July 11, 2020, 03:04:42 AM
Forum Rules: Read This Before Posting


Topic: FTIR  (Read 15180 times)

0 Members and 1 Guest are viewing this topic.

Offline Golden_4_Life

  • Full Member
  • ****
  • Posts: 131
  • Mole Snacks: +3/-4
  • Gender: Female
FTIR
« on: May 11, 2010, 11:19:56 AM »
Anyone ever work with an FTIR unit?
Seems like the prepared sample goes into a "disc plate" instead of a quartz cuvette as I'm used to doing with the UVVis. If so, then how does the disc plate hold/contain a liquified sample? How is the measurement done?  My first time looking into doing this
Golden4Life

Offline JGK

  • Chemist
  • Full Member
  • *
  • Posts: 738
  • Mole Snacks: +66/-19
  • Gender: Male
Re: FTIR
« Reply #1 on: May 11, 2010, 03:27:15 PM »
You can get attachments For FTIR that will take differing sample types Gases, liquids, solids, films etc
Experience is something you don't get until just after you need it.

Offline Mitch

  • General Chemist
  • Administrator
  • Sr. Member
  • *
  • Posts: 5294
  • Mole Snacks: +376/-2
  • Gender: Male
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: FTIR
« Reply #2 on: May 11, 2010, 05:24:26 PM »
Some instruments use NaCl plates to hold the liquid. You just put a small drop on one plate and then smush the other plate against it.
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

Offline Golden_4_Life

  • Full Member
  • ****
  • Posts: 131
  • Mole Snacks: +3/-4
  • Gender: Female
Re: FTIR
« Reply #3 on: May 12, 2010, 11:34:31 AM »
hmmmm though these procedures are simplistic, evidently they have worked for other investigators.  Thanks will try soon ;)
Golden4Life

Offline aeacfm

  • Full Member
  • ****
  • Posts: 226
  • Mole Snacks: +7/-2
  • Gender: Male
Re: FTIR
« Reply #4 on: May 16, 2010, 05:20:30 AM »
for liquid samples you have   2  ways

1 - using the discs like what said above and this is so good which has little noise and highly clear
2 - use the quartz cell but the problem with it that it has high noise .


there is advanced type of measurement for FTIR that is the ATR unit which uses diamond crystal to measure instead of cells and it is the best for both solids and liquids and the samples has no thing to do to prepare it it works by the Kublka Munk diffusion theory

Offline marquis

  • Full Member
  • ****
  • Posts: 331
  • Mole Snacks: +32/-3
Re: FTIR
« Reply #5 on: May 16, 2010, 04:40:40 PM »
You can get fluid cells for FTIR.  They are made from NaCl or KBr, but with a sealing spacer (usually teflon) between the plates to control the thickness.  Fluid cells work well, but some of them can be very hard to clean.  Try to see how to clean to cell before you buy it. 

When you fill a fluid cell, you usually elevate one side.  A pencil placed under one side to elevate works well.  This stops air bubbles from staying in the cell. 

We used this method to detemine silicone oil washed off pharmaceutical parts.

The other option is to use NaCl salt plates with a space between them.  We would die a o ring out of aluminum foil and use it as a spacer between two salt plates.  Works well for qualitative work, but not so well for quantitative work.

Offline Golden_4_Life

  • Full Member
  • ****
  • Posts: 131
  • Mole Snacks: +3/-4
  • Gender: Female
Re: FTIR
« Reply #6 on: May 17, 2010, 09:06:52 AM »
thanky all are under consideration
Golden4Life

Offline Golden_4_Life

  • Full Member
  • ****
  • Posts: 131
  • Mole Snacks: +3/-4
  • Gender: Female
Re: FTIR
« Reply #7 on: May 17, 2010, 06:55:27 PM »
I have obtained 'qualitative" data on the FTIR of PDMS which I extracted via methylene chloride, evap'd and reconstituted with the same.  The FTIR signal are clearly shown at wavenumbers: 1259.2 (a sizeable singlet peak), and at 1015 and 1095 (appearing as two doublets).  The issues that concern me are:
1) when I added double the amount of dimethicone I got a ~4x amount of signal compared to the 1x amount of dimethicone extracted by sample weight of the lotion.  This preliminary data suggests that the response to PDMS on FTIR is not linear---perhaps second order kinetics?
2) The singlet peak appeared in my reagent blank and on the sod.chloride plates only (though with a significantly less absorbance signal).

