Direct spectrometric analysis

Applications Specific applications of the direct spectrometric analysis methods of solid samples of Table 8.36 are given under the specific headings. One investigation that is practically only possible by direct solids analysis is checking the homogeneity of polymers [136,137] this is of significance for reference materials and for quality control. A method for the assessment of microhomogeneity should meet various requirements [223] ... 

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

Table 7.83 lists the main characteristics of TLC-FAB-MS/LSIMS. A key difference between EI/CI and FAB/LSIMS/LD is the fact that sampling in FAB and LSIMS is from a specified location that corresponds to the impact footprint of the primary particle beam. The natural compatibility of FAB, LSIMS and LD with the direct mass-spectrometric analysis of TLC plates is readily apparent. Most mass-spectrometric measurements are destructive in nature, but FAB and LSIMS are surface-sensitive techniques in which the material actually consumed in the analysis is sputtered only from the top few microns of the sample spot. The underlying bulk is not affected, and can be used for further probing. The major limitation of TLC-FAB depends on the capability of the compounds to produce a good spectrum. 

Krishnamurthy, T. Ross, P. L. Rapid identification of bacteria by direct matrix-assisted laser desorption/ionization mass spectrometric analysis of whole cells. Rapid Commun. Mass Spectrom. 1996,10,1992-1996. 

Basile,F. Beverly,M. B. Abbas-Hawks,C. Mowry,C. D. Voorhees,K. J. FIadfield, T. L. Direct mass spectrometric analysis of in situ thermally hydrolyzed and methylated lipids from whole bacterial cells. Anal. Chem. 1998, 70,1555-1562. 

Xu, M. Voorhees, K. J. Hadfield,T. L. Repeatability and pattern recognition of bacterial fatty acid profiles generated by direct mass spectrometric analysis of in situ thermal hydrolysis/methylation of whole cells. Talanta 2003,59, 577-589. 

Lee, S.W., Berger, S.J., Martinovic, S., Pasa-Tolic, L., Anderson, G.A., Shen, Y., Zhao, R., Smith, R.D. (2002). Direct mass spectrometric analysis of intact proteins of the yeast large ribosomal subunit using capillary LC/FTICR. Proc. Natl. Acad. Sci. USA 99, 5942-5947. 

Chapters 3 6 deal with direct mass spectrometric analysis highlighting the suitability of the various techniques in identifying organic materials using only a few micrograms of samples. Due to the intrinsic variability of artefacts produced in different places with more or less specific raw materials and technologies, complex spectra are acquired. Examples of chemometric methods such as principal components analysis (PCA) are thus discussed to extract spectral information for identifying materials. 

Neomycin is insufficiently volatile for direct mass spectrometric analysis. To overcome this problem Inouye- - prepared the volatile N-salicylidene Schiff s base, the M.S. of which, however, did not exhibit a peak for the molecular ion. To observe the molecular ion it was necessary to use the o-trimethylsilyl ether of the N-salicylidene Schiff s base. The spectrum of N-salicylidene neomycin was found to be dependant on the ion-chamber temperature indicating that thermal decomposition plays a significant part in the fragmentation process. 

Ayrton J. et al., 1998. Optimisation and routine use of generic ultra-high flow-rate liquid chromatography with mass spectrometric detection for the direct online analysis of pharmaceuticals in plasma. J Chromatogr A 8282 199. 

Analysis time is typically of the order of minutes to hours depending on the sample. Normally the time spent in actual AMS analysis is not the constraining factor, but rather sample purification prior to the spectrometric analysis. Accelerator mass spectrometers are space demanding facilities that typically occupy hundreds of square meters. Normally, dedicated personnel operate the device. Considerable effort is directed into refining the methods to allow operation by smaller, less costly facilities. 

T. Krishnamurthy and P. L. Ross. Rapid Identification of Bacteria by Direct Matrix-aAssisted Laser Desorption/Ionization Mass Spectrometric Analysis of Whole Cells. Rapid Commun. Mass Spectrom., 10(1996) 1992-1996. 

A second method uses permethylation of the dephosphated (48% aqueous HF, 48 h, 4°C) and 2H-reduced fipid A. This approach allowed the assignment of amide-bound fatty acids linked to GlcN(I) and GlcN(II), as well as the identification of the backbone structure as a HexpN disaccharide (85). Mass-spectrometric analysis of the products was performed by using either a short g.l.c. column (0.3 X 5 cm) or by direct insertion-probe analysis (87). In the case of C. violaceum (85), the mass spectra obtained from the permethyl-ated HexpN disaccharide bearing attached TV-methylacyl residues revealed unequivocally that both amino groups carried 12 0(3-OH). 

Beverly, M.B. Voorhees, K.J. Hadfield, T.L. Direct Mass Spectrometric Analysis of Bacillus Spores. Rapid Commun. Mass Spectrom. 1999,13, 2320-2326. 

A gas chromatograph (GC) can be used for the chromatographic separation of volatile analytes in complex mixtures prior to mass spectrometric analysis. This becomes especially advantageous if the GC elutes directly ( online ) into the ion source of a mass spectrometer, so-called GC-MS coupling. [63-65] Packed GC columns with a high flow can be connected via a jet-separator, but these are almost out of use at present. [66] Capillary columns provide flow rates in the order of a few milliliters per minute, therefore their back end can be connected directly at the entrance of the ion volume. 

Harrison KA, Murphy RC. 1996. Direct mass spectrometric analysis of ozonides application to unsaturated glycero-phosphocholine lipids. Anal Chem 68 3224. 

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