A Two-Step Pyrolysis-Gas Chromatography Method with Mass Spectrometric Detection for Identification of Tattoo Ink Ingredients and Counterfeit Products

Summary

This method for two-step pyrolysis online coupled to gas chromatography with mass spectrometric detection and data evaluation protocol can be used for multi-component analysis of tattoo inks and discrimination of counterfeit products.

Abstract

Tattoo inks are complex mixtures of ingredients. Each of them possesses different chemical properties which have to be addressed upon chemical analysis. In this method for two-step pyrolysis online coupled to gas chromatography mass spectrometry (py-GC-MS) volatile compounds are analyzed during a first desorption run. In the second run, the same dried sample is pyrolyzed for analysis of non-volatile compounds such as pigments and polymers. These can be identified by their specific decomposition patterns. Additionally, this method can be used to differentiate original from counterfeit inks. Easy screening methods for data evaluation using the average mass spectra and self-made pyrolysis libraries are applied to speed up substance identification. Using specialized evaluation software for pyrolysis GS-MS data, a fast and reliable comparison of the full chromatogram can be achieved. Since GC-MS is used as separation technique, the method is limited to volatile substances upon desorption and after pyrolysis of the sample. The method can be applied for quick substance screening in market control surveys since it requires no sample preparation steps.

Introduction

Tattoo inks are complex mixtures consisting of pigments, solvents, binders, surfactants, thickening agents, and, sometimes, preservatives¹. The increased popularity of tattooing in the last decades has led to the establishment of legislation addressing tattoo ink safety across Europe. In most instances, color-giving pigments and their impurities are restricted and therefore should be monitored by state laboratory market surveys to control their compliance with law.

Using the approach of online pyrolysis-gas chromatography mass spectrometry (py-GC-MS) described here, multiple ingredients can be identified simultaneous....

Protocol

1. Tattoo ink preparation and sample mounting

1. Use a 25 mm hollow glass pyrolysis tube as a sample holder and quartz wool for sample preparation.
   1. Grab the pyrolysis tube with the specialized tweezers for pyrolysis tubes (bake out for decontamination) and insert the necessary amount of quartz wool with pointed tweezers into the tube.
   2. Insert two steel sticks (bake out for decontamination) at each side of the pyrolysis tube and compress the wool into a 1–2 mm thick stopper. The stopper must be positioned at the lower third of the pyrolysis tube to achieve adequate heating during the pyrolysis process.
   3. Ignite a gas burner....

Results

The method includes a two-step chromatographic approach for each sample (Figure 1). In the first run, the sample is dried inside the injector system at 90 °C before volatile compounds are transferred onto the column. Since the drying process is incomplete in most cases, residual solvents and volatile compounds are transferred and analyzed. In the second run, the previously dried sample is subsequently pyrolyzed to facilitate the analysis of non-volatile organ.......

Discussion

Py-GC-MS is a useful screening method for a broad range of substances in tattoo inks that can also be used for the analysis of other products. Compared to other methods, py-GC-MS can be conducted with only minimal sample preparation. GC-MS devices can be found in most analytical laboratories compared to more specialized methods such as MALDI-ToF-MS and EDX.

The data evaluation of pyrograms may be challenging, since the list of possible ingredients is infinite in theory and library searches tha.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
99.999% Helium carrier gas	Air Liquide, Düsseldorf, Germany	-
5975C inert XL MSD with Triple-Axis Detectors	Agilent Technologies, Waldbronn, Germany	-
7890A gas chromatograph	Agilent Technologies, Waldbronn, Germany	-
AMDIS software (Version 2.7)	The National Institute of Standards and Technology, Gaithersburg, MD, USA	-	can be used for GC/MS peak integration, e.g. for transfer to pyrogram evaluation software
Cold Injection System (CIS)	Gerstel, Mühlheim, Germany	-
electron impact (EI) source	Agilent Technologies, Waldbronn, Germany	-
Enhanced ChemStation (E02.02.1431)	Agilent Technologies, Waldbronn, Germany	-	used to generate Average Mass Spektra (AMS), can be used for peak integration and standard GC/MS library search
J&W HP-5MS GC Column, 30 m, 0.25 mm, 0.25 µm, 5975T Column Toroid Assembly	Agilent Technologies, Waldbronn, Germany	29091S-433LTM
MassHunter Software	Agilent Technologies, Waldbronn, Germany	-	no Version specified, can be used for GC/MS peak integration and standard GC/MS library search
Microcapillary tube Drummond Microcaps, volume 2 µL	Sigma-Aldrich, St. Louis, MO, USA	P1549-1PAK
MS ChromSearch (Version 4.0.0.11)	Axel Semrau GmbH & Co. KG, Sprockhövel, Germany	-	specialized pyrogram evaluation software
NIST MS Search Program (MS Search version 2.0g)	The National Institute of Standards and Technology, Gaithersburg, MD, USA	-	used for MS and AMS library generation and corresponding substance search with selfmade and commercial libraries
NIST/EPA/NIH Mass Spectral Library (EI) mainlib & replib (Data version: NIST v11)	The National Institute of Standards and Technology, Gaithersburg, MD, USA	-	used commercial mass spectral library
Polystyrene (average Mw ~192,000)	Sigma-Aldrich, St. Louis, MO, USA	430102-1KG
Pyrolysis tubes, tube type - quartz glass - lenght 25 mm; 100 Units	Gerstel, Mühlheim, Germany	018131-100-00
Pyrolyzer Module for TDU	Gerstel, Mühlheim, Germany	-
Quartz wool	Gerstel, Mühlheim, Germany	009970-076-00
Steel sticks	Gerstel, Mühlheim, Germany	-
Thermal Desorption Unit (TDU 2)	Gerstel, Mühlheim, Germany	-
Transport adapter	Gerstel, Mühlheim, Germany	018276-010-00
Tweezers for Pyrolysis tubes	Gerstel, Mühlheim, Germany	009970-074-00
Zebron Z-Guard Hi-Temp Guard Column, GC Cap. Column 10 m x 0.25 mm, Ea	Phenomenex Ltd. Deutschland, Aschaffenburg, Germany	7CG-G000-00-GH0