Detection in SIMS

Liquid chromatography-thermospray-mass spectrometry has been successfully applied for the detection/ confirmation of nicarbazin residues in chicken tissues using negative-ion detection in SIM mode. Liquid chromatography-thermospray-mass spectrometry in SIM mode has also been used for the quantification of residues of moxidectin in cattle tissues and fat, ° and nitroxynil, rafoxanide, and levamisole in muscle. Confirmatory methods based on LC-TSP-MS have been further reported for the determination of penicillin G, cephapirin, and various penicillin derivatives " in mUk. Comparative evaluation of the confirmatory efficiency of LC-TSP-MS and LC-TSP-MS-MS in the assay of maduramycin in chicken fat showed the former approach to be marginally appropriate, whereas the latter is highly efficient. Tandem LC-MS-MS has also been successfully applied in the analysis of residues of chloramphenicol in milk and fish. ... 

The authors of the papers cited below describe different applications of both ES and APCI mass spectrometry. The LODs reported in these papers differ and are also hard to compare because of different ways of acquiring mass spectral data. Some report LODs as the smallest total amount detected, for example picogramme or femtomol, whereas others express LODs as the concentration injected (pg pl"0- Furthermore, the solvent flow rates differ substantially, from 1 plmin" to lOOOplmin" Finally, both the mass span over which data are collected and the total time of acquiring the data differ considerably. Typically, at a solvent flow rate of 400 pi min" and using gradient LC separation (20 pi loop) with mass spectrometric detection in SIM mode, a LOD of 5-10 fmol pr is achieved. Use of very low solvent flow rates (1 pi min" and the acquisition of data over, say, 3 min may result in lower LODs. This approach is very useful when the amount of sample is limited. 

Using masspectrometric detection in SIM mode for ions with m/z 77 and/or 92, detection and determination of picomol concentrations of F is possible in different analytical samples. 

Microanalytical LC-MS methods for the analysis of BR-derived boronates have also been developed, using either atmospheric pressure chemical ionization (APCl) or electrospray ionization (ESI). APCl-based LC-MS has been used to analyze naphthaleneboronate derivatives of BRs in this case, optimal results were obtained by using a reverse-phase HPLC acetonitrile-water gradient to elute a CIS column [67]. Typical ions observed in the positive-ion spectra of the naphthaleneboronates included a pseudomolecular ion [M -i- H]" and a fragment ion [M -i- H-H20]. The most abundant ion from the fragmentation of BL derivatives is the parent ion, while the [M -I- H-H20] ion is the most abundant in the mass spectra of CS, teasterone and typhasterol. Full-scan mass spectra were readily obtained from 400 ng of free BRs, whereas the limit of detection in SIM mode was around 2 ng. The most... 

The primary advantage associated with PCs lies in their robusmess and the fact that these detectors have essentially no upper counting limit (the limit typically stipulated is a result of the associated electronics). As a result, these are the only detectors used for both primary and secondary ion detection in SIMS instruments. As amplification is not carried out within these units, a current of > 1000 ions per second must strike the detector if a signal is to be recorded. Note 626 cps equates to lxlO A and the lower detection limit of the most sensitive electrometers is around a factor of two times this value. The practical lower limit of these detectors is, however, signal to noise dependent, hence, will also be a function of, among other things, the dwell time applied. 

When there is no concern about the matrix (fragrance concentrates and extracts, alcoholic perfumery) the complexity lies in the huge number of perfume constituents, which may interfere with PASs. GC with MS detection in SIM mode gives the most satisfactory results for routine determination. Among more sophisticated multidimensional approaches, GC-MS/MS lowers possible false positives and negatives, whereas... 

Ar, Cs, Ga or other elements with energies between 0.5 and 10 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection of atoms, ions and multi-atomic clusters (figure Bl.25.8). In SIMS, secondary ions (positive or negative) are detected directly with a mass spectrometer. 

Two different mixtures of peptides and alkaloids (qv) have been analy2ed by ce/uv/ms using sims to determine whether this technique can detect trace impurities in mixtures (85). The first mixture consisted of two bioactive peptide analogues, which included Lys-bradykinin (kahidin) and Met-Lys-bradykinin. The presence of 0.1% Lys-bradykinin was detected by sim ce/ms but not by ce/uv at 0.1% level as it migrated from the capillary column prior to the main component, Met-Lys-bradykinin. The second mixture consisted of two antibacterial alkaloids, berberine and palmitine. The presence of 0.15% palmitine was detected by ce/uv and sim ce/ms at 0.15% level as it migrated from the capillary column, following the main component berberine. This technique can provide a complementary technique for trace components in such sample mixtures. 

Moreover, an absorption band near 1375 cm-i is detected and it is assigned to the CH bending vibration present in cellulose and hemicellulose chemical structures (Sim et al., 1998). The prominent band at 1044 cm-i is also associated with hemicelluloses and is attributed to the C-OH bending. Finally, a sharp band at 897 cm-i, which is typical of b-glycosidic linkages between the sugar units in hemicelluloses, was detected in the anomeric region (Sun et al., 2005). 

