XpressCryogenics - Specialty Gas Mixtures

Xpress Cryogenics offers a wide variety of custom gas and liquid mixtures to suit the most stringent process control or analytical requirements. We supply three grades, covering most specification requirements. The Mixture Specifications Table shown below outlines each of the three grades - Primary Standard, Certified Standard, and Unanalyzed Mixture - and their tolerances.

If your requirements are different from those listed, simply call us or send an email to one of our Gas Specialists.

Primary Standards should be used when the application demands the highest mixture accuracy and reliability. Primary Standards are filled gravimetrically on a high-load, high-sensitivity scale, calibrated with NIST Class S weights. Gravimetric blending offers the closest tolerance available, sometimes better than some methods of analysis. A dual verification of mixture accuracy is also performed by routine quality control analysis.

Certified Standards, sometimes referred to as working standards, are analyzed calibration mixtures used routinely in science and industry. For the majority of applications, the tolerance of a Certified Standard is acceptable. These standards are generally prepared either by partial pressure or gravimetrically. Certification of the standard is usually done through quality control analysis.

Although prepared by the same techniques as Primary and Certified Standards, Unanalyzed Mixtures are not verified or checked by analysis. These mixtures should only be used in applications where the accuracy is not an issue.

Xpress Cryogenics conforms to two tolerances associated with all Primary and Certified Standards. First is the blend or preparation tolerance. This is the minimum acceptable uncertainty associated with the actual production of the blend. These uncertainties are accumulated during the manufacturing process because of the inherent inaccuracies of equipment used in production, such as pressure gauges, and the inherent inaccuracies due to the physical properties of the gases. Second is the analytical or certification tolerance, which is the minimum acceptable uncertainty associated with the analysis of the blend. This uncertainty is accumulated throughout the analytical procedure and includes instrument and calibration errors.

For most applications, the certification tolerance is of greater importance than the preparation tolerance because it represents the range in which the true or actual concentration may be in relation to the analyzed concentration. For some applications, such as those that require an upper or lower range of concentration that cannot be exceeded, the preparation tolerance becomes equally if not more important. A graphical representation of the interrelationship of the preparation and certification tolerances is shown in Figure 1.

Every gas mixture has a characteristic dew point curve, which is a series of temperature/ pressure relationships at which the gas will begin to condense to a liquid. Since components in the mixture may tend to condense at different rates, the result is a change in composition of the gas phase. Condensation is usually a problem for blends containing components that are liquefied compressed gases, for which the dew point temperature is close to the ambient temperature. To eliminate this effect, the pressure of the mixture must be kept below the dew point pressure.

For liquid mixtures containing components of widely varying vapor pressures, fractionation may be a problem. Fractionation may occur as liquid is being withdrawn from the cylinder. As the vapor space within the container increases, components with the higher vapor pressures will fill the vapor space to a greater degree, changing the composition of the liquid.

Due to the chemical and physical properties of gases, some mixture components when blended may react with other components in the blend or absorb onto the cylinder walls. Sometimes these reactions take place only under pressure or may be a function of the cylinder type and condition. Such reactions may result in the appearance of an unknown component or an unexplained decrease in concentration.

Blends containing an oxidant and a flammable can be prepared, but only under very stringent conditions. As a general rule, we will not prepare a mixture in which the fuel and oxidant concentrations exceed both the lower explosive limit of the flammable and the minimum oxidant required for combustion. Nor will we produce a mixture in which these two parameters are exceeded at high pressure during filling, although there are cases where reduced pressure blends are available. There are many criteria used to determine the safety of a blend containing an oxidant and a flammable.

Our matrix or base gases, such as nitrogen, argon, air, and hydrogen, will meet or exceed our UHP/Zero purity specifications. Other component gases are typically ultra-high, high-purity, and chemically pure reagents. Higher purities are available upon request, subject to availability. A detailed blend profile can be established specifically to our customers’ requirements. A Certificate of Analysis of the base gas (Product Source Analysis) can also be furnished at an additional charge.

Xpress Cryogenics, upon request, will provide a precise analysis of specified impurities at an additional charge for each component analyzed. The results will be recorded on the Certificate of Analysis, as well as the requested component concentrations.

Traceability is the connection between your blend and an accepted reference standard. All Primary and Certified Standards are traceable to standards produced by the National Institute of Standards and Technology (NIST). For instance, your blend may be analyzed on instrumentation that had been calibrated using gas mixtures produced by the NIST, called Standard Reference Materials (SRM). This is considered a direct form of traceability. Another direct form of traceability can be achieved by using NIST Traceable Reference Materials (NTRM), which are gas mixtures produced by a commercial gas supplier and verified by the NIST. Due to the cost and availability of SRMs, gas manufacturers may analyze your blend against a secondary blend, called a Gas Manufacturers Intermediate Standard, which had been analyzed against an SRM or NTRM. Although your blend is still traceable, this method is not a direct form of traceability. Components for which no SRM exists can still be traceable, although instead of establishing traceability through the analysis, traceability can be established through the production method. Blends produced gravimetrically, using scales calibrated by NIST Class S weights, are still considered traceable.

Grade	Concentration Range	Preparation Tolerance	Certification Tolerance
Primary Standards**	20 ppm to 4,999 ppm 5,000 ppm to 1.9% 2% to 49%	±5% of component ±2% of component ±1% of component	±1% of component ±1% of component ±0.02% absolute
Certified Standards**	1 ppm to 99 ppm 100 ppm to 0.99% 1% to 49%	±20% of component ±10% of component ±5% of component	±5% of component ±2% of component ±1% of component
Unanalyzed Mixtures	0.1% to 0.99% 1% to 50%	±15% of component ±10% of component	Not Applicable Not Applicable
These tolerances may vary from those stated in the table under the following situations: Mixtures containing low molecular weight components such as hydrogen and helium, depending on their concentration. Mixtures containing components that present adsorption, stability, or other blending problems. Multicomponent blends, depending on the type, number, and concentration of the components. *Depending on the component concentrations, it is sometimes possible to produce gas mixtures with tolerances that surpass those indicated.

If you require accuracies which exceed our stated tolerances, one of our Gas Specialists can assist you.