[Readers Insight] Why Are Ammonium Fluoride Added to the Mobile Phase in Modern LC-MS?

This article is written by Welch's contract writer Chromatography Mound. The content of the article presents a point of view from the author solely.

The choice of mobile phase additives in routine LC-MS analysis often comes down to two familiar options: formic acid and ammonium acetate. For positive electrospray ionization (ESI+), adding 0.1% formic acid can promote protonation and boost signal intensity. For negative electrospray ionization (ESI−), adding ammonium acetate (or ammonium formate) produces is a conventional choice.

But if we look closer at recent high-impact publications on untargeted metabolomics, lipidomics, oligonucleotide therapeutics, and pesticide residue analysis, we will find another mobile phase additive appearing with increasing frequency: ammonium fluoride (NH₄F).

Why has NH₄F, which is perhaps less prominent in inorganic chemistry, attracted such attention from mass spectrometrists seeking to improve sensitivity in negative-mode LC-MS? In this article, we will explore the principal advantages of ammonium fluoride and the mechanisms underlying its effects.

In mass spectrometry, negative ESI mode often delivers lower sensitivity than positive ESI. Many acidic or weakly acidic compounds, although able to lose protons readily, can be frustratingly difficult to detect with a strong response in ESI−. Traditional approaches in solving this issue involve using ammonium formate or ammonium hydroxide, whose performance may not be that satisfactory for some analytes.

Ammonium fluoride, on the other hand, offers an approach that potentially delivers a substantial increase in the signal intensity in ESI−. Fluoride ions feature a strong proton affinity. During the electrospray process, they can snatch hydrons (H⁺) from the target compound and form a highly volatile weak acid (hydrogen fluoride) that leaves the system. This can shift the ionization process toward the formation of [M−H]⁻, increasing the abundance of the target ion.

Compared with acetate and formate, fluoride’s ability to abstract protons in the gas phase is in a different league—it boosts ionization efficiency in an instant.

One of the most frustrating aspects of negative-mode MS analysis is the formation of competing adduct ions. We may be looking specifically for [M−H]⁻, only to find a mass spectrum crowded with formate adducts ([M+HCOO]⁻), acetate adducts ([M+CH₃COO]⁻), and even chloride adducts ([M+Cl]⁻) originating from trace contamination or residual ions in the system. Instead of concentrating in the ion of interest, the analyte signal may be distributed across several different ions, reducing the abundance of the ion used for quantification.

The addition of ammonium fluoride can alter these adduct-formation pathways. Because fluoride ion binds protons extremely rapidly, it effectively suppresses adduct competition from other trace anions in the system, focusing the signal into a single [M−H]⁻ peak. It should be also noted that fluoride ion itself does not readily form stable [M+F]⁻; adducts with most analytes.

Therefore, NH₄F does not only dramatically boost the response intensity of the target peak but also thoroughly simplifies the mass spectrum, making qualitative and quantitative analysis clear and straightforward.

Besides enhancing MS response, ammonium fluoride also improves chromatographic performance to some extent. Similar to ammonium formate and ammonium acetate, ammonium fluoride improves peak shape by suppressing secondary retention of the analyte on the column.

The ionic strength provided by NH₄F effectively shields residual silanol active sites on the surface of the column packing, reducing tailing of basic or amphoteric compounds, which in turn leads to higher signal-to-noise ratios.

However, the buffering capacity of ammonium fluoride is much weaker than those of ammonium formate and ammonium acetate. Therefore, when the target compounds already have poor peak shape, the use of this salt needs to be considered comprehensively.

Despite its potential benefits, ammonium fluoride is not a universal solution. It must be used carefully to avoid compromising analytical performance or damaging laboratory equipment.

Do not automatically apply the concentrations commonly used for ammonium formate, which are often around 10–20 mmol/L, to ammonium fluoride.

In many applications, a concentration of approximately 1 mmol/L is sufficient to produce a useful improvement in signal intensity. Excessive concentration not only causes severe ion suppression but can also damage the LC-MS system.

Although the corrosivity is low at millimolar concentrations, ammonium fluoride can still etch glass bottles when stored for a long time under alkaline conditions or with a high organic-phase proportion, leaching silicates that interfere with mass spectrometry.