When establishing quantitative methods in LC-MS, how to determine the quantitative ion pairs and qualitative ion pairs? What additives in the mobile phase can improve ionization efficiency?
When establishing a quantitative method in LC-MS, determining the quantitative ion pairs and qualitative ion pairs is crucial. Here are some recommendations on how to select them:
1. Quantitative Ion Pairs:
Typically, the ion with the highest intensity is chosen as the quantitative ion because it provides the best signal-to-noise ratio. In practice, you first need to obtain a full scan mass spectrum of the target compound and then identify the most abundant and representative ions. These ions can be molecular ions (such as [M+H]+ or [M-H]-) or characteristic fragment ions. When selecting quantitative ion pairs, ensure they exhibit a good linear relationship within the method's linear range.
2. Qualitative Ion Pairs:
Qualitative ions are usually used to verify the structure and identity of the target compound. Select ions with higher intensity and structural characteristics from the full scan mass spectrum as qualitative ions. Ensure that the selected qualitative ions are related to the structure and chemical properties of the target compound and have sufficient selectivity and specificity in practical applications.
In LC-MS analysis, the use of mobile phase additives can enhance ionization efficiency. Here are some common additives:
1. Acidic Additives:
When analyzing cationic compounds, acidic additives such as formic acid (0.1%) or acetic acid (0.1%) can be added to the mobile phase. These additives can facilitate the cationization process and increase the ion yield of the target compound.
2. Basic Additives:
When analyzing anionic compounds, basic additives such as ammonia (0.1%) or amino acids can be added to the mobile phase. These additives help in the anionization process, thus increasing the ion yield of the target compound.
3. Salt Additives:
In some cases, salt additives such as ammonium acetate or ammonium formate can be used. These salt additives help improve the solubility and ionization efficiency of the analytes.
Please note that the selection and concentration of additives need to be optimized based on the actual sample and target analytes. In practical applications, it may be necessary to try different types and concentrations of additives to achieve optimal ionization efficiency.
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