As many of you know, buffers are usually used in mobile phases to hold the pH constant. They are composed of a weak acid or weak base, combined with its conjugate base or acid in solution that is at least partially aqueous (at least 50%). This pair of compounds exists in equilibrium, so that when an acid or base is added, the equilibrium shifts to compensate, thus maintaining the same pH.
Buffering is commonly needed when analyzing ionizable analytes with reversed phase LC. For compounds like these, the pH of the mobile phase determines whether they exist in the ionized or non-ionized form. This does make a difference because the ionized species always elute from the column earlier for reversed phase LC. If the pH varies, the results would be inconsistent. Buffers are also sometimes necessary for applications because impurities or interfering compounds are ionizable. In this case, the buffer allows for consistent separation of the peaks of interest from the interferences.
The biggest question is usually which buffer to use. There are a couple of rules of thumb to follow:
For best results, use a buffer that has a pH at least 2 units away from the pKa for the analyte of interest. If the pH of the mobile phase is too close to the analyte’s pKa, you might observe split peaks or shoulders. If your analyte is acidic, then selecting a pH below its pKa results in the non-ionized form, while a pH above the pKa will result in the ionized species. Conversely, if your analyte is basic, then selecting a pH above its pKa results in the non-ionized form, while a pH below the pKa will result in the ionized species. If you have several analytes of interest, it usually works best to set your pH so that all of your analytes exist in the same form, either ionized or non-ionized. For our reversed phase columns, best results are achieved when acids are neutral and bases are charged.
Here are pka’s and pH ranges for some HPLC buffers that are commonly used with Restek columns:
|Operating pH range|
|11.3-13.3 (not advisable)|
|Tris (hydroxymethyl)- aminomethane||
|7.3-9.3 (use at 8.0 or below)|
With increasing use of LC/MSD and LC/MS/MS, volatility of buffers is critical. Introducing mineral salts into a mass spec system is usually detrimental and not advisable. Examples of acceptable volatile buffers are ammonium acetate, ammonium formate and ammonium citrate. Although modifiers like formic acid and acetic acid are not technically buffers, they are sometimes used to help control pH as well as ionization for LCMS.
The next question is usually what concentration to use. Generally, the lowest concentration that works reproducibly for your application is best. Higher concentrations are more prone to precipitating out of solution when the organic content increases, or perhaps even a drop in temperature occurs. For reversed phase, generally, concentrations of 2-50 mM are adequate. Concentrations are determined experimentally, keeping in mind that for reversed phase LC, increasing ionic strength of the buffer usually results in decreased retention for ionic species, while decreased strength results in increased retention.
Stay tuned for the next blog post, where we will explore preparation of these mixtures, potential pitfalls and share some helpful tips and good lab practices. Thank you for reading.