Frequently Asked Questions
 
We have been making and using standards for many years and have answered many of the questions you may have concerning the production, certification and use of our standards. Below you will find the answers to many of the most frequently asked questions about Alcoa Spectrochemical Standards. If you do not see your question below, please see our Technical Support page.

 

Q. How does Alcoa select what standards and compositions to offer as catalog items?

Q. Can I use a standard that doesn’t correspond exactly to my alloy? 

Q. What are the differences between "SS", "WA" and "KA" standards?How are they best used? 

Q. How are your standards produced and certified? 

Q. Are Alcoa standards traceable to NIST? 

Q. What do the brackets around certain compositions mean? 

Q. What are your "SQ" standards for? Why isn’t the composition certified?

Q. What does the uncertainty mean? Why do some values come with no uncertainty?

Q. How do I use the certified value and uncertainty to judge how well my instrument is performing?

Q. What is a matrix effect and how do I minimize or avoid this effect? 

Q. I have a piece of rolled or forged aluminum. Can I spark it directly and get the right composition?

Q. When I analyze your standard I do not get the same number you have on the certification sheet. Am I wrong or is the certification incorrect?      

Q. What’s the difference between a Certified Reference Material (CRM), a Standard Reference Material (SRM) and a Reference Material(RM)?

Q. Why do you have individual element standards?

Q. Why are some elements, such as sodium, not generally found in your alloy standards but only as individual element standards?

Q. My OES supplier says I have universal calibration curves. What does this mean? Do I still need standards?

Q. I am using your standards and have good precision and accuracy but my customer says I am still out of spec. What should I do?  

 

 

Q. How does Alcoa select what standards and compositions to offer as catalog items?


A. While Alcoa offers one of the widest ranges of standards in the industry, it is impossible to produce standards for every existing alloy and alloy variation. We attempt to choose the alloys that are most widely used within the aluminum industry and then supplement them with range standards in an attempt to cover compositional variations. Our "SS" standards are generally near the nominal registered Aluminum Association composition for that alloy, with some modifications for elements not covered in the registration. For those who need highly specific standards, we offer Alcoa Specialty Standards.

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Q. Can I use a standard that doesn’t correspond exactly to my alloy?


A. The most desirable situation is to have standards that match your particular composition as closely as possible. Recognizing that this may not be possible, standards that are close in composition or a series of standards that bracket the composition of interest can be used and can provide a high degree of analytical accuracy.

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 Q. What are the differences between "SS", "WA" and "KA" standards? How are they best used?


A. The "SS" standard is the nominal alloy composition and is generally close to the midpoint in Aluminum Association registered composition. The "WA" and "KA" standards are range standards and provide a means to vary composition within the same alloy type. They are also useful in analyzing related alloy compositions. The "Wx" and "Kx" designations are for wrought and casting alloys respectively.

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 Q. How are your standards produced and certified? 


A. Alcoa standards are produced from a continuously cast ingot with length and diameter optimized to assure compositional homogeneity from edge to center and along the length. Certification follows a set of statistically intensive procedures that check for both homogeneity and concentration. Compositional accuracy is assured through the use of NIST reference standards (when available), Alcoa master composition standards and most importantly the use of at least two independent analytical techniques.

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 Q. Are Alcoa standards traceable to NIST? 


A. Yes. NIST standard reference materials are used as part of the certification process. In many cases there are no NIST standards that correspond to the alloys we make. In those cases, the reference analytical methods, such as ICP, will always use NIST traceable solution standards. Traceability is further established through the use of certified and NIST traceable balances for all alloy charge production and analytical purposes. Note: Alcoa provides analysis for NIST standards as one part of NIST’s certification process.

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Q. What do the brackets around certain compositions mean? 


A. The brackets, e.g. (0.52), mean that this is an uncertified composition and is given for reference only. A reference value is established with at least one analytical technique, however,  analysis using a second or third independent verification would not have been performed. In the current Alcoa catalog, certified values are provided for most compositions. The exceptions to this are the Alcoa "SQ" standards which are used to compensate for instrument drift in the intensity domain.

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Q. What are your "SQ" standards for? Why isn’t the composition certified?   

 

A. In most spark optical emission systems the analytical response curves are established only once and will remain the same throughout the life of the instrument. This differs from many other quantitative analytical systems which are standardized prior to each use. Even though the shape of the analytical curves are fixed, no OES system is perfectly stable and the response curves will "drift" over time. In order to monitor and correct this drift, Alcoa provides calibration standards for drift correction -SQ standards. The reason SQ standards are not certified is because they are used simply to establish consistent instrument response. SQ standards also do not correspond to any particular alloy and therefore are generally not suited as standards for compositional determination. They are used at a frequency that depends on the drift behavior of the OES system. For current generation instruments, this is usually performed once per day (24 hours) or once per week.   

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 Q. What does the uncertainty mean? Why do some values come with no uncertainty?


A. The uncertainty reflects the errors associated with the certification of the composition for a particular element. Statistically, it contains the errors due to inhomogenity, instrumental precision and bias between analytical techniques used in the certification processes. In most cases the predominate error is the between technique bias. The given limits of uncertainty represent this combined uncertainty and seek to estimate, with a 95% confidence level, a range in which the true value may be expected to lie. In some cases no uncertainty is given because of limited data or the lack of a second independent measure. For those interested in more detail see the ISO document "Guide to the Expression of Uncertainty in Measurement" and NIST Technical Note 1297 "Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results."

