Drawback peak area

NMR has been used comparatively little for quantitative analysis although peak areas are directly proportional to concentration. The principal drawbacks are the expensive instrumentation and a lack of sensitivity. The latter can be improved with the aid of computers to accumulate signals from multiple scans or by using a pulsed (Fourier transform) technique. Relative precision lies in the range 3-8%. 

The increased ability of capillary GC to resolve organic species caused an unanticipated drawback for broad spectrum analysis. The amount of stationary phase on a capillary GC column is much less than on a packed column. This condition increases the likelihood of observing a decrease in chromatographic performance caused by the sample matrix. The altered stationary phase may cause reduced precision of retention times and peak areas. The changes in the chromatographic performance of the stationary phase are measured by the Grob general purpose test mix (9). 

With such pumps working at nearly constant pressure, the flow-rate changes continuously as the viscosity of the eluent varies with time. Quantitative analysis is very difficult, as the peak area, which normally depends on flow-rate, is not proportional to the amount of compound. This drawback is overcome in the... 

An alternative method [2] of determining Mi uses the fact that in power compensation DSC the proportionality constant between the transition peak area and Mi is equivalent to the constant which relates the sample heat capacity and the sample baseline increment. By measuring the specific heat capacity of a standard sapphire sample, an empty sample vessel and the sample of interest, from the difference in the recorded DSC curves of the three experiments Mi for the sample transition can be calculated. The advantage of this method is that sapphire of high purity and stability, whose specific heat capacity is very accurately known, is readily available. Only one standard material (sapphire) is necessary irrespective of the sample transition temperature. The linear extrapolation of the sample baseline to determine Mi has no thermodynamic basis, whereas the method of extrapolation of the specific heat capacity in estimating Mi is thermodynamically reasonable. The major drawbacks of this method are that the instrument baseline must be very flat and the experimental conditions are more stringent than for the previous method. Also, additional computer software and hardware are required to perform the calculation. 

Retention times of molecules separated over mesoporous silica are much longer than those obtained by using commercially available silica, this is due to the increased surface area of mesoporous silica, which in turn increases molecular capacity factors. Differences between capacity factors are also enhanced Thus, molecules which elute with similar retention times on commercial HPLC columns, with overlapping peaks, can be successfully separated by using HPLC columns slurry packed with mesoporous silica. The long retention times are somewhat of a drawback in that large amounts of solvent must be used and the peak shapes of molecules with long retention times can be broad. Mesoporous silica may not be ideal for routine analytical separations but provides an excellent and cost-effective preparative separation medium. 

Although the use of this function preserves the area of the peaks in the profile of the total pair-correlation function, one of the drawbacks of this method is that... 

This list is not complete. Many instrument manufacturers offer spectral databases packaged with their instruments. The publishers listed below offer their databases in both electronic and hardcopy formats, with CD versions and electronic versions becoming increasingly popular. The major drawback of the high-resolution databases for the beginner learning NMR spectral interpretation is the lack of peak expansion and area integrations on the spectra in many cases. The authors are deeply indebted to Aldrich Chemical Co., Varian Associates, and AIST for their permission to use their spectra in this chapter. 

Infrared absorbance detectors. For completeness we should note that IR detectors for HPLC do exist. As is the case for GC-IR (Section 12.8.2) a continuous rapid response HPLC-IR detector benefits from FTIR spectral acquisition and computerized data file storage. Such spectra as may be obtained have spectral peak widths more characteristic of a liquid matrix, instead of the more information rich, sharp gas phase GC-IR peaks. The major drawback to IR detection in HPLC is that most of the commonly useful HPLC mobile phases absorb strongly in many areas of the IR spectral region. Thus HPLC-IR can be used for only a very limited set of analytes. 

It is apparent that it would be difficult to resolve a large number of peaks such as is possible with NMR or mass spectrometry. This deficiency leads to two drawbacks of electrochemistry when only current is monitored. One is that it is usually difficult to monitor trace solution components when a trace constituent occurs in the presence of other electroactive materials, as might be the case in a biological or environmental sample. Pulse voltammetry has been used successfully for such samples at the ca. 10" M level, but the sample is usually subjected to a clean-up procedure, such as chromatography, to remove interferences. A second problem occurs in the area of organic reaction mechanisms which are initiated electrochemically, where several species may have oxidation or reduction potentials near the species of interest. It is often difficult to sort out a complex reaction mechanism in which several species have similar redox poten-... 

From 1 mg R, the quantity of each amino acid (corresponding to the area under each peak) can be determined with an accuracy of a few percent. Analysis is complete in about 20 hours. Analysis is further facilitated by on-line computation of results with direct readout of amino acid composition. The most common method of hydrolysis consists of heating the P. in 6 M HCl at 105-110°C for 20-70 hours in a sealed tube. A drawback of acid hydrolysis is that some ami-... 

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