ADV06-15_1-412

•

2008 vol. 1

GC Accessories

Septum bleed is not common, but when it occurs

it is observed as sharp, repetitive peaks in high

temperature portions of an analysis. Bleed peaks

can come from either the injection port septum or

the vial cap septum. Interfering peaks and inaccu-

rate data can result, so it is important to correctly

identify the source and understand how to elimi-

nate or minimize the bleed.

Diagnose the Bleed Source

The bleed from either septum shows a similar pat-

tern (Figure 1), but it is easy to determine the

source with a simple test. Isolate the injection port

by setting the instrument to perform a run with-

out an injection. Perform an analysis; if the bleed

disappears, then the vial cap septum was the

source. Determining if the vial cap septum is the

source of the bleed can save time by preventing

unnecessary troubleshooting and maintenance of

the injection port. If the vial cap septum is causing

bleed, the problem can be eliminated or mini-

mized with the following considerations.

Check Solvent-Septum Compatibility

Most of the time, septum bleed is negligible.

However, when a solvent and vial cap septum are

incompatible, extreme contamination can occur.

Figure 2 compares the first and fifth injections

from a vial containing a derivatized amphetamine

sample. In this case, the septum bleed peaks are

almost as large as the analyte peaks. This level of

bleed can interfere with analyses, especially those

geared for trace levels. Reduce the risk of septum

bleed by using a compatibility chart, such as the

one in the on-line version of this article

(www.restek.com/general

)

to determine which

septum material is compatible with the sample sol-

vent used.

Use Lined Septa

Most vial cap septa are lined with a protective layer

of polytetrafluoroethylene (PTFE) to prevent sol-

vent attack. As shown in Figure 3, PTFE effectively

prevents septum breakdown due to solvent expo-

sure. In comparison, unlined septa exhibit bleed

after just 24 hours at room temperature. Bleed lev-

els for unlined septa varied by material, but even a

low level of bleed can interfere with integration

and is of particular concern for trace analyses

(Figure 4).

How to Correctly Diagnose the Source of Bleed Contamination

By Amanda Rigdon, Innovations Chemist

• Avoid lengthy inlet troubleshooting.

• Reduce interference with correct solvent-septum compatibility.

Figure 1

Sharp, repetitive peaks are typical of septum bleed from

the vial cap or injection port.

Figure 2

Contamination from septum bleed can cause significant

interference with target analytes.

Columns:

Rtx

-5MS, 30m, 0.25mm, 0.25ìm (cat.# 12623)

Sample:

methylene chloride blank

Inj.:

1.0µL split (split ratio 10:1), 4mm split inlet liner w/ wool (cat.# 20782)

Inj. temp.:

240°C

Carrier gas:

helium, constant flow

Flow rate:

1.2mL/min.

Oven temp.:

70°C (hold 1 min.) to 290°C @ 20°C/min. (hold 13 min.)

Det.:

FID @ 250°C

Column:

Rtx

-5MS, 30m, 0.25mm ID, 0.25µm (cat.# 12623)

Sample:

100µg/mL each amphetamine, methamphetamine, MDA, MDMA, and MDEA

extracted from methanol and HFAA derivatized

Inj.:

1µL, splitless (hold 0.5 min.), 3.5mm custom splitless inlet liner w/ IP deacitvated wool

Inj. temp.:

220°C

Carrier gas:

helium, constant flow

Flow rate:

1.25mL/min.

Oven temp.:

70°C (hold 1 min.) to 290°C @ 15°C/min. (hold 4 min.)

Det:

Transfer

Line temp.:

280°C

Scan range:

43-450amu

Ionization:

Mode:

scan

1. amphetamine

2. methamphetamine

3. MDA

4. MDMA

5. MDEA

GC_EX00981

GC_EX00982

Injection

The Forgotten Septum