The Development of In-House Reference Materials in the Specialty
by Tom Dulski
Industrys needs far outweigh the availability of reference materials.
Tom Dulski of Carpenter Technology Corp. shows one way of creating
an adequate number of reference materials through traditional
The routine analytical methods used by todays metal industries
are, for the most part, faster, more accurate and precise, and
certainly achieve lower detection and quantitation limits than
any methods that have been developed in the past. They are, however,
also much more dependent upon reference materials to attain these
extreme performance levels. As a case in point, in the production
of iron-, nickel-, and cobalt-base specialty alloys of the sort
that have ended up in aircraft engines and prosthetic implants,
a chemical analysis may involve the determination of 25 elements
from major amounts to sub-part-per-million levels. The expected
level of uncertainty may range from 20 percent of the amount present
in the ppm realm down to less than 0.1 percent at the 30 percent
concentration level. As the molten metal is being refined, the
production personnel expect all this to be provided at near real
X-ray fluorescence and atomic emission spectrometers make this
sort of feat possible. But even with the invocation of fundamental
parameters calibration, few knowledgeable analysts would deny
a critical role to reliable matrix-matched reference materials,
particularly in validating high accuracy work with complex alloys.
The obvious first choice in most laboratory settings is to purchase
certified alloy reference materials to satisfy all calibration
and validation needs. Certified compositions in solid form are
available from national agencies (such as the National Institute
of Standards and Technology) and from commercial suppliers around
the world. The COMAR database, maintained by BAM (the Federal Institute for Materials
Research and Testing) in Germany, lists over 10,000 materials,
including many metals and alloys. Unfortunately, except for simple
matrices and a few high-volume production alloys, the reference
materials available for sale fall far short of meeting the current
requirements for the specialty alloy industry, which may involve
the analysis of 500 or more standard grades of widely varying
composition. Moreover, there are numerous modifications to base
compositions and entirely new experimental alloys constantly being
developed. Reference material needs here clearly represent a very
wide, but shallow, sea that no government or private commercial
source could ever adequately supply.
What choices, then, are available to laboratories that must play
this frenzied game that demands such extreme accuracy? The modern
minds initial response is to search for a technological solutiona
new, non-comparative methodology that will churn out those 25
numbers over five orders of magnitude in virtually any combination,
and do it with no sample preparation, memory effects, or down-time.
The search, no doubt, continues. But the near-term solution lies
An important point is that care needs to be exercised in using
current comparative technology. Most analysts know that making
a reference material from another reference material incorporates
a large potential for uncontrolled systematic error. But those
same analysts may look upon calibration with pure element solutions
as a simple, definitive answer to any analytical problem. However,
in the case of compositionally complex systems such as high temperature
alloys, such strategies must be applied with extreme caution.
One measure of the complexity of the analytical problem is the
number of required analytes in three, admittedly somewhat arbitrarily
defined, categories: major, minor, and trace. For purposes of
discussion we can regard major elements as those present at greater
than 1 percent concentration, minor as between 0.01 and 1 percent,
and trace as below 0.01 percent. In this segment of the metals
industry we can identify many alloys with five to eight major
constituents and 10 or more minor components. It is also frequently
necessary to measure more than 10 trace elements.
While the use of pure element solutions and comparative methodology
obviously can be useful for such alloy sample systems it is clearly
not simple and certainly not definitive. With sample systems such
as these, spectral and chemical effects abound in most instrumental
methodologies. These effects are compounded by the influence of
long- and short-term drift, particularly on the response of major
constituents. And, while software has reached a high level of
sophistication, no correction model is known that can seamlessly
handle all compositional combinations.
In addition, the preparation of high accuracy calibration solutions
from pure elements and compounds is not the trivial task that
it appears. High purity metals may bear undetected surface oxide
contamination, salts may be deliquescent, and oxides may be non-stoichiometric.
Commercially prepared solutions vary widely in quality, and are
subject to the same pipetting errors and chemical compatibility
issues in mixtures. Also, sole reliance on pure element calibration
makes any systematic error in the dissolution of the metal alloy
sample completely transparent. And this last issue can be extremely
relevant since unique procedures are often necessary to dissolve
some superalloy compositions.
What remains, then, as an alternative approach? It is time- and
labor- intensive classical wet chemistry, used to certify in-house
prepared alloy reference materials. Classical gravimetry and titrimetry,
supported and supplemented by instrumental techniques that are
augmented by separation science, can be the basis for producing
a large custom-tailored suite of reference materials to meet all
requirements for the high-speed production of specialty alloys.
This solution to the problem, of course, depends upon the presence
of an infrastructure of manpower, knowledge, and facilities that
today is no longer commonplace. Such an investment has a significant
return in the enhanced confidence that accrues to all the high-speed
Carpenter Technologys Program
The way reference materials are made at Carpenter Technology Corporation
involves a degree of systematic planning, involving close cooperation
between production and research. In most cases the prime driver
for the production of a new reference material is the process
control laboratory where calibration and validation needs are
usually the most urgent. Once the proposal for a new reference
material has been accepted the search begins for a suitable starting
material. Because micro-structural morphology can influence X-ray
fluorescence results, we take great pains to select materials
in the desired thermomechanical conditionin some cases producing
separate reference materials from the same heat, compositionally
certified separately as cast, wrought, and HIPed and forged forms.
