Recently, the trend in chromatograph analysis has been a shift from gas chromatography (GC) to liquid chromatography (LC). The reason is that for volatile substances, a GC can be used, but with refractory substances, the GC cannot be used unless some chemical treatment, such as derivatization, is conducted to make the refractory substance volatile. On the other hand, with LC, the derivatization is not needed, and the target sample area can be much wider than the one used with GC. However, in case of an LC, there are some aspects of the detector that create difficulties, so selection must be made to correspond to the sample and characteristics. But using an MS as a detector offers the potential for greater versatility and improved identification accuracy. In addition, the appearance of interfaces to effectively connect LC and MS, as well as performance improvements have led to a rapidly expanding demand for LC/MS.

LC/MS Interfaces

Since the 1970’s, interfaces for LC/MS have been a subject of intense study, and a variety of methods have been proposed and implemented. Among them are, the belt method, in which the mobile phase from LC is dropped onto a belt, heated and then measured using the electron ionization (EI) method; the spray method, in which the sample is sprayed as a gas aerosol and ionized using the EI method; and a thermo spray method. In late 1980’s, the atmospheric pressure ionization method (API) appeared, and has become widely used. It is the standard LC/MS interface today. While LC can use liquid at atmosphere pressure as the mobile phase, MS requires a high vacuum. The interface which successfully combines the conflicting natures of these is the atmospheric pressure ionization method ”API” method. There are two kinds of ionization method for API, one is Electrospray Ionization (ESI) method, and the other is the Atmospheric Pressure Chemical Ionization (APCI) method.

For example, when a compound with a molecular weight of 10,000 is observed as +10-valence ion, the m/z value (mass/charge ratio) of the obtained mass spectrum turns out to be 1,001. The spectrum of a multivalent ion can be obtained as a characteristic spectrum with a distribution, and allows a precise calculation of the molecular weight using a computer.

Multivalent ion spectra of ESI IgG2b antibody (approximately 150,000 MW) and molecular weight calculation results

2) APCI Method

APCI-LC/MS measurement example of green tea
(EC:epicatechin, EGC:epigallocatechin, ECg:epicathechingallate, EGCg:epigallocathechingallate)

As this illustrates, LC/MS can analyze the components of a wide range of samples from biological polymers to green tea, easily and with high sensitivity