Ir Spectrum For Aspirin

Getting a Clean IR Spectrum for Aspirin

Aspirin is one of the most common samples people run in teaching labs, which means there's a lot of mediocre advice floating around about it. Most of it isn't wrong per se, but it glosses over the things that actually make the difference between a usable spectrum and garbage. I'll walk through what I've learned doing this repeatedly, including the bit nobody mentions. The core issue with aspirin is that it's hygroscopic enough to pick up moisture if you're not careful, and water in an KBr pellet or Nujol mull shows up as a broad O-H stretch around 3200–3500 cm¹ that can be mistaken for residual salicylic acid. That's the first trap. Real salicylic acid contamination gives a sharp O-H around 3200 cm¹ too, but the shapes differ when you look closely. The broader the peak, the more likely it's just absorbed water. If you're running a solid-state sample, press your pellet quickly and keep it under desiccator conditions until you load it into the spectrometer. I once spent twenty minutes chasing a mysterious shoulder peak before realizing my KBr had been sitting uncapped on the bench for three days. Switching to freshly dried KBr eliminated it entirely.

Interpreting the Ir Spectrum For Aspirin

Aspirin, or acetylsalicylic acid, has a fairly distinctive IR profile that makes it relatively straightforward to identify if you know what to look for. Here's what matters: Carbonyl region (1600–1800 cm¹): Aspirin has two carbonyl peaks. The ester C=O stretch appears around 1750–1757 cm¹ and the carboxylic acid C=O stretch sits near 1685–1690 cm¹. These two are usually well-resolved from each other, which is one of the hallmarks. If you're seeing only one carbonyl peak, something's off — either your sample is degraded or you're looking at a different compound altogether. Aromatic region: C-H aromatic stretches show up around 3000–3100 cm¹, and you'll see aromatic ring breathing modes near 1500 cm¹ and 1600 cm¹. The pattern here is consistent with a disubstituted benzene ring, which matches the ortho substitution pattern of aspirin.

O-H stretch: The carboxylic acid O-H is typically a broad, strong band centered around 2500–3300 cm¹. It's often described as a "bearded" peak because it overlaps heavily with the C-H stretches and tails down to the low wavenumber side. If this peak looks sharp and narrow instead, your sample may be too dry or you're looking at something other than a free carboxylic acid. C-O stretches: The ester C-O and acid C-O stretches appear in the 1000–1300 cm¹ range. These are useful for confirmation but are less diagnostic on their own since they overlap with other functional groups. The practical workflow I use is to run a transmission spectrum with a KBr pellet at about 1–2 mg of sample per 200–300 mg of KBr. Grind them together in an agate mortar until the mixture looks uniformly speckled with no visible chunks of sample. Press at roughly 8–10 tons for one to two minutes. If the resulting pellet is cloudy, you either didn't grind long enough or there's moisture present. A good pellet should be transparent like glass.

Fourier transform instruments make this trivially fast now — a single scan at 4 cm¹ resolution takes about thirty seconds, and four scans averaged together give you a clean spectrum in under two minutes. Older dispersive instruments required patience I don't have, but most labs have moved on from those. If you're stuck with a single-beam FTIR, remember to run a background scan before every sample. Skipping this step is the fastest way to get weird baseline artifacts that look like peaks but aren't. There's a common misconception that you need to purify aspirin before running IR. You generally don't. IR is a bulk identification tool, not a purity test. If your commercial aspirin tablet has fillers like starch or cellulose, those will show up as extra peaks — starch gives a strong C-O band around 1030 cm¹, and magnesium stearate (a common lubricant) adds CH bands near 2920 and 2850 cm¹. To get a clean aspirin spectrum from a tablet, crush the tablet, extract the aspirin with a minimal amount of cold ethanol, filter, and let the filtrate evaporate. The residue will be substantially purer. I do this whenever I'm trying to teach students how to identify an unknown and the sample came from a pharmacy bottle rather than a reagent grade source. Another thing people miss: aspirin decomposes slowly on its own. Over months on a shelf, especially in humid conditions, it hydrolyzes back to salicylic acid and acetic acid. The IR spectrum of aged aspirin will show a growing salicylic acid signal — specifically a stronger and broader O-H stretch and a shifted carbonyl pattern. If you run a spectrum and it looks like something between aspirin and salicylic acid, check the expiry date on your sample before you blame the instrument. I've had grad students spend an entire afternoon troubleshooting a "mystery compound" that turned out to be a bottle of aspirin left open on a fume hood bench for six months.

If you're working with dilute solutions rather than solids, use a liquid cell with NaCl windows. Aspirin is soluble in ethanol and methanol at reasonable concentrations. Don't use water as a solvent if you can avoid it — water absorbs strongly across most of the IR region and will swamp your signal. Even trace amounts of water in an organic solvent can ruin a spectrum in the carbonyl region. Molecular sieve drying of your solvent beforehand is worth the ten minutes it takes.

What IR Can't Tell You

It's important to be honest about the limitations here. IR alone cannot reliably quantify the level of salicylic acid impurity in an aspirin sample. The peaks overlap too much in the carbonyl region, and the technique lacks the resolution needed for accurate quantitation without chemometric processing. If you need to verify USP-grade purity specifications on salicylic acid content, you're going to need HPLC. IR is fine for confirming that your sample is aspirin and roughly judging whether it looks contaminated, but it won't replace a proper analytical method for that purpose. Near-IR spectroscopy is another option worth mentioning briefly. It's faster for high-throughput environments and doesn't require sample preparation, but the spectra are messier and assignment is less intuitive. Most teaching labs and quality control departments that do routine aspirin checks have moved to Raman for this reason, or just stick with FTIR and accept the pellet prep time.