Retentate chromatography/mass spectrometry

In our particular application, we do not have an absolute method of calibration because the alkyl chain length affects the EO distribution retention. However, mass spectrometry would be an ideal third dimension. The automated combination of two-dimensional chromatography and mass spectrometry is the next step toward the future of simultaneous separation and identification of very complicated samples. 

Figure 2.2 shows the total ion current trace and a number of appropriate mass chromatograms obtained from the pyrolysis gas chromatography-mass spectrometry analysis of the polluted soil sample. The upper trace represents a part of the total ion current magnified eight times. The peak numbers correspond with the numbers mentioned in Table 2.1 and refer to the identified compounds. The identification was based on manual comparison of mass spectra and relative gas chromatographic retention times with literature data [34, 35] and with data of standards available. In some cases unknown compounds were tentatively identified on the basis of a priori interpretation of their mass spectra (labelled tentative in Table 2.1). 

We discussed the fundamentals of mass spectrometry in Chapter 10 and infrared spectrometry in Chapter 8. The quadrupole mass spectrometer and the Fourier transform infrared spectrometer have been adapted to and used with GC equipment as detectors with great success. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectrometry (GC-IR) are very powerful tools for qualitative analysis in GC because not only do they give retention time information, but, due to their inherent speed, they are also able to measure and record the mass spectrum or infrared (IR) spectrum of the individual sample components as they elute from the GC column. It is like taking a photograph of each component as it elutes. See Figure 12.14. Coupled with the computer banks of mass and IR spectra, a component s identity is an easy chore for such a detector. It seems the only real... 

A gas chromatography-mass spectrometry system is used to measure concentrations of target volatile and semivolatile petroleum constituents. It is not typically used to measure the amount of total petroleum hydrocarbons. The advantage the technique is the high selectivity, or ability to confirm compound identity through retention time and unique spectral pattern. 

Figure F2.4.1 Liquid chromatography/mass spectrometry (LC/MS) analysis of isomeric carotenes in a hexane extract from 0.5 ml human serum. Positive ion electrospray ionization MS was used on a quadrupole mass spectrometer with selected ion monitoring to record the molecular ions of lycopene, p-carotene, and a-carotene at m/z (mass-to-charge ratio) 536. A C30 HPLC column was used for separation with a gradient from methanol to methyl-ferf-butyl ether. The a -trans isomer of lycopene was detected at a retention time of 38.1 min and various c/ s isomers of lycopene eluted between 27 and 39 min. The all-frans isomers of a-carotene and P-carotene were detected at 17.3 and 19.3 min, respectively.

This method can be used when the enantiomers of interest are not coeluting with other compounds in the sample and when accurate quantitative information is not the highest priority of the analysis. The sample will have been prepared by an extraction method selected from those in unitgi.i and should have a concentration of 50 to 100 ppm. The identity of the components of the sample should be known from gas chromatography-mass spectrometry (GC-MS) together with their retention indices on the achiral stationary phase. Additional sample cleanup procedures may be needed to ensure the optimum results that are evaluated below ... 

TJ Schmidt, I Merfort, G Willuhn. Gas chromatography-mass spectrometry of flavonoid aglycones. II. Structure-retention relationships and a possibility of differentiation between isomeric 6- and 8-methoxyflavones. J Chromatogr 669 236-240, 1994. 

Both the liquid and gas products were analyzed by gas chromatography. The column for the liquid analysis was 20% Apiezon L on 60-80 mesh Chromosorb P. The column measured 1/4 inch by 7 feet. The gas analysis utilized a 1/4 inch by 10 foot column of 60-80 mesh Chromosorb 102. Temperature programming was required in both analyses. Identification of the GC peaks was based on retention time of pure compounds when these were available. In addition, two of the samples were analyzed by combined gas chromatography-mass spectrometry. By comparing the observed mass spectrometer fragmentation patterns with tabulated patterns it was possible to identify virtually every component in the product. Further details are available in the theses by Wu (23) and Early (J+). 

Palmblad M, Ramstrom M, Markides KE, Hakansson P, Bergquist J. Prediction of chromatographic retention and protein identification in liquid chromatography/mass spectrometry. Anal Chem 2002 74(22) 5826-5830. 

Kende, A., Z. Csizmazia, T. Rikker, et al. 2006. Combination of stir bar sorptive extraction—retention time locked gas chromatography-mass spectrometry and automated mass spectral deconvolution for pesticide identification in fruits and vegetables. Microchem. J. 84 63-69. 

Compound identifications were made by combined gas chromatography-mass spectrometry (GC-MS) based on relative retention times and mass spectral interpretations. The instrument used was a Finnigan 5100 computerized GC-MS system equipped with a 50 m x 0.32 mm i.d. fused silica capillary coated with CP Sil 8 CB (0.25 jim film thickness). Helium was used as carrier gas and the temperature program was as follows 110°C (2 min)- 3°C/min - 320°C. 

Thompson et al. [113] emphasise that even the use of two dissimilar gas chromatographic columns does not ensure irrefutable compound identification. For example, if the retention characteristics of a given peak obtained from two dissimilar columns suggest the possibility of the presence of a compound which appears wholly out of place in a specific sample, further confirmation is clearly indicated by such techniques as specific detectors, coulometry, p values, or gas chromatography-mass spectrometry or thin layer chromatography. 

Both fractions obtained from crackers extracted less than one week after baking had strong cracker-like aromas. The Freon 113 fraction had a cracker, roasted grain, cooked rice aroma while the ethyl acetate fractions had a cracker, sweet baked good, burnt butter aroma. A preliminary analysis of the two fractions by gas chromatography-mass spectrometry on a 25 m by. 22 mm methyl silicone column between 700 and 1800 retention indices showed the compounds listed in Table 2. 

The pyrolysate contained an appreciable amount of product of molecular weight 196, which could be either xanthone or the lactone 30. Directly coupled gas chromatography-mass spectrometry identified it as 30. The retention time and mass spectrum of this product agreed closely with those of an authentic sample of 30, synthesized by the method of Graebe and Schestakow (1895), which was clearly distinguishable from xanthone on both counts. The formation of 30 parallels that of fluorenone from phthalic anhydride and benzene, observed by Fields and Meyerson (1965). 

Combined gas chromatography-mass spectrometry (GC-MS) takes advantage of the separating power of the gas chromatograph and the identification power of mass spectrometry. The gas chromatograph separates the components and provides retention time data and the mass spectrometer identifies the components. The combined instrumentation has the potential to provide very useful information in FDR casework. Figure 16.5 illustrates a quadrupole mass spectrometer. 

The analytical data in the OCAD is derived from four different techniques. These are nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR), gas chromatography/mass spectrometry (GC/MS), and gas chromatography retention indices (GC(RI)). With a few exceptions, the OCAD contains only data of compounds that are listed in the schedules of the CWC and their derivatives of BSTFA and dimercaptotoluene. 

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