What’s important in a good ion source for mass spectrometry? Is it compatibility, robustness, or all-round performance…?

In this blog post we look at two of the most used ionisation sources in mass spectrometry – electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI). Let the battle commence…

ESI

Using this ion source, the analyte in solvent phase is passed through a charged capillary. The resultant spray contains charged droplets which then undergo subsequent losses in size as well as charge. Thanks to the dielectric properties of the solvent droplet, this is commonly considered a ‘soft-ionisation’ technique. What this means for the analyst is data from the molecular ion as well as multiply charged variants (therefore, more information!).

 

A typical example of an ESI source (left) and an APCI source (right) [1]

APCI

On the other hand, when using an APCI source for mass spectrometry, the sample is introduced through a heated needle to form an aerosol. When this is passed close to a charged pin a discharge is formed which ionises the analyte. Although APCI is a ‘soft ionisation’ technique, it is not known for producing multiply charged ions which limits its suitability for many applications. APCI however, is a very good ion source for non-polar analytes!

ESI vs APCI in Mass Spectrometry

Criteria	APCI	ESI
Ease of use	Multi-component source	Source alignment and optimisation required
Ionisation	Typically, only generates singularly charged ions.	Soft ionisation technique allows for the formation of multiply charged ions
Solvent compatibility	Acetonitrile has negative effect on ion formation [2]	Solvent choice is important. Non-polar solvents are to be avoided. Acetonitrile can cause suppression [3]
Flow rates	Works best with high flow rates, above 1000ul/min	Works best with low flow rates, 1000ul/min and below
Sample compatibility	Thermally labile samples can decompose due heated probe	Non-polar compounds to be avoided

 

Microflow ESI: Spraychip

In our compact mass spectrometer, the proprietary Spraychip is a MEMS developed microflow ESI source. Due to its microflow and unique design we believe in a lot of cases it outperforms the competition!

• Plug-and-play: Spraychip comes pre-aligned. No need to break vacuum to install and is fully aligned and optimised out of the box!
• Ultra-soft ionisation – most efficient ESI source on market
• Compatible with normal and reverse-phase solvents thanks to the MiDas interface
• Working range of 0.3 – 2µl/min (with the option to go up to 250000 ul/min with the MiDas) which covers almost every application!

To see the performance of the microflow ESI Spraychip and benefit other compact mass spectrometer devices for yourself, please contact Microsaic to arrange an on-site demonstration: info@microsaic.com

References

[1] University of Bristol, High Performance Liquid Chromatography Mass Spectrometry (HPLC/MS), http://www.bris.ac.uk/nerclsmsf/techniques/hplcms.html

[2]   Agilent Technical Overview: Making your LC method compatible with mass spectrometry, Edgar Naegale, Waldbronn, Germany

[3] Colizza K, Mahoney K E, et al., J. Am. Soc. Mass Spectrom. (2016) 27:1796Y1804