So:
1) Is the response to PDMS conc. via the FTIR detector "linear" or "quadratic"?
2) Why is there a signal for PDMS in my reagent blank and on the plates only? I KNOW I did not contaminate the solvents---but I must now presume that the silicone signal is coming from the glassware within which the samples were contained.

DOes this seem feaslible--my speculations?
Golden4Life

Offline marquis

  • Full Member
  • ****
  • Posts: 331
  • Mole Snacks: +32/-3
Re: FTIR
« Reply #8 on: May 17, 2010, 08:30:58 PM »
Several options.

The first is a method suggestion.  The analytical method we use is for a 350/360 viscosity PDMS.  If you are using a different PDMS, you may get different answers.

We wash the parts with petroleum ether.  The petroleum ether is then evaporated off.
A known amount of toluene (usually 5 mls) is added and swirled to dissolve the PDMS.
The toluene/ PDMS is then transferred to a liquid cell and the IR is taken. 

You want to use the sharp PDMS peak at 1254.  Above it, you will find a sharp toluene peak.  Draw a baseline for both peaks and measure the peak height.  Calculate the ratio of peak height PDMS to Peak height toluene.  Quantitate PDMS by plotting the concentration of PDMS vs (PDMS/ toluene peak height ratio). 

Your fluid cell needs to be relatively thin.  And the method is relatively sensitive.  If anything, try diluting the silicone.   

You can also analyze for PDMS using AA.  You wash off the silicone with the appropriate solvent and aspirate directly into the AA flame using a silicon lamp.  This requires a bit of care.  You need to cover all the beakers with parafilm.  High flame from the AA and flammable solvents require a lot of safety measures.

Good luck.

   

Offline Train

  • Regular Member
  • ***
  • Posts: 68
  • Mole Snacks: +2/-0
Re: FTIR
« Reply #9 on: May 17, 2010, 11:34:01 PM »
1) Is the response to PDMS conc. via the FTIR detector "linear" or "quadratic"?

Does PDMS stand for polydimethylsiloxane?  If so, I regularly got r > 0.999 on a 3-point calibration curve using the USP standard and the peak around 1259.  

Draw a baseline for both peaks and measure the peak height.  Calculate the ratio of peak height PDMS to Peak height toluene.  Quantitate PDMS by plotting the concentration of PDMS vs (PDMS/ toluene peak height ratio).  

Does your FTIR software do this for you, or do you you do it manually?  It was a source of frustration back when I had to run this test that I couldn't get the software to report absorbance at a particular wavelength.  Not to hijack the thread, but could you share what software you used and how you liked it?
« Last Edit: May 17, 2010, 11:49:44 PM by Train »

Offline Golden_4_Life

  • Full Member
  • ****
  • Posts: 131
  • Mole Snacks: +3/-4
  • Gender: Female
Re: FTIR
« Reply #10 on: May 18, 2010, 11:32:05 AM »
Ok, I have seen the peak at 1260; however, this same peak appeared in my reagent blank and plates only blank though they were smaller in height.  So, then using the 1260 peak is a questionable comparator--unless I ''subtract out" the signal from the reagent blank no?

In contrast, the peak doublet at 1014 and 1091.5, which is clearly visible does NOT show up in either the reagent blank nor plates only blank.  Arguably, this peak is the more justified as a comparator no?

To the lotion sample that contained 10%w/w PDMS, I treated with acid, then MeCl2, then separated layers, then evap'd off the solvent, then reconstituted with solvent (10 mL), then assayed about 1 small drop onto the plates.

The signal is not linear--though there was a qualitative difference between the 1x conc sample vs. the 4x conc sample.

I believe some of the PDMS signal is being contributed to by the glass erlenmyere flask (which contains Silicon) no?

Why would th use of Toluene instead of MeCl2 remove the aberrant signal?
I believe tha the PDMS signal from the doublet is the more defensible (and analytically valid) comparator because it provided a clean background and the PDMS signals were defelctions of it no?
Golden4Life

Offline Golden_4_Life

  • Full Member
  • ****
  • Posts: 131
  • Mole Snacks: +3/-4
  • Gender: Female
Re: FTIR
« Reply #11 on: May 18, 2010, 11:35:55 AM »
Ok, I have seen the peak at 1260; however, this same peak appeared in my reagent blank and plates only blank though they were smaller in height.  So, then using the 1260 peak is a questionable comparator--unless I ''subtract out" the signal from the reagent blank no?

