Books
in black and white
Main menu
Home About us Share a book
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
Previous << 1 .. 4 5 6 7 8 9 < 10 > 11 12 13 14 15 16 .. 83 >> Next

• Always correctly dispose of chemicals appropriately, e.g. organic solvents require special disposal arrangements - often arranged by the departmental safety officer.
• Consider other people who may be working nearby.
2.2 Quality Assurance
Quality assurance is about getting the correct result. In environmental analysis and monitoring, this involves several steps, including sample collection, treatment and storage, followed by laboratory analysis. A complete environmental protocol is shown in Figure 2.1. It is likely that the variation in the final measurement is more influenced by the work external to the analytical laboratory than that within the laboratory. Two important terms in quality assurance are accuracy and precision.
Accuracy is the closeness of a determined value to its ‘true’ value, while precision is the closeness of the determined values to each other. A determined result for the analysis of a polycyclic aromatic hydrocarbon in soil could produce precise (i.e. repeatable) but inaccurate (i.e. untrue) results.
SAQ 2.1
If the black circles shown in Figure 2.2 represent the results obtained and the centre of the rings represents the ‘true’ values, which of the results are accurate and which are precise?
In order to achieve good accuracy and precision - in the laboratory at least - it is desirable that a good quality assurance scheme is operating. The main objectives of such a scheme are as follows:
• to select and validate appropriate methods of sample preparation
• to select and validate appropriate methods of analysis
• to maintain and upgrade analytical instruments
• to ensure good record keeping of methods and results
• to ensure the quality of data produced
• to maintain a high quality of laboratory performance
Investigative Approach for Sample Preparation
15
Figure 2.1 A typical analytical protocol used for environmental analysis.
In implementing a good quality control programme, it is necessary to take into account the following:
• Certification of analyst competence. This is intended to assess whether a particular analyst can carry out sample and standard manipulations, operate the instrument in question and obtain data of appropriate quality. The definition of suitable data quality is open to interpretation but may be assessed in terms of replicate analyses of a ‘check sample’.
• Recovery of known additions. Samples are spiked with known concentrations of the same analyte and their recoveries noted. This approach will also allow
16
Methods for Environmental Trace Analysis
(a) (b)
(c) (d)
Figure 2.2 Accuracy and precision: the centres of the bull’s-eyes represent the ‘true’ values (cf. SAQ 2.1).
the operator to determine whether any matrix effects are interfering with the analysis.
• Analysis of certified reference materials. By definition, a certified reference material (CRM) is a substance for which one or more analytes have certified values, produced by a technically valid procedure, accompanied with a traceable certificate and issued by a certifying body (e.g. Figure 2.3).
DQ 2.1
Consider the quoting of numerical values and units for the major/minor constituents and the trace constituents in the CRM of Figure 2.3.
Answer
It is noted that numerical values are often grouped so that the numbers are within 1000 of each other with the appropriate choice of units, e.g.
0.013 figg-1 cadmiumiswithinafactorof1000forzinc(12.5 igg-1 )but not within a factor of 1000 for phosphorus (0.159 wt% or 1590 igg-1).
Investigative Approach for Sample Preparation
17
National Institute of Science and Technology Certificate of Analysis
Standard Reference Material 1515 Apple Leaves
Certified Concentrations of Constituent Elements1
Element
Calcium Magnesium Nitrogen (total) Phosphorus Potassium
Concentration (wt%)
1.526 ± 0.015 0.271 ± 0.008 2.25 ± 0.19 0.159 ± 0.011 1.61 ± 0.02
Element Concentration (gg g 1)2 Element Concentration (gg g
Aluminium 286 ± 9 Mercury 0.044 ± 0.004
Arsenic 0.038 ± 0.007 Molybdenum 0.094 ± 0.013
Barium 49 ± 2 Nickel 0.91 ± 0.12
Boron 27 ± 2 Rubidium 10.2 ± 1.5
Cadmium 0.013 ± 0.002 Selenium 0.050 ± 0.009
Chlorine 579 ± 23 Sodium 24.4 ± 12
Copper 5.64 ± 0.24 Strontium 25 ± 2
Iron 83 ± 5 Vanadium 0.26 ± 0.03
Lead 0.470 ± 0.024 Zinc 12.5 ± 0.3
Manganese 54 ± 3
1The certified concentrations are equally weighted means of results from two or more different analytical methods or the means of results from a single method of known high accuracy.
2The values are based on dry weights. Samples of this SRM must be dried before weighing and analysis by, for example, drying in a desiccator at room temperature (ca. 22 °C) for 120 h over fresh anhydrous magnesium perchlorate. The sample depth should not exceed 1 cm.
Figure 2.3 An example of a certificate of analysis for elements in apple leaves. Reprinted from Certificate of Analysis, Standard Reference Material 1515, Apple Leaves, National Institute of Standards and Technology. Not copyrightable in the United States.
Previous << 1 .. 4 5 6 7 8 9 < 10 > 11 12 13 14 15 16 .. 83 >> Next