High-Throughput Headspace Analysis of Volatile Impurities in Consumer Products Using Standard Additions

The method of standard additions (MoSA) enables quantification of volatile impurities in condensed-phase samples, such as emulsions, for which matrix-matched calibration standards are required. The MoSA technique is, however, expensive because it requires multiple analyses of each sample. Application of direct-injection mass spectrometry (MS)—in the form of selected ion flow tube MS (SIFT-MS)—transforms MoSA into a more cost-effective analytical approach because sample throughputs are higher, diverse functionalities can be analyzed simultaneously, and one instrument configuration covers a wide variety of analytes and matrices. This article describes MoSA-SIFT-MS analysis of several toxic volatile impurities (benzene, 1,4-dioxane, and formaldehyde) in a variety of consumer products and the implications for workflows.

Volatile organic compounds (VOCs) can be present in consumer products as active ingredients, fragrance components, carrier solvents, or impurities. Those that are known to be harmful to humans are subject to regulation, such as for cosmetic products under regulation number 1223/2009 of the European Parliament (1).

Many cosmetic formulations are emulsions, resulting in complex headspace partitioning behavior. This means that static headspace analysis (SHA) cannot be used for quantitative analysis of potentially harmful impurities because simple solvent-based calibration standards do not represent partitioning well (2). Hence the method of standard additions (MoSA), which involves preparing calibration standards in the sample matrix (2), must be used for headspace-based quantitation. The disadvantage of MoSA is that it requires multiple analyses to quantify the volatiles in each sample and hence is expensive for conventional gas chromatography mass spectrometry (GC–MS) methods in which the chromatography is usually the throughput-limiting factor. Only multiple-point standard curves are considered here, following Kolb and Ettre (2).

Utilization of selected ion flow tube mass spectrometry (SIFT-MS) as an alternative to GC–MS can significantly improve the analytical workflow for MoSA due to a reduction in runtime (less than 5 min), the ability to analyze diverse chemical functionalities simultaneously, and high calibration stability. This article provides a brief overview of the benefits of applying MoSA with automated SIFT-MS instruments to a variety of consumer products.

Experimental

The SIFT-MS technique is described in detail elsewhere (3). Briefly, rapidly switchable reagent ions (H₃O⁺, NO⁺, and O₂^+•) are utilized for gas-phase soft chemical ionization of VOCs in air and headspace. Real-time, high-throughput analysis is achieved because SIFT-MS is chromatography-free. Specificity is achieved through the combination of multiple ionization mechanisms and mass spectrometric detection. Commercial SIFT-MS instruments were used in the studies reviewed here (either Voice200ultra or Syft Tracer models; Syft Technologies, Christchurch, New Zealand).

Automated headspace-SIFT-MS analysis is best achieved using autosamplers based on syringe injection (4). In this work, automated MoSA analysis was carried out using a SIFT-MS instrument coupled with a multipurpose autosampler (MPS Robotic Pro, GERSTEL, Mülheim, Germany). Details of analysis conditions are given in the cited literature. In general, headspace analysis is conducted using 20-mL headspace vials, with sample extracted using a 2.5-mL headspace syringe, followed by steady injection (usually at a rate of 50 μL/s) into a flow of nitrogen or zero-air make-up gas (ca. 10-fold dilution in the sample inlet). Figure 1 shows example headspace-SIFT-MS data from a MoSA study. Optimized sample scheduling is achieved by the Maestro software package (GERSTEL).

Preparation of calibration standards in the sample matrix can be achieved in two ways:

• Pre-spiking of samples: Here, each calibration level is spiked into a different sample vial. This is the most robust approach, because it minimizes headspace perturbation. Furthermore, a single-head robotic autosampler can be utilized, and the sample and calibrations run back-to-back with high throughput analogous to SHA-SIFT-MS (5). However, it requires a larger quantity of each sample and a little more manual sample preparation. This approach was used unless otherwise stated.
• Just-in-time sample spiking (JITS): This approach involves automated preparation and analysis from a single vial containing the sample, followed by several incubate-pressurize-analyze-spike cycles to create the calibration curve. A dual-head autosampler (one headspace and one liquid injection) or an autosampler with automated tool change are the most efficient technologies for JITS. Although the JITS approach requires less sample and fewer vials, it results in reduced sensitivity because there is a reduction in headspace concentration from sample to sample. Automated analysis is also less efficient because calibrations are spiked sequentially, in contrast with parallel analysis when using pre-spiking.

