time tandem mass spectrometry

time tandem mass spectrometryis mainly accomplished using ion traps or FTICR. From a “hardware” perspective, only one mass analyzer is needed, so the instrument costs are lower than tandem mass spec, which is the biggest advantage of time-of-flight MS. Because of this,time tandem mass spectrometrycan achieve multiple stages (MSn), which tandem MS cannot match.

Let's focus on how tandem MS experiments are done in ion traps. It involves the following steps:

1. Based on the ion trap stability diagram, appropriate u and q values are chosen to store ions of a particular m/z in the ion trap, while ions of larger or smaller m/z are ejected from the trap.

2. Collision-induced dissociation (CID) is performed. Since the ion trap is filled with helium during operation, increasing the kinetic energy of the precursor ions enables CID. For example, an auxiliary tickle voltage with a sinusoidal waveform can be applied to the endcap electrode at the secular frequency of ion motion, so ions absorb energy through this resonant excitation process. By carefully controlling the amplitude of the tickle voltage, ions are excited but not ejected. This increases their kinetic energy for CID with helium.

3. A mass scan is done to obtain a product ion spectrum by scanning V.The above cycle is computer-controlled. Multiple stages (MSn) can be done if needed.

Because ions undergo cyclotron motion in the trap, collision efficiency is high.Time tandem mass spectrometrycan only do product ion scans, not precursor ion or neutral loss scans, but product ion scans are the most important.

Multi-Stage Mass Spectrometry

Developments in ion trap (ITMS) and Fourier transform ion cyclotron resonance (FTICR) MS enabled MSn (n>2), as both have ion storage and selective ejection capabilities. MSn experiments are software-controlled. After ionization of the sample, pulses eject all ions except the selected m/z. A pulse valve introduces collision gas to accelerate the selected ions to sufficient energy for CID and product ion formation. This can be repeated for MSn.

In-source CID for multi-stage MS utilizes the ion transfer region’s electric fields in API-MS, causing collisional activation at ~133Pa to produce CID spectra.

Advantages of this method are high collisional activation efficiency and sensitivity, simplicity without needing the m/z of M+, wide selectable energy range to suit compounds of different stability, and simple low-cost instruments. Thus, it can solve most qualitative and quantitative problems in drug analysis.

Disadvantages are complex results if impurities and coeluting peaks are present, and increased analysis time due to the need for chromatographic separation.

Regardless of the tandem method, a collision cell is required to collisionally activate ions separated by the first stage MS before the second stage MS for more information.

Reprinted from China Standard Material Website, original URL：https://www.gbw114.com/news/n4018.html