Other Comparisons with Column Methods

Berger [340] has examined the use of pSFC in polymer/additive analysis. As many polymer additives are moderately polar and nonvolatile SFC is an appropriate separation technique at temperatures well below those at which additives decompose [300,341,342], SFC is also a method of choice for additives which hydrolyse easily. Consequently, Raynor et al. [343] and others [284,344] consider that SFC (especially in combination with SFE) is the method of choice for analysing polymer additives as a relatively fast and efficient sample preparation method. Characterisation of product mixtures of nonpolar to moderately polar components encompassing a wide range of molecular masses can be accomplished by cSFC-FID. Unknown polymer additives may be identified quite adequately by means of cSFC-FID by comparison with retention times of standards [343], However, identification by this method tends to be time-consuming and requires that all the candidate compounds are on hand. SFC-FID of some low-to-medium polarity additives on reversed-phase packed columns... 

In HPLC, a sample is separated into its components based on the interaction and partitioning of the different components of the sample between the liquid mobile phase and the stationary phase. In reversed phase HPLC, water is the primary solvent and a variety of organic solvents and modifiers are employed to change the selectivity of the separation. For ionizable components pH can play an important role in the separation. In addition, column temperature can effect the separation of some compounds. Quantitation of the interested components is achieved via comparison with an internal or external reference standard. Other standardization methods (normalization or 100% standardization) are of less importance in pharmaceutical quality control. External standards are analyzed on separate chromatograms from that of the sample while internal standards are added to the sample and thus appear on the same chromatogram. 

The total contents of saponifiable and free acids in small biological samples were determined by MacGee and Allen [137]. A sample of tissue (3—5 mg) or serum (5—100 /xl) was hydrolysed with a lye, acidified and extracted with /7-hexane. Free acids were extracted from the /7-hexane into a small volume (5—10 ql) of trimethyl-(a,a,a-trifluoro-/ 7-tolyl)ammonium hydroxide. The quaternary salt was injected together with methyl propionate, and methyl esters produced by the pyrolysis were subjected to GC analysis (injection port temperature 240°C, column temperature 180°C, 10% EGSS-X). Comparison of this method with other esterification methods in Table 5.9 shows that it offers at least equally good results. 

Justification for new method or ATP can be related to, for example, the reduction of cost analysis, limitation of interferences, improvement of safety and waste management compared with the approved method. The method must be presented according to the USEPA format. For ATP, an in-depth comparison between ATP and USEPA approved methods must be produced with a two-column method comparison, providing a detailed discussion of a number of specific aspects each in its own section. The method development information is published in the Federal Register after proposal by USEPA to approve the ATP or new method. Information can be a detailed background and summary of the method, a discussion of the development of QC acceptance criteria, a description and discussion of the interlaboratory validation study, others studies (for instance of method limitations). The study plan is submitted to USEPA for review and comment, prior to conducting all studies. Lastly, the... 

Table I, in the column headed HSE-VW, shows the results of using Equations 2 through 6 to define the diameters with the HSE method to calculate the properties of an equimolar mixture of LJ fluids. The reference is a pure LJ fluid. Other columns show comparison with the machine-calculated results of Singer and Singer (8) in column MC. The van der Waals (VDW) one-fluid theory (9) and the VDW two-fluid theory (10) are in columns VDW-1 and VDW-2. The GHBL column gives the Grundke, Henderson, Barker, Leonard (GHBL) (11) pertu-bation theory results with each diameter determined by Equation 3.

Applicable for columns with uniformly distributed drops (fails in spray columns). Till now no published comparison with other methods available. Favoured in large columns.Recovery after sample taking may take up to 15 minutes. 

Although the arrested elution method has two drawbacks (the need of several runs to get a Dab value with a precision of 2%, and of constant flow rates over long periods for runs at various arrested times), in comparison with the continuous elution method, the former has also the following advantages 1) Effects of zone broadening other than axial molecular diffusion and non-uniform flow profile do not affect the measurement 2) no assumptions are made about the precise form of the flow profile, the smoothness of the column wall, or the accuracy in the knowledge of the column diameter. 

Today, lEC is used infrequently in comparison with other chromatographic methods. In most cases, IPC is more convenient because of its higher column efficiency, more stable and reproducible columns, and easier control over selectivity and resolution. There are, however, cases for using lEC instead of RP- or IP-HPLC, especially when organic ions have poor UV absorbance and need other detection (conductivity or MS). Then, completely volatile components of mobile phase are required. In such cases, lEC with volatile buffers fulfil this requirement, whereas ion-pair reagents are not sufficiently volatile in most cases also, when compounds are isolated or purified by HPLC separation, the removal of mobile phase is necessary. When multistep separation is required, the aqueous buffer-salt mobile phase used for ion-exchange allows direct injection of a sample fraction onto an RP column for the next step of separation. This may be difficult with IP systems. 

Evans et reported the HPLC-ED of plasma salicylate (2-hydroxybenzoate). A LoD of 4 ng on column was claimed. The eluent used was methanol-aq. sodium acetate (approximately 60mmolL pH 6.0) (4 - - 46). However, the column used was not stated, a protracted sample purification procedure was needed, no internal standard was used and the applied potential advocated (GCE, -I-1.35 V vs Ag/AgCl) was very high by comparison with other methods involving the ED of phenolic hydroxyl groups. 

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