Thiols porewaters

A DTNP method for HPLC has recently been reported (34). The sulfur compounds are stable after derivatization and the detection limits are quite low. The major disadvantages are that the derivatized thiols are detected by absorbance, and sediment porewaters and biological samples often contain large numbers of absorbing compounds which interfere with authentic thiols. In addition, sulfide is not detected. 

The methylene blue technique is a measure of hydrogen sulfide whereas DTNB should measure sulfide and any other organic or inorganic thiol which is present in porewater. Ideally, the bimane sulfide peak should correspond to the methylene blue concentration and the bimane total thiol concentration should correspond to the DTNB estimate. In saltmarsh cores, there was good general... 

Sprensen, J. (1988) Dimethylsulfide and methane thiol in sediment porewater of a Danish estuary. Biogeochemistry 6, 201-210. 

Thiols, as well as sulfide and sulfite, were determined in porewater samples by reversed phase high performance liquid chromatography (HPLC). The technique is based on precolumn derlvatization with an o phthalaldehyde/amine reagent (Figure 1) followed by HPLC and fluorometric detection. Derivatized porewater samples were Injected directly into the HPLC system the detection limit is 0.1 nM (for 100 ul injection). Details of the method are given in Mopper and Delmas (12). 

Figure 2. Upper Gradient separation of o-phthalaldehyde derivatives of 11 thiols and sodium sulfite according to Mopper and Delmas Peaks (1) sodium sulfite (100 pmol) (2) glutathione (7 pmol) (3) thloglycollate (200 pmol) (4) N-acetylcysteine (7 pmol) (5) 2-mercaptoethanesulfonate (Co-M) (10 pmol) (6) 3-mercaptoproplonate (10 pmol) (8) monothloglycerol (10 pmol) (9) 2 mercaptoethanol (10 pmol) (10) methanethiol (15 pmol) (11) ethanethlol (10 pmol) (12) 2-propanethlol (15 pmol) (13) 1 propanethlol (15 pmol). Middle Thiols In porewater in reducing sediment slurry from Biscayne Bay. Porewater water was filter-sterilized prior to derlvatlzatlon. Peak 7 sulfide (Note response factor is about 200 times lower than for thiols). Lower reagent blank in porewater matrix.

Table II Concentration Range of Thiols in Comparison to Other Low Molecular Weight Organics in Sediment Porewaters...

In order to provide additional evidence In support of the proposed Michael addition reaction, a sediment study was also performed. Acrylic acid was added directly to slurries of reducing sediments from Blscayne Bay and the formation of 3-mercaptopropionate In the porewater, relative to unsplked controls, was monltered by HPLC. The concentration of added acrylic acid was 0.1 mmol per liter slurry ( 0,2 mM In the porewater) and the slurries were Incubated under argon at 37 C for 2 hours prior to thiol analysis. Figure 6 clearly shows that addition of acrylic acid to reducing sediment gives rise to 3-mercaptopropionate, the main product expected from the Michael addition of HS to acrylic acid. 

Sediment slurries were Incubated at 30 C (approximate In situ temperature) with and without trlbutylphosphlne. Aliquots of porewater were periodically removed for thiol analysis over the following 2-4 days. During the course of this study, a total of 7 such Incubations were performed on freshly collected sediment. Results were similar In all cases and typically showed that trlbutylphosphlne Induced a dramatic and rapid release of bound (or oxidized) thiols (Table IV and Figure 8). Bound thiols were present at approximately 20 times greater concentrations than free thiols (I.e., 95% of all thiols released from sediment were Initially bound). If air Is not excluded during the Incubation, released thiols become reoxldlzed after several days (Figure 8) probably due to the oxidation of the protective phosphine. Addition of fresh trlbutylphosphlne regenerated the thiols. 

Addition of trlbutylphosphlne to extracted porewater (particle free) resulted In only a minor Increase In thiol concentrations. This result Indicates that the dramatic Increases obtained with slurries (Table IV and Figure 8) are probably due to release of thiols bound to particle surfaces, as opposed to release from disulfides dissolved In the Interstlal water. 

The formation of organic sulfur compounds in salt marsh porewaters is not as well understood(H) as the inorganic moieties although progress has been made in coastal marine sediments(12). Their presence as thiols and alkyl sulfides has been determined but generally in trace amounts(12-16). The importance of these trace organic sulfur compounds to atmospheric sulfur emissions is presently an area of intense research(, 15). Less appears to be known about organic sulfur compounds in salt marsh porewaters. 

Methods. In a recent study( ), polarography was used to determine Inorganic sulfur species in marine porewaters as in Tables I and II. In that study, no thiols or organic polysulfides were detected... 

Polarography of thiols. In an effort to duplicate the field results, several thiols which are commercially available were studied. The compounds were prepared in the pH 10 matrix which is used for the porewater analysis. The half-wave potentials are given for O.lmM solutions of each compound in pH 10 buffer. Our... 

In the Massachusetts marsh, only inorganic sulfide and thiosulfate occur as the major reduced forms of sulfur over the depth profile. Although this could be related to a lack of sulfide oxidation, sulfate depletion is not very high in this core. Therefore, neither process is the more dominant biogeochemical process. Previous studies(ip, ) have not found evidence for thiols in this marsh even when sulfide oxidation was prevalent, addition, cores were prepared from this marsh for controlled microcosm experiments. In cores in which oxidation was Induced, thiols were not detected in the porewaters. 

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