For laboratories performing routine elemental analysis, the answer depends on the method.
A single-element standard provides flexibility. A multi-element standard can reduce routine preparation. A custom ICP standard can take that efficiency further by combining method-specific analytes, concentrations, matrix, and volume into a formulation designed around a laboratory's workflow.
But convenience cannot come at the expense of chemistry. Concentration, matrix, elemental compatibility, oxidation state, stability, instrument requirements, and intended analytical use all need to be considered before choosing a formulation.
As discussed in our Guide to ICP Quality Control Standards, different standards serve different purposes within an ICP workflow. Once the required calibration or QC function is established, the next question is how that standard should be formulated.
What Is a Single-Element ICP Standard?
A single-element ICP standard contains one target element at a defined concentration and in a defined matrix.
Single-element reference materials give laboratories a high degree of flexibility because analysts can use individual standards to prepare calibration or QC solutions containing the concentrations required by a specific method.
They can be particularly useful when:
- Only a small number of elements are required
- Concentrations differ significantly between analytes
- Certain analytes are chemically incompatible
- The laboratory changes methods frequently
- Independent individual standards are desirable
CPI's Single Element ICP Standards include Peak Performance™ standards and CRMs for ICP-MS and ICP-OES calibration, trace-metals analysis, and QC.
What Is a Multi-Element ICP Standard?
A multi-element ICP standard contains several analytes in one solution.
For laboratories routinely analyzing the same groups of elements, multi-element standards can reduce:
- Pipetting steps
- Standard-preparation time
- Number of individual standards handled
- Opportunities for preparation errors
- Laboratory waste
Those benefits are especially useful in repetitive, high-volume workflows. However, the ability to measure many elements simultaneously does not mean every element can be combined into one stable reference solution.
Why Can't Every Element Be Combined?
The chemistry of the solution still applies.
When developing a multi-element inorganic standard, scientists may need to account for:
- Precipitation
- Hydrolysis
- Adsorption
- Oxidation and reduction
- Complex formation
- Matrix incompatibility
- Concentration-dependent instability
Chemical compatibility can also change with concentration.
A combination stable at relatively low analyte concentrations may not necessarily remain stable when those concentrations increase substantially. This is one reason unusual analyte combinations or concentrations can require formulation expertise rather than simply mixing existing single-element solutions.
How Do You Select the Matrix for an ICP Standard?
Matrix selection is one of the most important considerations when choosing an inorganic reference material.
Common matrices include:
Nitric Acid
Nitric acid is commonly used for inorganic elemental standards and is compatible with many metals and analytical applications.
Hydrochloric Acid
Certain elements can be better suited to hydrochloric-acid matrices. A chloride-containing matrix may also be appropriate when a laboratory is attempting to match aspects of its sample-preparation chemistry.
Mixed-Acid Matrices
Some applications require combinations of acids to maintain appropriate analyte stability.
Hydrofluoric Acid
Small quantities of HF can be required to stabilize certain elements or mixtures.
Laboratories using an HF-containing standard should verify that instrument components, sample-introduction components, and laboratory procedures are suitable for hydrofluoric-acid-containing solutions.
Water and Other Method-Specific Matrices
The correct choice ultimately depends on the analytes, their concentrations, analytical method, and intended use.
What Concentration Should an ICP Standard Be?
There is no single correct concentration for every ICP method.
Selection should consider:
- Analytical method
- Expected sample concentration
- Instrument working range
- Dilution procedure
- Required calibration range
- Analyte stability
A concentrated stock standard such as 1,000 µg/mL may be used to prepare lower-concentration working solutions. Alternatively, a custom formulation can be manufactured closer to the concentrations routinely required by a laboratory's method.
The second approach can be useful when analysts repeatedly perform the same preparation.
What Is a Custom Inorganic Standard?
A custom inorganic reference standard is formulated around the laboratory's analytical requirements rather than selected from a predefined catalog formulation.
Depending on the application, the laboratory can specify:
- Elements or analytes
- Concentration of each analyte
- Matrix
- Solution volume
- Packaging
- Required certification
- Analytical method or application
For a laboratory performing the same multi-element method routinely, that can replace repeated preparation from several individual stock standards with a formulation designed around the method.
How Can Custom Standards Improve Laboratory Workflow?
Suppose an analyst must prepare the same working mixture every week from multiple individual standards. Every preparation requires measuring, transferring, diluting, documenting, and handling those materials.
A suitable custom formulation can reduce preparation time and minimize opportunities for dilution or transcription errors.
The objective is not customization for its own sake.
It is to get the concentration, matrix, analyte combination, and volume the method requires so laboratory personnel can spend less time preparing standards and more time running samples.
What Should You Ask a Custom Standard Supplier?
Before requesting a formulation, define the analytical requirement as completely as possible.
Needed information includes:
- Which elements are required?
- What concentration is required for each?
- What matrix is needed?
- What volume does the laboratory use?
- What analytical method will use the standard?
- Is a CRM required?
- Are there specific packaging requirements?
- Are any unusual analyte combinations involved?
The manufacturer can then evaluate whether the requested combination is appropriate and stable.
CPI Custom and Single Element ICP Standards
CPI International's Peak Performance™ trace-metal standards use high-purity starting materials with NIST traceability. CPI offers single-element and multi-element solutions in matrices including nitric acid, hydrochloric acid, and specialized mixed-acid formulations.
CPI's custom-standard program allows laboratories to specify the analytes, concentrations, matrix, volume, and other formulation requirements appropriate to their analytical workflow.
CPI custom standards are available with no minimum order quantity, allowing laboratories to order only what they need while specifying the formulation required by the method.
Frequently Asked Questions
Are single element standards better than multi-element standards?
Neither is universally better. Single-element standards provide greater formulation flexibility, while properly formulated multi-element standards can reduce preparation steps. The appropriate choice depends on the analytical method.
Can I combine any ICP elements into one standard?
No. Element compatibility, concentration, matrix, oxidation state, and stability must be considered before a multi-element formulation is prepared.
Can CPI make a custom ICP standard?
Absolutely. CPI can formulate custom inorganic standards with specified analytes, concentrations, matrices, and volumes for laboratory-specific applications.
Why use a custom standard instead of preparing the mixture in-house?
For repetitive methods, an appropriate custom formulation can reduce preparation time and opportunities for dilution or transcription errors.
Your Method Should Determine Your Standard
The best reference material is not necessarily the formulation with the most elements or the highest concentration.
It is the one appropriate for the instrument, method, analytes, concentrations, matrix, certification requirements, traceability requirements, stability, and intended QC purpose.
Standardize Your Standards.
For more than 40 years, CPI International has supported the work scientists do. Today, that support includes catalog and custom reference standards designed to help laboratories obtain the materials their analytical methods require.
Explore CPI Reference Standards or configure a Custom Standard for your analytical method.