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The results obtained are tabulated using QROS in house software to present values in a ppm format and with sample homogeneity flags and site specific total metals exceedence warnings.
Sample homogeneity is a big issue when analysing samples containing made ground. Metals are usually present in their insoluble carbonate or oxide form and form clumps, unlike hydrocarbons that are usually more evenly dispersed. XRF analysis detects metals within the top few millimetres of the soil sample surface and relies on good sample preparation to obtain the best results. The QROS sample preparation and analysis protocol takes into account these factors and gives results that are usually very close to laboratory results.
There are certain situations where XRF and the laboratory analysis will not agree that well. The first situation is where soil samples contain a very high proportion of stone. Some l;aboratories crush the stone and this is included in the total result. Other laboratories remove the stone and only analyse the fine material. The QROS XRF method analyses only the fine material and reports the results as uncorrected for stone content.
Moisture content will also cause a variation. Soils containing moisture content will give a lower result compared to the same sample if it has been dried. Soils with a moisture content of 20% will therefore have a higher lab result than the XRF because the lab dries soil samples before analysis for metals. The QROS results are reported as uncorrected for moisture content. If an Mcerts laboratory analysis is used, the results should report moisture content and it is possible to compare data that way.
A final reason for a difference in results is down to the nature of the analysis. XRF will detect all metals in the sample, regardless of the matrix, provided they are at a high enough concentration. Laboratory analysis requires samples to be extracted and the extraction acid does not dissolve all of the soil. Metals can be trapped in the acid insoluble matrix and these will not be detected. Arsenic is commonly bound up in Silica Iron matrixes that are insoluble in the usual acids laboratories use to digest the sample.
Analysis by XRF is however a very rapid technique, typically generating results in 30 seconds. The QROS method takes the average of 3 separate analyses on a sample to obtain a more representative result for the sample. This increases the actual per sample analysis time to approximately 3 minutes, including sample preparation.
XRF is a very useful on site analysis tool and has a very good correlation with laboratory analysis. This has been demonstrated on numerous occasions. There are however some metals that XRF will struggle with. The most difficult metal for XRF to detect easily is Cadmium. The detection limit for this metal is usually in the twenty mg/kg range, which is not really sufficient for residential housing projects if Cadmium analysis is required. QROS uses a technique called Anodic Stripping Voltammetry for this metal if it is suspected. ASV can easily detect 1mg/kg in soil and as low as 10 microgram/litre in water.
Mercury is another important environmental pollutant that XRF can have difficulties with. Mercury and Bromine can overlap and unless a very specific calibration is used, the presence of Bromine can cause false positive results. Bromine has been used in fire retardants and these can fine their way into landfill. ASV can again be used to detect mercury very specifically and this may be an option if Bromine is suspected. ASV can detect 1mg/kg of Mercury in soil and 0.01mg/l in water.
Mercury is another important environmental pollutant that XRF can have difficulties with. Mercury and Bromine can overlap and unless a very specific calibration is used, the presence of Bromine can cause false positive results. Bromine has been used in fire retardants and these can fine their way into landfill. ASV can again be used to detect mercury very specifically and this may be an option if Bromine is suspected. ASV can detect 1mg/kg of Mercury in soil and 0.01mg/l in water.
Chromium is one of the lightest metals detected by XRF and although the XRF analyser can detect Chromium, the problem is that the X Rays emitted by Chromium are weak and cannot penetrate back through the soil matrix to the detector. This means only Chromium on the sample surface is detected, but this os often good enough for most applications. The main concern over Chromium is when it is in the toxic ionic form of Chrome (VI). QROS uses a colourimetric technique to measure Chrome (VI) that gives very accurate and reproducible results.
