The measurement and control of carrier gas fl ow is essential for both column effi ciency and for qualitative analysis. Column effi ciency depends on the proper linear gas velocity, which can be easily determined by changing the fl ow rate until the maximum plate number is achieved. Typical optimum values are:
FLOW CONTROL AND MEASUREMENT 17
75 – 90 mL/min for 1/4 - in. - outside - diameter (o.d.) packed columns; 25 mL/min for 1/8 - in. - o.d. packed columns; and 0.75 mL/min for a 0.25 - μ m - inside - diameter (i.d.) open tubular column. These values are merely guidelines; the optimum value for a given column should be determined experimentally.
For qualitative analysis, it is essential to have a constant and reproducible fl ow rate so that retention times can be reproduced. Comparison of retention times is the quickest and easiest technique for compound identifi cation. Keep in mind that two or more compounds may have the same retention time, but no compound may have two different retention times. Thus, retention times are characteristic of a solute, but not unique. Obviously, good fl ow control is essential for this method of identifi cation.
Controls
The fi rst control in any fl ow system is a two - stage regulator connected to the carrier gas cylinder to reduce the tank pressure of 2500 psig * down to a useable level of 20 – 60 psig. It should include a safety valve and an inlet fi lter to prevent particulate matter from entering it. A stainless steel dia-phragm is recommended to avoid any air leaks into the system. The fi rst gauge indicates the pressure left in the gas cylinder. By turning the valve on the second stage, an increasing pressure will be delivered to the gas chromato-graph and will be indicated on the second gauge. The second stage regulator does not work well at low pressures and it is recommended that a minimum of 20 psi be used.
For isothermal operation, constant pressure is suffi cient to provide a con-stant fl ow rate, assuming that the column has a concon-stant pressure drop. For simple, inexpensive gas chromatographs which run only isothermally, the second part of the fl ow control system may be a simple needle valve; this, however, is not suffi cient for research systems.
In temperature programming, even when the inlet pressure is constant, the fl ow rate will decrease as the column temperature increases. As an example, at an inlet pressure of 24 psi and a fl ow rate of 22 mL/min (helium) at 50 ° C, the fl ow rate decreases to 10 mL/min at 200 ° C. This decrease is due to the increased viscosity of the carrier gas at higher temperatures. In all temperature - programmed instruments, and even in some better isothermal ones, a differential fl ow controller is used to assure a constant mass fl ow rate.
Sometimes, however, it is not desirable to control the fl ow rate with such a controller. For example, split and splitless sample injection both depend on a constant pressure for correct functioning. Constant pressure maintains the same fl ow rate through the column, independent of the opening and closing of the purge valve. Under these conditions, the carrier gas pressure can be increased electronically during a programmed run in order to maintain a
* psi stands for pounds per square inch, and psig is an alternative abbreviation that emphasizes the fact that the pressure is read on a gauge (above atmospheric pressure). Actually, the standard unit of pressure in the SI is the pascal, Pa, and for those familiar with old units, the following conversions are as follows: 1 bar = 100 kPa; 1 atm = 101.3 kPa; 1 torr = 133 Pa; 1 psi = 6.9 kPa.
constant fl ow. An electronic sensor is used to detect the (decreasing) fl ow rate and increase the pressure to the column, thus providing a constant fl ow rate by electronic pressure control (EPC).
Flow Measurement
The two most commonly used devices are a soap - bubble fl owmeter and a digital electronic fl ow measuring device (Fig. 2.2 ). The soap - fi lm fl owmeter is merely a calibrated tube (usually a modifi ed pipet or buret) through which the carrier gas fl ows. By squeezing a rubber bulb, a soap solution is raised into the path of the fl owing gas. After several soap bubbles are allowed to wet the tube, one bubble is accurately timed through a defi ned volume with a stopwatch.
From this measurement, the carrier gas fl ow rate in mL/min is easily calculated.
Some electronic soap fi lm fl ow meters are based on the same principle, but the measurements are made with light beams, at a cost of around $ 50.
Another, more sophisticated electronic device uses a solid - state sensor coupled with a microprocessor to permit accurate fl ow measurements for a range of gases without using soap bubbles. A silicone - on - ceramic sensor can be used to measure fl ow rates of 0.1 to 500 mL/min for air, oxygen, nitrogen, helium, hydrogen, and 5% argon in methane. The cost for this device is about $ 700.
Very small fl ow rates such as those encountered in open tubular columns cannot be measured reliably with these meters. The average linear fl ow velo-city in OT columns, u¯ , can be calculated from equation (1) : nonretained peak such as air or methane (seconds). Since the fl ame detector does not detect air, methane is usually used for this measurement, but the column conditions must be chosen (high enough temperature) so that it is not retained. Conversion of the linear velocity in cm/sec to fl ow rate (in mL/min) is achieved by multiplying by the cross - sectional area of the column ( π r 2 ). See Appendix IV .
Compressibility of the Carrier Gas
Since the carrier gas entering a GC column is under pressure and the column outlet is usually at atmospheric pressure, the inlet pressure, p i , is greater than the outlet pressure, p o . Consequently, the gas is compressed at the inlet and expands as it passes through the column; the volumetric fl ow rate also increases from the head of the column to the outlet.
Usually the volumetric fl ow rate is measured at the outlet where it is at a maximum. To get the average fl ow rate, Fc, the outlet fl ow must b multiplied by the so - called compressibility correction factor, j :
FLOW CONTROL AND MEASUREMENT 19
(b)
Fig. 2.2 Flow meters: ( a ) Soap fi lm type. ( b ) Digital electronic type.
j
If one calculates a retention volume from a retention time, the average fl ow rate should be used, and the resulting retention volume is called the corrected retention volume, VRO:
VRO=jVR=jt FR c (4) This term should not be confused with the adjusted retention volume to be presented in the next chapter.