The addition of a mass spectrometer

Basic Protocols 3 through 9 are primarily useful as quality control measures. They are rapid, usually within 30 min, given reagent preparation. The results are used to monitor the quality of a process. These results support established values for high quality citrus oil. Basic Protocols 1 and 2 are more involved and are better suited for research purposes. The equipment is more sensitive and also more expensive. Furthermore, the strength of the GC analysis can be enhanced by the addition of a mass spectrometer to identify either contaminates or unknown compounds present in a sample. 

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the usual method of choice for the separation of anthocyanins combined with an ultraviolet-visible (UV-Vis) or diode-array detector (DAD)(Hebrero et al., 1988 Hong et ah, 1990). With reversed-phase columns the elution pattern of anthocyanins is mainly dependent on the partition coefficients between the mobile phase and the Cjg stationary phase, and on the polarity of the analytes. The mobile phase consists normally of an aqueous solvent (water/carboxylic acid) and an organic solvent (methanol or acetonitrile/carboxylic acid). Typically the amount of carboxylic acid has been up to 10%, but with the addition of a mass spectrometer as a detector, the amount of acid has been decreased to as low as 1 % with a shift from trifluoroacetic acid to formic acid to prevent quenching of the ionization process that may occur with trifluoroacetic acid. The acidic media allows for the complete displacement of the equilibrium to the fiavylium cation, resulting in better resolution and a characteristic absorbance between 515 and 540 nm. HPLC separation methods, combined with electrochemical or DAD, are effective tools for anthocyanin analysis. The weakness of these detection methods is a lack of structural information and some nonspecificity leading to misattribution of peaks, particularly with electrochemical... 

MASS DIRECTED AUTOPREP 8.7.1 The addition of a mass spectrometer... 

To deal with this problem we set about coupling an autoprep system to a mass spectrometer [14]. The addition of a mass spectrometer enabled the system to become far more specific and by inputting the molecular weight, the system would collect only the compound(s) of interest. In the majority of cases, only one desired component is required to be collected so this means that one sample will prcxluce only one purified fraction, two samples will produce only two and similarly, a plate of 80 samples will produce only 80 purified fractions. A mass-directed preparative HPLC (MS-prep) instrument would therefore eliminate the re-analysis and recombination steps from the purification process. 

As mentioned earlier, the addition of a mass spectrometer to an autoprep system greatly reduces the number of collected fractions and consequently simplifies some of the purification problems inherent in large sample batches. Nonetheless, if a microtitre plate containing 80 samples is to be purified, the tracking of the fractionated samples is a major consideration to avoid confusion. Con.sequently, we decided to adapt the way we collected fractions to minimise these problems. 

The detector has to comply with three different criteria firstly, the spatial resolution must be sufficient to distinguish the diffraction orders, secondly, it has to be efficient and thirdly, it has to be selective. It must not detect any molecule in the vacuum chamber but the fullerenes. This is a challenging task without the addition of a mass spectrometer, since even at a background gas pressure of typically 10-8 mbar the density of air molecules exceeds the density of fullerenes by far. 

In addition to explaining verbally the concepts of mass spectrometry, it is also helpful to explain them visually. Two ideas utilized in newborn screening, for example, is the ability of a mass spectrometer to sort molecules by their mass and determine how many of these compounds are present. One illustration uses coins while another uses jelly beans. Instructions on how to prepare and present these experiments are shown below. 

Shortly afterwards, this work was extended by the incorporation of a mass spectrometer into the system, thus enabling on-line NMR and MS data to be obtained with on-line collection of the eluent for off-line FT-IR spectroscopy [22]. The incorporation of the mass spectrometer required the addition of a small proportion of ammonium acetate, dissolved in methanol, to the deuterated chloroform used as the eluent in order to promote the ionisation of the analytes. The inclusion of methanol and ammonium acetate to the solvent obviously introduced new signals into the NMR spectra, and in addition resulted in the loss of exchangeable protons from the analytes which had been observable when chloroform alone was used as the solvent. This work demonstrated the feasibility of multiple hyphenation ( hypernation ) but the off-line nature of the FT-IR data acquisition, with the inevitable delay inherent in offline analysis, represents a slight disadvantage. In addition, volatile components may well be lost as the solvent is evaporated. This can be a problem that, together with analyte instability, is exacerbated with such interfaces when reversed-phase eluents are used since these require heating in order to ensure removal of the solvent. 

