ISO 5667-14:1998
(Main)Water quality — Sampling — Part 14: Guidance on quality assurance of environmental water sampling and handling
Water quality — Sampling — Part 14: Guidance on quality assurance of environmental water sampling and handling
Qualité de l'eau — Échantillonnage — Partie 14: Lignes directrices pour le contrôle de la qualité dans l'échantillonnage et la manutention des eaux environnementales
Kakovost vode - Vzorčenje - Navodilo za zagotavljanje kakovosti vzorčenja vode v okolju in ravnanja z vzorci
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 5667-14
First edition
1998-09-01
Water quality — Sampling —
Part 14:
Guidance on quality assurance of
environmental water sampling and handling
Qualité de l’eau — Échantillonnage —
Partie 14: Lignes directrices pour le contrôle de la qualité dans
l’échantillonnage et la manutention des eaux environnementales
A
Reference number
ISO 5667-14:1998(E)
---------------------- Page: 1 ----------------------
ISO 5667-14:1998(E)
Content
1 Scope . 1
2 Normative references . 1
3 Definitions . 1
4 Sources of sampling error . 4
5 Sampling quality control techniques. 5
6 Transport, stabilization and storage of samples. 12
7 Analysis and interpretation of quality control data. 13
8 Sample identification and records . 13
Annex A (informative) Control charts . 14
Annex B (informative) Bibliography . 18
© ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
---------------------- Page: 2 ----------------------
© ISO
ISO 5667-14:1998(E)
Foreword
ISO (the International Organization for Standardization) is a world-wide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 5667-14 was prepared by Technical Committee ISO/TC 147, Water quality,
Subcommittee SC 6, Sampling (general methods).
ISO 5667 consists of the following parts, under the general title Water quality — Sampling:
Part 1: Guidance on the design of sampling programmes
Part 2: Guidance on sampling techniques
Part 3: Guidance on the preservation and handling of samples
Part 4: Guidance on sampling from lakes
Part 5: Guidance on sampling of drinking water
Part 6: Guidance on sampling of rivers and streams
Part 7: Guidance on sampling of water and steam in boiler plants
Part 8: Guidance on sampling of wet deposition
Part 9: Guidance on sampling from marine waters
Part 10: Guidance on sampling of waste waters
Part 11: Guidance on sampling of groundwaters
Part 12: Guidance on sampling of bottom sediments
Part 13: Guidance on sampling of water, wastewater and related sludges
Part 14: Guidance on quality assurance of environmental water sampling and handling
Annexes A and B of this part of ISO 5667 are for information only.
iii
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© ISO
ISO 5667-14:1998(E)
Introduction
Quality control procedures are required for the collection of environmental water samples for the following reasons:
a) to monitor the effectiveness of sampling methodology;
b) to demonstrate that the various stages of the sample collection process are adequately controlled and suited to
the intended purpose, including adequate control over sources of error such as sample contamination, loss of
determinand and sample instability. To achieve this quality control procedures should provide a means of
detecting sampling error and hence a means of rejecting invalid or misleading data resulting from the sampling
process ;
c) to quantify and control the sources of error which arise in sampling. Quantification gives a guide to the
significance that sampling plays in the overall accuracy of data;
d) to provide information on suitably abbreviated quality assurance procedures that may be used for rapid
sampling operations such as pollution incidents or groundwater investigations.
This part of ISO 5667 is one of a group of International Standards dealing with the sampling of waters. It should be
read in conjunction with the other parts of ISO 5667 and in particular with Parts 1, 2 and 3.
The general terminology is in accordance with that published in ISO/TC 147, Water quality, and more particularly
with the terminology on sampling given in ISO 6107-2.
iv
---------------------- Page: 4 ----------------------
INTERNATIONAL STANDARD © ISO ISO 5667-14:1998(E)
Water quality — Sampling —
Part 14:
Guidance on quality assurance of environmental water sampling and
handling
WARNING: Consider and minimize any risks and obey safety rules. See ISO 5667-1 for certain safety
precautions, including sampling from boats and from ice-covered waters.
1 Scope
This part of ISO 5667 provides guidance on the selection and use of various quality assurance techniques relating
to the manual sampling of surface, potable, waste, marine and ground waters;
NOTE The general principles outlined in this part of ISO 5667 in some circumstances may be applicable to sludge and
sediment sampling.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this part of
ISO 5667. At the time of publication, the editions indicated were valid. All standards are subject to revision, and
parties to agreements based on this part of ISO 5667 are encouraged to investigate the possibility of applying the
most recent editions of the standards indicated below. Members of ISO and IEC maintain registers of currently valid
International Standards.
