Teaching
Objectives
-
To introduce students to the integral concept of traceability.
-
To highlight the crucial role of standards in Analytical Chemistry.
-
To describe the different types of standards relevant to Analytical Chemistry.
-
To integrate the different meanings of traceability and relate them to specific facets of Analytical Chemistry.
-
To relate traceability of a result to capital analytical properties.
3.1 Explanation of the Slides
Slide 3.1
Slide 3.2
3.2.1. These are the six sections of this
chapter.
3.2.2. The slide also shows the teaching
objectives as regards traceability, its use in Analytical Chemistry
and the key standards for this scientific discipline.
3.1.1 Introduction (1 Slide)
Slide 3.3
3.3.1. This is a general introduction to
the chapter contents, which encompass traceability and analytical
chemical standards. The aim is to summarize the relationship of
Traceability to Analytical Chemistry (particularly to the tangible
standards used for measurement).
3.3.2. Measurement standards play a
crucial role in Analytical Chemistry, an essentially metrological
discipline. Measurement standards constitute one of the three basic
types of standards described in Sect. 1.3 (see Slide
1.12).
3.1.2 The Integral Concept of Traceability (4 Slides)
Slide 3.4
3.4.1. This slide introduces
Sect. 3.2, which is concerned with the integral
concept of traceability.
3.4.2. This is a generic description of
“traceability”, a transversally applicable abstract concept that is
certainly difficult to define in a precise manner.
3.4.3. The definitions of “traceability”
in dictionaries and written standards are far for friendly.
Properly understanding what traceability is requires a more
detailed description of its two main connotations (“reference” and
“history”), which are used jointly in this chapter to build the
integral concept of traceability.
Slide 3.5
These are selected graphical examples
illustrating the integral concept of traceability.
- 1.
Traceability in a production chain: the chain links the raw materials obtained from a supplier to the finished product received by the client. Traceability in this context can be classified in two different ways, namely:
-
External and internal traceability. The former applies to inputs and outputs, and the latter to processes of the production system.
-
Backward and forward traceability around a circle connecting the raw materials to the product (forward) for quality-related purposes or the product to the raw materials (backward) for purposes such as dealing with clients’ claims.
-
- 2.
This is a more precise description of traceability of a product including transportation, distribution centres and points of sale, among others. The circular nature of traceability is clearer here. The arrows represent links in the traceability chain.
- 3.
Bar coding is essential for proper monitoring—an unavoidable requirement with a view to ensuring traceability. Such is the case, for example, with blood or urine collection in the first step of clinical analyses at hospitals and outpatient clinics. The urine cup and blood collection tubes for each patient are identified with a unique bar code in order to start the sample custody chain. The samples are sent to different laboratories and the analytical results compiled by appropriate software for delivery. In this way, traceability between patients and their results is assured.
- 4.
Traceability in the agri-food industry is not only essential but also a legal requirement as per a European Union directive and the recommendations of the Food and Agriculture Organization of the United Nations (FAO). For example, a hamburger must be unequivocally traceable to the cow from which the meat came. Traceability here is established by using ear tags and bar codes.
- 5.
A consumer-ready box of eggs on a supermarket shelf is labelled with a code identifying the type, country, place, farm and plant where the eggs were laid. In some countries, the eggs themselves have a printed bar code on the shell, but this identification system is less user-friendly.
- 6.
The last example illustrates the typical traceability chain for measurements of physical parameters such as temperature, time or current intensity. Each step in the pyramid is connected to the next through a traceability link that should be certified. In this example, a measurement made with a given piece of equipment, and its calibration, are connected to an SI unit held as a top-quality international standard at the top of the pyramid. The links in the traceability chain, which should never be broken, include the standards prepared and used by the laboratory, which should be successively compared with commercially available reference standards. The commercial standards in turn are connected to national standards kept by a national organization. Unfortunately, no such well-defined hierarchy can be constructed for chemical measurements. In fact, the highly diverse nature of potential samples (industrial, clinical, environmental) would require having a number of national centres for chemical metrology in each country.
Slide 3.6
3.6.1. This is an imaginary traceability
chain connecting A to B through an unbroken series of comparisons
with three intermediate references (R1, R2
and R3). Provided the chain is not broken, A and B can
be said to be traceable to each other.
