Applied Analytical Systems

The Effect of Air Leaks on the Performance of GC and GC/MS Systems

Introduction

In gas chromatography, air leaks can cause a cascading series of deleterious effects on system components and chromatographic results. Establishing and maintaining leak-free connections in GC systems is a basic yet critical aspect of gas-phase analysis. Leak-free systems provide consistent, reliable data and can improve productivity by increasing intervals between required maintenance. At elevated temperatures, generally higher than 260 °C, polysiloxane-based GC columns bleed and lose stationary phase, depending on the substituent group linked to the polymer . In the presence of oxygen, bleed increases dramatically at elevated temperature. Increased column bleed in turn leads to a shift to shorter retention for peaks of interest and an early demise of the GC column.

Liner activity is also impacted by the presence of oxygen in the flow path. Oxygen can strip deactivation layers from glass liners, leaving more active sites onto which polar analytes can sorb. This leads to increased tailing, poor peak integration, and inaccurate results. The need for more frequent inlet maintenance, along with system downtime, is a direct consequence of leaks, especially for analyses of active analytes, such as chlorinated pesticides

In GC/MS, air leaks in the flow path produce high levels of noise, increased bleed, shorter filament lifetime, more frequent source cleaning, and reduced electron multiplier life times. Cumulative effects of oxygen-contaminated carrier are illustrated in this application note to underscore the need to establish and maintain leak-free systems to the fullest extent possible.

Materials and Methods

System 1 consisted of a dual-channel FID Agilent 7890A GC equipped with a dual-tower Agilent 7693 Automatic Liquid Sampler and two inert split/splitless inlets. An inline three-way valve was installed in the carrier line leading to the rear inlet. One leg of the valve was plumbed to a pure helium (99.9999%) source, and the other was plumbed to a helium cylinder containing 1,000 μL/L oxygen. This installation enabled switching back and forth between pure helium and helium doped with oxygen to evaluate how lasting any effects might be. The front inlet was plumbed to the same pure helium source as the pure helium leg of the three-way valve on the rear inlet. Inlet maintenance and testing sequences were run simultaneously to ensure as close to a one-to-one comparison as possible.

System 2 consisted of an Agilent 7890B GC and an Agilent 5977A Series GC/MSD System equipped with a single-tower 7693 Automatic Liquid Sampler and an inert split/splitless inlet. An inline three-way valve was installed in the carrier line leading to the front inlet. One leg of the valve was plumbed to a pure helium source and the other was plumbed to a helium cylinder containing 1,000 μL/L oxygen. This installation enabled switching back and forth between pure helium and helium doped with oxygen to evaluate how lasting the effects might be

GC-FID conditions

Column: Agilent J&W DB-1701, 20 m × 0.18 mm, 0.18 μm

Carrier: Helium (front) versus 1,000 μL/L O2 in helium (rear),

constant flow 1.36 mL/min at 125 °C

Oven: 125 °C (0.34 min) to 275 °C (7.3 °C/min, 10.1 min hold)

Inlet: Pulsed splitless, 45 psi 0.32 min, 1 μL at 250 °C, total flow

54.4 mL/min, 3 mL/min switched septum purge, gas saver off,

50 mL/min purge flow after 0.33 min

Sample: CLP pesticide mix 4 μg/mL or endrin/DDT 20 μg/mL

Inlet liner: Ultra Inert splitless single taper with wool (p/n 5190-2293)

Dual FIDs: 300 °C at 40 mL/min H

GC/MS with Agilent J&W DB-1701 conditions

Column: Agilent J&W DB-1701, 20 m × 0.18 mm, 0.18 μm

Carrier: Helium versus 1,000 μL/L O2 in helium, constant flow

1.36 mL/min at 125 °C

Oven: 125 °C (0.34 min) to 275 °C (7.3 °C/min, 10.1 min hold)

Inlet: Pulsed splitless, 45 psi 0.32 min, 0.5 μL at 250 °C, total flow

63.9 mL/min, 3 mL/min switched septum purge, gas saver off,

60 mL/min purge flow after 0.33 min

Sample: CLP pesticide mix 4 μg/mL or endrin/DDT 20 μg/mL or

semivolatile mx

Inlet liner: Ultra Inert splitless single taper with wool (p/n 5190-2293)

