Linearity Studies are Done on New Instruments not Aging Instruments
Updated: Oct 21, 2022
Purpose:
To obtain admissions that the published studies dealing with the linearity of approved instruments assume a new instrument recently calibrated by the manufacturer.
Sample cross-examination of CFS expert on the inadequacy of studies relied upon by government scientists to maintain that approved instruments have good linearity - the studies were all done on new instruments, recently calibrated at the factory, not on aging instruments out in the field:
Q. Well, every study that we see having to do
with a new instrument, for example, Ms. Martin's, Terry
Martin's study, the published paper relating to the
Intoxilyzer 8000C...
A. Yes.
Q. ...she did linearity checks.
A. She did.
Q. She looked at the math and she did an actual
check to examine the linearity of the instrument?
A. Correct
Q. But my --
A. That's the purpose of an evaluation. An
evaluation is from the operational requirements of the
instrument.
Q. But the evaluation is always with respect to a
new instrument, an instrument that's been recently
calibrated, that's been sent by the manufacturer out for
evaluation.
A. Yes.
Q. The problem is, when we've got instruments out
in the field, if no one is checking linearity, and on a
regular basis, if no one is sending the instrument out with
basic regular either inspection intervals or regular
calibration intervals, it is not at all safe to assume that
the linearity of the response is going to be maintained.
A. Some of that is addressed at the time of the
preventative maintenance inspection done either on an annual
or biannual basis.
Q. And so that's why it's so important that some
of that being addressed is being done properly. And so if
the individual doing the annual maintenance is not properly
responding and documenting a problem when they see, say, the
numbers when they do an inspection at 50 milligrams per 100
mills or 40 milligrams per 100 mills being way out of spec,
and then they do nothing about it, that is problematic, I
would suggest to you, and that's why it is that the CFS
policy that every test stands on its own doesn't work out in
the field.
A. That's your opinion, yes.
MR. BISS: Your Honour, should we mark those
documents as exhibits?
THE COURT: Yes.
See also:
"Beer's Law and Spectrophotometer Linearity" by C.G. Cannon and I.S.C. Butterworth, 1953, Vol. 25, No. 1, Analytical Chemistry
"Does the Intoxilyzer 4011AS-A conform to the Beer-Lambert law?" by J Mack Cowan, Journal of the Forensic Science Society 1988: 28: 179-184