By : James W Zubrick
Email: j.zubrick@hvcc.edu
RUNNING THE SPECTRUM
There are many IR instruments, and since they are so different, you need your instructor’s help here more than ever. But there are a few things you have to know.
1. The sample beam. Most IRs are dual-beam instruments (Fig. 127).
Fig. 127 Schematic diagram of an IR.
The one closest to you, if you’re operating the instrument, is the sample beam. Logically, there is a sample holder for the sample beam, and your sample goes there. And there, a beam of IR radiation goes through your sample.
2. The reference beam. This is the other light path. It’s not visible light but another part of the electromagnetic spectrum. Just remember that the reference beam is the one farthest away from you.
3. The 100% control. This sets the pen at the 100% line on the chart paper. Or tries to. It’s a very delicate control and doesn’t take kindly to excessive force. Read on and all will be made clear.
4. The pen. There is a pen and pen holder assembly on the instrument. This is how the spectrum gets recorded on the chart paper. Many people get the urge to throw the instrument out the window when the pen stops writing in the middle of the spectrum. Or doesn’t even start writing. Or was left empty by the last fellow. Or was left to dry out on the top of the instrument. For those with Perkin-Elmer 137s or 710s, two clever fellows have made up generic felt-tip pen holders for the instruments. This way, you buy your own pen from the bookstore, and if it dries out it’s your own fault.
5. The very fast or manual scan. To get a good IR, you’ll have to be able to scan, very rapidly, without letting the pen write on the paper. This is so you’ll be able to make adjustments before you commit pen to paper. This fast forward is not a standard thing. Sometimes you operate the instrument by hand, pushing or rotating the paper holder. Again, do not use force.
Whatever you do, don’t try to move the paper carrier by hand when the instrument is scanning a spectrum. Stripped gears is a crude approximation of what happens. So, thumbs off.
I’m going to apply these things in the next section using a real instrument, the Perkin-Elmer 710B, as a model. Just because you have another model is no reason to skip this section. If you do have a different IR, try to find the similarities between it and the Perkin-Elmer model. Ask your instructor to explain any differences.
THE PERKIN-ELMER 71 OB IR
1. On-off switch and indicator. Press this once, the instrument comes on, and the switch lights up. Press this again, the instrument goes off, and the light goes out.
2. Speed selector. Selects speed (normal or fast). “Fast” is faster, but slower give^ higher resolution, that is, more detail.
3. Scan control. Press this to start a scan. When the instrument is scanning, the optics and paper carrier move automatically, causing the IR spectrum to be drawn on the chart paper by the pen.
4. Chart paper carriage. This is where the chart paper nestles while you run an IR. If it looks suspiciously like a clipboard, it’s because that’s how it works.
5. Chart paper hold-down clip. Just like a clipboard, this holds the paper down in the carrier.
6. Frequency scale. This scale is used to help align the chart paper and to tell you during the run where in the spectrum the instrument is.
Fig. 128 The Perkin-Elmer 71 OB IR.
7. Scan position indicator. A white arrow that points, roughly, to the place in the spectrum the instrument is at.
8. Line-up mark. A line, here at the number 4000, that you use to match up the numbers on the instrument frequency scale with the same numbers on the chart paper.
9. Pen and transmittance scale. This is where the pen traces your IR spectrum. The numbers here mean percentage of IR transmitted through your sample. If you have no sample in the sample beam, how much of the light is getting through? Those who said 100% are 100% correct. Block the beam with your hand and 0% gets through. You should be able to see why these figures are called the % transmission, or %T, scale.
10. The 100% control. Sets the pen at 100%. Or tries to. This is a fairly sensitive control, so don’t force it.
11. Sample beam aperture. This is where you put the holder containing your sample, be it mull or KBr pellet. You slip the holder into the aperture window for analysis.
12. Reference beam aperture. This is where nothing goes. Or, in extreme cases, you use a reference beam attenuator to cut down the amount of light reaching the detector.
USING THE PERKIN-ELMER 71 OB
1. Turn the instrument on and let it warm up for about 3-5 min. Other instruments may take longer.
2. Get a piece of IR paper and load the chart paper carriage, just like a clipboard. Move the paper to get the index line on the paper to line up with the index line on the instrument. It’s at 4000 cm-1 and it’s only a rough guide. Later I’ll tell you how to calibrate your chart paper.
3. Make sure the chart paper carriage is at the high end of the spectrum (4000 cm”1).
4. Put your sample in the sample beam. Slide the sample holder with your sample into the sample beam aperture (Fig. 126).
5. What to do next varies for particular cases. Not much, but enough to be confusing in setting things up.
6. Look at where the pen is. Carefully use the 100% control to locate the pen at about 90% mark when the chart (and spectrum) is at the high end (4000 cm-1).
The 100% Control: An Important Aside
Usually there’s not much more to adjusting the 100% control than is coming up in items 7 through 9, unless your sample, by its size alone, reduces the amount of IR reaching the detector. This really shows up if you’ve used the Mini-Press, which has a much smaller opening than that of the opening in the reference beam. So you’re at a disadvantage right from the start. The 100% control mopes around at, sometimes, much less than 40%. That’s terrible, and you have to use a reference beam attenuator (Fig. 129) to equalize the amounts of energy in the two beams. As you block more and more of the energy in the reference beam, the % Twill go back to the 100% mark. Stop at about 90%. Note that this is where you’d put the pen with the 100% control anyway, if you didn’t have these problems. Use the smallest amount of reference beam attenuation you can get away with.
