Pulsed valve

Detection limits in the lOOfg range can be obtained with a tuneable UV laser working at a wavelength of maximum absorption for the compounds of interest. Continuous supersonic beams require high gas loads and combination with a pulsed ionisation technique (e.g. REMPI) is unfavourable in terms of sensitivity. Pulsed valves are a better approach for a GC-UV-MS interface [1021]. 

The setup used for crossed beam experiments is basically the same apparatus used in the H2O photodissociation studies but slightly modified. In the crossed beam study of the 0(1D) + H2 — OH + H reaction and the H + HD(D2) — H2(HD) + D reaction, two parallel molecular beams (H2 and O2) were generated with similar pulsed valves. The 0(1D) atom beam was produced by the 157 photodissociation of the O2 molecule through the Schumann-Runge band. The 0(1D) beam was then crossed at 90° with the... 

Figure 1 is a schematic of the laser vaporization source. This diagram depicts a pulsed valve on the left which supplies high pressure helium flow directly towards the right. Several workers have also chosen to use continuous helium f ows(2,6,9). In general these sources are modifications of conventional supersonic beam sources. 

Figure. 1. Schematic of essential components of the Exxon group cluster laser vaporization source and fast flow tube chemical reactor. On the far left is a 1 mm diameter pulsed nozzle that emits an -200 ysec long pulse of helium which achieves an average pressure of approximately one atmosphere above the sample rod. Immediately before the sample rod position the tube is expanded to 2 mm diameter. The length of this extender section can be varied form 6 mm to 50 mm depending upon the desired integration time for cluster growth. The reactor flow tube is 10 mm in diameter and typically 50 mm long. The reactants diluted in helium are added and mixed with the flow stream via the second pulsed valve.

In addition to the high-pressure assembly, the modified system incorporates a new real-time data collection system coupled with a PC based computer. Experimental parameters, such as the valve firing sequence and the reactor temperature-control program, can be set from the computer. Reactants are introduced through two high-spe pulse valves or two continuous feed valves that are fed by mass flow controllers. In high-speed transient response experiments, the QMS is set at a particular mass value and the intensity variation as a function of time is obtained. In steady-flow experiments. 

Step Transient Experiments at 800 Torr. Activation of pretreated silver was performed under isothermal conditions at 493, 523, and 543 K and 800 torr using a step transient format. A typical spectrum collected at 493 K, obtained by simultaneously pulsing ethylene-d4 and oxygen-18 from separate pulse valves into a continuous helium flow, is plotted in Figure 4. In this example, the oxygen to ethylene ratio was 2 1. As observed in the steady-flow TPSR experiments, the pretreated sample is readily activated, while the preoxidi samples remain inactive. 

In addition to the EIEIO experiments, the dual cell has also been employed for collisional activation (CA) experiments. Selected parent ions may be transferred to the analyzer cell for CA. With pulsed valve introduction of the collision gas (argon) into the analyzer cell, it is possible to obtain very high daughter ion resolution. Figure 13 shows a C6H5C0+/C8H9+ doublet from CA of acetophenone and mesitylene, detected at a resolution of 500,000. 

T. J. Carlin and B. S. Freiser, "Pulsed Valve Addition of Collision and Reagent Gases in Fourier Transform Mass Spectrometry," Anal. Chem., , 571-574 (1983). 

Selected topics in Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry instrumentation are discussed in depth, and numerous analytical application examples are given. In particular, optimization ofthe single-cell FTMS design and some of its analytical applications, like pulsed-valve Cl and CID, static SIMS, and ion clustering reactions are described. Magnet requirements and the software used in advanced FTICR mass spectrometers are considered. Implementation and advantages of an external differentially-pumped ion source for LD, GC/MS, liquid SIMS, FAB and LC/MS are discussed in detail, and an attempt is made to anticipate future developments in FTMS instrumentation. 

Pulsed-Valve Cl and CID-Exneriments. Chemical Ionization (Cl), self-CI (SCI), and direct or desorption Cl (DCI) experiments in FTMS can be done equally well with the differentially-pumped external ion source described below, or with a pulsed-valve single cell arrangement (5,6). In our experiments, we admit a pulse of reagent gas via a piezoelectric pulsed valve with a minimum opening time of about 2.5 ms (7). Unlike solenoid pulsed valves, the performance of piezoelectric pulsed valves is not disturbed by the strong magnetic field of 4.7 Tesla. 

Examples of both, pulsed-valve positive ion Cl, using methane and ammonia as reagent ions, and pulsed-valve negative ion Cl, using a N20/CH<, mixture, are shown in Figure 3 a,b. In both examples the molecular ion was not stable with respect to electron impact at 70 eV, but the Cl spectra clearly show abundant quasi-molecular-ion peaks. 

Piezoelectric pulsed-valve inlet systems are equally useful in collision-induced dissociation (CID) experiments (8) where the CID target gas (usually Argon) is pulsed, and subsequently pumped away to permit high-resolution, high-accuracy acquisition of FTMS spectra. 

Recently, low pressure Cl and SCI experiments, which take advantage of the long reaction times (typically 10 to 60 seconds) possible in an FTMS, have been demonstrated (9). Figure 4 exhibits low-pressure El and DCI spectra of Riboflavin (Vitamin B2), taken without a pulsed valve. Only the DCI spectrum taken at 2x10"8 mbar contains the quasi-molecular ion at M = 377. 

Figure 3a. 70 eV El spectrum, self-CI, and pulsed-valve CH4CI... 

For experiments requiring higher pressures of Cl reagent gas, a second experiment sequence was written which incorporates the use of the pulsed valves supplied with the FTMS. In this sequence, the pulsed valves are opened for 6 msec, allowing a high pressure pulse of reagent gas to enter the source cell. A variable delay is introduced between the time the valves are opened and the time the electron beam is turned on. This enables the pressure at which the initial ionization occurs to be varied. Again, after ion formation in the source, the products may be transferred into the analyzer cell for detection if desired. 

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