Mass spectrometry sample purity requirements

The control of materials purity and of environmental conditions requires to implement physico-chemical analysis tools like ESC A, RBS, AUGER, SEM, XTM, SIMS or others. The principle of SIMS (Secondary Ion Mass Spectroscopy) is shown in Eig. 31 an ion gun projects common ions (like 0+, Ar+, Cs+, Ga+,. ..) onto the sample to analyze. In the same time a flood gun projects an electron beam on the sample to neutralize the clusters. The sample surface ejects electrons, which are detected with a scintillator, and secondary ions which are detected by mass spectrometry with a magnetic quadrupole. 

This interpretation was proved correct by considering the oxidation of a sample of diphenylmethane that had an isotopic purity of 97.0% a,a-dideuterio and 2.7% a-deuterio by mass spectrometry. The oxidation rate observed after the initial 15-second period (see Figure 2), during which the undeuterated and monodeuterated material were destroyed, yielded a second-order rate constant, ki = 0.0148 mole"1 per second. There is thus an appreciable isotope effect ku/kD of about 6 in the ionization of diphenylmethane by potassium ferf-butoxide in DMSO(80%)-tert-butyl alcohol (20% ) at 25°C. This compares with a value of fcH/ D of 9.5 reported for the ionization of triphenylmethane (16). The observation of primary isotope effects of this magnitude requires that the protonation of the diphenylmethide ion by tert-butyl alcohol in DMSO solution does not proceed at the diffusion rate which would, by the principle of microscopic reversibility, require the absence of an isotope effect in the deprotonation step. 

Authenticity evaluation has recently received increased attention in a number of industries. The complex mixtures involved often require very high resolution analyses and, in the case of determining the authenticity of natural products, very accurate determination of enantiomeric purity. Juchelka et al. have described a method for the authenticity determination of natural products which uses a combination of enantioselective multidimensional gas chromatography with isotope ratio mass spectrometry (28). In isotope ratio mass spectrometry, combustion analysis is combined with mass spectrometry, and the 13C/12C ratio of the analyte is measured versus a C02 reference standard. A special interface, employing the necessary oxidation and reduction reaction chambers and a water separator, was used employed. For standards of 5-nonanone, menthol and (R)-y-decalactone, they were able to determine the correct 12C/13C ratios, with relatively little sample preparation. The technical details of multidimensional GC-isotope ratio MS have been described fully by Nitz et al. (29). A MDGC-IRMS separation of a natural ds-3-hexen-l-ol fraction is... 

This paper is the only one in the liquid chromatography portion of this symposium which will attempt to deal with chromatography specifically from the viewpoint of the pesticide metabolism chemist. A residue analyst knows what compound he must analyze for, and develops his method with the properties of that substance in mind. On the other hand, the pesticide metabolism chemist has a different problem. At the conclusion of the treatment, exposure, and harvest phases of a radiolabeled metabolism study, he divides his material into appropriate samples, and extracts each sample with selected solvents to obtain the radioactive materials in soluble form. Typically these extracts consist of low levels (ppm) of carbon-14 labeled metabolites in a complicated mixture of normal natural products from the plant, animal, or soil source. The identity of each metabolite is unknown, and each must be isolated from the natural background and from other labeled metabolites in sufficient quantity and in adequate purity for identification studies, usually by mass spectrometry. The situation is rather like looking for the proverbial "needle in the haystack" when one does not know the size, shape,or composition of the needle, or even how many needles there are in the stack. At this point a separation technique must be selected with certain important requirements in mind. 

The molecular formula of a molecule can also be defined by high resolution mass spectrometry (hrms). The observed mass for the molecular ion or pseudo molecular ion must normally be within 5ppm of the calculated mass for El (electron impact) measurements, or within lOppm for Cl (chemical ionization) measurements. It is important to note that high resolution mass spectrometry confirms that some molecules of a particular molecular formula are present in the sample, but does not give any indication of purity. Some other evidence of compound purity will therefore be required. 

The general topic of sample purity, which is so important in any structural study, has already been addressed in Section 1.3. Here we are concerned with the preparation and handling of non-standard samples. This includes compounds that may exist for only a fraction of a second, and so will require us to design our experiments with due care so that data can be collected in a very short window of time. We consider three general themes that are applicable to a range of structural chemistry techniques. Note that the very nature of mass spectrometry measurements involves the generation of many unstable species from a stable sample, but our discussion of these technique-specific methods is restricted to Section 11.2.1. We then discuss some other specialized sample preparation techniques, including the treatment of non-volatile samples, which have to be vaporized, and ways in which to control the external parameters of temperature and pressure. 

Sample purity is definitely an issue, because GED always observes the superposition of everything interacting with the electron beam. In many cases, multi-component vapors have been successfully analyzed, and even the relative amounts of the individual components refined, but this always requires independent knowledge of what molecular species are present in the vapor. In this respect, combined experiments, in which mass spectrometry is used to determine the composition of the gas emerging from the nozzle, have been very successful. 

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