MoSA SIFT-MS Provides Higher Sample Throughputs

Faster SIFT-MS runtimes compared to chromatographic methods enable sample throughputs to be increased for two reasons. First, the shorter runtime results in faster progress through the standard additions. This is illustrated in Figure 2(a) for analysis of formaldehyde in a fragrance matrix using the sequentially spiked JITS approach (4,5). Second, advanced scheduling software enables multiple samples to be run in parallel because the pressurize-analyze-spike cycles for several samples can be accommodated within the incubation time for SIFT-MS analysis (Figure 2[b]). Within the 20-min incubation period shown, three additional samples were analyzed at a cost of only an additional 16 min, equating to a 2.9-fold throughput increase and at least 1.8-fold faster reporting of the first result compared to chromatographic approaches (5). The MoSA approach yielded formaldehyde concentrations of 65 µg/L, 87 µg/L, and 7 µg/L in three fragrance samples (4,5).

MoSA-SIFT-MS is Applicable to Diverse Matrices

Silva and Langford have demonstrated that the MoSA-SIFT-MS approach is applicable to diverse personal care product (PCP) matrices (6). In contrast with the pioneering work described in the previous section, which used standard additions to overcome challenges in quantifying formaldehyde in the presence of solvent levels that were above the instrument linear range (7), MoSA was essential because of the emulsion-based formulations (one syrup and two lotions). For these matrices, and a tablet, standard curves for analysis of carcinogenic benzene impurity are shown in Figure 3. Benzene impurities were quantified with good repeatability from ca. 20 to 600 ppb (in product) across the four commercial PCPs (6).

MoSA-SIFT-MS Can Accommodate Diverse Functionalities Simultaneously

Perkins and colleagues (8) have demonstrated that the very diverse volatile impurities—benzene, 1,4-dioxane, and formaldehyde—can be analyzed in the same MoSA-SIFT-MS run. Results for nine PCPs (hair and skincare retail categories) are summarized in Table 1. Benzene was quantified above the low-ng/g limit of quantitation (LOQ) in three products, dioxane above the sub-μg/g LOQ in all products, and formaldehyde above the low-μg/g LOQ in two products, providing quantitative analysis at concentrations relevant to consumer safety. Table 1 also illustrates the real-time specificity provided by SIFT-MS in that for benzene and 1,4-dioxane, four and five independent reagent ion-product ion pairs, respectively, can be utilized (ion codes: H = H₃O⁺, N = NO⁺, and O = O₂^+•; the number represents the product ion m/z).

MoSA-SIFT-MS Calibration: Further Enhanced Workflows

By analogy with the multiple headspace extraction approach (MHE) (9), Perkins and colleagues (8) have shown that “MoSA calibration” is feasible. Here, for each sample matrix, full MoSA on triplicate samples is recommended for determination of the MoSA calibration curve. Thereafter, just the sample can be used (SHA) and the prior calibration applied to the SHA result to give the concentration in the sample. Depending on the stability of the analyte and matrix, which should be determined in method development (8), this “MoSA calibration” may be stable for days to weeks, facilitating rapid quantitation of volatile impurities in the specific matrix. This is illustrated in Figure 4 using sequence for (a) GC–MS (30-min runtime) and (b) SIFT-MS methods that apply full MoSA to each sample, plus (c) temporal separation of “MoSA calibration” for SIFT-MS. In the case of the method in (8), the rapid analysis provided by SIFT-MS (Figure 1) can increase throughput 8- to 30-fold compared to conventional GC–MS analysis, while cutting (by at least two thirds) the time to report the first result. In addition to the economic advantages of delivering faster results to customers and generating more revenue through higher throughput, reduced calibration demand provides environmental advantages through reduced solvent usage.

Conclusion

Sample preparation using MoSA is readily implemented with SIFT-MS, enabling quantitative analysis of volatile impurities in emulsions and other matrices for which simple solvent calibration is not feasible. Compared to the conventional GC application of MoSA, SIFT-MS has advantages in terms of runtime, its ability to run multiple samples in parallel, and its ability to analyze diverse functionalities (benzene and formaldehyde) simultaneously. Combined, these serve to significantly reduce the time to report results as well as increasing the throughput compared to GC–MS. Additionally, the durability of “MOSA calibration” (dependent on analyte and matrix stability) with SIFT-MS, combined with the ability of SIFT-MS to quantify diverse analytes from one configuration, means that calibrations may be conducted weekly or monthly, further improving workflows (Figure 5[c]).

