ASTM E1329-00(2005)
(Practice)Standard Practice for Verification and Use of Control Charts in Spectrochemical Analysis
Standard Practice for Verification and Use of Control Charts in Spectrochemical Analysis
SIGNIFICANCE AND USE
Consistency in analysis depends on being aware of a significant change in instrumental response, such as that caused by drift or changes in analytical precision, or both, and taking corrective action. The usual corrective action for drift is standardization. Standardization, however, when there is no real need, can only broaden the spread of subsequent analyses. One purpose of this practice is to set guidelines that will avoid “ overstandardization.”
To control manufacturing processes, there must be confidence that a consistent material is being produced and that the analysis of the material is reliable. For assurance that the material meets specification, a purchaser may require the supporting record of control charts to assess that proper analytical control has been maintained.
Ideally, variations in analytical results may be held to chance causes. The concept of a confidence interval or limits on a control chart is based on what can be expected when all normal precautions are exercised. When results appear to go out of control, the analyst should consider and correct what might be an assignable cause. As experience is accumulated, however, it may not seem unusual for readings to drift with time as optics degrade, detector response changes, or excitations modify as, for example, when deposits build up on a counter electrode (a correctable assignable cause), or the longer range effects as an X-ray tube deteriorates.
SCOPE
1.1 This practice covers procedures for determining if a spectrochemical analysis is under statistical control.
1.2 Criteria are presented for determining when corrective action is required.
1.3 Control will be effected by using verifiers to test instrument response. It is recommended, although not required, that this be accompanied by the plotting of control charts.
1.4 The preparation of control charts is described.
1.5 Limitations—The procedures that are described do not apply to analyses that require a calibration each time a set of analyses is run. Reference is made specifically to optical emission spectroscopy, but the practice has a more general application.
1.6 This practice does not apply to validation procedures that monitor the correctness of calibration.
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Standards Content (Sample)
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Designation: E1329 – 00 (Reapproved 2005)
Standard Practice for
Verification and Use of Control Charts in Spectrochemical
Analysis
This standard is issued under the fixed designation E1329; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope MNL 7 Manual on Presentation of Data and Control Chart
Analysis
1.1 This practice covers procedures for determining if a
spectrochemical analysis is under statistical control.
3. Terminology
1.2 Criteria are presented for determining when corrective
3.1 Definitions—For definitions of terms used in this prac-
action is required.
tice, refer to Terminologies E135 and E456 and Practice E876.
1.3 Control will be effected by using verifiers to test
Refer also to the glossary of terms and symbols appearing in
instrumentresponse.Itisrecommended,althoughnotrequired,
MNL 7.
that this be accompanied by the plotting of control charts.
3.2 Definitions of Terms Specific to This Standard:
1.4 The preparation of control charts is described.
3.2.1 control limits—in control charts, the upper and lower
1.5 Limitations—The procedures that are described do not
limits of a statistic that are not expected to be exceeded,
apply to analyses that require a calibration each time a set of
designated as UCL and LCL respectively in this practice. For
analyses is run. Reference is made specifically to optical
the statistic that is the average of more than one reading or
emission spectroscopy, but the practice has a more general
determination, the upper and lower limits will be equidistant
application.
from a central line (CL) representing the expected average. For
1.6 This practice does not apply to validation procedures
the statistic of either standard deviation or range, the upper
that monitor the correctness of calibration.
limit will be farther from the central line if the lower limit is
2. Referenced Documents zero.
3.2.2 normalization—a procedure for correcting readings to
2.1 ASTM Standards:
a common basis. A special case of normalization is standard-
E135 Terminology Relating to Analytical Chemistry for
ization in which readings are made to conform to an existing
Metals, Ores, and Related Materials
calibration. Normalization permits gathering data in different
E158 Practice for Fundamental Calculations to Convert
periods of time and correcting for drift in a way that may be
Intensities into Concentrations in Optical Emission Spec-
3 independent of standardization routines.
trochemical Analysis
3.2.3 variation—difference in an observed value from a
E305 Practice for Establishing and Controlling Atomic
norm.
Emission Spectrochemical Analytical Curves
3.2.3.1 assignable cause—variation which can be identified
E456 Terminology Relating to Quality and Statistics
and corrected. It may be the result of a condition of an
E876 Practice for Use of Statistics in the Evaluation of
instrument or a method of operation. For example, signal
Spectrometric Data
intensities may be affected because a spectrometer is not
2.2 Other ASTM Documents:
profiled properly.
