Ultra-trace level detection of nonvolatile compounds studied by ultrasonic cutter blade coupled with dielectric barrier discharge ionization-mass spectrometry

In analytical mass spectrometry, an efficient desorption is needed for nonvolatile compounds at ultra-trace level detection. In this paper, an ultrasonic cutter-assisted non-thermal desorption method for ultra-trace level detection of different types of nonvolatile compounds such as drugs of abuse, explosives, pharmaceuticals, spinosad, cholesterol, rhodamine B, glucose and amino acids has been described. The relevant compounds were deposited on the ultrasonic blade except pharmaceutical tablets that were used directly, and gently touched on perfluoroalkoxy (PFA) made substrate with oscillation frequency about 40 kHz in order to desorb the solid molecules. The desorbed gaseous molecules were ionized using a home-made helium dielectric barrier discharge ionization (DBDI) source and then detected by an ion trap mass spectrometer. The synergistic effect caused by gaining the oscillation and frictional/mechanical energy enhanced desorption of the solid molecules into gaseous phase, thereby, resulting in detection at ultra-trace level. PFA made substrate showed better limits of detection (LODs) compared to that of wood made substrate. The LOD values for most of the target analytes were ranging from 20.00 ± 0.91 to 200.25 ± 9.04 pg with RSD values ≤ 5% except for pharmaceutical tablets where only depletion amounts were estimated. The LOD value of cocaine in urine was 39.88 ± 1.65 pg with RDS ≤4.56% showed to be a very promising analytical tool for analysis of drugs of abuse in biological samples under ambient conditions. Drugs of abuse, pharmaceuticals, spinosad, cholesterol, rhodamine B, glucose and amino acids were detected mostly as their protonated molecular ions while RDX and HMX were detected as their molecular cluster/adduct ions as [M + NO2]- and [M + NO3]-, and AN was detected as a cluster ion of HNO3 with NO3-, [HNO3 + NO3]-, without suffering from fragmentation. An effective mechanism of the enhanced sensitivity of the tribodesorption-DBDI-MS system in analyzing the nonvolatile compounds has been discussed.