Evaluation of an internal standard for qualitative DART-MS analysis of seized drugs
Graphical abstract
Introduction
Direct analysis in real time mass spectrometry (DART-MS) is one of several screening techniques that is seeing increased prevalence in seized drug analysis due its speed, simple sample preparation, and ability to provide accurate, high-fidelity information. Since the invention of the technique, the ability to analyze controlled substances has been shown [1]. Of the forensic applications where DART-MS has been demonstrated, seized drugs has the most prevalent footprint [2]. The large body of existing research highlights a range of applications from traditional qualitative seized drug analysis [3] to quantitative analysis of psychoactive substances in plant materials [4] to identification of emerging drugs[5] and analysis of drug residues for predictive screening [6].
As with any technique, maintaining data integrity is of utmost importance. For DART-MS analysis, especially when equipped with high-resolution mass spectrometers, multiple-layers of data integrity are employed. These include calibration of the mass spectrometer, use of a positive control to demonstrate mass accuracy or fragmentation consistency, and use of a negative control to demonstrate lack of carryover or contamination. An additional layer of data integrity that has not been adopted in qualitative analysis is the use of an internal standard. Internal standards are commonplace in quantitative analysis [2], [4], [5] and have been validated for qualitative gas chromatography mass spectrometry (GC–MS) analysis of drugs [7]. The use of an internal standard in high-resolution, qualitative, DART-MS analyses has several appeals: (1) it provides in-spectrum mass calibration verification for samples, (2) it demonstrates that the instrument is working when negative results are obtained, and (3) it provides an estimate of analyte concentration. Also, given that DART-MS data is typically processed by looking for peaks above a pre-defined relative intensity in the mass spectrum, the use of an internal standard can minimize the false identification of noise peaks – which occurs regularly in spectra where a peak significantly above background does not exist. There are potential pitfalls to inclusion of an internal standard, however, namely due to the possibility of competitive ionization. Prior research has shown that the signal of low proton affinity compounds can be inhibited or even eliminated in mixtures due to this phenomenon [8], [9].
In this work, the use of an internal standard for qualitative analysis of seized drugs was investigated. Through a series of studies, the utility and potential challenges of including an internal standard in the sample analysis routine was identified. These studies encompassed identifying the internal standard compound, tuning its concentration, and measuring competitive ionization against a suite of commonly seen drugs. To evaluate real-world use of the internal standard, a suite of 60 case extracts from mock or adjudicated samples were analyzed with and without internal standard. Ultimately, it was found that the benefits of incorporating an internal standard into qualitative screening analysis outweighed the potential challenges and provided an increased level of data integrity.
Section snippets
Instrumentation
For this work, a JEOL AccuTOF 4G LC-Plus mass spectrometer (JEOL USA, Peabody, MA, USA) coupled with a DART-SVP ion source (IonSense, Saugus, MA, USA) was used. Relevant DART settings included operation in positive ionization mode with helium as the source gas at a temperature of 400 °C and an exit grid voltage of +150 V. Mass spectrometer parameters included operation in positive ionization mode with an orifice 1 voltage of +30 V, a ring lens and orifice 2 voltage of +5 V, and an ion guide
Identifying an internal standard compound
Identification of an appropriate internal standard was completed by evaluating potential compounds that met several qualifiers given the current DART-MS workflow used at the laboratory. The workflow uses only positive ionization mode and uses methanolic extracts for sample analysis, therefore the internal standard needed to easily produce a positive ion and be readily soluble in methanol. Since the internal standard was to be used as a gauge for the approximate concentration of samples (to
Conclusions
In this work, tetracaine was identified and evaluated as a candidate internal standard for the qualitative analysis of seized drugs by DART-MS. Tetracaine was found to be inexpensive, soluble in methanol at the necessary concentration, and did not interfere with detection of any commonly observed compounds of interest. Studies were conducted to determine an appropriate concentration for the internal standard to allow for identification of the same compounds as GC–MS while also minimizing the
Disclaimer
Certain commercial products are identified in order to adequately specify the procedure; this does not imply endorsement or recommendation by NIST, nor does it imply that such products are necessarily the best available for the purpose.
Certain commercial products are identified in order to adequately specify the procedure; this does not imply endorsement or recommendation by Maryland State Police, nor does it imply that such products are necessarily the best available for the purpose.
A portion
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank Robert (Chip) Cody for his assistance in developing the post-processing method on the JEOL system.
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