Spectrometric error

Using spreadsheets to calculate unknown concentrations, and their standard deviations from the calibration curve, p. 481 Spectrometers (components) for UV, visible, and IR regions, p. 483 FTIR spectrometers, p. 499 Spectrometric error, p. 501 Fluorometry, p. 505 Optical sensors and fiber optics, p. 511... 

Table 2.9 Relative Concentration Error from 1% Spectrometric Error...

Elements with 1 predominant isotope can also, potentially, permit very precise atomic weight determinations since variations in isotopic composition or errors in its determination have a correspondingly small effect on the mass-spectrometrically determined value of the atomic weight. Nine elements have 1 isotope that is more than 99% abundant (H, He, N, O, Ar, V, La, Ta... 

Do of 1.40 eV for AI2 is within the error bounds of the experimental value of 1.55 0.15 eV determined by Stearns and Kohl (46) using a Knudsen cell mass spectrometric method and assuming a ground state. 

L234 are about 3 per mil lower than those calculated with commonly used X234 values, hence the revised modem sea water of 145.8 1.7 per mil (Cheng et al. 2000b), compared to earlier values about 3 per mil higher. In general half-lives are now known precisely enough so that their contribution to error in age is comparable to or smaller than typical errors in isotope ratios (determined with mass spectrometric techniques). 

Mass spectrometric measurements on corals typically result in errors in and °Th of 2 per mil or better (2a), with the exception that fractional error in °Th typically increases progressively from this value for samples progressively younger than several ka. This results from the low concentrations of °Th in very young corals. Errors in Pa are typically somewhat larger than those of the other isotopes, with errors of several per mil, except for corals younger than a few ka. 

Figure 15. Error in age vs. age, both on log scales (after Edwards et al. 1997). Each data point represents data from a particular sample analyzed by thermal ionization mass spectrometric techniques. Solid circles represent Pa ages. Contours of aniytical error in Pa/ U pertain to the Pa data points. Shaded squares represent °Th ages. See text for discussion.

There are a few developments on the horizon that will increase our ability to date bones and teeth reliability. Both y- and a-spectrometric methods can measure Pa/ U and °Th/U and concordance between dates calculated using the two can provide a measure of reliability. However, the discordance between the two is not very sensitive to different uptake regimes, and it is difficult to resolve, for example, bones that have undergone EU from those that have undergone LU with the analytical errors commonly encountered in measurements by y- and a-spectrometry. On the other hand, it has been shown recently that TIMS can measure both isotopic ratios with a precision usually better than 1% (Edwards et al. 1997). TIMS measurements of Pa/ U and °Th/U have yet to be routinely applied to dating fossil remains, but in the future, concordance between the two decay series will provide further evidence of the validity of a particular uptake model to a particular sample. 

After each series of experiments with beams of various intensity the section plate would be removed from the cell and disassembled, with radioactive silver washed out by nitric acid. Radioactivity of the solutions obtained was measured by a multichannel spectrometric scintillation y-counter with sensitivity of up to 10 G, i. e. around 10 of atoms which, according to calculations, is 10 times lower than sensitivity of ZnO sensor 10 G or 10 of Ag atoms respectively [28]. This difference in sensitivity lead to great inconveniences when exposing of targets was used in above methods. Only a few seconds were sufficient to expose the sensor compared to several hours of exposure of the scintillation counter in order to let it accumulate the overall radioactivity. It is quite evident that due to insufficient stability during a long period of exposure time an error piled up. 

Our first chapter in this set [4] was an overview the next six examined the effects of noise when the noise was due to constant detector noise, and the last one on the list is the first of the chapters dealing with the effects of noise when the noise is due to detectors, such as photomultipliers, that are shot-noise-limited, so that the detector noise is Poisson-distributed and therefore the standard deviation of the noise equals the square root of the signal level. We continue along this line in the same manner we did previously by finding the proper expression to describe the relative error of the absorbance, which by virtue of Beer s law also describes the relative error of the concentration as determined by the spectrometric readings, and from that determine the... 