3.6.2. Tracing one end of the chain to
the other requires using the “history” of the comparison made or
relationship established at each individual link. Obviously, this
is situation is unrealistic because traceability chains contain
many fewer links in practice.
3.6.3. Connecting links to
well-established references requires a sound knowledge of their
nature and meaning. Frequently, end B is a reference itself (e.g.,
when an analytical result A is traced to a certified reference
material B as described below).
Slide 3.7
3.7.1. From Slides 3.4 and 3.6 it clearly
follows that defining traceability in an integral manner entails
using its tracing and relational connotations jointly.
The tracing facet has to do with the
documented history of (a) a production process from the raw
materials or (b) the performance of an object or system. Such is
the case, for example, with a laboratory instrument, for which
there should be a detailed record of all actions including
installation, servicing, calibration and measurements.
The other facet is the relationship to
references, which are tracing landmarks (see Sect. 3.6.3) and,
most often, standards of some type.
3.7.2. The unambiguous presence of the
previous two facets in traceability is clearly apparent in its
translations into some languages of Latin origin. Thus, the
Portuguese word for “traceability” is rastreabilidade (tracing facet),
whereas the Italian word is riferibilità (referential facet).
3.7.3. Traceability in Analytical
Chemistry is related to such important concepts as accuracy,
uncertainty, calibration, representativeness, and laboratory
comparability and harmonization.
3.7.4. The integral concept of
traceability is applicable to analytical chemical entities such as
analytical results, standards, analytical methods, instruments and
samples. The specific connotations of traceability are all
discussed below.
3.1.3 Types of Standards and Their Traceability (4 Slides)
Slide 3.8
3.8.1. The pyramid of traceability in
physical measurements (Example 6 in Slide 3.5) is not directly
applicable to chemical measurements but can be replaced with a
simpler traceability chain connecting analytical chemical standards
(the tangible standards used in practice) to base standards (SI
units) through so-called “chemical standards”, which are intended
to serve as traceability links. The two types of chemical standards
(primary and secondary) can also be connected by a traceability
chain.
3.8.2. In this simple ranking of
traceability among standards in relation to chemical measurements,
nearness to the true value for each type of standard increases from
bottom (secondary chemical standards) to top (base standards).
Consequently, specific uncertainty decreases in the same direction,
and so do tangibility and accessibility.
Slide 3.9
This slide shows the seven base units
of the International System, namely: the metre for length, second
for time, candela for luminous intensity, ampere for electric
current, kelvin for thermodynamic temperature, mole for amount of
substance and kilogram for mass.
The kilogram and the mole are the two
most relevant to Metrology in Chemistry. The slide shows their
classical definitions. Note that defining the mole requires
mentioning the mass unit (the kilogram).
Slide 3.10
3.10.1. As stated in Slide 3.8, chemical
standards are intended to serve as traceability links between base
standards (SI units) and the analytical chemical standards used in
the laboratory. Consequently, chemical standards play a crucial
role in Analytical Chemistry because they determine the quality of
laboratory standards.
3.10.2. There are two types of chemical
standards according to tangibility:
-
Intangible standards, also referred to as “non-operational standards”, which are tabulated and include the mass of carbon-12, Avogadro’s number and atomic weights.
-
Tangible standards, also known as “operational standards”, which require some experimentation prior to use. The most salient standards of this type are the faraday, which requires electrochemical equipment for verification, and ultrapure (>99.999% silver), also known as “five nine silver”, which is extremely expensive.
Slide 3.11
This is an orderly depiction of the
three types of standards relevant to Analytical Chemistry in a
(traceability network) typical of chemical measurements. The mutual
connections between standards are represented by red chains.
-
As can be seen as regards SI standards, the mole cannot be defined without the kilogram.
-
Base standards (SI units) are connected to chemical standards through three traceability chains, namely:
-
one unequivocally linking the mole to the mass of carbon-12;
-
another between the ampere and the faraday (1 C = 1 A × 1 s); and
-
a third between the second and the faraday.
-
-
Most traceability links occur in the realm of chemical standards. Thus, the mass of carbon-12 is related to the atomic weights used in chemical calculations and to Avogradro’s number, which is in turn related to the faraday.