In contrast, the peak doublet at 1014 and 1091.5, which is clearly visible does NOT show up in either the reagent blank nor plates only blank.  Arguably, this peak is the more justified as a comparator no?

To the lotion sample that contained 10%w/w PDMS, I treated with acid, then MeCl2, then separated layers, then evap'd off the solvent, then reconstituted with solvent (10 mL), then assayed about 1 small drop onto the plates.

The signal is not linear--though there was a qualitative difference between the 1x conc sample vs. the 4x conc sample.

I believe some of the PDMS signal is being contributed to by the glass erlenmyere flask (which contains Silicon) no?

Why would th use of Toluene instead of MeCl2 remove the aberrant signal?
I believe tha the PDMS signal from the doublet is the more defensible (and analytically valid) comparator because it provided a clean baseline/background.
Golden4Life

Offline Golden_4_Life

  • Full Member
  • ****
  • Posts: 131
  • Mole Snacks: +3/-4
  • Gender: Female
Re: FTIR
« Reply #12 on: May 18, 2010, 12:47:36 PM »
SOrry to "quote" myownself--I did not intend to be "talking to myself" heheh.

At any rate the questions remain:

1) Is some of the vibrational signal coming from deformations in Si that are due to the glass erlenmeyer flask that the PDMS sample extract is contained in? If so, then how to get ride of this--use plastic containers? Suggestions please?

And,

2) Is the vibrational signal at 1259 of less "analytical rigor" than the doublets at 1014 and 1094 simply because the signal at 1259 is present in the negative controls?  Keeping in mind in your replies that the neither reagent blank nor plates only blank give a deformation signal in the doublet range.

In theory, the vibrational spectra of the Si-O bonds (waggling, bending, stretcing) is signified at wavenumbers 1014, 1094.
The deformations in the Si-CH3 bonds are signified at wavenumber 1259.

This is important because I must defend these results before group meeting on Wednesday.
Golden4Life

Offline Train

  • Regular Member
  • ***
  • Posts: 68
  • Mole Snacks: +2/-0
Re: FTIR
« Reply #13 on: May 18, 2010, 07:59:46 PM »
I used glass beakers, volumetric pipets and flasks with never any problem.

I think the linearity problem might be from just puting a drop on the plates.  It seems to me like there could be some variability in the amount of sample that ends up in the IR beam.  When I did this test I used a flow cell like Marquis described.  If you don't have one you could try measuring out 20 uL (or whatever is appropriate as long as it's the same) with a microsyringe or micropipet, and be careful when you put the plates together that the whole sample stays in the center.

If that's not it, then I would look at the sample prep.

But really, the AA method sounds much more fun.

Offline marquis

  • Full Member
  • ****
  • Posts: 331
  • Mole Snacks: +32/-3
Re: FTIR
« Reply #14 on: May 18, 2010, 08:46:06 PM »
Got a bunch of topics, so hopefully this makes some sense.

The broadness of the Si-O bonds makes quantitation much more difficult.  That's why we chose the 1259 peak, which represents the Si-CH3.  Glassware (silicates) contain Si-O bonds, but not Si-CH3.  We had no interference from erlenmeyer flasks, as long as they are cleaned well. 

The individual salt discs are ok for qual work, but will give problems with quantitation.  A flow cell of some kind is an absolute necessity for quantitative work. 

On Trains point on software.  We often had to break down and use a ruler to measure the heights.  Some of the software claimed to do it. But as Train mentioned, there were usually baseline problems.  As for scan parameters, our scans were set up for 1150 to 1450 reciprocal cm and we measured the PDMS peak at 1259 and the sharp toluene peak at ~1380.

As for solvents, the method works with a number of different solvents.  You will see versions of this method with carbon disulfide as the solvent.  I don't like CS2.  To me, there is little difference in toxicity between toluene and DCM (methylene chloride).  I can't speak for the DCM method, but have had 20 + years using the toluene method.

Hope this helps.

Sponsored Links