Sample application hydrodynamic injection

It is important to note that with hydrodynamic injection a volume of sample is injected, which has representative amounts (concentration) of the sample constituents. This characteristic, together with a better precision, makes it the most widely used injection technique in CE. However, owing to the poor detectability in terms of concentration of the widely used detectors (see further), its application is rather limited to the analysis of high-concentration samples. In trace analysis, for example, electromigration is favored over hydrodynamic injection [42],... 

The earliest applications of acoustic levitation in analytical chemistry were concerned with the development of various steps of the analytical process. Thus, Welter and Neidhart [72] studied the preconcentration of n-hexanol in methanol by solvent evaporation and the liquid-liquid extraction of n-hexanol from water to toluene in a levitated droplet, which they found to be efficient when using GC-FID with n-pentanol as internal standard. Solvent exchange of fluorescein from methylisobutyl ketone to aqueous sodium hydroxide was also accomplished. Sample concentration in an acoustically levitated droplet prior to injection into a CE equipped with an LIF or UV detector has also been accomplished [73,118]. The target analytes (namely, dansylated amino acids) were concentrated in the levitated drop and a limit of detection of 15 nM — much lower than the 2.5 pM achieved by hydrodynamic injection without preconcentration — was achieved following CE separation and quantification. For this purpose, 36000 sample droplets 2.3 pi in volume each were sequentially positioned in the acoustic Ievitator and evaporated. This example illustrates the potential of acoustic levitation for coupling to any type of detector for micro- or nanotrace analyses. 

Hydrodynamic injection methods are based on the creation of a pressure difference across the separation column with the column inlet located in the sample vial by any of three general methods raising the height of the sample vial above the level of the column outlet by overpressure of the sample vial by application of a vacuum to the detector end of the capillary. The average length of the sample zone, Li j, introduced is given by... 

The injection process introduces the prepared sample or reagent into the flowing carrier stream within the manifold. Ideally, the injector system should be designed so as to provide a high sample flow rate. Injection systems typically employ electrokinetic mobility or hydrodynamic pressure techniques. In the former systems, the sample flow into the microchannel is controlled by the application of an external electric field to the reservoir, while in the latter systems, a pressure difference is created in the reservoir using either a positive pressure (pistmi-type) technique or a suction pressure (vacuum) technique. 

Sample injection involves extremely small volumes (picolitre to nanolitre range) so as to not occupy more than a few percent of the total capillary volume (a 1 m capillary with d. of 50 jxm contains a volume of 2.5 xL). The two commonly used injection methods for CE are hydrodynamic (pressure) and electrokinetic. Each involves temporarily removing the bnffer reservoir from the inlet end (the cathode in the case illustrated in Figure 4.23) and replacing it by a vial containing the solution to be analyzed. Hydrodynamic injection involves application of a pressure difference between the two ends of the capillary. The volume amount of sample solution injected can be calculated using the Poiseuille equation ... 

In particular, SIA offers some additional advantages when coupled to CE when compared to other flow modes, such as the increased scope for sample pretreatment, lower sample volumes, and enhanced potential for hydrodynamic injection. A suitable interface is the most critical operational unit in achieving an efficient coupling of SIA to CE. The requirements for such an interface are as follows (a) the grounding electrode should preferably be inserted in the interface, to prevent failure of the electronic parts of the flow system (b) the capillary and the electrode should be positioned as close together as possible (c) the waste/outflow channel in the interface should be relatively wide to avoid (or minimize) hydrodynamic flow into the CE separation capillary. Different interfaces, sample pretreatment procedures, SIA-microchip CE approaches, and practical applications of SIA-CE have been reviewed (Kubafl and Karlberg, 2009 Ruzicka, 2014). 

When the sample size is small (10-20 pL), it becomes possible to effectively inject most of the ions in the sample in less than 30 s at 5 kV. A small water plug injected hydrodynamically has been shown to further improve sensitivity [51], though this was not the case for the above-mentioned application. Since the sample conductivity severely affects the amount of solute injected, internal standards are important for this mode of stacking. For quantitative analysis, make only a single injection out of each vial, as the sample becomes ion-depleted. 

Hydrodynamic loading is consistently reported to be the more precise injection method. Hettiarachchi and Cheung have concluded that hydrodynamic loading should always be used for high-precision quantitative applications because it is less dependent on the ionic concentration of the sample buffer (62), but running buffer is the recommended sample diluent. 

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