Comprehensive 2-dimensional gas chromatography fast quadrupole mass spectrometry (GC × GC-qMS) for urinary steroid profiling: mass spectral characteristics with chemical ionization

Abstract

Comprehensive 2-dimensional gas chromatography (GC × GC), coupled to either a time of flight mass spectrometry (TOF-MS) or a fast scanning quadrupole MS (qMS) has greatly increased the peak capacity and separation space compared to conventional GC-MS. However, commercial GC × GC-TOFMS systems are not equipped with chemical ionization (CI) and do not provide dominant molecular ions or enable single ion monitoring for maximal sensitivity. A GC × GC-qMS in mass scanning mode was investigated with electron ionization (EI) and positive CI (PCI), using CH(4) and NH(3) as reagent gases. Compared to EI, PCI-NH(3) produced more abundant molecular ions and high mass, structure-specific ions for steroid acetates. Chromatography in two dimensions was optimized with a mixture of 12 endogenous and 3 standard acetylated steroids (SM15-AC) relevant to doping control. Eleven endogenous target steroid acetates were identified in normal urine based on their two retention times, and EI and PCI-NH(3) mass spectra; nine of these endogenous target steroid acetates were identified in congenital adrenal hyperplasia (CAH) patients. The difference between the urinary steroids profiles of normal individuals and those from CAH patients can easily be visually distinguished by their GC × GC-qMS chromatograms. We focus here on the comparison and interpretation of the various mass spectra of the targeted endogenous steroids. PCI-NH(3) mass spectra were most useful for unambiguous molecular weight determination and for establishing the number of -OH by the losses of one or more acetate groups. We conclude that PCI-NH(3) with GC × GC-qMS provides improved peak capacity and pseudomolecular ions with structural specificity.

Figure 1

Chemical structures of the SM15 native steroids.

Figure 2

GC×GC-qMS total ion chromatogram (TIC) of a typical normal urine extract using (a) EI and (b) PCI-NH_3.

Figure 3

GC×GC-qMS TIC of (a) EI and (b) PCI-NH₃ in a typical CAH patient urine extract, and the PCI-NH₃ (c) extracted ion chromatogram (EIC, m/z = 259, 276, 336).

Figure 4

EpiT-AC, T-AC (in normal urine extract), and DHT-AC (in SM15-AC standard) mass spectra acquired using (a, d, g) EI (mass range m/z 50–390 amu), (b, e, h) PCI with methane reagent gas (PCI-CH₄) (mass range m/z 70–420 amu), and (c, f, i) PCI with ammonia reagent gas (PCI-NH₃) (mass range m/z 220–440 amu). PCI-NH₃ results in a very strong relative intensity for the MW + 18 ([M + NH₄]⁺) or MH⁺ mass ion, providing unambiguous assignment of molecular weight identification.

Figure 5

E-AC, A-AC (in normal urine extract), and EpiA-AC (in SM15-AC standard) mass spectra, acquired using (a, d, g) EI (mass range m/z 50–390 amu), (b, e, h) PCI-CH₄ (mass range m/z 70–420 amu), and (c, f, i) PCI-NH₃ (mass range m/z 220–440 amu).

Figure 6

19NE-AC (in SM15-AC standard), DHEA-AC and 11KE-AC (in normal urine extract) mass spectra acquired using (a, d, g) EI(mass range m/z 50–390 amu), (b, e, h) PCI-CH₄ (mass range m/z 70–420 amu), and (c, f, i) PCI-NH₃ (mass range m/z 220–440 amu).

Figure 7

5αA-DiAC, 5βA-DiAC, and 5βP-DiAC mass spectra in normal urine extract acquired using (a, d, g) EI(mass range m/z 50–390 amu), (b, e, h) PCI-CH₄ (mass range m/z 70–420 amu), and (c, f, i) PCI-NH₃ (mass range m/z 220–440 amu).

Figure 8

11OHA-AC (in normal urine extract) and Cne (in SM15-AC standard) mass spectra acquired using (a, d) EI (mass range m/z 50–390 amu), (b, e) PCI-CH₄ (mass range m/z 70–420 amu), and (c, f) PCI-NH₃ (mass range m/z 220–440 amu).