mike rusak and laura fairburn current obstacles on the application of ambient ionization ms in...
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Mike Rusak and Laura Fairburn
CURRENT OBSTACLES ON THE APPLICATION OF AMBIENT
IONIZATION MS IN FORENSICS
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SUMMARY OF THE DRAWBACKS
• Cost
• Professional experience is needed
• Lack of comparable data
• Ionization is affected by spray, chemical, and surface parameters.
• Problems with portability
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SUMMARY OF THE DRAWBACKS
• Utilization of standards
• Problems with quantitative analysis
• Lack of research on matrix effects
• Lack of field testing
• Information obtained is not yet applicable to current standards
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COST• Cost of the operator and the equipment can’t be
afforded by police departments
• The knowledge for just the calibration and operation is very specialized, increasing one’s expected salary
• Not a very practical purchase for a small police department
• Right now, the prototypes of the portable MS machines can only be afforded by the US military
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EXPERIENCE NEEDED
• Need to have a lot of experience and education
• You can’t just walk off the street, be given basic training and then be allowed to operate the equipment and deemed an expert
• Most evidence requiring MS is outsourced to specialized forensic labs
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LACK OF COMPARABLE DATA WHEN WORKING WITH UNKNOWNS
• Since this technique is still in the prototype phase, there aren’t many successful standard cases to compare one’s results as few labs use this technique
• All results need to be confirmed with other techniques such as IR and NMR, or GC-MS
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PARAMETERS THAT AFFECT IONIZATION
• All of the following parameters can be very difficult to control in the field
1.Geometric parameters
2.Spray parameters
3.Chemical parameters
4.Surface parameters
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PORTABILITY• The large volume and power supply needed
reduces the portability of the machine
• Currently, portable machines are very rare and experimental
• Compromises sensitivity and accuracy of readings
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UTILIZATION OF STANDARDS
• Standards are used to quantify and identify the analyte of interest
• There’s a dependence of the ion signal on the type of substrate present
• Eg) Thermal conductivity of substrate/matrix can influence the signal
• Roughness of substrate
• Hard to quantify an analyte use a conventional MS apparatus instead
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UTILIZATION OF STANDARDS
• If it isn’t possible to mix in a standard, an intrinsic compound of known concentration can be used, which isn’t ideal.
• Not very quantitative
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MATRIX EFFECTS
• Matrix: the material the analyte of interest is mixed with
• Influences desorption and ionization of the analyte
• Eg) cream, fillers (in the case of medicinal tablets), dirt, carpet, etc.
• Systematic matrix studies are rare
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MATRIX EFFECTS
• The matrix can cause overlap in the spectra due to unwanted compounds
• Can also cause ion suppression
• Less volatile compounds in the matrix disrupt droplet formation or evaporation, suppressing the amount of ions reaching the detector
• Can react with analyte during desorption or ion formation
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MATRIX STUDIES
• Yu et al. studied the effect of different biological matrices on the observed signal via DART analysis
• Showed that verapimil analyte signal can be reduced to 50% in bile, and 3% in brain tissue homogenate, compared to verapimil in neat solvent
• Also showed that matrix effects are compound dependent.
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MATRIX STUDIES
• Jecklin et al. showed how the limit of detection (LOD) varies when analyzing pesticides in different fruit juice matrices
• The authors noted that background intensities were not responsible for the observed changes
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LACK OF FIELD TESTING
• Explosives, toxic industrial compounds, chemical warfare agents, etc., have been explored by a variety of methods
• Analysis of these compounds is limited for that fact that the experiments are very controlled.
• Either done from standard solutions or complex matrices doped with standards
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LACK OF FIELD TESTING
• For drug analysis, pure chemicals are usually spiked onto food, skin, or other surfaces.
• More research needs to be done with real evidence and analytes in a non-standardized environment.
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APPLICABILITY
• Yield a type of information which may not be applicable to current standards• Eg) Food policy requires that pesticide levels be
measured in concentration (wt/wt), which requires homogenizing the fruit/veggies of concern.
• Sample preparation still needs to be done in some cases• Eg) To reduce ion suppression, especially for analytes
in a low concentration in complex matrices
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REFERENCES• Alberici, Rosana M. et al. Ambient mass spectrometry: bringing MS into the “real world”. Anal
Bioanal Chem. 2010, 398: 265–294
• Cooks et al., Forensic Applications of Ambient Mass Spectrometry. Anal Bioanal Chem (2009) 394, 1995-2008
• Chen et al., What Can We Learn From Ambient Ionization Techniques? J Am Soc Mass Spectrom (2009) 20, 1947-1963
• Gamez et al., Atmospheric pressure glow discharge desorption mass spectrometry for rapid screening of pesticides in food. Rapid Commun Mass Spectrom. (2008) 22, 2791-2798
• Takats, Zoltan et al. Ambient mass spectrometry using desorption electrospray ionization (DESI): instrumentation, mechanisms and applications in forensics, chemistry, and biology. J. Mass Spectrom. 2005, 40: 1261–1275
• Yu et al., Bioanalysis without Sample Cleanup or Chromatography: The Evaluation and Initial Implementation of Direct Analysis in Real Time Ionization Mass Spectrometry for the Quantification of Drugs in Biological Matrixes. Anal Chem. (2009) 81,193-202