Electrospray ionisation mass spectrometry: principles and clinical applications

Abstract

This mini-review provides a general understanding of electrospray ionisation mass spectrometry (ESI-MS) which has become an increasingly important technique in the clinical laboratory for structural study or quantitative measurement of metabolites in a complex biological sample. The first part of the review explains the electrospray ionisation process, design of mass spectrometers with separation capability, characteristics of the mass spectrum, and practical considerations in quantitative analysis. The second part then focuses on some clinical applications. The capability of ESI-tandem-MS in measuring bio-molecules sharing similar molecular structures makes it particularly useful in screening for inborn errors of amino acid, fatty acid, purine, pyrimidine metabolism and diagnosis of galactosaemia and peroxisomal disorders. Electrospray ionisation is also efficient in generating cluster ions for structural elucidation of macromolecules. This has fostered a new and improved approach (vs electrophoresis) for identification and quantification of haemoglobin variants. With the understanding of glycohaemoglobin structure, an IFCC reference method for glycohaemoglobin assay has been established using ESI-MS. It represents a significant advancement for the standardisation of HbA1c in diabetic monitoring. With its other applications such as in therapeutic drug monitoring, ESI-MS will continue to exert an important influence in the future development and organisation of the clinical laboratory service.

Figure 1. Mechanism of electrospray ionisation

Within an ESI source, a continuous stream of sample solution is passed through a stainless steel or quartz silica capillary

Figure 2

Operation of a quadrupole mass analyser. The ion (M⁺) travels from the source, through the 4 metal rods arrangement in the unique oscillating pattern, and reaches the detector.

Figure 3

Schematic diagram of a triple quadrupole system. The first (Q1) and third (Q3) are mass spectrometers and the centre (Q2) is a collision cell.

Figure 4

Schematic diagram of an ion trap mass analyser. The trap is made up of the 2 end cap electrodes and the ring electrodes. Inside the trap, the ions rotate and oscillate in an 8-shaped trajectory.

Figure 5

Mass spectrum of the cluster ions of a highly purified horse myoglobin protein standard solution (5 μmol/L in 50% acetonitrile solution containing 0.2% formic acid)

Figure 6

Mass spectrum of a mixture of serum alpha-foetal proteins purified by affinity chromatography. The top spectrum comprises the raw data showing the composite cluster ions of the different glycoforms. The bottom spectrum is the deconvoluted spectrum on the true mass scale after the Micromass MaxEnt processing.

Figure 7

Mass spectrum of normal haemoglobin. The top spectrum comprises the raw data showing the cluster ions of both α and β globins. The bottom spectrum is the deconvoluted spectrum on the true mass scale after software transformation.