a) Explain the following acronyms related to chromatography-mass spectrometry coupling:
- LC-MS: liquid chromatography-mass spectrometry
- GC-MS: gas chromatography-mass spectrometry
- RIC: reconstructed ion chromatogram
- TIC: total ion chromatogram (or current)
- SIM: selected ion monitoring
- MID: multiple ion detection
- HR-SIM: high-resolution SIM
b) How does an EI mass spectrum change when acquired during a GC-MS run as compared to the spectrum of the same compound from a direct probe?
Basically, the spectrum should not change. Therefore, comparison of spectra, e.g., with some library spectra, can be used for compound identification. However, GC peaks are transient and elution goes along with rapid changes of partial pressure in the ion source. Thus, care has to be taken to compensate for that. In unfortunate cases, spectra can be distorted due to autoprotonation or simply saturation close to the maximum of the chromatographic peak.
c) What complicates accurate mass measurements in GC-MS?
Scanning instruments offer better resolving power and higher mass accuracy at low scan rates.This presents some limitation for GC-HR-MS. In addition, changing concentrations of eluting compounds make it difficult to adjust the flow of an internal mass calibrant into the ion source at a level suitable for all components of a mixture.
d) Which type of mass analyzer is a promising alternative to magnetic sector instruments for GC-HR-MS work?
Orthogonal acceleration time-of-flight (oaTOF) analyzers can be used instead of scanning instruments. Modern oaTOFs offer high-resolution and sufficient mass accuracy. Depending on the design, they may alternatively be employed for fast GC-MS, GC x GC-MS or GC-HR-MS.