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Creating Accurate Low Level TO15 Standards with the Entech 4700 Precision Diluter
The analysis of Air Toxics TO-15 compounds in air by GCMS requires the accurate preparation of low level standards to fulfill 3 distinct requirements:
1 Create GCMS response factors for each target compound across the full range of expected concentrations
2 Validate the canister preconcentration system for consistent preconcentration and matrix management without loss of target compounds during analysis by GCMS
3 Provide inexpensive preparation of low level standard to allow recovery testing of all sampling canisters in a laboratory’s inventory for inertness validation
The ability to meet item #1 could simply be achieved by performing a loop injection of high concentration standards, but this would not meet the requirements of #2 and #3, which require the standards to be diluted down to ambient levels, often times below 1 PPBv. As described in more detail later, it is best to use at least 2 standards when creating a GCMS calibration curve, as this typically allows method calibrations over a wider concentration range, often times up to 1000x or more.
Large concentration ranges can reduce the number of dilutions and reruns that are necessary by a laboratory, thereby increasing productivity. For example, generating a calibration curve from 0.04PPBv to 40 PPBv would allow all samples within this range to be quantified without further dilution, whereas reducing the range to just 0.04PPBv to 4 PPBv would require any samples with target compounds in the range of 4 to 40 PPB to be rerun using smaller sample volumes, or after diluting a representative aliquot into a second canister.
Items #2 and #3 in the previous list are extremely important when performing accurate TO15 analyses, yet are often not given the consideration they deserve.
Creating standards that are as similar to the concentration and matrix of real samples is critical to prove that target compounds will be recovered properly during the preconcentration and matrix elimination process. Although calibration standards are stable in Silonite coated canisters without the presence of water, it is recommended that canisters be made up with between 50-70% relative humidity just so the sample preparation system can be challenged just as it will be with real samples. A discussion follows that describes how moisture levels in this range can be used to challenge the analytical system with extremely large or small amounts of total water just by varying the preconcentration volume.
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