Short-term precision

Short Term Precision The accuracy of the volume output of the pump over a few minutes. 

Short-term precision (major and minor elements) <1 % RSD... 

Detection limits at or below 1 ppt (1 pg/mL) are routinely attainable for many elements by ICP-MS as long as sources of contamination and reagent purity are carefully controlled. Detection limits as low as 10 ppq (10 fg/mL) are attainable in some cases. A linear dynamic range of up to 108 can be provided by ICP-MS. Short-term precision (relative standard deviation) of 1% to 3% is typical for clean samples. Long-term precision (relative standard deviation) of 5% or better over 8 hours is common for clean samples. Spectral overlaps, discussed previously, can... 

Method Sample type Stabilization time to 5% RSD, min Short-term -" precision, % RSD Long-term"" precision, % RSD External precision, % RSD LOD... 

Through the use of more than one diode and tuning them to several analyte lines, multielement determinations and the use of an internal standard become simple. In AAS work the latter also enables instrumental drift to be overcome and/ or the short-term precision to be improved as well. For the detection, all that is required is pulsing of the primary source and the use of lock-in amplification or a Fourier analyzer. 

In many interlaboratory studies, the short-term precision measured within a day or within batch is usually very small. However, when measured over 5-8 weeks, as in this programme, a more realistic and meaningful measure of the precision emerged. 

The value obtained for cr is an estimate of the precision of the method. If an analyst sets up a new analytical procedure and carries out 20 determinations of a standard sample, the precision obtained is called the short-term precision of the method. This is the optimum value of cr because it was obtained from analyses run at the same time by the same analyst, using the same instrumentation and the same chemicals and reagents. In practice the shortterm precision data may be too optimistic. Routine analyses may be carried out for many years in a lab, such as the determination of Na and K in serum in a hospital laboratory. Different analysts, different chemicals and reagents, and even different instmmentation may be used. The analysis of a standard sample should be carried out on a regular basis (daily, weekly, etc.) and these results compiled on a regular basis. Over several months or a year, the long-term precision of the method can be calculated from these compiled results. This is a more realistic measure of the reliability of the analytical results obtained on a continuing basis from that laboratory. 

In the analysis of simple solutions with known composition, for ICP-AES short term precision between 0.2 3 % relative standard deviation for analytes with concentrations ten times the detection Hmits may be achieved, while over several hours a 5 % RSD can be expected. This precision in combination with its robustness make ICP-AES superior to ICP-MS for the determination of minor and major components. With ICP-AES most elements (about 73) can be determined routinely at the 10 pg L level or better in solution with radial viewing. In favourable cases and/or axial viewing, detection Hmits < 1 pg can be achieved. Eor a single emission line, a Hnear range of four orders of magnitude may be easily attained and as many as six orders of magnitude can be observed in favourable cases. 

The value obtained for o is an estimate of the precision of the method. If an analyst sets up a new analytical procedure and carries out 20 determinations of a standard sample, the precision obtained is called the short-term precision of the method. This is the optimum value of o because it was obtained from analyses run at the same time by the same analyst, using the same instrumentation and the same chemicals and reagents. In practice, the short-term precision data may be too optimistic. 

Short-term precision (e.g., replicate analyses during the same day) is often referred to as the repeatability of a method. Long-term precision (e.g., over a week by the same laboratory or between laboratories) is called reproducibility. 

Two types of precision are normally encountered in ICP-MS analyses. The first, which is short-term precision, is the reproducibility of the measurement of consecutive determinations. Short-term precision is generally regarded as the best performance capable by the analysis system. It is often used as a parameter to evaluate the optimization of the instrument and methodology. 

The second type of precision is known as long-term precision. Long-term precision, is the reproducibility measured over a period of several hours, but within the domain of a single analysis session. Because of the nature of the analysis, long-term precision includes the instability from instrument drift, and is always poorer than the corresponding short-term precision. 

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