Gas Mixtures

Analytical Accuracy

Analytical accuracy is the difference between the analyzed concentration and the true concentration of a component. To make an analytical accuracy claim, the 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.

Concentration Ranges

You will notice that the specifications for blend tolerance, analytical accuracy and process accuracy are dependent on concentration. 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.