ISO 5667-1:1980, Water quality — Sampling — Part 1: Guidance on the design of sampling programmes.
ISO 5667-3:1985, Water quality — Sampling — Part 3: Guidance on the preservation and handling of samples.
3 Definitions
For the purposes of this part of ISO 5667, the following definitions apply.
3.1
accuracy
closeness of agreement between a test result and the accepted reference value
[ISO 3534-1]
NOTE The term accuracy, when applied to a set of test results, involves a combination of random and systematic error or bias
components.
3.2
bias
difference between the expectation of the test results and an accepted reference value
[ISO 3534-1]
1
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© ISO
ISO 5667-14:1998(E)
NOTE Bias is the total systematic error as contrasted to random error. There may be one or more systematic error
components contributing to the bias. A larger systematic difference from the accepted reference value is reflected by a larger
bias value.
3.3
precision
closeness of agreement between independent results obtained under stipulated conditions
[ISO 3534-1]
NOTE 1 The variation associated with test results from repeated sampling operations will be subject to variation from
analytical sources as well as from sources connected with the sampling process. A comparison of random error from repeated
sampling operations with that from repeated analysis of the same sample can be used to deduce the contribution of sampling
to overall random error.
NOTE 2 Precision depends only on the distribution of random errors and does not relate to the true value or the specified
value, (ISO 3534-1). The measure of precision is expressed in terms of a standard deviation value. Improved precision is
reflected in a smaller standard deviation value.
NOTE 3 The ‘independence’ of test results reflects the extent to which results are obtained in a manner not influenced by
any previous result on the same test object (ISO 3534-1). Quantitative measures of precision depend critically on stipulated
conditions. The well-known terms ‘repeatability’ and ‘reproducibility’ relate to specific types of stipulated conditions. The former
term corresponds to measurements made under the same controlled (same method, strictly adhered to in the same laboratory)
conditions; the latter term refers to the same method used in different laboratories.
3.4
representativeness
extent to which the condition of all the samples taken from the body of water reflects conditions in water of interest
3.5
comparability
degree of agreement with respect to control over random and systematic errors
3.6
Certified Reference Material
CRM
stable, homogeneous material, with a composition closely matching that of the sample to be analysed, for which the
concentrations of the determinands of interest in that material are known with a known degree of uncertainty
NOTE In most chemical analyses the traceability of measurement can be obtained by a series of calibrations that
demonstrates that no loss of determinand or contamination occurs during the sample treatment. This traceability can be based
on the analysis of a CRM.
3.7
blank
observed value obtained when measurement is made on a sample identical to the sample of interest, but in the
absence of the determinand.
NOTE Field blank samples are laboratory blank samples which are taken into the field, treated as samples and analysed as a
check on sampling procedures.
[ISO/TR 13530]
3.8
spike
known quantity of determinand which is added to a sample, usually for the purpose of estimating the systematic
error of an analytical system by means of a recovery exercise
[ISO/TR 13530]
2
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© ISO
ISO 5667-14:1998(E)
3.9
recovery
extent to which a known, added quantity of determinand in a sample can be measured by an analytical system.
NOTE Recovery is calculated from the difference between results obtained from a spiked and an unspiked aliquot of sample
and is usually expressed as a percentage.
[ISO/TR 13530]
3.10
control chart
chart, with upper and lower control limits, on which values of some statistical measure for a series of samples,
spiked samples and blanks, are plotted, usually in date or sample number order
NOTE 1 The chart frequently shows a central line to assist detection of a trend of plotted values toward either control limit.
[ISO 3534-2]
NOTE 2 In some control charts, the control limits are based on the within-batch data plotted on the chart; in others, the
control limits are based on adopted standard or specified values applicable to the statistical measures used.
NOTE 3 The cumulative sum (cusum) chart is a type of control chart. Cusum charts are used for detecting small permanent
shifts in trends that may remain undetected when using the Shewhart control chart. The basic principle of the most common
type of cusum chart is based on the plotted value, which is the cumulative sum of deviations of successive sample statistics
from a target value. When a process change is made, the sum is returned to zero. The ordinate of each plotted point
represents the algebraic sum of the previous ordinate and the most recent deviation from the target.
