As a general rule, a pronoun should refer clearly to a single, explicitly stated antecedent and should agree with that antecedent in number. In my scientific editing, I encounter one particular violation of this rule in nearly every paper that crosses my desk. Here’s an example:
The ZnO peaks for the samples that were dried for 5 days are smaller than that for the sample that was dried immediately.
The singular demonstrative pronoun “that” must have a singular antecedent, but the sentence contains no singular nouns. The author clearly meant “that” to refer to “ZnO peak,” which is implied but not explicitly stated. You might be inclined to leave this sentence unrevised, thinking that everyone will know what you mean. However, what about the following sentence?
The molar ratio in these complexes was smaller than those in the previously synthesized complexes.
Here, the author meant the plural demonstrative pronoun “those” to refer to an implied “molar ratios,” but some readers will pause momentarily, misreading “those” as referring to the closer plural noun “complexes.” Fortunately, there’s a simple, albeit slightly wordy fix for sentences like these: replace the pronoun with the appropriate noun or noun phrase:
The molar ratio in these complexes was smaller than the ratios in the previously synthesized complexes.
Here are three additional example sentences, with suggested revisions in italics:
The line shape of the band of the heterotrinuclear complex was intermediate between those of the homotrinuclear complexes.
The line shape of the band of the heterotrinuclear complex was intermediate between the line shapes of the homotrinuclear complexes.
In the homozygous mutant seeds, the fluorescence intensity in the rER was as weak as those in the amyloplasts.
In the homozygous mutant seeds, the fluorescence intensity in the rER was as weak as the intensities in the amyloplasts.
The massic activities of 210Pb in soils, fly ash, and coal were compared with that in aerosols.
The massic activities of 210Pb in soils, fly ash, and coal were compared with the massic activity in aerosols.
In scientific writing, clarity is sometimes more important than concision.