ATMOSPHERIC-PRESSURE ION-SAMPLING SYSTEM FOR LIQUID-CHROMATOGRAPHY MASS-SPECTROMETRY ANALYSES ON A BENCHTOP MASS-SPECTROMETER

A commercial benchtop GC/MS mass spectrometer has been modified for liquid chromatography/mass spectrometry (LC/MS) applications. The modifications and concepts are straightforward, and the cost of the modifications is minimal provided appropriate pumping is available. The system can sample gas-phase ions formed at atmospheric pressure either by pneumatically assisted electrospray (ion spray) or by atmospheric pressure chemical ionization (APCI). A two-stage vacuum system allows transfer of ions formed at atmospheric pressure through a heated ion-sampling capillary and a conical sampling orifice into the analyzer vacuum chamber. An applied 50-140 V dc potential between the ion-sampling capillary combined with a 2-15 V dc voltage on the ion-sampling cone transfers positive analyte ions into the analyzer region where an rf-only quadrupole array transmits the ion beam into the mass analyzer. The length of the ion-sampling capillary inside the first vacuum chamber is heated to approximately 180-degrees-C. On-line ion spray LC/MS analyses may be carried out with HPLC flows from low microliter/minute to 1 mL/min provided a flow of 10-50-mu-L/min is directed to the interface via an appropriate postcolumn split. APCI conditions using a corona discharge and a heated pneumatic nebulizer LC/MS interface can handle total HPLC effluent when the chromatographic flow ranges from 0.2 to 2.0 mL/min. Full-scan and selected-ion monitoring (SIM) LC/MS analyses of samples containing mixtures of sulfadrugs and of pesticides were carried out on injected sample sizes ranging from 10 ng to 1.5-mu-g per component using APCI while similar experiments with samples containing LSD, benzodiazepines, and selected pesticides were carried out under ion spray micro and conventional LC/MS conditions using injected sample levels ranging from 250 pg to 1-mu-g. When the potential difference between the ion-sampling capillary and the conical ion-sampling orifice is increased, collision-induced dissociation (CID) occurs, providing structurally informative mass spectra. Examples are shown from CID LC/MS experiments conducted under both ion spray and APCI conditions. SIM CID ion spray micro LC/MS experiments demonstrated a forensically valid detection limit for LSD of 2.5 ng injected on-column when four structurally significant ions were monitored and 250 pg when the protonated LSD molecule and its most abundant fragment ion were monitored.