What curve types are permitted by the EPA to generate calibration curves?

Ask the Expert Question:
We run several instruments like IC, ICP, GCMS, etc, for water analysis. What curve types are permitted (by EPA) to generate calibration curves. For example, EPA 300.1 ask for linear least squares fit. Can weighted models be use along with this?

Experts Response:
Tthe EPA method 300.1 does not allow any other calibration method other than linear regression. However, the EPA has been known to grant Alternative Test Protocol (ATP) status to use quadratic fit calibration models, but that is only granted on a case by case basis. You would need to apply for an ATP approval from EPA in order to use anything other than linear regression for method 300.1.

ICP, GCMS allow for linear regression models, as well as other calibration models. For GCMS it is most common to use Relative Percent Difference (RPD), weighted linear regression, and quadratic curves. I suggest that you read SW846 method 8000c for an explanation of the various calibration models acceptable for organic analysis.

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When can you exclude calibration points?

Ask the Expert Question:
When is it permissible to exclude calibration points?

Expert Response:
Removal or replacement of calibration points (point(s)) from the middle of a calibration curve (i.e., points other than the highest or lowest) is not permitted unless an injection or instrument problem confined to that point was directly observed and can be clearly documented. In this case, the failed standard must be re-run within 24 hours and before any samples and inserted into the initial calibration. If that new point is not useful, recalibration is required.

Removal of points for individual analytes from levels other than the highest and lowest is not permitted in any event.