Ultratrace Quantitative Analysis of Catalyst Poisoners using a Dedicated GC–MS Analyser

Apr 01, 2012
Volume 25, Issue 4

A dedicated GC–MS analyser was developed to address the increasing need for more sensitive catalyst poisoner analysis. The system combines the separation power and robustness of a classic backflush configuration with the selectivity and sensitivity of mass spectrometry.

The use of high-yield metallocene catalysts has dramatically increased both efficiency and selectivity of polymerization processes (1). Unfortunately, these catalysts are extremely prone to poisoning by feedstock impurities, such as arsine (AsH3), phosphine (PH3), oxygenates (for example, dimethylether) and sulphur-containing compounds (mercaptanes, sulphides, etc) (2,3). Minute amounts of these compounds are sufficient to impose undesirable effects and induce immediate loss of catalytic activity and reaction yield. At the same time, trace contaminants at the part-per-billion (ppb) concentration levels can end up in the polymers and alter subsequent polymer properties and characteristics.

For decades, process chemical and petrochemical analysts used to address their analytical challenges mainly by relying on superior chromatography and smart tools such as valve switching, backflush and Dean's heart-cut. In combination with relatively cheap, robust and selective detectors, they were capable of providing all information necessary to control and tweak petrochemical processes.

Table 1: Typical specifications for catalyst poisoners in polymer grade hydrocarbons (6).
Organic catalyst poisoners are usually determined using dedicated chromatographic analysers. These systems are, typically, equipped with a dual capillary column configuration with backflush and fitted with a flame ionization detector (FID). Under these conditions, limits-of-detection are usually situated around 100 ppb, depending on the compound investigated and the complexity of the matrix that is introduced (4). Unfortunately, this is far from sufficient to protect the latest catalysts, which start to deteriorate as soon as fed with low ppb amounts (5,6). An overview of some typical specifications for catalyst poisoners in polymer-grade hydrocarbons is given in Table 1.

Mass spectrometry (MS) is hardly used in petrochemical QC laboratories, which is primarily because of its apparent complexity and higher cost-of-ownership. Nonetheless, MS detection has several distinct advantages over classic analogue detectors. In full scan acquisition mode, for example, it allows tracking and identification of unknown components using spectral deconvolution and subsequent library matching. In selective ion monitoring (SIM) mode, on the contrary, MS permits trace and ultratrace quantification of target analytes which is often superior to classic selective detectors. Furthermore, MS permits the use of mass labelled internal standards (ISs) that behave identically to their native analogues, which has a positive effect on overall method precision and accuracy. It is no surprise that instrument manufacturers have invested substantially in solutions aimed at reducing overall MS complexity and total cost-of-ownership in the last couple of years. Easy tune and calibration functionalities, increased sensitivity and speed, new acquisition modes and elegant solutions that eliminate downtime, such as vacuum lock technology, have contributed largely in this respect.

This article gives an overview of the main characteristics and performance of a new gas chromatography mass spectrometry (GC–MS) analyser that has been recently developed. The system combines the chromatographic separation power and backflush/Dean's heart-cut capabilities of a classic oxygenate analyser with the orthogonal separation power, sensitivity, selectivity and overall robustness of the latest generation single quadrupole mass spectrometers.

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