Starting materials may include billet sections from in-process
production heats, finished bars from inventory, R&D heats of up
to 400 pounds [180 kg], or other sources.
First, any undesirable surface features, such as end-porosity,
pipe, and adherent scale are cut off. The material is then cut
up into small solids of a size suitable for instrumental analysis,
maintaining a record of their spatial location in the original
work piece. Homogeneity testing typically involves both X-ray
fluorescence and optical emission spectrometry. ASTM E 826, Practice for Testing Homogeneity of Materials for Development
of Reference Materials, and similar criteria are applied to the
For small lot projects intended for infrequent use with rarely
encountered compositions, only 30 to 40 solid specimens may be
involved. In this range, 100 percent of the lot is homogeneity
tested. However, with larger lots intended for the analytical
control of high volume compositions, nearly 600 solids may be
produced, and homogeneity testing is based on a sampling plan
designed to represent the original uncut solid form. In the case
of 100 percent testing, deviant data may lead to the rejection
of one or two solids. In the case of sampled testing, deviant
data may lead to 100 percent testing. But, in general, homogeneity
testing is a pass/fail procedure.
About 100 grams of full-cross section millings are obtained from
four or more pieces of passed material. The milling operation
is performed slowly and at settings designed to favor uniform
chip size and to avoid excessive heating. The combined millings
are degreased with low-residue methylene chloride, dried, and
mixed, and then submitted for chemical testing.
A Modern Classical Approach
With the exception of hot extraction methods for carbon, sulfur,
nitrogen, and oxygen, all the analytical techniques employed in
the certification process require the preparation of an aqueous
solution of the alloy. This may involve hot-plate or microwave
digestion with acids, molten salt fusion, or any of several specialized
approaches. The complex compositions encountered in this industry
segment often require very careful planning of the dissolution
to ensure complete solvation of the analyte without loss as a
volatile or precipitated species, and without the introduction
of any interfering substances.
Our laboratory utilizes about 60 analytical methods that can be
described as classical wet chemistry. Of these, about half would
be considered definitive in the sense that they do not rely upon
reference materials or pure element solutions to obtain a result,
namely gravimetric methods, and volumetric methods based on normalities.
The remainder are spectrophotometric methods that rely on a calibration
curve prepared from pure element solutions.
In addition, we have developed numerous hybrid techniques that
use classical separations to reduce background and circumvent
interference and sensitivity problems with instrumental approaches.
Many of these are the same or similar separation schemes as those
used for classical gravimetric methodsion exchange, mercury cathode
electrolysis, solvent extraction, and precipitation reactions.
In inductively coupled plasma (ICP) emission the mercury cathode
can be used to great advantage, particularly for low levels of
aluminum, titanium, zirconium, and the rare earths. Traces of
tramp elements can even be accessed by flame atomic absorption
and very low levels of alloying additions by X-ray fluorescence.
Such approaches can serve as a valuable cross-check on modern
direct techniques like electrothermal atomic absorption and inductively
coupled plasmamass spectrometry (ICP-MS). In all, about 150 distinct
procedures have been used in the Carpenter reference materials
program. Confidence in a certified value can be significantly
enhanced when completely independent procedures, especially those
based on different chemical or physical principles, achieve the
same result. It must be borne in mind, however, that analytical
approaches can vary widely in expected uncertainty and in the
cost and effort of their application.
Analytes are determined with a three-replicate minimum, and always
accompanied by a complement of certified reference materials.
We also attempt to account for the total mass balance in the alloy
by exhaustively measuring all major and minor components. Thus,
for example, we routinely determine iron in iron-base alloys.
Our goal is a total certified composition in the range of 99.9
to 100.1 percent.
Reference Materials and Certificates
Once the analytical work is complete, a certificate is drafted
following the guidelines of ISO Guide 31, Reference materialsContents
of certificates and labels, and ASTM E 1831, Guide for Preparing Certificates for Reference Materials Relating
to Chemical Composition of Metals, Ores, and Related Materials.
Uncertainties are included and calculated from a sum of both the
analytical and homogeneity test variances. Our certificates are
unique, however, in that the appended wet chemical test report
contains all replicate data from the test material and all the
traceability data obtained from the certified reference materials
that were analyzed at the same time and in the manner as the test
material. Two classes of certificates are issued: one for complete
compositional analysis and one for element-specific compositional
analysis. In the latter case, one, or only a small array, of elements
are certified. In addition to the paper copy, certificate data
are recorded in a searchable database that is accessible from
any PC in the company.
The solid disks or rectangular solids are electrolytically etched
with the company logo, a grade designation, and an identifying
code number. These, and the corresponding certificate, are distributed
to the various laboratories within the company where they are
put into routine use for calibration, validation, and verification
of comparative instrumental methodology. The remaining solid specimens
are stored in a large inventory cabinet and are supplied to the
company laboratories as needed. In some cases these materials
are made available for sale to outside parties.
The unused portion of the chips is also stored and sometimes proves
useful in re-check work, and for the validation or verification
of routine solution work or thermal extraction analysis.
The Carpenter In-House Reference Material Program has been used
to ensure the accuracy of product composition for many decades.
As appropriate new methodologies and protocols have evolved, they
have been incorporated into the program, extending its value and
usefulness. Today, the program provides both a direct audit trail
to NIST Standard Reference Materials and a database that documents
our analytical capabilities. //