2. ICP Quality Issues There are several quality issues that are important with respect to multi-element chemical standards: Accuracy Purity Chemical Compaitibility Stability Availability Documentation Traceability Many of the topics above have been discussed in other publications on our site. Please use the links provided throughout this article to gain a better understanding of the issues discussed below.
Accuracy The accuracy of a certified reference material (CRM) standard is dependent upon:
The method(s) used for certification (Method Validation).
Proper preparation of the elemental standard (a QC sample or second source sample should be analyzed against the single or multi-element blend).
Identification and expression of all Random and Fixed Errors. See Understanding Error Budgets for definitions. (Note that uncertainty calculations will be discussed in part 3 of this series).
Chemical and physical stability -- Container Transpiration plays an important role in chemical stability. See Stability of Elements at ppb Concentration Levels for detailed information on physical stability.
Packaging and storage -- Container Material Properties are important to consider.
Absence of blunders (training, written procedures, detailed records, internal audits, etc.). Having a good quality system in place helps to prevent laboratory blunders. See ISO Simplifed to learn more about out which International Organization of Standardization (ISO) standards are most important for trace analyses.
Purity Purity becomes an issue when using starting materials of single element blends to prepare multi-element blends. The degree of importance increases as the relative order of magnitude of the components increases. Known purity and hopefully very clean materials are critical in the execution of ICP-OES spectral interference studies. These studies typically involve the aspiration of a 1000 µg/mL solution of a single element while collecting the spectral regions of analytes that may be interfered with.
Inorganic Ventures' laboratory has purchased many materials claiming a purity of 5 to 6-9's. However, it's never a bad idea to confirm a manufacturer's claims. For more information regarding purity considerations, please consult the following online articles:
» Environmental Contamination
» Contamination From Reagents
» Contamination From the Analyst and Apparatus
Chemical Compatibility It's important for the multi-element blends to be compatible with the containers in which they are prepared and stored. It's equally important that they are compatible with the introduction system of the instrument(s) used to analyze the blend and with the other analytes within the blend. Some points to consider:
Is the matrix of the standard compatible with glass or quartz? Glass is not compatible with HF and caustic matrices.
Are there possible reactions between the chemical components of the standard that may adversely alter the standard with time? Photo-reduction of Ag in high HCl matrices, ppt of Ag in trace Cl matrices, ppt of Pb and Ba with trace levels of sulfate or chromate, ppt of the alkaline and rare earths with F- in HF matrices, ppt of fluorinated elements like Sn(F)x-y in the presence of elements that would complex with the fluoride and therefore 'pull it away' from the metal stabilized as the fluoride complex, etc.
Does the standard contain components that could form volatile compounds? A classic example is the oxidation of osmium chloride to the very volatile and toxic OsO4 when nitric acid is added. Volatile compounds may not be lost from the standard solution but will give false high readings due to a disproportionate amount of the element making it to the plasma where the nebulization efficiency is greater due to the added mode of transport to the plasma as the vapor state.
Stability How stable is the standard blend? When a blend is made for the first time and then remade at a later time, a comparison of the two should be made to confirm stability. If there are chemical concerns from the beginning then a fresh blend should be prepared the next analytical day for comparison. Refer to Stability of Elements at ppb Concentration Levels for more information.
Availability Consider the following:
Does the blend have to be kept away from the air (oxygen) or light?
Must it be refrigerated or frozen?
Is it sensitive to fungal growth?
How much time is required for preparation and how often is preparation required?
If you purchase your standards, how quickly do you receive them?
Some of these questions may appear as if they belong in other sections but they all impact the availability of the standard in important ways. For example, blends that must be kept refrigerated or frozen cannot be used until allowed to come to room temperature. This is often the case with blends manufactured within the biological pH range of 4-10.
Documentation Although documentation may seem less important than the above topics, it is paramount for less obvious reasons. Think about the following questions:
Is there a potential for litigation?
Does a customer or certifying body audit your laboratory?
Is all the information on hand when it's needed?
What documentation around or about your chemical standard is needed?
ISO has issued a document referred to as ISO Guide 31. This document details what the international scientific community considers to be critical to the analyst when using chemical standard solutions or CRMs. Our guide to Certificate of Analysis Components offers explanations of each section of an ISO Guide 31-compliant Certificate of Analysis.
Traceability Is it possible that the scientific, commercial, or legal communities will scrutinize your data? If so, the issue of traceability may be more critical than you realize. Traceability has been defined as "the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties." This definition has achieved global acceptance in the metrology community.
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LC-MS实验瓶颈的突破与优化