Cusum charts are generally interpreted by masks superimposed on the chart, a signal occurring if the path of the cusum inter-
sects or touches the boundary of the mask (ISO 3534-2).
3.11
Shewhart control chart
control chart to show if a process is in statistical control
[ISO 3534-2 and ISO 8258]
NOTE It may be a chart using attributes (for example, proportion nonconforming) for evaluating a process, or it may be a
chart using variables (for example, average and range) for evaluating a process. Examples are:
X-bar chart - the sample means are plotted in order to control the mean value of a variable.;
R chart - the sample ranges are plotted in order to control the variability of a variable;
s chart - the sample standard deviations are plotted in order to control the variability of a variable;
2
s chart - the sample variances are plotted in order to control the variability of a variable;
C chart - the number of defectives (per batch, per day, per machine, etc.) is plotted.
3.12
action control upper [lower] limit
limit above [below] which, or the limits outside which, the statistic under consideration lies when action should be
taken
[ISO 3534-2]
NOTE These limits are based on the assumption that only 0,3 % of normally distributed results will fall outside these limits.
Such an occurrence would strongly indicate that additional, assignable causes of variation may be present and that action may
be required to identify and reduce them.
3
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© ISO
ISO 5667-14:1998(E)
3.13
warning control upper [lower] limit
limit below [above] which (upper [lower] limit), or the limits between which, the statistic under consideration lies with
a high probability when the process is under control
[ISO 3534-2]
NOTE 1 When the value of the statistic computed from a sample is outside the warning limits but inside the action limits,
increased supervision of the process is generally necessary and rules may be applied for action in particular processes.
NOTE 2 At the warning limits, attention is called to the possibility of out-of-control conditions, but further action is not
necessarily required.
NOTE 3 The warning limits will always be within the action limits.
NOTE 4 Warning limits are usually set so that only 5 % of results should normally fall outside them. Under certain
circumstances, successive results outside warning limits indicate that assignable causes of variation are present and that
actions are required to identify and reduce errors.
NOTE 5 The limits are calculated from the standard deviation of the statistic under consideration of at least 20 samples.
Warning and action control limits are applied to individual sampling results.
3.14
central line
line on a control chart representing the long-term average or a prespecified value of the statistical measure being
plotted
[ISO 3534-2]
4 Sources of sampling error
Sources of error include the following:
a) Contamination
Contamination can be caused by sampling equipment materials (sampling containers and sample containers)
by cross contamination between samples and by sample preservation and inappropriate storage and transport
arrangements.
b) Sample instability
The type of sampling vessels and containers used can affect the stability of the determinand between sampling
and analysis due to the inherent instability of the sample itself and the conditions in which samples are stored
and transported.
c) Incorrect preservation
The choice of sampling vessels and containers affect the integrity of the determinand and the options for
preservation which may be available, as detailed in ISO 5667-3.
d) Incorrect sampling
Deviation from the sampling procedure, or the procedure itself, may be a source of error.
e) Sampling from nonhomogenized water bodies
f) Sample transportation (see clause 6).
4
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© ISO
ISO 5667-14:1998(E)
5 Sampling quality control techniques
5.1 General
Sampling is defined in ISO 5667-2 as the process of removing a portion, intended to be representative, of a body of
water (or sludge or sediment) for the purpose of examination for various defined characteristics.
Guidance is given below with respect to quality control procedures which can be used to identify and quantify errors
associated with sampling.
A complete overview of quality assurance applied to sampling is outside the scope of this document. However, it is
important to emphasize that the quality control measures discussed below should ideally be applied in the context of
a well organized approach to quality control. This would include a review of the whole approach to sampling with
respect to its fitness for the intended purpose. Within this, the choice of sampling techniques, sampling locations,
numbers and types of sample taken, training of sampling staff, sample transport, preservation and storage should
be considered. The chosen approach should be adequately documented and a system of record-keeping
established. A suitable quality control programme could contain any or all of the techniques listed below. The effort
expended on sampling quality control is dependent on the objectives of the programme, but it is recommended that
at least 2 % of analytical efforts should be devoted to quality control for sampling.
As noted earlier, quality control measures in sampling have three main objectives:
a) to provide a way of monitoring and detecting sampling errors and hence a means of rejecting invalid or
misleading data;
b) to act as a demonstration that sampling errors have been controlled adequately; and
c) to indicate the variability of sampling and thereby to give a guide to this important aspect of error.
