Matrix-ionization chamber

Another example is the matrix ionization chamber system (vanHerk, 1991 1992). The ionization chamber consists of two printed boards which enclose a thin liquid film. Each plate carries a system of 256 parallel conducting strips. The principle is shown in Figure 8. The strips of one plate are connected to 256 charge-sensitive amplifiers. Voltage pulses are applied successively by a multiplexer to the 256 strips of the other plate. This way, 256 x 256 = 65,536 crossing points exist which function as mini-ionization chambers. The sensitive volume is irradiated continuously by X-rays and the ionization produced in each minichamber is read out. Such a device has been applied as a portal imaging device in radiation therapy (Meertens et al., 1990). 

For HPLC MS/MS assays the use of stable isotope labeled internal standards is by far the best method to overcome any potential matrix effects and random variation in the MS/MS detector. If for any reason this stable isotope internal standard is not available, an analog compound with a mass different from the analyte can also be used. The chromatographic retention time of the internal standard, however, should be as close as possible to the retention time of the analyte. This ensures, that time dependent random variation in the ionization chamber, or whereever else in the MS/MS detector, are compensated by the internal standard. In a toxicokinetic assay described by Chi et al. (2003), for example, an internal standard was used which showed the same retention time as the analyte. 

The conventional XAS experiment involves the direct measurement of the incident and transmitted beam intensity using ionization chambers. The first chamber contains a weakly absorbing gas, which permits <70% of the incident radiation to fall on the sample, and the second ionization chamber contains a mixture of inert gases that will absorb virtually all of the transmitted intensity. The measured absorption coefficient comprises that due to the matrix (/um) and that due to the atom of interest (/u,a). The application of transmission method is ultimately limited by the incident number of photons and the ratio of fiM to fi. In cases where = 1, it is difficult to use the transmis-... 

A schematic of a MALDI-TOF-MS instrument is depicted in Figure 11.2b. Samples, consisting of a few microlitres of analyte solution (with or without matrix), are deposited on a MALDI target (Figure 11.2a). After the solvent has evaporated the sample plate, carrying the solidified samples, is introduced into the MALDI ionization chamber via load-lock. The ionization process takes place in a high-vacuum chamber to which the plate is introduced via a prechamber kept at a pressure lower than atmospheric. Analyte ions are then accelerated as they are formed and pumped into the TOF analyzer, where they are separated based on their mass-to-charge ratio. 

In addition, co-eluting matrix compounds are often present in a sample from biological material. This will most likely suppress the total ion current formed in the ionization chamber, thus lowering the sensitivity of the method of analysis (Buhrman, Price and Rudewicz, 1996 Chan, 1996 Knebel, Sharp and Madigan, 1995). By using chromatography, ion suppression is reduced. Furthermore, unexpected compounds, metabolites and/or artefacts may be easier to identify with HPLC-MS. With flow or loop injection, these compounds may be overlooked with loss of (important) data as a result. 

Figure 20.13 Smoke detector. The radioactive source in a smoke detector is americium-241 oxide, embedded in a gold foil matrix. Americium is an alpha and gamma emitter. The radiation ionizes the air in the ionization chamber, producing an electric current. When smoke particles interact with the ions, the current is reduced, and the alarm is triggered. 

There are two principal components of mass spectrometers the ionization chamber, where ionization of the sample occurs, and the mass analyzer, where ion sorting and detection occur. Mass spectrometer instruments vary in design with regard to both of these components. Thus far we have mentioned only one ionization technique, electron impact (El). In Section 9.18A we discuss El ionization in more detail, as well as discuss two other important ionization methods electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI). 

Matrix effects on measured Mg isotope ratios due to the presence of Fe in laser ablation targets require further study. Young et al. (2002a) formd that adding Fe via solution to the plasma to yield FeY Mg+ up to 2.0 resulted in no measurable shifts in Mg/ Mg and Mg/ Mg of laser ablation products. Norman et al. (2004) showed that there is a shift in instrumental fractionation in measured Mg/ Mg on the order of +0.06%o for every 1% decrease in Mg/ (Mg+Fe) of the olivine target. These disparate results might be explained by ionization in different locations in the torch when samples are introduced by aspiration of solutions rather than by gas flow from a laser ablation chamber. Different matrix effects for solutions and laser... 

Figure 6 The SELDI technology. This type of proteomic analytical tool is a class of mass spectroscopy instrument that is useful in high-throughput proteomic fingerprinting of serum. Using a robotic sample dispenser, 1 p,L of serum is applied to the surface of a protein-binding chip. A subset of the proteins in the sample binds to the surface of the chip. The bound proteins are treated with a matrix-assisted laser desorption and ionization matrix and are washed and dried. The chip, which contains multiple patient samples, is inserted into a vacuum chamber where it is irradiated with a laser. The laser desorbs the adherent proteins and causes them to be launched as ions. The TOF of the ion before detection by an electrode is a measure of the mass-to-charge (m/z) value of the ion. The ion spectra can be analyzed by computer-assisted tools that classify a subset of the spectra by characteristic patterns of relative intensity (adapted from www.evmsdoctors.com).

The ICP-AES and ICP-MS techniques may also suffer from matrix effects, such as spray chamber effects caused by the different viscosity of the samples and the calibration standards. The careful choice of internal standards can reduce this problem. The effects caused by high amounts of easily ionized elements may be solved by internal standardization or by the use of matrix-matched calibration curves. An additional specific problem with ICP-AES is the risk of spectral overlaps. 

Figure 16.20 FAB and MALDI techniques, (a) The principle of fast-atom beam formation with xenon (b) The formation of fast atoms of argon in a collision chamber and subsequent bombardment of the sample by this atom beam, usually of 5-10 kV kinetic energy (c) MALDI or ionization by effect of illumination with a beam of laser generated light onto a matrix containing a small proportion of analyte. The impact of the photon is comparable with that of a heavy atom. Through a mechanism, as yet not fuUy elucidated, desorption and photoionization of the molecules is produced. These modes of ionization by laser firing are particularly useful for the study of high molecular weight compounds, especially in biochemistry, though not for routine measurements.

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