Conversion factor method using molarity

The acetylene reduction approach is based on the abihty ofnitrogenase to reduce substrates with triple N bonds. During the measurement, acetylene gas (HC=CH) is reduced to ethylene (H2C=CH2) in a theoretical molar ratio of 3 1 relative to N2 gas (N=N). To estimate N2 fixation with this approach, a water sample is sealed in a gas-tight container and acetylene is added (Capone, 1993 Montoya et al, 1996). At the end of the incubation, the concentration of acetylene and ethylene is measured using flame ionization gas chromatography. The rate of N2 fixation is then calculated using a conversion factor to convert acetylene reduction to N2 gas fixation. Release processes do not affect the acetylene reduction method such that the rate measured approximates a gross N2 fixation rate. 

Between this chapter and Chapter 10, we have now seen three different ways to convert between a measurable property and moles in equation stoichiometry problems. The different paths are summarized in Figure 13.10 in the sample study sheet on the next two pages. For pure liquids and solids, we can convert between mass and moles, using the molar mass as a conversion factor. For gases, we can convert between volume of gas and moles using the methods described above. For solutions, molarity provides a conversion factor that enables us to convert between moles of solute and volume of solution. Equation stoichiometry problems can contain any combination of two of these conversions, such as we see in Example 13.8. 

The raw GC data was converted into compositional data using the internal standard method with either N2 or Kr as the reference species. Once the species compositions were determined, values for the reactant conversion, product selecti-vities, and product yields were evaluated using the standard expressions. Errors in these parameters were calculated by closing the mass balance on the reaction. This was done by comparing the feed gas composition with the measured product gas compositions. Large errors in the mass balance (> 5%) were indicative of physical problems with the system, which were typically attributed to leaks. Smaller errors (< 5%) were caused by uncertainties in the GC molar response factors as well as fluctuations in flow rates of the MFCs on the Feed Gas Mixing Board. 

The instability and chemical conversion of some OPA derivatives imply that a denvatized compound may, in fact, result in one fluorescent and two radioactive peaks (Simson and Johnson, 1976, Fig. 1). The chemical rearrangement of the derivatives may, however, be a minor factor with respect to retention and the fluorescent and nonfluorescent derivatives may coelute. The use of more chemically stable amino acid derivatives, i.e. those formed by reaction with FMOC chloride, eliminates this problem. When the radioactivity of an amino acid is measured, it is often desirable and necessary to inject larger concentrations of amino acids than in a routine expenment. With the OPA method it is then critical to (a) make sure that OPA is present in the required molar excess (Lindroth and Mopper, 1979), (b) lower the pH of the reagent mixture to spare the top of the column, and (c) use the same or lower proportion of organic solvent in the sample as in the beginning of the gradient in order to obtain a concentration of the derivatives on the column top. 

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