The phrase "are difficult to substitute" is correct and usable in written English.
It can be used when discussing items, concepts, or elements that cannot easily be replaced or exchanged for something else.
Example: "Certain ingredients in this recipe are difficult to substitute without altering the flavor significantly."
Alternatives: "are hard to replace" or "are not easily interchangeable."
This paper will focus on a set of critical materials, mostly named as single elements, that are metals, at risk of supply constraints, have environmental implications, financially costly, price volatile, deemed economically important and are difficult to substitute as a result of their unique properties or for economic reasons.
Where it is difficult to substitute natural resources, the skills of planning and managing natural resources will take priority over other strategies.
Patent sharks seem to have adopted a new strategy that they are likely to begin following more frequently: trapping manufacturers with patent-protected technology that is difficult to substitute for under any circumstances not only under time pressure but also in the long run once it is integrated into a complex product or technology standard.
However, it would be difficult to substitute those species for annual grain commodities, because they are much more difficult to harvest, transport, store and process mechanically.
Interestingly, SD13M is included in the region of chromosome 13 that was difficult to substitute from strain B6 to MSM during the course of establishing a consomic strain [ 21].
Dairy products are particularly difficult to replace, says Daniel Granato, a researcher at the Federal University of Paran?Brazil, because the intrinsic attributes of dairy foods come from proteins and fats that are difficult to mimic in a nondairy substitute.
Dairy products are particularly difficult to replace, says Daniel Granato, a researcher at the Federal University of Paraná, Brazil, because the intrinsic attributes of dairy foods come from proteins and fats that are difficult to mimic in a nondairy substitute.
Therefore, when NDFD measurements are difficult to obtain, ADL could adequately serve as a substitute.
Second, use of substituted phthalic anhydrides yields isomeric mixtures of rhodamines that are difficult to separate, requiring tedious chromatography.