-
Ultrapure silver is at the boundary between chemical standards and analytical chemical standards. In fact, it is so pure that it can be considered a chemical standard itself—one that is related to the chemical standards atomic weights and the faraday. Likewise, it can be exceptionally used in practice to standardize primary chemical standards by experimentation (E).
-
Primary chemical standards can be traced to atomic weights and ultrapure silver. On the other hand, secondary standards can only be traced to primary standards (through experimentation). Primary and secondary standards are defined in the following section.
3.1.4 Analytical Chemical Standards and Their Integration (10 Slides)
Slide 3.12
3.12.1. This slide defines and classifies
analytical chemical standards. By definition (see Slide 3.8), they
are at the bottom of the significance hierarchy of standards
relevant to Metrology in Chemistry because their associated values
are the farthest from the value held as true and also the most
uncertain; by contrast, they are the most tangible and accessible,
and hence the most commonly used in practice. Analytical chemical
standards can be traced to SI units through chemical standards (see
Slide 3.11).
Analytical chemical standards are
confusingly or even contradictorily defined in the scientific
literature. An integral approach to their characteristics enables
their classification according to three different criteria, namely:
(A) intrinsic properties, (B) reliability and (C) nature. Each
classification is discussed in one of the next three slides.
3.12.2. For example, potassium hydrogen
phthalate is a primary standard commonly used to standardize
solutions of sodium hydroxide (classification A). Also, however, it
can be considered a reference material (classification B) and a
pure substance (classification C).
Slide 3.13
The analytical chemical standards used
in the laboratory can be of two types according to their intrinsic
properties, namely:
-
Primary standards. These are chemical substances fulfilling two essential requirements:
- 1.
a high purity (above 99 or 99.5%) that makes them traceable to atomic weights and ultrapure silver (Slide 3.11); and
- 2.
stability against atmospheric agents (water, oxygen, carbon dioxide).
- 1.
-
Secondary standards. These are neither pure nor stable but can be useful for some purposes. However, they can only be made traceable by connection to a primary standard through experimentation (E in Slide 3.8).
Slide 3.14
There are two types of analytical
chemical standards according to reliability or traceability to base
standards (see classification B in Slide 3.13), namely: reference
materials (RMs) and certified reference materials (CRMs). The
former (RMs) are usually commercially available with a label
stating their characteristics. The latter (CRMs) are produced by
renowned international organisms that supply them with certificates
stating with their associated values and their uncertainty. CRMs
are usually of the sample matrix type.
Analytical chemical standards can be
of three types according to nature (classification C), namely:
-
Physical standards. These are used as received for equipment verification (calibration). Thus, a UV–visible spectrophotometer can be checked with holmium and didymium filters for correct operation of its wavelength monochromator. Similarly, a balance is typically calibrated with so-called “transfer weights”, which are traceable to the kilogram standard (an SI unit).
-
Pure substances or their mixtures. These are analytical chemical standards (see Slide 3.13)—and hence, pure, stable substances—that can be used for both equipment and method calibration (e.g., with a calibration curve as in Slide 2.36). Some are mixtures of pure substances (e.g., vials containing several C3–C6 hydrocarbons for calibrating gas chromatographs).
-
Sample (matrix) standards. These are either certified reference materials (CRMs), which are described in Slide 3.17, or laboratory-made materials (working standards). Matrix standards are high-quality—and expensive—materials mimicking the composition of a sample and having the value of an associated quantity certified by a competent organization. Such is the case, for example, with a soil standard having certified contents and uncertainties in polycyclic aromatic hydrocarbons (PAHs), a milk standard with a certified aflatoxin content or a liophilized serum standard with a certified cholesterol content.
Slide 3.15
3.15.1. This scheme harmonizes the
classifications of Slides 3.12 and 3.14 by classifying the three
types of analytical chemical standards according to nature
(classification C) in terms of reliability (classification B).
Thus, physical standards are largely reference materials, whereas
pure substances and their mixtures can be reference materials (RMs)
or certified reference materials (CRMs), and matrix standards are
mostly CRMs.