Fig. 129 Using a reference beam attenuator with a KBr window.
7. OK, at 4000 cm”1 the %T (the pen) is at 90%.
8. Now slowly, carefully move the pen carriage manually so that the instrument scans the entire spectrum. Watch the pen! If the baseline creeps up and goes off the paper (Fig. 130), this is not good. Readjust the 100% control to keep the pen on the paper. Now keep going, slowly, and if the pen drifts up again, readjust it again with the 100% control and get the pen back on the paper.
9. Now go back to the beginning (4000 cm-1). If you’ve adjusted the 100% control to get the pen back on the paper at some other part of the spectrum, surprise! The pen will not be at 90% when you get back. This is unimportant. What is important is that the pen stay within the limits, between the 0 and 100%T lines, for the entire spectrum.
10. In any case, if the peaks are too large, with the baseline in the proper place, your sample is just too concentrated. You can wipe some of your liquid sample or mull off one of the salt plates or remake the KBr pellet using less compound or more KBr. Sorry.
11. When you’ve made all the adjustments, press the scan button, and you’re off.
Fig. 130 An IR with an unruly baseline.
CALIBRATION OF THE SPECTRUM
Once the run is over, there’s one other thing to do. Remember that the index mark on the paper is not exact. You have to calibrate the paper with a standard, usually polystyrene film. Some of the peaks in polystyrene are quite sharp, and many of them are very well characterized. A popular one is an extremely narrow, very sharp spike at 1601.4 cm-1 (6.24 pim).
1. Don’t move the chart paper or this calibration will be worthless.
2. Remove your sample, and replace it with the standard polystyrene film sample. You will have to remove any reference beam attenuator and turn the 100% control to set the pen at about 90%, when the chart is at 4000 cm-1.
3. With the pen off the paper, move the carriage so that it’s just before the calibration peak you want, in this case 6.24 fim.
4. Now, quickly, start the scan, let the pen draw just the tip of the calibration peak, and quickly stop the scan. You don’t need to draw more than that (Fig. 131). Just make sure you can pick your calibration peak out of the spectral peaks. If it’s too crowded at 1601.4 cm-1, use a different polystyrene peak — 2850.7 or 1028.0 cm-1 (3.51 or 9.73 fim). Anything really well known and fairly sharp will do (Fig. 132).
5. And that’s it. You have a nice spectrum.
IR SPECTRA: THE FINISHING TOUCHES
On IR chart paper there are spaces for all sorts of information. It would be nice if you could fill in
1. Operator. The person who ran the spectrum. Usually you.
2. Sample. The name of the compound you’ve just run.
3. Date. The day you ran the sample.
Fig. 131 A calibration peak on an IR spectrum.
4. Phase. For KBr, say “solid KBr.” A Nujol mull is “Nujol mull.” Liquids are either solutions in solvents or “neat liquids,” that is, without any solvents, so call them liquids.
5. Concentration. For KBr, a solid solution, list milligrams of sample in 100 mg of KBr. For liquids, neat is used for liquids without solvents.
6. Thickness. Unless you’re using solution cells, thin film for neat liquids. Leave this blank for KBr samples (unless you’ve measured the thickness of the KBr pellet, which you shouldn’t have done).
7. Remarks. Tell where you put your calibration peak, where the sample came from, and anything unusual that someone in another lab might have trouble with when trying to duplicate your work. Don’t put this off until the last day of the semester when you can no longer remember the details. Keep a record of the spectrum in your notebook.
You now have a perfect IR, suitable for framing and interpreting.
Fig. 132 IR of polystyrene film pointing out many calibration peaks.
Fig. 133 The finishing touches on the IR.
INTERPRETING IRs
IR interpretation can be as simple or as complicated as you’d like to make it. You’ve already seen how to distinguish alcohols from ketones by correlation of the positions and intensities of various peaks in your spectrum with positions listed in IR tables or correlation tables. This is a fairly standard procedure and is probably covered very well in your textbook. The things that are not in your text are
1. Not forgetting the Nujol peaks. Mineral oil will give huge absorptions from all the C—H bonds. They’ll be the biggest peaks in the spectrum. And every so often, people mistake one of these for something that belongs with the sample.
2. Nitpicking a spectrum. Don’t try to interpret every wiggle. There is a lot of information in an IR, but sometimes it is confusing. Think about what it is you’re trying to show, then show it.
3. Pigheadedness in interpretation. Usually a case of, “I know what this peak is so don’t confuse me with facts.” Infrared is an extremely powerful technique, but there are limitations. You don’t have to go hog wild over your IR, though. I know of someone who decided that a small peak was an N—H stretch, and the compound had to have nitrogen in it. The facts that the intensity and position of the peak were not quite right, and neither a chemical test nor solubility studies indicated nitrogen, didn’t matter. Oh well.
