Helium seawater concentration

Soo [96] determined picogram amounts of bismuth in seawater by flameless atomic absorption spectrometry with hydride generation. The bismuth is reduced in solution by sodium borohydride to bismuthine, stripped with helium gas, and collected in situ in a modified carbon rod atomiser. The collected bismuth is subsequently atomised by increasing the atomiser temperature and detected by an atomic absorption spectrophotometer. The absolute detection limit is 3pg of bismuth. The precision of the method is 2.2% for 150 pg and 6.7% for 25 pg of bismuth. Concentrations of bismuth found in the Pacific Ocean ranged from < 0.003-0.085 (dissolved) and 0.13-0.2 ng/1 (total). 

While the gases used in stripping are usually air, nitrogen, or helium, electrolytically evolved hydrogen has been used as a collector for hydrocarbons [49]. In this technique, the gas is not passed through a column of adsorbent, but instead collects in the headspace of the container. Since the volume of seawater and of hydrogen are known, the hydrocarbon concentration in the headspace can be used to calculate the partition coefficients and the concentration of hydrocarbon in the seawater. 

Zsolnay and Kiel [26] have used flow calorimetry to determine total hydrocarbons in seawater. In this method the seawater (1 litre) was extracted with trichlorotrifluoroethane (10 ml) and the extract was concentrated, first in a vacuum desiccator, then with a stream of nitrogen to 10 pi A 50 pi portion of this solution was injected into a stainless steel column (5 cm x 1.8 mm) packed with silica gel (0.063-0.2 mm) deactivated with 10% of water. Elution was effected, under pressure of helium, with trichlorotrifluoroethane at 5.2 ml per hour and the eluate passed through the calorimeter. In this the solution flowed over a reference thermistor and thence over a detector thermistor. The latter was embedded in porous glass beads on which the solutes were adsorbed with evolution of heat. The difference in temperature between the two thermistors was recorded. The area of the desorption peak was proportional to the amount of solute present. 

Top Z, Clarke WB (1981) Dissolved helium isotopes and tritium in lakes Further results for uranium prospecting in Central Labrador. Econ Geol 76 2018-2031 Top Z, Eismont WC, Claike WB (1987) Helium isotope effect and solubihty of helium and neon in distilled water and seawater. Deep-Sea Res 34 1139-1148 Torgersen T (1980) Controls on pore-fluid concentration of ""He and Rn and the calculation of " He/ Rn ages. J Geochem Explor 13 57-75... 

For calibration, microlitre amounts of a gravimetrically prepared standard solution of DMS in ethanediol are injected via the T-fitting into the helium line (see Fig. 24-1), as a sample of degassed (DMS-free) seawater is loaded into the purge vessel. This procedure provides a matrix-matched calibration and minimizes systematic errors by automatically correcting for degassing efficiency and potential DMS losses within the purge and trap unit. To avoid subsequent DMS release from particulates, the DMS-free seawater should be prepared from a filtered sample, ideally from deep water of low DMS concentration. 

Another example of the benefits of using more reactive gases, such as an ammonia and helium mixture over pure helium or hydrogen gas, is in the determination of vanadium in a high-concentration chloride matrix. The collision mode using helium works reasonably well on the reduction of the C1 0+ interference at 51 amu. However, when 1% NHj in helium is used, the interference is dramatically reduced by the process of charge/electron transfer. This allows the most abundant isotope, to be used for the quantitation of vanadium in matrices such as seawater or hydrochloric acid. Vanadium detection capability in a chloride matrix is improved by a factor of 50-100 times using the reaction chemistry of NH3 in helium compared to pure helium in the collision mode. T z... 

---