Additive determination, laser probe

Two-step laser mass spectrometry has been employed by Zenobi et al. [783] for direct spatially resolved in situ analysis of a variety of additives in different polymers cfr. also Chp. 3.4.3). Sheng et al. [784] have proved that direct laser probe FTMS analysis of industrial samples can rapidly determine... 

The laser-based methods discussed thus far can be used to probe surface and interstitial contaminants as well as for the direct determination of additives in a complex matrix. It is now common to have a CCD camera and video display that gives a microscopic view of the sample when it is in the mass spectrometer. This allows contaminants, defects, and areas of interest to be observed and manipulated while under the probing beW of the laser (25). A number of industrial examples have I ved that direct Laser Probe FT/MS analyses can rapidly determine many additives directly, even when the combination of laborious classical wet chemical techniques with other modem instrumental methods have proved difficult and time consuming. 

Figure 7 illustrates the direct determination of two antioxidants in a cross-linked polymer. The protonated molecule is observed for Tinuvin 900 and the odd-electron molecular ion is observed for Tinuvin 770. In this example, a carbon dioxide laser was used to ablate the sample, and following the ablation step an electron beam was turned on to ionize the ablated materials. Exact mass measurements confirmed the identity of the two additives present at about 2%. Each laser shot ablates very small quantities of the sample producing a mass spectrum as illustrated in Figme 7. The molecular ion signals observed correspond to the detection of about 30 picomoles of additive in each laser ablation event. In some extreme cases, we have determined detection sensitivities that are sub-attomolar in particular when determining an UV absorbing surface species witii an UV laser probe. Table II illustrates that a wide variety of additives have been determined by Laser Probe FT/MS. 

In addition to the surface/interface selectivity, IR-Visible SFG spectroscopy provides a number of attractive features since it is a coherent process (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region, (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29-32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33-37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

Another LDI instrument that was similar in principle to LAMMA was developed by Perchalski (1985) that featured the additional selectivity of two stages of mass analysis provided by a triple quadrupole mass spectrometer (QqQ). The LDI QqQ was shown to have potential for use as a probe-type analyzer for molecular analysis of mixtures, as demonstrated by the detection of a mixture of nine antiepileptic drugs by monitoring the precursor ion/product ion pair for each drug (Perchalski et al., 1983). The LDI—QqQ, however, was determined to be too slow to adequately characterize molecules ionized by cationization or anionization after desorption by a single-shot laser. Also, the vaporization/ionization process on the LDI—QqQ was unable to ionize polar, nonvolatile, and/or thermally unstable molecules (Perchalski, 1985). 

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