# Direct Quantification of Ammonia and Alkylamines in Aqueous Environments Using Solid-Phase Microextraction-Gas chromatography-Mass spectrometry

Doha Zidouhia, Etienne Grau, Dario M. Bassani

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Doha Zidouhia, Etienne Grau, Dario M. Bassani. Direct Quantification of Ammonia and Alkylamines in Aqueous Environments Using Solid-Phase Microextraction-Gas chromatography-Mass spectrometry. Chemistry–Methods, 2025, 5 (8), pp.e202400095. 10.1002/cmtd.202400095. [https://hal.science/hal-05348179v1](https://hal.science/hal-05348179v1)

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# Direct Quantification of Ammonia and Alkylamines in Aqueous Environments Using Solid-Phase Microextraction-Gas chromatography-Mass spectrometry

Doha Zidouhia, Etienne Grau,* and Dario M. Bassani*

The detection of ammonia in aqueous solutions using solid-phase microextraction (SPME)-GC-MS (gas chromatography-mass spectrometry) is described. This technique enables the direct detection of ammonia without the need for derivatization or the use of colorimetric methods, which can be prone to interference from transition metal ions or organic cosolvents typically used in nitrogen reduction catalysis. The use of a fused-silica SPME fiber coated with polydimethylsiloxane/divinylbenzene is found to

allow fast and direct quantification of ammonia with a limit of detection = 0.17 ppm and a limit of quantification = 0.5 ppm in water. The method is straightforward to implement and can be readily automated using commercial instrumentation. It is also capable of detecting isotopically labeled ¹⁵NH₃ as well as simple alkylamines such as methylamine or ethylamine which may be produced or released during the course of a reaction.

## 1. Introduction

Ammonia (NH₃) is a crucial raw material used in fertilizers and as feedstock in the pharmaceutical and chemical industries. Its production, mostly through the Haber-Bosch process, is essential for agriculture and reached 163 Mt/y worldwide in 2022.[1] The Haber-Bosch process requires significant energy for producing hydrogen from steam and methane and then reacting it with nitrogen at high temperatures and pressures. It accounts for ≈1.2% of global CO₂ emissions[2,3] and future uses of ammonia as an alternative energy carrier may potentially further increase its environmental impact. Over the last decade, several alternative methods for ammonia production have been investigated, in particular using electrochemical[4-7] or photochemical synthesis.[8-10]

To compare and optimize alternative synthetic pathways for ammonia production, safe, fast, and reliable techniques to quantify ammonia formed under a variety of reaction conditions and in the presence of potentially interfering species are essential.[11]

Numerous approaches for the detection of ammonia in gas[12-15] or in solution exist.[16-21] However, the presence of excess sacrificial reductants, such as amines, electrolytes, or alcohols used in photo- or electrochemical nitrogen reduction can interfere with the detection of low levels of NH₃, particularly in aqueous environments.[22-24] Beyond ammonia production, accurate quantification of NH₃ is also crucial for ensuring health safety since ammonia is a corrosive substance and irritant that poses risks to both animals and humans.[25,26] High concentrations can lead to chronic respiratory conditions, early-onset asthma in children, and severe illnesses such as COPD and lung cancer.[27]

Current methods to quantify ammonia can be divided into spectroscopic and chromatographic techniques. Other electrochemical techniques using modified electrodes are more sensitive to interference and thus not widely used for NH₃ detection in complex matrices.[28] Common spectroscopic approaches require the derivatization or reaction of NH₃ with a reactive organic compound such as dansyl chloride,[29] mercury ions (Nessler's reagent), or phenol (indophenol blue method). The latter two solution-based methods are the most widely employed by researchers studying photo- or electrochemical nitrogen fixation despite their numerous shortcomings as pointed out by Zhang and coworkers.[23] To summarize, Nessler's reagent involves the use of a toxic mercury reagent and is sensitive to the presence of even relatively low concentrations of transition metal ions including Ce³⁺, Ru³⁺, and Fe²⁺ (10 mM), and numerous other metal ions when they are present at higher concentrations. Furthermore, the reaction is also very sensitive to the presence of alcohols, amines, and organic cosolvents (DMSO, DMF) typically used in photo- or electrocatalysis experiments.[22,23] Finally, the reaction time affects the precision of the measurement and must be carefully monitored. Unlike Nessler's reagent, the indophenol blue method relies on the reaction between ammonia and phenol or a phenol derivative to produce a colored species under neutral or alkaline pH conditions.[30]

