Determination of acetate by LC?

[Mattias]

I guess there are better ways to determine acetate ions than with LC (CE?), but that is the technique that I have availiable.

I have done some test shots with an Agilent SAX column, using a 50 mM phosphate buffer pH 6.5 as mobile phase (UV 210 nm). I get something that looks quite awful. The peak seems to be split into at least three peaks, and also the baseline makes a dip just before the peak. The sample is dissolved in water.

I have used Google, but didn''t find anything useful. Has anyone got a good LC method for this purpose?

[mardexis]

quite a few vendors have columns for organic acids which work fairly well. I believe they tend to operate by ion exclusion. The separation should not be difficult but detection certainly is. RI or conductivity are you best bets.

[danko]

Hi Mattias,

From you description, I gather, your test is performed under isocratic conditions and that is the main reason for the “awfulâ€

[ljc]

By ion suppressing the acetic acid using 50 mM H3PO4 in water as the eluent, and using a polar column such as a MacMod PS-C18 or ES Inds EPIC Polar column, acetate will have a k' of around 1 to 1.5 (RT about 4 to 6 mins). 210 nm as suggested above is good (204 is lambda max for acetic acid in pH 2 H3PO4). Another way is to use a Hamilton PRPX300 exclusion column with 10 mM H2SO4 as eluent at 1 mL.min. Again UV detection is OK. Depending on the matrix you might want to acidify the sample before injection. 10 uL to 100 uL injection can be used depending on the sample matrix.

[XL]

I worked with organic acids for some time in the past, and have collected some information on this subject.
In general, the following methods can be considered for this purpose:
1. Anion-exchange IC method using suppressed conductivity detection and IonPac AS-11 HC http://www1.dionex.com/en-us/columns_ac ... s3609.html
2. Ion-Exclusion LC using UV detection or IC method using suppressed conductivity detection and IonPac ICE-AS1 or AS6 columns http://www1.dionex.com/en-us/columns_ac ... s3630.html
3. RPLC method using UV detection in ion-suppression mode http://www1.dionex.com/en-us/columns_ac ... s6953.html
4. Mixed-Mode Chromatography LC method using UV detection (see Figure 7 in http://www1.dionex.com/en-us/webdocs/48 ... 021407.pdf)

Usually the K' of acetic acid on a RP column is around 1 to 1.5 as ljc suggested above. Considering the application requirement and complexity of the sample , it is often not adequate. For this application, I believe Acclaim Mixed-Mode WAX-1 column is a good candidate (see the last link above). Chromatographic conditions include: Column dimension, 4.6x150 mm; mobile phase, 25 mM phosphate buffer, pH6; flow rate, 0.8 mL/min; UV at 210 nm. The K' is around 3. The interference from sample matrix can also be minimized on this column.

You are welcome to contact me regarding this application.