The use of vanadium (III) has been proposed recently as a suitable alternative to cadmium for the reduction of NO 3 to NO 2 during spectrophotometric analysis. However, the methods proposed suffer from decreased sensitivity and additional steps for the measurements of nitrite and nitrate. We have developed an improved fast and sequential protocol that permits the determination of low concentrations of nitrite and nitrate in marine and freshwater samples using small volumes. NO 2 concentration is firstly determined using the common Griess reaction. The subsequent addition of a 2% VCl 3 solution in 6N HCl in the same sample and the reaction at 60ºC for 25 minutes results in an efficient reduction of the NO 3 to NO 2-(> 95%), which is also detected by the already added Griess reagents. The method has a detection limit <0.05 µM, a high precision (ranging from 0.2 to 11%) and accuracy (0.07 µM) for the determination of NO 3-+ NO 2 concentrations lower than 30 µM. Comparison of the proposed method with the established Cd column method using samples from a variety of environments (fresh water reservoir, sediment freeze lysable pore water, estuarine water samples and samples from an acid mine drainage impacted reservoir) showed good agreement between the two methods, with a difference between methods of 0.073 ± 0.099 µM. The analysis can be performed in large batches (~60 samples) using small sample volumes (≤1 mL) for the determination of both NO 3 and NO 2 in less than one hour.

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SummaryRight-side-out plasma membrane vesicles isolated from Zea mays roots were used to study membrane potential (Av)-dependent Ca2+ transport. Membrane potentials were imposed on the vesicles using either K+ concentration gradients and valinomycin or SCNconcentration gradients, and the size of the imposed A v was measured with ['4C]tetraphenylphosphonium. Uptake of 45Ca2+ into the vesicles was stimulated by inside-negative Ay. The rate of transport increased to a maximum at a A y of about -80 mV and then declined at more negative Av. When extravesicular Ca2+ concentration was varied, uptake was maximal in the range 100-200 pM Ca2+. Neither dihydropyridine nor phenylalkylamine Ca2+ channel blockers had any effect on Ca2+ uptake but 30 pM ruthenium red was completely inhibitory with half maximal inhibition at 10-15 pM ruthenium red. Calcium transport was also inhibited by inorganic cations. Zn2+, Gd3+ and Mg2+ inhibited by a maximum of 30% while La3+, Nd3+ and Mn2+ inhibited by 70%. The inhibitory effects of La3+ and Gd3+ were additive. Lanthanum-insensitive Ca2+ tive Ca2+ transport was totally inhibited by 80 pM Gd3+ and showed maximum activity at a A~J of -60 mV, with less uptake at both higher and lower Av. Lanthanum and Gd3+ also inhibited Ca2+ uptake into protoplasts isolated from Zea roots and their individual and combined effects were similar in extent to those observed with plasma membrane vesicles. It is concluded that