LC-MS Platform

Authors

Creative Biogene

Summary

Creative Biogene provides an advanced LC-MS platform to assist our clients in many industries such as pharmaceuticals, biopharmaceuticals, forensic, industrial, food, and environmental sectors.

Introduction

The elements of an LC-MS system include the autosampler, the HPLC system, the ionization source (which interfaces the LC to the MS) and the mass spectrometer. Ideally, these elements are all under the control of a single computer system. It should be noted that to interface HPLC with MS, there are some restrictions on the flow rate and mobile phases that can be used. Typical reversed phase HPLC systems connected to MS would use some combination of water and either methanol or acetonitrile as the mobile phase. There are limitations on the mobile phase modifiers. For example, in most cases the modifiers have to be volatile. Mobile phase modifiers are chemicals added to the mobile phase that are used primarily to improve the chromatography of the analytes of interest. Typical mobile phase modifiers would include ammonium acetate, acetic acid and formic acid.

There are various types of ionization sources that can be used as the interface between the HPLC eluant and the mass spectrometer. The two most common sources are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI); both of these source types are now standard equipment on mass spectrometers that are used for LC–MS applications. For both ESI and APCI, the ionization occurs at atmospheric pressure, so these sources are often referred to as atmospheric pressure ionization (API) sources. For both ESI and APCI, some combination of high voltage and heat is used to provide the ionization that is needed to produce the ions that are assayed by the MS system. In ESI, the high voltage field (3–5 kV) produces nebulization of the column effluent resulting in charged droplets that are focused toward the mass analyzer. These droplets get smaller as they approach the entrance to the mass analyzer: as the droplets get smaller, individual ions emerge in a process referred to as 'ion-evaporation' – these ions are then separated by the MS system. In APCI, heat is used to vaporize the column eluant and then a corona discharge is used to ionize solvent molecules, which then produce the analyte ions via chemical ionization mechanisms. More recently, a third type of ionization source, termed atmospheric pressure photoionization (APPI), has become available. In APPI, heat is used to vaporize the column eluant (similar to APCI) but the ionization is produced by way of an ultraviolet (UV) lamp that produces 10 eV photons. Depending upon the solvent system used, the 10 eV photons will either ionize the mobile phase solvent or a dopant (a compound such as toluene that can be ionized by the 10 eV photons) added to the column effluent; these ions then produce the analyte ions through various ionization mechanisms including charge or proton transfer.

There are many types of mass spectrometers available for interfacing with HPLC. One of the more common systems used for HPLC–MS is the single quadrupole mass spectrometer. This system will provide a mass spectrum for each chromatographic peak that elutes from the LC column and is analyzed by the MS system. The second type of system is the time-of-flight (TOF) mass spectrometer, which has the added capability of providing a higher mass resolution spectrum from each component that is assayed. The third system is the triple quadrupole MS–MS system, which is most often used for bioanalytical assays but can also be used for metabolite identification assays. The fourth MS system is called an ion-trap mass spectrometer and has the unique capability of producing MSn data that are important when performing structural elucidation assays.

In addition to these four types of mass spectrometers, there are a growing number of additional types, including hybrid systems that have unique capabilities. Hybrid mass spectrometers combine two of the basic types of mass spectrometer to make a specialty system; an example of a hybrid mass spectrometer is the 'Q-TOF' MS–MS system, which combines a quadrupole mass spectrometer with a TOF mass spectrometer.

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