Automatic peroxyacetyl nitrate (PAN) GC analyzer

Front view of the PAN-GC with the automatic calibration unit.

In 1992 the PAN-GC was developed, funded by the German EPA (UBA). Since that time the system has under- gone a number of updates, funded by the German Bundesumwelt Stiftung (DBU). The calibration procedure was tested in-depth by research institutions [1, 2, 3]. Instruments are operated successfully in many countries worldwide.

The automatic PAN-Analyzer is suited for 19"-rack-mounting and combines a PC-controlled, fully automated analytical instrument with a reliable and easy to handle calibration method. The limit of detection for PAN is < 50 ppt (< 30ppt under optimum conditions) and the linear dynamic range extends to 10 ppb or more. The dynamic calibration unit is based on the photochemical synthesis of PAN from an NO calibration gas in a flow system.

The PAN analyzer comprises gas chromatographic separation on capillary columns at sub-ambient temperature with electron capture detection [1]. The columns are mounted in a compact temperature controlled oven, cooled by Peltier elements. Temperature fluctuations are less than 1 K. The allowed ambient temperature range is 20-30 °C. An optional heater permits the operation of the GC over a wider range of ambient temperatures (5- 30°C). Purified and pre-conditioned nitrogen is used as the carrier and make-up gas. Back flushing of the pre- column prevents contamination and reduces analysis time by preventing substances with long retention times from entering the main column and the detector. Sampling, injection and column switching is accomplished with a pneumatically actuated 10 port valve (VALCO). A by-pass arrangement for the sample flow provides short residence times in the sample line.

Automatic operation

A new PC-Software (Win XP and higher) was designed that facilitates the control of the GC-system, as well as data- acquisition and reduction. A number of different I/O-boards (USB – DA or PCMCIA/PC card) manufactured by Measurement Computing Inc., USA are implemented in the software. The following modules are part of the software package:

  • Control of the GC and its continuous operation, collection of control parameters for remote operation
  • Data acquisition of the ECD-signal and automatic integration of the peak area.
  • Control of the calibration device and automatic initiation of the calibration procedure, provision of control parameters
  • Climate Module to view the raw data and to re-integrate peaks, if necessary
  • Online Module to evaluate time-series of the integrated peaks, e.g. CCl4, PAN

Peak finding occurs automatically after the approximate position and width of the peaks have once been marked in a test chromatogram on the PC-screen. During the subsequent automatic operation of the GC, the exact peak position is automatically optimized and integration occurs automatically by a least squares fitting. Peak areas are stored along with the concentrations derived from calibration factors. The flows of the sample air and the slope of the baseline (as an indicator of the performance of the ECD) are also stored and are used to derive quality-flags that can be transmitted to a remote computer.

Specifications

  • Detection limit: ca. 50 ppt
  • Detection limit with pre-concentration unit: ca. 5 ppt
  • PC/notebook- or PC104-controlled
  • Automatic peak integration, 6 measurements per hour standard
  • Up to 10 measurements per hour as option
  • PAN-calibration: Photolytic production of PAN from NO calibration gas

Calibration unit

The calibration unit produces a continuous, stable flow of known PAN concentrations in air. It is based on the
photochemical synthesis of PAN from an NO calibration gases (ppm range) in a flow reactor with subsequent dynamic dilution to concentrations of 0.1 to 30 ppb in an air flow of 1-10 L/min. NO is used instead of NO2, due to the better stability of NO calibration gases. Purified ambient air can used for dilution purposes in order to save expensive zero air. The procedure is semi-automatic. The PAN calibration unit is connected to the PAN-GC via a PFA valve. In the calibration mode samples are drawn from the overflow assembly of the calibration unit.
The required peroxyacyl radicals are produced by photolysis of acetone. The peroxy radicals obtained from the photolysis first oxidize the NO to NO2, which then forms the PAN. The PAN concentration is obtained from the NO mixing ratio in the NO standard µNO, the flow of the standard ΦNO, the total flow after dilution Φtotal, and the reaction yield for PAN YPAN.

µPAN = µNO*YPAN*(ΦNO/Φtotal)

The calibration unit was thoroughly evaluated during the QA-program of the German Tropospheric Research Focus using two independent techniques [24]. The PAN yield was determined to be 92 % ±3 %. It was found to be stable over a wide range of operating conditions, i.e. 0.1 - 30 ppb PAN in 10 L/min. A comparison of several calibration units [3] yielded < 10% disagreement, including possible errors in the calibration gases used by the different investigators.

 

[1] A. Volz-Thomas, I. Xueref, R. Schmitt, Automatic Gas Chromatograph and Calibration System for Ambient Measurements of PAN and PPN, Environ. Sci. Pollut. R. 94 (2002) 72

[2] H.W. Pätz, A. Lerner, N. Houben, A. Volz-Thomas, Charakterisierung eines neuen Verfahrens zur Kalibrierung von Peroxiacetylnitrat (PAN), Gefahrstoffe - Reinhaltung der Luft  62 (2002) 215

[3] H.-J. Kanter, V.A. Mohnen, A. Volz-Thomas, W. Junkermann, K. Glaser, C. Weitkamp, F. Slemr, Quality Assurance in TFS for Inorganic Compounds, J. Atmos. Chem. 42 (2002) 235