Methanol organic trace analysis

Betalactam antibiotics comprise several classes of compounds, among which the cephalosporins and the penicillins are most important. Both classes contain bulky side chains attached to the 7-aminocephalosporanic acid or 6-aminopenicillanic acid nuclei, respectively. The betalactams have limited stabihty, especially in organic solvents like methanol and acetonitrile, which may hamper accurate trace analysis. 

While water has been used as a solvent more than any other media, nonaqueous solvents [e.g., acetonitrile, propylene carbonate, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or methanol] have also frequently been used. Mixed solvents may also be considered for certain applications. Double-distilled water is adequate for most work in aqueous media. Triple-distilled water is often required when trace (stripping) analysis is concerned. Organic solvents often require drying or purification procedures. These and other solvent-related considerations have been reviewed by Mann (3). 

Several existing protocols require a solvent (acetone, methanol, isopropanol) rinse as part of equipment decontamination for VOC sampling and 1 10 percent hydrochloric or nitric acid rinse for metal analysis sampling (DOE, 1996 USACE, 1994). These practices, successful as they may be in removing trace level contaminants, create more problems than they are worth. Organic solvents are absorbed by the polymer materials used in sampling equipment construction and appear as interferences in the VOC analysis. Acid destroys the metal surfaces of soil sampling equipment and induces corrosion. The use of solvents and acids is a safety issue and it also creates additional waste streams for disposal. 

The crystalline dipentanoate (89-102 g, 0.20 mol) is dissolved in 1 L of methanol containing 6.6 g (0.12 mol) of sodium methoxide. After 4 hr at room temperature, analysis of the solution by thin layer chromatography (Note 3) shows only traces of the mono- and diester. The solution is neutralized to pH <7 (test paper) with -10 mL of coned hydrochloric acid. The solution is diluted with 1 L of 0.1 M phosphate buffer (pH 7), transferred to a 4-L separatory funnel and extracted with a mixture of 1 L of ethyl ether and 500 mL of toluene. The organic layer Is washed with a 1-L portion of water, dried over magnesium sulfate, concentrated to 300 mL and cooled to 4 C. 

Figure 5 Fast SFC analysis of the S-enantiomer of clevidipine after hydrolysis into its corresponding acid. Column and conditions Chiralpak AD 50 x 4.6 mm ID at 30°C, mobile phase carbon dioxide with 28% of 2-propanol, flow rate 4.0 ml min backpressure 150 bar, UV detection at 240 nm. Sample preparation and work-up 2 mg of S-clevidipine substance was dissolved in 0.5ml of methanol followed by 50 pi of 1 mol r sodium hydroxide. After 10 min, 1 ml of water and 50 pi of sulfuric acid, 1 mol l was added and 0.5ml of dichloromethane as extraction medium. After brief vortexing and centrifugation 5 pi of the lower organic phase was loaded manually and injected. Upper trace sample, lower trace sample with 0.1% of S-acid added. (Reproduced with permission from Gyllenhaal O (2001) Fast enantioselective separation of clevidipine and a dihydropyridine substituted acid by SFC on Chiral pak AD. Fresenius Journal of Analytical Chemistry 369-. 54-56 Springer.)...

A differential technique is helpful in the analysis of organic compounds for traces of iron in the presence of interfering background absorption. The iron in the sample solution (solvent, 1+5 hydrochloric acid in methanol) is present as the strongly absorbing ferric chloride complex and exhibits an absorption band at 360 m/n. The reference liquid is equivalent to the sample solution except that phosphoric acid is substituted for hydrochloric. The iron phosphate complex is transparent. The phosphoric acid is presumed not to affect the spectrum of the organic background and the difference spectrum represents only the iron absorption. 

Sodium borohydride (1.51 g, 40 mmol) is added slowly in small portions to a cooled stirred solution of tricarbonyl[(3 ,5 )-4-methyl-3,5-heptadien-2-one]iron (2.64 g, 10 mmol) in methanol (200 mL), and the mixture is allowed to warm to room temperature. After 2.5 h of stirring, water (400 mL) is added. The mixture is stirred for an additional 15 min and then extracted with diethyl ether. The organic layer is washed with water, dried (magnesium sulfate), and filtered. Evaporation of the solvent affords 2.46 g (92%) of complex as a dark yellow oil, which contains traces of the 4 -exo-isomer (TLC analysis). Purification by dissolving the oil first in a minimum amount of dichloromethane, then adding Skellysolve F (petroleum ether) dropwise, gives the V-endo-iron complex as bright yellow crystals mp 64 5 °C 2.02 g (76%). ... 

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