Gas and Liquid Mixture Basics

To make an analytical accuracy claim, the gas mixture you order must undergo an independent laboratory analysis using an instrument, such as a gas chromatograph or FTIR, or using a chemical analysis method. In addition, the analysis must be calibrated with a reference material whose true value has been established, and the key contributors to analysis variability must be quantified and controlled. This data can then be used to calculate the analytical accuracy of the blend. Analytical accuracy should not be confused with certified or certified accuracy. The term certified is used in many different contexts in the specialty gas industry. Some vendors supply certified mixtures and imply an analytical accuracy, when in fact no independent laboratory analysis was done. It is important to understand how the manufacturer defines the term certified before you purchase your next calibration standard.

Analytical traceability is achieved by laboratory analysis of your mixture on an instrument directly calibrated with gas reference material from a recognized metrology organization such as NIST, VSL or NPL. For example, when the analysis is performed with a NIST SRM or NTRM, the resulting product is said to be "directly NIST traceable." For specialized applications, traceability can also be linked to consensus standards created by trade or industry groups, where no national or international reference standards are available. Examples include reformulated gasoline calibration standards for measurement of oxygenate and sulfur levels in gasoline.

The high cost of reference standards like SRMs makes it economical for gas suppliers to blend intermediate standards – so-called Gas Manufacturers Intermediate Standards (GMISs) – and analyze them against SRMs. The unbroken chain of traceability can still be maintained even if GMIS-type standards are used, however, the accuracy of this laboratory analysis will be lower. Every step that intervenes between the product analysis and the original reference standard results in more uncertainty for the gas mixture you buy. In addition, the resulting products will be traceable, but not directly traceable. If your application requires direct traceability like many environmental measurements, be sure your gas supplier has access to and uses the correct reference standard.

Blend tolerance is the difference between the certified concentration and the requested concentration for a given component. For example, if you order a mixture at a concentration of 100 ppm with a blend tolerance of 5%, then we must produce a product with a certified concentration between 95 and 105 ppm (i.e. [100-95] / 100 = 0.05 or 5% relative) to be in specification. Blend tolerance will vary by the component and concentration you order, and will also be dependent on the manufacturing method used to make the blend.

The reason for this is that gas mixtures become progressively more difficult to blend and analyze as the concentration decreases, and hence, specifications must be relaxed accordingly. Said another way, a higher concentration results in a tighter blend tolerance, higher analytical accuracy, and higher process accuracy (i.e. smaller values), while a lower concentration results in a looser blend tolerance, lower analytical accuracy and lower process accuracy (i.e. larger values). Other factors affect specifications as well, including characteristics of the minor component, the type of balance gas or liquid used, instrument limitations, reference standard accuracy, and the purity of the raw materials used to make the blend.

But in some applications, the presence of other trace impurities in the mixture can interfere with the proper functioning of analytical equipment and could result in poor quality analytical results. For example, one application may require a mixture of 500 ppm of carbon monoxide in nitrogen, with dual-certification, ±1% accuracy and direct NIST-traceability, but trace sulfurs must be below 1 ppm to avoid contamination or interference. In this case, the trace sulfurs constitute the critical impurities. In this instance we would check the quality of all raw materials for trace sulfurs, and then analyze the final blend to assure that your trace sulfur impurity specification was met. Trace level impurities can originate from the raw materials used, the balance gas/liquid used or from the blending process. Air Liquide's manufacturing processes track and keep trace impurities to a minimum, and offers the analysis of critical impurities as an option for some gas mixtures.

Components such as atmospherics, hydrocarbons, inert gases and even reactive components such as nitric oxide, hydrogen sulfide and 1,3-butadiene are available. Click here to see our standard gas mixtures. We can also make blends of components you specify that can include rare or unusual materials. In addition, we can manufacture mixtures using raw materials that you supply.

Our raw materials are carefully selected and tested for trace impurities prior to use. For our gravimetric products, we can go one step further. Raw materials are analyzed and certified not just for trace impurity concentrations, but also for the accuracy of raw material purity. The accuracy is a critical element of achieving our process accuracy specifications.

