An analytical methodology based on an on-line sample enrichment of water samples by means of an imprinted polymer, and the separation of benzimidazole compounds within a C(18) column by ion-pair reversed-phase liquid chromatography, has been developed. The molecularly imprinted polymer has been synthesized by precipitation polymerization using thiabendazole as template molecule, methacrylic acid as functional monomer, and divinylbenzene as cross-linker. Initial experiments carried out by solid-phase extraction on cartridges demonstrated a clear imprint effect for thiabendazole, as well as the ability of the imprinted polymer to selectively rebind several benzimidazole compounds. The developed methodology has been applied to the quantification of thiabendazole, carbendazim, and benomyl in river, tap, and well water samples within a single analytical run at concentration levels below the legislated maximum concentration levels. In this sense, detection limits of 2.3-5.7 ng.L(-1) have been obtained for the analysis of benzimidazole fungicides in different water matrices. Recoveries obtained for the determination of benzimidazole fungicides in spiked samples ranged from 87% to 95%, with RSD below 5% in all cases.

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Geochemistry of the extensive peralkaline pyroclastic flow deposit of NW Mexico, based on conventional and handheld X-ray fluorescence. Implications in a regional context Geoquímica del extenso depósito de flujo piroclástico hiperalcalino del NW de México, basada en fluorescencia de rayos X convencional y portátil. Implicaciones en un contexto regional Abstract Chemical analyses conducted on the surface of rock slabs under a combination of two X-ray spectrometry methods, wavelength dispersive XRF and energy dispersive XRF, are used to establish a geochemical correlation between the studied samples. This proves to be an excellent method for the characterization of volcanic glasses, particularly when particles of exotic origin are present, because the effect of these is not easily eliminated by conventional whole rock analysis. Analyses of glassy rhyolites (ignimbrites and lava flows) in northwestern Mexico establish a geochemical signature for the samples, providing criteria that allow us to: a) correlate them with a peralkaline volcanic event, previously reported, that occurred during Middle Miocene time; b) distinguish them from other metaluminous varieties in the region and, c) propose a correlation between all the peralkaline vitrophyres that crop out within the studied area, of at least 50.000 km 2 , validating the hypothesis that they are related to the same volcanic event. Finally, based on the results of this study and previous geological investigations, it is proposed that a distance of more than 100 km between the geographical location of the thickest peralkaline deposits in Sonora, is probably related to a displacement along transtensional dextral faults during the Late Miocene.. Keywords: Geochemical correlation, ignimbrites, peralkaline rhyolites, NW México ResumenEl análisis químico realizado en secciones de roca bajo la combinación de dos métodos de espectrometría de fluorescencia de rayos x, por dispersión de longitudes de onda y por dispersión de energías, permitió establecer una correlación geoquímica entre las muestras estudiadas, resultando ser un excelente método para la caracterización de vidrios volcánicos, en particular cuando contie-

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