Sampling of gases and liquids

Abstract The sampling and initial sample preservation procedures for gases, liquids, suspensions and solids can be quite different from each other. The sampling and ST / SP of gases and vapours is normally more complex compared with the sampling and clean-up of liquids. The most important reasons are that the sample frequently is not visible not homogeneous. Detailed discussion on the techniques mentioned below can be found in the following chapters.



Sampling of gases and liquids

During the sampling of gases and vapours it is important that the volume of the container or the flow rate of the sampling system is exactly known. The flow rate should be kept constant during the sampling procedure. There are a number of possibilities to do this. A simple and effective way is by using a restrictor at the end of a sampling needle or by using a glass capillary. The volume of the container can be determined by weighing the container before and after the sampling. 

Approaches for volatile organics and gases include: grab sampling, solid-phase trapping, liquid trapping, headspace sampling, purge and trap, and thermal extraction. The various modes of head-space sampling will be discussed in a separation section of this Topic Circle.

  • Solid-phase trapping
     Using solid-phase trapping a gaseous sample is passed through a tube packed with adsorbent (e.g., silica gel, activated carbon) and the trapped analytes are eluted with a strong solvent. In this approach the gas flow rate is critical for the trapping efficiency. Important parameters to be watched are aerosol formation, adsorbent overloading, and irreversible adsorption of reactive analytes. Chemical complexing reagents may be useful to improve trapping efficiency as well as the purge and trap technique.
  • Liquid trapping
    In liquid trapping a gaseous sample is passed through solution, which is a good solvent for analytes that remain behind. The gas usually passes through the solution unabsorbed. It is important that the flow rate should be low enough so that no foams or aerosols are created. Complexing agents may be added to solvent aid trapping and the temperature can be lowered for very volatile species.
  • Volatile Organic Sampling Train

Liquids are frequently easier than gaseous samples because a dissolution or extraction step is not needed. In many cases dilution with a suitable solvent is sufficient. For liquid samples the removal of interferences, concentration / dilution of the samples and compatibility with the final analytical techniques are the most important features. Typical approaches for liquids include: solid-phase extraction (SPE), LLE, dilution, evaporation, distillation, microdialysis and lyophilization. 

  • In SPE a liquid is passed through a solid phase, which selectively retains the analyte. Thereafter, the analyte can be eluted with a strong solvent. In some cases interferences are retained and analytes allowed passing through solid phase unretained. The mechanism of SPE is comparable with LC. The advantages of SPE are that a wide variety of sorbents is available for the selective removal of inorganic, organic, and biological analytes. The good selectivity and efficiency of SPE can be explained by the fact that various modes like reversed-phase (RP), normal-phase (NP), ion-exchange (IE), restricted-access (RAM), Immunoaffinity (IA) and molecular-imprinting (MIP) can be applied. Techniques like SPME and stir bar sorption extraction (SBSE) are relatively new developments approaches to perform extraction procedures without the need for using organic solvents. Furthermore, a number of different sampling formats (e.g., packed syringes and cartridges, disks, pipette tips and 96-well plates) allowing off-line, at-line, on-line and in-line sampling procedures. This means that SPE can be considered to be the most versatile sampling and sample manipulation approach.
  • Liquid-liquid Extraction
    Still the most popular technique is LLE. In this case the sample is partitioned between two immiscible phases. The extraction solvent and extraction conditions are chosen in such a way that a maximum difference in solubility is obtained. In order to obtain reliable results one should beware of the formation of emulsions.  Possibilities to break them are heat, addition of salt; change of the KD value by using different solvents or chemicals affecting the equilibrium (such as buffers for pH adjustment, salts for ionic strength, complexing agents, ion-pairing agents, etc.). LLE can be performed manually by using a separatory funnel in case a relatively small number of samples must be analyzed or in an automated way by using packed cartridges or 96-well plates in case larger number of samples must be analyzed.
  • Dilution
    An additional technique is dilution in which the sample is diluted with a solvent compatible with the eluent of the separation system (e.g. LC) to avoid system overloading or to be in linear range of detector. For example, to avoid band broadening the solvent should not be too strong for the LC eluent and should be miscible with LC eluent; “dilute and shoot” is a typical ST method for simple liquid samples such as pharmaceutical formulations.
  • Evaporation.
    In evaporation the liquid is removed by gentle heating at atmospheric pressure with flowing air or inert gas or under vacuum. Evaporation should not be performed too quickly and bumping can result in sample losses. Sample losses can also occur on the wall of the container. Evaporation should be done at moderate temperatures, by using an inert gas (e.g. N2) by using a rotary evaporator or an automated system (e.g. Turbovap).
  • Distillation
    In distillation a sample is heated to the boiling point of the solvent, and volatile analytes are concentrated in the vapor phase, condensed, and collected. This approach is mainly used for samples that can easily be volatilized. Problems are that a sample can decompose if heated too high. This means that vacuum distillation can be used for low-vapor-pressure compounds, while steam distillation is rather gentle since maximum temperature is 1000C.
  • Microdialysis
    Microdialysis is a technique in which a semipermeable membrane is placed between two aqueous liquid phases and low-molecular weight molecules transfer from one liquid to the other based on a concentration difference over the membrane. Enrichment techniques such as SPE are required to concentrate the dialysate. Microdialysis is used, for example, for the examination of extracellular chemicals in living plant and animal tissue and in fermentation broths. It has been used on-line with LC. Dialysis with MWCO membranes can also be used for on-line deproteination of samples.
  • Lyophilization (freeze-drying)
    In lyophilization (freeze-drying) an aqueous sample is frozen and water removed by sublimation under vacuum. This technique can be used for nonvolatile organics, the concentration of inorganics and large sample volume can be handled. A potential problem can be the loss of volatile analytes.