The following quality control techniques are described below.
the collection of replicate samples as a check on the precision of sampling;
the use of field blank samples to monitor sources of sample contamination;
the use of spiked samples as quality controls to assess sample stability during transport and storage.
5.2 Replicate quality control samples
This term can be used to cover a range of approaches to quality control which aim to assess the random error
associated with different levels of the sampling process:
a) analytical variance: replicate analyses of the same sample prepared in the laboratory can be used to estimate
short-term analytical error;
b) analytical + subsampling/transport variance: analyses of replicate samples taken in the field (B and B ) from
1 2
the bulk sample (B) (the sample obtained by a single application of the sampling procedure). The difference
between such data gives an estimate of analytical plus sampling variance (includes storage but excludes the
effect from sampling containers).
c) analytical + total sampling variance: analysis of bulk samples obtained by separate application of the sampling
procedure. This provides an indication of the variance of the whole process of sampling and analysis (A
1
and A ).
2
The relationship between the different sampling variances in examples b) and c) is illustrated schematically in
figure 1.
5
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© ISO
ISO 5667-14:1998(E)
Figure 1 — Relationship between different sampling variances
The difference between A and A gives an estimate of total sampling variance (sampling, containers, storage and
1 2
analysis).
The difference between B and B (expressed as the mean of b & b and b & b ) gives an estimate of
1 2 11 12 21 22
analytical plus sampling variance (including storage, excluding sampling container).
The difference between replicate analyses b & b and b & b gives an estimate of analytical precision.
11 12 21 22
The analysis of replicate samples provides an estimate of the contribution of analytical error for all of the examples
in figures 2 to 7.
Comparison between the different estimates of variance described above can be used to identify the most important
sources of measurement uncertainty. This is illustrated in tables 1 and 2 below.
Table 1 — Analytical variance
Sample
...
SLOVENSKI STANDARD
SIST ISO 5667-14:1999
01-marec-1999
.DNRYRVWYRGH9]RUþHQMH1DYRGLOR]D]DJRWDYOMDQMHNDNRYRVWLY]RUþHQMDYRGHY
RNROMXLQUDYQDQMD]Y]RUFL
Water quality -- Sampling -- Part 14: Guidance on quality assurance of environmental
water sampling and handling
Qualité de l'eau -- Échantillonnage -- Partie 14: Lignes directrices pour le contrôle de la
qualité dans l'échantillonnage et la manutention des eaux environnementales
Ta slovenski standard je istoveten z: ISO 5667-14:1998
ICS:
13.060.10 Voda iz naravnih virov Water of natural resources
13.060.45 Preiskava vode na splošno Examination of water in
general
SIST ISO 5667-14:1999 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
SIST ISO 5667-14:1999
---------------------- Page: 2 ----------------------
SIST ISO 5667-14:1999
INTERNATIONAL ISO
STANDARD 5667-14
First edition
1998-09-01
Water quality — Sampling —
Part 14:
Guidance on quality assurance of
environmental water sampling and handling
Qualité de l’eau — Échantillonnage —
Partie 14: Lignes directrices pour le contrôle de la qualité dans
l’échantillonnage et la manutention des eaux environnementales
A
Reference number
ISO 5667-14:1998(E)
---------------------- Page: 3 ----------------------
SIST ISO 5667-14:1999
ISO 5667-14:1998(E)
Content
1 Scope . 1
2 Normative references . 1
3 Definitions . 1
4 Sources of sampling error . 4
5 Sampling quality control techniques. 5
6 Transport, stabilization and storage of samples. 12
7 Analysis and interpretation of quality control data. 13
8 Sample identification and records . 13
Annex A (informative) Control charts . 14
Annex B (informative) Bibliography . 18
© ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
---------------------- Page: 4 ----------------------
SIST ISO 5667-14:1999
© ISO
ISO 5667-14:1998(E)
Foreword
ISO (the International Organization for Standardization) is a world-wide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 5667-14 was prepared by Technical Committee ISO/TC 147, Water quality,
Subcommittee SC 6, Sampling (general methods).
