Comparison of relative retentions

Figure 4. Comparison of relative retentions of halogenated hydrocarbons with DB-1 and DB-1701 stationary phases.

COMPARISON OF RELATIVE RETENTIONS AND FID AND ECD RESPONSES OF TESTOSTERONE AND CHOLESTEROL HALOGENATED ACYL DERIVATIVES [355]... 

FIGURE 4-7. Comparison of relative retention due to the polarity (hydrophobicity) of Cj8, phenyl, and cyano (CN) bonded phases. Column Nova Pak family (4/a) 3.9 mm ID x 15 cm. (a) C, phase, (b) Phenyl phase, (c) CN phase. Mobile phase— acetonitrile water, 35 65 flowrate 2 mL/min detection 254 nm sample 1. benzyl alcohol, 2. acetophenone, 3. p-tolualdehyde, 4. anisole. Nova-Pak CN is the least hydrophobic and therefore the least retentive, followed by the more hydrophobic Nova-Pak Phenyl, and Nova-Pak C18 with the most hydrophobicity and the longest retention times. 

Fig. 4-18. Comparison of relative retentions of amino acids on anion and cation exchangers.

The retention volume of a compound, determined as shown in Fig. 1 (see p. 97), is a characteristic property when all of the conditions of the separation can be duplicated exactly. To avoid slight differences that may be caused by minor variations in the operating conditions, retention volumes are usually made relative to that of a standard compound which is assigned a value of unity. An unknown may therefore be identified by comparison of its retention volume with that of an authentic sample, both made relative to the same standard and obtained under the same conditions. The condition most difficult to duplicate is the column packing, and this situation makes it unlikely that relative retention volumes reported can be reproduced with numerical exactitude. However, the relative orders of a series of such values should be reproducible, provided that the same liquid phase is used. It should be emphasized that identification by the comparison of relative retention volumes does not obviate the need for confirming the identification by conventional methods. Wherever possible, the separated components should be collected from the effluent gas-stream and be identified by infrared spectroscopy and by the preparation of... 

Assignment is by comparison of retention (comparison of relative retentions - the reference substance is the internal standard). Reproducibility 0.02 - 0.03 min. (applies to isothermal and programmed-temperature... 

When pure explosives are analyzed, a small aliquot of the sample is diluted with ethyl acetate, and identification is based upon the comparison of relative retention times with those of explosives in a standard solution analyzed both before and after the sample. The standard solution is prepared by the dilution of the 11 most common explosives (EGDN, NG, PETN, RDX, 2,4-2,6-3,4 DNT, TNT, o, m, and p nitrotoluene) in ethyl acetate. Also, a mixture of two retention references markers is coinjected with every sample and standard solution, and retention times are measured relative to these substances. The markers commonly used to provide reference peaks in GC analyses are 2 fluoro-5-nitrotoluene (FNT) and 2,6 dinitro-3,4,5-trimethyl-ferr-butylbenzene (MT). 

A solvent solution of the polymer is applied to the pyrolysis filament and pyrolysed at a controlled temperature. Comparison of relative retention time and peak height ratio data for the unknown with that for known polymers enables die unknown to be identified. 

Peaks are identified from absolute or relative retention times by comparison with data from previously run standards stored in RAM or in libraries on disk. To take account of the variability of retention times from successive runs, retention time windows are used. These are defined as being /R x% for a standard, the unknown being positively identified if its retention time falls within the specified range. The size of the window can be varied by the user to conform with the degree of certainty required. Reference peaks can be selected for the calculation of relative retention times or as internal standards in quantitative analysis (pp. 9, 114). 

Identification (ID) tests in Category IV require only specificity for their validation. Identification by HPLC usually involves comparison of the retention time (%) or relative retention time (RRT) of a sample and standard injection. The increasing use of photodiode array (PDA) detectors in HPLC methods also allows identification by comparison of UV spectra for standards and samples, in addition to retention characteristics. The information required for either ID test by HPLC can be gathered while performing any other HPLC method for a given sample. Identification tests are often incorporated into the assay method and the satisfactory completion of specificity for the assay will meet the requirements for ID as well. 

Chromatographic retention times are characteristic of the compounds they represent but are not unique. Coincidence of retention times of a test and a reference substance can be used as a feature in construction of an identity profile but is insufficient on its own to establish identity. Absolute retention times of a given compound vary from one chromatogram to the next. Comparisons are normally made in terms of relative retention, which is calculated by the equation... 

The simplest analytical information that can be obtained with the aid of FFF is the homogeneity of the sample or evidence for the presence of a compound of interest in the fractionated sample by the appearance of a peak in the expected interval of retention volume. In some cases, comparison of the retention volume and the peak shape of the investigated component with the peak shape of a reference sample can provide sufficient qualitative analytical information on sample purity and homogeneity. The peak areas in the fractogram can be used to evaluate quantitatively concentrations of the detected components provided that the relationship between detector response and concentration or quantity of the detected component is known. This relationship is usually determined by a calibration procedure. However some sample is lost in the void peak so that it is not possible to relate the detected concentration to that of the original sample consequently, concentration determinations can more advantageously serve to compare the relative concentrations of the fractionated components. 

