Scouting techniques

It is essential to start a series of such scouting experiments under conditions at which very low retention may be anticipated for all solutes. In this way, no late eluting peaks will be overlooked. It is much more practical to increase short capacity factors than it is to decrease large ones. 

The main disadvantage of such a series of sequential scans is the large number of experiments required to establish the area of optimum capacity factors (see for example refs. [520] and [521]). 

In order to achieve the same degree of separation in an isothermal analysis as in a programmed temperature run, a temperature 45 °C below the retention temperature of a pair of peaks should be selected. 

Expressed as a formula, if Tris the retention temperature, i.e. the temperature of the oven at the time of elution, and T0 the recommended isothermal temperature, then 

According to eqn.(5.4), if the result of a programmed temperature scanning experiment in GC is a bunch of peaks eluted around a column temperature of 195 °C, then a chromatogram in which all the peaks appear with roughly optimal capacity factors may be expected to result from an isothermal experiment at 150 °C. 

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If you do not have a gradient system, I have developed a fast isocratic scouting technique. You select the same column and detector wavelength, but equilibrate the column in 80% acetonitrile in water for our first injection. A strong solvent composition is selected to blow everything off quickly. Look at the peaks if they are resolved, quit. If they are still unresolved, mix the mobile phase with an equal volume of water making 40% acetonitrile, reequilibrate, and shoot again. This time, the peaks should be much farther apart. If not, do another equal dilution with water to 20%, reequilibrate, and reinject the sample. 

A second approach is to use TLC as a scouting technique. A good correlation from TLC to HPLC is generally obtained when mobile phases contain solvents equal to or less polar than ethyl acetate [347], For solvents more polar than ethyl acetate, the correlation is not as useful because the more polar solvent absorbs preferentially to the TLC sorbent [347]. Under these circumstances, compensation for this change in TLC can be made by using a LC solvent mixture less polar than the TLC solvent mixture. Correlations of mobility between TLC and HPLC separations for cephalosporin antibiotics and steroidal hormones have been reported [348-350]. If all else fails a three part series on the basics of separations should be consulted [351-353]. 

In some cases we may speed up the selection of appropriate primary parameters with the help of programmed analysis, i.e. temperature programming in GC or solvent programming in LC. Another useful scouting technique may be thin layer chromatography (TLC). Possibilities for establishing the appropriate values of the primary parameters will be discussed in section S.4. 

The first field of application involves the use of programmed analysis as a scanning or scouting technique for unknown samples. In this case the (volatility or polarity) range of... 

The DAD has brought compound identification to HPLC. Previously, mass spectrometry was the sole domain in peak identification for gas or liquid chromatography (GC-MS or LC-MS). This can now be achieved as part of the HPLC analysis, and at a lower cost, because there is the ability to use an HPLC-DAD system as a scouting technique to check the possible identity of an unknown sample. 

The mode of peak identification that the diode array detector provides allows identification of unknown peaks, not only by retention time but also by their UV spectra. The UV spectmm is very reproducible and its full shape is much more compound-specific than is generally assumed. It is a powerful tool that, beyond other applications, it can also be used as a scouting technique to find out the possible identity of an unknown sample. However, spectral identity is a necessary but not a sufficient precondition for compound identification. The information that is acquired by the comparison of an unknown and a known spectrum can be used as strong indication of compound similarity or confirmation of identity. Combinatorial estimation of retention time and spectral comparison results will give us a more definite idea. 

Arizona cauliflower scouting represented low crop activities. Five orange harvesters and five lemon tree pruners were tested using concurrent dosimetry and urinary monitoring techniques. The re-entry intervals were 43, 2, and 1 days for the orange harvesters, lemon tree pruners, and scouts, respectively. Re-entry workers were monitored for 5- to 6-hr work days in California and for a 4-hr work day in Florida and Arizona. 

My scouting gradient technique was developed when I had to make separations in a customer s laboratory to sell an HPLC system. I only had a few hours to make a separation to convince the customer that he should consider buying a system. But, it provides useful insight for developing a method to use in your laboratory. 

Results of scouting grafting onto experiments indicated that soluble poly(butadiene- -styrene) s can readily and quite efficiently be obtained by effecting styrene polymerization with the r-BuCl/Et2 A1C1 initiating system in the presence of various polybutadienes. Thus it appeared of interest to follow up this lead and to investigate the mechanism of this technique, in particular to elucidate the effect of experimental conditions on grafting details. 

Specific advances in MDCT have been directed toward lowering radiation dose. Among these, automatic tube current modulation is more important for body than for head imaging. With this technique, the tube current is adjusted according to body diameter on the scout image, so as to maintain the same total amount of noise in the image (proportional to photon flux) for any given slice (Table 4.2) [40]. 

The video esophagram, or modified barium swallow, is the standard technique to evaluate dysphagia. This examination is more sensitive than clinical evaluation of aspiration (DeMatteo et al. 2005), and is also more sensitive than the conventional upper GI series for the detection of aspiration (Vazquez and Buonomo 1999). A scout radiograph of the chest should be obtained to assess for evidence of aspiration. For the examination, the child must be securely... 

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