Sulfur-bridged cubane-type molybdenum-gallium clusters [Mo(3)GaS(4)(H(2)O)(12)](5+) (2) and [Mo(3)GaS(4)(H(2)O)(12)](6+) (3) have been synthesized from [Mo(3)S(4)(H(2)O)(9)](4+) (1) and gallium metal and have been isolated as 2(pts)(5).14H(2)O (2.pts) and 3(pts)(6).17H(2)O (3.pts) (Hpts = p-toluenesulfonic acid), whose structures have been characterized by X-ray crystallography. Crystal data for 3.pts: orthorhombic, space group Pca2(1), a = 11.188(1) Å, b = 30.936(4) Å, c = 23.987(2) Å, V = 8301(2) Å(3), Z = 4, D(calcd) = 1.628 g cm(-3), D(obsd) = 1.63 g cm(-3), R (R(w)) = 5.8% (6.3%) for 6102 reflections (I > 3.0sigma(I)). Crystal data for 2.pts: triclinic, space group P&onemacr;, a = 16.406(4) Å, b = 16.743(3) Å, c = 13.173(4) Å, alpha = 90.64 degrees, beta = 98.40(2) degrees, gamma = 89.32(2) degrees, V = 3579.2(14) Å(3), Z = 2, D(calcd) = 1.681 g cm(-3), D(obsd) = 1.68 g cm(-3), R (R(w)) = 6.7% (10.1%) for 7612 reflections (I > 3.0sigma(I)). The Mo-Ga distances (3.pts, 3.60[2] Å; 2.pts, 3.52[2] Å) are much longer than the corresponding Mo-Mo (3.pts, 2.679[7] Å; 2.pts, 2.713[3] Å) distances. At low H(+) concentrations, such as 0.5 M HCl, 2 is stable, and at H(+) concentration higher than 2 M HCl, 2 is oxidized by H(+) to give 3 with evolution of hydrogen gas. Chloride ion plays an important role in the oxidation of 2. Electronic spectra of 3.pts and 2.pts are very similar. Peak positions and epsilon values (lambda(max), nm (epsilon, M(-1) cm(-1))): 3.pts, 762 (447), 550 (204); 2.pts, 760 (404), 548 (205). The cyclic voltammogram of 3 shows three reduction processes (the cathodic peak potentials, E(pc), are -0.83, -1.20, and -1.74 V, respectively), where the first and second processes are quasi-reversible (the anodic peak potentials, E(pa), are -0.55 and -1.00 V, respectively). The reduction current of each process is close to 1:1:3.

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A method has been developed for the determination of trace selenium by generation of its gaseous hydride using either metallic zinc or sodium borohydride as a reductant and followed by the introduction of the hydride into a premixed argon (entrained air)-hydrogen flame where all atomic fluorescence lines of selenium are simultaneously detected by use of a nondispersive system. The comparison of the zinc and the sodium borohydride reduction methods is discussed in terms of detection limit, precision, and interference. The best attainable detection limits for selenium are 0.3 ng (15 pg/ml) and 0.4 ng (20 pg/ml) with the zinc and sodium borohydride methods, respectively. Analytical calibration curves obtained by measuring peak heights and integrated peak areas of the fluorescence signals are linear over the range of approximately 4 orders of magnitude from the detection limits. Most mineral acids examined in the range up to 2.0 M do not interfere; however, the nitric acid is an exception which gives a depressing interference. The interference was found to be much more severe in the zinc method than in the sodium borohydride technique. The presence of several elements including other hydride-forming elements in 1000-fold ratio to selenium causes a depressing interference, whereas an enhancing interference from tellurium is observed only in the zinc reduction method. The present method has been successfully applied to the determination of selenium in wastewaters and foods.

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Table 11. Precision and Accuracy Dataa compound hexachlorobutadiene hexachlorocyclopentadiene octachlorocyclopentene hexachlorobenzene hexachlorobu tadiene hexachlorocyclopentadiene octachlorocyclopentene hexachlorobenzene recov-std. ery, % dev., % 0.1 ppb 90 0.03 80 0.01 88 109 0.02 1.0 ppb _ --85 0.15 94 0.23 99 0.15 96 0.24 1 0 PPB hexachlorobutadiene hexachlorocyclopentadiene octachlorocyclopentene hexachlorobenzene hexachlorobu tadiene hexachlorocyclopentadiene octachlorocyclopentene hexachlorobenzene ___ 125 89 0.24 119 0.12 86 0.13 correlation coefficien tb 0.997 0.995 0.996 0.996Data represent three relicates for each compound at each concentration.Correlation coefficient represents how closely the experimental fits with the expected values and is equal to m o x / o y where m = slope of the line and o x and oy are the standard deviations of x and y array of the data points. a strong preferential solubility in the organic phase (such as the chlorinated organic compounds), extraction efficiencies of 90-100% can be obtained in one 30-s step. This makes it possible to minimize the volume of organic solvent used, greatly reducing the time required to perform the extraction. Also, by using hexane, hexane-benzene, or hexane-toluene instead of hexanemethylene chloride, the concentration step can be eliminated except for GC/MS analysis where only concentration by nitrogen purge would be required. For compounds with less favorable distribution coefficients, pH adjustments and multiple extractions might be necessary and these factors should be examined for individual cases.Because of the very short extraction time and the design of the Polytron Homogenizer, essentially no heat is transferred to the sample during the extraction process. This eliminates the potential for thermal degradation or evaporation loss of the compounds of interest, an effect occasionally observed Table 111. Time and Cost Comparisona Polytron homogenizer EPA pro-procedure tocol steps in procedure 7 11 pieces of glassware required 2 6 volume of organic solvent used, 30 150 mL min time to perform one extraction, 10 44 approximate cost per analysis $9 $31 a Costs will be based on individual laboratory costs.when ultrasonic extraction techniques are used. Table I1 is a summary of the statistical analysis of the data. The percent recovery and standard deviation a t each concentration level and the correlation coefficients for the linearity of each compound over the range studied are given. This demonstrates that the Polytron Homogenizer procedure produces results which meet or exceed the accuracy and precision requirements necessary for trace level environmental monitoring.We have used the technique with success for the determination of the four compounds discussed in both plant effluent and groundwater samples. Additionally the technique has also been used to extract chlorinated organics from soils efficiently with the emulsions formed easily broken with centrifugation. Table I11 is a summary of the time and cost analysis comparison between the EPA...