ACATS-IV  4 channel gas chromatograph

ACATS-IV is a four-channel gas chromatograph that measures CCl3F (chlorofluorocarbon (CFC) -11), CCl2FCClF2 (CFC-113), CH3CCl3 (methyl chloroform), CCl4 (carbon tetrachloride), CH4 (methane), H2 (hydrogen) and CHCl3 (chloroform) every 140 s. N2O (nitrous oxide), CCl2F2 (CFC-12), CBrClF2 (halon-1211) and SF6 (sulfur hexafluoride) are measured every 70 s. ACATS-IV operates as follows.

Air external to the aircraft is delivered to the instrument by an external, variable speed, 2-stage, Teflon diaphragm pump (KNF Neuberger), driven by a brushless 28 VDC motor. The pump is mounted on the aft wall of the ER-2 Q-bay (Figure 1) and is turned on by the ACATS-IV onboard computer when the ER-2 aircraft ascends through 87 kPa of atmospheric pressure. Ambient air is routed through about 2 m of the 4.5 mm internal diameter (ID) stainless steel tubing from outside the aircraft fuselage to the series of sample loops inside the instrument (Figure 1, dashed lines). The flow of the air through the sample loops is limited to about 180 sccm by using an absolute pressure relief valve (Tavco, set to about 150 kPa). Sample flow is measured and recorded through the flight to ensure adequate flushing of sample loops.

Figure 1
Figure 1 - Location of ACATS instrument on ER-2 aircraft

All samples throughout a flight are injected at the same sample loop pressure. To achieve this, the loops prior to the injection of a sample are isolated from the pump (using solenoid valve (SV) 6, Figure 2) and allowed to depressurize through the exhaust, lightweight, proprietary pressure controller (PC1, Figure 2) to 87 kPa. All pressure controllers of ACATS-IV are identical (PC1 through 5, Figure 2), rely upon a pressure gradient across them, and will not function properly unless the exhaust pressure is sufficiently lower than the inlet pressure. Therefore, to make measurements in the troposphere, where the ambient pressure is close to 87 kPa, a miniature diaphragm pump (KNF Neuberger, 24 V brushless DC motor, model UNMP30) is used to provide the consistent exhaust vacuum to all five pressure controllers.

About 9 s after SV6 is closed, the desired pressure is reached in the sample loops and they are isolated from the exhaust using SV5 (Figure 2). Sample injection pressure (measured by the pressure controller) and temperature are recorded for later normalization of detector responses. Then, samples are injected onto each chromatography column using a twelve-port, two-position gas sampling valve (GSV, Valco model EWC12TGA).

Two packed chromatography columns are employed per channel: a pre-column and main column, selected to rapidly separate compounds of interest. Channel # 1 also employs a short, 0.15 m post-column, operating at higher temperature than pre- and main columns (Table 1, Figure 2) to separate N2O and SF6. Detectors and columns for each chromatography channel are enclosed in individual, thin-walled, insulated aluminum housings (ovens) and are heated using 50-100W electric heaters.

Figure 2
Figure 2 - ACATS instrument schematic
Temperature control of the ovens is crucial for the stability of chromatography, and is accomplished by using five computer-interfaced controllers (Omega Engineering, Inc., Stamford, CT, model CN76122-485), one for each of the detectors and chromatography columns. Temperature variations are within ±0.1°C. The characteristics of columns are shown in Table 1 and channel configurations are shown in Table 2.

Table 1
Table 1
Table 2
Table 2

The instrument detects part per trillion (ppt) levels of gases of interest in the ambient air samples using electron-capture detectors (ECDs). Channels #2-4 use Valco ECDs (model 140BN by Valco Instruments Co. Inc, Houston, TX) with ultra-high purity (UHP, 99.999%) N2 carrier gas. Channel #1 uses a Shimadzu ECD (Shimadzu GC-mini-2, Tokyo, Japan) with UHP P-5 (argon+5% methane) carrier gas. Channel # 4 is doped with about 30 parts per million dry mole fraction (ppm) N2O to enhance the detection of H2 and methane. The ECD signals are sampled at 8 Hz. Exhaust pressures of the ECDs are controlled by pressure controllers PC2-PC5 (Figure 2), to prevent flows through ECDs from changing in response to ambient pressure fluctuations during the flight.

Fast rate of measurements is achieved in ACATS-IV through the use of a chromatographic technique, defined here as 'foldback' chromatography, which allows the next sample to be injected soon after late-eluting gases from the previous sample injection leave the pre-column and enter the main chromatographic column (Figure 3). With foldback, late-eluting peaks from a previous injection lie on a nearly flat baseline in front of the air peak of the next injection. Foldback is implemented on the ACATS-IV channels 3 and 4 (CH3CCl3, CCl4 and CH4 , Table 2, Figure 4), reducing the time between injections to 140 s while the length of chromatograms remains at 210 s. Channels #1 and 2 (N2O, SF6, halon-1211, and CFC-12) have chromatogram lengths of 70 s and do not use foldback.

