Mass Spectrometry & Spectroscopy 19
The two publications from Johannes Kepler University [48,49] demonstrated the surprising observation that HPLC/DART does not exhibit sample suppression from nonvolatile buffers (such as phosphates), a problem that plagues common HPLC/MS methods such as APCI and ESI. The lack of a commercially available HPLC/ DART interface may explain why HPLC/DART has not been more widely used. Given the simplicity of the interface: a zero dead volume connection, stainless steel capillary, mounting brackets and a solvent drain, it is not clear why a commercial interface has not been offered. The problem may be circular: the development may be waiting for customer demand, yet customer demand will not occur until an interface is available. Perhaps this situation will change in the near future and DART will find a place among LC/ MS detection methods.
Figure 6. Thermal desorption/pyrolysis DART analysis of a particle of duct tape showing the reconstructed ion chromatograms for selected components (centre) and their corresponding mass spectra.
Chromatography with DART
In the absence of chromatography, DART analysis relies on the specificity of the mass spectrometer - that is, resolving power, mass accuracy and tandem mass spectrometry (MS/MS) - to achieve specificity. However, DART has been combined with the most common forms of chromatography including thin- layer chromatography [32-47], gas chromatography [23], liquid chromatography [48,49] and capillary electrophoresis [50]. Of these, the coupling with thin-layer chromatography has been the most common. Figure 7 shows the extracted ion chromatograms for 9 drugs separated on a silica TLC plate with ethyl acetate/ methanol/ammonium hydroxide (85:10:5). The TLC plate was cut to a 1 cm width prior to analysis. After separation, the plate was sprayed with 4% glycerol in methanol to facilitate desorption of the drugs from the silica substrate and then scanned horizontally through the DART gas stream at a rate of 3 mm s-1
using the
DART-SVP linear rail (IonSense LLC, Saugus MA USA). The DART gas heater was set to 450°C. The chromatograms are normalised to show all compounds on the same scale because the response of the weakest signal is only 1/100 relative to the strongest signals. In contrast to fluorescence (inset), DART analysis reveals all 9 drugs.
References
1. Cody, R. B.; Laramee, J. A.; Durst, H. D. Versatile New Ion Source for the Analysis of Materials in Open Air under Ambient Conditions; Analytical Chemistry 2005, 77, 2297-2302.
2. Domin, M. A.; Cody , R. B. Ambient Ionization Mass Spectrometry; Royal Society of Chemistry, 2015.
3. Penning, F. M. Über Ionisation durch Metastabile Atome; Naturwissenschaften 1927, 15, 818.
4. Pierce, C. Y.; Barr, J. R.; Cody, R. B.; Massung, R. F.; Woolfitt, A. R.; Moura, H.; Thompson, H. A.; Fernandez, F. M. Ambient generation of fatty acid methyl ester ions from bacterial whole cells by direct analysis in real time (DART) mass spectrometry; Chem. Commun. 2007, 807 - 809.
5. Fraser, D.; DeRoo, C. S.; Cody, R. B.; Armitage, R. A. Characterization of blood in an encrustation on an African mask: spectroscopic and direct analysis in real time mass spectrometric identification of haem; Analyst 2013, 138, 4470-4474.
6. Kim, H. J.; Park, S. R.; Jang, Y. P. Extraction-free In situ Derivatisation of Timosaponin AIII Using Direct Analysis in Real Time TOF/MS; Phytochemical Analysis 2014, 25, 373-377.
7. Zeng, S.; Wang, L.; Chen, T.; Qu, H. On-line coupling of macroporous resin column chromatography with direct analysis in real time mass spectrometry utilizing a surface flowing mode sample holder; Analytica Chimica Acta 2014, 811, 43-50.
8. JEOL USA Inc. Using Solid Phase Microextraction with AccuTOF-DART® Fragrance Analysis (Application Note).
Figure 7. Reconstructed ion chromatograms (RICs) for 9 drugs spotted onto a TLC plate and analysed by DART compared to fluorescence detection (inset). The RICs are normalised to the same scale with relative scale factors as shown in the inset table. The plate is scanned at a rate of 3 mm s-1 shown for the spots at 1.35 minutes and 1.98 minutes, respectively.
. Mass spectra are for
9. Cajka, T.; Riddellova, K.; Tomaniova, M.; Hajslova, J. Recognition of beer brand based on multivariate analysis of volatile fingerprint; Journal of Chromatography A 2010, 1217, 4195-4203.
10. Rodriguez-Lafuente, A.; Mirnaghi, F.; Pawliszyn, J. Determination of cocaine and methadone in urine samples by thin-film solid-phase microextraction and direct analysis in real time (DART) coupled with tandem mass spectrometry; Analytical and Bioanalytical Chemistry 2013, 1-5.
Conclusion
Direct Analysis in Real Time (DART) mass spectrometry can be used for more than simply exposing a sample to the heated DART gas stream. When combined with simple sample handling and sample preparation methods such as in-situ derivatisation, solid-phase microextraction, and thermal desorption and pyrolysis, DART selectivity and detection limits can be greatly improved. Dopant- assisted argon DART and oxygen adduct formation are two variations on DART that permit the selective detection of nonpolar compounds such as saturated hydrocarbons. While DART is often used as a stand-alone ion source, it has been combined with the most common forms of chromatography with characteristics that complement other mass spectrometric detection methods.
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