How does mass spectrometry select an ion source based on the sample?
??1. How to choose the ion source according to the sample?
Analysis:
Can be based on molecular weight and polarity. APCI is suitable for small molecules and compounds with low polarity; ESI is suitable for analysis of larger molecular weight ranges and molecules that require a certain polarity. Generally, ESI analysis is considered first, and if the polarity is too small, APCI is considered.
??2. Comparison between temperament and liquid systems?
Analysis:
In addition to the different separation methods used (gas phase and liquid phase), the main difference lies in the vacuum system and ionization method. The temperament vacuum system is relatively simple, requiring only a small mechanical pump and a molecular turbine pump. The mechanical pump for liquids is larger than that for gases, requiring two molecular turbopumps. The ionization methods of temperament include electron ionization (EI) and chemical ionization (CI). Liquid ionization methods include electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI).
??3. What are the differences between APCI and ESI?
Analysis:
1) The way ions are produced is different. APCI utilizes corona discharge ionization, gas phase ionization. ESI utilizes ion evaporation, liquid phase ionization.
2) The types of compounds that can be analyzed are different. APCI is suitable for weakly polar, small molecule compounds with a certain volatility; ESI is suitable for polar compounds and biological macromolecules.
3) The flow rate is different. ESI generally has a small flow rate, about 0.001 to 0.25 mL/min, and APCI is relatively large, about 0.2 to 2 mL/min.
4) Multiple charges. APCI cannot generate a series of multi-charged ions, so it is not suitable for analyzing macromolecules; ESI can generate a series of multi-charged ions, which is especially suitable for biomolecules such as proteins and peptides.
??4. What is the difference between CID and CAD?
Analysis:
CID, collision-induced dissociation. CID phenomenon usually occurs when the voltage is adjusted at the vacuum interface, usually to remove the solvent. If the voltage is increased, fragment ions will also be generated. This is the principle of primary mass spectrometry. CAD collision activation dissociation. When doing secondary mass spectrometry, after the selected precursor ion enters Q2, it is activated by collision to produce product ions. This process is called CAD. Taking the API3200 spectrometer as an example, CID refers to the collision-inducing gas in Q0, and CAD refers to the collision-inducing gas in Q3. In other words, CID&CAD here refers to nitrogen.
??5. How to quantify using tandem mass spectrometry?
Analysis:
When quantified by tandem mass spectrometry, the fragment peaks (product ions) produced later are used for quantification. However, this product ion is a characteristic fragment produced by the parent ion in the collision chamber, so the quantitative sensitivity of MRM is much better than that of SIM. The steps to establish the method are: use a standard solution of a certain solubility (1-10 ug/mL) to tune the primary mass spectrometry conditions of the compound and find the optimal mass spectrometry conditions for the parent ion. Then the parent ion is fragmented, the collision energy is optimized, and its characteristic product ions are obtained. Finally, the mass spectrometry conditions and the precursor ion->product ion pair are used for quantification.