Monday, February 13, 2012

Fast GC-PID/FID Analyses using Resistively Heated Columns for Rapid Analyses in the Field


Fast GC-PID/FID Analyses using Resistively Heated Columns for Rapid Analyses in the Field

Oral presentation in Real-Time Detection Systems
 Authors:
John N.  Driscoll, PID Analyzers, Sandwich, MA 02653
Stan Stearns, VICI, Houston, TX 77055
Philip Smith, OSHA Labs, Salt Lake City, UT 84081

Objective- The analysis times for NIOSH ketone and aromatics methods (# 1301, 1501 & 2005) are between 25-35 minutes. The time consuming operations are the long ramping and cooling times required. The other difficulty is that if peaks are not adequately resolved, other components could interfere with the analysis. We will employ resistively heated columns to reduce the total analysis times by 50-60% to 12-15 minutes and use the PID/FID response ratios to enhance identification of components of interest. These ratios have been used previously (1) to identify the molecular structure of hydrocarbons. 

Methods- A PID Analyzers compact GC (<20#) with an external laptop will be used for the analysis that incorporates  new technology developed by VICI  that takes  polyimide-coated fused silica (FST) and removes  the polyimide layer. Then the (FST) is electroplated with nickel.  As a result of the superior heat transfer of the electroplated nickel, we are able to  rapidly and efficiently heat and achieve excellent resolution for a 30M capillary column. A mini-FID was designed that will easy attach to the outlet of the PID taking a single detector port and providing improved identification of components.

Results –For  the initial runs with the series of 12 ketones (NIOSH #1301), we were able to separate 10 of the components by just increasing the ramp rate from 10o/min to 200 /min. The final time for analysis was 11 minutes. We do have 8 ramp rates and 4 cooling fan rates so with an additional ramp, we should be able to separate methyl amyl ketone and ethyl amyl ketone and use a faster fan rate to reduce the cool down time. For NIOSH Method #1501, we were able to separate 15 of the 17 compounds in 14 minutes instead of 35 minutes. We were not able to separate nitrobenzene and naphthalene but there is a such a significant difference in the PID/FID response ratio that each of the components can be easily identified.

Conclusions 
We have shown that the resistively heated columns can be used to improve both separations & time of analysis. The use of a second detector (FID) for confirmation of peak identity is more helpful for faster separations where complete resolution is not attained.

1.   Driscoll, et al., "Gas Chromatographic Detection and Identification of Aromatic and Aliphatic Hydrocarbons in Complex Mixtures by Coupling Photoionization and Flame Ionization Detectors," J. Chrom., 158, 171 (1978).

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