Canadian Forest Service, Street, Edmonton, Alberta, Canada T6H 3S5

Transcription

1 Scientific registration no: 2122 Symposium no: 12 Presentation: poster Improvement of laboratory performance through soil and plant check sample programs Amélioration des performances de laboratoire par des programmes de contrôle d échantillons de sols et de plantes KALRA Yash Canadian Forest Service, Street, Edmonton, Alberta, Canada T6H 3S5 Introduction As discussed in a recent paper (Kalra 1997), our laboratory has participated in the following soil and plant analysis check sample programs in the last 25 years: Acid Rain Direct/Delayed Response Project (Palmer 1987), AOAC INTERNATIONAL (Kalra 1995, 1996) Alberta Institute of Pedology (McGill 1987), Canada Soil Survey Committee (McKeague et al 1978), Energy Resources Conservation Board (Korchinski and Banister 1993), Expert Committee on Soil Survey (Sheldrick and Wang 1987), International Soil Exchange (Houba et al. 1992), International Union of Forestry Research Organizations (Kalra and Edwards 1974; van Goor et al. 1971), Laboratory Exchange Program (Pleijsier 1985), Long Range Transport of Air Pollutants (Kalra et al. 1994), Utah State University (James 1985), and Western Enviro- Agricultural Laboratory Association (Kalra and Peters 1981). The objective of the present paper was to determine the sources of variability in the results of some of theses studies and hence improve laboratory performance. Results and Discussion Although our laboratory participated in several programs, sources of variability in only some of those studies will be discussed. In a study carried out for the Canada Soil Survey Committee, McKeague et al. (1978) found that the major source of variability in the data submitted by 24 laboratories was the analyst. They found that even for an apparently simple determination such as soil ph, results by a single method varied by as much as 3 units of ph. Differences due to the method of analysis on these 28 samples were apparent in the case of several properties; ph measured in water was consistently higher than in calcium chloride, as expected. For a study by the Expert Committee on Soil Survey, Sheldrick and Wang (1987) compiled the results for the following properties: ph, C, N, extractable Fe and Al, exchangeable cations, CEC, calcium carbonate equivalent, EC, extractable P (Table 1), loss on ignition, particle-size distribution, water retention, and total elemental analysis. The eight samples were from the following Great 1

2 Groups: Humo-Ferric Podzol, Ferro-Humic Podzol, Humic Gleysol, Gray Brown Luvisol, Melanic Brunisol, Dystric Brunisol, Black, and Solonetz. Table 1. Bray P, ug/g (Source: Sheldrick and Wang 1987). Sample Laboratory <1 < < * Range < Mean Standard Deviation Tentative Best Value *Data rejected by Q test. Sheldrick and Wang (1987) concluded that the variability in analytical results was due to (1) different analytical procedures employed (2) modification of the analytical techniques by individual laboratories (3) inter-laboratory bias, i.e., the variation between analyses performed by different laboratories employing the same methods and (4) within-laboratory bias, i.e., the variation in analytical results that occurs when the analyses are performed in the same laboratory, but on different days, by different analysts, using different standard solutions, etc. They pointed out that variabilities (1) and (2) can be reduced by recommending the use of a standardized compilation of analytical methods for the required soil attributes. The interlaboratory bias can be checked by referring to the results obtained by other laboratories on the same soil samples. The within-laboratory bias can be controlled by proper quality control procedures in the individual laboratories. In general, soil heterogeneity and method-dependent differences are the two major sources of variation in analytical data. In the first collaborative study of the Western Enviro-Agricultural Laboratory Association, Kalra and Peters (1981) distributed well-mixed soil samples and provided copies of the methods to be used, thus eliminating these two sources of variation. It was found that for certain properties the variation in data between contributors was much greater than experimental error. They emphasized that this area warranted attention. Some possible sources of variability in the results were (1) interpretation of methods (e.g., "What is a saturated paste" depends on the interpretation of the analyst) (2) units of expressing results (e.g., us/cm or ms/cm) (3) mix-up in samples in some laboratories (4) Decimal errors (5) dilution errors (6) types of analyses (7) contamination in the laboratory (8) competence of analysts and (9) inaccuracy in calculation and transferring of data from work sheets to report sheets. 2