References

(1) European Parliament. Regulation (EC) No. 1223/2009 of the European Parliament and of the Council of 30 November 2009 on Cosmetic Products. European Parliament, 2019. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32009R1223 (accessed 2024-08-01).

(2) Kolb, B.; Ettre, L. S. Static Headspace-Gas Chromatography – Theory and Practice (2nd ed.); John Wiley & Sons: New York, 2006.

(3) Smith, D.; Španě, P.; Demarais, N.; Langford, V. S.; McEwan, M.J. Recent Developments and Applications of [SIFT-MS]. Mass Spec. Rev. 2023, e21835. DOI: 10.1002/mas.21835

(4) Perkins, M. J.; Langford, V. S. Application of Routine Analysis Procedures to a Direct Mass Spectrometry Technique: [SIFT-MS]. Rev. Sep. Sci. 2021, 3 (2), e22001. DOI: 10.17145/rss.21.003

(5) Perkins, M. J.; Langford, V. S. High-Throughput SIFT-MS Analysis of Formaldehyde in Fragrances Using the Method of Standard Additions. Syft Technologies application note. 2023. https://bit.ly/3Y9a0Ka (accessed 2024-08-01)

(6) Silva, L.; Langford, V. High-Throughput, Quantitative Analysis of Benzene in Personal Care Products Using Headspace-SIFT-MS. Syft Technologies application note. 2022. https://bit.ly/3Xv3Cew (accessed 2024-08-01)

(7) Perkins, M. J.; Silva, L. P.; Langford, V.S. Evaluation of Solvent Compatibilities for Headspace-SIFT-MS Analysis of Pharmaceutical Products. Analytica 2023, 4, 313–335. DOI: 10.3390/analytica4030024

(8) Perkins, M. J.; Hastie, C. J.; Langford, V. S. Headspace-Selected Ion Flow Tube Mass Spectrometry Workflows for Rapid Screening and Quantitation of Hazardous Volatile Impurities in Personal Care Products. Analytica 2024, 5, 153–169. DOI: 10.3390/analytica5020010

(9) Langford, V. S.; Perkins, M. J. A Fast, Simplified Approach to Quantitative Headspace Analysis of Volatile Impurities in Drug and Packaging roducts. The Column 2023, 19 (9), 2−8. https://www.chromatographyonline.com/view/fast-simplified-quantitative-headspace-analysis-volatile-impurities-drug-packaging-products (accessed 2024-08-01)

About the Authors

Vaughan S. Langford is a Principal Scientist at Syft Technologies in New Zealand. He joined Syft in late 2002 after completing his PhD in Physical Chemistry at the University of Canterbury, and post-doctoral fellowships at the Universities of Geneva, Western Australia, and Canterbury. He has 42 peer-reviewed publications (including nine book chapters or reviews) on a wide range of SIFT-MS applications, plus has contributed articles for industry publications and numerous conference papers. Direct correspondence to: vaughan.langford@syft.com

Leslie Silva is Applications Team Lead at Syft Technologies, Inc. in the United States. She joined Syft in 2020 after completing her PhD in biochemistry and molecular biology at the University of Calgary, a postdoctoral fellowship at MD Anderson Cancer Center, and research positions at Lawrence Berkeley National Laboratory and a food & beverage start-up, Endless West. She has 23 peer-reviewed publications and numerous patents, application notes, and conference papers. Direct correspondence to: leslie.silva@syft.com

Mark Perkins is a senior applications chemist and SIFT-MS expert at Element Lab Solutions (formerly Anatune Limited), based in Cambridge, United Kingdom. Mark graduated from the University of Southampton, UK, with a PhD in electrochemistry. He was with the Malaysian Rubber Board's UK research center for 12 years, first as a senior analyst, then as head of the analytical section. He joined Anatune/Element in early 2015 in a role that supports and expands the analytical capability of SIFT-MS–with a particular focus on autosampler integration and the development of automated test methods. Direct correspondence to: mark.perkins@element.com