Abundance sensitivity specifications

FIGURE 7.9 A low abundance sensitivity specification is critical to minimize spectral interferences, as shown by (a), which represents a spectral scan of 50 ppb of at 75.5 amu,... 

Several basic types of methods have been employed to idenfify low-abundance, tissue-specific transcripts. The more classical differential hybridization techniques (e.g., 1) are mostly limited to the detection of moderately abundant transcripts representing >0.05% of the mRNA population (2). Subtractive hybridization techniques can increase the detection sensitivity by 10- to 100-fold and make the identification of quite rare genes possible (2). A specialized form of subtractive hybridization, the Gene Expression Saeen (3), can detect both upregulated and downregulated transcripts. A number of protocols have been devised in recent years to simplify and expedite the process of transcript identification by subtractive hybridization (4-8). Here we present a comprehensive set of methods that have proven quite successful in our laboratories and that may serve as an entry point for future refinement. 

Desrosiers [23] considers GC-MS in selective ion monitoring (SIM) mode as being the best analytical system for the quantification of in-polymer additives in polyoleflnic materials. The method is extremely sensitive and can be used for highly accurate quantitative work. Levels in the ppb range may be measured and samples in the low or sub-ppm range may be truly differentiated provided homogeneous samples are available. However, GC-SIM-MS is a most difficult system to calibrate and to maintain. In selected ion monitoring, total ion scans are obtained of each additive and byproduct of interest. A concentrated ion, specific to that additive or byproduct at the time of elution from the analytical column, is chosen as the quantitative ion. Usually, in addition, two other ions with lower abundances and specific of the component of interest, at the... 

As described in Chapter 7, there are two very important performance specifications of a quadrupole—resolution and abundance sensitivity. Although they both define the ability of a quadrupole to separate an analyte peak from a spectral interference, they are measured differently. Resolution reflects the shape of the top of the peak and is normally defined as the width of a peak at 10% of its height. Most instruments on the market have similar resolution specifications of 0.3-3.0 amu and typically use a nominal setting of 0.7-1.0 amu for all masses in a multielement run. For this reason, it is unlikely you will see any measurable difference when you make your comparison. 

The thermal ion mass spectrometer was specifically developed for the measurement of isotope abundances and is capable of excellent precision. Although the spark source mass spectrometer used in this work lacks some of this precision, it has proved very useful in stable isotope dilution work. It has a number of advantages, including greater versatility, relatively uniform sensitivity, and better applicability to a wide range of elements. 

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