Today, HPLC is the dominant analytical technique used for the analysis of most classes of compounds. The analyses can be carried out at room temperature and the collection of fractions for reanalysis or further manipulation is straightforward. The main reason for the slow acceptance of the HPLC technique for Upid analysis has been the detection system. Traditionally, HPLC used ultraviolet/visible (UV/vis) detection, which requires the presence of a chromophore in the analyte. Most lipid molecules do not contain chromo-phores and therefore would not be detected by UV/vis. Modern HPLC detection techniques, such as the use of a mass spectrometer as the detector, derivatization techniques to introduce chromophores, and the availability of pure solvents to reduce interference, have allowed HPLC to compete with and/or complement GC and other traditional methods of lipid analysis. In addition to analytical HPLC, preparative HPLC has been used extensively to collect pure samples of the lipids for the derivatization or synthesis of new compounds. 

Mass Spectrometers for Stress MS. The requirements of a mass spectrometer for stress MS are more stringent than those for conventional mass spectrometry. The events to be monitored are short-lived, typically < 1 sec, and the total amount of evolved compounds is small, between 10" and 10" g. In addition, a source vacuum housing versatile enough to accept readily the various devices for mechanically loading polymeric samples is required. The mass spectrometer must have a large, open ion source. 

Destructive vi. nondestructive-. Some detectors destroy the analyte as part of the process of their operation (e.g., by burning it in a flame, fragmenting it in the vacuum of a mass spectrometer, or by reacting it with a reagent). Others leave it intact and in a state where it may be passed on to another type of detector for additional characterization. 

Calibration of the instrument is the first essential step of a quantitative procedure. Like any other analytical instrument, the response of a mass spectrometer is not absolute and might deviate with time. In addition, the sample matrix has a variable influence on the mass spectrometry response. Calibration involves determination of the correlation between a known concentration of the analyte and the resulting mass spectrometry signal. In ideal situations, the sample and the analyte standards are both analyzed under identical experimental conditions. Depending on the levels of accuracy and precision that are required, the calibration might be performed by one of the methods described below. 

A similar situation exists for alloys where a component pressure is not measurable under the temperatures for measurable vapor pressures of other components. Examples are Hf in Ni-Al-Hf [85]), Cr in Mn-Cr [104], and rare earth (RE) in Mg-RE alloys [105]. The latter study was based on a Knudsen cell without the use of a mass spectrometer. Nonetheless the approach is applicable to a mass spectrometric study. Depending on the alloy system, several approaches can be taken. In some cases the effect of controlled additions of the low-pressure component on a fixed ratio of the measured elements is the only required information [100,102]. Albers et al. [85] and Zaitsev et al. [104] did a Gibbs-Duhem integration to obtain the activities of the low vapor pressure component. Pahlman and Smith [105] assumed Raoultian behavior in the terminal RE-Mg solution and moved across the phase diagram to derive the activity of the RE component in each two-phase region. 

Accurate quantitation in GC/MS requires the addition of a known quantity of an internal standard to an accurately weighed aliquot of the mixture (matrix) being analyzed. The internal standard corrects for losses during subsequent separation and concentration steps and provides a known amount of material to measure against the compound of interest. The best internal standard is one that is chemically similar to the compound to be measured, but that elutes in an empty space in the chromatogram. With MS, it is possible to work with isotopically labeled standards that co-elute with the component of interest, but are distinguished by the mass spectrometer. 

Direct pyrolysis in the ion source of a mass spectrometer (QMS) was used to analyse PE/(dicumylperoxide, Santonox R) and PVC/DIOP [259]. In-source PyMS is an analytical tool for fast analysis of flame retardants in unknown mixtures of polymers [223, 265], Heeren and Boon [224] used in-source filament pyrolysis FTMS for high-speed, broadband screening of additives in polymeric household appliances. 

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