Lactic acid acetaldehyde salty separation

[HPLCSlave]

Sorry, I should have mentioned earlier...The samples are without salt...They are a mixture of acetaldehyde, lactic acid, acrylic acid at a concentration of about 0,1M each in distiled water. I thought I should separate them in pure water first.
As for the shape of the peaks: I've tried more than one column...and the result is always similar: the higher the pH the worse the peak shape.
I didn't inject the particular substances for each conditions but I have chromatograms of the pure substances at some of them.
The baseline is that of an old Knauer RI detector...I know that is't totally unesthetical.

[HW Mueller]

What else didn´t you tell us? For instance, did you take spectra of the first or second type chromatogram above? Why didn´t you use UV if you got "tell tale" specs? What else is in the mobile phase? How is the pH determined? What Uwe said is essential for the acids, but should not play much of a role for acetaldehyde unless it is doing some "strange" things which aldehydes do at some conditions.

This may be a personal preference, but to me it seems licentious not to inject individual standards, besides their mixtures.

[HPLCSlave]

I'd try to answer the questions asked by Mr. Müller.

1. Do you mean UV Spectra when you ask if I took the spectra for my chromatograms? I also have a UV detector, but no PDA. Nevertheless, it has a low signal to noise ratio so it helps to identify acrylic acid but lactic acid only if it elutes in a reasonable time at the given conditions.

2.I havent used UV because for the chromatogram at pH=5 I was first concerned with the form of the peaks not with their nature( if one can call those peaks...)

3. I have calculated the pH by the methods used for weak or storng acids, depending on what I have used. I have determined with a WTW pH 300(senTix 41 electrode) pH-meter that the pH of the H2SO4 solution used was about 4,7.
I haven't used other substances than the mentioned ones in the mobile phase(i.e. H2SO4 or H3PO4)

As for the use of acetate buffer instead of H2SO4, I was unable to see any progress towards separation.

Thank you very much for your time and energy.

[HW Mueller]

So you didn´t use an acetate buffer?

With H2SO4 at such low concentration you are inviting horrendous changes in pH. For instance the opposite of what I suggested above (when I thought you coinjected NaCl) might easily take place:
When you inject the stuff in H2O the pH will jump toward pH = 7 in a cetain region. The SiO- will immediatly rise there so that you get exclusion of part of your acid anions.
There is still the problem of the acetaldehyde which you could resolve via UV and UV spectra, + maybe MS, and NMR. I wouldn´t forgo injection of individual st. substances either.

[HPLCSlave]

I have also used acetate, but I couldn't notice a better separation.
I see...so you mean that these unretained acid anions would mess up my chromatograms...

Once again, thank you very much for your time and advices.

[HW Mueller]

Just to prevent a possible misunderstanding: I was talking about a regional (where the pH of the mobile phase is disturbed by the injection of a neutral sample, if it was acidic the picture would be different, but not less disturbing) exclusion of part of the ions. I could have just called it a pH incompatibiliy. With this sort of mobile phase you will get an incompatibility, practically no matter what you do. For instance if you adjust the pH of the sample to be that of the mobile phase you will certainly have a ionic strength incompatibiliy.
Incidentally, at what pH was your acetate buffer?

[HPLCSlave]

The acetate buffer had a pH of 4,74

[HW Mueller]

If you had a rasonable concentration of this buffer than I am at the end of the line, as I can´t tell how hard (varied) you tried to do these experiments. Also, there is the unresolved problem of what these broad peaks, above, really are.

[XL]

HPLCSlave,

We finally got time to do your application. First tried was an aqueous compatible RP column. We had similar observation to yours - acetaldehyde and lactic acid co-elute. Changing mobile phase pH or organic solvent composition didn't improve the situation. Then we tried Acclaim Mixed-Mode WAX-1 (a reversed-phase and weak anion-exchange mixed-mode column). And it worked. Here are the conditions:

Column: Acclaim Mixed-Mode WAX-1, 5-micron, 4.6x150-mm
Mobile phase: 39.1 g acetonitrile, 950 g water, 2.30 g NH4H2PO4 (20 mmol), 0.660 g (NH4)2HPO4 (5.0 mmol)
Flow rate: 1 mL/min
Detection: UV at 210 nm

Result:
1. Acetaldehyde tR= 3.7 min (k' ~ 0.8
2. Lactic acid tR=4.2 min (k' ~ 1)
3. Acetic acid (oxidation impurity of acetaldehyde) tR=4.4 min (k'~1.1)

The use of a 4.6x250-mm column will further improve the resolution.

I hope this will help.