Calibration, Check Standards and Quality Control

Last revised July 22, 1997

Page contents: (click to skip down)

[General quality assurance policy]

[Mass spectrometry samples]

[Plant/soil total C, total N]

[Samples for colorimetry]

[Samples for atomic absorption spectrophotometry]

General quality assurance policy

Detailed information is given below on the methods used by this laboratory to ensure the accuracy of its analyses. Where U.S. E.P.A.-certified analytical methods exist for a given analyte, they are used (these are called out in the methods for each particular analyte.)

[Nitrogen, Nitrate-Nitrite. EPA Method 353.2]
[Nitrogen, Ammonia. EPA Method 350.1]
[Phosphorus, All Forms. EPA Method 365.1]
[Calcium: EPA Method 215.1]
[Magnesium: EPA Method 242.1]
[Potassium: EPA Method 258.1]

In general we feel that quality control of our sample analyses is our responsibility, while quality control of any given research project is the investigator's responsibility. If an adequate system of quality control is not built into a project from the beginning, including field replicates, field blanks, split samples and so on, we cannot provide it. The following sources may aid investigators in preparing quality control strategies for their projects:

Allen, S. E., et al. 1974.

Analysis of soils: sample collection and initial treatment pp.13ff. In Chemical Analysis of Ecological Materials. John Wiley and Sons, New York USA.

Allen, S. E., et al. 1974.

Analysis of vegetation and similar materials: sample collection and initial treatment. pp. 71ff. In Chemical Analysis of Ecological Materials. John Wiley and Sons, New York USA.

American Public Health Association. 1992.

Collection and preservation of samples. pp.1-18 -- 1-23. In Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC USA.

James, D. W. and K. L. Wells. 1990.

Soil sample collection and handling. pp.25-44. In R. L. Westerman, ed., Soil Testing and Plant Analysis. Third ed. Soil Science Society of America, Madison, WI USA.

Peterson, R. G. and L. D. Calvin. 1986.

Sampling. pp.33-51. In A. L. Page et al., eds., Methods of Soil Analysis: Part 2, Chemical and Microbiological Properties. Agronomy, A Series of Monographs, no.9 pt.2, Soil Science Society of America, Madison, Wisconsin USA.

U. S. Environmental Protection Agency. 1979.

Handbook for Analytical Quality Control in Water and Wastewater Laboratories. EPA-600/4-79-019. U.S. E.P.A., Cincinnatti, Ohio USA.

U. S. Environmental Protection Agency. 1983.

Sample preservation. pp. xv-xx. In Methods for Chemical Analysis of Water and Wastes. EPA-600/4-79-020. U.S.E.P.A., Cincinnati, Ohio, USA.

Stable isotope analysis of 13C and 15N

• Standards presently in use are: for nitrogen-15 samples, ammonium sulfate (0.36630 At%15N); for carbon-13 and dual-element samples, N.B.S. 1572 citrus leaf standard (0.36820 At%15N, -26.50 delta13C.)

• Five empty tin-capsule blanks are analysed initially in each batch, to allow correction for traces of C originating from the tin capsules and for the small amount of N2 gas introduced as an impurity in the oxygen pulse. Any detectable 15N or 13C in these blanks is subtracted from the sample and standard values to give a true zero baseline.

• Drift correction during each run is accomplished by including recalibrant standards once every 10 unknowns. Recovery check standards are included twice for every 10 unknowns, before and after recalibration.

• The driver software for this instrument uses single-point calibration for both initial and drift-correct standards.

• Clients are referred to the mass spectrometer [sample preparation] instructions for methods of sample treatment that will ensure the best possible precision and accuracy.

Micro-Dumas analysis for total C and total N

• Standards presently in use are ultrapure acetanilide (C8H9NO, 71.09% C, 10.36% N) and ultrapure atropine (C17H23NO3, 70.56%C, 4.84% N).

• Four empty tin-capsule blanks are analysed initially in each batch, to allow correction for traces of C originating from the tin capsules and for the small amount of N2 gas introduced as an impurity in the oxygen pulse. Any detectable N or C in these blanks is subtracted from the sample and standard values to give a true zero baseline.

• Initial calibration is accomplished by linear regression on a descending series of four calibrant standards. Recovery check standards are included once for every 10 unknowns.

• Clients are referred to the micro-Dumas combustion [sample preparation] instructions for methods of sample treatment that will ensure the best possible precision and accuracy.

Samples for colorimetric analysis

• 1000ppm standard stocks are made from appropriate dry reagents (KNO3, NaNO2, (NH4)2SO4, and KH2PO4.) These stocks are checked by analyzing Environmental Protection Agency certified Nutrient-1 quality control (QC) solutions of known analyte values. If the determined value of the QC solution differs from the known value by > 1% the stock is discarded.

• Working standards are made daily from standard stocks

• Calibration of each run is accomplished by linear regression on a descending sequence of three working standards and a blank at the start of the run. Within-run drift correction is achieved by one recalibrant standard and baseline blank every twenty samples. Recovery check standards are included once for every twenty samples.

• Digests for total nitrogen and total phosphorus are also checked by digesting and analyzing E.P.A. Nutrient-2 QC solutions formulated to challenge digestion techniques alongside every batch of unknowns, one EPA-QC2 standard per twenty unknowns.

• Clients are referred to the colorimetric analysis [sample preparation] instructions for methods of sample treatment that will ensure the best possible precision and accuracy.

Samples for atomic absorption spectrophotometry

• 1000ppm standard stocks are purchased from reputable vendors (Aldrich AA/ICP calibration/check standards: 40412-8 for Ca, 40398-9 for K, 40403-9 for Mg.)

• Working standards are made daily from standard stocks

• Calibration of each run is accomplished by linear regression on a descending sequence of two working standards and a blank at the start of the run. Within-run drift correction is achieved by one recalibrant standard and baseline blank every twenty samples. Recovery check standards are analyzed once per every twenty unknowns

• Clients are referred to the atomic absorption spectrophotometry [sample preparation] instructions for methods of sample treatment that will ensure the best possible precision and accuracy.