GC-coupled mass spectrometry with electron ionization (EI-MS) or with electron capture, negative ionization (ECNI-MS), and GC with electron capture detection (ECD) have been the major techniques for the analysis of aryl methyl sulfone fractions isolated from tissue (Sect. 5.1). GC/ECD detection and identification of MeS02-PCBs relies on a comparison of GC retention times relative to authentic standards, which is dependent on the absence of co-eluting interfe-rents. GC/MS techniques have provided important structural information for MeS02-PCBs, especially in the absence of authentic standards. 

Determination of Relative Reaction Ratios of Olefins Comparison of relative reaction rates was done by gas chromatographic analysis of reaction mixtures with addition of a suitable internal standard such as biphenyl. A mixture of the olefin to be compared and norbornene (both olefins in excess) were readed with a defidency of ethyl mercaptoacetate at various temperatures. Relative retention times were compared with those of independently prepared and characterized mercaptoacetate-olefin adducts. Comparison of the relative amounts of each product in the reaction mixture gave the relative, competitive reaction rate. The independently synthesized adducts were also examined to determine that... 

Separation of fatty add methyl esters was achieved on an Agilent J W DB-23 fused silica capillary column (60 mxO.251 mm i.d., 0.25 pm Agilent, Santa Clara California, USA). The oven temperature program was initially 120 °C for 5 min, raised to 180 °C at 10 °C min-i, then to 220 °C at 20 °C min-i and finally isothermal at 220 C for 30 min. The injector and detector temperatures were maintained at 220 and 225 °C, respectively. The carrier was high purity helium with a linear flow rate of 1 ml min i and spht ratio 1 50. Fatty add methyl esters were identified using fatty add methyl esters standards (Sigma, St Louis, Mont, USA) by comparison of the retention times of the relative peaks (Nasopwulou et aL, 2011). 

The composition is determined based on relative response in the chromatograms. The identification of each component is based on direct comparison of the retention time and mass spectral fragmentation with the data published by Adams [12] and others [13-16]. The mass spectral library fit (Wiley 139 Library) for all identified components was more than 90%. It should be mentioned that the GC column used in this study (Rtx -5, 30m x 0.25 mm, 0.25pm film thickness) was similar to that reported by Adams [12]. 

A comparison of the molar volumes of 2-, 4-, and 5-alkylthiazoles with their relative retention volumes shows that these values also vary in the same direction (see Fig. III-2). 

Nepeta (Lamiaceae) is a genus of perennial or annual herbs found in Asia, Europe and North Africa. About 250 species of Nepeta are reported of which, 67 species are present in Iran. Some species of this genus are important medicinal plants and their extracts have been used for medicinal purposes. Aerial parts of Nepeta sintenisii Bornm. was subjected to hydrodistillation and the chemical composition of isolated essential oil has been analyzed by GC/MS method for first time. Identification of components of the volatile oil was based on retention indices relative to n-alkanes and computer matching with the Wiley275.L library, as well as by comparison of the fragmentation patterns of the mass spectra with those reported in the literature. 

In cases where a mixture has a large number of components, or pure standards are not available, published retention data must be consulted. The uncorrected retention time, tR (p. 86), is not suitable for this purpose because it cannot be compared with data from different columns and instruments. Valid comparisons can be made using relative retention data which are dependent only on column temperature and type of stationary phase. An adjusted retention time, / R, is first obtained by subtracting from tR the time required to elute a non-retained substance such as air (Figure 4.26)... 

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). 

A further common problem the analyst faces when integrating SPC into analytical procedures for the determination of LAS is the scarcity of available reference compounds, thereby complicating their determination. Therefore, the identification of the analytes has to be performed by comparison of retention time and absolute peak area ratio between the deprotonated molecular ion and the fragment ion, relative to the ratio obtained from the authentic standard ( 20%). Retention times of SPC, for which no standards were available, can be determined once by mass spectrometric identification in full-scan mode. 

TABLE 6 Comparison of Retention Times and Relative Retention Times (RRT) Between a Simulated Chromatogram and an Experimentally Obtained Chromatogram... 

Mass spectra of the cis and trans isomers of the two substituents are virtually indistinguishable, but the IR spectra allow easy differentiation in the region near 1320 cm . The relative stereochemistry of the natural alkaloids can be determined by comparison of gas chromatographic behavior with synthetic material of known stereochemistry, since the two isomers have different retention times. 

BAs are identified by comparison of retention times with those relative to the standards added to the sample. 

Chemical Analysis of Extracts. The extracts were analyzed by capillary column GC-MS for OCs, TAAPs, and PAHs (see the list on page 313). The GC-MS parameters used at the two laboratories are shown in Table II. The identification and quantitation were all done by using automatic routines based on a mass spectra library created from authentic standards of the selected compounds. Compounds were located by searching the reconstructed ion chromatogram for each library entry within a narrow retention time window relative to the internal standard (anthracene-dio or phenanthrene-dio). Quantitation was achieved by comparison of characteristic ion areas in the field samples with ion areas of the internal standard. These ion areas were normalized by response factors established by comparison of ion ratios of a standard mixture of all 66 analytes at a concentration of 2.5 ng//zL. 

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