Atomic absorption spectrometry has been rapidly developed both at home and abroad since its inception in the mid-1950s. It is widely used due to its advantages of sensitivity, accuracy, selectivity, anti-interference ability and fastness. Various fields of chemical analysis are applied, and some are listed as standard analytical methods. In recent years, atomic absorption spectroscopy has also received extensive attention and application in the field of water quality testing. Many basic water quality testing departments have equipped this instrument and have become a routine analysis method and method.
The analysis of samples by atomic absorption spectroscopy is indeed a simple and easy to grasp analytical method, but it is by no means a highly sophisticated analytical method. The main reason is that the atomic absorption spectrometer has many interference factors, such as using flame atoms. When the absorption is analyzed, the fluctuation of the flame, the amount of solution lift, etc.; when the graphite furnace atomic absorption is used for analysis, the quality of the graphite tube, spectral interference and the like are not easily controlled.
According to my actual work experience, this paper talks about several aspects that should be paid attention to when using AAS to conduct quality control assessment:
1. For all kinds of samples, there is an analytical method that is most suitable for it. To understand the application range of atomic absorption spectroscopy, consider its adaptability. It is well known that the absolute detection limit of graphite furnace atomic absorption is very high. From this point of view, it is erroneously considered that a sample with a high concentration can also be determined by graphite furnace atomic absorption method, or erroneously considered as graphite furnace atomic absorption. The method has a wide dynamic range and high accuracy.
For example, when a manufacturer purchases an instrument, it is considered to be a good instrument with a good detection limit. It is felt that a solution with a low concentration can certainly be measured for a high concentration solution. However, it is only when the instrument is bought back that this judgment is wrong. For a high concentration solution, it must be diluted to an appropriate concentration range to be determined. Therefore, for the determination of high-concentration samples, the use of high-precision measurement methods, such as the choice of spectrophotometry is better than the atomic absorption method. This is because atomic absorption spectroscopy measures the absorption of light, and the ratio of the half width of the absorption line to the emission line of the hollow cathode lamp is not more than about 10, so that it is not possible to simultaneously measure a sample having a wide concentration range as in the case of emission spectroscopy.
2, draw the correct working curve
Since the linear range of the atomic absorption method is narrow, it is particularly important to draw the correct working curve. Pay attention to the following points when doing the work curve:
(1), draw a working curve to take at least 5 to 7 points, and each point to be repeated two or more times, until the measured value of the parallel sample meets the requirements, and then the next point is measured.
(2) The same solvent system must be used for the standard sample and the sample to be tested.
(3) The concentration range selected for the working curve should include the concentration of the sample to be tested. The ideal linear range of the atomic absorption method is within 0.1~0.5 of the absorbance. If the concentration is higher, the standard curve is significantly bent. Therefore, the atomic absorption method can only be narrower than the concentration range determined by spectrophotometry. One of the remedies is to connect various absorption lines with different sensitivities to achieve a wide range of concentrations. However, this method is not suitable for alkali metal and alkaline earth metal elements with low absorption lines, and can only be applied to elements such as lead, copper, iron, manganese, and platinum. Another remedy is to add a few more points where the working curve begins to bend in order to draw the correct working curve, or use a quadratic equation to draw the working curve.
3. Effect of sample dilution on analytical results
Atomic Absorption In the field of water quality testing, two analytical methods, flame atomic absorption and graphite furnace atomic absorption, are commonly used. Since the sensitivity of the two methods is different, the corresponding analysis method should be selected according to the concentration range of the sample.
Different instruments of the same project have different working ranges. Before making a sample, you should first understand the working range of the instrument you are using. If the concentration range of the sample is not within the working range of the instrument, then it is necessary to consider diluting the sample so that the concentration range of the diluted sample is within the working range of the instrument. It is worth noting that the dilution factor is not too large, which is especially important when using graphite furnace atomic absorption for detection. This is because the sensitivity of the graphite furnace atomic absorption is high, and the distilled water, deionized water and acid used must contain impurities, which causes measurement errors.
4, the effect of acid on the determination
(1), the impact on the blank value
A few years ago, the standard curve for lead in graphite furnace atomic absorption spectrometry was that it suddenly found that the blank value was much higher. At that time, it was suspected that the container was contaminated, and the container was rewashed, and the blank solution was re-formulated (determination of the blank value with 1% HNO3 deionized water). The result was still the same, and the replacement of the graphite tube did not work. After repeated trials, it was finally found to be interference with nitric acid. The nitric acid used at the time was newly opened, and it was not a manufacturer with the last use.
(2), the impact on sensitivity
Since the amount of impurities contained in nitric acid produced by some manufacturers is higher than the amount indicated on the label, the sensitivity of the measurement is lowered when measured by graphite furnace atomic absorption spectrometry. If this is the case, replace the nitric acid or use as little or no nitric acid as possible during the pretreatment of the water sample.
Due to the acid produced by different manufacturers, the content of impurities is not the same. Therefore, when using a graphite furnace for water sample analysis, it is important to note that the acid added to the standard series plus the acid added to the water sample must be the same batch of acid from the same manufacturer. Only then can the acid be tested against the standard series. The error in the determination of the water sample is controlled at the same level. This is especially important in graphite furnace atomic absorption analysis.
In short, when using atomic absorption spectroscopy for sample analysis, on the one hand, we must have a sufficient understanding of the performance of the instrument; on the other hand, we must continue to sum up experience and improve analytical skills in practice. Only in this way can a satisfactory analysis result be obtained.
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