Sample volume maximum allowed

Equation (22) allows the maximum sample volume that can be used without seriously denigrating the performance of the column to be calculated from the retention volume of the solute and the column efficiency. In any separation, there will be one pair of solutes that are eluted closest together (which, as will be seen in Part 3 of this book, is defined as the critical pair) and it is the retention volume of the first of these that is usually employed in equation (22) to calculate the maximum acceptable sample volume. 

The maximum allowable dispersion will include contributions from all the different dispersion sources. Furthermore, the analyst may frequently be required to place a large volume of sample on the column to accommodate the specific nature of the sample. The peak spreading resulting from the use of the maximum possible sample volume is likely to reach the permissible dispersion limit. It follows that the dispersion that takes place in the connecting tubes, sensor volume and other parts of the detector must be reduced to the absolute minimum and, if possible, kept to less than 10% of that permissible (i.c.,1 % of the column variance) to allow large sample volumes to be used when necessary. 

Add the sample to the column in a maximum volume of 2.5 ml. If the sample volume is less than 2.5 ml, do not adjust it at this time. Allow the sample to enter the gel bed completely. 

Unfortunately, the magnitude of the variance contribution from each source will be different and the ultimate minimum size of each is often dictated by the limitations in the physical construction of of the different parts of the apparatus and consequently not controllable. It follows that equipartition of the permitted extra column dispersion is not possible. It will be seen later that the the maximum sample volume provides the maximum chromatographic mass and concentration sensitivity. Consequently, all other sources of dispersion must be kept to the absolute minimum to allow as large a sample volume as possible to be placed on the column without exceeding the permitted limit. At the same time it must be stressed, that all the permitted extra column dispersion can not be allotted solely to the sample volume. 

The equations also make clear that the sample mass, i.e. the product C U should be large. The column may be overloaded with regard to the main component as long as the resolution of the trace peak is not affected (Figure 19.2). However, the injection volume is restricted if band broadening needs to be avoided. The maximum allowed injection volume which restricts band broadening to 9% is given by " ... 

In the analytical chromatogram the injected sample volume is so small that the peak widths are not influenced. Moreover, the sample mass is too low to give overloading. The maximum allowed injection volume has been calculated in Section 19.2 and the... 

Usually the injection quality can be assumed to correspond with K=2 but for excellent injection it is lower, thus allowing larger sample volumes. However, it is more important that the maximum injection volume depends on the retention volume of the solute therefore, for capillary columns with their extremely small retention volumes (since Ur decreases with the square of the capillary diameter), the allowed injection volume has values in the nanolitre range. 

Conventional packed column injection ports are used for the sample introduction on packed and megabore columns. In case of capillary columns, unless a special injection technique is used, the low maximum allowable sample volume which may be introduced on the column negatively affects the experimental detection limits as only a tiny fraction of the derivatized extract is finally processed in the hyphenated system. Online preconcentration and injection of derivatized organolead species can solve that problem. It consists of three consecutive processes taking place in the injection hner sample injection, solvent venting and release of the analytes on to the column. Up to 25 fil can be processed at a time and larger amounts can be handled by successive injections of 20 to 25 /a1 volumes at 1 min intervals to remove the solvent. 

The sample volume should be small, although sufficient to perform all the assays required. The maximum amount of sample to be extracted from critical patients and children is 1 mL. This should be divided Into parts sufficiently large to allow the different analyses required (e.g. glucose, CO2) to be carried out. The chief problems resulting from the use of small sample volumes arise from the lack of homogenization when the sample must be reconstituted or thawed after lyophllizatlon and freezing, respectively. 

We must NEVER allow our tube to exceed the maximum allowed sample depth in the spinner because this error may cause damage upon sample insertion. If we have an excess of solution in our tube, we cannot center our solution volume about the midpoint of the detected region because this will exceed the maximum allowable depth instead we put the tube into the spinner only down to the maximum depth. The result will be that the distance from the meniscus to the center of the detected region will be greater than the distance from the tube bottom to the center of the detected region. 

The volume analyzed for each individual sample is a tightly controlled variable. Although the four-liter marlnelli beakers (a water containing volume having a center cavity which will house the photon detector) used in the analysis are currently filled to a specified height (14.1 cm), the allowed variation does not (by protocol) exceed 0.2 cm. For chemically pure water specimens, this translates to a maximum allowable difference of 1.4 percent by volume. This maximum allowable uncertainty has been used for the final estimation of the relative systematic uncertainty in the volume term of the activity or LLD equation. 

The method for measuring pollution from sulphur dioxide must be both specific for this gas and allows its concentration measure. Its specificity depends on the presence of a physical property or a chemical reaction that detects sulphur dioxide without interference with other atmospheric constituents. The concentration measurement involves the determination of SO2 in a given volume of air, and therefore the use of an adequate instrument comprising a sampling mbe, an absorption device, a meter air and a pump. In order to track pollution changes, it is desirable that each step (measure) is carried out over a short period. However a period of 24 h is the maximum allowable. 

Low frequency Most measurements for dosimetric purposes have so far been made by standard ESR spectrometers at X-band. The diameter of the dosimeter pellets used in these spectrometers is limited to ca 5 mm. Film dosimeters have a maximum width of 5-6 mm and a thickness less than 0.5 mm. Due to these size limitations at X-band, ESR spectrometers working at lower frequency might be more useful. Such spectrometers allow larger sample volumes, are smaller and therefore more portable, and presumably less expensive. The loss of sensitivity, compared to the X-band, may be compensated for by the increased sample dimensions, and for samples with significant g-anisotropy also by a narrower line-width as schematically shown in Fig. 9.8 for a g-anisotropy typical for the spectrum observed in lithium formate. 

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