Spectrum-Analyzer Devices

There is only one reported set of field data that compares measurements by a spectrum analyzer device and a standard laboratory analysis procedure (McKnight et al, 1989). A laboratory study was also carried out at NIST similar to the one described for lead-specific analyzers. In the laboratory study, the counting time was selected so that the precision (standard deviation) of individual replicate measurements of the same sample of g)fpsum wallboard was about 0.1 mg/cm2 the results are given as follows  

Estimated total variances were determined for each substrate by pooling the variances over lead concentrations. The precision (square root of the total variance) reported in the preceding table is that expected for an individual reading as opposed to that for the mean of three individual readings. The analysis of variance showed that essentially all variability was due to variability of individual replicate measurements. Hence, the precision of laboratory measurements could be improved by increasing the counting time. 

The same method of analysis as previously described, i.e., paired eompar-ison, was used to determine estimates of precision and bias for field measurement proeedures where one individual PXRF measurement was taken from a given sample. The eounting time was chosen just as it was chosen for the laboratory study. For this partieular PXRF device, for lead concentrations less than 2 mg/em over wood or plaster, the standard deviation of the differenee distribution is 0.3 mg/em, and the mean of the difference distribution is 0.1 mg/cm, with two outliers exeluded. 

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There are two general types of commercially available instruments for measuring lead in paint films that are based on exciting k-shell electrons lead-specific and spectrum analyzer devices. The lead-specific type checks only for lead, while the spectrum analyzer device can be programmed to analyze for... 

Based on data for the spectrum analyzer device, a substrate correction may not be needed for some substrates, but based on laboratory data, it may be needed for other substrates, such as steel. From the limited data available, the true lead concentration is expected to be within 0.7 mg/cm of the experimental value 95% of the time for measurements over wood and plaster when the precision of replicate measurements over the same substrate is no poorer than 0.1 mgkm. ... 

The above considerations deal only with the predetection signal-to-noise ratio. The measuring device (spectrum analyzer or autocorrelator) will, for instance, introduce further errors. In addition, thermal noise in the electrical circuits has been ignored. In general, multichannel spectrum analyzers and autocorrelators give the best postdetection signal-to-noise ratios. 

Each element, Kirchhoff showed, produced a characteristic pattern of bright lines when heated to incandescence, a pattern different from that of any other element. Kirchhoff had thus worked out a method of fingerprinting each element by the light it produced when heated. Once the elements had been fingerprinted, he could work backward and deduce the elements in an unknown crystal from the bright lines in its spectrum. The device used to analyze elements in this fashion was named the spec--troscope. (See Figure 17.)... 

Other demonstrated prototype devices include optical gyroscopes (144), broadband acoustic spectrum analyzers (145), 1x2 Y-fed directional couplers (146) and polarization-insensitive electrooptic modulators (147,148). For long-haul telecommimication applications, it is difficult to maintain polarization control thus, a need exists for polarization-insensitive modulators. Indeed, polarization insensitivity was one of the advantages claimed for gallium arsenide elec-troabsorptive modulators. By using different poling schemes, overall polarization insensitivity has been achieved for polymeric modulators (147,148). 

The appHcations of SAW devices are numerous, their implementation as deflectors, tunable filters, and frequency shifters, as described, are just a few of their possible uses. Other important device applications are time multiplexer, pulse compression of chirped signal, correlators, spectrum analyzer, and isolators. 

There are scores of microwave devices that need to be measured. The most common measurements tirat are done are the frequency measurements, but we are also interested in phase measurements depending on the device under test (DUT). We can use various equipments like spectrum analyzer, vector network analyzer, etc. [33]. Most of the devices that we measure will be two-port devices that is, the input is applied at one port and the output is taken at the other port. So we will basically be measuring devices, a group of devices that form a part of tiie system, and microwave circuits. 

The isomerization proclivity of a reactive intermediate can be interrogated by measuring the tandem mass spectrum of the survivor ion. This capability requires that the instrumentation available be equipped with an additional mass analyzing device after the one used for the mass separation of the NR products. 

McKnight, M. E. Byrd, W. E. and Roberts, W. E. 1990. Measuring lead concentration in paint using a portable spectrum analyzer X-ray fluorescence device. NISTIR W90-650, National Institute of Standards and Technology, Gaithersburg, Maryland. 

Ions in a TOF analyzer are temporally separated according to mass. Thus, at the detector all ions of any one mass arrive at one particular time, and all ions of other masses arrive at a different times. Apart from measuring times of arrival, the TDC device must be able to measure the numbers of ions at any one m/z value to obtain ion abundances. Generally, in TOF instruments, many pulses of ions are sent to the detector per second. It is not unusual to record 30,000 spectra per minute. Of course, each spectmm contains few ions, and a final mass spectrum requires addition of all 30,000 spectra to obtain a representative result. 

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