GC×GC–TOF MS with Select-eV Variable-Energy Electron Ionization for the Characterization of Allergens in Cosmetics

An EU Directive listing a total of 27 allergenic compounds that have to be avoided in fragrances was released in 2003. The Directive urged the labeling of allergens at quantities > 10ppm for “leave-on” products such as perfumes and >100ppm for “wash-off” products such as shower gels. Therefore, precise identification and quantification of allergens are essential to meet the standards of the Directive.

Allergens present in complex cosmetics extracts can be studied using two-dimensional gas chromatographic technique with time-of-flight mass spectrometry (GC*GC-TOF MS).

The combination of two columns with different stationary phase facilitates separation of allergens, avoiding the need for erroneous sample preparation procedures. However, it is difficult to examine individual compounds in complex cosmetic samples containing multiple compounds that have weak molecular ions or similar spectra. Using soft ionization technique to minimize ion fragmentation can overcome this problem, but is a laborious process.

This article discusses the characterization of allergens in cosmetics using GC×GC–TOF MS with Select-eV variable-energy electron ionization.

Features of BenchTOF Instruments

BenchTOF time-of-flight mass spectrometers from Markes International are best suited for the two-dimensional gas chromatographic analysis of complex cosmetics samples due to their unique characteristics:

  • Speed – BenchTOF manipulates narrow peaks generated in GCxGC couplings due to its ability to acquire full-range mass spectral data to very high densities. Maximum information on weak, matrix masked signals can be obtained corresponding to high stored-to-disk data rate through sophisticated data-mining and spectral deconvolution algorithms.
  • Spectral quality – The ‘reference-quality’ spectra generated by BenchTOF offers rapid and accurate matching of unknown and target, and complies with that in Wiley or NIST libraries.
  • Sensitivity - Unlike quadrupole instrument, BenchTOF provides full-range spectra to identify trace-level analytes in a single run.

Experimental Methods

Using acetone at concentration levels of 0.2 to 10ppm, a set of calibration standards consisting of allergenic compounds was prepared in addition to two cosmetic extracts – one for perfume and the other for cream. The experimental conditions used are as follows:

GC:

Instrument Agilent 7890A
Injector Split/splitless injector
Liner 4.0mm i.d. liner, 1µL injection
Carrier gas He, constant flow at 1.0mL/min
Mode Split
Inlet temperature 230°C
Septum purge On, 3mL/min

2D column set:

1st dimension SGE SolGelWax, 30m × 0.25mm × 0.25µm
2nd dimension DB1, 2.6m × 0.1mm × 0.1µm
Modulation loop As for 2nd dimension
Column set Equivalent pneumatic impedance to 35m × 0.18mm (calculated from K factor look-up charts for 1st- and 2nddimension columns used)

Temperature program:

Main oven 40°C (1.0 min), 6°C/min to 150°C, 4°C/min to 250°C (20 min)
Secondary oven No offset
Hot jet 140°C (1.0 min), 6°C/min to 210°C, 4°C/min to 250°C (hold time matched to total run time)
Cold jet Dewar fill: high, 45%; low, 35%
Modulation period 2.5s, hot-jet pulse 350ms
Total run time 64min

TOF MS:

Instrument BenchTOF-Select (Markes International)
Filament voltage 1.8V
Ion source 250°C
Transfer line 280°C
Mass range m/z 40–600
Data rate 50 Hz with 200 spectra per data point

GC Image™ software was used for image processing.

Experimental Results

Separation and Identification of Compounds

A calibration set with four standard solutions was prepared with respect to the described conditions. All 29 compounds were separated using the reverse-phase column set.

BenchTOF GC time-of-flight mass spectrometers can provide ‘classical’ EI spectra without mass discrimination. Comparisons of allergens spectra were performed and are shown in the Figure 1. Table 1 summarizes the library match results and retention times of all 29 target compounds.

Figure 1. Comparison of the deconvolved spectra for four target analytes with respect to the NIST 2011 library spectra (bottom, blue).

Table 1. Summary of results for all target allergens in the 10ppm calibration standard, compared with the NIST library.

