Retention time column

Perhaps the most revolutionary development has been the application of on-line mass spectroscopic detection for compositional analysis. Polymer composition can be inferred from column retention time or from viscometric and other indirect detection methods, but mass spectroscopy has reduced much of the ambiguity associated with that process. Quantitation of end groups and of co-polymer composition can now be accomplished directly through mass spectroscopy. Mass spectroscopy is particularly well suited as an on-line GPC technique, since common GPC solvents interfere with other on-line detectors, including UV-VIS absorbance, nuclear magnetic resonance and infrared spectroscopic detectors. By contrast, common GPC solvents are readily adaptable to mass spectroscopic interfaces. No detection technique offers a combination of universality of analyte detection, specificity of information, and ease of use comparable to that of mass spectroscopy. 

Figure 2.3. Capillary gas chromatogram of Si(OCH3)4 (3M) (a) after hydrolysis/ condensation with H20 (1.8 M) and HC1 (0.05 M) showing assignments of molecular formulas and structures and (b) with H20 (1.5 M) and catalysis conditions shown. For (a), linear and cyclical structures are indicated along the x-axis. All plots illustrate relative concentrations of species (y-axis) as a function of GC column retention time (x-axis). Higher mass species (e.g., hexamers (Si6) and pentamers (Si5)) demonstrate longer retention times. [Reprinted from Ref. 72, with permission.]...

Column performance is affected by the carrier gas flowrate and there is always an optimum flowrate for every column. Retention times also are affected by the carrier gas flowrate. A 1% change in carrier gas flowrate will cause a 1% change in retention time. For all these reasons it is important to keep the flow of the carrier gas constant. There are basically two ways to assure... 

The electrical signal from a detector is amplified and fed into a recorder or computer for analysis. A typical recorder trace is shown in Figure 3.5. Each peak represents a component in the original mixture. A peak is identified by a retention time, the time lapse between injection of the sample and the maximum signal from the recorder. This number is a constant for a particular compound under specified conditions of the carrier gas flow rate temperature of the injector, column, and detector and type of column. Retention time in GC analysis is analogous to the R value in thin-layer or paper chromatography. 

Problem 9 Column retention time and plate count changing. 

Fig. 8. Products of iV-acetyl-L-tyrosine treatment with NaOCl studied by HPLC method. Peak denoted 4 represents A -acetyltyrosine, peak denoted 6 is iV-acetyl 3-chlorotyrosine. Minute peak denoted 8 represents 3,5-dichlorotyrosine. Graph respresents column eluate light absorption at 280 nm plotted versus column retention time. (From Drabik and Naskalski, unpublished results.)...

The CH2C12 solution was analyzed by vapor phase chromatography (VPC) methods using 6 ft X % inch 30% Carbowax or 6 ft X 4 inch 20% SE-30 columns. Retention times of the reaction products were matched on both columns with precalibrated chromatographs of known... 

Comprehensive protocols for the analysis of plastics/polymers need to be developed. In the past analytical protocols included extractions performed with a polar and a non-polar solvent which were used to extract organic compounds from a polymer for subsequent analysis by GC using a flame ionization detector (FID). But FID alone may not be a definitive test, since the identity is based on column retention time, which is not a unique characteristic for many of these complex organic compounds. 

The advantage of GC/MS over LC/MS is that extensive libraries of El data are available for searching (see Section 9.10.4.3.4). Where necessary, this can be complemented by molecular weight data from Cl. Library identification then requires confirmation by comparing the column retention time and MS and MS data with that of a standard. If no library match is found, then a similar process of determining elemental formulas (see Section 9.10.4.3.3) and interpretation of the fragmentation data from first principles must be followed (see Section 9.10.4.3.4). 

