Kendrick mass

Key takeaways

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

Reference excerpt

The Kendrick mass is defined by setting the mass of a chosen molecular fragment, typically CH2, to an integer value in Da (dalton). It is different from the IUPAC definition, which is based on setting the mass of 12C isotope to exactly 12 u. The Kendrick mass is often used to identify homologous compounds differing only by a number of base units in high resolution mass spectra. This definition of mass was first suggested in 1963 by chemist Edward Kendrick, and it has been adopted by scientists working in the area of high-resolution mass spectrometry, environmental analysis, proteomics, petroleomics, metabolomics, polymer analysis, etc.

Definition According to the procedure outlined by Kendrick, the mass of CH2 is defined as exactly 14 Da, instead of the IUPAC mass of 14.01565 Da. To convert an IUPAC mass of a particular compound to the Kendrick mass, the equation

Kendrick mass = IUPAC mass × 14.00000 14.01565 {\displaystyle {\text{Kendrick mass}}={\text{IUPAC mass}}\times {\frac {14.00000}{14.01565}}}

is used. The mass in dalton units (Da) can be converted to the Kendrick scale by dividing by 1.0011178. Other groups of atoms in addition to CH2 can be used define the Kendrick mass, for example CO2, H2, H2O, and O. In this case, the Kendrick mass for a family of compounds F is given by

Kendrick mass (F) = (observed mass) × nominal mass (F) exact mass (F) {\displaystyle {\text{Kendrick mass (F)}}={\text{(observed mass)}}\times {\frac {\text{nominal mass (F)}}{\text{exact mass (F)}}}} . For the hydrocarbon analysis, F=CH2. As an example, Kendrick analysis has been used for visualizing families of halogenated compounds of environmental interest that differ only by the number of chlorine, bromine or fluorine substitutions. It has been suggested that Kendrick mass be expressed in Kendrick units with symbol Ke.

Kendrick mass defect The Kendrick mass defect is defined as the exact Kendrick mass subtracted from the nominal (integer) Kendrick mass:

Kendrick mass defect = nominal Kendrick mass − Kendrick mass {\displaystyle {\text{Kendrick mass defect}}={\text{nominal Kendrick mass}}-{\text{Kendrick mass}}}

In recent years the equation has changed due to rounding errors to:

Kendrick mass defect = nominal mass − Kendrick exact mass {\displaystyle {\text{Kendrick mass defect}}={\text{nominal mass}}-{\text{Kendrick exact mass}}}

The members of an alkylation series have the same degree of unsaturation and number of heteroatoms (nitrogen, oxygen and sulfur) but differ in the number of CH2 units. Members of an alkylation series have the same Kendrick mass defect. The Kendrick mass defect has also been defined as

Kendrick mass defect = ( nominal Kendrick mass − Kendrick mass ) × 1,000 {\displaystyle {\text{Kendrick mass defect}}=({\text{nominal Kendrick mass}}-{\text{Kendrick mass}})\times 1{,}000} . The abbreviations KM and KMD have been used for Kendrick mass and Kendrick mass defect, respectively.

Kendrick mass analysis

In a Kendrick mass analysis, the Kendrick mass defect is plotted as function of nominal Kendrick mass for ions observed in a mass spectrum. Ions of the same family, for example the members of an alkylation series, have the same Kendrick mass defect but different nominal Kendrick mass and are positioned along a horizontal line on the plot. If the composition of one ion in the family can be determined, the composition of the other ions can be inferred. Horizontal lines of different Kendrick mass defect correspond to ions of different composition, for example degree of saturation or heteroatom content. A Kendrick mass analysis is often used in conjunction with a Van Krevelen diagram, a two- or three- dimensional graphical analysis in which the elemental composition of the compounds are plotted according to the atomic ratios H/C, O/C, or N/C.

Kendrick mass defect analysis of polymers and alternative base units Because Kendrick mass defect analysis can be carried out by substituting any repeating unit for CH2, KMD analysis is particularly useful for the visualizing the data from polymer mass spectra. For example, a Kendrick mass defect plot of an ethylene oxide/propylene oxide copolymer can be created by using ethylene oxide (C2H4O) as the base unit and calculating the Kendrick mass as:

Kendrick mass = IUPAC mass × 44.00000 44.02621 {\displaystyle {\text{Kendrick mass}}={\text{IUPAC mass}}\times {\frac {44.00000}{44.02621}}}

where 44.02621 is the calculated IUPAC mass for C2H4O. Alternatively, a KMD plot can be constructed for the same copolymer by using propylene oxide as the base unit. Polymer mass spectra containing multiple charge ions exhibit isotopic splitting.

Fractional base units and referenced KMD plots Kendrick mass defect plots created by using fractional base units exhibit enhanced resolution. Referenced Kendrick mass defect plots (KMD plots referenced to the terminal group and adduct composition) with fractional base units can be used to obtain an overview of copolymer composition.

Worked examples

Example 1 — a first encounter with Kendrick mass

Start with the simplest possible case. Write down what Kendrick mass claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Kendrick mass 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 Kendrick mass 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 Kendrick mass

In research: Kendrick mass appears in science 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 Kendrick mass 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: Kendrick mass is common in secondary-school and first-year university syllabi. It links to neighbouring topics Mass spectrometry, Metrology, Units of mass, so understanding it makes those chapters shorter.
In everyday life: Look for Kendrick mass 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.

How to study Kendrick mass in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kendrick mass 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 Kendrick mass out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kendrick mass in simple terms?

The Kendrick mass is defined by setting the mass of a chosen molecular fragment, typically CH2, to an integer value in Da (dalton). It is different from the IUPAC definition, which is based on setting the mass of 12C isotope to exactly 12 u.

Why does Kendrick mass matter?

Because it connects several science 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 Kendrick mass?

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 Kendrick mass.