No. Compound RT 1 (min) RT 2 (s) Forward match
1 Limonene 13.08 1.00 925
2 Linalool 20.63 0.52 915
3 Methyl 2-octynoate 23.08 0.54 934
4 β-Citral 23.92 0.60 892
5 α-Citral 25.00 0.60 896
6 1,2-Dibromobenzene 25.04 0.54 913
7 Citronellol 25.42 0.54 914
8 Geraniol 27.08 0.50 925
9 α-Isomethyl ionone 27.79 0.92 931
10 Benzyl alcohol 28.04 0.34 932
11 Safrole 28.25 0.54 917
12 Hydroxycitronellal 29.50 0.48 897
13 Methyl eugenol 31.13 0.56 895
14 Cinnamaldehyde 32.00 0.44 937
15 Lilial 32.13 0.72 889
16 Majantol 33.75 0.52 894
17 Eugenol 34.58 0.48 914
18 Amylcinnamaldehyde 36.71 0.72 906
19 p-Anisyl alcohol 36.88 0.42 914
20 Cinnamyl alcohol 37.00 0.42 891
21 Farnesol (isomer 1) 37.63 0.74 937
22 Farnesol (isomer 2) 37.88 0.72 916
23 Isoeugenol 38.42 0.48 911
24 Farnesol (isomer 3) 38.46 0.74 915
25 Hexylcinnamaldehyde 38.83 0.78 876
26 Coumarin 40.88 0.48 900
27 Lyral (isomer 1) 40.92 0.60 894
28 Lyral (isomer 2) 41.17 0.60 832
29 Amylcinnamic alcohol 42.17 0.60 865

Target compound quantitation

The quantitation of BenchTOF- Select was carried out using seven target compounds such as limonene, two isomers of lyral, geraniol, two isomers of citral and cinnamyl alcohol.

Using GC Image data analysis software, a qualifier expressions template was created to allow continuous identification of allergens and obtain fast results. Following this, allergens present in the samples were quantified using the calibration curves of seven target analytes (Figure 2). The contour plots of cream and perfume extracts were provided in Figure 3.

Figure 2. Calibration curves of seven target allergens

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Figure 3. Quantified allergens in the cream (A) and perfume (B) extracts with values given in ppm

Robust sample preparation method ensures cosmetic extract samples with low background matrix. GC×GC–TOF MS technique delivers a high degree of chromatographic resolution required for the separation of target compounds from potential interferences. For instance, separation using traditional one-dimensional techniques may lead to co-elution of citronellol with geranyl acetate in perfume sample. Figure 4 shows mass spectra of separated citronellol and geranyl acetate compounds from perfume extract.

Figure 4. Corresponding mass spectra of separated citronellol and geranyl acetate compounds from perfume extract, using one-dimensional GC system

Variable-Energy Electron Ionization

Identical mass spectra of isomeric compounds at 70eV ensures accurate identification of individual isomers. However, it is possible to produce low energy EI spectra using Select-eV technology, obtaining extra information useful for distinguishing isomers. Low energy mass spectra of farnesol isomers are demonstrated in Figure 5.

Figure 5. Separation of farnesol isomers showing comparison of mass spectra obtained at ionisation energies of 70eV (top) and 15eV (bottom).

In GC×GC–TOF MS analyses, the selectivity and sensitivity delivered by low-energy electron ionization supports the highly structured separation space held by complex samples.

have high sensitivity, but can achieve even lower detection limits when coupled to Select-eV technology in the analysis of allergens. Comparison of 70eV and 15eV spectra of safrole and coumarin are shown in Figure 6.

Figure 6. Comparison of mass spectra obtained at ionization energies of 70eV (top) and 15eV (bottom) for safrole and coumarin

Reduced fragmentation at 15eV and stronger molecular ion indicates improved signal-to-noise ratios, lower detection limits and better selectivity of the system. At low voltages, chromatographic background escapes ionization thereby further improving signal-to-noise and minimizing noise.

Conclusion

This article demonstrated the spectral quality, chromatographic resolving power and sensitivity of GC×GC–TOF MS, required for quantitative and qualitative analysis of allergens and other compounds present in cosmetic extract samples.

The generation of complementary spectra for improved compound identification, using Select-eV is also explained, in addition to the methods for enhancing the sensitivity and selectivity of the system.

About Markes International Limited

Markes International is a specialist manufacturer of analytical thermal desorption instrumentation and associated sampling equipment. The company's comprehensive portfolio of products is designed to automate and enhance the measurement of trace level volatile and semi-volatile organic chemicals (VOCs and SVOCs) in real-world samples.

For over a decade, Markes International has pioneered analytical thermal desorption (TD) instrumentation and associated sampling equipment. Markes has also the widest range of thermal desorption consumable products on the market; the company is widely regarded as being the World leader in thermal desorption technology.

This information has been sourced, reviewed and adapted from materials provided by Markes International Limited.

For more information on this source, please visit Markes International Limited.

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