Norboma-2,5-diene (0.95 mL, 9.4 mmol) and phenylacetylene (1.0 mL, 9.1 mmol) were added under to tris(acetylacetonato)cobalt (7.1 mg, 0.02 mmol) and (- -)-bicyclo[2.2.1]hept-5-ene-2,3-diylbis(diphenyl-phosphane) (14mg, 0.03 mmol) in dry THF (1 mL). The catalytically active cobalt species was then generated by adding 1 M diethylaluminum chloride in hexane (5 mL). The reaction mixture was stirred for 4h at 35 °C. i-PrOH (5mL) was added dropwise to decompose the excess of diethylaluminum chloride. The volatile constituents were removed by vacuum distillation at rt. The product was then isolated by vacuum distillation at lOO C in a Kugelrohr apparatus [a] 56.3 (c = 1, CHClj, 20°C). The H NMR and mass spectrum of the product were identical to those of a reference sample yield 100%, enantiomeric purity 98.4% ee, both determined by GC with naphthalene as standard using a 40-m Lipodex-C column. Retention times (—)-product 120.7 min, (-l-)-product 123.7 min (column temperature 104 C, carrier gas Hj, flow rate 4-5 mL min at 1.7 bar, injection temperature 170°C). 

Due to a lack of information on vapor pressure for a variety of drugs of abuse and related compounds, the authors determined this parameter by an indirect method based on a system using gas chromatography and relative retention times. This approach is a modification those described by others (Hamilton 1980 Westcott and Bidleman 1981 Bidleman 1984). The original method is based on in the relationship between solid vapor pressure and GC column retention time (or volume retention time, Vr), and has been used in determination of vapor pressures for herbicides, pesticides, and a variety of nonpolar organic compounds. The vapor pressure (Pv) of two substances at the same temperature (as well as their latencies of vaporization, Ly) are related by... 

Column 150 X 4.6 5 p-m Adsorbosphere CIS (PEEK column) (retention times are longer and peaks broader with stainless steel column)... 

Figure 21.2. Chromatogram of the three-component mixture of Figure 21.1. to = time for solvent to traverse the column, = retention time of substance B, ty,g = peak basewidth of substance B, h = peak height.

Another chemometric technique that has been applied to GCxGC-MS data is PARAFAC2, which differs from PARAFAC in that one dimension need not be strictly trilinear [27,28]. This allows for analysis of data where there is shifting of peak profQes or changes in retention time in one separation dimension due to, for example, temperature programming, which affects the second-column retention times, or misalignment across samples [4,29]. PARAFAC2 can be used in most cases where PARAFAC is used, but is computationally more complex and expensive. 

Extraction Derivatization Column/ Retention Time Detection Reference... 

The methylated (CH2N2) low-molecular-weight compounds were analyzed on a Kratos MS-30 equipped with a Perkin Elmer Sigma 3 gas chromatograph and a 3 ft X 1/8 inch OV-1 column. Retention times of the sample peaks identified by GC-MS were compared to those of known standards on a 6 ft X 1/8 inch OV-1 coliamn on a Tracor model 560 GC for confirmation. Lignin content was measured by the UV method (10). 

Detectors discussed in Chapter 22 respond to solutes as they exit the chromatography column. A chromatogram shows detector response as a function of time (or elution volume) in a chromatographic separation (Figure 21-3). Each peak corresponds to a different substance eluted from the column. Retention time, fp is the time needed after injection for an individual solute to reach the detector. 

Flame ionization detection 280° or 290°. Cel-esticetin (I) and 7-O-demethylcelesticelin were separated using a 3% PPE column Retention time... 

The determination of double-bond position in unsaturated fatty acids has traditionally seemed to be an intractable problem for MS alone. Recourse to GC/MS and comparison of the column retention times of unknowns with those of standards has usually solved the problem. However, several techniques have recently managed to overcome the difficulties without using GC. 

For size-exclusion chromatography (Topic D7), the retention volume, is used to characterize solutes. This is the volume of mobile phase required to elute the solute from the column. Retention times are directly proportional to retention volumes at a constant flow rate, so equation (2) can be re-written as... 

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