Capillary transfer conventional

In the case of the capillary blood, it is extremely important that the specimen not be allowed to stand for extensive periods of time before centrifugation. If the blood is to be transferred to the pH meter, then the collecting tube is sealed at both ends during transportation. It is then aspirated into the pH instrument as soon as practicable since one needs a smooth even flow in order to aspirate a specimen into the conventional micro pH meter. After the whole blood has been sampled for various purposes, it is important that the remaining blood be centrifuged promptly. If not, it will clot. Subse-quentially, centrifuging with a clot will tend to hemolyze the blood. Erythrocytes will adhere to the wall and as they are pulled down by the clot, they will be ruptured. Those who do not observe these precautions will find that it is rather difficult to obtain unhemolyzed blood. 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

The majority of commercial developments which relate to the automation of GC and HPLC pay little attention to sample preparation. There are few examples where pretreatment is not required. A fully automated system was developed by Stockwell and Sawyer [23] for the determination of the ethanol content of tinctures and essences to estimate the tax payable on them. An instrument was designed and patented which coupled the sample pre-treatment modules, based on conventional AutoAnalyzer modules, to a GC incorporating data-processing facihties. A unique sample-injection interface is used to transfer samples from the manifold onto the GC column. The pretreated samples are directed to the interface vessel hy a simple hi directional valve. An ahquot (of the order of 1 ml) can then he injected on to the GC column through the capillary tube using a time-over pressure system. 

On-line coupling between a gas chromatograph and an atomic spectrometry detector is fairly simple. Typically, the output of the CG capillary column is connected to the entrance of the atomisation-ionisation system simply via a heated transfer line. When separation is performed by liquid chromatography (EC), the basic interface is straightforward a piece of narrow-bore tubing connects the outlet of the EC column with the liquid flow inlet of the nebuliser. Typical EC flow rates of 0.5-2 ml min are within the range usually required for conventional pneumatic nebulisation. 

Figure 27.11 illustrates a third dual-electrode arrangement that permits enhancement of the response by reversible redox cycling. Many more electrons are therefore transferred than would be the case with a single electrode, and the current is amplified dramatically. This concept does not work well with conventional LC columns because the volume flow rate is too large to permit a significant number of redox cycles. Nevertheless, the concept is certainly interesting, and, as reversed-phase capillary columns are developed, it may well have some practical value. A detailed treatment of multiple-electrode LCEC has been published [24]. 

The early phase of development can be characterized by a transfer of concepts from conventional CE to the planar format, such as capillary gel electrophoresis, micellar electrokinetic chromatography, sample stacking and pre- and postcolumn sample derivatization. Emphasis was laid on the demonstration of the specific advantages mainly from the separation science point of view. With only very few exceptions, detection has received much less attention yet. LIF detection with confocal imaging has been used in most of the early work owing to its high sensitivity and its relatively easy implementation. If not explicitly mentioned otherwise, all experiments described in the following sections were carried out with LIF detection [28,29]. 

Drugs possessing a steroid structure are particularly easy to separate by CEC (see also the section on Steroids). Euerby et al. [203] published the separation of tipredane and five related compounds. A conventional capillary packed with 3 mm Spherisorb ODS-1 can be used for this purpose using acetonitrile-Tris pH 7.8 buffer (8 2) (50 mmol/1). Under these conditions it is also possible to separate the C-17 diastereoi-somer of the active compound without the addition of a chiral modifier (b-cyclodextrin is needed to achieve a comparable result in other separation modes) (Fig. 10.25). The elution order of individual compounds was exactly the same as with reversed phase chromatography, and it was concluded that with unionized species HPLC methods should be directly transferable to the CEC mode. 

For some applications flat heat pipe panels (HPP) have advantages over conventional cylindrical heat pipes, such as geometry adaptation, ability for localized heat dissipation and the maintenance of an entirely flat isothermal surface (Fig. 14). The liquid-vapour interface formed in capillary channels inside the heat pipe panel is capable to generate self-sustained thermally driven oscillations. Thin layer (several mm) of the sorbent between mini-fins on the outer side of the heat pipe panel ensures an advanced heat and mass transfer during the cycle adsorption/de sorption. 

Apparatus for continuous addition and transfer of one phase, involving a helix of narrow-bore tubing, has been described by Ito and Bowman. With a coil of 8000 turns and continuous phase separation provided by centrifugation they obtained separations equivalent to about as many as 5000 tubes used in the conventional Craig manner this number would be expected imder some conditions if each turn in the capillary acts as one equilibrium vessel under conditions of continuous transfer. 

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