Figure 3
Figure 3 - ACATS chromatogram valve timing

Foldback requires precise timing of GSV switches so that late-eluting molecules can pass completely through the pre-column before it is backflushed by switching a GSV into the 'load' position (Figure 2, GSV 1). Backflushing, the passing of carrier gas through pre-columns in the direction opposite to the normal flow, is necessary to expel from the pre-column all molecules eluting later than peaks of interest. The next sample can then be injected onto a clean pre-column. The fact that pre-columns are in use until the last eluting peaks are transferred into main columns, leaves a very short time (30 s) to backflush them. Sufficient backflushing is achieved if the volume of flushing carrier gas is at least 1.5 times the volume of carrier gas passed through a pre-column in the 'forward' direction. Therefore, backflush flow rates are maintained as high as 90-100 sccm.

Although providing adequate pre-column cleansing, the high backflush flows create strong inverted pressure gradients in the pre-columns that hinder the 'forward' carrier gas flow through the pre-column / main column series when the GSV is switched into the 'inject' position. The resulting drop in the 'forward' flow through columns and ECD produces an increase of the baseline of the chromatogram, which can be detrimental to the detection of overlaying foldback chromatographic peaks. To accelerate the recovery of 'forward' flow rates, the 'forward' carrier gas flow at injection time is increased up to 200 sccm for 4-7 s, and then decreased back to normal in a series of steps. This carrier gas flow programming helps remove baseline aberrations that can affect the foldback peaks. Flow programming is performed using proprietary low-weight mass flow controllers controlled by the instrument computer.

Even with sophisticated flow programming, the high sensitivity of ECDs to small carrier gas flow changes sometimes results in disturbances of the baseline under foldback peaks (Figure 4, channel 3, dashed lines). These small changes in the baseline can make consistent integration of the peaks difficult for some molecules (CH3CCl3, CCl4) and may increase the uncertainty of the measurements. To remedy this problem, baseline subtraction is performed prior to peak integration for these molecules. The 'clean' baseline (with no analyte peaks) is obtained from zero-air (from Scott-Marrin Company, CA) injections, which are performed under conditions identical to ambient air injections (Figure 4, dashed lines). Baseline subtraction proved necessary only for channel #3, where the detection of CH3CCl3 and CCl4 is complicated by a baseline increase (Figure 4, Channel #3). See Section 4 below for details of the integration and zero air subtraction methods.

The serial arrangement of sample loops (Figure 2) made it possible for P-5 carrier gas (pressurized at 620 kPa, compared to 150 kPa sample pressure) from the #1 pre-column to backstream into the #4 sample loop (which is used to measure methane), once the GSV #1 switches into the 'load' position. The 5% CH4 concentration in P-5 causes methane contamination of the sample on channel #4. To prevent this, a solenoid valve (SV7 on Figure 2) is used to isolate sample loops #1 and #2 from loops #3 and #4. The valve, SV7, is programmed to close shortly before the switching of GSV to 'load' on channel #1, and remains closed for 12 s until the P-5 carrier gas bleeds out of sample loops. After that SV7 is opened and all loops are filled and flushed with sample air.

ACATS-IV measures 84 x 49 x 33 cm, weighs 48 kg and is installed on the ER-2 aircraft next to the NOy instrument inside a common frame. The frame also supports a compressed gas cylinder rack with pressurized, 3 and 1.5-liter, AcuLife-IV (Scott Specialty Gases, Plumsteadville, PA) treated, Kevlar-reinforced, aluminum bottles that contain ACATS-IV and NOy carrier gases (Figure 1). An additional 1.5-liter external gas bottle was added to the underside of the ACATS-IV instrument in 2000 due to increased demand on the nitrogen carrier gas caused by faster chromatography. The entire ACATS-IV-NOy assembly is mounted to three support eyelets in the instrument bay (Q-bay) in the bottom forward section of the ER-2 aircraft fuselage (Figure 1 and Figure 2, inset).

The instrument operation is pre-programmed to allow automatic in-flight analyses of ambient air from the outside the aircraft, a standard gas and zero air from the gas rack. Instrument control and data acquisition are handled by an on-board computer.

Figure 4
Figure 4 - ACATS sample chromatograms

Further details on the operation, calibration, data processing and error analyses for ACATS-IV are available in the article
In Situ Measurements of Long-Lived Trace Gases in the Lower Stratosphere by Gas Chromatography