19 Flashcards
(10 cards)
What determines whether a GC detector like an FID or an ECD is chosen?
The detector choice depends on analyte properties: FID is universal for organic compounds (responds to carbon content), while ECD is highly sensitive to electronegative species (e.g., halogenated compounds).
Why are plasma-based detection methods (e.g., ICP-MS) less common in small-molecule GC/LC?
Plasma-based detectors (e.g., ICP-MS) target elemental analysis rather than molecular structure, making them more suitable for inorganic or metal analysis rather than organic small-molecule profiling.
How does isoelectric point (pI) play a role in ion-exchange chromatography?
At a given pH, analytes with net charges opposite to the stationary phase’s charge will bind; adjusting pH to the analyte’s pI reduces its net charge and elutes it from the column, enabling selective separation.
In the context of small-molecule chromatography, what is the benefit of using capillary electrophoresis (CE)?
CE separates analytes based on electrophoretic mobility (charge-to-size ratio) in an electric field, offering high efficiency and rapid separation for charged small molecules without a packed stationary phase.
Describe one scenario where mass spectrometric detection reveals compounds that UV/Vis cannot.
Non-chromophoric analytes—molecules lacking strong UV-absorbing groups—remain invisible to UV/Vis but can be ionized and detected by MS, enabling identification of otherwise undetectable species in complex mixtures.
How does a detector’s dynamic range influence the choice of chromatography-detection combination?
A detector with a wide dynamic range (e.g., MS with linear response over several orders of magnitude) can quantify both trace and abundant analytes in the same run, whereas narrow-range detectors may saturate or miss low-level components.
What is ‘matrix suppression’ in LC-MS, and how can chromatographic choices mitigate it?
Matrix suppression occurs when co-eluting matrix components reduce ionization efficiency of analytes in the mass spectrometer. Optimizing chromatographic separation (e.g., using longer or different-phase columns) reduces co-elution and minimizes suppression.
Why is GC not suitable for analyzing nonvolatile or thermally labile small molecules without derivatization?
Nonvolatile or thermally unstable compounds decompose or fail to vaporize in GC injectors. Derivatization converts them into more volatile, thermally stable derivatives, enabling GC analysis but adding complexity.
What considerations determine whether to use GC-MS versus LC-MS for a given small-molecule analysis?
GC-MS is ideal for volatile, thermally stable analytes that can be vaporized; LC-MS is chosen for nonvolatile, polar, or thermally labile compounds that cannot readily undergo GC analysis.
Summarize the key factors in selecting an optimal chromatography method for small molecules.
Choose stationary/mobile phases to exploit analyte polarity or charge; balance resolution against run time and sample throughput; account for matrix complexity and potential interferences; and match detection method (UV/Vis, DAD, or MS) to analyte properties and required sensitivity.
