The precision of this method is critically dependent on the sampling, thermal history, and handling of the test sample. It should be noted that there are some anomalies between the bunker transfer standard ISO 13739 and this method. Specifically, this method (ASTM D 7621 / IP 570) states that to minimize the loss of H2S gas, do not homogenize and avoid shaking the sample before taking a test portion; and do not transfer the sample to another container. However, the sampling procedure in an Annex of ISO 13739 states that the custody transfer sample shall be thoroughly shaken or stirred to promote homogeneity and then is to be poured in small, equal portions into at least four sample bottles, making three or four passes to fill each bottle in turn to obtain nominally identical samples. Clearly, the custody transfer sample specified in ISO 13739 is unsuitable for H 2 S testing per ASTM D 7621 / IP 570 and if asked to do this we must advise our client(s), in writing, of these anomalies and discuss alternative sampling arrangements with them. Sampling for H 2 S is neither simple nor straightforward and requires very careful and specific sample handling and storage requirements as stated in the test method that will be used to analyse the sample. While these methods vary, they do have some common themes; such as:
• Time is critical - from four hours to three days; therefore, storage beyond three days is not allowed, even if the sample is refrigerated. • One sample per test; therefore, re-testing of a sample is not permitted. • Full or nearly full sample containers; therefore, HSE and transportation regulations may impact or prohibit this. • Samples must be handled carefully as agitation, elevated temperature, oxidation, etc. can all lead to loss of H 2 S and erroneous results. Finally, please remember that this is guidance material and that the definitive documents for sampling for H 2 S are the test methods themselves.
ASTM D 7621 / IP 570 Requirements
Sampling shall be performed according to normal practice and care shall be taken to ensure that the integrity of the material is maintained and the possible loss of H 2 S is kept to a minimum. • Draw the sample directly into a suitable clean H 2 S inert container, of a minimum volume of 500 mL with an impervious gas-tight closure. • Suitable containers include amber glass bottles and epoxy lined containers. Epoxy lined containers shall be visually inspected to ensure that the lining has not been damaged and that the containers are not dented. • The closure aperture shall allow the drawing of a test specimen with a syringe or pipette. • To ensure sample integrity fill the sample container to approximately 95 % full and cap immediately. • If a dedicated H 2 S sample cannot be taken, then the H 2 S measurement shall be the first test carried out on the sample as any additional handling can lead to loss of H 2 S and low results. • Take the samples to the laboratory as soon as is practicable after sampling; test immediately if possible. • If samples are not tested immediately, store in a cool place such as a refrigerator and analyse within 3 days after sampling.
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