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  • Stephen Biss

Instrumental Deviations from Beer-Lambert Law


Purpose:

To explore the reasons why measurement instruments in the field may deviate from the linearity alleged by the Beer-Lambert law

To introduce a university textbook dealing with inter alia:

External Calibration

Creating a Calibration Curve

Limits to Beer's law

No longer linear when the molar absorptivities differ

Instrumental Deviations from Beer-Lambert law

Polychromatic Radiation rather than Monochromatic IR

Sample cross-examination of CFS expert on university textbook dealing with deviations from linearity - deviations from Beer-Lambert Law. CFS takes the position, without empirical evidence, that such deviations will never be gross deviations:

MR. BISS: Q. Mr. Palmentier, this is a...

A. Skoog an West.

Q. ...a book that I got from the University of

Toronto, Fundamentals Of Analytical Chemistry. And I just

want to ask you, please, to take a look at page 171, where in

the textbook they talk about external standard calibration.

A. Okay.

Q. I just want to ask you, on the first paragraph

there under 8D-2, is that the kind of calibration that we use

when we are calibrating -- or when the factory is calibrating

an Intoxilyzer 8000C?

A. Yes. That first paragraph, yes. That's part

of the calibration of the instrument done by an authorized

person or at the factory.

Q. The second paragraph talks about what the

calibration function is that's created and talks about it as

sometimes being called the calibration curve, sometimes being

called a working curve?

A. All right. So what's your question?

Q. My question is, it says:

"It is often desirable that the calibration curve

be linear in at least the range of the analyte

concentrations."

A. Yes. That's something that we use in our

analytical laboratory all the time. But sometimes, depending

on the response, again, because you're looking at

concentration of drugs from, say, therapeutic to a fatal

level, the curve often is not linear at all.

Q. All right. Going further down that paragraph,

then, that relationship, called the linear relationship:

"...is then used to predict the concentration of

an unknown analyte solution."

That's what we do with an Intoxilyzer 8000C, but

it's done electronically, rather than manual.

A. Yes.

Q. And then it says down at the next paragraph

after that:

"Computerizing numerical data analysis has largely

replaced graphical calibration methods..."

[As read]

And that's exactly what we have in an Intoxilyzer

8000C?

A. Well, yes, yeah. We don't do anything by

paper and rulers anymore. Everything is done electronically.

Q. Right. So just move on, please, to page 669

under paragraph 24C-3, Limits (indiscernible) as well, page

669?

A. Sorry, you said 6...

Q. Six six nine. Page 669. Just keep going and

you'll find it?

A. Oh, okay, sorry, because the page numbers...

Q. Yeah. I did not reproduce the whole textbook.

THE COURT: Thank you goodness for that.

MR. BISS: It's rather expensive when you're

(indiscernible). Page 669?

A. Yes, Lambert-Beer Law.

Q. Limits to Lambert -- limits to Beer's law. Is

Beer's law the same thing as Lambert-Beer Law?

A. Yes, it depends on which scientist you talk to

about which name should go first and who discovered it first,

yes.

Q. All right. So the first sentence says:

"There are few exceptions to the linear

relationship between absorbance and path length at

a fixed concentration."

Then it says:

"We frequently observe deviations from the direct

proportionality between absorbance and

concentration."

However, even though the path length... In other

words, the length of the sample chamber is constant. Do you

agree with that comment?

A. Yes.

Q. Right.

A. And it's fixed, yes, so we know.

Q. Right. And they talk about some of those

deviations are real deviations. Then it says that others are

as a result of the method that we used to measure absorbance

instrumental deviations. Right?

A. That's what it says, yes.

Q. Right. Talking about instrumental deviations,

if we could move ahead, please, to page 671.

A. Yes.

Q. Under Instrumental Deviations, Polychromatic

Radiation. It says:

"Beer's law strictly applies only when

measurements are made with monochromatic source

radiation. In practice, polychromatic sources

that have a continuous distribution of wavelengths

are used in conjunction with a grating or a filter

to isolate a nearly symmetric band of wavelengths

surrounding the wavelength to be employed.

[As read]

So my suggestion to you is that Intoxilyzer 8000Cs

use a filter to isolate a nearly symmetric band of

wavelengths surrounding the wavelength to be employed.

A. Sorry, one more time?

Q. Intoxilyzer 8000Cs use a filter...

A. Yes.

Q. ...an optical filter...

A. Yes.

Q. ...to isolate a band of wavelengths

surrounding 9.4 microns.

A. Or 3.4 micrometres, yes.

Q. All right. On page... it's covered up by the

staple, but 672.

A. Yes.

Q. Halfway down the page it says, "As shown in

figure 24..."

A. Sorry. Sorry. Sorry. As... 672?

Q. Page 672.

A. Okay. Sorry. There. "In addition"?

Q. Yes.

A. It's the third line down.

Q. All right. On page 672, the reference to

figure 24-16. Do you see where in the text it refers to

figure 24-16?

