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Ensuring good data collection
Ensuring good data collection
When recording spectra it is important to check that data has been correctly recorded for a sample, and that any issues are corrected at the data recording stage. This page provides guidance on checking recorded spectra and examples of common issues encountered during data acquisition.
Good spectra
Good spectra
Throughout this page two compounds will be used in the examples, these being 3-nitrobenzaldehyde and 3-nitrobenzyl alcohol, and good examples of ATR-IR spectra for both compounds are available below.
3-Nitrobenzaldehyde
3-Nitrobenzaldehyde
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3-Nitrobenzyl alcohol
3-Nitrobenzyl alcohol
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Further down the page are examples of spectra showing commonly encountered recording problems. It is useful to work through to
Things to check which suggest a good spectrum has been recorded:
A baseline that is mostly between 90 and 100% transmission
The y-axis covers a good portion of the transmission percentage range with some peaks descending into transmission values <50%
No peaks going above 100% transmission
Background and sample recording
Background and sample recording
Many modern spectrometers don't display the acquired background or raw sample recording during data acquisition. It is useful to have some understanding of this data. Below are a background recorded on a ATR with a diamond, and the corresponding sample recording for 3-nitrobenzaldehyde. It is worth comparing the background and raw data with the resulting spectra for the compound. The absorbances from the diamond crystal and background air dominate the recording before the subtraction. The carbonyl peak for the aldehyde can be identified before the subtraction, but little of the other data is easily discernible.
Background recording on an ATR with a diamond crystal and air recording.
Data acquisition for a sample of 3-nitrobenzaldehyde prior to background subtraction.
Commonly encountered issues
Commonly encountered issues
Throughout this page two compounds will be used in the examples, these being 3-nitrobenzaldehyde and 3-nitrobenzyl alcohol, and good examples of these are available towards the top of the page.
No background recorded
No background recorded
With modern instruments it tends to be very difficult to record a spectrum without a background to subtract, and the example below has been extracted from the datafile. A spectrum where no background has been recorded will be the sum of the peaks observed in the background, in this case absorbances from the diamond crystal, along with absorbances from the air (particularly water vapour and CO₂), along with absorbances from the compound. It is likely low transmission values would be observed for the y-axis (values omitted here).
Good background, poor sample contact
Good background, poor sample contact
With solid samples, it is necessary for force to be applied to the sample (using the clamp) in order to achieve sufficient contact with the crystal surface for Evanescent waves to be absorbed by the compound. In the sample below, insufficient pressure was applied to the clamp, which resulted in low levels of signal. This can be seen from the limited transmission values on the y-axis (strongest bands are only seeing about a 3% drop in transmittance), and results in a very noisy baseline. The same effect can also be observed if sufficient pressure is observed, but there is very little compound on the crystal, e.g. the compound gets knocked off the crystal as pressure is applied.
Background contaminated with the same sample as the data acquisition
Background contaminated with the same sample as the data acquisition
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Background contaminated with a different compound
Background contaminated with a different compound
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Solvent for cleaning the ATR not allowed to dry before background recorded
Solvent for cleaning the ATR not allowed to dry before background recorded
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Compound containing solvent or water
Compound containing solvent or water
In this recording, a sample of the 3-nitrobenzaldehyde has been recorded which is wet with water:
Below is a spectrum of water, and by comparing the peaks in this spectrum with the contaminated sample above and the good spectrum for 3-nitrobenzaldehyde, the contamination of the 3-nitrobenzaldehyde spectrum with water is fairly clear, with the OH absorbance centred around 3300 cm⁻¹ is evident. Similar effects can be observed if samples are wet with other solvents, e.g. ethanol or ethyl acetate. Usually the easiest solution is to allow the compound to dry (e.g. by spreading out on a watch glass) and repeating the measurement when the compound has had more time to dry. The sample data folders contain spectra for a range of common solvents.
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