D. Zidouhia, D. M. Bassani

Univ. Bordeaux

CNRS

Bordeaux INP

ISM UMR 5255

F-33400 Talence, France

E-mail: dario.bassani@u-bordeaux.fr

D. Zidouhia, E. Grau

Univ. Bordeaux

CNRS

Bordeaux INP

Laboratoire de Chimie des Polymères Organiques (LCPO)

UMR 5629

16 Av. Pey Berland, 33607 Pessac cedex, France

E-mail: etienne.grau@u-bordeaux.fr

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© 2025 The Author(s). Chemistry - Methods published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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Despite the need for the addition of sodium nitroprusside as a catalyst to intensify the color change and citrate buffer to stabilize the pH of the reaction solution, the method is both sensitive and reproducible. However, it requires relatively lengthy reaction times (several hours) and is also sensitive to interference by alcohols and various metal ions.[23]

Recently, surface-enhanced Raman spectroscopy was shown to allow fast, in situ ammonia detections at concentrations below 1 ppm by selectively monitoring the N-H stretching modes.[31] This method is fast and provides in situ real-time monitoring of ammonia concentration using a confocal Raman microscope setup that is particularly useful for monitoring catalytic and electrocatalytic processes. However, there are challenges associated with this method in addition to the instrumental complexity inherent to reliably quantifying the efficiency of the surface-plasmon Raman enhancement and interference by the background water signal. In particular, the thin silver film used is susceptible to oxidation and adsorption of organic or inorganic species that potentially impact its durability.

To date, chromatographic techniques for quantifying ammonia concentrations focus on the use of ion chromatography (IC).[32,33] This technique uses ion-exchange resins coupled to a conductivity detector to separate ions according to their charge or ionic strength and quantify their concentration. Although this technique gained popularity for ammonia quantification in N₂-electroreduction studies owing to its potential for automation, it faces challenges from interference with high concentrations of salts, like sodium or potassium ions, and variations in pH.

Clearly, there is a need for a robust method for quantifying ammonia at mM and µM levels that does not require excessive sample preparation or suffer from interference by organic or inorganic species. The use of mass spectrometric detection coupled to chromatographic separation generally affords the optimum level of sensitivity and separation for many applications. This is widely considered to be a robust analytical technique but it has not been exploited for ammonia detection. Possible reasons may include the difficulty of isolating a volatile gas or ion when using electrospray for liquid chromatography-mass spectrometry (LC-MS) and the impossibility of directly injecting aqueous samples in gas chromatography-mass spectroscopy (GC-MS).[34] We now report that this limitation can be solved by using a commercial solid-phase microextraction (SPME) polymer fiber to selectively adsorb ammonia from aqueous samples and allow their quantitative determination using a GC-MS instrument. This straightforward SPME-GC-MS method detects ammonia as well as simple amines without the need for additional sample handling other than to make the solution alkaline for low NH₃ levels (≤10 ppm). The analyzes can be easily automated using commercial instrumentation and, thanks to the separation and selective detection of the GC-MS, they are insensitive to the presence of numerous organic and inorganic species. Although aliphatic and aromatic amines have been previously quantified using SPME, we could not find reports of its application to the analysis of ammonia in water.

# 2. Experimental Section

## 2.1. Methods and Materials

Ammonia (33% solution in water) and methylamine (40% solution in water) were purchased from Tokyo Chemical Industry, ethylamine (70% solution in water) was purchased from Sigma-Aldrich, and potassium carbonate (99%) was purchased from Carl Roth. The SPME system consists of a fused-silica fiber coated with polydimethylsiloxane/divinylbenzene (PDMS/DVB) polymer designed to optimize analyte adsorption. The fiber (65um PDMS/DVB, Stableflex 24 G) and manual fiber holder were purchased from Supelco. The plunger allowed the fiber to be deployed from a hollow needle into the sample solution, ensuring controlled exposure to the sample for effective extraction of volatile compounds.

Analyzes were performed on a TraceGC instrument (Thermo Scientific) equipped with a 10 m PoraPLOT column specific for amines (Agilent). The GC was coupled to a Multispec MS instrument (Thermo Scientific) using electron impact ionization (70 eV). The oven conditions for the GC analysis allowed for the separation of the peak of NH₃ from that of water and other amines or organic species: Initial temperature = 40 °C (0 min), final temperature = 200 °C (5 min), gradient = 20 °C min⁻¹. A splitless injector held at 200 °C was used to precondition the fiber and for desorption of the analytes.

