Mobile phase selection in LC-MS
The use a volatile mobile phase is a good starting point. As well as essential mobile phases such as methanol, water, acetonitrile, and acetic acid are also generally used to regulate pH levels. For buffer solutions, ammonium acetate and ammonium formate are commonly used since they are volatile salts.
Besides, to make easy the separation of polar analytes by reversed-phase liquid chromatography, the volatile ion-pair reagents can be added to the mobile phase. These reagents have a polar ionic group and a long hydrophobic tail, which will attach to the stationary phase. In the presence of ion-pair reagents, the polar molecules interact with the charged ionic groups of the ion-pair reagent and are separated in the reversed-phase HPLC column.
With a focus on ionization efficiency, solvents are necessary for producing reaction ions for APCI, and polar solvents are required for ESI because they are essential to dissolve polar molecules.The most generally used solvents consist of LC-MS grade acetonitrile and water, including ion pairing agents such as formic acid, trifluoroacetic acid. Below are some examples of the mobile phases which are used in the analysis of LC-MS.
- Trifluoroacetic Acid (TFA)
- Formic Acid (FA)
- 0.1% TFA in Acetonitrile,
- 0.1%TFA in Water,
- 0.1% FA in Acetonitrile
- 0.1% FA in Water
Try to keep away from using phosphate buffers in liquid chromatography-mass spectrometry as there is the accumulation of in-volatile species, so they block sample orifice.
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