Mass spectrometry direct probe method

Phenyl Mercuric Chloride, An organomercury compound was believed responsible for a large fish kill in Boone Lake, Tennessee in 1968. Residue obtained from empty, unlabeled drums found in the lake was analyzed by MS using the direct probe method. Mercury has six isotopes of enough abundance to be observed by mass spectrometry (5). The mercury isotope pattern was observed in the molecular ion region of the spectrum (M" 314, Figure 3) where it is distorted by the chlorine isotope... 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

In an acetone extract from a neoprene/SBR hose compound, Lattimer et al. [92] distinguished dioctylph-thalate (m/z 390), di(r-octyl)diphenylamine (m/z 393), 1,3,5-tris(3,5-di-f-butyl-4-hydroxybenzyl)-isocyanurate m/z 783), hydrocarbon oil and a paraffin wax (numerous molecular ions in the m/z range of 200-500) by means of FD-MS. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out (Chapter 2). The method of Dinsmore and Smith [257], or a modification thereof, is normally used. Mass spectrometry (and other analytical techniques) is then used to characterise the various rubber fractions. The mass-spectral identification of numerous antioxidants (hindered phenols and aromatic amines, e.g. phenyl-/ -naphthyl-amine, 6-dodecyl-2,2,4-trimethyl-l,2-dihydroquinoline, butylated bisphenol-A, HPPD, poly-TMDQ, di-(t-octyl)diphenylamine) in rubber extracts by means of direct probe EI-MS with programmed heating, has been reported [252]. The main problem reported consisted of the numerous ions arising from hydrocarbon oil in the recipe. In older work, mass spectrometry has been used to qualitatively identify volatile AOs in sheet samples of SBR and rubber-type vulcanisates after extraction of the polymer with acetone [51,246]. 

Mass spectrometry combines exquisite sensitivity with a precision that often depends more on the uncertainties of sampling and sample preparation than on those of the method itself. Mass spectrometry is a supreme identification and recognition method in polymer/additive analysis through highly accurate masses and fragmentation patterns quantitation is its weakness. Direct mass spectrometry of complex polymeric matrices is feasible, yet not often pursued. Solid probe ToF-MS (DI-HRMS) is a breakthrough. Where used routinely, mass spectrometrists are usually still in charge. At the same time, however, costs need to be watched. 

If we consider only a few of the general requirements for the ideal polymer/additive analysis techniques (e.g. no matrix interferences, quantitative), then it is obvious that the choice is much restricted. Elements of the ideal method might include LD and MS, with reference to CRMs. Laser desorption and REMPI-MS are moving closest to direct selective sampling tandem mass spectrometry is supreme in identification. Direct-probe MS may yield accurate masses and concentrations of the components contained in the polymeric material. Selective sample preparation, efficient separation, selective detection, mass spectrometry and chemometric deconvolution techniques are complementary rather than competitive techniques. For elemental analysis, LA-ICP-ToFMS scores high. 

One problem with methods that produce polycrystalline or nanocrystalline material is that it is not feasible to characterize electrically dopants in such materials by the traditional four-point-probe contacts needed for Hall measurements. Other characterization methods such as optical absorption, photoluminescence (PL), Raman, X-ray and electron diffraction, X-ray rocking-curve widths to assess crystalline quality, secondary ion mass spectrometry (SIMS), scanning or transmission electron microscopy (SEM and TEM), cathodolumi-nescence (CL), and wet-chemical etching provide valuable information, but do not directly yield carrier concentrations. 

There are several methods available to probe the actual interphase to demonstrate the existence of interpenetrating networks directly. Among these techniques are depth profiling by SIMS (secondary ion mass spectrometry) or SNMS (sputtered neutral mass spectrometry), and the use of X-ray photoelectron spectroscopy (XPS) depth profiling or Auger electron spectroscopy (AES) depth profiling. 

The ionization methods reported for IMS included MALDI [41,76-80], Secondary Ion Mass Spectrometry (SIMS) [19, 81-86], Matrix-enhanced (ME)-SIMS [87, 88], Desorption Electrospray Ionization (DESI) [89-99], Nanostructure Initiator Mass Spectrometry (NIMS) [100-102], Atmospheric Pressure Infrared MALDI Mass Spectrometry (AP-IR-MALDI-MS) [103], Laser Ablation-inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) [104-106], Laser Desorption Postionization (LDPI) [107], Laser Ablation Electrospray Ionization Mass Spectrometry (LAESI) [108, 109], and Surface-assisted Laser Desorption/ioniza-tion Mass Spectrometry (SALDI) [110-112], Another method was called probe electrospray ionization (PESI) that was used for both liquid solution and the direct sampling on wet samples. 

The determination of TCDD in the final extract was by high resolution mass spectrometry multiple scan averaging. No gas chromatography was employed the sample was vaporized via the direct insertion probe (12.13). This method is inherently more prone to interferences than GC-MS (17.18) the response observed may not be due to TCDD. The results do serve as an indication, however, that if TCDD is present in human milk it is at 1 ppt or less. Thus any methods developed must be capable of this detection limit. 

Frequently industrial hygiene analyses require the identification of unknown sample components. One of the most widely employed methods for this purpose is coupled gas chromatography/ mass spectrometry (GC/MS). With respect to interface with mass spectrometry, HPLC presently suffers a disadvantage in comparison to GC because instrumentation for routine application of HPLC/MS techniques is not available in many analytical chemistry laboratories (3). It is, however, anticipated that HPLC/MS systems will be more readily available in the future ( 5, 6, 1, 8). HPLC will then become an even more powerful analytical tool for use in occupational health chemistry. It is also important to note that conventional HPLC is presently adaptable to effective compound identification procedures other than direct mass spectrometry interface. These include relatively simple procedures for the recovery of sample components from column eluate as well as stop-flow techniques. Following recovery, a separated sample component may be subjected to, for example, direct probe mass spectrometry infra-red (IR), ultraviolet (UV), and visible spectrophotometry and fluorescence spectroscopy. The stopped flow technique may be used to obtain a fluorescence or a UV absorbance spectrum of a particular component as it elutes from the column. Such spectra can frequently be used to determine specific properties of the component for assistance in compound identification (9). 

Field ionization (FI) is a method that uses very strong electric fields to produce ions from gas-phase molecules. Its use as a soft ionization method in organic mass spectrometry is principally due to Beckey [8], Like El or Cl, FI is only suitable for gas-phase ionization. Therefore, the sample is introduced into the FI source by the same techniques that are commonly used in El and Cl sources, for example using a direct probe that can be heated or the eluent from a gas chromatograph. 

One of the most significant developments in mass spectrometry in the recent years is the introduction of a new class of ionization methods where samples in either solid or liquid state can be directly ionized in their native environment under ambient conditions (rather than inside a mass spectrometer) without any sample preparation. This new class of ionization methods is often referred to as ambient ionization methods [1,2], Because these methods generally ionize analytes on the surface or near the surface of the samples at atmospheric pressure, they have also been called atmospheric pressure surface sampling/ionization methods or direct/open air ionization methods [3], Since the first reports on ambient ionization with desorption electrospray ionization (DESI) [4] and direct analysis in real time (DART) [5], numerous reports have been published on the applications of these new ionization methods as well as the introduction of many related ambient ionization methods such as desorption atmospheric pressure chemical ionization (DAPCI) [6], atmospheric solid analysis probe (ASAP) [7], and electrospray-assisted laser desorption/ionization (ELDI) [8], Recently, two reviews of the various established and emerging ambient ionization methods have been published [2,3],... 

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