High-throughput analytical approach combining automated sample preparation and gas chromatography with universal carbon response

Highlights

• Automation of sample preparation, gas chromatography analysis, and data processing.

• Quantification using universal carbon response and internal standard.

• Method development using modeling tools for fast optimization and implementation.

• Robust gas chromatography method for separating 13 compounds in 8 min.

Abstract

Specifying effective analytical techniques and methods can be very challenging when a large number of sample streams need to be analyzed with high frequency. Combining those requirements with the need to monitor multiple components over a wide range of concentrations makes at-line analytical approaches preferred by many analytical scientists. Traditionally, at-line analytical support requires sample preparation and operator time. Recently developed technologies enabled automated sample preparation tools coupled with gas chromatography; thus, eliminating sample preparation steps and increasing productivity. Recently a commercial micro-reactor was introduced that can be combined with a flame ionization detector, providing the ability to quantify components without the need to perform a standard calibration, which saves significant time and materials. In this manuscript, we describe a unique analytical capability that combines automated sample preparation and gas chromatography with flame ionization detection and universal carbon response to provide high flexibility and accuracy when there is minimal information about the unknowns or reference materials are not available. Some of the challenges and performance monitoring techniques for this technology are also discussed in this manuscript.

Introduction

In recent years, the chemical, pharmaceutical, and agricultural industries have been rapidly evolving to more complex processes and products [1], [2]. This higher complexity in modern synthesis processes often translates into the necessity of identifying and analyzing a significant number of side products and impurities. To overcome the increase in workload added by these challenges, many companies and research organizations are heavily investing in automation and high-throughput capabilities. Accurate measurement of analytical process parameters is critical to any successful product development, and this work has traditionally been very time consuming due to the need for standards calibration, sample preparation, and data processing. It has become a tremendous challenge in modern analytical laboratories to generate more data with higher complexity, limited workforce, and tight timelines for process and product development.

Gas chromatography is a mature and established analytical technique commonly practiced in most industrial and academic laboratories. Recent developments in equipment and software automation capabilities, in combination with advancements in the detection and quantification for gas chromatography, have made significant improvements in streamlining the process of evaluating complex samples. Robots that can automate the sample injection process (autosamplers) have been available for a few decades, and in some cases can perform a variety of simple sample preparation steps. The complexity of the operations that need to be performed and the need for advanced training for qualified personnel are common limitations for adopting these technologies.

Over the last few years, further advancements in robotic autosamplers have been made with technologies that enable, for a first time, the automatic exchange of multiple tools to perform the most common injection techniques, including split/splitless, on-column, headspace, and solid phase micro extraction [3]. Basic operations such as using several syringes of different volumes to perform dilutions or avoiding carry over across different types of samples have provided significant advantages over previous technologies. This level of automation enabled these systems to combine multiple injection techniques in the same sequence, without requiring the user to change any of these tools manually. These advancements in robotics and autosampler instrumentation can be readily coupled to a gas chromatograph to automate several common steps required during the sample preparation for a large number of samples and to perform the injection to initiate the method.

The flame ionization detector (FID) is the most popular detector for gas chromatography. Some advantages of using the FID are its large linear dynamic range (10⁷), robustness, signal stability, sensitivity, ease of use, affordability, and low maintenance cost [4]. The main drawback is that it does not provide any selectivity or information about the compounds detected other than the fact that they can emit ions when burning in a hydrogen flame. While the FID is strictly not a universal detector, nearly any compound that contains carbon and hydrogen will provide a response. Another important aspect of the FID is that the signal generated by the compounds is proportional to the amount of carbon present in the gas stream being analyzed [5]. This property can provide a reasonable estimation of the amount of each compound in the sample when compared to an adequate surrogate substance if the standard is not available (e.g., analysis of complex samples) [6]. While such estimation can provide reasonably good accuracy for the analysis of hydrocarbons, the error can be drastically larger and is affected not only by the elemental composition of the analyte but also by the nature of the specific functional groups present in the molecules [7].

Recently, Dauenhauer et al. [8] published an article highlighting the implementation of a two-step micro-reactor that oxidizes all the analytes eluting from the GC column to carbon dioxide in a first step and then converts them into methane in a second step. This method not only enables the FID to have a more uniform response per carbon base, but also significantly improves the sensitivity for the detection of compounds that typically have a low response to the FID (such as low molecular weight aldehydes and organic acids). Most importantly, the use of this micro-reactor to convert the components eluting from the GC column to methane (and water) eliminates the need for calibration and analytical standards since the response is proportional to the moles of carbon atoms contained in each one of these molecules [20].

In this manuscript, we focus on a case study that describes the implementation of a high-throughput GC system equipped with a robotic autosampler and the micro-reactor coupled to an FID. This configuration enables the instrument to perform a number of sample preparation steps using a variety of diluents, internal standards, and different injection techniques without user intervention. The micro-reactor in-line with the FID eliminates the necessity of the external standard calibration and increases the response for several species. This capability resulted in a dramatic improvement in the analytical throughput with minimal operator involvement by eliminating the need for standard calibration and sample preparation.