• Stephen Biss

Traceability of Measurement Result on an Approved Instrument


To obtain an admission from the CFS scientist that the traceability of a measurement result on an approved instrument or a truck weigh scale is through the calibration of the instrument at the factory or a proper recent re-calibration by the manufacturer or factory authorized service centre. At the time of calibration a calibration curve is created to establish a relationship between the electrical signal coming off the detector in the AI or the weigh scale and the digital indication. That isn't done using controls at time of use.

To clarify that calibration of a gas chromatogaphy instrument used for blood, urine, serum analysis is done quite differently at time of use. A calibration curve on a gas chromatograph is created electronically at time of use.

Traceability of a measurement result on an AI is therefore through the manufacturer's Certificate of Calibration or re-calibration certificate. The Certificate, reference standards, and methodology at the time of calibration or re-calibration are therefore relevant (and essential) to reliability of the measurement result.

Q. Right? And – and again, what’s measurement traceability? Can you help us with that please? A. So, in that case, the analysis that’s performed and the accuracy of that analysis is related to a specific parameter. So, you have an alcohol standard solution that is certified by the manufacturer to have this concentration in it, and that is something that you can then use to relate to the concentration of the unknown in a particular sample. So, you produce a calibration curve that we described earlier, right, well, varying – increasing concentration and those standards are certified to contain a certain amount of alcohol that can then produce a response, in relation to the concentration and then you compare your unknown to that to get the actual concentration. Q. But measurement traceability, the traceability is between the measuring device and the

standards, the reference standards that were used in the calibration of that device, right? A. That's correct. So, we have standards that are certified. We also have pipets that are calibrated and checked on a very regular basis to make sure that they haven’t drifted at all. Q. Well, I understand that from the perspective of controls, but I just want to go back to the measurement itself. The measurement itself is done by an instrument, whether it’s a gas chromatograph or whether it’s an Intoxilyzer. But the concept of measurement traceability relates to the traceability of the measurement, not the traceability of the control. A. Well, the measurement is traced back to the controls that are certified, and they’re traceable to a standard that is – that they’ve been certified to be of a certain concentration. So, you can rely on the accuracy of the results.

[expert is talking about a gas chromatograph not an approved instrument] Q. Well, I’m suggesting that - you’re talking about controls. I’m talking about the calibration of the instrument itself. So.... A. Well, the instrument itself isn’t calibrated. The instrument just simply produced a response in – in response to whatever’s put into it. Q. You’re talking about on a gas chromatograph. A. Yes. Q. And that’s – and perhaps I misunderstand. A. Okay. Q. But you actually create a calibration curve at the point in time when you are setting up a gas chromatograph to use it?

A. Yes. Q. All right. Do you do that manually? Electronically? A. Well, what you do is, you run – you set up a run sequence... Q. Yes. A. ...that has a control to check whether or not the instrument is sensitive enough, you run a blank, or a negative control, then you have a series of calibration curve standards and then another blank and then you have your controls, either a low, medium or a high control, followed by your samples, and then bracketed by another control. Q. All right. So, I guess there are two different models. If I use the – the – go back to the truck and the paperclips, and the measurement reference. If the paperclips are the response of the instrument, the electrical response of the instrument, the mass of the truck – we can’t measure the mass of the truck until a calibration curve has been created and that’s – the calibration curve relates the paperclips, which are the electrical signal to the measurement standards. Have you got my analogy? A. Well, somebody has to certify the scale, that, you know, when you put a thousand pound weight on it, that that equals a thousand pounds.

Q. So, that takes.... A. That’s what shows up on the scale. Q. So, that takes place during calibration of the instrument. And so, if we compare that to the model – that model is different from the model that you’re using in the laboratory with the gas chromatograph, where you are actually calibrating the electrical signal to the measurement standard each time you’re using it.

A. Correct. Q. Whereas with the weigh scales, the truck and the paperclips and the measurement standard, or if we’re talking about an Intoxilyzer 8000C the connection between the paperclips and – which are the electrical signal, and I’m saying that because the idea of what an electrical signal is, is kind of hard to imagine, but we – I think we can think of a number of paperclips corresponding with an actual reference standard, like, say a kilogram, we could calculate the number of paperclips that relate to a kilogram, or the number of paperclips that relate to a gram, but it’s that relationship between the electrical signal and the reference standard, that’s something done at the time of calibration, and the case of an Intoxilyzer 8000C, it’s probably back - at least in this case, it’s back when the instrument was manufactured in relationship to the truck on the weigh scale, it could be something a whole lot more recent. Have I got the analogy right? A. Yes.

Q. I’ve separated out the concepts between what you’re doing in the laboratory with your reference standards and creating a calibration curve in the laboratory with respect to the setting up of the calibration curve on an Intoxilyzer 8000C or on a weigh scale at some previous point in time. A. Yes.

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