Emitter heating current

Ktimmler, D. Schulten, H.-R. Correlation Between Emitter Heating Current and Emitter Temperature in Field Desorption Mass Spectrometry. Org. Mass Spectrom. 1975,10, 813-816. 

Fig. 14. Evaporation profile of about 350 fg [Cs]. The emitter heating current is raised from 0 to 100 mA linearly with 0.19 mA/s ...

The ions are detected using a channel electron multiplier (Valvo) and a combined counter/ratemeter registering unit (Ortec). The channel electron multiplier is operated at —3 kV. A linear emitter heating current (EHC) programmer is employed for the desorption of the samples. In all cases the EHC is raised at 0.19 mA/s from 0 to 100 mA (see Fig. 14). All measurements for the calibration curve of [Cs] (see Fig. 13) are made with one FD emitter starting from small sample amounts. 

Although the desorption of thallium occurs at relatively low emitter heating currents (20-25 mA) no interference from organic ions can be observed in the investigation of the brain tissue homogenizate. For a FD mass spectrometric investigation, however, the concentration of about 90 mg/1 thallium is high as compared to sample amounts usually required for an alkali metal determination. In order to as-... 

In FD-MS analysis the emitter is normally heated resistively to aid in the desorption process. Chemicals that are reasonably volatile and not too polar will desorb with the emitter held at ambient temperature. Most organics, however, will require the application of some emitter heating current (EHC) to get them to desorb. The amormt of heat needed is much less than that... 

Emitters can be heated by applying an electric current via the emitter holders. It is somewhat difficult to establish a precise calibration of emitter temperature versus emitter heating current (EHC) [56,57]. The actual temperature not only depends on the emitter material, but also on diameter and length of the emitter as well as on length and area density of the whiskers. A useful estimate for tungsten emitters with carbon whiskers is given below (Fig. 8.7). 

The complex spectrum in Fig. 3 (emitter heating current 18-30 mA) can be considerably simplified when only the fraction of ions produced with an emitter heating current between 18 and 21 mA is recorded. Fig. 4 shows that under these conditions only a few significant ions are detected that enable the identification of cationic and the anionic moiety of the sulphate salt. 

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