The interference of various transition metal ions or organic cosolvents was examined using aqueous solutions containing 1000 ppm of ammonia and 3.3 mM of either Ru(2, 2'-bipyridine)₃Cl₂, Zn(CH₃CO₂)₂, MnCl₂, FeCl₂, or NiCl₂, 1000 ppm of either hydrazine, formamide, or urea, or in 10% aqueous acetonitrile, methanol, or tetrahydrofuran (THF) solutions.

## 2.2. General Procedure for the Quantification of NH₃

A solution to be analyzed was placed in a screw-cap vial equipped with a septum. The SPME needle/fiber assembly was lowered into the solution and the fiber was exposed for 5-10 min. While not critical, the exposure time should be identical for all samples and the calibration curve to ensure reproducibility. At the end of the exposure time, the fiber was retracted into the needle and the assembly was placed into the GC injector. For dilute NH₃ solutions (≤10 ppm), the pH of the solution to be analyzed was made alkaline (pH between 10 and 11) by adding powdered potassium carbonate (K₂CO₃) prior to incubation of the fiber. For more concentrated NH₃ solutions, this step was not necessary.

For concentrations above 100 ppm, the ammonia peak was directly visible on the GC spectrum obtained after analysis. However, for lower concentrations, it may be necessary to search for the peak at *m/z* = 16 using the analysis software, as this peak is specific to ammonia. This was chosen instead of 17 *m/z* (NH₃⁺) because the latter was often present throughout the spectrum due to moisture. For greater accuracy, plotting the signal at *m/z* = 16 was more appropriate. An example of a typical GC chromatogram showing the peak for NH₃ and H₂O is shown in Figure 1.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 1.** A) GC-MS signal obtained from a 1000 ppm aqueous ammonia solution at pH 7 using a PDMS/DVB SPME fiber. B) MS chromatogram at *t* = 4.5 min.</figcaption></figure>

### 3. Results and Discussion

Solid-phase microextraction relies on the selective adsorption of some compounds in a solution or gaseous matrix to a natural or synthetic polymer coated on a fused-silica fiber.[35] This allows these compounds to be concentrated onto the fiber material during an incubation period and thus easily extracted from the matrix. Subsequent exposure of the fiber to the heated environment of a gas chromatography injector induces desorption of the analytes which are then transferred from the fiber to the GC column for separation and detection. This technique is particularly useful for the analysis of biological samples,[36] foodstuffs,[37] and wines,[38] for which the direct GC analysis of aqueous solutions is not possible. The fiber assemblies are commercially available for use with entirely automated sample analysis systems, or as a manual kit suitable for use with nonautomated GC instruments. For the analysis of ammonia in aqueous samples, an SPME fiber coated with a PDMS/DVB polymer was selected as this material is suitable for low-molecular weight amines.[39–41]

Comparison of head-space or solution SPME analysis for the quantification of ammonia revealed that the latter allows its quantification and detection at the sub-ppm level. In a typical procedure, the fiber is initially preconditioned by placing it into the heated GC injector for 2–3 min to desorb any contaminants and then incubated in the solution to be analyzed for 5–10 min. The fiber is then placed into the GC injector (5 min) to allow desorption and analysis of the adsorbed species. After this, the fiber can immediately be reused without preconditioning. Analysis of aqueous ammonia solutions over a large concentration range (10–2000 ppm) shows a linear correlation between the area of the peak at RT = 4.5 min assigned to NH₃ and the analyte concentration (*r* = 0.997, **Figure 2**).

Understandably, the detection of ammonia in aqueous samples is rendered difficult by the presence of water, which both competes with NH₃ for adsorption to the SPME fiber and also masks it through H-bonding and protonation. Despite the hydrophobic nature of the PDMS/DVB polymer, a broad peak corresponding to H₂O adsorbed onto the fiber can be observed at longer RT (6.5–8.5 min, Figure 1) in the chromatograms.