MSD temps: Transfer line 280 °C, source 300 °C, quad 180 °C

Mode: Full scan, 10 to 450 amu

Results and Discussion

GC/FID

A 7890A GC dual-channel FID, a 7890B GC, and a 5977 Series GC/MSD system were set up with a three-way valve in the carrier line to enable switching between pure helium, and helium doped with 1,000 μL/L oxygen to simulate an air leak. Oxygen in helium at 1,000 μL/L is representative of a 5% by volume air leak into each respective system. This approach was chosen to rapidly demonstrate the deleterious effects of having oxygen in the carrier gas. Organochlorine pesticides included in US EPA Method 8081 were chosen as a test case for the dual-GC-FID system. DB-1701 columns (14% (cyanopropyl-phenyl) methylpolysiloxane phase) were used to demonstrate the effects of having oxygen in the carrier, with respect to endrin/DDT breakdown, bleed profiles, and retention time stability.

Figure 1 shows a typical FID trace for US EPA 8081 pesticides on a 20 m × 0.18 mm, 0.18 μm Agilent J&W DB-1701 column. The nominal concentration of the pesticide mix in this chromatogram was 4 μg/mL.

Almost immediately, oxygen in the carrier gas was observed to have a subtle impact on the column bleed performance with temperature cycling on the DB-1701 stationary phase. There was an increase in bleed on the oxygen-exposed column as the oven temperature climbed to 275 °C. The effect persisted, even after purging with pure helium carrier, indicating permanent column damage.

Figure 2 is an overlay of blank injection FID chromatograms with and without

oxygen in the carrier.

GC/MS

A 7890B GC with a 5977 Series GC/MSD system was set up with a three-way valve in the carrier line to enable switching between pure helium, and 1,000 μL/L oxygen-doped helium to simulate an air leak. The three-way valve was switched back and forth between the oxygen-doped and pure helium carrier gas. At first, the oxygen-doped carrier was inline only during acquisition sequences. Gradually, exposure to oxygen-doped carrier gas was increased to overnight, then weekends, and finally for five days continuously until auto tune no longer functioned. The cumulative total of oxygen exposure for the  GC/MS was 15 days before the system would no longer tune. The highest EMV (electron multiplier voltage) obtained was 2,350 volts, at which time source cleaning and filament replacement were required to return the instrument to normal function with pure helium.

GC/MS testing was conducted on a DB-5ms column with a 1 μg/mL GC/MS semivolatile analyzer check-out mix. Here, the impact of oxygen in the carrier gas was readily apparent in that the high signal background all but wiped out the analyte signal. At a level of 1 μg/mL, this represents an alarming loss of sensitivity.

Figure 3 is an overlay of the total ion chromatograms for an injection of 1 μg/mL GC/MS semivolatile mix with oxygen in the carrier and without oxygen in the carrier.

Conclusions

Air leaks into GC and GC/MS systems have a dramatic and cumulative effect on system performance. Permanent column damage, retention time drift to shorter retention, and increased inlet activity are characteristic of oxygen exposure at elevated temperature in both GC and GC/MS systems. All of these effects were observed. Dramatic signal loss, high background noise and rapid increase in electron multiplier voltage were seen on the GC/MS system.

The conditions used in these experiments were chosen to simulate an air leak of approximately 5% air into the system, and to rapidly visualize the deleterious effects. Both GC column and liner lifetimes were shortened, requiring more frequent maintenance. Under these conditions, after 15 days of cumulative exposure to oxygen, electron multiplier voltage climbed to 2,350 volts and the filament failed, dictating a major service event in just over two weeks. Having to clean the source and replace a filament twice a month would most certainly have a dramatic impact on system productivity. All of these deleterious effects build a strong case for doing everything possible to keep air and the oxygen it contains out of GC and GC/MS systems.

Reference:

Culled from: Impact of Air Leaks on the Productivity of GC and GC/MS Systems by Ken Lynam. Application Notes. Agilent Technologies Inc.

Written by Muyiwa Adebola,

www.aasnig.com, [email protected]

07084594004, 07084594001

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