Automated equipment, limits

Standardization of a Titrant For wet chemistry analytical methods, a titration is often used and the titrant, or the solution to which an unknown sample is compared, must be standardized. This can be done by comparing it with another standard. The important thing here is that the standard with which it is compared is ultimately traced to a SRM. The procedure utilizes volumetric glassware heavily, and thus the analyst must be assured that these are properly calibrated, as discussed above. Auto-titrators can be used (Figure 5.4). In this case, the automated equipment can be calibrated against manual equipment, i.e., volume readings obtained with the auto-titrator must match the volume readings obtained with a calibrated buret for the same sample. If they do not match (within accepted limits), the auto-titrator must be taken out of service and repaired, just like the defective balance. 

Most of these assays can be performed using the same automated equipment that is used for plasma, but the assays will need to be validated separately for urine to ensure that the matrix (urine versus plasma) does not interfere with the method and that the enzyme levels present in urine, either endogenously or following injury, are within the limits of linearity. Immunoassays (ELISA) are commercially available for a, tt or i -glutathione-S-transferase in the rat and human (Biotrin International, Dublin). 

Only the imagination of the system designer will limit the scope of these applications. They will vary from simple conveyor systems for flow-or spray-coating adhesive on the substrate to very complex curve-following, automated equipment for intricate shapes. 

The first technique is the offline mode, whereby fractions are collected and later reinjected into the second dimension. This can either be done manually or in an automated fashion. This approach puts low demands on the instrumental set-up and there are no limitations on separation speeds in each dimension. Off-line techniques are prone to sample losses by vial contamination, low reproducibility, and long analysis times. Furthermore, sample dilution and eluent incompatibilities may have a negative impact However, many off-line experimental approaches have been described throughout the literature and fully automated equipment is now commercially available. Figure 2 displays the principle of fractionation and reinjection in 2D-HPLC, as used in the off-line approach. 

In spite of the growing popularity of mercury porosimetry and the ready availability of excellent automated equipment, the interpretation of the mercury intrusion-extrusion data is still far from clear. The values of surface tension and contact angle which must be inserted in the Washburn equation are still uncertain - as are the limits of applicability of the equation itself. Other problems include the reversible or irreversible deformation of the pore structure, which undoubtedly occurs with some corpuscular or weakly agglomerated systems. 

VPD-TXRF is also a facile technique for interface analysis [4.78, 4.79]. Automated VPD equipment (Fig. 4.16) improves both the detection limit (upper range 10 atoms cm ) and the reliability (by > 50%) of the VPD-TXRF measurement [4.14]. Current research focuses on sample holders [4.80, 4.81] and light-element detection capability [4.82-4.84]. 

Cassettes Cassette is a term used to describe two different cross-flow membrane devices. The less-common design is a usually large stack of membrane separated by a spacer, with flow moving in parallel across the membrane sheets. This variant is sometimes referred to as a flat spiral, since there is some similarity in the way feed and permeate are handled. The more common cassette has long been popular in the pharmaceutical and biotechnical field. It too is a stack of flat-sheet membranes, but the membrane is usually connected so that the feed flows across the membrane elements in series to achieve higher conversion per pass. Their popularity stems from easy direct scale-up from laboratory to plant-scale equipment. Their limitation is that fluid management is inherently very limited and inefficient. Both types of cassette are very compact and capable of automated manufacture. 

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