Since target compounds cannot be analyzed unless they are ionized, first investigate:
(1) Will target compounds be ionized?
(2) Which detection mode is appropriate, positive or negative ion mode?
If a Shimadzu LCMS-2010EV(A) system is used, positive and negative ions can be measured simultaneously by using the Multi-Sequence Mode, and good analytical results can be obtained without fussing over setting ionization parameters, such as DCL voltage and Q-array DC/RF voltage, by using the C (constant) mode or S (scan) mode, which use optimized values according to the ion mass. FIA parameters are indicated in Table 1.
In FIA, the sample solvent can have a large influence on the ionization efficiency. Therefore, sample preparation is extremely important. If necessary, an acid or base is added to adjust pH or a dissolving assistant to ensure complete dissolution. It can easily be determined from total ion chromatograms (TIC) whether ions are generated, but because separation is not performed for FIA, it is necessary to confirm that the ions are from the target compounds. Since lower sample concentration makes this determination more difficult, it is probably better to experiment with sample concentrations in the tens to hundreds of ppm until proficient. Figure 1 shows the FIA results for a certain compound. It shows the TIC for each ionization and a chromatogram for the mass number with the highest ion intensity. It is easy to determine from this data that some sort of ions are observed.
|Mobile Phase Flowrate: |
First-Choice Mobile Phase:
Nebulizer Gas Flowrate:
Drying Gas Flowrate:
Block Heater Temperature:
Q-array DC Voltage:
Q-array RF Voltage:
|0.2 mL/min |
0.1 % Aqueous formic acid solution / acetonitrile = 1/1
+4.5 / -3.5 kV
m/z 10 - 1000
Cases Where Only Positive Ions Can Be Detected
Cases Where Both Positive and Negative Ions Can Be Detected
Cases Where Only Negative Ions Can Be Detected
Cases Where No Ions are Detected
(Note: In Figure 1, [ESI-P] m/z 411 is 388 + 23, which is a sodium adduct ion, and [ESI-N] m/z 423 is 388 + 35, which is a chlorine adduct ion.)
If it can be determined that observed ions are generated from a target compound, then that achieves the first phase of LC-MS analysis. Not performing this first phase can have terrible consequences. Since the analysis only takes a few minutes, we recommend that it be performed without fail. (Mu)
Supplemental Explanation: Based on past analytical experience and other factors, if proper detection of target compounds is expected, sometimes we do not perform flow injection analysis and verify whether gradient conditions with mobile phases A and B can be used to separate samples. In that case, perform flow injection analysis if there is a problem with ionization.