Determination of trace el ements using atomic absorption spectrometry with graphite furnace
BS EN ISO 15586:2003 pdf free.Water quality一 Determination of trace el ements using atomic absorption spectrometry with graphite furnace.
Pre-treatment and analysis of samples with especially low element concentrations should be carried out under ‘clean laboratory” conditions. The mclean laboratory” technique requires that the laboratory be supplied with filtered air, and that the samples be continuously protected from contamination originating from various sources. In some cases, dean benches with filtered laminar airflow under a weak over-pressure, may be used as a suitable alternative.
Trace elements in water samples are analysed in one or more of the following fractions.
a) Preserved by addition of acid (non-filtered). Preserve the sample by addition of nitric acid. Particles should be allowed to sediment before analysis.
b) Filtered (dissolved). Filter the sample through a membrane or capillary filter and preserve the filtrate by addition of nitric acid.
c) Digested in acid. Digest the preserved sample with nitric acid or aqua regia.
Store preserved water samples in cool conditions in accordance with ISO 5667-3 until analysis (1 C to 5 SC).
Filtration of samples is necessary if the dissolved forms of trace elements are analysed. Filter the sample immediately after sampling and before preservation. Avoid equipment where the sample may come in contact with metal parts. To reduce the risk of contamination, pressure filtration is preferable to vacuum filtration.
Prepare at least one blank test solution by filtration (and preservation) of water (5.1) in the same way as the test samples.
Preserve water samples in accordance with ISO 5667-3. To obtain a pH <2 in the samples. add 0,5 ml of concentrated nitric acid (5.2) per 100 ml of sample. For the preservation of water samples with high alkalinity, it may be necessary to add more acid. It is important that sufficient acid is added to the sample to avoid loss of elements through adsorption effects. Report the amount of acid added.
Preferably preserve the samples in the laboratory in a clean atmosphere to avoid contamination risks. For the reagent blank solution, preserve water (5.1) in the same way as the test samples.
7.2.4 Digestion of water samples
Methods for digestion of water with aqua regia and nitric acid are specified in ISO 15587-1 and ISO 15587-2, respectively. Since chloride may cause severe interference in the graphite furnace technique, digestion with nitric acid is recommended. For some elements (e.g. Sb in this International Standard) nitric acid is not suitable and aqua regia should be used.
Prepare at least one blank test solution by digesting water (5.1) in the same way as the test samples. Before analysis of the digests, make up to volume with water (5.1).
7.3.1 Storage of sediment samples
After sampling, store sediment samples in their original containers (6.2) in a refrigerator, or frozen until further treatment (ISO 5667-15).
If the determination is to be perfomed on a dry sample, preferably freeze-dry the sample , or alternatively dry it at 105 °C for 24 h. Crush the dried sample in an agate mortar (6.5). homogenize and sieve it if necessary.
Dried sediments are hygroscopic and will absorb moisture if stored. Freeze-dried samples contain a few percent water. It is necessary to control the water content by drying a sub-sample at 105 °C, before digestion and analysis.
7.3.2 Digestion of sediment samples See Annex B.
8 Chemical modification
Chemical modifiers are used to overcome spectral and/or non-spectral interferences in a sample (matrix effects).
By measuring a sample with and without addition of an analyte and by comparing the recovery of the analyte with a calibration standard, it is often possible to recognize the existence of a non-spectral interference. In order to ascertain that the modification works, the same procedure is repeated with the addition of a chosen chemical modifier.
In general, the aim of chemical modification is to allow a pyrolysis temperature that is high enough to remove the bulk of concomitants before the atomization step. The combination of Pd and Mg(N03)2 is regarded as a “universar modifier that is used for a number of elements. The combination of Pd with a reducing agent, e.g. ascorbic acid, is sometimes used instead of Pd/Mg(N03)2. The background absorption tends to be high with Mg(N03)2. Other modifiers are also used. Some of them (e.g. Ni compounds) may be disadvantageous, because they contain elements that are frequently determined with the same equipment and can cause contamination of the furnace. In Table 2 some recommendations of chemical modifiers are given for the elements in this International Standard. Other chemical modifiers may be used if they show consistent results.
If chemical modifiers are used, add them both to test samples, reagent blank solutions, blank test solutions, calibration solutions, and blank calibration solutions. To achieve the recommended amounts in Table 2, 10 p1 of modifier solution shall be added. Preferably inject the modifier solution with the autosampler directly into the atomizer after the sample is delivered.
Preferably, as a start, use the temperatures and times recommended by the manufacturer. Interrupt the argon flow during the atomization step.
Always use background correction.
Alternative wavelengths (with different sensitivities) may be used. For example, for lead, the wavelength 217,0 nm may be used, where the sensitivity is about twice of that at 283,3 nm. However, the noise is higher and the risk for interferences is greater. In the case of high concentrations a wavelength with lower sensitivity may be used, i.e. 307,6 nm for Zn and 271.9 nm or 305.9 nm for Fe.
For evaluation the integrated absorbance (peak area) is recommended
10.1 Standard calibration technique
Perform the calibration with a blank calibration solution (5.14) and 3 to 5 equidistant calibration solutions (5.13) for an appropriate concentration range. It should be stressed that the linearity of the calibration curve is often limited.
Correct the absorbance values of the calibration solutions by subtracting the absorbance value of the blank calibration solution. For plotting of a calibration curve or foi calculation of the calibration function, use the resulting values together with the analyte concentrations of the calibration solutions.
10.2 Standard addition technique
To reduce the effect of non-spectral interferences, where chemical modification is not used or does not eliminate matrix effects, the standard addition technique may be applied provided the calibration curve is linear in the absorbance range used. The standard addition technique cannot be used to correct for spectral interferences, such as unspecific background absorption, and shall not be used if interferences change the signal by a factor of more than three.
Transfer equal volumes of the test sample to three vessels (e.g. autosampler cups). Add a small amount of standard solution to two of the vessels so as to obtain corresponding absorption values that are 100 % and 200 % higher than that which would be expected from the original sample Add an equal amount of water (5.1) to the third vessel. Mix the solutions well. Measure the integrated absorbance of each solution, and then plot the concentration added along the abscissa against the measured absorbarice along the ordinate, as illustrated in Figure 1. Determine the analyte concentration in the reagent blank solution or blank test solution in the same way. In Figure 1 the analyte concentration of the test sample solution is 6,67 pg/i, and the blank test solution is 0,36 pg/I.BS EN ISO 15586 pdf free download.Determination of trace el ements using atomic absorption spectrometry with graphite furnace