Paul Gunston
Carlos VargasThis Information Applies To:
Agilent Spectroscopy Systems (AA50,55,240,280)
Issue:
Typical QA/QC procedures demand calibration through several steps including running interference check samples, calibration verification, running calibration standards, and determination of linear dynamic range.
The idealized calibration or standard curve follows Beer's law, which states that the absorbance of an analyte is proportional to its concentration. Deviations from linearity usually occur for various reasons including unabsorbed radiation, stray light, or disproportionate decomposition of molecules at high concentrations. Figure 1 shows an idealized Absorbance concentration curve (3) and deviation to this idealized response curve (4). The curvature shown in (4) can be minimized, although it is impossible to avoid completely. It is desirable to work in the linear response range. The rule of thumb is that a minimum of five standards and a blank should be prepared to have sufficient information to fit the standard curve appropriately.
Figure 1. Absorbance concentration curve. 1 Absorbance axis, 2 Concentration axis, 3 Idealized, 4 Deviation
Discussion:
To create the optimum calibration curve, that is, the line of best fit, an algorithm is used. SpectrAA Software allows the user to choose any of eight different curve fitting algorithms depending on specific conditions. The optimum curve fit occurs with when curve correlation coefficient (calculated by the software) is closest to the value of 1.0.
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Tip: For more information on the details of the algorithms, see the SpectrAA Software Help
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Steps to follow:
- Select Develop > Edit Methods and from the Calib. Algorithm drop down menu (Figure 2). Select the preferred calibration type from the eight available algorithms. New Rational will typically return the most accurate calibration for AA analysis.
Figure 2. Curve Fitting Algorithms. 1 Edit Methods icon, 2 Calibration tab
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New Rational: Provides a least squares line of best fit.
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Rational: Point-to-point fit with some intermediated curve fit. This curve fit is not recommended for normal analysis as it does not smooth or minimize any errors in the standards.
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Linear: Provides a least squares line of best fit.
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Linear Origin: Provides a least squares line of best fit. Only recommended for analyses within the linear range for the analyte.
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Quadratic: Provides a second order least squares line of best fit. Suitable for most concentration mode calibrations.
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Quadratic Origin: Provides a second order least squares line of best fit forced through zero.
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Cubic: Provides a third order least squares line of best fit.
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Cubic Origin: Provides a third order least squares line of best fit forced through zero. Use with caution as it can exhibit upward curvature.
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Tip: New Rational, Rational, Linear origin, Quadratic Origin, Cubic origin are for those calibrations passing through the origin. |
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Learn how to effectively maintain, troubleshoot and operate your Agilent Spectroscopy System: Guidelines for Troubleshooting and Maintenance of AA Systems, Agilent 240/280 Series AA Users Guide, Troubleshooting and Maintenance of Atomic Absorption Systems
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