Column, capillary flow rate

A gas chromatograph (GC) can be used for the chromatographic separation of volatile analytes in complex mixtures prior to mass spectrometric analysis. This becomes especially advantageous if the GC elutes directly ( online ) into the ion source of a mass spectrometer, so-called GC-MS coupling. [63-65] Packed GC columns with a high flow can be connected via a jet-separator, but these are almost out of use at present. [66] Capillary columns provide flow rates in the order of a few milliliters per minute, therefore their back end can be connected directly at the entrance of the ion volume. 

Some detectors, such as the FID, are constructed to provide maximum performance with a carrier gas flow rate in the order of 30 ml/min. When using capillary columns, this flow rate is obtained at the exit of the column by using a makeup gas either identical to or different from the carrier gas. 

Syringe pump or HPLC pump capable of flow rates of 1 to 10 pl/min Reversed-phase (typically Cl8) column (since flow rate into continuous-flow FAB source must be <10 pl/min, capillary column must be used or flow must be split post column)... 

Fig. 4. Analysis of an anion standard solution by IC (a) and CE (b) [48]. IC conditions aVydac 302IC4.6 column, a flow-rate of 2.5 ml/min, an injection volume of 25 xl, an isophthalic acid mobile phase, UV detection at 280 nm. CE conditions an electrolyte of potassium dichromate, sodium tetraborate, boric acid and the DETA (diethylenetriamine) EOF modifier, pH 7.8 65 cmX75 xm I.D. capillary 20 kV indirect UV detection at 280 nm. Anions 1, chloride 2, nitrite 3, chlorate 4, nitrate 5, sulfate 6, thiocyanate 7, perchlorate 8, bromide.

Column Technology. Increased sensitivity and component resolution have resulted from advances in solid-state electronics and column and detector technologies. In the field of column technology, the capillary column has revolutionized toxicant detection in complex samples. This column generally is made of fused silica 5 to 60 m in length with a very narrow inner diameter (0.23-0.75 mm) to which a thin layer (e.g., 1.0 11 in) of polymer is bonded. The polymer acts as the immobile or stationary phase. The carrier gas flows through the column at flow rates of 1 to 2 ml/min. 

Requires capillary column (eluent flow rate <5 jl/min) or split devices... 

The driving forces for the rapid development and growth In mlcrobore column HPLC are (1) savings In solvent consumption a total saving of up to 99.9% can be achieved when narrow-bore microparticle packed columns or open—tubular micro-capillary columns are used (2) the high separation power using long column and small particles (e.g., 3 nm) (3) the compatibility of the column eluent flow rates with a mass spectrometer and flame based detectors and (4) opportunities In new detector development,... 

Make-up gas. To provide maximum performances, the three detectors described above should be served with a gas flow of at least 20mL/min, which is far superior to that within capillary columns. This flow rate is attained by mixing, at the outlet of the column, a make-up gas either identical or different (such as nitrogen), from the carrier gas. 

Gas chromatography requires that samples remain stable when volatilized in a stream of helium, first in the injector used to introduce the analytes onto the GC column, and then during the time that the sample components traverse the column as it is heated progressively inside an oven. The requiranent for volatility means that only nonpolar or moderately polar compounds can be analyzed by GC-MS. For capillary GC columns the flow rate of the gas that moves compounds through the column (the carrier gas) is low, about 1 ml/min. Such a quantity of gas can be introduced into the ion source directly, without compromising the vacuum in the instrument. This simplifies the interfacing of GC with MS so that a heated interface tube can be used to link the two instruments and through which the GC column is run so that it abuts the ion source. 

Figure 10.3A shows a common interface in use today. Most GC-MS systems use capillary columns, and fused silica tubing permits an inert, high efficiency direct transfer between the two systems. For capillary flow rates of 5 mL/min or less, a direct interface is possible. Bench-top GC-MS systems can easily handle these low flow rates, and they provide better sensitivity (transfer of total sample) and better preservation of GC results. 

In micro-HPLC with narrow bore or capillary columns, lower flow rates should be used at comparable linear mobile-phase velocities. The flow rate should decrease in proportion... 

Typical dimensions of packed columns and flow rates in pc-SFC are given in Table 12. This technique is becoming more and more popular compared to capillary SFC. A wide spectrum of stable stationary phases, showing different selectivities, is available commercially. Reproducible, quantitative sample introduction by means of the HPLC sample loop technique is no problem, nor is independent pressure or How control. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

In general, the longer a chromatographic column, the better will be the separation of mixture components. In modem gas chromatography, columns are usually made from quartz and tend to be very long (coiled), often 10-50 m, and narrow (0.1-1.0 mm, internal diameter) — hence their common name of capillary columns. The stationary phase is coated very thinly on the whole length of the inside wall of the capillary column. Typically, the mobile gas phase flows over the stationary phase in the column at a rate of about 1-2 ml/min. 

It is clear that the separation ratio is simply the ratio of the distribution coefficients of the two solutes, which only depend on the operating temperature and the nature of the two phases. More importantly, they are independent of the mobile phase flow rate and the phase ratio of the column. This means, for example, that the same separation ratios will be obtained for two solutes chromatographed on either a packed column or a capillary column, providing the temperature is the same and the same phase system is employed. This does, however, assume that there are no exclusion effects from the support or stationary phase. If the support or stationary phase is porous, as, for example, silica gel or silica gel based materials, and a pair of solutes differ in size, then the stationary phase available to one solute may not be available to the other. In which case, unless both stationary phases have exactly the same pore distribution, if separated on another column, the separation ratios may not be the same, even if the same phase system and temperature are employed. This will become more evident when the measurement of dead volume is discussed and the importance of pore distribution is considered. 

A six-port valve was first used to interface the SEC microcolumn to the CZE capillary in a valve-loop design. UV-VIS detection was employed in this experiment. The overall run time was 2 h, with the CZE runs requiring 9 min. As in the reverse phase HPLC-CZE technique, runs were overlapped in the second dimension to reduce the apparent run time. The main disadvantage of this yu-SEC-CZE method was the valve that was used for interfacing. The six-port valve contributed a substantial extracolumn volume, and required a fixed volume of 900 nL of effluent from the chromatographic column for each CZE run. The large fixed volume imposed restrictions on the operating conditions of both of the separation methods. Specifically, to fill the 900 nL volume, the SEC flow rate had to be far above the optimum level and therefore the SEC efficiency was decreased (22). 

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