Plasma desorption mass spectrometer

Mass Spectrometiy. Samples were introduced into a Californium-252 time-of-flight plasma desorption mass spectrometer in a 1% TFA solution applied to a nitrocellulose-covered aluminized Mylar foil. TTie samples were run at 15 kV accelerating volts with a 30-cm flight path, and the signal was accumulated for 19 hr (1),... 

Plasma desorption mass spectrometer Biolon 20 and aluminized polyester targets are from Biolon. The spin apparatus was homemade (description available from the author). Gilson adjustable automat pipettes (2, 10, 20, 100, and 2(X) 1) are from Gilson Medical Electronics. The Heto Vac VRl vacuum centrifuge equipped with a Heto CTllO cold trap is from Heto Lab). 

FIGURE 4.2 Schematic diagram of a plasma desorption mass spectrometer. Because the TDC is a multistop device, each ionization and recording cycle produces time-interval measurements for several ions. In the example shown. 3 and 4 ions are recorded in the first and second cycles, respectively. 

Concentration detection limits in CE-MS with the ESI interface are similar to those with UV detection. Sample sensitivity can be improved by using ion-trapping or time-of-flight (TOE) mass spectrometers. MS analysis can also be performed off-line, after appropriate sample collection, using plasma desorption-mass spectrometry (PD-MS) or matrix-assisted laser desorption-mass spectrometry (MALDI-MS). 

Another early desorption technique is that of plasma desorption mass spectrometry (PDMS). The sample is deposited onto a thin aluminum or aluminized polyester foil (0.5-1 pm in thickness) and placed just in front of a californium-252 emitter which is located in a time-of-flight (TOF) mass spectrometer (Figure 7). Californium is an a-emitter that decays into two highly energetic a-particles that are expelled in diametrically opposite directions. One of these particles collides with and ionizes the sample while the other hits a collection plate and triggers the timing circuit for the TOF mass spectrometer. 

Much of the current interest in time-of-flight mass spectrometers is driven by instruments which desorb nonvolatile molecules (particularly peptides and other biological molecules) from surfaces. These methods include plasma desorption mass spectrometry (PDMS), laser desorption (LD), and matrix-assisted laser desorp-tion/ionization (MALDI), and they greatly simplify the design of time-of-flight mass spectrometers, since they effectively eliminate both the time- and spatial-distribution problems. 

G. Brinkmalm, P. Hakansson, J. Kjellberg, P. Demirev, B. U. R. Sundqvist, and W. Ens. A Plasma Desorption Time-of-Flight Mass Spectrometer with a Single-Stage Ion Mirror Improved Resolution and Calibration Procedure. Int. J. Mass Spectrom. Ion Proc., 114(1992) 183-207. 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

Mass spectrometry (MS) is a gas-phase technique in which atoms or molecules present in the spectrometer chamber are ionized, and follow a trajectory through applied electric and magnetic fields which separates them according to their mass/charge ratio. A number of procedures have been developed to enable MS to be used for analysing species in the liquid and solid phases, and are based on species extraction into the gas phase. These include plasma desorption, ion bombardment, thermospray and electrospray ionization, and laser desorption. In this section we concentrate on techniques useful to electrochemistry. 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

TOF analysers are directly compatible with pulsed ionization techniques such as plasma or laser desorption because they provide short, precisely defined ionization times and a small ionization region. However, to take advantage of TOF analysers, it is interesting to combine such powerful analysers with continuous ionization techniques. These ionization techniques can be compatible with TOF analysers but require some adaptations to pulse the source or to transform a continuous ion beam into a pulsed process. For instance, the coupling of an ESI (or any other API) source with a TOF mass spectrometer is difficult, because ESI yields a continuous ion beam, whereas the TOF system works on a pulsed process. 

After focusing the accelerating potential (V) is applied for a much shorter period than that used for ion production ca 100 nsec) so that all the ions in the source are accelerated almost simultaneously. The ions then pass through the third electrode into the drift zone and are then collected by the sensor electrode. The velocity of the ions after acceleration will be inversely proportional to the square root of the ion mass. With modern ion optics and Fourier transform techniques Erickson et al. (6) could sum twenty spectra per second for subsequent Fourier transform analysis. The advantage of the time of flight mass spectrometer lies in the fact that it is directly and simply compatible with direct desorption from a surface, and thus can be employed with laser desorption and plasma desorption techniques. 

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