Technical Note
0712
Understanding Laser Diode Thermal Desorption 1
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Pierre Picard , Patrice Tremblay , Réal Paquin 1
Phytronix Technologies, Québec Canada, 2 Laval University, Québec, Canada
Keywords: LDTD, Thermal desorption, High throughput, Parametric characterization
How Does LDTD Work ? The Laser Diode Thermal Desorption (LDTD) ionization source (Figure 1) allows the analysis by mass spectrometry of samples at atmospheric pressure. A sample in solution is introduced into a well (96-well plate especially designed with a metal sheet insert). It is allowed to dry at room temperature. A Laser Diode is used to heat the back-side of the well to produce a rapid thermal desorption of the dried sample. Intact desorbed molecules are cooled and carried by the air flow to an APCI region to undergo ionization. The APCI is achieved without the presence of solvent or mobile phase. This enhances the ionization performance and allows the analysis of many compounds that are not performing in conventional APCI. All these processes are operated at room temperature. A piston head seals the well during the thermal desorption process.
Why Does LDTD Work ?
LazWell Sample Plate
Carrier Gas The functionality of the LDTD is a combination of 3 key features. Transfer Tube First, the maximum heating rate is IR Laser Beam 3000 °C/sec (traditional thermal Mass probe is 1000 °C/sec). This Spectrometer characteristic contributes to limit Inlet the thermal degradation products. Corona Discharge In second, the gas dynamic is Needle Piston Piston head optimized to remove and thermalize the molecules leaving the surface. Finally, the physical Figure 1 Schematic of the Laser Diode Thermal Desorption ionization source. structure of the sample deposit is in the nanoscale range. The behavior of the matter in that dimension leads to a much lower volatilization enthalpy and thus prevents thermal degradation of the molecules. Figure 2 propose a typical desorption profile in witch the laser is operated during 4-6 seconds leading to an ultra fast sample desorption producing a narrow signal.
Parametric characterization Carrier Gas Temperature Monitoring Through the LDTD The temperature at several points into the LDTD ionization source has been taken under typical operation conditions (laser power of 25 % operated for 4 seconds with a carrier gas flow at 2 L/min). Theses conditions allow temperature equilibrium of the carrier gas throughout the ionization interface and lead to the temperature presented in Figure 3. The most significant data is the fast cooling of the gas at 1 mm from the well surface to 39 °C. Unlike others API o sources, operating at temperature higher than 200 C, the LDTD interface does not put extra energy to the desorbed neutrals. This phenomenon contributes to limit the thermal degradation of the desorbed molecules in the gas phase. Figure 2 Typical laser desorption profile.
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LazWell Sample Plate Center of tube 32ºC
Thermal Desorption Under Analyte Melting Point Temperature Tube exit 30ºC
The LDTD process allows to thermally desorb neutrals molecules even though the temperature reached into the well is lower than the reported analyte melting point (Figure 5) [sulfadiazine reported melting point is 2520 256 C]. This behaviour has been observed for hundreds of compounds up to now.
Surface 100ºC
Tube entrance 36ºC 1 mm from surface 39ºC
Film Deposit Into a LazWell Well
Figure 3 Carrier gas temperature through out the LDTD interface.
The usual sample volume deposited into a well vary up to 10 µL. At that range, for a compound in ng/mL range in concentration, the film deposit presents crystals in the nanoscale size (Figure 6). The effective melting temperature of such crystal has been reported to be well below the bulk melting point reported in textbooks [Goldstein et al. 1992, Science, 256, 1425]. From that point, the LDTD analysis of compounds that should decompose before melting is observed. As an example, the prednisone is ionized and quantified down to 1 pg on the plate.
LazWell Well Temperature under Laser Irradiation
Surface well temperature (oC)
Operating the diode laser induces heat transfer to the well metal sheet. The temperature relation is linear with the diode laser power up to 60 % where surface absorption properties are modified (Figure 4). Typical working power used in the analysis is around 30% 0 leading to a surface temperature of 120 C.
Laser power % (1 second ramp to % and stay at % for 5 seconds)
Figure 4 Well surface temperature monitored as function of the laser power. Figure 6 Electron microscopy of dried prednisone (500 pg) into a well.
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Area count (million)
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Conclusions
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It is obvious from the data that the thermal desorption process involved in LDTD produce mainly intact desorbed molecules at temperature below the reported melting point. This behavior constitutes the key feature of the LDTD where the high heating rate, the carrier gas flow and the nanoscale deposit combined effect allows the exceptional properties of the LDTD.
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Well surface temperature ( C)
Figure 5 Sulfadiazine thermal desorption as function of the diode laser heat transfer to the well surface.
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