We have also carried out preliminary experiments in which we have detected the laser desorption of ethylene, cyanogen, methanol, and benzene from the Pt(s)[7(111) x (100)] surface. These spectra are shown in Figure 9. In the experiments involving ethylene, cyanogen, and methanol only neutral species are desorbed. In the case of benzene we observe the molecular parent ion in the absence of the electron beam. We believe that this is due to resonance multiphoton ionization of the benzene by the laser after desorption (resonance multiphoton ionization of benzene is very efficient with 249 nm radiation). These spectra are in marked contrast to the results of SIMS experiments which produce a wide variety of complex metal-adsorbate cluster ions. In the case of ethylene, our experiments were performed at 140 K, and under these conditions ethylene is known to be a molecular x-bonded species on the surface. In SIMS under these conditions the predominant species is CH (15)t but in the laser desorption FTMS experiments neutral ethylene is the principal species detected at low laser power. 

Because of the complexity of sample preparation, backscatter measurement geometry is the choice for an in sim planetary Mossbauer instrument [327]. No sample preparation is required, because the instrument is simply presented to the sample for analysis. Both 14.41 keV y-rays and 6.4 keV Fe X-rays are detected simultaneously. 

Tables 6.27 and 6.31 show the main characteristics of ToF-MS. ToF-MS shows an optimum combination of resolution and sensitivity. ToF-MS instruments provide up to 40000 spectra s-1, a mass range exceeding 100000 (in principle unlimited), a resolution of 5000, and peak widths as short as 200 ms. This is better than quadruples and most ion traps can handle. Unlike the quadrupole-type instrument, the detector is detecting every introduced ion (high duty factor). This leads to a 20- to 100-times increase in sensitivity, compared to QMS used in scan mode. The mass range increases quadratically with the time range that is recorded. Only the ion source and detector impose the limits on the mass range. Mass accuracy in ToF-MS is sufficient to gain access to the elemental composition of a molecule. A single point is sufficient for the mass calibration of the instrument. ToF mass spectra are commonly calibrated using two known species, aluminium (27 Da) and coronene (300 Da). ToF is well established in combination with quite different ion sources like in SIMS, MALDI and ESI.

Kottegoda S, Aoto PC, Sims CE, Allbritton NL (2008) Biarsenical—tetracysteine motif as a fluorescent tag for detection in capillary electrophoresis. Anal Chem 80 5358-5366... 

In practice, some precautions are necessary. The first arises directly from the physical principle of the technique. As only the first few nanometres of the surface are analysed, ToF-SIMS is greatly affected by surface pollution. A prolonged contact of the sample with the atmosphere can lead to disturbing pollution. Poly(dimethylsiloxane)s are classical pollutants which can be detected in numerous studies [Sodhi 2004]. As a consequence, fresh prepared surfaces are recommended for ToF-SIMS analyses. 

The presence of characteristic peaks from palmitic and stearic acids is consistent with the hypothesis of the use of oil as binding media. The lack of any characteristic ions issued from egg tempera means that ToF-SIMS does not allow detection of egg tempera in this sample. However, it could be present but is not detected due to high degradation occurring in very old egg tempera. The presence of short chain fatty acids, which are not detected in the new reference sample, is attributed to oil ageing. The distribution of fatty acid ions in the cross-section is well correlated with the distribution of lead. The ions are not detected in the ground layer (Figure 15.9). 

In addition to these numerous results, two other points are discussed by the authors fatty acid speciation and oil identification. These two aspects are developed in another publication written by the same authors [Keune et al. 2005]. The fatty acid speciation is based on the positive ion ToF-SIMS analysis and aims to prove if the fatty acids detected exist as free fatty acids, ester bound fatty acids or metal soaps. On account of the study of different standards, it is shown that when free fatty acids are present, the protonated molecular ion and its acylium ([M-OH]+) ion are detected. In cases of ester-bound fatty acid only the... 

With the assumption that all of the deuterium detected by SIMS in the near- surface layer is incorporated in platelets, this yields —350 deuterium atoms per platelet, which corresponds to one or two D atoms per Si—Si bond. 

Neutralization of zinc acceptors in InP by hydrogen occurs also in nonintentionally hydrogenated materials grown by organometallic vapor phase epitaxy (Antell et al., 1988 Cole et al., 1988). Levels of hydrogen comparable to the zinc concentration have been detected by SIMS experi ... 

PAHs are widely distributed in the environment as evidenced by their detection in sediments, soils, air, surface waters, and plant and animal tissues. However, the ecological impact of PAHs is uncertain. PAHs show little tendency for bioconcentration despite their high lipid solubility (Pucknat 1981), probably because most PAHs are rapidly metabolized. Sims and Overcash (1983) list a variety of research needs regarding PAHs in soil-plant systems. Specifically, research is needed to establish the rates of PAH decomposition in soils the soil PAH levels above which PAH constituents adversely affect the food chain and enhancement factors that increase degradation rates of PAHs, especially PAHs with more than three rings. Once these factors have been determined, PAH disposal into soils may become feasible at environmentally nonhazardous levels. 

Figure 4.1 The principle of SIMS. Primary ions with an energy between 0.5 and 10 keV cause a collisional cascade below the surface of the sample. Some of the branches end at the surface and stimulate the emission of neutrals and ions. In SIMS, the secondary ions are detected directly with a mass spectrometer, whereas in SNMS the secondary neutrals are ionized before they enter the mass spectrometer.

Figure 9.26 Cr 2p spectra and negative SIMS spectra of two model catalysts and a blank reference. The blank (bottom) shows only Si,0, fragments on a chromium-loaded catalyst CriCf fragments appear after thermal activation (Ar/02). If desorption of chromium is made impossible (in oxygen-free argon), Cr2Or clusters can also be detected. In combination, this is strong evidence that chromate anchors to the silica surface as a monomer (courtesy of P.C. Thiine and R. Linke, Eindhoven).

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