An Introduction to Mass Spectrometry (cont.)

III. Tandem Mass Spectrometers

A typical tandem mass spectrometer is shown.

As mentioned before, a tandem mass spectrometer is very useful for gaining structural information about molecules. In the first stage, a collection of ions is created in the ion source of the mass spectrometer. The ions are allowed to pass through the first mass analyzer and collision cell, and their m/z values are measured in the second mass analyzer. Based on the data collected in the initial measurement, the first mass spectrometer is set to pass just one m/z value. This ion enters the collision cell and collides with argon. The kinetic energy of ions is converted to vibrational energy and the ions fragment. The m/z values of fragment ions are then determined in the second mass spectrometer.

Many types of tandem mass spectrometers have been developed and new innovations in tandem mass spectrometers allow greater automation and efficiency in data acquisition. Data can be generated in a data-dependent manner through interaction of the m/z data in each scan with a computer program to control the type of experiment performed. For example, a scan of the mass range can reveal the presence of several ions above a preset ion-abundance threshold. The computer can signal to the instrument to perform tandem mass spectrometry on each of the ions, thus improving the efficiency of data acquisition, particularly during separations when ions appear for only a brief period of time.

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