Helium damping gas

The quadrupole ion trap is a three dimensional analogue of the linear quadrupole mass analyzer [71,72]. It consists of a cylindrical ring electrode and two end-cap electrodes. Both end-cap electrodes contain a whole for injecting and ejecting ions into and out of the ion trap (Fig. 8.11). A relatively high pressure of helium damping gas (about 0.1-0.4 Pa) is present in the ion trap in order to kinetically cool the trapped... 

We use a GC Top 8000 gas chromatograph coupled with a PolarisQ ion-trap mass spectrometer and equipped with an AI3000S autosampler (Thermofinnigan www. thermo.com). The steroids are separated on a DB-1 crosslinked methyl-silicone column, 15 mx 0.25 mm i.d., film thickness 0.25 pm (J W Scientific marketed by Agilent). Helium is used as a carrier gas at a constant pressure of about 35 kPa. A 1-pl aliquot of the final derivatized extract is injected into the system operated in splitless mode (valve opened at 2 min). The GC temperature program is the same described before for the quadrupole GC-MS system. The injector and transfer lines are kept at 260°C and 280°C, respectively. The ion source temperature is 225°C. A damping gas flow of helium is applied to the ion trap. 

Figure 6.21 Partial spectra illustrating the dramatic effect of use of helium as a damping gas in a Paul trap on its paformance as a mass spectrometer using the mass selective instability scan mode. Peaks on the left are for mtz 69, those on the right for m/z 502. The top spectra were obtained without damping gas, and the bottom spectra by using 10 torr of helium. Reproduced from Stafford, Int. J. Mass Spectrom. Ion Proc. 60, 85 (1984), copyright (1984), with permission from Elsevier.

The damped oscillations are converted to an electrical signal by a non-drag optical transducer light is passed through a pair of polarizers, one of which serves as the inertial mass of the pendulum, to a photo-detector. The temperature, humidity and gas (usually helium) surrounding the specimen are closely controlled. 

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