3.15.2. The use of each type of standard
is described in detail below. In any case, RMs are used mainly for
equipment and method calibration, whereas CRMs are typically used
for the overall assessment of analytical methods.
3.15.3. The quantities involved can be
physical (particularly with RMs), chemical (more commonly with
CRMs) or, very often, physico–chemical.
Slide 3.16
These are the ten desirable or
indispensable properties for an analytical chemical standard.
The indispensable properties are as
follows:
-
Usefulness for the task concerned (that is, suitability for purpose) (1).
-
Stability (7), homogeneity (8) and a well-defined uncertainty (6) for primary standards, in addition to certified values (5) for CRMs.
-
Experimental traceability to primary standards (10) for secondary standards.
-
Detailed storage and use instructions (3).
-
Wide variety and availability (2).
The desirable properties include
accessibility (4) and ease of preparation and use (8).
Slide 3.17
This is a brief description of
certified reference
materials (CRMs), which are standards having associated
values and their uncertainties endorsed (certified) by a renowned
non-profit international organization responsible for assuring that
these atypical standards meet their requirements. They correspond
to the so named “referential quality” (see Slide 1.17). The
associated values and their uncertainties can only be established
via interlaboratory exercises involving the use of different
analytical processes. CRMs are usually expensive because they take
long to prepare and can only be obtained in small amounts.
-
Most CRMs are of the sample matrix type and mimic actual samples to be analysed by a laboratory. The analyte may be already present in the sample or externally added later. Some organisms also certify the purity of solutions containing substances such as dioxins or PAH.
-
The slide shows the four requirements for a matrix-type CRM, which have to do with the sample matrix, homogeneity and stability in the material, the associated data and the history of the material from preparation to delivery.
Interestingly, a CRM has several
analytical connotations of traceability. Thus, the value for the
associated quantity (e.g., the content, in ng/kg, in toxic dioxins
of ash from an industrial incinerator) should be unequivocally
connected to a chemical standard (referential facet): also, its
characteristics and production should be accurately known (tracing
facet).
Slide 3.18
These are the three main uses of
analytical chemical standards in the laboratory. The first two are
discussed in Slides 3.19–3.23. The third (Slide 3.23) is the use of
primary standards such as potassium hydrogen phthalate to
standardize secondary standards such as sodium hydroxide, which are
those used in practice (delivered from a burette) to determine
acids in samples by titration.
Slide 3.19
Analytical chemical standards are
useful for both equipment and method calibration, which should be
clearly distinguished by analytical chemists.
Thus, as implied by the designation,
the target of equipment
calibration or verification is an instrument or
apparatus (the two are distinguished in Slide 1.25). The aim is to
assure proper functioning of the instrument or apparatus concerned.
For example, if the measurement delivered by an instrument in
response to a standard (usually a physical reference material)
departs from the value for the associated quantity, then the
instrument should be adjusted to have the response coincide with
that expected from the standard. An instrument delivers analytical
information whereas an apparatus produces non-analytical
information. There follows an example of each type of equipment.
-
pH buffering solutions with a certified value including two decimal figures for calibrating pH-meters. If the experimental response does not coincide with the certified value, then the potentiometer of the instrument should be adjusted until it does.
-
Calibrated thermocouples for monitoring the temperature inside stoves. If the temperature in the digital display of the apparatus departs from the reading of the thermocouple, then the stove should be adjusted to have the two temperatures coincide.
The target of
method calibration is a chemical
method of analysis. The aim is to characterize in an
unequivocal manner the relationship between the instrument response
and the presence and/or concentration of an analyte in the sample.
This entails constructing a signal–concentration curve for
calibration (see Slide 2.36 in relation to the analytical property
“sensitivity”). This calibration procedure is not applicable to
apparatuses because the information to be processed is purely
analytical. The standard usually contains the analyte (unless, for
example, a secondary standard such as a sodium hydroxide solution
is to be standardized with a primary standard such as potassium
hydrogen phthalate).
These two types of calibration are
exemplified in the next slide.
Slide 3.20
The following example distinguishes
equipment calibration from method calibration with the
spectrophotometric determination of iron in water by formation of a
coloured chelate.
