I am working on developing a Robust method to determine 500 - 8000 ppm acetic acid in aqueous formulations. GC columns I tried, HP FFAP col, 1 u film or Restek Rtx 1301. Neither gives reproducible results. It seems that the acid is adsorbed onto the col giving me lower area counts after few days. Samples are prepared in acetonitrile and dried using Na2SO4. What causes loss of response over time? Which column should I use?
By Jason Ellis on Monday, January 17, 2000 - 07:46 am:
What other components (organic or inorganic) are in your samples? It is possible that you may be contaminating the inlet and column with something which is causing the adsorption. This is a very common cause of response loss over time.
An FFAP-type column is strongly recommended for analysis of acetic acid -- non-FFAP phases typically give poor response and peak shape. I ran some samples in my lab at a nominal concentration of 20 ppm and saw very good signal-to-noise response for acetic acid using a DB-FFAP.
By Anonymous on Tuesday, January 18, 2000 - 04:50 am:
Thanks for your response. The samples have about 25% salts, i.e. mag. nitrate and chloride. However most of it ppt. out when samples are prepared in acetonitrile. Also it has about 15% five membered ring organic compounds which chromatographs well by GC.
Is small amount of water, say 1% in sample prep bad for the FFAP column? I am sure salts deposit onto the liner and then on the first few inches of the column which could create a problem. Cutting a few inches off the col. does help. With all these precautions and care method works in research lab but has problem when transferred to a QC lab where they may not be as careful.
By Jason Ellis on Tuesday, January 18, 2000 - 08:54 am:
1% is quite a significant amount of contaminant material to deposit onto the liner and column. The water in the sample will not harm the FFAP column, however the water will carry with it some of your salt material. The fact that you regain performance when you trim the column pretty much confirms that this is what is happening. While most of this material will deposit onto the liner, some will make it into the front of the column.
There may not be much that you can do other than performing regular "front-end" maintenance on your system (e.g. changing the liner and clipping the column). Some difficult methods require this type of preventative maintenance almost daily, depending on the nature of the samples and the sample load.
Alternately, you could try going to a drier solvent I suppose, or you could try running your sample through an ion exchange resin to selectively remove the salts. The resin procedure could get tricky -- I'd recommend contacting the folks at Biorad (or another manufacturer of the resins) for reference. Any way you could further dry your sample prior to injection would help as well.
By Anonymous on Tuesday, January 18, 2000 - 10:47 am:
Is acetic acid the only acidic species in your matrix? If it is, perhaps a simple titration would be the way to go. Your sample throughput would be much higher and would free up your GC for other work.
By Anonymous on Tuesday, January 18, 2000 - 04:55 pm:
It might be necessary to extract the acetic acid and the organic component with something other than ACN to get away from the salt problem.
For pesticides, we extract veges in ACN, then ADD several grams NaCl to cause the water and ACN to separate. The mixture is about 80 mL water and 200 ACN at that point. So it's a similar problem. But then, we evaporate the ACN on a hot plate, and re-constitute the soluble material in another solvent, such as methanol. The acetic acid would go away with that much heat so that wouldn't work, but it might give you some ideas.
Will the salts precipitate when the solution is chilled? What effects do different types of GC inserts have?
By Ulf Houlberg on Wednesday, January 19, 2000 - 05:58 am:
Acetic acid (and acetates)can easily be converted quantitatively into Methylacetate using Methanol and Sodiumhydrogensulfate and then analysed by headspace GC. The sample preparation take less than a minute pr sample and sub-ppm levels up to 1000ppm can be measured. We use the method rutinely to monitor acetate and formate in low levels. Revert if you want details.
By Anonymous on Friday, January 28, 2000 - 12:33 pm:
Thanks to all of you for your responses. The sample do contain other acids, however, we do have a titration procedure for routine use but if there is a process upset, the titration will not work since it is not specific enough.
I like the derivatization procedure. If Ulf Houlberg would kindly send the method to me at email@example.com, I would appreciate it. The samples do have ethanol (2000 ppm) and ethyl acetate (less than 500 ppm). The derivatization may convert some of the ethanol and HOAc to ethyl acetate? Then the HOAc results will not be accurate.
By jwarwick on Monday, January 31, 2000 - 12:59 pm:
I would look at using ion exchange HPLC with low UV detection as we are able to detect down to 10 ppm in solution this way with virtually no sample prep.
By Anonymous on Monday, February 7, 2000 - 10:14 am:
J warwick: Can you give me details on the ion exchange method? Sounds interesting.
By Laubscher on Monday, February 21, 2000 - 04:43 am:
Re: Acetic acid analysis with HPLC: We always prefer to analyse volatiles with GC, but HPLC has proved to be an excellent tool for short chain fatty acids dissolved in water. UV @ 210nm and eluent water, pH=2.5 (phosforic acid) which must be prepared daily to prevent bacterial growth. No extraction or sample pre-treatment is necessary. Just rinse your HPLC column (reverse phase C18) at the end of each day. We can detect down to 10 mg/l acetic acid. Using this approach we have "discovered" even things like formic acid in some samples which otherwise would have slipped through undetected using GC.
By Venkitesh on Sunday, July 23, 2000 - 04:27 am:
Can I separate Acetic and Acetic Anhydride by using capillary GC
By Anonymous on Tuesday, July 25, 2000 - 09:59 am:
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