Matrix ions

Mass spectrum obtained from the NIST Hasteloy Ni-basad standard alloy, using electron-gas SNMSd (Laybold INA-3). The sputtering energy was 1250 V, increasing the sputtered atom flux at the expense of depth resolution. Matrix ion currents ware about 10 cps, yielding background limHed detection at about 2 ppm. 

Figure 4 Mass spectrum obtained from the Aiuminium Peehiney standard Ai 11630, using eiectron-gas SNMSd with a sputtering energy of 1250 V. The AI matrix ion currerrt was significantly greater than 10 cps. yielding a background courrt rate limit less than 1 ppm.

The charges of matrix ions are ez% = ez = ez and the density of the matrix subsystem is p (p+ = p- = p /2). We define the functions y (r) describing the interactions between particles. In particular, the interactions between matrix ions are given as... 

The high sensitivity and selectivity of the EPR response enables diamagnetic systems to be doped with very low concentrations of paramagnetic ions, the fate of which can be followed during the progress of a reaction. The criteria [347] for the use of such tracer ions are that they should give a distinct EPR spectrum, occupy a single coordination site and have the same valency as, and a similar diffusion coefficient to, the host matrix ion. Kinetic data are usually obtained by comparison with standard materials. 

FAB has been used to analyse additives in (un) vulcanised elastomer systems [92,94] and FAB matrices have been developed which permit the direct analysis of mixtures of elastomer additives without chromatographic separation. The T-156 triblend vulcanised elastomer additives poly-TMDQ (AO), CTP (retarder), HPPD (antiozonant), and TMTD, OBTS, MBT and A,lV-diisopropyl-2-benzothiazylsulfenamide (accelerators) were studied in three matrix solutions (glycerol, oleic acid, and NPOE) [94]. The thiuram class of accelerators were least successful. Mixture analysis of complex rubber vulcanisates without chromatographic separation was demonstrated. The differentiation of matrix ions from sample ions was enhanced by use of high-resolution acquisition. 

The indexed relative sensitivity factor approach obviates the necessity of measuring the relative sensitivity factors from all possible matrices, by transferring relative sensitivity factors for elements between different matrices by using the matrix-dependence of characteristic intensity ratios in the spectra. Calibration curves are constructed relating RSFs for an element in a matrix to the matrix ion species ratio (e.g. M2+/M+ for element M) generated from a single standard. 

To overcome these difficulties, Ito and Sunahara [102] suggested the use of the matrix ions as the eluent ions, for example the use of relatively high concentrations of sodium chloride solution. 

Compared with LC/MS/MS methods, nanoelectrospray/MS/MS methods offer additional benefits such as no sample carry-over and low sample and solvent consumptions. The major concerns surrounding columnless analysis of biological samples are matrix ion suppression and direct interference from endogenous components or metabolites of the dosed compound. Therefore, an extensive sample clean-up process must be in place to ensure the accuracy and precision of the assay. A nine-fold gain in terms of sample throughput was achieved with a nanoelectrospray/MS/ MS method that produced accuracy, precision, and detection limits comparable to those of a traditional LC/MS/MS method.14... 

Direct injections using RAM or TFC have simplified sample preparation and increased throughput. Matrix ion suppression was greatly reduced or eliminated in several cases compared with traditional off-line sample cleanup procedures such as PPT, SPE, and LLE. Method development time was minimized with generic methods15 that suit most applications. Detailed applications can be found in a recent review.8... 

Data-dependent acquisition ability has been developed and incorporated into most software packages [MetaboLynx, Xcalibur, and Analyst Information Dependent Acquisition (IDA)]. In data-dependent acquisition mode, a mass spectrometer decides on the fly whether to collect MS/MS or MSn data, remain in full scan MS mode, or conduct other survey scans based upon user-defined criteria. Product ion spectra of potential metabolites can be automatically acquired in a single LC/ MS run. However, false positives may be generated due to highly intense matrix ion signals that may inadvertently trigger MS/MS or MSn scan functions. 

Figure 2.9. Schematic of a matrix-assisted laser desorption/ionization (MALDI) event. The SEM micrograph depicts sinapinic acid-equine myoglobin crystal from a sample prepared according to the dried drop sample preparation method. In the desorption event neutral matrix molecules (M), positive matrix ions (M+), negative matrix ions (M-), neutral analyte molecules (N), positive analyte ions (+), and negative analyte ions (-) are created and/or transferred to the gas phase. Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

Some spectrometers are equipped with delayed extraction. Ions are accelerated from the target only 200 to 300 nsec after the laser beam hits the target. During the short delay, light matrix ions dissipate. This helps to avoid massive collisions during the acceleration between the large molecules and the matrix ions. 

Reflectron time-of-flight mass spectrometers can be used to generate fragment spectra. Before the introduction of delayed extraction, it was observed that the ions had a tendency to disassemble into fragments while they were traveling to the reflectron (Spengler et al., 1992). This behavior is called postsource decay (PSD). The ions collide with matrix ions in the gas phase when they are accelerated with several kilovolts. Their decay occurs in the first field-free drift region. 

Berkenkamp, S. Menzel, C. Hillenkamp, F. Dreisewerd, K. Measurements of Mean Initial Velocities of Analyte and Matrix Ions in Infrared MALDI-MS. J. Am. Soc. Mass Spectrom. 2002, 75, 209-220. 

The high throughput capability of MALDI and the different ionization mechanisms make this technique also an attractive alternative to electrospray ionization for the analysis of low relative molecular mass compounds (LRMM) [44]. However, interferences of matrix ions and the ionization of the low relative molecular mass compounds are the challenges of this technique [45, 46]. 

FAB peptide preparation is accomplished by dissolving a relatively pure (>70%) peptide sample into a 3-nitrobenzyl alcohol (NBA) matrix. There are many matrices used in FAB analyses, however, 3-nitrobenzyl alcohol has become a standard for FAB peptide analysis (Figure 12). The peaks associated with the NBA matrix can make the spectra more cumbersome to interpret. However, to the experienced mass spectroscopist the matrix ions can act as reference ions and are a useful indicator of accuracy. Rarely are the NBA matrix ions observed above m/z 1000. 

Interpenetrated Mall Matrix. Ion-exchange hollow libers can be produced by polymerizing an ionic monomer within (he porous wall matrix of a hollow fiber. Requirements of such a fabrication are th the monomers should not dissolve or plastici/c die polymer from which the fibers arc made. f2i the heat generated during the polymerization and contraction prior to the formation of new interpenetrating polymer should be minimized and ( ) the polymerization should not occur within the lumen (and hence cause plugging of the fiber). One drawback of such fibers is brittleness. 

The fragmentation patterns and characteristic fragment ions for the carotenoids observed in FAB-MS and LSIMS tandem mass spectra are also observed in the tandem mass spectra obtained following ESI (see Basic Protocol 4), APCI (see Basic Protocol 5), and other methods. A detailed account of structure determination of carotenoids using FAB ionization with CID and MS/MS is presented in van Bree-men el al. (1995). Finally, another advantage of MS/MS is that matrix ions formed during FAB-MS or LSIMS, and any other contaminating ions, are eliminated, which simplifies interpretation of the mass spectrum. 

Whether using FAB-MS, LSIMS, or MALDI, matrix ions might interfere with the... 

Receptor Action In Analytical Membranes. This work has Identified the Importance of some of the relevant parameters associated with organized lipid matrix ion conduction. Receptors embedded In these matrices should function to perturb one or more of the three compartmenta 1 zones of the BLM. A number of features significant to electrochemical receptor operation follow ... 

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