protein precipitation using MeCN and TCA following formaldehyde derivatization was used for the determination of AMP in milk samples. The use of MeCN and TCA described in this method resulted in a clear supernatant and the highest recoveries (93% with CV of 6.1%). The derivatization reaction was done at 100°C for 30 min. The fluorescence of AMP derivatives was at least 20 times higher than that of AMO derivative thus, no preconcentration step was needed, resulting in the simplicity of this assay. A coextracted interfering unknown compound was observed at or near the retention time of the AMP derivative. This did not affect the accuracy (less than 10%) of the determination (74). A similar procedure was reported for beef, pork, chicken, and catfish tissue samples (75). 

Several countries have fixed the maximum allowed concentration for some of the BAs in wine e.g., Switzerland recommends 4 mg/L for His, Netherlands 5 mg/L, Germany 2 mg/L, and France 8 mg/L. High-performance LC techniques are largely used to determine BAs content in wines, with ion exchange (96,107) or RP columns withpre- (100,108-110) or post- (10) column derivative formation or without derivatization (98,4,111,112) with different detection means, mainly UV (50,110,113,114) or fluorescence (97,100,50,108,109). Nevertheless, if BAs are to be determined at low levels with no interference from other compounds, e.g., AA, previous cleanup and preconcentration steps are required. 

Also, wine is a very complex matrix and so cleanup or preconcentration steps must be undertaken prior to chromatography. Many analytical methods include these steps, mainly using liquid-liquid extraction (LLE), both before and after derivatization (49,100,103,5,122,123). Nowadays SPE is preferred, and studies have been carried out using this method before and after derivatization (37,48,51). 

Another method (37) that involves cleanup of wine by cationic exchange and a preconcentration step under controlled vacuum, before derivatization of the amines with PA and RP-HPLC elution with fluorimetric detection, (DL 25-50 fig/L), was used to identify and quantify BAs in several red wines of Tarragona region. 

Solid-phase microextraction capillary gas chromatography (SPME-GC) is also an interesting preconcentration method. After derivatization with tetraethylborate, tetrapropylborate, or tetraphenylborate, the ethylated compounds are extracted by SPME on a silica fiber coated with polydimeth-ylsiloxane (PDMS). SPME can be performed either in the aqueous phase or in the headspace. After SPME extraction, species are thermally desorbed, separated by GC, and analyzed.106... 

Derivatization may also be necessary as a means of preconcentrating very polar analytes, such as the herbicide glyphosate and its metabolite aminomethylphosphonic acid (AMPA), which are otherwise too polar to be retained on an SPE cartridge.126... 

Perez-Sirvent, C. and M.-J. Martlnez-Sanchez. 2007. Comparison of two derivatizing agents for the simultaneous determination of selenite and organoselenium species by gas chromatography and atomic emission detection after preconcentration using solid-phase microextraction. J. Chromatogr. A 1165 191-199. 

Sample preparation in CE is similar to sample preparation in HPLC (Section 3.8). It is usually performed off-line in CE, because commercial instrumentation does not allow for on-line sample preparation. The exception is sample preconcentration, which is readily performed on-line. Sample preparation includes any manipulation of the sample, such as weighing, filtration, dilution, or derivatization, prior to introduction into the CE system. 

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