Instrument performance

These comments apply to the running of a low resolution quadrupole ICP-MS, capable of both solution and laser ablation analysis, since this is the most widespread form of the instrument. High resolution MC ICP-MS is, as they say, a whole other ball game  

Quantification of the limits of detection (LOD), or minimum detectable levels (MDL statistically defined in Section 13.4), is an important part of any analysis. They are used to describe the smallest concentration of each element which can be determined, and will vary from element to element, from matrix to matrix, and from day to day. Any element in a sample which has a value below, or similar to, the limits of detection should be excluded from subsequent interpretation. A generally accepted definition of detection limit is the concentration equal to a signal of twice (95% confidence level) or three times (99% confidence) the standard deviation of the signal produced by the background noise at the position of the peak. In practice, detection limits in ICP-MS are usually based on ten runs of a matrix matched blank and a standard. In this case  

Historically, solution-based analytical techniques such as ICP-MS have quantified data using the parts per million/billion (ppm/ppb) system, rather than the SI unit of quantity the mole (see Section 13.1). It is worth repeating that the ppm/ppb system is derived from mass (e.g., 1 ppm is 1 ig in 1000 ml), whilst the mole system is related to number of atoms or molecules (1 mole = 6 x 1023 atoms or molecules). 

A common source of error in ICP-MS is that negative masses are produced for unknown solutions. This occurs when the calibration line crosses the vertical axis at a value significantly above zero (Fig. 9.6(b)), which is often the result of high counts from the instrument blank, usually caused by an uncorrected interference. Any unknown samples which have a CPS below this value will then produce negative masses. One solution to this is to computationally force the line through the origin, or to remove outliers from the calibration line, but it is much better to identify the true cause of the problem (i.e., find the interference, if this is the source), and to rerun the samples. 

If it is not possible to include a particular element in the calibration solutions, it is possible to perform a semiquantitative analysis. This uses the response of those elements which are in the calibration solution, but predicts the sensitivity (defined as cps/concentration) for the missing element(s) by interpolating between the sensitivities of known elements. By plotting sensitivity against mass for all the elements present in the calibration solutions (Fig. 9.7) and fitting a curve through the points, it is possible to predict the sensitivity of the instrument for any particular mass number, and hence use this sensitivity to convert cps to concentration at that mass number. As can be seen from the figure, however, this is a very crude approximation, and any data produced in this way must be treated with some caution. 

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Still be smooth, free of fouling material, and the edges sharp and square. It will be diffieult to achieve, but remember the better the instrumentation performs, the less the data scatter that will require rationalization later. 

The performance of a measuring instrument can be expressed in several ways. The precision or accuracy describes the instrument performance in a general and qualitative sense. Thus, these expressions cannot be characterized using numbers. 

CurreU, G., Analytical Instrumentation Performance Characteristics and Quality, AnTS Series, Wiley, Chichester, UK, 2000. 

Pure element and compound solutions for calibration and instrument performance, trace elements in glass, glasses, glass filters... 

These conditions may be changed to obtain optimal instrument performance and to maximize sensitivity. The actual conditions used for sample analysis are recorded in the raw data. 

Generate calibration curves for isoxaflutole, RPA 202248 and RPA 203328. After the instrument performance criteria are met, a minimum of four standards over a range... 

In a GLP-compliant laboratory, a data system must meet explicit requirements guaranteeing the validity, quality, and security of the collected data. Operational qualification (OQ) must be performed after any new devices are installed in the laboratory system and whenever service or repair are performed. The role of OQ is to demonstrate that the instrument functions according to the operational specifications in its current laboratory environment. If environmental conditions are highly variable, OQ should be checked at the extremes in addition to normal ambient conditions. Performance qualification (PQ) must be performed following any new installation and whenever the configuration of the system has been changed. PQ demonstrates that the instrument performs according to the specifications appropriate for its routine use. 

Establish control charts of instrumental performance. Day-to-day variations in pump flow rate, relative response factors, absolute response to a standard, column plate counts, and standard retention times or capacity factors are all useful monitors of the performance of a system. By requiring that operators maintain control charts, troubleshooting is made much easier. The maintenance of control charts should be limited to a few minutes per day. 

An SFE instrument can be designed as a single standalone instrument performing a range of manual processes extraction, fractionation, concentration, solvent exchange, reconstitution and derivatisation. 

F. Garofolo, Bioanalytical method validation, in Analytical Method Validation and Instrument Performance Verification (eds. C. C. Chan, H. Lam, Y. C. Lee and X.-U. Zhang), Wiley-Interscience, Hoboken, NJ, 2004, pp. 105-138. 

Studying the effects of instrumental performance should be the province of the manufacturers. Unfortunately, the perception is that it is to their benefit to release such results only if they turn out to be good , and there is little incentive for them to perform studies whose only purpose is to increase scientific knowledge. Thus it is up to academia to pick up this particular ball, if there is any interest in it at all. 

Action limit, n - the limiting value from an instrument performance test, beyond which the instrument or analytical method is expected to produce potentially invalid results. 

Check sample, n - a single pure compound, or a known, reproducible mixture of compounds whose spectrum is constant over time such that it can be used as a quality or validation or verification sample during an instrument performance or function test. 

Regular calibration and verification ensures that the parameters measured by a particular instrument can be related to a recognized standard. The frequency of instrument calibration may be quite varied, depending largely on the application. If, during the verification of instrument performance, it has been shown that the instrument stays in calibration for about three months, the calibration would be repeated at approximately two-monthly intervals. However, verification (system suitability) will be carried out each time samples are analysed. For some critical analyses, calibration may be performed for each batch of samples or, in an extreme case, for each separate sample. 

Instrument performance checks and calibration procedures are carried out at appropriate intervals and show that calibration is maintained and day-to-day performance is acceptable. Appropriate improvement/corrective action is taken where necessary. 

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