According to linguists, the early languages of humans did not have articles to accompany nouns, the necessary information concerning number and specificity being conveyed through context or helper words somewhere else in the sentence. This admirable systems still exists in many languages of the world, including Russian, Hindi, Chinese, and Japanese, but at some point other languages evolved a set of words to precede nouns to indicate this information.
In English the definite article “the” originated from the word “that” which was one of the words used to specify the location of an object. (English used to have three ways of indicating location—“this,” right here; “that,” further off; “yon,” much further off.) The word “that” eventually evolved into the definite article “the,” which indicates that a certain specific object is being referred to. The alternative is the indefinite article “a/an,” which does not specify any particular object, but means one of any number of possibilities. (A/an was derived from the word “one,” and it can still have this meaning.)
The distinction between using a definite vs. an indefinite article can be subtle. If you are assembling a distillation apparatus and you need a condenser you might tell your assistant, “Bring me the condenser.” Since you used the definite article, this implies that want a certain specific condenser, perhaps the one he put in the drying oven a few hours earlier. He probably knows which one you want, but if he doesn’t he’ll ask. On the other hand, if you say “Bring me a condenser,” this implies that there are more than one condensers available in the lab and that it doesn’t matter which one he brings. Any condenser will do.
Compared with many languages article usage in English is pretty simple. German has twelve articles. English has only three, and there is no difference in meaning between a/an—use “a” with a noun that begins with a consonant sound; use “an” with a noun that begins with a vowel sound. It’s the sound that’s important, not the letter the word starts with. Thus, “an NMR spectrum” since the abbreviation NMR is pronounced “en-em-are.”
There are a few places where no article appears—the so called “zero-article.” Plural nouns generally don’t take articles—“Spectra are shown in supplemental information.” Singular nouns of a general nature denoting something which is made up of many parts (“mass nouns”) are treated like plural nouns in this respect—“Chromatography is a good way to purify organic compounds.” No native English speaker would say, “The chromatography is a good way to purify organic compounds.” But if you asked someone why he omits the definite article here, he might have a hard time explaining it to you—“I don’t know. Nobody says it that way. It wouldn’t sound right.” Since plural and mass nouns refer to many objects there is usually no need for definite or indefinite articles with them, and it would be confusing if articles were used. This is why article use here “wouldn’t sound right.”