Sample apertures

The TRAPI was developed by Matsuoka and co-workers " and has been used to determine the rate constants of about a dozen IM reactions at atmospheric pressure. As a first approximation, the TRAPI experiment might be described as an atmospheric pressure version of the PHPMS with initial ionization caused by a pulsed X-ray source. The X-rays cause relatively even ionization throughout the 6.4-cm ion source volume by penetrating through thin sections of the ion source walls formed by 25- im thick molybdenum foil. A 16-pm ion-sampling aperture is located at the center of one of these thin walls. The ions that pass through this aperture are measured by an associated mass spectrometer as a function of time after the X-ray pulse. 

Even though the TRAPI and the PHPMS both involve a pulsed high-pressure ion source, they are fundamentally different methods in that they are based on different underlying principles. In the TRAPI method, ions are transported through the sampling aperture by convective flow with the buffer gas, rather than by diffusion through the buffer gas. Also, second-order ion-ion or ion-electron recombination... 

Kanazawa et al. [124] determined omeprazole and its metabolites in human plasma by liquid chromatography-three-dimensional quadrupole mass spectrometry with a sonic spray ionization interface. The analytical column was YMC-Pack Pro Cig (5 cm x 2 mm) using acetonitrile-50 mM ammonium acetate (pH 7.25) (1 4) at a flow-rate of 0.2 ml/min. The drift voltage was 30 V. The sampling aperture was heated at 110 °C and shield temperature was 230 °C. In the mass spectrum, the molecular ions of omeprazole, hydroxyomeprazole, and omeprazole sulfone were clearly observed as base peaks. The method is sufficiently sensitive and accurate for pharmacokinetic studies of omeprazole. 

Fig. 8.5. Schematic of an API interface for CEC—MS. 1, introduction of column effluent from CEC and spraying device 2, atmospheric pressure region 3, ion sampling aperture 4, atmospheric pressure to vacuum interface 5, skimmer 6, ion transfer optics (adapted from ref. [11]).

Fig. 9. Schematic diagram of the free jet flow reactor used by Mark Smith and co-workers for very low temperature reaction kinetic measurements [58]. The jet originates from a pulsed beam valve 1, and ions are produced by REMPI using a focussed pulsed laser. The reaction zone is bounded by a repeller plate 2 and an endplate 3 ions are propelled, by a pulsed voltage on the repeller, towards a sampling aperture in the endplate which leads to a TOF-MS 4...

A three-stage differentially-pumped vacuum system, consisting of a molecular-beam stage between ion-sampling aperture and a skimmer, evacuated by a rotary pump, an ion optics region, and a mass analyser region, both evacuated by turbomolecular pumps. This design is most frequently used today. 

Since that time, several other API instmments were described. The most successful commercial instrament was the TAGA (trace atmospheric gas analysis) spectrometer, built by Sciex in Canada. Important features of this system are the 20-mVs cryopump, which allowed the use of a 100-200-pm-ID sampling aperture, and the air curtain to prevent clogging of the sampling aperture. 

Immediately after their production, the ions in the humid atmospheric-pressure ion source will attract solvent molecules by ion-dipole interactions. These solvated ions must be desolvated prior to entering the mass analyser. This is achieved by ion-molecule collisions in the transition region, especially between the ion-sampling aperture and the skimmer. A small potential difference between the nozzle and the skimmer is applied to enhance declustering. 

The diameter of the sampling aperture should be as large as possible to maximize the analyte signal and to minimize orifice clogging, while keeping the pumps used for this section to a reasonable size. 

Although contemporary FT-IR spectrometers and microscopes are well matched, for spectra measured when using sample apertures that approach the diffraction limit (<20gm) even a 30 s collection may result in a spectrum with a rather poor SNR. It may be noted that if the measurement of each spectrum takes 30 s and a 64x64 map is required at 20 pm spatial resolution, it would take over 34h to acquire all the spectra required for the image ... 

The sample holder system used contains six sample apertures. Five samples are maximally placed at the same time in this holder to keep one aperture free for the reference measurement, see Figure 4.9. This whole sample holder system is lifted into a special thermostat bath provided with a liquid nitrogen cooling coil. This cooling possibility extends the lower temperature limit of these measurements from 20°C to about -50°C. The bath is filled with a mixture of water/ethylene glycol (l/l) for measurements between -50°C and 80°C. Silicone oil (100 cS.) is used as medium for measurements between 0eC and 200°C. The sample temperature is measured by a platinum resistance thermometer, placed as close as possible to the sample in the ultrasonic beam. 

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