GC036 Flow Measurement

There are two ways of measuring the flows independently. The first one is an inexpensive soap-bubble flowmeter which consists of a calibrated tube (usually a modified pipet or buret) through which the carrier gas flows. With a rubber bulb we can create a bubble, which is raised into the path of the gas. After that, the ascension of a particular bubble to a defined volume is measured with a stopwatch. The carrier gas in mL/min is easily obtained from this measurement. There are also available electronic soap film flowmeters at a cost around $50. The second alternative is the use of a sophisticated device, composed of a solid-state sensor and a microprocessor to accurately flow measurements without using soap bubbles. Silicone-on-ceramic sensor can be used to measure flow rates of 0.1-500 mL/min for air, nitrogen, oxygen, helium, hydrogen and 5% argon in methane. The cost for this device is around $700. Very small flow rates, like the ones found in tubular columns, cannot be measured reliably. The average linear flow velocity in OT columns is calculated with:

Linear velocity = L / tM

Where L is the length of the column (cm) and tM is the retention time (seconds) for a non-retained peak such as air or methane. Conversion of the linear velocity in cm/s to flow rate, Fc, in mL/min requires the multiplication by the cross-sectional area of the column:

Fc = pi * rc^2 * L /tM

Where rc is the radius of the column.

Comments

Post a Comment

Popular posts from this blog

GC013 Acronyms for GC

LC - Liquid chromatography   LSC - Liquid‐solid chromatography   MDQ - Minimum detectable quantity   MP - Mobile phase MS - Mass spectrometry MSD - Mass selective detector OT - Open tubular PDMS - Polydimethyl siloxane PLOT - Porous layer open tubular PTV - Programmed temperature vaporization RSD - Relative standard deviation SCOT - Support coated open tubular SD - Standard deviation SEC - Size exclusion chromatography SP - Stationary phase TCD - Thermal conductivity detector TF - Tailing factor TOF - Time of flight (MS) TPGC - Temperature programmed gas chromatography WCOT - Wall coated open tubular
Read more

GC010 Chromatographic terms and symbols - Part 3

  In a typical chromatogram for a single solute A, we can see a small peak at the beginning, short time after the injection (which is performed at time zero). The solutes are characterized by their retention times or retention volumes (tr, or Vr). These quantities are depicted as the distance between the injection point and the peak maximum. The formula to calculate the retention volume Vr is: Retention volume equals (“retention time” multiplied by “gas flow rate”) Or we can express it in terms of the retention time as: retention time equals (“Retention volume” divided by “gas flow rate”) Or in formula Vr = tr x Fc   or   tr = Vr / Fc; where Fc is the gas flow rate The small initial peak corresponds to a component that does not is sorb into the stationary phase, thus it is an un-retained component. The IUPAC defines Vm, the holdup volume as: “the volume of the mobile phase (MP) required to elute the un‐retained compound from the chromatographic column and reported at column t
Read more

GC001 GC Introduction to gas chromatography

 Gas chromatography (GC) is a technique used for separation and analysis of volatile compounds. It can be used to analyze gases, liquids or solids, which are usually dissolved in volatile solvents. The molecular weight of the compounds analyze can vary in the molecular weight range from 2 to over 1000 Da. For example, it can be used to separate and analyze more than 450 components in coffee aroma.
Read more