Water sampling

In the case of water sampling it is important that collection containers are pretreated before the sample can be collected. In principle, only polyethylene or PTFE containers should be used and they should be washed and stored in 10% of HNO3 for 2 days and rinsed with double distilled deionised water. Following collection, acidification of the sample (normally with 2 mL of 10% HNO3 or 5 M HCl) will reduce or eliminate trace element adsorption and hydrolysis. 

Depending on the type of water precautions must be taken. For the collection of tap water, the first water running from the tap must be avoided because there will be a high concentration of trace elements from the pipes, soldering and welds. Normally, sampling is performed by running the tap for 5 – 30 min, before the actual sample is taken. 

Most water samples require filtration immediately after collection to remove bacteria, algae and particulate matter. In most cases 0.5 mm membrane filters are used. Stabilizing agents like nitric, hydrochloric and sulphuric acids are frequently added to lower the pH to about 1 – 3.5. Before storage all sample containers should be completely full, because the presence of air may chemically or biologically alter the sample. Water samples shouldbestored in the dark, either by refrigeration (40C) or by deep-freezing (-200C). 

The most important problem during the sampling of surface water is that, in principle, no samples may be taken from a stagnant water source because in those cases contaminants from valves, connectors, pipes, lubricants, etc. can be dissolved in the water. The system must therefore always be flushed for a while before taking the sample. The material of the container, used for storage, normally is not critical. However, the container must be carefully closed using aluminum or PTFE cap to avoid that pollutants fro the cap will pollute the sample. It is important that the sampling is performed at the same temperature as the surroundings. Using pressurized systems the sample must be done at flow rates of 500 mL/min or higher. 

In principle, there are three ways of sampling: grab, composite and continuous. Also in this case it is of great importance that a representative sample is taken. The most critical parameters, in this respect are; the sampling frequency, the time point of sampling and the storage of the sample (without degradation or contamination) until the actual analysis. 

  • The composite methods mean that individual samples, taken at different time points, are nixed and analyzed. The result of measuring a composite sample is a mean value which can provide less reliable data for non stabile compounds. Surface water samples are often collected over a period of 24 h, while the sampling of industrial processes should be conducted over at least a full process cycle. 
  • Sampling from flowing systems means that it is important that on different time points samples of the exactly the same volume are taken. In addition it is important that the concentration of the analytes and flow of the system are fluctuating too much in time. The minimum time for the sampling procedure depends on:
    • The speed of the sampling; 
    • The concentration of the analytes;
    • The sensitivity of the analytical system.  

Gaseous and liquid samples can be trapped on an adsorbent. This process is normally more efficient for gases compared with liquids. In the case of water samples a certain volume of water (usually 500 – 1500 mL) is flushed over the adsorbent (e.g. XAD), which can be packed in a small cartridge or column. After sampling the adsorbed analyte(s) can be determined directly or indirectly. In Table 1.6 an overview is given of some of the adsorbents that can be used for the sampling of aqueous samples. 

