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Several commonly used technical indicators in the analysis of ICP spectrometer
In the process of using ICP spectrometer, we often encounter several commonly used conceptual things. We all even know what it means, and many people don’t understand when they say it specifically. Let’s try to explain it here. These commonly used indicator concepts. In fact, these indicators are not only encountered in the analysis of ICP spectrometer, most of the instruments need to be described with these indicators.
1, the detection limit
The detection method detection capability is usually expressed by the concept of detection limit. It means that a method can detect the minimum or minimum amount of a substance to be tested at a given confidence level. This is called the detection limit of the substance. The relative detection limit is expressed in terms of concentration. The quality is called the absolute detection limit. The detection limit is a qualitative concept, which only indicates that the response signal of this concentration or amount can be distinguished from the blank signal, and quantitative analysis cannot be performed near the detection limit.
In 197, the International Union of Theoretical and Applied Chemistry (IUPAC) adopted a recommendation on the detection limit. According to this recommendation, the detection limit of the method refers to the lowest concentration or content of the element that can be detected with appropriate confidence. In other words, the factory limit is defined as the concentration of the substance required to produce the lowest signal that can be resolved. The detection limit is expressed in two ways, the absolute detection limit (expressed as the mass of the analyte) relative to the detection limit (expressed as the analyte concentration).
The detection limit can be divided into the detection method detection limit and the instrument detection limit.
Instrument detection limit: refers to the minimum or minimum concentration of the analyte that the analytical instrument can detect. Instrument detection limits are generally used for performance comparison of different instruments. Generally, the standard deviation of the background response is obtained by a plurality of blank tests, and the three-fold blank standard deviation (ie, 3δ) is used as an estimated value of the detection limit. It is also possible to record the measured signal intensity S and noise (or background signal) intensity N of the measured sample with a known concentration of the sample and the blank test to achieve the lowest sample at S/N=2 or S/N=3. The concentration is LOD (Limit of Detection). When using a non-instrumental analysis method, the lowest detectable level is determined as the detection limit by analysis of the sample at a known concentration. The representation method is often:
(1) Minimum detection concentration: When the minimum detection limit is met, the concentration of the test solution to be injected, common units: mg/mL, ng/mL, mol/L, etc.
(2) Minimum detection amount: minimum detection amount = minimum detection concentration × injection amount, common units: ng, pg, fg, etc.
Method detection limit: The method detection limit is not only related to the noise of the instrument, but also depends on the whole process of sample measurement, such as sampling amount, extraction and separation, and optimization of measurement conditions. The actual experimental conditions should be indicated in the actual work. For example, when detecting a compound XY, the method stipulates that 100 mg is sampled, and after the extraction treatment, the volume is adjusted to 10 ml, and the detection limit of the method is 1 μg/g. If the method is changed to increase the sample size to 1 g, the method detection limit is 0.1 μg/g. If the method is changed to increase the sampling amount to 1 g and the volume is 1 ml after the extraction treatment, the detection limit of the method is 0.01 μg/g.
The detection limit mainly depends on three aspects:
1 The selectivity and specificity of the analytical method.
2 sensitivity of the analytical method.
3 The precision of the analytical method. The instrument detection limit does not take into account the influence of any sample preparation steps. The detection limit is generally determined by the solvent blank, so the value is always lower than the method detection limit.
Generally, the standard deviation of 3 times the blank measurement is used as the detection limit, and the standard deviation of 10 times is the limit of determination (LOD). When the measurement result is not greater than the detection limit, it is reported as undetected; when the measurement result is greater than the detection limit and not greater than the lower limit of quantitative determination, the report is qualitative detection; when the measurement result is greater than the lower limit of quantitative determination, the quantitative result is reported.
The quantitative results of the samples should be within the range of the standard curve. The extrapolation calculations are not allowed. The extrapolation results are not verified by methodology and the accuracy cannot be determined. If the sample is too concentrated, it should be diluted. If it is too thin, it should be concentrated to make it fall within the range of the standard curve. Therefore, the lower limit of quantitation should be the lowest concentration point of the standard curve.
2. Accuracy:
Refers to the degree to which the average value of multiple measurements under certain experimental conditions is consistent with the true value, expressed as error. It is used to indicate the magnitude of the systematic error. In actual work, the control test is usually carried out by standard materials or standard methods. In the absence of standard materials or standard methods, the pure substances added to the components to be tested are usually subjected to a recovery test to estimate and determine the accuracy. It refers to the degree of conformity between the measured value and the true value, and is an important indicator of the evaluation method.
Accuracy Commonly used relative error measure: relative error = (xu) × 100% / u where X is the measured value of the sample and u is the true value.
Accuracy is a comprehensive reflection of system error and random error in the analysis process, which determines the reliability of the analysis results. The method has better precision and eliminates the systematic error before it has better accuracy.
3, precision:
Refers to the lowest concentration or quality of an element that can be detected with appropriate confidence. Or the concentration or quality of the elements needed to resolve the lowest signal.
Precision is often expressed as relative standard deviation. The RSD of the method is a function of measuring the signal value (or concentration value) and the number of measurements. When using RSD to indicate the precision of an analytical method, the corresponding concentration level and number of measurements should be indicated. The RSD of the method is 1% (2 x 10-6 g/ml, n = 11).
4. Sensitivity is another important indicator of analytical methods, but for a long period of time, the two related but different concepts of detection limits and sensitivity are often confused or equated. To correct this situation, IUPAC The sensitivity is also specified. It is pointed out that the sensitivity S of the method indicates the change of the measured signal caused by the change of the concentration or content of the measured element by one unit, that is, S=dx/dc. The sensitivity can also be understood as the calibration curve. Slope. The high sensitivity of an analytical method means that a change in the unit concentration or content of the measured element can cause a more significant change in the analytical signal. In addition, in the practice of analysis, there are specific expression methods for the sensitivity of a certain type of analytical method. For example, for the spectrophotometric method, the molar absorptivity is used; for the atomic absorption spectrum, it is expressed by the characteristic concentration or the characteristic amount. The sensitivity of the method is relatively stable only under certain conditions, so it is not often used as a characteristic index of the characterization method.
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