3.2.3.2 chance or common cause—random variation which
consistently affects a system, contributing to the imprecision in
This practice is under the jurisdiction of ASTM Committee E01 on Analytical
a predictable way. In the application of control charts, the
Chemistry for Metals, Ores, and Related Materials and is the direct responsibility of
assumption is made that chance causes of variation are
Subcommittee E01.22 on Laboratory Quality.
normally distributed.
Current edition approved May 1, 2005. Published June 2005. Originally
approved in 1990. Last previous edition approved in 2000 as E1329 – 00. DOI:
10.1520/E1329-00R05. 4. Significance and Use
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
4.1 Consistency in analysis depends on being aware of a
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
significantchangeininstrumentalresponse,suchasthatcaused
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Withdrawn. The last approved version of this historical standard is referenced
on www.astm.org. ASTM Manual Series, ASTM, 6th edition, 1990.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1329 – 00 (2005)
by drift or changes in analytical precision, or both, and taking measurements, the averages of a set of before and after
corrective action. The usual corrective action for drift is readings will be used for normalization.
standardization. Standardization, however, when there is no
NOTE 1—If there appears to be a drift between readings of standardants
real need, can only broaden the spread of subsequent analyses.
obtained before and after a set of calibrants has been run, an instrument
One purpose of this practice is to set guidelines that will avoid
problem may have to be investigated and corrected or the operational
“overstandardization.”
environment improved. Reliable calibration data can be obtained only if
4.2 To control manufacturing processes, there must be an instrument shows a stable operation. Practice E876 describes ways to
test for drift.
confidencethataconsistentmaterialisbeingproducedandthat
the analysis of the material is reliable. For assurance that the
6.1.1.1 Unless a curve fitting routine is being used that
material meets specification, a purchaser may require the
requires “standardizing” before running a set of reference
supporting record of control charts to assess that proper
materials it is recommended that no normalization be done
analytical control has been maintained.
until all calibration data has been recorded. Strictly speaking
4.3 Ideally, variations in analytical results may be held to
standardization, as defined in Terminology E135, only can be
chance causes. The concept of a confidence interval or limits
doneafteracalibrationhasbeenestablished.Ifanormalization
on a control chart is based on what can be expected when all
to some prescribed set of readings is done as if it were a
normal precautions are exercised. When results appear to go
standardization before each time a set of reference materials is
out of control, the analyst should consider and correct what
run, the resulting record of readings can be treated as if no
might be an assignable cause. As experience is accumulated,
standardization had been done.
however, it may not seem unusual for readings to drift with
6.1.2 Choose one set of averages of before and after
time as optics degrade, detector response changes, or excita-
readings of 6.1.1 as the norm. A grand overall average of the
tions modify as, for example, when deposits build up on a
sets may be used if that seems like a reasonable median of all
counter electrode (a correctable assignable cause), or the
sets.Excludeanyreadingsforaelementinareferencematerial
longer range effects as an X-ray tube deteriorates.
that does not show comparable repeatability to what was
observed for that element in other materials. For higher level
5. Problems in Applying Control
readings, the comparison should be made to observed relative
5.1 A complication in effecting verification control or in
repeatabilities.
using control charts with spectrochemical analyses is that the
6.1.2.1 For an ideal normalization of readings, determine
measurements being taken are not absolute. Determinations
the regression fit of a set of observed readings, x, to expected
depend upon comparisons of one measurement to another: the
readings, y. This linear regression, which is also supported by
relativeintensityofananalyticallinetotherelativeintensityof
Practice E305, commonly is done on electronic calculators or
an internal standard line in optical emission spectroscopy; the
computers by the following equations to determine a slope, m,
interrelationship of counts in X-ray spectroscopy under some
and a constant, k, which can be used to correct observed
specified condition of maintaining a fixed intensity from an
readings to an established norm:
irradiating source and holding to a consistent response from a
n(xy – (x~(y!
detector with or without pulse height analyzers and with or
m 5 (1)
2 2
n( x – (x
~ ! ~ !
without an external monitor; the nonlinear relationship of
emulsion blackening to radiation in photographic measure-
and
ments; and the relative response in integrating for fixed times
k 5 ~(y –m(x!/n (2)
with ostensibly constant radiation sources. Added to these is
the complication of background signal in all techniques.
where the summations of functions of x and y are as fol-
5.2 Itisimportanttorecognizethatthereareseveralsources
lows:
of random variation, including variations from the measuring
x = the observed average readings of an element in a
method as well as inhomogeneity in the specimens.The device
calibration set,
being used to test analytical response is the analytical system y = the expected normal readings for that element, and
itself.Thisdiffersfromnormalstatisticalprocesscontrolwhere n = the number of pairs of x and y readings.
an independent and usually more accurate measuring device is 6.1.2.2 Apply the appropriate m and k corrections to the
used to verify the process variability.
averages of the verifiers and standardants, as well as to the
calibrants in each calibration set, as follows:
6. Verifiers
R 5mR 1k (3)
N O
6.1 It is recommended that readings for all potential verifi-
ers as well as standardants be established by measuring them where:
R = normalized reading, and
along with the calibrants.