The target of
equipment calibration here
is the spectrophotometer and the calibration tools are physical
standards rather than the analyte. A UV–visible absorption spectrum
is obtained with a holmium filter (shown in the slide) in place for
comparison with the spectrum associated to the standard. If the
maxima in the two spectra fail to coincide, then the monochromator
of the instrument is adjusted accordingly.
The purpose of
method calibration in this
example is finding the relationship between the absorbance and the
concentration of iron by using a calibration curve constructed from
standard solutions of the analyte. The iron concentration in the
sample is determined by interpolating its absorbance into the
curve.
Slide 3.21
Another purpose
of analytical chemical standards is the overall assessment of
analytical processes by using standards (usually matrix-type
certified materials) as references. As can be seen, the procedure
involves comparing the result obtained from n aliquots of the CRM with its
certified value, either qualitatively or statistically (see next
slide). If the two values coincide, then the method in question is
traceable to the CRM and has thus been “validated”.
It should be noted that only a very
low proportion of existing analytical processes can be assessed in
this way. In most cases, an alternative solution must be found (see
Slide 3.31).
Slide 3.22
This slide
compares in statistical terms the certified value of a reference
material to the experimental value obtained by analysing
n aliquots of the CRM with
an analytical method for validation. The null hypothesis is that
the two values will be identical and the alternative hypothesis
that they will not.
A more rigorous
comparison can be made by considering the specific uncertainty of
the experimental value, using tabulated Student’s t-values. The specific uncertainty is
used to determine the uncertainty interval around the result at a
given probability level (e.g., 95%).
The null hypothesis (H0)
and the alternative hypothesis (H1) can be represented
graphically for easier interpretation.
-
If the value associated to the CRM (C CRM) falls within the uncertainty interval, then the method (H0) will be traceable to the CRM and hence validated.
-
If the associated value falls outside the uncertainty interval (H1), then the method will be subject to positive or, as shown in the slide, negative errors, so it cannot be validated.
Slide 3.23
To standardize means to establish a
(generally experimental) link from a standard to another of a
higher rank (nearness to the true value) in the hierarchy. This is
the third potential use of analytical chemical standards.
Standardizing is indispensable with a
view to unequivocally connecting a secondary standard highly
suitable for a given purpose (e.g., an NaOH, KMnO4 or
Na2S2O3 solution) to a primary
standard (potassium hydrogen phthalate, sodium oxalate and
potassium iodate, respectively).
This procedure is also known as
“factoring” because it involves calculating an experimental
non-dimensional factor (below 1) by which the approximate
concentration of the secondary standard is to be multiplied in
order to determine the actual concentration. The slide shows an
example of factoring: the standardization of an NaOH solution with
potassium hydrogen phthalate.
3.1.5 Specific Meanings of Traceability in Analytical Chemistry and Their Integration (10 Slides)
Slide 3.24
This slide illustrates different
meanings of traceability in Analytical Chemistry.
Traceability of a result is the only
possible, “orthodox” meaning in the realm of Metrology in general.
The other uses of the word “traceability” in the slide are
unorthodox because they are adapted to the specificities of
Metrology in Chemistry. All are associated to the integral concept
of traceability, which includes the tracing and referential
facets—by exception, traceability
of a sample (aliquot) only possesses the tracing facet and
is thus the most unorthodox term.
These concepts are described in
detail in Slides 3.25–3.33
Slide 3.25
These are the three basic meanings of
the integral concept of traceability of a result.
- (1)
A relationship to standards (referential facet), which is the classical, most orthodox definition.
- (2)
A documented history of the production of the result (the tracing facet), which directly influences its quality.
- (3)
Comparability and harmonization of laboratories (a practical consequence).
The integral concept of traceability
can only be properly understood by considering all three meanings,
which are discussed in detail in the following slides.
Slide 3.26
The most
orthodox definition of traceability of a result is that based
on its relationship to standards. In some cases, the relationship
is established through intermediate landmarks (lower-rank
standards). The description of this relationship constitutes the
tracing facet. This is apparent from the ISO definition, shown in
the slide, which holds quite well for physical measurements—in
fact, it was issued specifically for them (see Example 6 in Slide
3.5).
As can be seen, the definition
contains concepts extraneous to Metrology in Chemistry. What is a
national (or international) standard? How distant is an analytical
chemical laboratory from a base (SI) standard? Only with physical
standards such as transfer weights for calibrating balances can the
pyramid in Example 6 of Slide 3.5 be constructed.