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Calibration curve for the quantification of NH₃ in aqueous solutions from 10 to 2000 ppm at pH 7 using a PDMS/DVB SPME fiber. Straight line corresponds to the best fit described by $y = 1435.6x + 21484$ ($r² = 0.9975$).</figcaption></figure>

The protonation of ammonia is also expected to compete with adsorption onto the fiber as the ammonium ion is not adsorbed by the polymer. For more concentrated samples, for example in which [NH₃] ≥ 1000 ppm (59 mM), the pH was measured to be ≥10, and we can calculate that the proportion of protonated ammonia is proportionally small (1.8%) with respect to the overall quantity in the sample. **Figure 3** shows the effect of the pH on the detection of ammonia for a 1000 ppm solution. As it can be seen, for solutions where the pH ≤ pKb of ammonia (9.3), a majority of the ammonia in solution is present as ammonium hydroxide and escapes detection.

For more dilute solutions, particularly those with [NH₃] ≤ 10 ppm (0.59 mM) in which the proportion of NH₄⁺ nears 5% at pH 7, it is advisable to adjust the pH to alkaline values through the addition of potassium or sodium carbonate so as to increase the pH ≥ 10 as determined using a pH meter. With this protocol, and by specifically detecting the *m/z* = 16 characteristic of ammonia, we find that detection limits below 1 ppm are readily accessible.[42] Results for ammonia concentrations between 1 and 10 ppm are shown in **Figure 4** with measurements done in triplicate to
<figure><img src="image_8.png" /><figcaption>Figure 3. Analysis of NH₃ (1000 ppm) in aqueous solutions at different pH values.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 4. Calibration curve for the quantification of NH₃ in aqueous solutions from 1 to 10 ppm at pH ≥ 10 using a PDMS/DVB SPME fiber. Error bars and data points were determined from the average of three samples. Straight line corresponds to the best fit described by y = 1.138 × 10⁶ + 538 161; (r² = 0.997).</figcaption></figure>

provide the error bars shown. From the slope (S) and variance (s) of the linear regression obtained, we determine the limit of detection (LOD) and the limit of quantification (LOQ) to be 0.17 and 0.51 ppm, respectively, according to Equation (1) and (2), where s and S are the variance and slope of the linear fit to the calibration curve, respectively.

$$ LOD = 3.3 \frac{\sigma}{S} \qquad (1) $$

$$ LOQ = 10 \frac{\sigma}{S} \qquad (2) $$

A specific advantage of using SPME to extract analytes of interest from a complex matrix is the inherent adsorption selectivity of the material used in the microfiber. This allows concentration of the species of interest with respect to both its solution- or gas-phase environment, but also toward other species present. This is particularly useful for avoiding interference by metal ions or electrolytes that may be present in the solution, including those transition or rare-earth metal ions whose hydroxide salts may precipitate under basic aqueous conditions.

The results for the analysis of aqueous solutions containing 1000 ppm of ammonia and various metal ions are shown in Figure 5. As it can be seen, the method is insensitive towards the presence of Fe²⁺, Ni²⁺, Ru²⁺, Mn²⁺, or Zn²⁺ at 3.3 mM concentrations. Similarly, no interference was found from the presence of 1000 ppm of either hydrazine, acetamide, or urea which are plausible side-products formed during ammonia production. Hydrazine could be detected from hydrazine reference solutions, establishing that the SPME fiber can extract this species, although less efficiently than ammonia. Finally, the presence of 10% methanol, tetrahydrofuran (THF), or acetonitrile in the aqueous solutions did not interfere with the analyzes. These results suggest that the technique may be directly used to quantify ammonia in the crude reaction mixtures used for photo- or electrochemical production of NH₃, although it is important to note that its validity must be verified under the specific conditions used.

In studies involving the electrochemical or catalytic production of ammonia, it can be of interest to unambiguously assign the formation of small amounts of NH₃ to specific reaction processes using isotopically labeled starting materials such as ¹⁵N₂. This can, for example, exclude the release of NH₃ from nitrogen-containing ligands or ambient contaminants. Obviously, colorimetric or IC cannot distinguish between different nitrogen isotopes. Thus, the identification of ¹⁵NH₃ is currently achieved using NMR techniques, but these require significant sample amounts and the use of high-field NMR spectrometers.[21,43,44] In principle, MS detection can readily discriminate between different isotopic contributions. To verify this, we proceeded to analyze an aqueous solution of ¹⁵NH₃. Analysis of the chromatogram was conducted at m/z = 18 although this results in an important peak for water being detected (Figure 6).

