PPB Level Process Monitoring by Ion Mobility Spectroscopy (IMS)

PPB Level Process Monitoring by Ion Mobility
Spectroscopy (IMS)
Tad Bacon & Kurt Webber
Molecular Analytics
14550-A York Road, Sparks, MD 21152
(800) 635-4598
(410) 472-7300
info@ionpro.com
www.ionpro.com
ABSTRACT
Requirements for high purity products, and the need to monitor for catalyst poisons have driven
the need for process monitoring to the low ppb range. Traditional on-line process analyzers such
as GC, IR, etc. often cannot provide the required sensitivity. This paper describes the use of IMS
for real time continuous process monitoring. This technique has been field proven to provide
dependable, specific, low maintenance performance in even the harshest environments. Limits of
detection of 1 ppb and below are routinely achieved. Typical applications include: ammonia in
ethylene, hydrogen and other light hydrocarbons; HCl in vinyl chloride and other
chlorofluorocarbons; HF in vinyl fluoride, 134A and other chlorofluorocarbons, Cl2 in hydrogen
and others. This technique has also been successfully applied to ambient air and stack
monitoring.
THEORY OF OPERATION
IMS is an ionization-based time of flight technique, performed at atmospheric pressure. A
description of the IMS cell is seen in Figure 1. The sample is drawn over a semi permeable
membrane by way of an internal eductor. The membrane serves several purposes. It serves to
protect the interior of the cell from particles and high moisture levels, provides a degree of
selectivity, and allows various levels of sensitivity based on permeation rate. The molecules of
interest permeate through the membrane, and are picked up by the carrier flow, which sweeps
the other side of the membrane. The carrier stream delivers the sample molecules to the reaction
region of the cell, which contains a small Ni 63
radioactive source. There the sample is ionized as
a result of a series of ion-molecule reactions. In most cases, compounds known as dopants are
added to the carrier stream. These dopants enter into the ion-molecule chain of reactions to
provide a degree of selectivity based on the charge affinity of the analyte. Dopants can be
selected to provide either specific or generic detection of the acid gases. Once the sample has
been ionized, the ions begin to drift towards the opposite end of the cell due to the influence of an
electrostatic field. A shutter grid is located in the tube, which can be biased electrically to either
block the ions, or allow them to pass through. This shutter grid is pulsed periodically to allow the
ions into the drift region. There, they begin to separate out based on their size and shape while
flowing counter to a drift gas flow, which is introduced at the end of the drift tube. A collector
plate located at the end of the tube detects the arrival of the ions by producing a current. This
current is amplified to produce a time of flight spectrum. Ions are identified by their characteristic
drift time position in the spectrum .A typical series of spectra is seen in Figure 2. This series also
illustrates the charge sharing phenomenon characteristic of IMS. The Ni 63
source provides a finite
amount of charge. Accepting charge from the dopant ion ionizes the sample gas. Thus, as the
concentration of the sample gas increases the sample ion peak height increases, while the
dopant ion peak height decreases.

Figure 1 - IMS CELL

Figure 2 - - INCREASING CONCENTRATIONS OF HYDROGEN FLUORIDE

HARDWARE
An IMS analyzer produced by Molecular Analytics is seen in Figure 3. The analyzer consists of
two enclosures joined together. The right hand enclosure is temperature controlled and holds the
pneumatic components of the device including the cell. Carrier and drift flows are provided by an
external source of instrument air which is controlled by the internal regulator and precision valves.
The scrubbers provide a final cleaning stage for the instrument air. The dopant is introduced into
the carrier stream by way of an internal permeation tube. The left hand enclosure contains the
electronic portion of the analyzer. The microprocessor interprets the spectra to identify the
desired component, and determines the concentration based on peak height. The concentration
is displayed on the front of the instrument. Remote indication is provided by way of a 4-20 mA
signal loop. Two alarm levels are user settable and are indicated by contact closure. The
analyzer also provides on board fault detection, which is indicated by a flashing fault code on the
display. Remote indication of fault is indicated by contact closure. Most process measurements
are easily made directly, without the need for sample pre-treatment. However, monitoring in the
high temperature and humidity conditions seen in some process streams typically requires the
use of a sampling system. For these measurements, a dilution type probe is most often
indicated. Use of dilution probes is a good match with IMS because of the ppb level sensitivity
possible with the technique.
Figure 3 - IMS Analyzer