ISO 5667 consists of the following parts, under the general title Water quality — Sampling:
Part 1: Guidance on the design of sampling programmes
Part 2: Guidance on sampling techniques
Part 3: Guidance on the preservation and handling of samples
Part 4: Guidance on sampling from lakes
Part 5: Guidance on sampling of drinking water
Part 6: Guidance on sampling of rivers and streams
Part 7: Guidance on sampling of water and steam in boiler plants
Part 8: Guidance on sampling of wet deposition
Part 9: Guidance on sampling from marine waters
Part 10: Guidance on sampling of waste waters
Part 11: Guidance on sampling of groundwaters
Part 12: Guidance on sampling of bottom sediments
Part 13: Guidance on sampling of water, wastewater and related sludges
Part 14: Guidance on quality assurance of environmental water sampling and handling
Annexes A and B of this part of ISO 5667 are for information only.
iii
---------------------- Page: 5 ----------------------
SIST ISO 5667-14:1999
© ISO
ISO 5667-14:1998(E)
Introduction
Quality control procedures are required for the collection of environmental water samples for the following reasons:
a) to monitor the effectiveness of sampling methodology;
b) to demonstrate that the various stages of the sample collection process are adequately controlled and suited to
the intended purpose, including adequate control over sources of error such as sample contamination, loss of
determinand and sample instability. To achieve this quality control procedures should provide a means of
detecting sampling error and hence a means of rejecting invalid or misleading data resulting from the sampling
process ;
c) to quantify and control the sources of error which arise in sampling. Quantification gives a guide to the
significance that sampling plays in the overall accuracy of data;
d) to provide information on suitably abbreviated quality assurance procedures that may be used for rapid
sampling operations such as pollution incidents or groundwater investigations.
This part of ISO 5667 is one of a group of International Standards dealing with the sampling of waters. It should be
read in conjunction with the other parts of ISO 5667 and in particular with Parts 1, 2 and 3.
The general terminology is in accordance with that published in ISO/TC 147, Water quality, and more particularly
with the terminology on sampling given in ISO 6107-2.
iv
---------------------- Page: 6 ----------------------
SIST ISO 5667-14:1999
INTERNATIONAL STANDARD © ISO ISO 5667-14:1998(E)
Water quality — Sampling —
Part 14:
Guidance on quality assurance of environmental water sampling and
handling
WARNING: Consider and minimize any risks and obey safety rules. See ISO 5667-1 for certain safety
precautions, including sampling from boats and from ice-covered waters.
1 Scope
This part of ISO 5667 provides guidance on the selection and use of various quality assurance techniques relating
to the manual sampling of surface, potable, waste, marine and ground waters;
NOTE The general principles outlined in this part of ISO 5667 in some circumstances may be applicable to sludge and
sediment sampling.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this part of
ISO 5667. At the time of publication, the editions indicated were valid. All standards are subject to revision, and
parties to agreements based on this part of ISO 5667 are encouraged to investigate the possibility of applying the
most recent editions of the standards indicated below. Members of ISO and IEC maintain registers of currently valid
International Standards.
ISO 5667-1:1980, Water quality — Sampling — Part 1: Guidance on the design of sampling programmes.
ISO 5667-3:1985, Water quality — Sampling — Part 3: Guidance on the preservation and handling of samples.
3 Definitions
For the purposes of this part of ISO 5667, the following definitions apply.
3.1
accuracy
closeness of agreement between a test result and the accepted reference value
[ISO 3534-1]
NOTE The term accuracy, when applied to a set of test results, involves a combination of random and systematic error or bias
components.
3.2
bias
difference between the expectation of the test results and an accepted reference value
[ISO 3534-1]
1
---------------------- Page: 7 ----------------------
SIST ISO 5667-14:1999
© ISO
ISO 5667-14:1998(E)
NOTE Bias is the total systematic error as contrasted to random error. There may be one or more systematic error
components contributing to the bias. A larger systematic difference from the accepted reference value is reflected by a larger
bias value.
3.3
precision
closeness of agreement between independent results obtained under stipulated conditions
[ISO 3534-1]
NOTE 1 The variation associated with test results from repeated sampling operations will be subject to variation from
analytical sources as well as from sources connected with the sampling process. A comparison of random error from repeated
sampling operations with that from repeated analysis of the same sample can be used to deduce the contribution of sampling
to overall random error.
NOTE 2 Precision depends only on the distribution of random errors and does not relate to the true value or the specified
value, (ISO 3534-1). The measure of precision is expressed in terms of a standard deviation value. Improved precision is
reflected in a smaller standard deviation value.