Beyond the direct detection of NH₃, the use of coupled GC-MS detection allows the separation of various volatile amine derivatives that may be either formed during a reaction, liberated from ligands or from sacrificial donors. As a simple test, we proceeded to analyze solutions containing methylamine or ethylamine in aqueous media over a concentration range of 1–1000 ppm. The same protocol was used as for the detection of ammonia but using the signal at m/z = 30 that is specific for these amines.

<figure><img src="image_10.png" /><figcaption>Figure 5. SPME-GC-MS quantification of NH₃ (1000 ppm) in aqueous solutions in the presence of various transition metal ions (3.3 mM), nitrogen compounds (1000 ppm), or cosolvents (10% v/v).</figcaption></figure>
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 6.** A) GC-MS signal obtained from a 1000 ppm aqueous ¹⁵NH₃ solution using a PDMS/DVB SPME fiber. B) MS chromatogram at *t* = 4.5 min.</figcaption></figure>

Under the conditions used for separation of NH₃, these compounds can also be identified from their retention times (5.8 vs. 7.0 min for MeNH₂ and EtNH₂, respectively). Satisfactory linear correlations are obtained for both compounds (Figure 7) which can be used to estimate the corresponding LOD and LOQ (Table 1).

The LOD and LOQ shown in Table 1 are of course dependent on the sensitivity of the instrument used and were roughly optimized with respect to the residual signal level of the MS detection. Along with the effect of pH and the possible interference of organic or inorganic species present in the aqueous matrix, we note an additional limitation of the technique related to the longevity of the SPME fiber. With repeated daily use, the polymer coating of the SPME begins to degrade after about one month. This results in a reduced ability of the fiber to adsorb ammonia or other amines. Prior to any analysis, we recommend analyzing a reference sample with a known ammonia concentration. If there is a significant discrepancy between the reference sample and the calibration curve, a new fiber should be used and/or new calibration curves should be generated to ensure the accuracy of the analyzes.

<figure><img src="image_13.png" /><figcaption>**Figure 7.** Calibration curve for the quantification of ethylamine (blue triangles) and methylamine (black squares) in aqueous solutions from 10 to 1000 ppm using a PDMS/DVB SPME fiber. Straight line corresponds to the best fit described by $y = 7773.8[MeNH_2] - 173689 (r^2 = 0.9911)$ or $y = 28459[EtNH_2] - 630516 (r^2 = 0.9952)$.</figcaption></figure>

<table><caption>**Table 1.** Detection (LOD) and quantification (LOQ) limits for various amines in aqueous media using SPME-GC-MS.</caption><thead><tr><th>Amine</th><th>LOD [ppm]<sup>a)</sup></th><th>LOQ [ppm]<sup>a)</sup></th></tr></thead><tbody><tr><td>Ammonia</td><td>0.166</td><td>0.505</td></tr><tr><td>Methylamine</td><td>0.020</td><td>0.066</td></tr><tr><td>Ethylamine</td><td>0.055</td><td>0.166</td></tr></tbody></table>

## 4. Conclusion

Our results show that the use of an SPME fiber allows the use of a routine GC-MS instrument to reliably quantify the concentration of ammonia in aqueous solutions. The technique is relatively fast (≈30 min per analysis) and easily automated using commercially-available SPME injectors. Furthermore, the use of a fiber to extract the ammonia from the sample renders the method robust toward interference by the presence of various transition metal ions or organic cosolvents that may be used for catalytic ammonia production. Thanks to chromatographic separation and MS detection, different amine derivatives as well as isotopically labeled ¹⁵NH₃ can be differentiated and quantified. In our hands, a routine benchtop GC-MS system readily afforded sub-ppm quantification limits which can likely be improved using more advanced instrumentation.

## Acknowledgements

This work received financial support from the French government in the framework of the University of Bordeaux's France 2030 program/GPR "Post-Petroleum Materials". The University of Bordeaux and the CNRS are also gratefully acknowledged for their financial support. The authors are also grateful to Dr. Christelle Absalon (Univ. Bordeaux) for helpful discussions on the use of SPME techniques and to the CESAMO analytical facilities of the Univ. of Bordeaux for their assistance.

<footnote>a) Determined in aqueous solutions at pH > 10.4 using Equation (1) and (2).</footnote>
<figure><img src="image_14.png" /></figure>

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** ammonia detection · catalysis · nitrogen reduction · solid-phase microextraction

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Manuscript received: December 23, 2024

Revised manuscript received: April 18, 2025

Version of record online: