Results and discussion
Column selection
A narrow-bore (0.18 mm i.d.), thick-film column coated with DB-1301 (equivalent to G43) was selected. The low phase ratio (β = 22) resulted in better resolution for the first-eluting (but frequently detected) solvents, i.e., methanol, ethanol, diethylether, acetone, isopropyl alcohol, and acetonitrile, while the low internal diameter resulted in an analysis time below 30 min.
No stationary phase is able to separate all 60 solvents in approximately 30 min. In most practical cases, however, only 3–5 solvents need to be determined. The method described allows the analysis of 56 solvents, and eventually the same column can also be utilized for the analysis of ethylene oxide or other solvents using liquid injection. For this reason, the column selection and separation conditions here can be considered generic.
Figure 2 - Fifty-six-component solvent mixture analyzed by SHS-GC-FID-MS. a) Total ion chromatogram (TIC) from GC-scan-MS, b) TIC from GC-selected ion monitoring (SIM)-MS, c) FID trace.
The separation obtained by SHS-GCFID- MS for a solvent test mixture in DMSO:water is shown in Figure 2. The concentration of the solutes was 16 ppm for the five class 1 solvents and 560 ppm for the class 2 and 3 solvents. With one injection, three data files were obtained. The upper trace shows the TIC obtained by MSD in scan mode, the middle trace shows the corresponding SIM data, and the lower chromatogram is the FID trace. Toluene eluted at 16.16 min in all three chromatograms, and the offset in retention time between FID and MS was smaller than 0.03 min or 2 sec for all solutes.
Figure 3 - Detailed FID chromatogram (16 ppm class 1 and 560 ppm class 2 and 3 solvents). Peak identification: see Table 2 (*: octane).
A detailed view of the separation (Figure 3) shows three elution windows of the FID trace. All solutes are labeled on the chromatograms.
Validation
Limits of detection, linearity, and repeatability were evaluated with the different modes of detection. Repeatability was checked at the 3-ppm level (μg/g, equivalent to 0.15 μg/mL in vial) for class 1 solvents and 100 ppm for the others (n = 6). Five-point calibration curves (plus a blank) were measured between 0.15 and 15 ppm for class 1 solvents, and between 6 and 600 ppm for most others. For some solutes giving low response, linearity was tested in the 500–5000 ppm range. For coeluting compounds, the experiments were performed with single-compound solutions in order to allow peak integration with FID. The results are summarized in Table 2.
Table 2 - Validation results for 56 solutes
Column 4 in Table 2 indicates possible coelution of target compounds. Most solutes were chromatographically resolved and could thus be quantified by either FID or MS when present in the same sample. In some cases, coelution was observed (labeled “C”). In these cases, quantification was still possible by MS after ion extraction or in SIM mode.
Figure 4 - Comparison of overlapped peaks in FID and individual SIM ion chromatograms for 1,1,1-trichloroethane (m/e = 97) and cyclohexane (m/e = 56).
Figure 5 - Comparison of overlapped peaks in FID and individual SIM ion chromatograms for benzene (m/e = 78); dimethoxyethane (m/e = 45); and 1,2-dichloroethane (m/e = 62).
In columns 5–8, the LODs (determined from the lowest calibration level at S/N = 3) for the three detection modes are compared to the ICH limits. In most cases (43 of the 56 analytes), the LODs were well below the ICH limits for all three detection modes. For the class 1 solvents, however, it is clear that MS in SIM mode is preferred. This is also illustrated in Figure 4 for some class 1 solvents. At 10.6 min, 1,1,1-trichloroethane coeluted with cyclohexane, as seen in the FID trace. However, both solutes can be accurately measured with extracted ion chromatograms using m/e = 97 for 1,1,1-trichloroethane and m/e = 56 for cyclohexane. The same can be observed in Figure 5 for the overlapped peaks of benzene; 1,2-dimethoxyethane and 1,2-dichloroethane at 11.5 min in the FID trace and the extracted ion peaks of m/e = 78 for benzene; m/e 45 for 1,2-dimethoxyethane; and m/e = 62 for 1,2-dichloroethane (2-methoxyethanol was not detected at this level).
