Regulated laboratories, Rules concerning calibration curves

http://dnr.wi.gov/regulations/labcert/documents/guidance/-lodguide.pdf
Non-linear, or quadratic curves require a minimum of five calibration standards to fully
characterize the curve. For most inorganic analyses, the blank should be included as a point on the calibration curve. It is not acceptable to force any calibration curve through zero.

http://dnr.wi.gov/regulations/labCert/documents/guidance/08Calibration.pdf
Chapter NR 149 allows the use of any calibration model that can be chosen to be
indicative of a detectors or analyte response, as long as the choice is not used to
compensate for saturation of signal, lack of sensitivity or malfunction. The Chapter
minimizes the risks of using higher order models inappropriately by requiring more
stringent calibration verification for non-linear models. 

When options to use more complex calibration functions are available, a
laboratory must choose a linear function, unless it can demonstrate that a nonlinear
function defines the calibration range better. A laboratory may use
weighted algorithms or reduction techniques. However, using non-linear functions
or weighted algorithms to compensate for instrument saturation, insensitivity, or
malfunction is not allowed.