Quantification of nitrite and nitrate in seawater by triethyloxonium tetrafluoroborate derivatization-headspace SPME GC-MS.

Triethyloxonium tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography-mass spectrometry (GC-MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO(2)). Isotopically enriched nitrite [(15)N] and nitrate [(15)N] are employed as internal standards and for quantification purposes. HS-GC-MS provided instrumental detection limits of 0.07 µM NO(2)(-) and 2 µM NO(3)(-). Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.

Application of mercury cold vapor atomic fluorescence spectrometry to the characterization of mercury-accessible -SH groups in native proteins.

A new analytical approach has been applied to the determination and characterization of mercury-accessible -SH groups in pure native protein samples (ovalbumin, hemoglobin, glyceraldehyde-3-phosphate dehydrogenase, aldolase, pyruvate kinase, hexokinase, lactate dehydrogenase, alcohol dehydrogenase, creatine phosphokinase, lysozyme, and cytochrome c). The method is based on the selective reduction of Hg(II) in the presence of Hg(II)-thiol complexes with alkaline sodium tetrahydroborate, to give Hg(0) in a continuous flow reaction system coupled with atomic fluorescence spectrometric (AFS) detection. The method is fast and specific and allows one to work with nanomole amounts of a single protein without any preliminary incubation and without any separation of Hg(II) from thiol-complexed mercury. The meaning of the results obtained in the determination of the accessible -SH groups in native proteins by using chemical probes is discussed.

Elimination of hydrofluoric acid interference in the determination of antimony by the hydride generation technique.

For the determination of antimony by hydride generation techniques a pretreatment procedure has been developed for the reduction of Sb(V) to Sb(III) in order to remove the effect of hydrofluoric acid which strongly interferes with the reduction of pentavalent antimony to the trivalent state. It is based on the combined action of l-cysteine and boric acid at 80 degrees C. The pretreatment is effective in both nitric and hydrochloric acid media. Quantitative recoveries are obtained in less than 60 min. Under these conditions antimony is reduced to the trivalent state in acid media containing both nitric and hydrochloric acid. The method has been applied to the determination of total antimony in certified reference materials of sediments after pressurized microwave digestion with HNO(3)-HCl-HF. Good agreement is obtained by using both analytical techniques: continuous flow hydride generation atomic fluorescence spectrometry and flow injection electrothermal atomic absorption spectrometry with in-situ trapping of stibine in a graphite atomizer.

On the determination of total dissolved tin in natural waters by direct hydride generation and non-dispersive atomic-fluorescence spectrometry.

The analytical response of inorganic tin and of eleven organotin compounds of the type R(n)SnX(4-)(n) (with n = 1, 2, 3 and R = methyl, ethyl, butyl and phenyl) was compared for direct hydride generation with non-dispersive atomic fluorescence detection. Most of these compounds showed behaviour resembling that of inorganic tin, with the exception of tributyltin and the phenyltin compounds. A simple pretreatment with 10(-3)M bromine and 0.033M nitric acid at 70 degrees for 60 niin prevents any risk of underestimation and the total dissolved tin in natural waters can be determined with recoveries better than 90%, with inorganic tin as calibration standard.

Behaviour of some dialkyl- and trialkyl-lead compounds in the hydride-generation procedure using a non-dispersive atomic-fluorescence detector.

The method of generating covalent hydrides by reduction with sodium tetrahydroborate was applied to aqueous solutions containing traces of R(3)Pb(+) and R(2)Pb(2+) compounds (R = methyl, ethyl). For each compound the effects of sample acidity, sodium tetrahydroborate and hydrogen peroxide concentrations, reaction-vessel and transfer-line materials, were measured and the experimental conditions defined for obtaining maximum sensitivity. Experimental evidence indicating the formation of organolead hydrides during the reduction step at room temperature was adduced. The reaction efficiency was found to be 90% for trimethyl-, triethyl- and diethyl-lead and 59% for dimethyl-lead. In the case of inorganic lead the efficiency was only 27%. Without an intermediate preconcentration step, the detection limits obtained for trimethyl-, triethyl-, dimethyl- and diethyl-lead (3 times the standard deviation) were 3-5 ng of lead per litre. A procedure for discriminating between R(3)Pb(+), R(2)Pb(2+) and Pb(2+) compounds is proposed.

Simultaneous determination of arsenic, selenium, tin and mercury by non-dispersive atomic fluorescence spectrometry.

A procedure is described for simultaneous determination of arsenic, selenium, tin and mercury in aqueous solution by non-dispersive atomic-fluorescence spectrometry. Radiofrequency-excited EDLs, 100% modulated in the kHz region, were used for atom excitation. Sodium tetrahydroborate was used as reductant and a hydrogen-argon miniflame as atomizer. In the optimized procedure, which uses 1 ml of sample, the limits of detection (three times the standard deviation of the blank) were 0.04, 0.08, 0.1 and 0.1 ng ml for arsenic, selenium, tin and mercury respectively. The linear dynamic range was greater than three decades for all analytes and the precision was better than 7% (typically 3%) for concentrations 1 ng ml . Results for mutual interference effects are reported. Copper, nickel, lead and cobalt interfered only with selenium (5 ng ml ), when present in at least 200-fold weight ratio to it. Using 5 ml of sample improved the limits of detection for selenium and arsenic (0.01 and 0.02 ng ml respectively), but at the expense of greater interference. Recovery from spiked natural water samples was better than 95% at the ng ml level, except for selenium in sea-water, when the recovery was only 85%. Determination of the four elements, including standard-addition and background measurements, requires about 10 min.

A simultaneous multielement non-dispersive atomic-fluorescence spectrometer using modulated sources and frequency discrimination of fluorescence signals.

Commercial radiofrequency-excited electrodeless discharge lamps can be run from a square-wave modulated power supply so as to give a low level of continuous emission when modulated in the frequency range 3-10 kHz. Use of a different modulation frequency and lock-in amplifier for each lamp allows multielement non-dispersive atomic-fluorescence spectrometry to be performed. Very low detection limits have been obtained for arsenic, selenium, tin and mercury. The use of low-cost electronic components in the system largely offsets the high cost of the individual excitation power supplies and tuned a.c. detectors.