3 Eighty five countries participated in a study in which soils from Brazil, Canada, France, Hungary, Kenya, Malaysia, Syria, the Netherlands, and USA were analyzed for several properties including cation exchange capacity (Table 2). In the compilation of statistics for all properties, Pleijsier (1985) recommenced that it is best to use median of the data because it is a good estimate of the centre of the data as it is less sensitive to extreme and outlying values than mean. Table 2. Cation exchange capacity, meq/100 g (Source: Pleijsier 1985). Sample Median Median Absolute Deviation Median Median Absolute Deviation The International Union of Forestry Research Organizations has been interested in plant analysis for several years. In 1970, foliage samples of Corsican pine, Douglas fir, Japanese larch, poplar, and Scots pine were analyzed for several properties including Zn (Table 3). Significant differences between and within laboratories were observed in the results reported for some samples. Kalra and Edwards (1974) evaluated the results compiled by van Goor et al. (1971). It was indicated that possible sources of errors in data from some of the participants included sample heterogeneity, methods of determination, contamination in the laboratory, dilution factors, calculation errors, and instrument settings. Kalra and Edwards (1974) concluded that the results indicated a need for greater awareness of these sources to attain the highest accuracy and precision. Table 3. Zinc in tree foliage, ppm (Source: van Goor et al. 1971). Lab Scots pine 1 Douglas fir Corsican pine Scots pine 2 Poplar Japanese larch In an inter-laboratory study coordinated by the Canadian Forest Service, Kalra et al. (1994) reported the dependence of data on the quantity of a particular element present in the sample. 3

4 On average, coefficients of variation suggested reasonable reproducibility among laboratories with respect to C, P, K, Ca, Mg, Cu, and Al; some laboratory-to-laboratory variation with respect to N, S, Fe, Mn (Table 4), and Zn; and considerable laboratory-to-laboratory variation with regard to other elements. In general, samples with low concentrations presented difficulties. Table 4. Concentration of Mn (ug/g) in plant materials analyzed by 10 laboratories (Source: Kalra et al. 1994). Sample Mean Median Standard Deviation Standard Error Mean Conclusions To ensure that a laboratory produces credible analytical results, it is important to participate in collaborative studies utilizing check samples as a method of implementing a quality control procedure. The check samples provide a reference base to participants. We use them for quality control in our regular analytical work and also for methods development. Moreover, on the basis of these samples, laboratories can prepare their own reference samples for use as a quality control check in batches of samples to generate analytical results that are correct and reproducible. A well-documented quality control program is essential to attain laboratory credibility. To improve laboratory performance, soil and plant analysts must be alert to all sources of variability. References Houba, V.J.G., J. Uittenbogaard, and P.J. Pellen International Soil-analytical Exchange (ISE).Department of Soil Science and Plant Nutrition, Wageningen Agricultural University, Wageningen, the Netherlands. James, D.W Soil testing laboratory coordination program. Utah State University, Logan, Utah, USA. Kalra, Y.P Determination of ph of soils by different methods: Collaborative study. J. AOAC INTERNATIONAL 78: Kalra, Y.P Soil ph: First soil analysis methods validated by the AOAC INTERNATIONAL. J. For. Res. 1:

5 Kalra, Y.P Canadian Forest Service's participation in check sampleprograms over a 25-year period. Paper presented at the 5th International Symposium on Soil and Plant Analysis, Bloomington, Minnesota, USA, August 3-7, Kalra, Y.P. and I.K. Edwards Possible sources of errors in chemical analyses of IUFRO plant samples. International Union of Forestry Research Organizations (Vienna, Austria) News 7: Kalra, Y.P., G.J. Koteles, I.K. Morrison, and J.R. Ramakers Forestry Canada's interlaboratory study for the analysis of plant materials. Commun. Soil Sci. Plant Anal. 25: Kalra, Y.P. and M.K. Peters WEALA check sample program: A collaborative study. Western Enviro-Agricultural Laboratory Association, Edmonton, Alberta, Canada. Rep. WEALA-1. Korchinski, M. and K. Banister Results of a collaborative study. Energy Resources Conservation Board, Calgary, Alberta, Canada. Rep. OW-2. McGill, W.B Performance testing for soil testing laboratories in Alberta. Alberta Institute of Pedology, Alberta Agriculture, Edmonton, Alberta, Canada. McKeague, J.A., B.H. Sheldrick, and J.G. Desjardins Compilation of data for CSSC (Canada Soil Survey Committee) reference soil samples. Agriculture Canada, Soil Research Institute, Ottawa, Ontario, Canada. Palmer, C Acid Rain Direct/Delayed Response Project. Environmental Research Center, University of Nevada, Las Vegas, Nevada, USA. Pleijsier, L.K The laboratory methods and data exchange program (LABEX). International Soil Reference and Information Centre, Wageningen, the Netherlands. Interim report on the exchange round Sheldrick, B.H. and C. Wang Compilation of data for ECSS (Expert Committee on Soil Survey) reference soil samples. Agriculture Canada, Land Resources Research Institute, Research Branch, Ottawa, Ontario, Canada. van Goor, C.P., M. de Wit, and J. van den Burg International comparison of methods for soil and plant analysis. Report of activities in Forest Research Station, De Dorschkamp, Wageningen, the Netherlands. Keywords : quality control, check sample programs, soil analysis, plant Analysis Mots clés : contrôle de qualité, programme de contrôle d échantillons, analyse de sols, analyse de plantes 5