acetoacetate product M.W. determination

[foxp]

I'm looking for a way to determine the M.W. of my product which contains multiple acetoacetate groups. It seems to be temperature sensitive over ~120C. On GCMS, it decomposes before it evaporates off the column. On LCMS, the detector (I'm told) needs to be heated between 400 and 500C. What can I do?

[james little]

We have analyzed a lot of acetoacetate groups. The problem is that they can exist in their keto and enol forms as they chromatograph often giving two peaks with a valley between them.

We have had some luck trapping the enol as its bistrimethylsilyl derivative and doing GC-MS

Some information on my website about silylation and silylation artifacts.

http://users.chartertn.net/slittle

[Peter Apps]

As long as your product is not contaminated with anything significant (and I presume that you have it reasonably pure) you could do solid probe MS, or approximate solid probe MS by having a very short column (or even a length of uncoated silica) in a GC with all the temperatures set below the danger level.

If you just want MW, it would be good to limit fragmentation by reducing the EI voltage if your MS allows this.

Peter

[james little]

On LCMS, should work OK by ESP. Even though temperature of ESP gas might be high (250-450 C on our Waters type instruments for flows of 100 - 400 ul/min flow), the time that the compound "sees" the high temperature if very low and most of the heat used to convert the sample into a vapor or aerosol. This is illustrate by the Lidenfrost effect..

http://www.varsity.utoronto.ca/archives ... coals.html

Still might have a problem with keto-enol forms and double peaks. Might have to play with pH and type of column.

In GC-MS, if you don't get a Molecular ion, might have to use ammonia chemical ionization. I have some CI information on my web page..

http://users.chartertn.net/slittle

However, if you silylate, most compounds give a M-CH3 ion even if no molecular ion in EI mode with trimethysilyl derivatives.

If relatively pure, could do solid probe EI and CI or even electrospray by infusion and skip the column separaton. Also proton NMR might be very useful since integration gives one a very good idea if compound is pure.

[james little]

One other thought, if you do solid probe, might want to try direct chemical ionization on a "wire" type probe instead of a standard solid probe. You can heat the former quicker. In CI, can heat quickly and desorb before the compound has time to decompose.

[foxp]

Thank you very much for your responses. I am trying to make glycerol derivative bearing three AcAc groups. This part of the project is already patented so I guess it's OK to say that.
The M.W. data I am getting from LCMS suggests that the M.W. is 1.5 to 2 times higher than it "should" be. It's not that I just don't see the molecular ion, but that I get peaks with high M.W. and they are odd numbers which tells me that they are fragments also.
From what I've gathered, acetoacetate groups break down at temp>120C to O=C=CHC=OCH3 and whatever alcohol can be formed from the other ester oxygen. What I suspect is that at 400 or 500C this happens. The alcohol then dehydrates and (in my molecule) forms 2HC=C=CH2. They are very unstable and react/rearrange and give me these larger molecules. The sample is not pure. It also contains t-butyl AcAc starting material that I am in the process of trying to remove. I don't see a molecular ion for that either.
Does this seem reasonable?
Would these ketenes react/recombine in these conditions? Or was my sample a mess before it made it to the LC?

Argh.

[james little]

don't think the desired molecule would be that unstable in electrospray conditions.

First dissolve the sample at about the 1 mg/ml level in acetonitirle. Then take 5 ul of this and dissolve in 1 ml of 90:10 acetonitrile:water containing 4.5 mmolar ammmonium acetate. Run the electrospray source at low source voltage to minimize fragmentation.

The ammonium acetate should give you a good M+ammonium ion.

Could also try injeting the sample on a 50 x 4.6 mm C18 column starting with 25% acetonitirle and 75% (97:3 water:acetonitrile containing 2.6 mmolar ammonium acetate) going to 100% acetonitrile in 15 minutes hold for 15 minutes at 100% with flow 1.5 ml/min. We usually add 0.1 ml/min via a "t" of 14 mmolar ammonium acetate in methanol to aid ionization at 100% organic.

Example of this "survey" method see page 20 of

http://littledomain.com/james/files/pit ... _point.pdf

Might get a double peak with a valley for the product, but I think it should work. We make a lot of acetoacetate derivatives at our company..