A. At the top, yes.

Q. "As shown in figure 24-16, however, the

relationship between Am and concentration is no

longer linear when the molar absorptivities

differ. In addition, as the difference between...

the two E's ...increases, the deviation from

linearity increases."

Do you understand that?

A. So this is referring to... where are E prime

and E double prime referred to.

Q. (Indiscernible).

A. Equation.

Q. Right back on page 671. Yeah, I recognize

that some of the formulas are somewhat complicated, because

they use logarithmic relationship. But perhaps I can

simplify my question...

A. Okay.

Q. ...to this. In the theory that is being

described in this textbook, they're suggesting that even

though we hope to have a linear relationship as best we can,

because that's how or nearly linear relationship, because

that's how we can use infrared spectroscopy or some other

kind of spectroscopy for purposes of reliable measurement,

the problem is that if the molar absorptivities, in other

words, what is described in the training aid as a constant,

if those numbers differ, then the problem is that the

deviation from linearity increases.

A. Wow. I think that as it's used here in our

manual is an oversimplification of the actual process that's

taking place, compared to what's written here. Again, I

think you're getting into the area where the engineers would

have figured all of this out at the time of design of the

instrument and the manufacture of the instrument to sort this

out. This is way beyond me with respect to figuring this

out. This is my superficial understanding of the

Lambert-Beer Law with respect to how solutions react or how

compounds react relative to concentration, but this takes it

to a whole new level. And, again, this would have been

established by the engineers and all this kind of

mathematics. They would have people who specialize in this

kind of thing. This is way beyond my area of expertise for

being able to establish this and what kind of curves and how

this is used to establish the concentration of an unknown.

Q. All right. Let's look at the next paragraph

then.

A. Which page are we on now?

Q. We're on 67...

A. Three?

Q. ...2.

A. Oh, okay.

Q. "If the band of wavelength selected for

spectrophotometric measurements corresponds to a

region of the absorption spectrum in which the

molder absorptivity of the analyte is essentially

constant, departures from Beer's law will be

minimal."

In other words, if we look at the infrared

signature for ethyl alcohol, and we pick the region at 9.4

microns, we've got a question of is... is the shape of that

fingerprint, is the shape of that spectrum relatively

constant at that point or if what we're dealing with are

wavelengths that are including places other than the peak of

the value, the valley or the peak that's being used to

analyze, then the problem is that there will be departure

from Beer's law.

A. That sounds logical. If I may point out that

the absorption spectrum shown on page 40 of 238 of the Centre

of Forensic Sciences' training aid would have been obtained

using a spectrophotometer, which would then, as -- what it

does is it will generate infrared light wavelength by

wavelength. So this would have been obtained by scanning all

the wavelengths of infrared light one at a time in order to

get that. That would be completely different physics from

how the instrument actually calculates or how it sees the

absorption of infrared light.

Q. Right. Because the instrument is just looking

at a broad number of wavelengths, not a specific point of

wavelength. Not a specific wavelength, but a collection of

wavelengths, and doing its calculation based on that?

A. Correct. Yes.

Q. So the bottom of that paragraph, the last

sentence there it says:

"On the other hand, some absorption bands in the

UV-visible region and many in the IR region are

very narrow, and departures from Beer's law are

common, as illustrated for Band B in figure

24-17."

So, for example, if you look at the top of page

673, you can see that if what's being analyzed using the

filter is a very, very narrow region at the tip of a peak or

a bottom of a valley in the infrared spectrum, you get a

very, very different kind of effect than if the filter is

analyzing something on the slide slope of the valley or the

peek?

A. Correct.

Q. Right. So I want to suggest to you that

that's the theory behind why linearity is such an essential

concern with respect to any kind of instrument that uses

spectroscopy.

A. Could you say that one more time?

Q. I want to suggest to you that that is the --

that is part of, at least part of the theory of the

chemical -- of the analytical chemistry theory behind the

reason that we need to be checking linearity on instruments

on a regular basis.

A. Again, I'm going to go back to that safeguards

in place at the time of testing determine whether or not

there's been any significant changes in the instrument that

would require that it be taken out of service and out of use.

Q. And the --

A. That's the only way I can answer that

question.

Q. All right.

A. Again, you're getting into -- you've obviously

done a lot of research here and spent a lot of time on this.

But, again, a lot of this is beyond my scope of being able to

answer some of these questions.

Commentary: CFS/ATC experts need to be cross-examined as to the "safeguards" allegedly in place at time of testing. In the 8000C, one of these safeguards is the internal standards or internal test procedure sustem (the ITPs). Quaere: How often are the ITPs re-calibrated and what external standards are used? Does an 8000C cease to be reliable if the ITP system needs re-calibration? Do the ITPs run across the measuring interval or are they only applicable at 100 mg/100mls. Lawyers need to be familiar with the inadequacies of such internal standards systems.

Here is an excerpt from the 8000C Training Aid ITP page:

#BeerLambert #linearity #crossex

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