Long-term precision

Short- and long-term precision of migration time and peak area ( = repeatability, intermediate/day-to-day... 

Schoeller, D. A. (1980). Model for determining the influence of instrumental variations on the long-term precision of isotope dilution analyses. Biomed. Mass Spectrom. 7, 457—463. 

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... 

Generally, dissolved solid concentrations should be kept below 0.2% for ICP-MS. Higher dissolved solid concentrations can lead to deposition of sample on the sampling and skimmer orifices, partial orifice plugging, or deposition of material on ion lenses that degrade sensitivity and medium-term to long-term precision. Furthermore, relatively small concentrations of a heavy element (100 ppm or greater) in a sample can cause a decrease in analyte sensitivity, particularly... 

The precision has been defined in the ISO 5725-1 standard as the closeness of agreement between independent test results obtained under stipulated conditions [28]. Many factors may contribute to the variability of test results obtained with the same method on identical samples, including (but not limited to) the operators), the equipment, the reagents, the RM(s), the environment, the time between measurements, and the laboratories. The maximal variability of test results is explained by the reproducibility (R) of the method. All factors that have influence on the variability of a test method should be taken into consideration when assessing the reproducibility. The repeatability (r) is assessed by keeping all the above-mentioned factors constant (e.g., same operator, same equipment, same laboratory, short time interval). It is a measure of the minimum variability of a method. The intermediate precision is situated between the two extreme measures of precision repeatability and reproducibility. The terms within-laboratory reproducibility (w), long-term precision, and so on, are often used to demonstrate the intermediate precision of a method. For a correct interpretation of the intermediate precision, the factors that have been taken into account should be known. 

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

Another major change in the final rule was the elimination of an earlier provision that would have required the FDA to review a manufacturer s QC instructions. That was a key provision for allowing laboratories to simply follow a manufacturer s directions. However, with elimination of that provision, laboratories now have more responsibility for establishing effective QC systems that will monitor the complete analytical process, take into account the performance specifications of the method, detect immediate errors, and monitor long-term precision and accuracy. 

Blake GM, Roe D, Lazarus CR. Long-term precision of glomerular filtration rate measurements using Cr-EDTA plasma clearance. Nuclear Medicine Communications 1997 18 776-84. 

The project began with a simple series of exercises to evaluate the analytical precision and bias of the participating laboratories. The long-term precision was compared with the documented precision given by the participants in the detailed questionnaire [54]. 

We have defined the precision of a method. Repeatability is the long-term precision over several weeks. Ruggedness refers to the precision of one lab over multiple days. 

Reproducibility. Both the short- and long-term precision with which saturated solutions can be generated and measured are better than 3%. The results presented in Table VIII demonstrate this fact. 

Comprehensive tests were performed for between-day imprecision and accuracy with several participating laboratories. With normal contents long-term precision was around 6% and recovery rates were between 88.4 and 104.6 %. 

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. 

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. 

For these reasons, it is critical that when short- and long-term precision are evaluated, you know all the potential sources of imprecision and drift. It is therefore important that you choose either a matrix that is representative of your samples or one that will genuinely test the instrument out. Typical sample matrices include the following ... 

The cone-spray nebulizer is a more efficient design of the previously described slot nebulizer. A diagram of this type of nebulizer is shown in Figure 5.16. Instead of a slot, where sample approaches the orifice from only one direction, the cone spray consists of a three-dimension funnel-shaped depression made in an inert material, such as sapphire, with an orifice positioned in the bottom of the funnel. Sample is introduced into the funnel from above and is concentrated on the orifice as it flows to the bottom. A commercially available version of this nebulizer has an orifice with a 216-p,m diameter and operates at about 32 psi.The long-term precision of this nebulizer has been shown to be about 1% over an 8-h period of time. 

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. 

Figure 10.1 shows a plot of the % RSD as a function of concentration for Cd.The nature of this plot is typically characteristic of most elements. It is clearly seen from this plot that as lower concentrations are approached, the % RSD increases approximately exponentially. When approaching the detection limit, the % RSD becomes close to 100, which is consistent with its definition. At nominal concentrations, most elements can be determined on a short-term basis with a % RSD of approximately 1%. As an estimate, for long-term precision, the values are about a factor of 3 greater. 

Thirlwall, M., Ancziewicz, R., Vance, D., and Munday, D. (2002) Accuracy and long-term precision of MC-ICP-MS isotope ratios. Geochim. Cosmochim. Acta, 66(15A), A771. 

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