17th International Mass Spectrometry Conference :: Prague, 2006
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|Speaker:||Alexander B. Fialkov|
|Presentation date:||Mon, Aug 28, 2006|
|Presentation time:||12:10 – 12:30|
Alexander B. Fialkov1, Aviv Amirav1, Urs Steiner2, Larry Jones21 Tel Aviv University, Tel Aviv, Israel
Correspondence address: Alexander B. Fialkov, Tel Aviv University, School of Chemistry, Ramat Aviv, Tel Aviv, 69978 Israel.
Keywords: Ionization, Electron; Mass Spectrometry, Gas Chromatography; Molecular Beam; MS/MS, Gas Chromatography.
Novel aspect: A new type of GC-MS with record setting performance. GC-MS analysis of thermally labile and low volatility compounds, ex. hydrocarbons up to C84H170. Record sensitivity for trace analysis. Soft EI and Cluster CI ionization.
The current Achilles Heel of gas chromatography mass spectrometry is its limited capability with respect to analyzing thermally labile as well as low volatility large molecules. In addition, the molecular ion, which is the most important mass spectral feature, is often missing or weak with standard EI Ion sources.
In order to address these challenges we have combined the benefits of supersonic molecular beam (SMB) interface and its fly-through EI ion source with the advanced features of the Varian 1200 GC-MS and MS-MS and created a new type of GC-MS named 1200-SMB, with record setting performance:
The GC column output was mixed with helium make up gas, total of 90 ml/min, and transferred through a heated transfer line to a supersonic nozzle. The sample compounds expanded from the supersonic nozzle into a vacuum chamber, were vibrationally cooled, skimmed and collimated into a SMB. The vibrationally cold sample compounds passed a fly-through dual cage electron ionization ion source where they were ionized by 70 eV electrons with 10-15 mA emission current. The ions were focused by ion lens system, deflected 90 degrees by an ion mirror and entered the original Varian 1200 for their mass analysis. The 1200-SMB is characterized by the following main features:
GC/MS of compounds over 1000 amu: High GC column flow rates lower the elution temperatures and allow much larger molecules to elute. We analyzed a mixture of heavy linear chain hydrocarbons all the way to C84H170, all with dominant molecular ions.
GC/MS of thermally labile compounds: The lower elution temperatures greatly enhance the range of labile compounds amenable for analysis such as carbamate pesticides, high explosives, underivatized steroids and polar drugs.
Improved S/N: The features of enhanced molecular ion and reduced noise provide significant improvement in the detection sensitivity via SIM or RSIM on the molecular ion. OFN detection limit of below 1 femtogram in SIM mode was obtained, LOD of 2 fg was achieved for a more realistic compound such as diazinon and the use of the 1200-SMB resulted in an improvement factor of over 50 times in the RSIM detection limit of cholesterol on its molecular ion at m/z=386. In general, the harder the compound for analysis the greater is the gain in sensitivity of the 1200-SMB.
MS/MS: Having an enhanced molecular ion, significantly improves the MS-MS selectivity and sensitivity. We found that MS-MS on the molecular ion is much more effective than on fragment ions, and thus, the enhancement of the molecular ion directly improves the MS-MS sensitivity.
Fast GC/MS: Fast GC-MS analysis was achieved through the use of very high column flow rates for fast splitless injections and the high system selectivity due to the combination of enhanced molecular ion and MS-MS. We demonstrated a few seconds GC-MS-MS analysis of diazinon, spiked at 10 ng/g in a mixed fruit and vegetable extract.