The Split Mull Method

FIGURE 4.8 The infrared spectrum of an aspirin tablet obtained using amineral oil mull. 

FIGURE 4.9 The infrared spectrum of mineral oil, which is used as a mulling agent. Note its bands around 3000, 1400, and 720 cm, which can interfere with the spectra of samples. 

FIGURE 4.10 The infrared spectrum of Fluorolube, amulling agent. Note the bands below 1350 cm and the lack of bands above 1350 cm .  

A problem with the split mull technique is that by the time the mulls are prepared and the spectra are measured and spliced together, it may be faster and easier to obtain the sample spectrum using a KBr pellet. 

FIGURE 4.11 The spectrum of mineral oil from 4000 to 1350 cm electronically spliced to the spectrum of Fluorolube from 1350 to 400 cm . Note the lack of features except for the small mineral oil peak at 720 cm .  

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Fortunately, a way around the problem of mineral oil s infrared bands is a technique called the split mull method. The sample is first prepared as a mull in mineral oil and its spectrum is measured. Then a second mull of the sample is made using a mulling agent called Fluorolube , which is a mixture of long-chain fluorocarbons made by replacing all the C-H bonds in long-chain alkanes with C-F bonds. The infrared spectrum of Fluorolube is shown in Figure 4.10. 

Like other transmission techniques, mulls can be time-consuming. The only way to know if the mull was made properly is to measure its spectrum. If the spectrum is bad, more time must be spent preparing a new sample. In a pair of skilled hands each sample will take several minutes via this technique. Using the split mull method at a minimum doubles sample preparation time. Mulls are typically used for qualitative work, but not for quantitative work since concentrations and pathlengths are difficult to reproduce with this method. The advantages and disadvantages of mulls are summarized in Table 4.4. 

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