Precision and sensitivity

The sensitivity of a radioreceptor assay is primarily dependent on two factors, one of which is the affinity of the receptor for the radioligand. Thus, if 20% displacement of pH]-GABA is taken as the least amount that can accurately be determined, the sensitivity of the radioreceptor assay for GABA, as illustrated in Fig. 2, is approximately 10 pmol using a 2 ml incubation volume. If the receptor affinity could be increased tenfold, then the concentration of GABA necessary to inhibit 20% would also be reduced a similar amount, making for a more sensitive assay. 

Perhaps the major determinant in deciding on an appropriate volume for assay is the specific activity of the radioligand. For example, if in a 2 ml incubation volume total specific radioligand binding is 1000 cpm/mg protein, only 100 cpm/mg protein will be bound using a 0.2 ml volume. 

With respect to precision, radioreceptor assays are similar to other analytical techniques. In a given assay the amount of displacement of radioligand will normally vary less than 5% among triplicate samples. When the same sample is analyzed on different days, the variation between results averages less than 20%. Again, the greater the number of dilutions analyzed, the more precise will be the value obtained. 

The major drawback of radioreceptor assays relates to their specificity. While receptor binding assays are specific in that only a limited number 

Exogenously administered substances are not the only agents which might interfere with a receptor binding assay. For example, opiate receptor binding as a radioreceptor assay for morphine would appear to be a relatively specific procedure since only opiates can interfere with the attachment to this receptor site. However, the discovery of endogenous opioid peptides makes the morphine assay less feasible with brain tissue because of the potential for these substances to be present in the tissue extracts. 

---

The Mattauch-Herzoggeometry (Fig. 3.20) enables detection of several masses simultaneously and is, therefore, ideal for scanning instruments [3.49]. Up to five detectors are adjusted mechanically to locations in the detection plane, and thus to masses of interest. Because of this it is possible to detect, e. g., all isotopes of one element simultaneously in a certain mass range. Also fast, sensitive, and precise measurements of the distributions of different isotopes are feasible. This enables calculation of isotope ratios of small particles visible in the image. The only commercial instrument of this type (Cameca Nanosims 50) uses an ion gun of coaxial optical design, and secondary ion extraction the lateral resolution is 50 nm. 

However, I believe that enough has been described to support my contention that modern methods of characterisation are absolutely central to materials science in its modern incarnation following the quantitative revolution of mid-century. That revolution owed everything to the availability of sensitive and precise techniques of measurement and characterisation. 

The measuring procedure used must be appropriate to the chemical agent to be measured, its limit value, and the wotkplace atmosphere so that the results show the concentration of the agent in the same terms as the limit value. Also, the limits of detection, sensitivity, and precision of the measuring procedure must be appropriate to the limit value. 

Overall, the RDE provides an efficient and reproducible mass transport and hence the analytical measurement can be made with high sensitivity and precision. Such well-defined behavior greatly simplifies the interpretation of the measurement. The convective nature of the electrode results also in very short response tunes. The detection limits can be lowered via periodic changes in the rotation speed and isolation of small mass transport-dependent currents from simultaneously flowing surface-controlled background currents. Sinusoidal or square-wave modulations of the rotation speed are particularly attractive for this task. The rotation-speed dependence of the limiting current (equation 4-5) can also be used for calculating the diffusion coefficient or the surface area. Further details on the RDE can be found in Adam s book (17). 

Several analytical methods are available to quantify chlorophylls and choice depends on the information needed. For quality control in industries and legislation attendance, simple and cost-effective methods represent widely used problem-solving approaches. For research purposes, more sensitive and precise methods are necessary to identify chlorophylls and derivatives simultaneously and individually. 

Immunoassays designed for environmental applications are mostly sold as some variation of the ELISA format. ELISA-like formats dominate the field because they are inexpensive and because they provide high sensitivity and precision without requiring complex instrumentation. The basic ELISA format supports both field and laboratory-based applications but is limited by multiple steps and inadequate sensitivity for some applications, excessive variability and sometimes long analysis times. Some of the other formats discussed in this article may replace the ELISA for selected applications however, because many laboratories are familiar with the ELISA technology, there will be a significant delay before alternative formats are widely accepted. 

Methods for Determining Biomarkers of Exposure and Effect. Analytical methods with satisfactory sensitivity and precision are available to determine the levels of americium in human tissues and body fluids. However, improved methods are needed to assess the biological effects of americium in tissues. 

Comparisons of CE and HPLC on similar analytical problems have been reported [879-883]. In contrast to CE and HPLC, GC is best suited for analysis of nonpolar, lower MW, volatile compounds. HPLC and GC have detection limits roughly 100-1000 times lower than CE, while traditional electrophoresis has detection limits comparable to CE. As already mentioned, the inferior detection sensitivity and precision of CE when compared with HPLC are caused by the technique s nanoscale. 

