Analytical instrument and method of calibrating an analytical instrument

• US 5,210,778 A
• Filed: 04/12/1991
• Issued: 05/11/1993
• Est. Priority Date: 04/20/1990
• Status: Expired due to Term

• Current Assignee: Panalytical B.V. (Spectris PLC)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Massart, Desire J. L.
• Primary Examiner(s): Porta, David P.
• Application Number:
  US07/685,266
• Time in Patent Office: 760 Days
• Field of Search: 378/53, 378/54, 378/56, 378/207, 73/1 R, 364/571.04, 364/571.05, 364/571.02, 250/252.1, 356/312, 356/315, 356/316
• US Class Current: 378/53
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/3103 Atomic absorption analysis
• CPC Class Codes: G01N 2201/129 Using chemometrical methods

Abstract

An analytical instrument, for example an atomic absorption spectrophotometer, comprises means for measuring the absorbance of a plurality of standards of known concentration (100) and plotting the measured absorbance against concentration (101). A straight line is fitted to the plotted points (102) and a quality co-efficient calculated (103). If the quality co-efficient is acceptable (104) the calibration line is used for measurement of samples (105). If not, then the slope of the line joining each point to the origin is determined and if the slopes are random (107) then a robust regression technique is used to fit the calibration line (108). If outliers are then detected (109) it is determined which points are outliers (110) and appropriate action taken, for example to restrict the range if the last point(s) is/are outliers (111). If the slopes determined in step (106) are not random, then, provided more than fourpoints remain (113), the slope of each point with respect to the first point is determined (114). If they are again not random (115), then a curved calibration line is diagnosed while if they are random, a straight line not passing through the origin is diagnosed (117). In atomic absorption spectroscopy a straight line not passing through the origin indicates a problem with the blank solution, for example, contamination.

Claims

1. An atomic absorption spectrophotometer comprising means for storing measured absorbance values of a plurality of standards in association with the concentration values of corresponding standards, means for determining, using statistical techniques, a best straight calibration line, means for determining the quality of the calibration line, means for determining the ratio of absorbance to concentration for each of the measured standards and means for determining whether the ratios have a given order, whereby it can be determined whether a calibration line which is not of acceptable quality is caused by lack of measurement precision.

2. An atomic absorption spectrophotometer as claimed in claim 1 comprising means for determining whether the ratios of each of the measured standards with respect to the first measured standard are in a given order whereby it can be determined whether a blank problem exists.

3. An atomic absorption spectrophotometer as claimed in claim 1 in which the number of samples lies between four and six inclusive.

4. An atomic absorption spectrophotometer as claimed in claim 1 in which the statistical technique is the least squares technique.

5. An atomic absorption spectrophotometer as claimed in claim 1 comprising means for applying a robust regression technique to the measured points if the quality of the calibration line is not acceptable and the ratios are not ordered and means for detecting whether and in what position an outlier occurs in which if an outlier(s) is/are detected at the highest concentration sample(s) curvature of the calibration line at high concentrations is indicated.

6. An atomic absorption spectrophotometer as claimed in claim 5 comprising means for indicating the region of a linear calibration curve.

7. An atomic absorption spectrophotometer as claimed in claim 4 comprising means for indicating a blank problem if zero concentration, zero absorbance is detected as an outlier.

8. An atomic absorption spectrophotometer as claimed in claim 1 comprising means for storing measured absorption values of a plurality of standards added to the sample, means for determining, using statistical techniques, a best straight standards addition line, means for determining whether the slopes of each measured sample plus standard point with respect to the sample point is in a given order whereby it can be determined whether curvature of the standard addition line occurs.

9. An atomic absorption spectrophotometer as claimed in claim 8 comprising means for determining the quality of the standards addition line and for comparing its slope with that of the calibration line if both are satisfactory to determine whether matrix interference problems exist.

10. An atomic absorption spectrophotometer as claimed in claim 9 in which the statistical technique is the least squares method.

11. An analytical instrument comprising means for storing radiation intensity of a plurality of standards in association with the concentration values of corresponding standards;

means for determining, using statistical techniques, a best straight calibration line;

means for determining the quality of the calibration line;

means for determining the ratio of absorbance to concentration for each of the measured standards; and

means for determining whether the ratios have a given order, whereby it can be determined whether a calibration line which is not acceptable quality is caused by lack of measurement precision.

12. An analytical instrument as claimed in claim 11, comprising an X-ray spectrometer, wherein the radiation intensity values are intensity of X-rays received by a detector.