Slide 3.27
The second facet of traceability of a
result is the documented history of its production. It is thus a
tracing facet. The questions to be answered in this respect are as
follows:
-
Who performed the analytical process?
-
What materials and equipment were used?
-
When and how was the process performed?
Example: A laboratory determining a
dioxin content of 0.1 ng/kg in a properly identified sample
(e.g., code rd33245f-2012) of landfill ash analysed on February 4,
2012 should keep an accurate record of the persons taking part in
the process of obtaining the result (who?); the materials
(reagents, standards) and equipment used (what?); and the
analytical method and environmental conditions in the laboratory
(how?). Computers and, especially, bar codes, are indispensable for
monitoring purposes in this context (that is, for the tracing facet
of traceability).
Properly answering the previous
questions is unavoidable with a view to fulfilling the requirements
of laboratory accreditation in ISO 17025:2014 (see Chap.
8). Oddly, this standard imposes
traceability in the results but does not refer specifically to the
concept.
Slide 3.28
3.28.1. The practical consequence of
traceability of a result is
the ability to compare, harmonize and trace laboratories to one
another.
For example, if three laboratories in
Beijing, Barcelona and San Francisco (Labs 1–3 in the slide) are
independently traceable through the unbroken green chains (A) in
the determination of the same analyte in the same sample to an SI
unit—only in theory—or the same CRM through different intermediate
standards (e.g., standards obtained from national suppliers),
then
3.28.2. The intermediate standards will
be comparable and traceable to one another (blue chains, B);
and
3.28.3. The results of the three
laboratories for the same analysis or determination will be
comparable and traceable to one another. Therefore, the three
laboratories will be in harmony, which will make them highly
competitive on the international market (the tree can be
mutually confident). Thus,
acceptance by a German dealer in alcoholic drinks of the results of
an analysis for total tannins in wine conducted by the in-house
laboratory of a producer in La Rioja (Spain) may lead to the dealer
deciding to import the wine.
Slide 3.29
Traceability
among standards is dealt with at length above (see, for example,
Slide 3.8). This slide is simply a reminder of its significance to
a metrological discipline such as Analytical Chemistry, where
traceability of a result is
also crucial.
The slide focuses on traceability in
primary and secondary analytical chemical standards.
Slide 3.30
The traceability of an instrument
delivering analytical chemical information provides an excellent
example of the combination of the two basic facets of traceability.
-
The tracing facet is inherent in the requirement set by ISO 17025:2004 for laboratory accreditation and involves recording the whole “history” of an instrument since it was installed.
-
The referential facet focuses on the standards used to calibrate or verify the instrument (Slide 3.19).
The two facets are connected by
calibration, which should be properly documented as well.
Slide 3.31
The terms “standard method” and
“official method” formerly used to assure quality in analytical
methods have been gradually replaced with “a method traceable to…” and led to the
quality of a traceable method being judged by the particular
reference at the end of the traceability chain.
- (1)
Ideally, the final reference should be an SI unit, but this virtually impossible in the chemical realm.
- (2)
The most realistic degree of quality is traceability to a certified reference material. However, the scarcity of CRMs makes it difficult to accomplish.
- (3)
A more affordable target is traceability to a body of laboratories using the same method to analyse aliquots of the same sample in an intercomparison exercise supervised by a widely acknowledged national or international competent organization.
- (4)
Next in the reliability ranking is traceability to a primary, absolute method using no analytical chemical standards (see Slide 5.12). Such is the case with gravimetric methods, for example. The problem arises when the same laboratory has to perform the target method and the primary method in the absence of external references.
- (5)
Traceability to a specialized international reference laboratory (a distinction issued by a competent international organization such as the European Union) is also limited in scope because most laboratories deal only with highly specific types of samples (e.g., bovine meat to be analysed for antibiotics or anabolic steroids).
As shown in the slide, reliability in
the traceability chain decreases from 1 to 5.
Slide 3.32
3.32.1. The traceability of the sample (aliquot)
subjected to an analytical process is very interesting for a number
of reasons, namely:
- (1)
It is rather unorthodox in that it only encompasses the tracing facet of the integral concept of traceability.
