Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: MCB2012 on September 03, 2015, 11:38:30 PM
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I work for a water testing company and have been tasked with Herbicides analysis (EPA method 515.1), it is a fairly simple liquid-liquid extraction using diethyl-ether. Since I began, I have had difficulty recovering Dinoseb and Acifluorfen. It has gotten to the point where I might lose my job if I can't get 70% of expected recovery...
We extract a calibration standard at 50x the state reporting level, dilute that extract to a 5 point calibration. I also extract a laboratory fortified blank at 10x and it is this that I have the problems with. I have played with so many variables, NaSO4 acidity, bake time and temp, NaCl baking, acid washing all glassware scrupulously, comparing fresh/ older diazomethane. My last ditch effort was to lower the calibration standard to 20x, hoping the relative recovery of the 10x would improve enough, but no cigar :(
Any advice on the method or even resources on best practices of using syringes, seperatory funnels, quantity of drying agent, cleaning glassware eyc, anything at all. My trainer's suspicion has always been "water in the extract", but ultimately no one knows.
Thanks to anyone who has anything at all to say.
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Diazomethane is notoriously difficult to work with. I have less success using this than any other reagent. Maybe that's just me.
My understanding of the method is that trimethylsilyldiazomethane is an alternative. You could try that.
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I don't know this method nor the compounds, but it seems to me that your problem may well be with the diazomethane. How do you generate it?
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I too work for a water testing company, albeit a small one, and I am the technical manager. The buck stops with me and if people are failing to perform then we work back to when the test was working. Has there been a change in supplier, a new batch of reagent? Look up the purchasing records for the chemicals and ask to see what findings (if any) were picked up the last time the method was audited.
While this probably doesn't help your immediate situation, if you are inexperienced and surrounded by clowns then you are going to run into this again and again.
Any idiot can follow a procedure, however trainers and management have not trained or managed your training sufficiently so they are entirely at fault, not you.
Are other people in your department achieving the 70% recovery? Your trainer for example?
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I don't know this method nor the compounds, but it seems to me that your problem may well be with the diazomethane. How do you generate it?
Probably using a small scale generator from Aldrich, anything else is too hazardous.
These give very poor reproducibility in my experience.
Tetramethlyammonium hydroxide made neutral with careful addition of a buffer gives a quaternary complex with your acid/ phenol. This rearranges in the GC injection port to give very reliable methylation reactions in my experience.
Not the EPA method though.