restek corporation, 110 benner circle, bellefonte pa … inlet seal for 1/16” micro-packed columns...

An inlet seal for 1/16” micro-packed columns allows the thermocouple to pass into the liner from the bottom. No column nut was installed.

A Matter of Degrees, but do Degrees Really Matter? A Study of GC Inlet Temperature Profiles, Inlet-to-Inlet Temperature Variability, and Their Effects on Chromatography

Scott Grossman, Jack Cochran, Gino Tambourine Restek Corporation, 110 Benner Circle, Bellefonte PA 16823

0

10

20

30

40

50

60

70

80

90

80 105 130 155 180 205 230 255 280

Measured From the Bottom

Measured from the Top

Therm

ocouple

Positio

n I

nsid

e t

he L

iner

(mm

, 78 m

m ≈

top o

f lin

er)

Thermocouple Temperature ( C)

Abstract: For gas chromatographers, the inlet liner is the vessel in which their liquid sample must vaporize reproducibly and without discrimination or sample loss to ensure precise and accurate analyses. Many parameters affect how successfully that happens, but inlet temperature is certainly a significant one. When the inlet temperature is set, how much of the inlet is actually at that set-point? If not the whole inlet, what is the inlet’s actual thermal profile? Finally, among inlets with different designs but ostensibly similar performance, how similar are the thermal profiles? While information on GC inlet thermal profiles has been published, we were surprised to see that seemingly significant differences can exist between inlets of a similar type for a variety of reasons. By probing these differences chromatographically, we explore what effect such thermal profile differences can have on injection phenomena for compounds across a wide boiling point range.

1. The First Measurements, and the Motivation to Continue! Within the context of another experiment, temperature measurements were taken from the inside of two Agilent split/splitless injection ports. In the graph in this section, these initial two inlets are labeled “7890 Split/Splitless 2” and “6890 Split/Splitless (Missing Top Insulation Ring and Bottom Oven Wall Insulation). At the time of the initial measurements, the missing insulation was not observed immediately. Rather, the extreme temperature differences were noted, which lead to an investigation. During the investigation, a number of other inlets were measured, and their data is also shown in this section’s graph. This motivated us to want to explore these temperature measurements more, and more importantly, to observe how they might affect the chromatography of a set of analytes with a very wide molecular weight range.

0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)

Thermocouple Temperature (°C)

7890 Split/Splitless 1


6890 Split/Splitless with EZ Twist Top 1

6890 Split/Splitless with EZ Twist Top 2(Suspected Missing Oven Wall Insulation)6890 Split/Splitless

6890 S/SL (Missing Top and Bottom Oven WallInsulation)6890 S/SL (Top Insulation Replaced)

Inlet Setpoint0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)







Inlet Setpoint

0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)







Inlet Setpoint

0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)







Inlet Setpoint0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)







Inlet Setpoint

0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)





6890 Split/Splitless with EZ Twist Top 2(Suspected Missing Oven Wall Insulation)

6890 Split/Splitless

6890 S/SL (Missing Top Insulation Ring andBottom Oven Wall Insulation)

6890 S/SL (Top Insulation Replaced)

Inlet Setpoint

0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)





6890 Split/Splitless with EZ Twist Top 2(Suspected Missing Oven Wall Insulation)

6890 Split/Splitless

6890 S/SL (Missing Top Insulation Ring andBottom Oven Wall Insulation)

6890 S/SL (Top Insulation Replaced)

Inlet Setpoint0

10

20

30

40

50

60

70

80

80 100 120 140 160 180 200 220 240 260 280

Th

erm

oco

up

le P

osit

ion

in

In

let

Lin

er (

mm

-w

ith

78

mm

= T

op

of

Lin

er)







Inlet Setpoint

2. The Next Measurements and a Difference is Discovered! With an interest in measuring inlet temperatures, we decided to explore some of the effects the manner in which the measurements were taken has on the measurement itself. The example shown below in the figure and graph illustrates the differences in temperature measurements based on whether or not the thermocouple was inserted from the bottom of an Agilent injection port or from the top. The details of the setups are given in the figure, but the lesson we learned is that while we believe the relative differences observed when measuring inlet-to-inlet variability are real as long as the measurement technique is consistent, the actual temperature may be different. Other factors affect the temperature measurement, but they are not covered in this poster.

Bottom Up

The thermocouple is inserted into the inlet from the bottom, pushed all the way to the top of the liner, and pulled down in 5 mm increments (ending at 75 mm from the top of the liner, or 3.5 mm from the bottom). With no column, measurements were made without gas flow in this scenario.