- (2)
It represents traceability to two rather than one final reference: the information required and the results of two traceability chains, one corresponding to the capital property representativeness (Slide 7.10) and the other being the so-called “sample custody chain”.
- (3)
It is cyclic: provided the partial traceability chains are not broken, the main goal of Analytical Chemistry (Slide 1.8) is reached. Such a goal is ensuring that the results fulfil the information demand because they are traceable to the specific socio–economic problem addressed (Slide 7.10).
3.32.2. Therefore, the sample aliquot
that is subjected to the analytical process should be traceable to
the information demand (the socio–economic problem) through
unequivocal relationships to the bulk sample, the object and the
analytical problem (see Chap. 7).
3.32.3. The second traceability chain
starts at the aliquot, which should be unequivocally related to its
results through the sample custody chain. The custody chain, which
relies on the use of bar codes and computers, is essential for
automated laboratories processing large numbers of samples each
day. Thus, a clinical laboratory may lead to a healthy person being
diagnosed with diabetic coma—or vice versa—if it does not ensure
traceability of each sample (aliquot) to the patient from whom it
was obtained.
3.32.4. The consequence of the two
traceability chains that start at the sample aliquot not being
broken is that the results are consistent with the information
required and hence that the information demand is fulfilled. This
is a permanent challenge for Analytical Chemistry (see Chap.
7, devoted to analytical
problem-solving).
Slide 3.33
This hierarchical arrangement of the
analytical meanings of traceability facilitates their
integration.
At the top of
the hierarchy is traceability of a
result, which is the most orthodox concept and relies on the
traceability of the standards, equipment and methods used to obtain
it.
Traceability between tangible standards
(analytical chemical standards) provides support for
-
traceability of a result;
-
traceability of equipment in its referential facet; and
-
traceability of methods in its referential facet.
Traceability of equipment and methods
relies on traceability of standards, which in turn rests on
traceability of a result and traceability of a method.
Traceability of methods rests on
traceability of standards and traceability of equipment, and
provides support for traceability of a result.
This approach
would be incomplete without an unequivocal relationship between the
sample (aliquot) and the result in the context of traceability of the sample aliquot,
which is an undeniable foundation of its traceability. It is
separated from the previous types of traceability because it is a
completely unorthodox concept—it lacks the typical referential
facet of other meanings of traceability in Analytical
Chemistry.
3.1.6 Traceability and Capital Analytical Properties (1 Slide)
Slide 3.34
This section harmonizes the contents
of this chapter (Traceability. Reference materials) with those of
the previous one (Analytical Properties).
3.34.1. Based on the general scheme of
analytical properties in Slide 2.4, the capital properties
(accuracy and representativeness) are attributes of the
results.
3.34.2. Based on the integral view of the
concepts behind traceability of a
result (Slide 3.33), capital analytical properties are
consistent with this form of traceability. Thus,
-
accuracy is the first part of the support for traceability of a result, which relies on that of standards, equipment and methods; and
-
representativeness corresponds to consistency of the sample aliquot with the information required and the results themselves.
Both properties are crucial (see
Slide 2.13) with a view to assessing the quality of the results and
building the integral concept of traceability.
3.2 Annotated Suggested Readings
BOOKS
Principles of Analytical Chemistry
M. Valcárcel
Springer-Verlag, Berlin, 2000.
This was the first book to start the
teaching of Analytical Chemistry with its foundations before
dealing with methods and techniques in order to provide students
with an accurate notion of what Analytical Chemistry is and
means.
This chapter overlaps to a great
extent with Chap. 3 of Valcárcel’s book, from which it
borrows the title. This is a simplified version of that chapter,
which, however, has been expanded in some respects to better
illustrate abstracts concepts such as traceability, validation of
analytical methods, and the relationship of traceability to
analytical properties. Valcárcel’s book can be used for direct
consultation of the contents of this chapter.
Metrology in Chemistry and Biology: A practical
approach
M. Valcárcel and 17 other
authors.
O.O.P.E.C., UE (Luxemburg), 1999.