Using MS, all compounds, except 2-methoxyethanol, can be detected at LOD under the ICH limit. Some of the other more polar solutes (2-ethoxyethanol, DMF, DMAC, and 1-methyl-2- pyrrolidone) also gave a low response in FID, whereas MS detection limits were satisfactory.
The LOD and slope of the calibration curve, however, depend on the solvent used. In general, for apolar solvents in the most polar matrix (water), the lowest LODs will be obtained. For the (apolar) class 1 solvents, the LOD can be up to 10 lower (more sensitive) if static headspace is performed in DMSO:water versus pure DMSO.
Repeatability of the SHS-GC-FIDMS method was excellent. RSDs were mostly below 5%, both for GC-FID and GC-MS (SIM and scan modes). The average RSDs were 4.4% for scan, 3.8% for SIM, and 3.0% for FID. Again, the values were higher for some more polar solutes. Finally, good linearity was obtained for most compounds. Except for 2-methoxyethanol, 2-ethoxyethanol, DMF, DMAC, and 1-methyl-2-pyrrolidone (compounds with higher LODs), linearity was excellent with the three types of detection (R² > 0.99).
Application
Figure 6 - Analysis of levamisol sample analyzed by SGS-GC-FID-MS.
As an illustration, SHS-GC-FID-MS analysis was performed on a sample of levamisol (tetramisole). The FID chromatogram (a 100-mg sample was dissolved in 2 mL DMSO:water, 1:1) is shown in Figure 6. In the sample, a trace amount of toluene was detected. The concentration was 66 ppm, well below the 890-ppm ICH limit. It is interesting to note that a trace amount of dimethyl sulfide (DMS) was also detected. DMS is an impurity in the DMSO solvent used to dissolve the samples. In addition, an “unknown” was detected at 4.5 min. Through MS spectral library searching, the peak was identified as 2-chloropropane. This solvent is not included in the ICH solvent list. However, the ability to unequivocally identify this unknown in the sample clearly demonstrates the advantage of using parallel MS detection.
Conclusion
More than 50 residual solvents can be determined in pharmaceutical products in a single run using an SHS-GC-FID-MS configuration. Quantification can be performed routinely by FID for most target compounds, while MS is especially suited for the trace-level determination of class 1 solvents and for the identification of unknowns. Mass spectrometry also excels for the determination of coeluting peaks through the use of extracted ion or SIM ion chromatograms, thereby eliminating the need for additional analyses on dissimilar columns. The quantitative data, including repeatability, linearity, and LOD, are excellent, meeting or exceeding ICH guidelines.
The retention-time locked method presented can be considered generic, since it covers most solvents and can be used both for identification (by MS) and for quantification (by FID and/or MS) of residual solvents across a wide concentration range.
References
- ICH Harmonised Tripartite Guideline, Q3C (R3); www.ich.org/LOB/media/MEDIA423.pdf.
- USP Method 467, U.S. Pharmacopoeia, updated June 2007, USP 32—NF18.
- Firor, R.L. The determination of residual solvents in pharmaceuticals using the Agilent G1888 Network Headspace Sampler; Agilent Application Note, publ. no. 5989–1263EN, 2004.
- Gudat, A.E.; Firor, R.L.; Bober, U. Better precision, sensitivity, and higher sample throughput for the analysis of residual solvents in pharmaceuticals using the Agilent 7890A GC system with G1888 headspace sampler in drug quality control; Agilent Application Note, publ. no. 5989– 6023EN, 2007.
- Firor, R.L.; Gudat, A.E. The determination of residual solvents in pharmaceuticals using the Agilent G1888 HS/6890GC/5975 inert MSD system; Agilent Application Note; publ. no. 5989– 3196EN, 2005.
- Gudat, A.E.; Firor, R.L. The determination of extractables and leachables in pharmaceutical packaging materials using headspace/GC/MS; Agilent Application Note, publ. no. 5989–5494EN, 2006.
Dr. David is R&D Director, Ms. Jacq is a GC expert, and Prof. Sandra is Director, Research Institute for Chromatography, Pres. Kennedypark 26, B-8500 Kortrijk, Belgium; tel.: +32 56 20 40 31; fax: +32 56 20 48 59; e-mail: frank.david@richrom.com. Dr. Klee is New Technology Program Manager, Agilent Technologies, Wilmington, DE, U.S.A.