Here are the Basic Principles of LC-MS Method You Must Know

LC-MS is a technique that combines the separation power of Liquid Chromatography (LC) with the sensitive detection of Mass Spectrometry (MS). Improvements in the LC or MS techniques can be incorporated into the LC-MS technique. As an example, modern systems often use HPLC (High-performance Liquid Chromatography) for their LC-MS techniques, though other systems like UHPLC (Ultra-HPLC) are available and used as well.

LC-MS analysis covers a broad range of application areas. Pharmaceuticals is a major area for the deployment and use of this technique. The method is best used for non-volatile organic compounds that cannot be analyzed using gas chromatography (GC).

Optimizing The LC-MS Method

Being cautious is the basic way of using this method. LC and MS are both separate techniques and can have different requirements. Putting these two together requires a fair amount of work and experience.

As an example, LC uses high pressure and creates a high gas load to separate a liquid phase. Meanwhile, MS relies on a vacuum and limited gas load.  Another issue is the fact that LC prefers ambient temperature while MS needs a high temperature to operate. There are other differences between these two techniques, thus requiring a careful interface for the LC-MS method.

Several conditions and parameters need consideration when setting up an LC MS assay. There is no single, straightforward method to achieve this. Each case requires special attention depending on the nature of the analyte. Focusing on the analyte and its individual properties play a major role in the optimization of the LC-MS method.

An ionization source is often employed to interface the LC and MS sections of the method. This is a way to handle the differences between both techniques. This is a part of the mass spectrometer itself, which works to get better results from the LC-MS technique.

Basics Of Operation For LC And MS Techniques

LC, or more accurately for the modern sense, HPLC technique often uses a granular solid material. A sample is injected with solvent and is adsorbed on the solid material (stationary phase). As the solvent passes, it separates the compounds involved, based on their solvent affinity.

HPLC uses a pump to inject the solvent. The type of pump used depends on the technique employed, though syringe pumps are the most popular. Other available options include a direct gas-pressure system, pneumatic intensifier, and reciprocating pumps.

In some cases, the sample can be introduced directly to the MS in the liquid phase. However, the vast majority of LC-MS assays ionization of the sample. The techniques available for this include:

• Atmospheric Pressure Ionization (API)
• Atmospheric Pressure Photoionization (APPI)
• Atmospheric Pressure Chemical Ionization (APCI)
• Electrospray Ionization (EI)

API is quite popular with pharmaceutical development. Whichever ionization technique is used, the result is usually gas-phase ions under atmospheric pressure conditions.  Most Mass Spectrometers work with cations, though there are a few that prefer anions.

The MS separates these ions according to the mass and charge ratio. This is followed by a detector, that identifies the ions and their quantity.

It may have some complexity setting up, but the LC-MS technique has excellent pay-off for the effort. LC-MS assays tend to be highly accurate and reproducible. The technique also offers excellent specificity and selectivity. Another advantage of the technique is that it allows the multiplexing of several analytes per run. This reduces cost as well as equipment time.

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Maria Brown Joined: April 1st, 2020