N
R = observed average reading.
6.1.1 Ideally, the full set of potential standardants and
O
The grand averages of the normalized readings of the
verifiers should be run before and after a series of calibrants to
permit normalizing all calibration data to a common basis. To standardants and verifiers will become the values used for
standardizing.
achieve the best normalization of data, readings should be
recorded for all elements of interest on every standardant and 6.1.3 If the analytical system only can support the early
verifier, even if there is no knowledge of expected concentra- convention of “two-point” standardization, and if the only
tions. Unless there is a marked change in the before and after permissible normalization is a quasi-standardization, before
E1329 – 00 (2005)
collecting calibration data it is still advisable to record all termsofconcentrationbymultiplyingbytheslopeoftheofthe
readings for all elements in all reference materials to establish calibration equation at the point of the verifier reading. Details
a full record of what can be expected for all the reference are given in Annex A1.
materials (see 8.6). The initial set of “normal” readings are 6.3.1 If a deliberate change is made in the slope of a
reasonable starting points. Neither the preferred method of calibration curve after the collection of data, such as might be
using a regression fit nor the recommendation of waiting until done in the transformation in 6.1.4, the effective standard
all data have been logged before assigning normal values are deviation of the reading will be the previous observed standard
infallible. Modification of these values always should be an deviation divided by the factor used to change the slope of the
option as more experience is gained. It is expected, however, curve. Thus, if a standard deviation has been calculated as
thatthepreferredmethodswillarriveattheidealnormalvalues being 0.6 when a curve slope (change of concentration divided
earlier. by change in reading) at some point was 0.4, it would become
6.1.3.1 If the operating system is based on two-point stan- 0.3ifthecurvewasmadetwiceassteep,thatis,whentheslope
at the same point was changed to 0.8.
dardization, Eq 3 still would be used to normalize or standard-
ize readings. The generation of slope and constant corrections,
7. Use of Confidence Interval to Control Spectrochemical
however, would be as follows:
Analysis
m 5 H –L / H –L (4)
~ ! ~ !
R R O O
7.1 Practice E876 uses Student’s t-table to establish the
and
range of reading or concentration around an average that will
include the true reading or concentration at some confidence
k 5H –m~H ! (5)
R O
level. The calculation includes the standard deviation of the
where:
measurement.To be effective, the standard deviation should be
H = Reference or normal reading of the high standardant,
R
estimated with at least 16 df. The interval straddling the
L = Reference or normal reading of the low standardant,
R
average will be 6ts/ n , where t is a factor from the t-table
=
H = Observed reading of the high standardant, and
O
for some probability level, s is the estimate of standard
L = Observed reading of the low standardant.
O
deviation, and n is the number of readings taken for one
6.1.4 Ifdataarelatertransformedbyaslopeandinterceptto
observation. If control of a method depends upon observing an
give a different scaling for the calibration, the same transfor-
intensity reading, the confidence interval may be in terms of an
mation must be applied to the readings of standardants and
intensity reading. If a method uses a computer to display
verifiers.
concentration, the confidence interval should be in terms of
6.2 If a verifier (or a new standardant) is established after a
concentration.
calibration has been defined, the expected reading can be
7.1.1 If the confidence interval is used to judge when drift
established as follows:
has occurred, it will be appropriate to use a confidence level of
6.2.1 Shortly after a standardization, run the verifier in
95 % to anticipate when control may be in jeopardy.
replicate and keep a record of its average reading. Average
7.1.2 Itwillbesatisfactorytouse2.0asanapproximationof
about ten such observations made after new standardizations to
a t-factor for the 95 % confidence. When it is observed that a
obtain a good representation of the expected reading.
verifier has exceeded this range, often referred to as a
6.2.1.1 Normalization coefficients are determined by mak-
“2-sigma” limit, a standardization should be done. This can be
ing a linear regression fit of normal readings as a function of
a half correction if the verifier reads less than three times the
observed readings, such as is done in Practice E305 in
standard deviation, the “3-sigma” limit. Practice E305 de-
establishing a straight line relationship by the method of least
scribes procedures for half corrections. See also 8.5.3.
squares. The “normal” set of readings
...
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