Column retention

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.]...

Correlation to HAS and IAM column retentivity VD is a function of albumin binding and partitioning into phospholipid membranes, and these can be measured using affinity HPLC columns Retentivity on human albumin and immobilized artificial membrane HPLC columns [33]... 

Guard column accumulates nonvolatile substances that would contaminate chromatography column Retention gap improves peak shape by separating volatile solvent from less volatile solutes prior to chromatography... 

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. 

Retention volume. The volume of mobile phase required to elute a particular component from the column. Retention volume is a measure of the attraction of a sample component for the packing material when a specific mobile phase and set of operating conditions is used. Vr is given by the... 

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... 

In Section 2.1 the main chromatographic descriptors generally used in routine HPLC work were briefly discussed. Retention factor and selectivity are the parameters related to the analyte interaction with the stationary phase and reflect the thermodynamic properties of chromatographic system. Retention factor is calculated using expression (2-1) from the analyte retention time or retention volume and the total volume of the Uquid in the column. Retention... 

Thus, the SEC data obtained from its LS detector determine the MWD, whereas the VISC detector characterizes conformation and branching. The efficiency of SEC is a consequence of no column calibration requirement for the determination of M and MWD. The precision of the system is limited only by the signal-to-noise ratios of the LS and RI detectors, not by chromatographic variables such as flow rate and column retention. Sophisticated software is required to display the SEC picture of molecular structure. 

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. 

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