Geoanalytical Recommendations

The analytical requirements for the purpose of producing internationally comparable geochemical baseline data are as follows:

• Given the many applications of baseline geochemical data, a comprehensive multi-element approach is essential.  Analytical requirements are considered in two categories for the, i.e.,

(a) Global or Geochemical Reference Network (GRN) using the Global or Geochemical Terrestrial Network (GTN) and Global or Geochemical        Marine Network (GMN) grid cells of 160 x 160 km, and

(b) national or regional surveys conducted by national or international organisations.

• Samples collected as part of the GRN should ultimately provide abundance data for as many elements that can be reliably determined, i.e., most elements in the periodic table, using methods with limits of detection significantly below presently estimated crustal abundances. Abundance data are not required for the following: H, O; inert gases other than Rn, Tc, Pm, Po, At, Fr, Ac and Pa.

• Analytical requirements for national or regional surveys must be fully compatible with those for the GRN. Where possible identical methods should be used, but alternative multi-element schemes based on the techniques of XRF, ICP-AES, ICP-MS, NAA, etc. can be employed as long as the criteria for precision and accuracy are met. In order to portray the spatial distribution of elements as completely as possible, detection limits must be as low as possible. In the FOREGS/EuroGeoSurveys Geochemical Atlas of Europe project all samples of the same medium were analysed for the same suite of elements in the same laboratory. This is the only practical way to produce harmonised and compatible data across political boundaries for the establishment of the Global Geochemical Reference Network, which is similar to a geodetic network that the surveyors use for producing the topographical maps.

• A proposal is made for a standard list of elements to be determined; elements are classed into List 1 (51 elements) and List 2 (20 elements) (see Darnley et al., 1995 – The Blue Book); Ru, Rh, Re, Os and Ir require the establishment of satisfactory methods and detection limits; Ra and Rn require radiometric methods.

• For national surveys, if analytical facilities are insufficient to cover all List 1 elements initially, missing data should be added at a later date. List 2 is of lower priority, but potentially important.

• For the purpose of establishing reproducible baseline data of permanent value, analytical methods should be employed, which provide the total concentration of each element present. Sample decomposition, where required, must be total.

• Partial decomposition (partial extraction) methods have many variants in different laboratories and are difficult to standardise, so they are NOT recommended for any data that may be used for international compilation or correlation.

• With respect to national and regional surveys, it is recommended that if more than 20% of the reportable values for any element determined fall below the limit of detection, the results for that element should be considered as unsatisfactory and alternative methods considered.

• Strict quality control, through the use of appropriate standard materials, i.e., Primary Reference Materials (PRMs) and Secondary Reference Materials (SRMs), as well as splits of duplicate-replicate samples is essential, and the manner in which all these materials have been used, and the resulting quality control statistics, must be reported with each data set generated.

• The Chinese series GSD, GSS and the Canadian STSD standard samples are recommended to be used as primary SRMs in international geochemical mapping. For national surveys, primary and secondary SRMs should be used to monitor the analytical accuracy: (a) primary SRMs to monitor the international or interlaboratory bias, and (b) secondary SRMs to monitor the routine between-batch drift within a laboratory.

• Radioelements are to be determined by gamma ray spectrometry.