Even if removal of inorganic carbon is complete, the analysis of the remaining carbon is difficult. At a concentration of 1 ppm, a normal value for surface water in the open ocean, a 1 ml sample will contain 1 ig carbon. If we are interested in differences between samples, we must strive for a precision of 5% or 0.05 xg C per ml sample. This requirement places severe constraints on the sensitivity and precision of instrumentation. 

Matrix absorption, secondary fluorescence and scattering phenomena limit sensitivity and precision in many cases, especially with dense matrices. The sensitivity falls off with atomic number elements with Z < 15 are particularly difficult to analyse. Analysis is characteristic of surface layers (5-500 pm depth) only for a solid specimen. Instruments are often large, complicated and costly. 

The calibration curve was linear up to 5000 /tg/mL. Recoveries for cyclosporine ranged from 90 to 110 %. The limit of detection was below 30 /.ig/mE. Within-run and between-run coefficients of variation were less than 8%. About 100 whole blood samples could be analyzed within 3 hr with very high efficiency, sensitivity, and precision. 

Previous derivatization of the extract is necessary to improve the stability of the compounds and the sensitivity and precision of subsequent GC-MS analysis. Silyl derivatives formed for example with MSTFA [43], halogenated alkene derivatives produced with heptafluorobutyric anhydride (HFBA) [36] or pentafluoropropionic acid [58] or anhydride (PFPA), as well as acetate derivatives formed using acetic anhydride [48] have been widely employed. 

Improvements in these methods are likely to take the form of improved specificity for galactose-l-phosphate uridyl transferase and improvements in preparing the red cell hemolyzates. Sensitivity and precision of the assays are now good enough to reveal a puzzling feature some galactosemics appear to have a very small amount of enzymatic activity in their red cells. This is discussed further in Section 5. 

A special type of micro DTA-holder (Fig. 13 c) uses large-surface, vapor-deposited thermopiles as the AT sensor which assure excellent heat contact with the crucibles. No undesirable heat exchange occurs at the sensor leads which are also vapor-deposited. This results in high sensitivity and precision. 

Photoemissive tubes are necessary for work in the ultraviolet range and they show greater sensitivity and precision than photoelectric cells. A simple photo-emissive tube consists of two electrodes in a vacuum. A silver cathode coated with an alkali metal is maintained at a potential difference of about 100 V from the anode, which is a plain silver wire and serves to collect the electrons (Figure 2.26(a)). 

GC/ECD and GC equipped with a microcoulometric detector have been used to determine heptachlor and heptachlor epoxide in a variety of human tissues, including the liver, brain, adrenals, lungs, heart, kidneys, spleen, and pancreas (Curley et al. 1969 Klemmer et al. 1977 Radomski et al. 1968). Details of a sample preparation method were not reported for GC equipped with a microcoulometric detector (Curley et al. 1969). Sample preparation steps for GC/ECD include homogenization, extraction with petroleum ether or hexane, usually followed by a clean-up procedure (Klemmer et al. 1977 Radomski et al. 1968). Recovery, sensitivity, and precision data were not reported (Curley et al. 1969 Klemmer et al. 1977 Radomski et al. 1968). 

Henry Dale first established that there were two different receptors for acetylcholine, the muscarinic and nicotinic, and it is now known that there are hve different classes of muscarinic and an ill-dehned number of nicotinic receptors. Different receptors were originally identihed by pharmacological methods but molecular biological techniques now provide very sensitive and precise means of identify-... 

As already discnssed, intrinsic LIF bioreactor monitoring is the most common application within biopharmaceutical manufacturing. Other extrinsic PAT LIF methods are also possible for biopharmacenticals snch as varions flnorescence immunoassays facilitated by flow injection analysis " or other real-time approaches. As PAT begins to emerge within biopharmaceuticals the wider used of sensitive and precise in-line intrinsic and extrinsic approaches such as optodes and at-line extrinsic methods is likely to occnr. ... 

Because of the differences in the construction of various purge and trap devices, actual recoveries may vary significantly from those shown in Figure 3 and Table I. Therefore it is required that individual investigators determine recoveries of compounds to be measured as a function of flow rate with their apparatus. Operation in the optimum flow rate range will assure maximum sensitivity and precision for the compounds measured. 

No studies were located that identified biomarkers specific to thorium-induced disease states. If a biomarker for thorium-induced effect in humans were found, this effect could also be used as an indicator of exposure to thorium. Therefore, there is a need to develop biomarkers that will serve as indicators of exposure and effect from exposure to uranium. It may be necessary to develop analytical methods of satisfactory sensitivity and precision for the quantification of thorium-induced effects in humans. 

Because of its sensitivity and precision, ion chromatography (EPA Method 300.0) is a good technique for analyzing chlorine dioxide in water. Ion chromatography utilizes the ability of certain ion exchange resins... 

---