NOTE 3 The ‘independence’ of test results reflects the extent to which results are obtained in a manner not influenced by
any previous result on the same test object (ISO 3534-1). Quantitative measures of precision depend critically on stipulated
conditions. The well-known terms ‘repeatability’ and ‘reproducibility’ relate to specific types of stipulated conditions. The former
term corresponds to measurements made under the same controlled (same method, strictly adhered to in the same laboratory)
conditions; the latter term refers to the same method used in different laboratories.
3.4
representativeness
extent to which the condition of all the samples taken from the body of water reflects conditions in water of interest
3.5
comparability
degree of agreement with respect to control over random and systematic errors
3.6
Certified Reference Material
CRM
stable, homogeneous material, with a composition closely matching that of the sample to be analysed, for which the
concentrations of the determinands of interest in that material are known with a known degree of uncertainty
NOTE In most chemical analyses the traceability of measurement can be obtained by a series of calibrations that
demonstrates that no loss of determinand or contamination occurs during the sample treatment. This traceability can be based
on the analysis of a CRM.
3.7
blank
observed value obtained when measurement is made on a sample identical to the sample of interest, but in the
absence of the determinand.
NOTE Field blank samples are laboratory blank samples which are taken into the field, treated as samples and analysed as a
check on sampling procedures.
[ISO/TR 13530]
3.8
spike
known quantity of determinand which is added to a sample, usually for the purpose of estimating the systematic
error of an analytical system by means of a recovery exercise
[ISO/TR 13530]
2
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SIST ISO 5667-14:1999
© ISO
ISO 5667-14:1998(E)
3.9
recovery
extent to which a known, added quantity of determinand in a sample can be measured by an analytical system.
NOTE Recovery is calculated from the difference between results obtained from a spiked and an unspiked aliquot of sample
and is usually expressed as a percentage.
[ISO/TR 13530]
3.10
control chart
chart, with upper and lower control limits, on which values of some statistical measure for a series of samples,
spiked samples and blanks, are plotted, usually in date or sample number order
NOTE 1 The chart frequently shows a central line to assist detection of a trend of plotted values toward either control limit.
[ISO 3534-2]
NOTE 2 In some control charts, the control limits are based on the within-batch data plotted on the chart; in others, the
control limits are based on adopted standard or specified values applicable to the statistical measures used.
NOTE 3 The cumulative sum (cusum) chart is a type of control chart. Cusum charts are used for detecting small permanent
shifts in trends that may remain undetected when using the Shewhart control chart. The basic principle of the most common
type of cusum chart is based on the plotted value, which is the cumulative sum of deviations of successive sample statistics
from a target value. When a process change is made, the sum is returned to zero. The ordinate of each plotted point
represents the algebraic sum of the previous ordinate and the most recent deviation from the target.
Cusum charts are generally interpreted by masks superimposed on the chart, a signal occurring if the path of the cusum inter-
sects or touches the boundary of the mask (ISO 3534-2).
3.11
Shewhart control chart
control chart to show if a process is in statistical control
[ISO 3534-2 and ISO 8258]
NOTE It may be a chart using attributes (for example, proportion nonconforming) for evaluating a process, or it may be a
chart using variables (for example, average and range) for evaluating a process. Examples are:
X-bar chart - the sample means are plotted in order to control the mean value of a variable.;
R chart - the sample ranges are plotted in order to control the variability of a variable;
s chart - the sample standard deviations are plotted in order to control the variability of a variable;
2
s chart - the sample variances are plotted in order to control the variability of a variable;
C chart - the number of defectives (per batch, per day, per machine, etc.) is plotted.
3.12
action control upper [lower] limit
limit above [below] which, or the limits outside which, the statistic under consideration lies when action should be
taken
[ISO 3534-2]
NOTE These limits are based on the assumption that only 0,3 % of normally distributed results will fall outside these limits.
Such an occurrence would strongly indicate that additional, assignable causes of variation may be present and that action may
be required to identify and reduce them.
3
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SIST ISO 5667-14:1999
© ISO
ISO 5667-14:1998(E)
3.13
warning control upper [lower] limit
limit below [above] which (upper [lower] limit), or the limits between which, the statistic under consideration lies with
a high probability when the process is under control
[ISO 3534-2]
NOTE 1 When the value of the statistic computed from a sample is outside the warning limits but inside the action limits,
increased supervision of the process is generally necessary and rules may be applied for action in particular processes.
