In some cases, excreta monitoring may be the only measurement technique for those radionuclides which have no g-ray emissions or which have only low energy photon emissions. Excreta monitoring programmes usually involve analysis of urine, although faecal analysis may also be required if the material is relatively insoluble. Other samples may be analysed for specific investigations. Examples are the use of nose blow or nasal smears as routine screening techniques. Blood can be sampled in the case of suspected high level contamination, although activity concentrations are generally difficult to relate to body content or intakes (see also publications of IAEA [14-16]).

The collection of urine samples involves three considerations. Firstly, care must be taken to avoid adventitious contamination of the sample. Secondly, it is usually necessary to assess or estimate the total activity excreted in urine per unit time from measurements on the sample provided. For most routine analyses, a 24 h collection is preferred but, if this is not feasible, it must be recognised that smaller samples may not be representative. Where a 24 h sample is not easily collected then the first morning voiding is preferable for analysis [15]. The total daily excretion of creatinine, produced as a metabolic product in muscle metabolism, may be less variable than the volume of fluid lost in urine, although some individuals may still exhibit wide daily variations. For some radionuclides, adequate sensitivity can be achieved only by analysis of several days’ excreta (e.g see Duke, [17]). Measurement of creatinine concentration in urine has frequently been used to estimate 24 h excretion of radionuclides from urine samples collected over part of a day. Tritium is an exceptional case for which it is usual to take only a small sample and to relate the measured activity concentration to the concentration in body water. Thirdly, the volume required for analysis depends upon the sensitivity of the analytical technique.

The analysis of faecal samples for routine monitoring involves uncertainty in interpretation owing to daily fluctuations in faecal excretion. Ideally, therefore, collection should be over a period of several days. However, this may be difficult to achieve in practice and interpretation may need to be based on a single sample. Faecal monitoring is more often used in special investigations, particularly following a known or suspected intake by inhalation of moderately soluble, Type M or insoluble, Type S compounds. In these circumstances measurement of the quantity excreted daily may be useful in the evaluation of clearance from the lungs and in the estimation of intake. Early results may be useful in identifying exposed individuals.

Table 3:                 Typical lower limits of detection for in vitro measuring techniques

Radionuclides that emit g-rays may be determined in biological samples by direct measurement with scintillation or semiconductor detectors. Analysis of a- and b-emitting radionuclides requires chemical separation followed by appropriate measurement techniques. Measurement of so-called total a or b activity may occasionally be useful as a simple screening technique, but there is no single method that will determine accurately all the a- and b-activity in the sample. The results cannot always be interpreted quantitatively, but can be useful for providing confirmation of satisfactory working conditions, an unusual result indicating the need for further investigation which would include radiochemical analysis.

Measurement of activity in exhaled breath is a useful monitoring technique for some radionuclides such as ²²⁶Ra and ²²⁸Th since the decay chains of both these radionuclides include gases which may be exhaled [18,19]. It can also be used to monitor ¹⁴CO₂ formed in vivo from the metabolism of ¹⁴C‑labelled compounds [20, 21].

Increasing use is being made of mass spectrometric techniques for the analysis of excreta samples. Inductively Coupled Plasma - Mass Spectrometry (ICP-MS, Fig. 7) can achieve much lower detection limits for long-lived radionuclides than is possible with alpha spectrometry. For example, for ²³⁸U and ²³²Th, detection limits are in the region of 5-10 mBq per sample [22]. Measurement times are in the region of a few minutes, whereas an alpha spectrometry measurement typically takes several days. The more advanced mass spectrometric techniques such as multiple collector ICP-MS or sector field ICP-MS have the capability to detect very small changes in isotopic ratios and so can detect small amounts of depleted or enriched uranium in urine samples [23]. In the case of ²³⁹Pu the use of Thermal Ionization Mass Spectrometry (TIMS), which has a minimum detection limit of about 4 μBq per sample, allows detection of an intake of Type S that would give rise to a committed effective dose of about 0.2 mSv. The more complicated technique of accelerator mass spectrometry (AMS) can be used to measure ¹⁴C in small samples, mg-size, with low activities down to ~0.1mBq/ml with high accuracy (<2%). The AMS technique may need to be considered for routine monitoring in situations where exposure to [¹⁴C]-compounds that deposit extensively in adipose tissue, or other tissues with very slow metabolic turnover, biological half-times of 60->150 d, is likely, since it must be expected that the elimination of ¹⁴C from such tissues will occur by metabolism to [¹⁴C]CO₂ and exhalation in the breath.