Measurement techniques background contamination

In PIGE the y-emission is usually prompt. If very low amounts of trace elements have to be detected it can be advantageous to use a delayed decay. In this case, the technique is called charged particle activation (CPA) and is an analogue to neutron activation analysis (NAA). It has the advantage that the prompt background from interfering reactions is completely removed as irradiation and analysis are completely separated in time. This also allows to remove external contaminants in the short time between irradiation and measurement which further improves detection limits. A comprehensive description of the technique can be found in the ion beam analysis handbook [2], For 19F CPA is conceivable in special cases via the 19F(d,dn)18F reaction. However, we have found only one application in the literature [64],... 

Few papers on the analysis of PCAs or their measurement in environmental samples have reported on techniques to minimize contamination. PCAs (C10-C13,60-70% Cl) levels ranging from 4 ng g 1 to 25 ngg 1 in sodium sulfate were found in procedural blanks used in sediment extractions [28]. PCAs (C10-C13,60-70% Cl) were also detected in DCM (0.15 pg 1 ) left to evaporate in an open flask overnight it was unclear, however, whether contamination was a result of airborne PCAs or was from the DCM itself [28]. Similar problems have been encountered with airborne PCB contamination of analytical labs [65]. Significant procedural blanks result in higher method detection limits, i. e., the mean plus three times the standard deviation in the background signals from procedural blanks (sodium sulfate) [14,66,67]. 

The methods available in most literature are not necessarily suitable for all samples and often do not include the most modern sample preparation techniques. Most methods often require proof of their reliability by including recovery studies to support confidence in the analysis. Analysis of an unknown plastic material with unknown levels of metals often requires extreme care in all stages of analysis when compared with a known plastic and using a well tried and developed method that has background information. In some cases where measurements of extremely low levels of metals are required, entirely different and more sensitive methods are often used taking special precautions in avoiding contamination and interference. 

The detection of Kr, produced by cosmic rays, is of great importance for dating samples from the Earth s crust, groundwater, and polar ice cores. Measurement of Kr in groundwater is a unique analytical problem as its concentration is ca. 1400 atoms per liter. The problem is further complicated by isotopic interference of stable krypton isotopes ( Kr/ Kr is about I0 ). Prior to final analysis by RIS-TOF [8] several consecutive steps of field sampling and isotope concentration were necessary. All steps required extreme care to avoid sample contamination. Water samples of 50- 100 L were necessary to detect Kr above the background. This value is more than 100 times less than for the routine technique of Ar dating. 

This paper reviews experience gained from using the fiill range of commercial analytical techniques, to identify how contamination can be distinguished from background, using y-spectrometry and gross a+P measurement. 

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