Internal Standard Utilization Strategies for Quantitative Paper Spray Mass Spectrometry

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Forensic Science

Background:

• Premixing internal standards (ISTD) with samples for paper spray mass spectrometry (PS-MS) is inefficient and not universally applicable.
• Independent ISTD deposition on paper, while feasible, can compromise quantitative accuracy and precision.

Purpose of the Study:

• To evaluate different ISTD utilization strategies for PS-MS to identify sources of irreproducibility and bias.
• To compare the quantitative performance of premixed, predeposited, and postdeposited ISTD methods.
• To assess the impact of robotic versus manual pipetting on ISTD deposition accuracy.

Main Methods:

• Investigated ISTD deposition strategies (premixed, predeposited, postdeposited) using drugs of misuse as analytes.
• Assessed performance based on precision (CV) and accuracy (bias) with varying ISTD volumes.
• Compared robotic liquid handling systems with manual pipetting for ISTD deposition.
• Utilized strategy-matched calibrations to correct for systematic biases.

Main Results:

• Premixed ISTD yielded the best results (average CV = 1%, |bias| = 5%), significantly outperforming independently deposited ISTD (average CV = 18%, |bias| = 61%).
• Predeposited ISTD with a robotic handler showed optimal performance at lower volumes (CV = 8% for 2 μL).
• Manual pipetting of low-volume predeposited ISTD resulted in poor performance, while postdeposited ISTD was generally inferior.

Conclusions:

• Predeposition of ISTD, particularly using automated systems, enhances quantitative performance in PS-MS.
• Strategy-matched calibration effectively corrects biases across different ISTD utilization methods.
• Achieved American Academy of Forensic Sciences (AAFS) accuracy and precision requirements in most evaluated scenarios.