AMMONIA IN VARIOUS PROCESS STREAMS
APPLICATIONS
PPB levels of ammonia in ethylene, hydrogen, and other light hydrocarbons cause downstream
process problems due to catalyst poisoning and other factors. IMS easily performs this
monitoring, with a limit of detection of 1 ppb. The analyzer has been used in a wide variety of
different process streams, without any evidence of interference from any co-existing compounds.
Figure 4 shows a typical spectrum for this analysis.
FIGURE 4- AMMONIA IN ETHYLENE
HCl IN VARIOUS HYDROCABONS, VINYL CHLORIDE, AND VARIOUS
CHLOROFLUOROCARBONS
HCl can also cause problems in a number of processes. Catalyst regeneration in the reformer
process can produce HCl, which is very corrosive to downstream piping. HCl has undesirable
properties in processes involving vinyl chloride. Ultra-pure gasses needed for the semiconductor
industry such as hexafluoroethane must be monitored for HCl contamination. These ppb
measurements have all been addressed by IMS. Figure 5 shows spectra obtained from various
concentrations of HCl in tetrafluoroethylene.

2% TFE BLANK
20 PPB HCl IN 2% TFE
50 PPB HCl IN 2% TFE
100 PPB HCl IN 2% TFE

4% TFE BLANK
40 PPB HCl IN 4% TFE
130 PPB HCl IN 4% TFE
200 PPB HCl IN 4% TFE
FIGURE 5- HCl IN TETRAFLUOROETHYLENE

HF IN VINYL FLUORIDE, 134A, AND HYDROCARBONS
HF contamination of vinyl fluoride produces similar problems to these seen by HCl contamination
of vinyl chloride. The refrigerant 134a must also be monitored for HF to prevent corrosion in air
conditioners and other refrigeration devices. HF contamination of light hydrocarbons resulting
from alkylation processes is also a concern. On-line IMS analyzers have been utilized for all of
these applications.
Cl2 IN HYDROGEN AND OTHER PROCESSES
Hydrogen is often produced by electrolysis. Chloride compounds in the water present the
possibility of Cl2 contamination. IMS has been used to monitor for this problem. IMS has also
been used to monitor for Cl2 in other industrial processes including precious metal recovery, TiO2
manufacture, and in corrosion testing.
OTHER APPLICATIONS
IMS is not limited to the applications described above. The technique has also been successfully
applied to a number of unique measurements for specific users, and has wide application in other
process as well as environmental measurements.
REFERNCES
1. V. Harris, S. Klamm, J. Flora, Field Evaluation of Hydrogen Fluoride Continuous Monitoring
Systems for Fugitive Emissions from Primary Aluminum Potrooms, (Draft Report) EPA Contract
No. 68-D2-0165, Midwest Research, September 26, 1995
2. V. Harris, J. Wilner, "Laboratory and Field Evaluation of Ion Mobility Spectroscopy for
Measuring Hydrogen Fluoride Emissions" presented at the AWMA conference in San Antonio,
Tx. 1995.
3. T. Bacon, "Real Time Monitoring of Chlorine Dioxide Without Interference from Chlorine",
Pittsburg Conference, New Orleans, 1992, Paper # 326P.
4. G. Gorden, G. Pacey, B. Bubnis, S. Laszewski, J. Gaines, "Safety in the Workplace: Ambient
Chlorine Dioxide Measurements in the Presence of Chlorine", presented at Chemical Oxidation
Fourth International Symposium, Vanderbilt University, Nashville Tenn., 1994
KEYWORDS
Analyzer, Ion Mobility Spectroscopy, Ammonia, Hydrogen Chloride, Hydrogen Fluoride, Chlorine,
Process Monitoring