Optimized Measurement Protocol

FIGURE 20.9 Time-resolved analysis of 30 nm gold nanoparticles by sp-ICP-MS, showing the pulse is fully characterized in less than 1 ms. (Courtesy of PerkinEhner, Inc., 2003-2012. All rights reserved. With permission.) 

As mentioned previously, sp-lCP-MS analysis is still in its infancy as an analytical tool, even though a number of researchers are actively working in this area and pushing the capabilities of the technique. However, it will probably take another few years before it will be in the hands of the routine labs. In the meantime, ICP-MS manufacturers are going to be keeping an eye on this rapidly growing application area to better-understand how the technique will be used to solve real-world analytical problems encountered by the environmental monitoring community. 

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This chapter presents, in a natural order, the different steps for obtaining an exploitable parametric map i.e., the modeling of acquired data in terms of noise and signal (Section 2), the optimization of the measurement protocol parameters (Section 3) and the actual methods of reconstruction that lead from the acquisition space to the parametric space (Section 4). The last part looks at certain applications and the current limits of these approaches (Section 5). 

A general measurement protocol consists of collecting n noisy samples by changing the value of the explanatory variables. To a vector x then corresponds a vector of measurements y in each voxel of the image. This section presents several strategies for choosing x in order to optimize the quality of estimation a priori. 

To optimize or compare measurement protocols, an index of quality must be quantified for each estimated parameter. For a parameter taken separately, the performance of estimation is a function of both the variance (or precision) and the bias (or accuracy) of the estimate. The covariance matrix of each unbiased estimator 0 checks the inequality of Frechet-Cramer-Rao 24... 

This example has been chosen to show that it is possible to calculate the optimal performance of a measurement protocol in conditions close to those in the field. For a set value of 6 (actually the unknown parameter) it is then possible to seek the value of x that minimizes CRB for a set number n. This optimization is often impossible to carry out analytically when n>2 and/or when matrix I has a complex... 

One of the difficulties of quantitative imaging is that each of the steps that result in parametric mapping is closely dependent on the context, and especially on the model and the distortions induced by the measurement protocol. To our knowledge, no software package is currently available that will manage all the tasks described in this chapter. As efficient softwares for such optimization and reconstruction becomes more generic and commonly available, such techniques will be certainly applied with increasing frequency and efficiency. 

Adhering to these rather strict requirements for optimal optical density, spectral range and minimizing spectral artifacts requires permanent verifications of the reality of the CD signal measured. Good CD results require formulation and keeping of the measurement protocol. It is recommended that the vhole series of samples that are to be compared is measured under the same conditions. For example, a broken sample cell can distort the vhole series. 

It should be emphasized that these direct measurements of ( ), C and/or p02 (or SO2) are not used routinely in clinical PDT, although various sub-sets of them have been used in some trials, in which they have helped both to optimize the protocols and to interpret the resulting responses and variations thereof In part the limitation in their clinical application have been due to lack of expertise in their use and in part to lack of available commercial clinical instruments. 

It is clear that the analytical demands put on ICP-MS are probably higher than any other trace element technique, because it is continually being asked to solve a wide variety of application problems at increasingly lower levels. However, by optimizing the measurement protocol to fit the analytical requirement, ICP-MS has shown that it has the unique capability to carry out rapid trace element analysis, with superb detection limits and good precision on both continuous and transient signals, and still meet the most stringent data quality objectives. 

So, when evaluating isotopic ratio precision, it is important that the measurement protocol and peak quantitation procedure are optimized. Isotope precision specifications are a good indication regarding what the instrument is capable of, but once again, these will be defined in aqueous-type standards, using relatively short total measurement times (typically 5 min). For that reason, if the test is to be meaningful, it should be optimized to reflect your real-world analytical situation. 

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