NOTE 2 At the warning limits, attention is called to the possibility of out-of-control conditions, but further action is not
necessarily required.
NOTE 3 The warning limits will always be within the action limits.
NOTE 4 Warning limits are usually set so that only 5 % of results should normally fall outside them. Under certain
circumstances, successive results outside warning limits indicate that assignable causes of variation are present and that
actions are required to identify and reduce errors.
NOTE 5 The limits are calculated from the standard deviation of the statistic under consideration of at least 20 samples.
Warning and action control limits are applied to individual sampling results.
3.14
central line
line on a control chart representing the long-term average or a prespecified value of the statistical measure being
plotted
[ISO 3534-2]
4 Sources of sampling error
Sources of error include the following:
a) Contamination
Contamination can be caused by sampling equipment materials (sampling containers and sample containers)
by cross contamination between samples and by sample preservation and inappropriate storage and transport
arrangements.
b) Sample instability
The type of sampling vessels and containers used can affect the stability of the determinand between sampling
and analysis due to the inherent instability of the sample itself and the conditions in which samples are stored
and transported.
c) Incorrect preservation
The choice of sampling vessels and containers affect the integrity of the determinand and the options for
preservation which may be available, as detailed in ISO 5667-3.
d) Incorrect sampling
Deviation from the sampling procedure, or the procedure itself, may be a source of error.
e) Sampling from nonhomogenized water bodies
f) Sample transportation (see clause 6).
4
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SIST ISO 5667-14:1999
© ISO
ISO 5667-14:1998(E)
5 Sampling quality control techniques
5.1 General
Sampling is defined in ISO 5667-2 as the process of removing a portion, intended to be representative, of a body of
water (or sludge or sediment) for the purpose of examination for various defined characteristics.
Guidance is given below with respect to quality control procedures which can be used to identify and quantify errors
associated with sampling.
A complete overview of quality assurance applied to sampling is outside the scope of this document. However, it is
important to emphasize that the quality control measures discussed below should ideally be applied in the context of
a well organized approach to quality control. This would include a review of the whole approach to sampling with
respect to its fitness for the intended purpose. Within this, the choice of sampling techniques, sampling locations,
numbers and types of sample taken, training of sampling staff, sample transport, preservation and storage should
be considered. The chosen approach should be adequately documented and a system of record-keeping
established. A suitable quality control programme could contain any or all of the techniques listed below. The effort
expended on sampling quality control is dependent on the objectives of the programme, but it is recommended that
at least 2 % of analytical efforts should be devoted to quality control for sampling.
As noted earlier, quality control measures in sampling have three main objectives:
a) to provide a way of monitoring and detecting sampling errors and hence a means of rejecting invalid or
misleading data;
b) to act as a demonstration that sampling errors have been controlled adequately; and
c) to indicate the variability of sampling and thereby to give a guide to this important aspect of error.
The following quality control techniques are described below.
the collection of replicate samples as a check on the precision of sampling;
the use of field blank samples to monitor sources of sample contamination;
the use of spiked samples as quality controls to assess sample stability during transport and storage.
5.2 Replicate quality control samples
This term can be used to cover a range of approaches to quality control which aim to assess the random error
associated with different levels of the sampling process:
a) analytical variance: replicate analyses of the same sample prepared in the laboratory can be used to estimate
short-term analytical error;
b) analytical + subsampling/transport variance: analyses of replicate samples taken in the field (B and B ) from
1 2
the bulk sample (B) (the sample obtained by a single application of the sampling procedure). The difference
between such data gives an estimate of analytical plus sampling variance (includes storage but excludes the
effect from sampling containers).
c) analytical + total sampling variance: analysis of bulk samples obtained by separate application of the sampling
procedure. This provides an indication of the variance of the whole process of sampling and analysis (A
1
and A ).
2
The relationship between the different sampling variances in examples b) and c) is illustrated schematically in
figure 1.
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SIST ISO 5667-14:1999
© ISO
ISO 5667-14:1998(E)
Figure 1 — Relationship between different sampling variances
The difference between A and A gives an estimate of total sampling variance (sampling, containers, storage and
1 2
analysis).
The difference between B and B (expressed as the mean of b & b and b & b ) gives an estimate of
1 2 11 12 21 22
analytical plus sa
...
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