How can I optimize my Large Volume Injection (LVI) ?
Optimization Routine For Large Volume InjectionThis LVI routine is based on OPTIC and fast (not speed controlled) injections. A packed liner (sintered liner is also a packed liner) should be used to hold the solvent during evaporation. (check for the LVI training manual on www.glsciences.eu )
1. OBTAIN A REFERENCE CHROMATOGRAMEither from previous work, or by using an on-column injection of a 1µl standard solution, obtain a reference chromatogram forcomparison purposes. (if no on-column injector available use a 1µl splitless injection).
2. CHECK THE LINER PACKING IS INERT WITH RESPECT TO THE SAMPLEFit a packed liner and inject 1µl of the standard in the cold splitless mode. Compare the results with the reference chromatogram.Differences between the two chromatograms can indicate non-optimised splitless transfer conditions. For example: If the peakareas tend to be smaller than the reference chromatogram, the Optic temperature could be too low, or, the splitless time is tooshort: If some of the peaks are smaller or absent, when compared to the reference chromatogram, the liner packing may not beinert to all of the analytes of interest.
3. DETERMINE THE LIQUID CAPACITY OF THE LINER (Vmax)Without turning off the carrier gas, remove the column from the injector and inject 150µl of solvent. Look for solvent droplets atthe injector outlet. Progressively reduce the amount injected until no droplets are observed. This volume is Vmax.
4. DETERMINE THE SOLVENT ELIMINATION TIME (Vent time)Reconnect the column to the injector. Set the column temperature to the injector initial temperature. Set the detector tominimum sensitivity. Set the vent flow to 100 ml/min. Rapidly inject a volume of pure solvent equal to the volume to be used inlarge volume injection (do not exceed 90% VMAX). Measure the peak width of the solvent in seconds.
5. LARGE VOLUME INJECTION OF STANDARDDilute the standard by a factor equal to the number of µl to be used in the large volume injection, using clean solvent. Set the venttime to the solvent elimination time. Set the detector sensitivity to that used to obtain the reference chromatogram. Inject theselected volume of dilute standard (do not exceed 90% Vmax).Compare the chromatogram obtained with the reference chromatogram.
6. FINE TUNINGIf the chromatogram from the large volume injection looks like the reference chromatogram, no further optimisation is required. Ifvolatile components are lost, reduce the vent time in steps of 3-5%. If the more heavy compounds are lost (end of the chromatogram) it could be that the Splitless time of the LVI method was too short.
- Do not be tempted to take short-cuts.
- Set the vent flow to 100 ml/min.
- Set the split flow to 50 ml/min.
- Set the initial injector temperature in the range 20-40°C below the solvent boiling point.
- Set the final temperature to the liner packing maximum temperature or 50°C above the elution temperature of the latest running peak.
- Set the inlet ramp rate to 4°C/sec.
- Vmax is usually 60-125µl.
- Vent time is usually 20-50 seconds. (Or use the solvent sensor to determine the vent time)
- Polar solvents have larger elimination times.
- When using GC-MS, the rise in source pressure can be used to determine the solvent elimination time
---------------------------------------------------------To start as a kind of general LVI method;Liner = sintered glass liner, solvent Hexane. Method Type Large VolumeMethod Name Method1Equilibration Time 00:05 min:secEnd Time 20:00 min:secInitial Temperature 45 ºCRamp Rate 5.0 ºC/sFinal Temperature 325 ºCTemperature Control Keep Current TemperatureSeptum Purge Flow 5 mL/minVent Mode Fixed TimeVent Time 00:35 min:secCarrier Control Mode Flow ControlSample Sweep Column Flow 1.0 mL/minTransfer Column Flow 1.0 mL/minTransfer Time 02:00 min:secStart Column Flow 1.0 mL/minEnd Column Flow 1.0 mL/minVent Flow 50.0 mL/minSplit Flow 25.0 mL/min Start with this method and if you do not see the first compounds in your chromatogram; lower the Vent Time. The Sintered liner should be installed with the left side to the top (o-ring must be there) and the notch at the bottom of the inlet (for Split flow).