One common characteristic of many advanced scientific techniques, as indicated in Table 2, is that they are applied at the measurement frontier, where the net signal (S) is comparable to the residual background or blank (B) effect. The problem is compounded because (a) one or a few measurements are generally relied upon to estimate the blank—especially when samples are costly or difficult to obtain, and (b) the uncertainty associated with the observed blank is assumed normal and random and calculated either from counting statistics or replication with just a few degrees of freedom. (The disastrous consequences which may follow such naive faith in the stability of the blank are nowhere better illustrated than in trace chemical analysis, where S B is often the rule [10].) For radioactivity (or mass spectrometric) counting techniques it can be shown that the smallest detectable non-Poisson random error component is approximately 6, where ... 

All samples were dried for 72 hours at 80°C, and dry weights were calculated. Dried samples were milled to spectrometrically measure the specific activity of i Cs. The standard error of specific activity was in the range 10-20%. Statistical analysis used the software package MS Excel. 

Note that, during mass spectrometric analyses, the isotopic composition (230rh/232-ph) is measured independently from (2 U/2 2 ph) and thus the errors of these two ratios are not correlated, even though they have the same denominator in 2 h. (22 U/22 Th) is obtained by isotope dilution method while (2 h/2 2jjj is measured directly by mass spectrometers. 

Routine spectrometric analyses currently run in industry may involve mixtures of up to 20 or 30 components. The solution of systems of 20 or 30 simultaneous equations by hand calculations is so tedious an operation, and so subject to human error, that it is impractical to accomplish it on a routine basis. The necessary calculations are, however, a well-defined arithmetic procedure easily adapted to digital calculators. Analog computers can also be used in solving simultaneous equations but are subject to accuracy limitations. 

An easy calibration strategy is possible in ICP-MS (in analogy to optical emission spectroscopy with an inductively coupled plasma source, ICP-OES) because aqueous standard solutions with well known analyte concentrations can be measured in a short time with good precision. Normally, internal standardization is applied in this calibration procedure, where an internal standard element of the same concentration is added to the standard solutions, the samples and the blank solution. The analytical procedure can then be optimized using the internal standard element. The internal standard element is commonly applied in ICP-MS and LA-ICP-MS to account for plasma instabilities, changes in sample transport, short and long term drifts of separation fields of the mass analyzer and other aspects which would lead to errors during mass spectrometric measurements. 

A major topic in isotope mass spectrometry is the determination of the half-lives of long-lived radionuclides. De Bievre and Verbruggen34 determined the half-life of 241 Pu for 3-decay in the isobaric radionuclide 241 Am on material from Oak Ridge that had initially been about 93% isotopically enriched. Due to the isobaric interference of 241 Pu and 241 Am radionuclides during mass spectrometric measurements by TIMS, Am had to be removed by chemical separation immediately (less than 48 h) prior to measurements as described in reference 34. On the basis of all the measurements performed over an extended period of more than 20 years and after considering the possible effects of systematic errors during these measurements, a half-life for the 3 decay of 241 Pu of (ti/2 = 14.290 0.006 a) was reported.34... 

J. R. Troost and E. Y. Olavasen [ Gas Chromatographic/Mass Spectrometric Calibration Bias, Anal. Chem. 1996,68, 708] discovered that a chromatography procedure from the U.S. Environmental Protection Agency had a nonlinear response on a variety of instruments. The assumption of constant response factor led to errors as great as 40%. 

Several formal and informal intercomparisons of nitric acid measurement techniques have been carried out (43-46) these intercomparisons involve a multitude of techniques. The in situ measurement of this species has proven difficult because it very rapidly absorbs on any inlet surfaces and because it is involved in reversible solid-vapor equilibria with aerosol nitrate species. These equilibria can be disturbed by the sampling process these disturbances lead to negative or positive errors in the determination of the ambient vapor-phase concentration. The intercomparisons found differences of the order of a factor of 2 generally, and up to at least a factor of 5 at levels below 0.2 ppbv. These studies clearly indicate that the intercompared techniques do not allow the unequivocal determination of nitric acid in the atmosphere. A laser-photolysis, fragment-fluorescence method (47) and an active chemical ionization, mass spectrometric technique (48) were recently reported for this species. These approaches may provide more definite specificity for HN03. Challenges clearly remain in the measurement of this species. 

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