Carr

ier

Gas F

low

Used in T

op D

ow

n A

ppro

ach

The thermocouple is inserted into the inlet from the top through a modified weldment , pushed to the bottom of the liner, and lifted 1 mm to ensure unobstructed gas flow. The column, is installed lower than usual to accommodate the thermocouple but still allow for gas flow. The thermocouple is pulled up in 5 mm increments (ending 75 mm from the bottom, or 3.5 mm from the top of the liner). This arrangement best mimics operation conditions and was used for subsequent investigations.

Top Down

How To Take an Inlet’s Temperature…

A difference was observed, which we believe is significant and not just measurement error, since repeat measurements under the same conditions were very reproducible.

3. Making Deliberate Changes and Measuring Their Effects! Our initial observations suggested that temperature differences could exist from inlet-to-inlet, and that often times the greatest differences were observed when insulation was missing, which is not a surprise. But how do these differences affect chromatography? To answer those questions, a mixture of 17 straight-chain hydrocarbons ranging from C8 to C40 (counting by the evens) dissolved in carbon disulfide were analyzed under varying chromatographic conditions (e.g. split vs. splitless), varying inlet liner choices (e.g. straight with wool vs. single taper with wool), and varying insulation conditions (e.g. nut warmer vs. no nut warmer). Some of the results are shown below, but more data will be made available on our website (www.restek.com). All temperature measurements shown in this section were collected by measuring using the Top Down approach.

Effect of Oven Temperature on Bottom of Liner With and Without the Nut Warmer

The aluminum cup that mounts to the upper wall of the oven and surrounds the bottom of the inlet is often discarded for column installation convenience, but does it affect liner temperature, and if so, does that affect chromatography?

3a. Nut Warmer ?

In the graph directly above, the traces are temperatures taken with the oven door open, while the two open shape data points are temperatures taken at the very bottom of the liner with the oven door closed and set to 40 °C! While the oven air is hotter than ambient, the circulation caused by the oven fan is more actively cooling the very bottom of the inlet, and hence the bottom of the inlet liner, and that difference is exacerbated by the absence of the nut warmer. These temperatures better represent operational conditions.

2 4 6 8 10 12 14

pA

-400

-300

-200

-100

0

100

200

300

400Without nut warmer cup

With nut warmer cup

C8

C40

C28

Inlet Setpoint = 250 °C

Effect of Nut Warmer Cup on Chromatography Splitless with Single Taper with Wool

Time (min)

FID

Response (

PA)

3b. Nut Warmer & Top Insulation Ring Next we removed the top, fibrous insulation ring that sits just under the inlets perforated disk on Agilent 6890 and 7890 instruments. We kept the nut warmer off to try to show a cumulative effect.

min2 4 6 8 10 12 14

pA

-250

0

250

500

750

1000

1250

1500

1750

Fully insulated(inlet @ 300 °C)

No nut warmer (inlet @ 300 °C)

No nut warmer & no top insulation (inlet @ 250 °C)

No nut warmer & no top insulation (inlet @ 270 °C)

Time (min)

FID

Response (

PA)

Effect of Nut Warmer Cup & Top Insulation Ring on Chromatography Splitless with Single Taper with Wool

0

10

20

30

40

50

60

70

80

100 150 200 250 300

Top Insulation Ring & NoNut Warmer

No Top Insulation Ring &No Nut Warmer

Fully Insulated


Therm

ocouple

Positio

n I

nsid

e t

he L

iner

(mm

, 78 m

m ≈

top o

f lin

er)

Note the higher recorded temperatures! This was puzzling indeed!


0

10

20

30

40

50

60

70

80

100 150 200 250 300

No Top Insulation but WITHNut Warmer Cup

No Top Insulation Ring &No Nut Warmer


Therm

ocouple

Positio

n I

nsid

e t

he L

iner

(mm

, 78 m

m ≈

top o

f lin

er)


Interestingly, with the nut warmer replaced, the thermal profile looks almost identical to

the trace without the nut warmer. However, the inlet

was able to achieve 300 °C without

trouble, suggesting that heat loss had

indeed been curbed.

The temperature profile data for the inlet without the nut warmer and the top insulation ring was not what we had expected, but the temperature measurements were confirmed by chromatography. To the left, the series of chromatograms shows how an inlet without two major pieces of insulation and set 30 °C lower (blue trace) than the fully insulated condition (purple trace) is able to produce almost identical chromatographic results, both for peak shape and amount transferred to the column. Compare that to the condition where just the nut warmer was missing and the inlet was hotter (green trace), and it’s conclusive that the temperature inside the inlet was indeed hotter under these conditions.