The sampling of a representative tap water sample can be performed by flushing the tap, first, during 10 min with a flow of 500 mL/min, to remove all the air and solid particles present in the piping system. A positive feature is that during this flushing the temperature of the water will b stabilized. In order to guarantee the quality of the sample the conductivity and the pH should be measured and a sufficiently large sample should be taken allowing, if necessary, a duplicate or triplicate analysis.

Table 6: Sorbents for the sampling of aqueous samples







Ambersorb XE-340

CHCl3, C5H11, CH3OH

Thermal desorption


Organic compounds

Desorption with organic solvents

Molecular sieves 13X


Thermal desorption

Porous polymers



Chromosorb 106

CHCl3, C5H11, CH3OH

Thermal desorption

Macroporous polymer

Aromatic hydrocarbons

Thermal desorption, ppb level

Porapak Q,
Chromosorb 101



Styrene-divinylbenzene copolymer



Porous polyurethane foam



Tenax GC




CHCl3, C5H11, CH3OH

Thermal desorption, ppb level


Petroleum, n-C6, acetone

Recoveries 90-100%, diphenylamine



Thermal desorption ppb level


Volatile organic components


XAD resins




Haloform, Dioxines



Organic pollutants

Desorption with organic solvents

Normally the sample containers should be completely filled to avoid the volatilization of volatile compounds. However, during transport it may be necessary to fill a sample container for only 90% to allow temperatures.            

An important group of methods for the sampling of gases and vapors are the cryogenic techniques. Sampling and concentration of samples using these techniques results in a direct availability of the sample for the next step in the analytical procedure. A disadvantage can be that in case large volumes of water are present, sample losses can occur because of aerosol formation. In Table 7 gives an overview of a number of reagents that can be used for the fast freezing of samples. Liquid nitrogen can be used to replace atmospheric air.

Table 7: Stabilization of organic compounds.




Neutral compounds

pH > 2, thiosulfate

40C, max. 7 days before analysis


pH = 3 (monochloro acetic acid), thiosulfate

-100C, max. 28 days before analysis

Chlorinated acids


40C, max. 14 days before extraction

Chlorinated dioxins and furans


200C, max. 90 days before extraction

Diquat and paraquat

pH = 2, thiosulfate

40C, max. 7 days before extraction, after that 30 days



40C, max. 7 days before analysis



40C, max. 14 days at -100C max. 18 months

Halo acetic acids

Ammonium chloride

40C, max. 28 days before extraction

Neutral chlorinated compounds


40C, max. 6 weeks before extraction

Organochlor pesticides


40C, max. 7 days before extraction

Organophosphor pesticides


40C, max. 14 days before extraction


Chloroform / Sulfuric acid

3 weeks

Phenoxyacid herbicides

pH = 2 (sulfuric acid)

40C, max. 50 days

Phenylurea herbicides

pH 5 – 9

40C, max. 7 days before extraction

Phthalates and adipate esters


40C, max. 14 days before extraction



40C, max. 14 days before extraction


pH < 2, thiosulfate

40C, max. 7 days before extraction


pH 5 – 9

40C, max. 7 days before extraction

Volatile organic compounds


40C, max. 14 days before extraction

The techniques that can be used for suspensions include: filtration, centrifugation and sedimentation:

  • In filtration a liquid is passed through filter, made of a suitable material to avoid unwanted absorption / adsorption effects, to remove suspended particulates. It is highly recommended to prevent backpressure problems and to preserve the lifetime of separation columns to filter eluents before a chromatographic or electrophoretic separation. Filter materials must be compatible with the used solvents and may not dissolve during the experiments.  Large porosity (> 2  mm) filters should be used for maximum flow or small-porosity filters (< 0.2  mm) to remove bacteria.
  • During centrifugation a sample is placed in suitable container (e.g. tapered centrifuge tube) and spun at high velocity in an appropriate centrifuge. Subsequently the supernatant liquid is decanted. The quantitative removal of solid sample from a tube sometimes presents a practical problem.
  • In sedimentation the sample is allowed to settle when left undisturbed in a sedimentation tank. The settling rate depends on the Stokes’ radius. It is an extremely slow process and in particularly the particle size is determining the settling rate. 
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