Mike Rusak and Laura Fairburn

CURRENT OBSTACLES ON THE APPLICATION OF AMBIENT

IONIZATION MS IN FORENSICS

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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.

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

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