APPENDIX I
Carbon Dioxide in Nitrogen
Introduction
This appendix describes the preparation of Certified Reference Materials (CRM's) consisting of carbon dioxide in nitrogen. The reader should be familiar with the general description of the preparation and analysis of CRM's given in the main text of this document and the material in Appendix G which provides procedures for extending the certification of CRM's and for dateof sale certification of CRM's.
Current SRM's
Following is a list of SRM's of carbon dioxide in nitrogen for which CRM's may be made. The concentrations shown are in mole percent. The actual concentration of a particular SRM may differ by more than several percent relative from the nominal value and it is therefore essential that the exact concentration of the SRM to be duplicated be known before attempting the preparation of a CRM. The eight SRM's covering 2619a through 2626a listed below were originally developed for use in calibrating combustion efficiency measurements; the remaining originally developed for use in calibrating mobile source emission measurements.
| SRM Number | Nominal Concentration of CO2 |
|---|---|
| 1674b | 7.0 mole percent |
| 1675b | 14.0 mole percent |
| 2619a | 0.5 mole percent |
| 2620a | 1.0 mole percent |
| 2621a | 1.5 mole percent |
| 2622a | 2.0 mole percent |
| 2623a | 2.5 mole percent |
| 2624a | 3.0 mole percent |
| 2625a | 3.5 mole percent |
| 2626a | 4.0 mole percent |
| 2632 | 300 ppm |
| 2633 | 400 ppm |
| 2634 | 800 ppm |
The preparer of carbon dioxide in nitrogen CRM's should note that the CO2 concentrations in the eight combustion efficiency SRM's (2619a through 2626a) are certified to within 0.1 percent relative at the 95 percent confidence level; the CO2 concentrations in the remaining five SRM's are certified to within ±1 percent relative at the 95 percent confidence level.
Cylinders
The CRM's must be packaged in new aluminum cylinders equipped with valves of appropriate material that conform to CGA recommendations for these particular mixtures; alternatively, aluminum cylinders previously used for carbon dioxide in nitrogen CRM's or that have been used only for carbon dioxide in nitrogen mixtures may be used for these CRM's, provided that the history of a reused cylinder is well known, and, in case of a CRM cylinder, an analysis must be performed of the remaining gases to assure that the contents have not been diluted or contaminated. Note: The concentration of the remaining gas mixture must lie within plus or minus one percent relative of the concentration of the original CRM, as certified. In the case of reused cylinders, the cylinder must be evacuated before refilling, and the concentration of the new CRM gas mixture must lie within one order of magnitude (within 0.1 to 10 times) of the concentration of the previous CRM or other carbon dioxide in nitrogen mixtures previously contained.
Preparation and Analysis
Prepare a lot of at least 10 cylinders all at identical concentration. The concentration of carbon dioxide must lie within ±1 percent relative of the concentration of the specific SRM with which the lot is to be compared. Analyze at least 10 samples from the lot, if the lot contains more than 20 samples. If the lot contains fewer than 20 samples, it is recommended that all samples be analyzed. Analyze the samples by direct comparison to the SRM or by comparing samples in the lot to a single sample selected at random from the lot (internal standard from the lot). Analyze each sample at least two times. Analyze for impurities as required in Section F.
After the first analysis, the lot should be put aside to "incubate" for at least one month after which time all samples in the lot are analyzed and at which time the impurities are redetermined. If the two sets of analyses are the same, then it may be assumed that the lot is homogeneous and stable. If the analysis was performed by direct comparison to the SRM, then the analytical value for the sample will be the certified value. If the analyses were performed by comparison to an internal standard for the lot, then it will be necessary to determine the concentration of the internal standard. This is done by repeated comparison of the internal standard to the SRM. The concentration of the internal standard must be determined at the time of the first and second analysis by comparing it to the appropriate SRM's. The measured concentration of the internal standard should be the same for both analyses within the limits of error of the analyses. The concentration measured for the internal standard is multiplied by measured ratio of each sample to the internal standard to obtain the concentration of each sample.
The uncertainty of the CRM includes the error of the SRM and the uncertainty due to the imprecision of the intercomparison. The error of the SRM, ESRM, is given on the certificate for the particular cylinder and is defined as the estimated upper limit of the total uncertainty. The estimated upper limit of the uncertainty for the CRM should be calculated as follows:
E2 (Total Error) = 2 SQRT({ESRM/2}2 + a2 + b2 + c2)
where E2 is the estimated upper limit of the uncertainty of the CRM; a is the imprecision of the intercomparison of the internal standard with the lot; b is the imprecision of the intercomparison of the internal standard with the SRM; and c is the error of the calibration.
Analytical Method
Analysis of these CRM carbon dioxide mixtures may be done by gas chromatography, paramagnetic analyzers, or any other methods specified for carbon dioxide determination. Analysis of SRM's 2632-2634 also employed a nondispersive infrared technique. The method chosen should have adequate sensitivity and precision, and the nature of the instrument response must be known for the region between the SRM and the CRM. The precision and sensitivity are evaluated during analysis and are reflected directly in the "imprecision of intercomparison." The instrument response characteristic is evaluated over the small range between the SRM and CRM by observing the response over a wide range using SRM's as calibration standards.
Impurities
| Carbon monoxide | 0.1 ppm |
| Nitrous oxide | 0.5 ppm |
| Hydrocarbons as methane | 1 ppm |
| Water vapor | 5 ppm |
| Total other impurities | 10 ppm |
Period of Certification
Experience at NIST indicates that carbon dioxide in nitrogen mixtures contained in aluminum cylinders are stable for periods of at least two years. If no change in concentration of the CRM's is indicated by the results of the producer's first and second analysis, then stability for a period of two years from the date of the second analysis is assured with a high degree of probability. Any samples not sold within two years of the second analysis should be recertified (see section h., below) and the period of certification should be appropriately extended.
Recertification (see Appendix G for further details)
CRM's of carbon dioxide in nitrogen that have exceeded the period of certification may be recertified by the original supplier according to the following provisions.