Standard reference material absorption method

Wet chemical methods involve sophisticated sample preparation and standardization with National Bureau of Standards reference materials but are not difficult for the analytical chemist nor necessarily time consuming (Figure 1). The time from sample preparation to final results for various analytical methods, such as GFAA (graphite furnace atomic absorption), ICP (inductively coupled plasma spectroscopy), ICP-MS (ICP-mass spectrometry), and colorimetry, ranges from 0.5 to 5.0 h, depending on the technique used. Colorimetry is the method of choice because of its extreme accuracy. Typical results of the colorimetric analysis of doped oxides are shown in Tables I and II, which show the accuracy and precision of the measurements. 

If the calculated value of t (without regard to sign) exceeds a certain critical value (defined by the required confidence limit and the number of degrees of freedom) then the null hypothesis is rejected. For example, a method for determining lead by atomic absorption returned the following values for a standard reference material containing 38% Pb 38.9, 37.4, and 37.1%... 

You are developing a procedure for determining traces of copper in biological materials using a wet digestion followed by measurement by atomic absorption spectrophotometry. In order to test the validity of the method, you obtain an NIST orchard leaves standard reference material and analyze this material. Five replicas are sampled and analyzed, and the mean of the results is found to be 10.8 ppm with a standard deviation of 0.7 ppm. The listed value is 11.7 ppm. Does your method give a statistically correct value at the 95% confidence level ... 

NAA is a quantitative method. Quantification can be performed by comparison to standards or by computation from basic principles (parametric analysis). A certified reference material specifically for trace impurities in silicon is not currently available. Since neutron and y rays are penetrating radiations (free from absorption problems, such as those found in X-ray fluorescence), matrix matching between the sample and the comparator standard is not critical. Biological trace impurities standards (e.g., the National Institute of Standards and Technology Standard Rference Material, SRM 1572 Citrus Leaves) can be used as reference materials. For the parametric analysis many instrumental fiictors, such as the neutron flux density and the efficiency of the detector, must be well known. The activation equation can be used to determine concentrations ... 

Gill and Fitzgerald [481] determined picomolar quantities of mercury in seawater using stannous chloride reduction and two-stage amalgamation with gas-phase detection. The gas flow system used two gold-coated bead columns (the collection and the analytical columns) to transfer mercury into the gas cell of an atomic absorption spectrometer. By careful control and estimation of the blank, a detection limit of 0.21 pM was achieved using 21 of seawater. The accuracy and precision of this method were checked by comparison with aqueous laboratory and National Bureau of Standards (NBS) reference materials spiked into acidified natural water samples at picomolar levels. Further studies showed that at least 88% of mercury in open ocean and coastal seawater consisted of labile species which could be reduced by stannous chloride under acidic conditions. 

Since the weighing of such small sample quantities can influence the analytical error, it is desirable to avoid the need to weigh the sample. One channel is occupied by the reference absorption measurement, for example with pure iron or low-alloy steels an iron hollow-cathode lamp is used and the measurement takes place at 372 nm. In this case, only one further element can be determined. The carrying out of the method is simple (Fig. 6). Tests of this compact sample technique with standard materials, of which there are at present only a few containing certified trace elements, show satisfactory agreement (Fig. 7). As regards traces, the chips of standard materials are very homogeneous. 

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