A reactive component is one that exhibits erratic behavior, most commonly observed as instability – a significant drop in the expected concentration when blended into mixtures. There are several ways that a component can earn a reactive designation. Some compounds react with other components of the mixture or with the balance material, and these reactions can be accelerated in a high-pressure cylinder. Other components are "sticky," that is, they attach themselves to the cylinder wall or delivery system and drop out of the mixture. Both of these conditions lead to unstable mixtures. Our proprietary blending techniques minimize or compensate for component reactivity.

For all laboratory analyzed products, reference standards are used that are equal to or more stringent than the required analytical accuracy specifications. For EPA protocol mixtures, only SRMs, NTRMs, PRMs and EPA GMISs are used as reference standards. For some selected gas mixtures, we use Air Liquide Reference Standards (ALRSs), or accurately prepared temporary standards as reference standards, all of which meet our stringent accuracy specifications. These are the best reference standards available in the industry, short of a NIST or VSL certified standard.

This number specifies how well the concentration of your current cylinder will agree with that of your replacement cylinder, when compared on your instrument. Reproducibility is not a value that is measured directly, but it is a necessary result of the analytical accuracy and/or process accuracy.We are able to guarantee reproducibility because our accuracy specifications measure how well we have determined the true value of your mix.

The benefit of the reproducibility specification becomes apparent in an application operating under statistical process control (SPC). Use of a replacement calibration standard, if significantly different in concentration from the current standard, could necessitate a change in SPC calibration and control parameters, causing a process upset. The reproducibility specification available exclusively can save you time and money by avoiding these potentially costly incidents.

A common limitation in preparing mixtures is that one or more components has a low vapor pressure at room temperature. Such mixtures can frequently be prepared by reducing the total pressure of the mixture to avoid possible condensation of the less volatile components. When this situation occurs, we alert you by printing actual mixture properties (i.e. dew point temperature, vapor pressure restrictions) on the label and Certificate of Accuracy. For applications where mixture pressure and dew point are critical, a full mixture phase envelope can be provided for some gas mixtures.

To make a mixture with a process accuracy claim, the blending process used must have tight manufacturing controls and must be calibrated with reference materials, such as NIST-certified weights, whose true value has been established. In addition, all the key contributors to process variability must be quantified and controlled, so that the accuracy can be calculated. A gravimetric scale, in many respects, is similar to an analytical instrument. The device is highly sensitive, can be calibrated, and can be used to measure both the concentration and the accuracy of a component. A properly controlled process, such as GRAVSTAT, can yield tight accuracy specifications, sometimes even tighter accuracy specifications than analytical accuracy, and is an example of a manufacturing technique for which a process accuracy can be calculated.

In order to have process traceability, the blending method used for your mixture must also be directly calibrated with reference material from a recognized metrology organization such as NIST, VSL or NPL. For example, if a gravimetric blending scale is calibrated with NIST Class 1 weights, or the older Class S weights, the resulting product is said to be NIST-traceable by weight. Process traceability is extremely important for product classes, because traceability for this single-certified product can only be achieved through weight-traceability. 

Our shelf life claims are based on formal stability studies, and on more than 50 years of history and experience blending specialty gas mixtures. Some mixtures also have an expiration date that is usually set by regulations (i.e. EPA protocols). We employ ACULIFE™ and other proprietary cylinder treatment processes to guarantee mixture shelf life and stability.

For all laboratory analyzed products, Air Liquide uses Scott brand developed analytical procedures to achieve concentration and accuracy specifications. But for some applications, customers require certification of their mixture by a specific analytical procedure. For instance, refinery customers measuring sulfur emissions often require analysis of H2S by EPA Method 11, the EPA's approved reference method. Environmental mixtures are certified using standard EPA protocol procedures required to produce a compliant product. For other mixtures, Air Liquide also offers further analysis using industry standard methods as specified or requested by the customer. These additional analyses add value by making your mixture perfectly matched to your application.

Laboratories, for example, must establish traceability of measurements to earn ISO/IEC 17025 accreditation. Traceability is the pathway or linkage back to the true value of a measurement. For traceability to exist, there must be an unbroken chain of comparisons between the measured value and an accepted reference that is recognized in the national or international measurement system. In specialty gases, there are two types of traceability, analytical traceability and process traceability. Analytical traceability can be achieved through use of gas reference material, such as a NIST SRM, while process traceability can be achieved by measurement of a fundamental physical parameter, such as mass, temperature or pressure.