Capillary columns flow rate through

Fig. 10.17. Capillary electrochromatography of PTH-amino acids with gradient elution. Column, 207 (127) mm x 50 pm i.d. packed with 3.5 pm Zorbax ODS particles, 80 A pores. Starting eluent (A), 5 mM phosphate, pH 7.55, 30% acetonitrile gradient former (B), 5 mM phosphate, pH 7.55, 60% acetonitrile flow-rate (through inlet reservoir), 0.1 ml/min gradient, 0-100% B in 20 min voltage 10 kV current, 1 pA temperature, 25°C UV detection at 210 nm electrokinetic injection, 0.5 s, 1 kV. Peaks in order of elution formamide PTH-asparagine PTH-glutamine PTH-threonine PTH-glycine PTH-alanine PTH-tyrosine PTH-valine PTH-proline PTH-tryptophan PTH-phenyialanine PTH-isoleucine PTH-leucine. The concentration of the PTH-amino acids dissolved in the mobile phase was 30-60 pg/ml. Reprinted with permission from Huber et al. [68]. Copyright 1997 American Chemical Society.

In contrast to LC detectors, GC detectors often require a specific gas, either as a reactant gas or as fuel (such as hydrogen gas as fuel for flame ionization). Most GC detectors work best when the total gas flow rate through the detector is 20-40 mL/min. Because packed columns deliver 20-40 mL/min of carrier gas, this requirement is easily met. Capillary columns deliver 0.5-10 mL/min thus, the total flow rate of gas is too low for optimum detector performance. In order to overcome the problem when using capillary columns, an appropriate makeup gas should be supplied at the detector. Some detectors use the reactant gas as the makeup gas, thus eliminating the need for two gases. The type and flow rate of the detector gases are dependent on the detector and can be different even for the same type of detector from different manufacturers. It is often necessary to refer the specific instrument manuals for details to obtain the information on the proper selection of gases and flow rates. All detectors are heated, primarily to keep the... 

If peaks are symmetrical, as is frequently the case with modern capillary columns, only the peak height must be converted. When recording the chromatograms on a strip chart recorder, the chart speed on the recorder should be increased to obtain an accurate measurement of the peak width. The area can then be obtained by triangulation. Other required data are the flow rate through the detector and the moles of sample in the detector. For samples that are split the measurement of the split ratio is also required to obtain the number of moles injected. The... 

Since the TCD Is a flow sensitive (concentration) detector, the low flow rates generally associated with capillary systems should enhance the observed response of a low volume (30) TCD. The observed response, l.e. peak height, or peak area, is Inversely proportional to the gas flow rate at the detector (assuming the same carrier gas flow rate through the column, with any additional gas being Introduced as a make-up flow just prior to the detector). Optimum detector response should be obtained at those flow rates Just sufficient to efficiently sweep-out the dead volumes associated with the detector and connecting tubing. 

The liquid-junction interface consists of a metal tee with an electrical connection and outlet buffer reservoir. The ends of the separation column and electrospray capillary are positioned opposite each other at the center of the tee with a gap between them of 10-25 p,m. The outlet reservoir provides additional makeup flow to the separation buffer through the third branch of the tee. The interface decouples many aspects of the separation system from the electrospray source and can be used with a wide range of column flow rates. On the other hand, it is rather cumbersome to use, particularly the alignment of the capillaries. In addition, it is difficult to avoid band broadening at the open connection and the makeup liquid dilutes the sample concentration at the sprayer. 

Nitrogen was used as carrier gas. The gas manifold used Brooks mass-flow controllers to deliver metered amounts of carrier and feed gases. After mass-flow controllers, all streams were heated by heat tracings. This was essential to prevent condensation for 12DCP. The partial pressure of the feed was determined by its fraetion of the total molar-feed-flow rate through the sample bed and the total pressure. The sample bed outlet was directed to a backpressure regulator for maintenance of the system pressure. Part of the sample bed effluent was split either to vent or to the inlet of a gas-chromatography unit. The concentrations in the outlet stream were analyzed by an on-line GC, in which a 30 m PoraPLOT-Q capillary column was used. 

Requirement for auxiliary gases Some detectors do not function well with the carrier gas composition or flow rates from a capillary column effluent. Makeup gas, sometimes the same as the carrier gas, may be required to increase flow rates through the detector to levels at which it responds better and/or to suppress detector dead volume degradation of resolution achieved on the column. Some detectors require a gas composition different from that used for the GC separation. Some detectors require both air and hydrogen supplied at different flow rates than the carrier to support an optimized flame for their operation. Makeup flow dilutes the effluent but does not change the detection mechanism from concentration to mass-flow detection. 

Fig. 4.11 demonstrates a twofold increase in the mobile phase flow rate through monohthic capillary columns when increasing their pore diameter firom 250 nm to 1.3 Jim. This finding contradicts the generally accepted perception that in traditional CEC on packed columns, the rate of... 

In contrast to LC detectors, GC detectors often require a specific gas, either as a reactant gas or as fuel (such as hydrogen gas as fuel for flame ionization). Most GC detectors work best when the total gas flow rate through the detector is 20-40 ml/min. Because packed columns dehver 20-40 ml/min of carrier gas, this requirement is easily met Capillary columns dehver 0.5-10 ml/min ... 

The flow rate through a capillary column whose inner diameter is less than 0.53 mm is difficult to measure accurately and reproducibly by a conventional soap-bubble meter. Instead, the flow of carrier gas through a capillary column is usually expressed as a linear velocity rather than as a volumetric flow rate. Linear velocity may be calculated by injecting a volatile nonretained solute and noting its retention time, tM (seconds). For a capillary column of length L in centimeters. 

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