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Q. How do I use the certified value and uncertainty to judge how well my instrument is performing? 


A. Since there are always errors associated with both the certification process and the actual analyses done on your instrument, it is unlikely that you will consistently reproduce the certified value exactly. Certainly, your value should fall within the given uncertainty but you must make a judgment on whether that is sufficient criteria for acceptance. For instance, an analyses of Si may yield 8.62% against a certified value 8.55% with an uncertainty of 0.09%. If a control chart showed that this was a random value and that on average (day-to-day) your analyses were consistent with the certified value you would judge this to be acceptable. On the other hand if you consistently showed an average of 8.62% you may be seeing a bias in your analyses and may want to take action to correct the response.

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Q. What is a matrix effect and how do I minimize or avoid this effect? 


A. There is some confusion when people speak of matrix effects. In most cases in OES we are simply speaking of the fact that different alloys can produce different responses for the same elemental concentration. This effect has less to do with instrumentation and more to do with the micro and macro grain structure and the formation of second phases associated with different alloys. These are what give these alloys properties such as corrosion resistance, strength, fatigue resistance, etc. The best way to avoid the undesirable analytical effects of matrix effects is to use standards whose compositions are similar to that of the alloy being analyzed. This is the reason why Alcoa offers so many different alloy standards.

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Q. I have a piece of rolled or forged aluminum. Can I spark it directly and get the right composition?


A. Maybe - this brings us back to matrix effects. Processes such as rolling, forging, heat treating can significantly affect the grain structure and the formation of second phases. Even large scale castings can show significant macro segregation. Sparking directly on a fabricated piece of aluminum may subject the analyses to errors due to these metallurgical effects.  That’s why we always recommend chilled cast samples taken directly from the molten metal. These samples will have structures most similar to standards. Alloy identification can be done on fabricated pieces but we would recommend that certified composition be established using chilled cast samples.

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Q. When I analyze your standard I do not get the same number you have on the certification sheet.  Am I wrong or is the certification incorrect?     

 

A.  This depends on the magnitude of the difference- see the discussion above on uncertainty. When a customer comes to us with this problem, we most often find that there’s either a problem with instrumentation, calibration curves or a matrix effect that has not been considered. Occasionally, we are wrong. We certify literally thousands of standards and tens of thousands of elements each year and rarely have a problem because of the numerous verification steps we have in place. If you have a problem with a standard, we would be happy to help you resolve the issue whether it is within your laboratory or with our standard.

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Q. What’s the difference between a Certified Reference Material (CRM), a Standard Reference Material (SRM) and a Reference Material(RM)?

 

A. These definitions came under considerable debate when ASTM E1724-95 "Standard Guide for Testing and Certification of Metal and Metal Related Reference Materials" was being drafted. Basically a CRM is a standard which is accompanied by a certificate which expresses both the value of the property being measured and an uncertainty. It is generally believed that it must come from a recognized certifying body with demonstrated capability to make these measurements. Some believe that it can only come from a government agency such as NIST. We do not. Alcoa Standards are CRM’s. By definition, an SRM is a certifed reference material produced at a national metrology laboratory such as NIST. An RM is a material that has shown to have a property which is sufficiently homogeneous and established that it can be used to establish a calibration curve or judge the performance of the measurement system. In the absence of a CRM they can be used to assign values to unknowns.

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Q. Why do you have individual element standards?   


A. They provide a convenient means of establishing response (calibration) curves over a fairly wide range of concentrations. Remember if you choose to use them for day-to-day analytical work, you need to determine that there are no matrix effects since individual element standards are most often not in a particular alloy matrix. There are some exceptions.

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Q. Why are some elements, such as sodium, not generally found in your alloy standards but only as individual element standards?


A.
Some elements are simply difficult to alloy into aluminum and can cause undesirable effects in the standards such as segregation through an ingot or formation of inclusions. Sodium for instance has a tendency to burn off during casting and interfere with silicon grain refining or modification when using strontium or phosphorus. Boron on the other hand, readily forms borides with elements such as titanium. This might not be an issue in general production but can cause significant problems in standards where high homogeneity and consistency are mandatory. Individual element standards allow us to tailor our casting methodology specifically to avoid these problems.   

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Q. My OES supplier says I have universal calibration curves. What does this mean? Do I still need standards?


A. What your supplier has done is to generate calibration curves for each element using a large number of different alloy standards. This gives a generic calibration curve that provides pretty good analyses but will generally not give the same accuracy as those generated for a specific alloy. Standards for your particular alloy are therefore essential. The supplier’s software will allow you to perform a “type standardization” that helps tie the generic calibration curve to a specific alloy. If done correctly, this will greatly increase the accuracy of the analyses.    

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Q.  I am using your standards and have good precision and accuracy but my customer says I am still out of spec. What should I do?


A.
First assure yourself that you have taken into consideration all possible sources of error. Is the lab analyzing good quality samples representative of the material being taken and prepared according to industry standards such as ASTM E716? Are you analyzing chilled cast samples while your customer is looking at a fabricated piece - see above? Are you using the right standard? Have you verified your analyses using an independent method? If you feel comfortable with these answers, one of the best ways to handle such a problem is to independently verify accuracy between laboratories by analyzing the same Alcoa standard.

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