[christina]

What would cause the peaks in a IR analysis to be smaller than they should be? My sample had the right peaks where they should be and they were the right shape but were shorter then they should be.

Something different is what the y axis says on the IR analysis that they gave us so we would know how the IR was supposed to look for our samples. But the y axis on the one they gave us said "percent transmittance" while the IR graph that I had after the analysis had "percent reflectance". Not sure if it's the same.

If they are though then an example of the difference in peak size is for my IR:

my sample of 4-bromo-2-chloroacetaniilide had
1 middle sized peak at: 3275.24 cm-1
and the intensity was : 69.608 % reflectance

The given IR card that was given for the pure sample showed the peak going all the way down to about 30% percent transmitance ( I don't have the exact value)

Thanks 

[agrobert]

Hopefully you realized 70% R (Absorption) and 30% (Transmittance) are related somehow.

http://en.wikipedia.org/wiki/Transmittance

Do you understand now?

[ARGOS++]

Dear ~Christina~;

One of the most common reasons why IR Spectra from the identical Compound look different is that both spectra are not taken with the same “solvent”. For Example: one in KBr and the other in Nujol, CCl4, etc.
Some Effects can also be caused if you “directly compare” Reflectance with Transmittance.

In IR you are also not able to compare Absorbance-Strengths, that’s why the peaks are normally labelled with (s) for strong, (m) for medium, (w) for weak, and maybe some other.

But what would be important, if the case, is if you really measured in Reflectance:
Reflectance (R) is, a little simple spoken, when the light is coming out on the same side of the sample as it entered its surface, whereas Transmittance (T) is, when the light passes through the sample and is measured on its backside.

To measure Reflectance a so called “Ulbricht-Sphere” is required and that’s quite uncommon for normal IR. Where possibly Reflectance is more common, it’s for IR-Spectroscopy (IRS), but you didn’t say anything in this direction. I never did IRS myself, but I easy believe that such Spectra look also different.
But could it be that only the wrong Paper was used for the Instrument?

A thumb-rule for converting Reflectance to Absorption is  A = ≈1/R.  But that’s quite far from exact!
A little more precise would be the “Kubelka-Munk”-Transformation.
I think you know how you have to convert from Transmittance to Absorption, but that’s maybe not necessary because (s), (m), and (w).

I hope it may be of help to you.

Good Luck!
                    ARGOS⁺⁺

[christina]

Dear ~Christina~;

One of the most common reasons why IR Spectra from the identical Compound look different is that both spectra are not taken with the same “solvent”. For Example: one in KBr and the other in Nujol, CCl4, etc.
Some Effects can also be caused if you “directly compare” Reflectance with Transmittance.

I'm sorry but I didn't mention that no solvent was used for the IR.

In IR you are also not able to compare Absorbance-Strengths, that’s why the peaks are normally labelled with (s) for strong, (m) for medium, (w) for weak, and maybe some other.

But what would be important, if the case, is if you really measured in Reflectance:
Reflectance (R) is, a little simple spoken, when the light is coming out on the same side of the sample as it entered its surface, whereas Transmittance (T) is, when the light passes through the sample and is measured on its backside.

To measure Reflectance a so called “Ulbricht-Sphere” is required and that’s quite uncommon for normal IR. Where possibly Reflectance is more common, it’s for IR-Spectroscopy (IRS), but you didn’t say anything in this direction. I never did IRS myself, but I easy believe that such Spectra look also different.
But could it be that only the wrong Paper was used for the Instrument?

The IR done was ATR but I don't know much about it. (the lab tech did the actual analysis)

A thumb-rule for converting Reflectance to Absorption is  A = ≈1/R.  But that’s quite far from exact!
A little more precise would be the “Kubelka-Munk”-Transformation.
I think you know how you have to convert from Transmittance to Absorption, but that’s maybe not necessary because (s), (m), and (w).

unfortunately not. I'm not sure what the Kubelka-Munk Transformation is exactly so I think I'd have to look that up. And I did not know that I had to convert from transmittance to absorption either.

[/quote]

Thanks ARGOS

[hmx9123]

The more sample you have, the more absorbtion you will have.  If the wavenumbers line up, then you have the right sample.

For example, if the standard has 0.005g of sample in it to 100mg of KBr, and your sample has 0.0005g per 100mg KBr, your sample will have smaller peaks.

[ARGOS++]

Dear ~Christina~;

The ATR technique I had only for IRS in mind and not as a first priority technique for “normal” IR’s.

Simply spoken ATR is only a very, very thin cuvette but with the disadvantage that its path-length is wavelength dependent!  As its path-length is sooo thin, it’s very common that several (~5 – 30) reflections are count together for Non-IRS.
ATR should normaly be a Transmittance measurement.
Till I have found better references you may read:    "Attenuated Total Reflection (ATR)”.
But keep in mind that therein a few things are simplified (maybe toooo), also some known problems.

But on the other hand: Normal FT-IR-ATR instruments should do always the whole translation to “comparable” Transmittance by itself if then requred.   So I’m a little astonished!
Could it be in this moment that also the path-length compensation is not really done?

But still: Are you sure that you compared a spectrum from a solid with a spectrum of a solid?
I think especially about that, because you told that one spectrum was in Transmittance and the other in Reflectance!  That would anyway “act as” different Solvents.
And unfortunately ATR spectra of solids are also force dependent too!

Don’t try to understand at the moment the “Kubelka-Munk”-Transformation for your Application (I’m using it quite frequently.)!
In case you really need any kind of Concentration measurements you does it better with normal calibration a la Beer-Lambert as you can see on: "Corrected Beer's Law”
There you will also see how to translate Transmittance to Absorption and vice versa.
(But not from Reflectance!)
But keep in mind, that Peaks in the IR goes very fast into “Saturation”!, that’s why very short “Cuvettes” are required.

I have added an Example of one Compound in two “Solvents”, and if you take a look at you can easily imagine how much also its solid spectrum would differ.
And the similar is also valid for your spectra!

Dear hmx9123: That's true of course, but if it really will be Reflectance, then you should also translate it to that for her.


Good Luck!
                    ARGOS⁺⁺