From above, the higher recorded temperatures at the top and bottom of the liner when even more insulation was removed was puzzling, but another interesting observation was that when the inlet was set to 300 °C, instead of the 250 °C used to take the temperature measurements, the inlet shut down because it could not achieve 300 °C. This suggested that even though the top and bottom were hotter, heat was escaping at an accelerated rate (which is what we did expect) and so the operation of the inlet was compromised.

3c. Putting it all back…

0

10

20

30

40

50

60

70

80

100 150 200 250 300

Fully Insulated BeforeRemoving Any Insulation

Fully Insulated After HavingRemoved Insulation andThen Replaced


Therm

ocouple

Positio

n I

nsid

e t

he L

iner

(mm

, 78 m

m ≈

top o

f lin

er)

min4 6 8 10 12 14

pA

-400

-300

-200

-100

0

100

200

300

400Fully Insulated After Removing & Replacing Insulation

Fully Insulated Before Disturbing Any Insulation

Time (min)

FID

Response (

PA)

Interestingly, after all of the insulation was replaced, the temperature profile, corroborated by the chromatography, was different, suggesting that even without ostensibly changing anything, the conditions within the inlet can change if the inlet is disturbed

C40

C8

!

4. Split/Splitess vs. the Multimode Inlet

0

10

20

30

40

50

60

70

80

40 90 140 190 240 290

Fully Insulated S/SL (BeforeAny Insulation Removal)

Fully Insulated MMI


Therm

ocouple

Positio

n I

nsid

e t

he L

iner

(mm

, 78 m

m ≈

top o

f lin

er)

min2 4 6 8 10 12 14

pA

-400

-300

-200

-100

0

100

200

300

400

Split/Splitess

MMI

Time (min)

FID

Response (

PA)

Comparison of S/SL and MMI Inlets Splitless with Single Taper with Wool

C40

C8

Because the Agilent Multimode Inlet (MMI) uses the same inlet liners as a split/splitless inlet (S/SL), and because it is capable of performing traditional hot split and hot splitless injections as well as other injection modes traditionally performed with PTV inlets, we wanted to see how the MMI’s temperature profile, fully insulated, compared to the S/SL inlet. From the charts below, under the same analytical conditions, the differences are very apparent from a temperature profile and chromatography perspective. These differences merit more investigation to better understand whether method transfer from a S/SL inlet to a MMI would be a reliable option.

Analytical Run Conditions: Autosampler – 5 µL syringe, 1 µL injection, 3 pre- and post-injection solvent washes, 1 sample wash, 6 syringe pumps, fast autosampler (hexane rinse solvent) Inlet - 300 °C, gas saver off, splitless, hold time = 1.5 min, 60 mL/min purge flow (these conditions were used for both split/splitless and multimode inlets) Column – Rxi – 5SIL MS, 30 m, 0.25 mm, 0.25 µm, constant flow @ 2 mL/min Oven – 40 °C (hold 1.5 min) to 70 °C @ 75 °C/min to 115 °C @ 45 °C/min to 175 °C @ 40 °C/min to 300 °C @ 30 °C/min to 340 °C @ 20 °C/min (hold 5 min) = 15.1 min analysis (oven program set to reported max ramp rates, although the oven did lag at hotter and hotter temperatures – this may be the cause of some of the observed retention time shifting for higher molecular weight compounds) FID – 350 °C, H2 @ 40 mL/min, Air @ 350 mL/min, N2 @ 45 mL/min Sample – 10 ng/µL mix of 17 straight-chain hydrocarbons (C8-C40, evens) in CS2

5. Conclusions Even though the technique used to take measurements can affect the actual value, it’s clear that differences in inlet temperature do exist between inlets, especially when the insulation is disturbed. Those differences can affect chromatography, but it seems they largely affect the heaviest compounds the most, in some cases almost completely eliminating their transfer to the column. This poster explores only a portion of what we investigated, so for more information regarding this work, please contact: [email protected]

0

10

20

30

40

50

60

70

80

100 150 200 250 300

With Nut Warmer

Without Nut Warmer

Nut Warmer (Oven @ 40 C)

No Nut Warmer (Oven @ 40 C)

Inlet Temperature Profile With and Without Nut Warmer

Temperature taken at the very bottom of the liner with the oven door closed and oven at 40 °C


Therm

ocouple

Positio

n I

nsid

e t

he L

iner

(mm

, 78 m

m ≈

top o

f lin

er)

restek corporation, 110 benner circle, bellefonte pa … inlet seal for 1/16” micro-packed columns...

Documents