Sampling Techniques Ppt

Sampling is usually the most important step of any analytical procedure. Errors committed at this stage cannot be corrected later during the analysis.

  There is a plethora of sampling

methods, which depends greatly on the physico-chemical nature of these analytes and their expected concentrations in air.

Most widely used methods for VOCs sampling are either pre-concentration on sorbent tube or whole air sample taken in canister. In some special cases compounds are collected by chemisorptions.

  Active sampling is done using a pump for drawing a

specified volume of the air to be sampled through an adsorbent tube with a specified, usually low air flow rate.

Grab sampling is done using polished stainless steel or aluminium canisters which allows to sample for a very short periods of time (down to 10-30 seconds) The VOCs are enriched afterwards in the laboratory.

Passive sampling consists of adsorbents normally contained in a thin tube. The cross section of the tube and the distance between the opening of the tube and the adsorbent surface determines the sampling rate of the passive sampler.

Canister sampling involves the collection of the whole air matrix in a pre-cleaned evacuated cylinder. This technique is especially useful for the most volatile species, such as propane, butane etc.

Canister sampling is performed in two modes: either grab sampling or time integrated sampling. Sample can be collected either on sub-atmospheric (passive) pressure or pressurized (active) with pump.

  By the process of thermal desorption analytes are

transferred into the GC-column.

The actual air sample is collected without any breakthrough;

No degradation problem of trapping material;

Moisture has no effect upon sampling; Duplicate analyses of the sample can be

performed.

Requires complex sampling apparatus; Severe clean up procedure between sampling is

necessary; The flow tends to decrease towards the end of

sampling To obtain good compound recovery from

canister, the sample pressure should be as high as possible without causing precipitation of liquid water within the canister;

Because of the presence of reactive species, gas phase reaction and subsequent transformation and loss of target compounds may occur.  

Plastic bags (such as Tedlar, Teflon or aluminized Tedlar) are simple to use, inexpensive and are available in various sizes, normally from 500 ml to 100 l of volume.

These bags are mostly useful for measurement of strongly polluted air or relatively high trace gas concentrations.

Sampling on sorbents allows larger volume of gas than with canisters to be collected; in addition, sorbent tubes are easier to handle than canisters

  The quantity of the substance which can be

captured on the adsorbent depends on the ambient conditions (pressure, temperature, concentration, humidity etc.), amount of sorbent, sampling speed etc.

Breakthrough of the analytes has to be avoided.

Some adsorbents may show degradation problems that have significant impact on adsorption process.

Contamination problem of sorbents must be avoided before and after sampling.

Retention of moisture content on sorbents must be avoided as it may pose undesirable effects on chromatographic analysis.

1 Active tube sampling

2 Passive tube sampling (diffusive sampling)

Both short term and long term samples can be collected on adsorbents packed in appropriate tube with the use of sampling pump.This technique is particularly convenient for personal monitoring because all of the equipments required while sampling can be worn by the worker.

Active sampling allows control over the sampling rate and sampling time.

Passive sampling may be defined as any sampling technique based on free flow of analyte molecules from the sampling medium to a collecting medium, as a result of a difference in concentration of the analyte between the two media.

The advantages of passive tube sampling are lower cost and greater worker acceptability, as it does not require bulky and expensive pumps that are subjected to regular checking and possible error in flow rate.

In this process the substance to be measured is absorbed irreversibly in a liquid phase and then it is altered by a chemical reaction with other components present. In this chemisorption process impingers and bubblers containing liquids have been usually used to collect high-boiling, reactive or polar substances.

1 On-line gas chromatography analyzer

2 Differential optical absorption spectroscopy

On-line gas chromatography analyzers are special automatic portable gas chromatographs designed tomeasure selected organic compounds in situ. The most frequently used analyzers are BTEX analyzers measuring benzene, toluene, ethylbenzene and xylene.

On-line gas chromatography analyzers can measure sample regularly in any given time interval (e.g.every 15 or 30 min ) so that results obtained can be interpreted in relation to meteorological conditions, vehicular emissions etc.