Until recently, what we now know as mass was called “weight.” A beam balance measures the amount of matter in an object. The more an object has, the more it will weigh.

Unlike weight, which is determined by the strength of gravity, mass stays the same regardless of where an object is on Earth or Mars. This is why objects with different masses will weigh differently on each planet.

Accuracy

The accuracy of a mass measurement is the degree to which the measured value conforms to its true value. For example, the smallest part of a sample is expected to weigh exactly the same as its larger counterpart (with 180 grains making a shekel or gin and 600 grains making a pound).

Even though routine single- and tandem-quadrupole instruments have mass accuracy in the range of 3-10 ppm, which can assist in compound identification, it is often difficult to determine precisely what a compound’s formula is. For that reason, spectral accuracy, which is the ability to separate closely spaced peaks in a mass spectrum, has become an important quality indicator for MS.

NIST has developed a system for democratizing accurate, precise, and cost-effective mass calibrations. In partnership with the Army, it is deploying a portable unit that can calibrate torque wrenches, eliminating the need for the Army to send equipment to NIST for expensive and lengthy mass measurements.

Precision

Precision measures how close repeated measurements of the same thing are to each other. The more precise a measurement, the closer it is to its true value. It is important to understand that accuracy and precision are not the same thing.

In the field of mass measurement, scientists have made significant improvements in both accuracy and precision. One way to improve accuracy is to minimize systematic errors, which can be caused by improper calibration or experimental techniques. Another way is to use more precise instruments, such as high-quality balances.

Scientists have also improved the measurement of Planck’s constant, which is used to calculate the amount of matter needed to make a given amount of energy (Avogadro’s number). This is an important step towards replacing the platinum-iridium artifact that currently defines the kilogram with a new definition based on fundamental natural quantities. NIST’s new measurement of this quantity has an uncertainty less than 13 parts per billion.

Technology

The field of mass measurement continues to see technological advances that push the boundaries for accuracy and precision. This includes enhancing the sensitivity of instruments through new ionization techniques and other enhancements, as well as improving data processing algorithms.

PNNL researchers use sophisticated high-resolution mass spectrometry (MS) instruments to analyze intact proteins, thousands of other molecules, and complex mixtures. They also develop a variety of MS-related technologies to improve analytical sensitivity so that not even an invisible molecule can hide in a sample.

The TwoMP Auto system combines the efficiency and ease of automation with the sensitivity and speed of mass photometry for high-throughput measurement of multi-sample biomolecules such as adeno-associated viruses (AAVs). This automated, robotic platform allows users to set up their own protocols for mixing, transferring, and measuring samples, including the preparation of buffers, standards, and solutions, and reduces error due to multiple clamping steps. The system then automatically runs the mass measurements and returns results to the operator for analysis.

Applications

Mass measurement is used to determine the inertial mass of a body, which is related to its weight. This is why you’d find a balance in a gym or an astronaut’s weightmeter (Tsiolkovsky State Museum of Cosmonautics).

The most common application for accurate mass measurement is protein and nucleic acid identification. Mass spectrometry uses its ability to detect, identify and quantify molecules based on their mass-to-charge ratio (m/z) to analyze peptides and proteins.

Another popular application for mass photometry is to quantify protein oligomerisation and aggregation mechanisms, characterise sample heterogeneity, monitor stability of component components, or to test for experimental modifications on the molecular structure of a biomolecule (Higuchi, et al. 2021; Naftaly, et al. 2021). Nucleic acids can also be analysed using mass photometry, such as on the Sequenom MassARRAY27and Ibis T500028 platforms. These use MALDI-TOF and ESI-TOF, respectively. The methods are similar in that neither require labelling the molecules. They are therefore able to deliver results quickly, with a whole measurement workflow often taking just minutes.