chemistry

Internal standard

Internal standard is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Internal standard rather than just read about it. In short: In a chemical analysis, the internal standard method involves adding the same amount of a chemical substance to each sample and calibration solution. The internal standard responds proportionally to changes in the analyte and provides a similar, but not identical, measurement signal.

Key takeaways

Internal standard belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Internal standard to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Internal standard from memory before moving on to harder problems.

Reference excerpt

In a chemical analysis, the internal standard method involves adding the same amount of a chemical substance to each sample and calibration solution. The internal standard responds proportionally to changes in the analyte and provides a similar, but not identical, measurement signal. It must also be absent from the sample matrix to ensure there is no other source of the internal standard present. Taking the ratio of analyte signal to internal standard signal and plotting it against the analyte concentrations in the calibration solutions will result in a calibration curve. The calibration curve can then be used to calculate the analyte concentration in an unknown sample. Selecting an appropriate internal standard accounts for random and systematic sources of uncertainty that arise during sample preparation or instrument fluctuation. This is because the ratio of analyte relative to the amount of internal standard is independent of these variations. If the measured value of the analyte is erroneously shifted above or below the actual value, the internal standard measurements should shift in the same direction. Ratio plot provides good way of compensation of detector sensitivity variation, but may be biased and should be replaced by Relative concentration/Relative calibration calculations if the reason of response variability is in different mass of analysed sample and traditional (not internal standard) calibration curve of any analyte is not linear through origin.

History The earliest recorded use of the internal standard method dates back to Gouy's flame spectroscopy work in 1877, where he used an internal standard to determine if the excitation in his flame was consistent. His experimental procedure was later reintroduced in the 1940s, when recording flame photometers became readily available. The use of internal standards continued to grow, being applied to a wide range of analytical techniques including nuclear magnetic resonance (NMR) spectroscopy, chromatography, and inductively coupled plasma spectroscopy.

Applications

Nuclear magnetic resonance spectroscopy In NMR spectroscopy, e.g. of the nuclei 1H, 13C and 29Si, frequencies depend on the magnetic field, which is not the same across all experiments. Therefore, frequencies are reported as relative differences to tetramethylsilane (TMS), an internal standard that George Tiers proposed in 1958 and that the International Union of Pure and Applied Chemistry has since endorsed. The relative difference to TMS is called chemical shift. TMS works as an ideal standard because it is relatively inert and its identical methyl protons produce a strong upfield signal, isolated from most other protons. It is soluble in most organic solvents and is removable via distillation due to its low boiling point. In practice, the difference between the signals of common solvents and TMS are known. Therefore, no TMS needs to be added to commercial deuterated solvents, as modern instruments are capable of detecting the small quantities of protonated solvent present. By specifying the lock solvent to be used, modern spectrometers are able to correctly reference the sample; in effect, the solvent itself serves as the internal standard.

Chromatography In chromatography, internal standards are used to determine the concentration of other analytes by calculating response factor. The selected internal standard should have a similar retention time and derivatization. It must be stable and not interfere with the sample components. This mitigates the uncertainty that can occur in preparatory steps such as sample injection. In gas chromatography-mass spectrometry (GC-MS), deuterated compounds with similar structures to the analyte commonly act as effective internal standards. However, there are non-deuterated internal standards such as norleucine, which is popular in the analysis of amino acids because it can be separated from accompanying peaks. Selecting an internal standard for liquid chromatography-mass spectrometry (LC-MS) depends on the employed ionization method. The internal standard needs a comparable ionization response and fragmentation pattern to the analyte. LC-MS internal standards are often isotopically analogous to the structure of the analyte, using isotopes such as deuterium (2H), 13C, 15N and 18O.

Inductively coupled plasma Selecting an internal standard in inductively coupled plasma spectroscopy can be difficult, because signals from the sample matrix can overlap with those belonging to the analyte. Yttrium is a common internal standard that is naturally absent in most samples. It has both a mid-range mass and emission lines that don't interfere with many analytes. The intensity of the yttrium signal is what the signal from the analyte gets compared to. In Inductively coupled plasma-mass spectrometry (ICP-MS), species with a similar mass to the analyte usually serve as good internal standards, though not in every case. Factors that also contribute to the effectiveness of an internal standard in ICP-MS include how close its ionization potential, change in enthalpy, and change in entropy are to the analyte. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) internal standards can be selected by observing how the analyte and internal standard signals change with varying experimental conditions. This includes making adjustments to the sample matrix or instrumentation settings and evaluating whether the selected internal standard is reacting in the same way the analyte is.

Example of internal standard method

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Internal standard

Start with the simplest possible case. Write down what Internal standard claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Internal standard before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Internal standard ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Internal standard

In research: Internal standard appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Internal standard in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Internal standard is common in secondary-school and first-year university syllabi. It links to neighbouring topics Analytical chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Internal standard outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Internal standard in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Internal standard means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Internal standard out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Internal standard in simple terms?

Why does Internal standard matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Internal standard?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Internal standard.