As MS operates in high vacuum, it is necessary to ensure that the LC parameters and mobile phases (liquid inlet) are compatible to MS. Usually when switching from LC to LCMS analysis, the current LC conditions can be used if the mobile phase is volatile. If involatile (e.g. phosphate buffer), modifications (e.g. chromatography mode, column, type and volatility of mobile phase) is required. This should be investigated stepby- step to allow better interfacing with LCMS. As mentioned in Chapter 1, there are a variety of separation modes for LC (Table 1) and the types of stationary phase and mobile phase can be modified based on sample characteristics, the desired level of separation, ionization level and MS compatibility.
A great starting point would be to use volatile mobile phases. A list of mobile phases suitable for API and LCMS are summarized in Table 3. In addition to the fundamental mobile phases such as water, methanol and acetonitrile, acetic acid is also commonly used to adjust the pH level. For buffer solutions, volatile salts such as ammonium acetate and ammonium formate are used. Also, volatile ion-pair reagents can be added to the mobile phase to facilitate the separation of polar compounds using reversed-phase LC. These reagents, which have a long hydrophobic tail and a polar ionic group, tend to attach to the stationary phase of the column with the ionic group sticking out. In the presence of ion-pair reagents, polar compounds interact with the charged ionic groups of the ion-pair reagent and gets retained and separated in the reversed-phase column. With focus on the ionization efficiency, protic solvents are essential for generating reaction ions for APCI, and polar solvents are essential for ESI since they are required to dissolve polar or ionic compounds.