Scientists from 9 European countries
led by Spain produced the first official publication on Metrology
in Chemistry and Biology, which discusses coincidences with and
differences from traditional Metrology (that is, Metrology in
Physics). The most salient conclusion is the need to adapt general
metrological principles to the specificities of chemical,
biochemical and biological measurements. Some annexes to standards
issued in the XXI century have echoed the recommendations. Many
parts of this chapter are inspired by this document.
3.3 Questions on the Topic (Answered in Annex 2)
- 3.1.
What are the main purposes of a sample matrix standard with a certified analyte content (a CRM)? Tick the correct answers.
-
[ ] Calibrating an instrument
-
[ ] Globally assessing an analytical process
-
[ ] Calibrating a method
-
[ ] Standardizing secondary analytical chemical standards
-
- 3.2.
What is a matrix standard? What is its main use?
- 3.3.
What are the essential requirements for establishing the traceability of an instrument?
- 3.4.
Tick the type correct type of standard in each case.StandardBasicChemicalAnalytical chemicalPrimarySecondaryCarbon-12A 0.1 mol L−1 solution of KMnO4Potassium hydrogen phthalateUltrapure silverThe faraday
- 3.5.
Describe the traceability network among standards relevant to Analytical Chemistry with emphasis on the connections between basic, chemical and analytical chemical standards.
- 3.6.
How would you define “traceability of an analytical method (CMP)”?
- 3.7.
The total free acid content of a wine sample is determined by acid–based titration with a sodium hydroxide solution previously standardized with potassium hydrogen phthalate. What standards are used in the process?
-
Chemical:
-
Primary analytical chemical:
-
Secondary analytical chemical:
-
- 3.8.
Define “equipment calibration” and relate it to or distinguish it from “method calibration”.
- 3.9.
What are the purposes of equipment calibration (verification)? Tick the correct answer(s).
-
[ ] Constructing a calibration curve
-
[ ] Adjusting faulty equipment
-
[ ] Globally assessing an analytical method
-
[ ] Distinguishing error types in Analytical Chemistry
-
- 3.10.
Connect each of the following standards to its type in the column on the right.
- 3.11.
Rank the reliability of the following types of standards with a score from 1 (least reliable) to 4 (most reliable).StandardReliabilitySecondary analytical chemical standardChemical standardCRMPrimary analytical chemical standard
- 3.12.
What role do analytical chemical standards play in the traceability of a result?
- 3.13.
What type of standard (basic, chemical or analytical chemical) has the greatest associated uncertainty? Why?
- 3.14.
A sample of powdered milk with a protein content certified in a document issued by a renowned independent organization is
-
[ ] A primary standard
-
[ ] A certified reference material
-
[ ] A secondary standard
-
[ ] A reference material
-
- 3.15.
Name the types of chemical standards, state their differences and give some examples.
- 3.16.
Give an example of each complementary criterion used to classify analytical chemical standards.
- 3.17.
Comment on the tracing facet of traceability of a result. What should it be consistent with?
- 3.18.
Describe a procedure for assessing (validating) a new analytical method in terms of its relationship to matrix-type certified reference materials.
- 3.19.
What is the main limitation of CRMs for establishing the traceability of methods?
- 3.20.
What types of standards prevail among (a) reference materials (RMs) and (b) certified reference materials (CRMs)?
- 3.21.
Which base standard is the most relevant to Chemical Metrology? Why?
- 3.22.
Why are secondary standards used even though they have unsuitable properties (e.g., instability, impurity)?
- 3.23.
What are the requirements for a matrix-type CRM?
- 3.24.
What are the three most salient general uses of analytical chemical standards?
- 3.25.
What are the three principal meanings of traceability of an analytical result?
- 3.26.
How is an analytical method assessed to assure reliability?
- 3.27.
What analytical properties are related to traceability? Explain your answer.
- 3.28.
On what should mutual recognition of the results of two or more laboratories rest?
- 3.29.
What feature and twofold meaning does traceability of the sample aliquot subjected to an analytical process have?
3.4 An Abridged Version of the Chapter
The contents of this chapter can be
shortened for teaching Analytical Chemistry to students not
majoring in Chemistry, albeit to a lesser extent than those of
others because of its transversal conception. The following 5
slides (15% of all) can be omitted for this purpose: