12 Sample handling
12.1.1 The method of handling samples between the point at which they are extracted or drawn and the laboratory test bench or sample storage are devised to ensure that the nature and integrity of the sample are maintained.
12.1.2 The method of handling a sample will depend on the purpose for which it has been taken. The laboratory analytical procedure to be used will often require a special handling procedure to be associated with it. For this reason consult the appropriate method of test so that any necessary instructions as to sample handling tan be given to the person drawing the sample. If the analytical procedures to be applied have conflicting requirements, then draw separate samples and apply the appropriate procedure to each sample.
12.1.3 Take particular care in respect of the following:
a) liquids containing volatile material, since loss by evaporation can occur;
b) liquids containing water and/or sediment, since separation tends to occur in the sample container;
c) liquids with potential wax deposition, since deposition can occur if a sufficient temperature is not maintained.
12.1.4 When making up composite samples take great care not to lose light ends from volatile liquids and not to alter water and sediment content. This is a very difficult operation and should be avoided if at all possible.
12.1.5 Do not transfer samples of volatile liquids to other containers at the sampling location but transport them to the laboratory in the original sample container, cooled and inverted, if necessary. Great care is necessary if a sample contains both volatile components and free water.
12.2 Homogenization of samples
Procedures are specified for the homogenization of samples that may contain water and sediment, or are in any other way non-uniform, before transfer from the sample container to smaller containers or into laboratory test apparatus. Procedures for verifying that the sample is satisfactorily mixed before transfer are given in 12.3.
It is not possible to agitate manually samples of liquids containing water and sediment sufficiently to disperse the water and sediment within the sample. Vigorous mechanical or hydraulic mixing is necessary in order to homogenize the sample prior to transfer or sub-sampling.
Homogenization may be by various methods. Whichever method is used it is recommended that the homogenizing system produces water droplets below 50 μm, but not less than 1 μm. Water droplets of below 1 μm will give a stable emulsion and the water content cannot then be determined by centrifuge methods.
12.2.2 Homogenization by high-shear mechanical mixer
Insert a high-shear mechanical mixer into the sample container so that the rotating element reaches to within 30 mm of the bottom. A mixer with counter-rotating blades operating at about 3000 min(-1) is usually suitable. Other designs may be used if the performance is satisfactory.
NOTE - High-shear mixers frequently produce stable emulsions and water contents after mixing cannot be determined by the centrifuge method (ISO 3734).
Avoid any significant rise in temperature during the mixing.
12.2.3 Circulation with external mixer
This method tan be applied to both permanently sited sample containers and portable containers; for the latter, use a quick-disconnect coupling. Circulate the contents externally using a small pump through a static mixer installed in small-bore piping. Various designs are suitable; follow the manufacturer's operating instructions.
Use a circulation flow rate sufficient to circulate the contents at least once per minute. A typical mixing time is 15 min, but this will vary according to the water content, the type of hydrocarbon and the design of the system. When the whole sample is thoroughly mixed, run off the required quantity of sub-sample from a valve in the circulating line whilst the pump is running. Then empty the container and thoroughly clean the entire system by pumping solvent round until all traces of the hydrocarbon and water have been removed.
12.3 Verification of the efficiency of mixing
12.3.1 Whatever means are chosen for obtaining a sub-sample from a non-homogeneous mixture, verify the suitability of the mixing technique and the time required to obtain a suitably mixed sample.
12.3.2 If the sample remains homogeneous and stable after mixing (e.g. where completely miscible components such as lubricant additives have been blended), continue the mixing procedure until successive samples drawn from the main bulk of the sample give consistent results. This establishes the minimum mixing time.
NOTE - As the sample is homogeneous after this time, and will remain so, transfers from the main bulk tan be made without further mixing.
12.3.3 If the sample does not remain homogeneous for more than a short period of time after mixing (for example if water and sediment are part of the mixture), use the special method for the verification of mixing described in 12.3.4.
NOTE - lt may be necessary, owing to the characteristics of the hydrocarbon, to sub-sample while mixin is still in progress.
12.3.4 Ensure that the sample as drawn fills the containers to about three-quarters full, and homogenize the sample for a known period of time, which should be recorded. During this period draw off small portions at regular intervals and test each immediately for water content by a standard method (see 12.3.5). When the test results are consistent record the value obtained as the blank water content.
Add an accurately measured quantity of water, between 1 and 2 %, and homogenize for the same period of time as for the blank and take samples as before. If there is good agreement between the water content determined, taking account of the blank water content, and the quantity of water added, then repeat by adding a further accurately measured quantity of water, again between 1 and 2 %. If the results continue to give good agreement then assume that the mixing time is adequate.
If the results do not show good agreement (within the repeatability of the method), then discard them. Revert to the beginning of the procedure, and use a longer period of mixing.
12.3.5 Do not determine water content by the centrifuge method (ISO 3734) for this verification of the mixing system, as the method cannot be relied upon to give the total water content.
12.4 Transfer of samples
12.4.1 If the sample container is not portable, or if it is inconvenient to take samples directly from the container into laboratory test apparatus, transfer a representative sample into a portable container for transport to the laboratory.
12.4.2 At every stage of transfer of a sample it is essential to homogenize the contents of the container from which the sample is being taken using one of the methods specified in 12.2.
12.4.3 Verify the mixing time for each combination of container and mixer by one of the methods specified in 12.3.
12.4.4 Complete any transfer of sample within the period during which the mixture is known to be homogenous and stable. This period is short; do not take longer than 20 min to complete any transfer.