Zero dead volume union

The contribution of the equipment between injection unit and detector cell should be negligable in relation to the column for a sufficient column characterization short connections with narrow capillaries and zero dead volume unions are the precondition for reliable plate numbers. Every end fitting of a column causes additional band broadening. In the past a column type was offered that could be directly combined without any capillary links unfortunately, it has disappeared from the market. 

The Waters company recommends a system check of the chromatographic equipment that is used for plate number determination and analyses (2) the columns in the GPC unit used are replaced by a zero dead volume union. Then the test sample is injected under the same conditions such as a plate number determination. The 5a peak width measured on a suitable recorded peak is evaluated this 5or width of a 20-/a1 injection should be lower than 150 /a1. 

First, disconnect the column and replace it with a zero-dead-volume union. 

Figure 3.1 Compression fittings, (a) Male fitting (b) female fitting (c) zero dead volume union.

Supercritical fluid grade carbon dioxide (Scott Specialty Gases, Plums teadvi lie, PA) was used as the carrier fluid. A Lee Scientific Model 501 supercritical fluid chromatograph equipped with a flame ionization detector (FID) and a nitrogen-phosphorus detector (NPD) was the instrument utilized for these studies. Fused silica capillary columns (50 pm i.d.) were employed for all the experiments. Three column types with stationary phases of three different polarities were used SB-Methyl-100, SB-Biphenyl-30 and Carbowax 20M (0.25 pm films). Frit restrictors were used to maintain pressure and proper flow rates in the column. The restrictor was connected to the end of the column via a zero dead-volume union. The end of the restrictor was positioned in the detector at 1 mm below the end of the flame jet. The detector was operated at 325-350 C with nitrogen make-up gas at 25 mL/min. Split injection was used in these experiments with 0.2 pL injection rotor and a split ratio of approximately 10 1. 

One instrument configuration utilized in this laboratory is shown in Figure 5. In this instrument the column was mounted in a constant temperature gas chromatograph oven, which also served to heat the air circulated through the DFI probe. A zero dead volume union is typically used to connect the column to a short length of 4-8 ym i.d. or contoured (tapered) fused silica restrictor. The restrictor and probe tip are heated to compensate for cooling due to decompression of the fluid during the DFI process. 

End-column detection is done by coupling the outlet of the capillary to an external detection cell. The properties of this detection cell (path length, material, etc.) can be tailored to allow for high sensitivity and low analyte limits of detection. The careful selection of coupling and transfer fluidics, such as zero dead-volume unions, ensures that the high resolution afforded by CE is maintained within the end-column detector. 

Figure 3.10. Calculated electrolysis efficiency curves for a (1) fused silica emitter with a 0.075-cm-long stainless-steel zero dead volume union (2) a 3.5-cm-long platinum emitter (3) a channel flow-by cell with 6-mm width and 6-mm length containing a 6-mm-wide electrode using a 16 pM gasket (4) a pmous flow-through electrode with 0.508-mm diameter, 0.36-tmn width, 40% total porosity, and 800 nm average pore radius and (5) the same cell as in (4), but the current is limited to 6pA. n = 2, D = 5 x 10 cm s , and e = 5pM were used for creating the curves.

Prepare 1% acetone in water as mobile phase B, and water alone as mobile phase A. A column under measurement is removed from the line, and the tubing is connected with a zero dead volume union. A gradient of 10 min is run from 100% A to 100% B at the flow rate of 2 mL/min and held for 10 min at 100% B with detection wavelength at 260 nm. The difference between 5 min and the time at half height between the initial baseline and the plateau times the flow rate is the dwell volume for the HPLC system. The typical dwell volume for most HPLC systems is approximately 1 mL. 

Figure 3.4 Effect of extra-column volume on migration times and band-broadening for a series of columns of different internal diameter operating at the same linear velocity. Conditions A zero-dead volume union was used in place of a column to connect the injector to the detector premixed 50/50 (v/v) water to acetonitrile as the mobile phase 0.05 mg/mL uracil as an analyte. From Wu, N., et al. J. Sep. Set 2012, 35, 2018-2025, with permission.

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