Gas chromatography is an ideal tool for analysis of gases. The versatility of the technique extends its capability to analyse samples such as dissolved solids, liquids and gases. The scope of applications extends from pharmaceuticals, foods, environmental monitoring, petroleum refining, and workplace environment monitoring for safety of workers. Gas samples require special handling as gases have different physical and chemical properties in comparison with solids and liquids.
Types of Gas Sampling
Spot sampling is adopted when compositional details of a sample are required at any given point of time.It is commonly applied to observe the composition of a gas stream in industrial manufacturing processes. A tube similar Pitot tube,used for measuring flow velocities of fluids is inserted into the process pipeline and the representative sample is collected in the sampling cylinder for analysis in the laboratory.
Continuous sampling has gained popularity in recent years and is mainly applied in process stream monitoring. The sample is withdrawn continuously and led to the detector for real-time analysis. The method provides real-time changes in the composition and helps in taking instantaneous quality control measures for consistent quality of product. The only disadvantage of this approach is that you have to place reliance on the accurate response of the instrument to changes in composition of the gas stream.
Three approaches are commonly used for collection of gas samples. One requires a direct collection for analysis in a laboratory and the other involving liquid displacement and another using adsorption or absorption on a solid support followed by desorption. In the present article direct collection is covered in some detail.
Several devices are used for grab sampling such as evacuated flasks, metal cylinders, plastic bags, etc.
Evacuated glass bulbs with varying capacities have one heat sealed and the open and drawn to a tip and sealed. The sealed tip is broken in the required environment and the allowed air is filled inside the bulb. On resealing the container is submitted for analysis to the laboratory. The sample is drawn using a gas tight syringe by penetrating the butyl rubber septum on the bulb.
Plastic bags specially designed for the purpose are available in different capacities depending on applications in hand. These can be used successfully for sampling of both organic and inorganic gases. Such bags are made from materials like polyester, Teflon,Fluorocarbons,etc. The choice of material Is based on the absorption or reaction of the gas with the bag material. The bags are connected to a pump for drawing gas samples inside.
Tips to Reduce Sampling Errors
Like samples of liquids there are potential sources of errors with Gas sampling. Typically these errors can be minimised by:
- Running the sample pump for some time before start of sampling to optimise the representative sample collection
- The sampling bag material should be inert to the gases collected and before transfer samples to the laboratory the bulbs or bags should be sealed tightly with suitable caps and stoppers
- Samples should be stored under appropriate temperature conditions and protected from exposure to light to prevent decomposition
- Whenever possible the sample should be analysed at the earliest opportunity after collection to avoid any changes in composition during storage and transportation.