Instrumental baseline 666 INDEX

The standard flow cell for the Michrom instrument has a path length of 2mm. Modified flow cells of 4 mm, 5mm and 6mm were evaluated to effect increased signal response. The 5 mm flow cell was selected as the best compromise between increased signal with a tolerable level of baseline drift due to the refractive index changes across the gradient. Data was collected with the EZ Chrom data and control system available from Michrom. 

Very often baseline problems are related to detector problems. Many detectors are available for HPLC systems. The most common are fixed and variable wavelength ultraviolet spectrophotometers, refractive index, and conductivity detectors. Electrochemical and fluorescence detectors are less frequently used, as they are more selective. Detector problems fall into two categories electrical and mechanical/optical. The instrument manufacturer should correct electrical problems. Mechanical or optical problems can usually be traced to the flow cell however, improvements in detector cell technology have made them more durable and easier to use. Detector-related problems include leaks, air bubbles, and cell contamination. These usually produce spikes or baseline noise on the chromatograms or decreased sensitivity. Some cells, especially those used in refractive index detectors, are sensitive to flow and pressure variations. Flow rates or backpressures that exceed the manufacturer s recommendation will break the cell window. Old or defective source lamps, as well as incorrect detector rise time, gain, or attenuation settings will reduce sensitivity and peak height. Faulty or reversed cable connections can also be the source of problems. 

The first term of the Fourier transform essentially defines a baseline shift of very broad bandwidth and is a very sensitive measure of the absorbance due to Ught scattering. A plot of the first Fourier transform term versus amount of compound added works much better as an index of precipitation than a simple plot of absorbance versus amount of compound added. The UV absorbance method is also somewhat dependent on the instrumental design and works best when there is a large distance between the sample cell and the UV detector. 

Assuming a correct baseline is drawn, the question arises as to how to compare the MMD estimated from a tof mass spectrometer to that obtained from size exclusion chromatography (sec) using a UV or differential refractive index (DRI) detector. A sec is the instrument most commonly used to obtain the MMD for synthetic polymers. The most obvious difference between the two techniques is that the tof detector counts numbers of ions of n-mers while the DRI or UV detectors on sec instruments measures mass concentration of the polymer. The mass concentration of the polymer is proportional to the product of the molecular mass of the -mer and the number of molecules of that n-mer per unit volume. Thus, in the ms we obtain the number MMD while in the sec we normally obtain the mass MMD. Furthermore, the raw signal of each must be corrected for the transformation from time to mass, which is different for each instrument. This transformation has been discussed in detail (58). 

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