Instrument baseline, checking

It is recommended to scan the DSC under the proposed experimental conditions to check the curvature and noise level of the instrument baseline before analysing samples. There are several reasons why the instrument baseline may be curved and/or display a high noise level. Trace amounts of residue from a previous experiment may be attached to the sample holder. Decomposed or sublimated compounds frequently condense on the sample holder, distorting the shape and quality of the instrument baseline. If the instrument baseline is still not satisfactory, following cleaning with ethanol or acetone, there may be a problem with the purge gas flow. The linearity of the instrument baseline will be... 

As a consequence of the development of extraction methods for STA based on mixed-mode SPE columns, as well as of the recent introduction of instruments for the automated sample preparation allowing efficient evaporation and derivatization of the extracts, full automation of STA methods based on GC-MS analysis is also available. It needs GC-MS instalments equipped with an HP PrepStation System. The samples directly injected by the PrepStation are analyzed by full scan GC-MS. Using macrocommands, peak identification and reporting of the results are also automated. Each ion of interest is automatically selected, retention time is calculated, and the peak area is determined. All data are checked for interference, peak selection, and baseline determination. 

Satisfactory results for a method can only be obtained with well-performing equipment. Therefore, before an instrument is used to validate a method, its performance should be verified using universal standards (47). Special attention should be paid to the equipment specifications that are critical for the performance of the method. For example, if detection limit is critical for a specific method, the detector specifications for baseline noise and the response to the specified compounds should be checked. Furthermore, any reagent or reference standard used to determine critical validation parameters should be double-checked for accurate composition and purity. 

Consult your instructor on the proper operation of your instrument. Handle the infrared cell carefully, avoiding contact with water and the fingers. Fill the cell with pure m-xylene and obtain a spectrum on this from 2 to 15 p,m, being sure to record the last peak just before 15 pm (692 cm" )- Each time you run a sample, be sure to check 0% T by placing a card in the sample beam and adjust the pen to 0% T. Empty the cell, rinse and fill with p-xylene, and run a spectrum on this. Repeat for o-xylene. Run spectra on each of the standard mixtures. From the spectra of the pure substances, choose a peak of each isomer to measure. Using the baseline method (see Figure 16.11), measure PqIP for the peak for each compound. Prepare a calibration curve of the ratio of og PJP) J og PolP ri,o and of log(Po/P)para/ log(Po/E)ortho versus concentration for the meta and para isomers, respectively. See Chapter 20 and your CD for spreadsheet preparation using an internal standard. 

The third stage of control is to measure the result of instrument settings, e.g. gas flow and temperature. The instrument can then be held after new settings have been made until gas flow and temperature are stable It may also be used to check the baseline before sampling 92). 

This instrument uses a helium sparging device with four separately controlled solvent lines, ensuring optimal solvent degassing and improving check valve reliability at low flow rates, diminishing baseline disturbances at low ultraviolet (UV) wavelengths. 

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