Retention Time/Index Compression??

[sarakuhl]

I'm running an isothermal method looking at crude oil from C6-C20 on a ZB1-inferno (30x0.25x0.25), 150 split ratio. Identifying the n-alkanes is easy. They're obviously visible. But consistent ID of inter-alkane peaks becomes difficult, particularly after system maintenance. I've tried calculating retention indeces. I say plural because I've done Kovats, Linear, Linear interpolation, cubic spline, bessel spline, etc. When I calculate them for the same peaks on a run of the same sample after inlet maintenance (column clip included), the indeces for most peaks is fine, (+/- 0-2 points). But for the heavier inter-alkanes, the indeces shift upwards of 8 points. Is this a "normal" phenomena? Does this mean there is something "wrong" with my system? Do I need to change a setting in my method? Being able to consistently id these smaller peaks from one sample set to the next is critical to my experiments and any assistance would be GREATLY appreciated!

[James_Ball]

I'm running an isothermal method looking at crude oil from C6-C20 on a ZB1-inferno (30x0.25x0.25), 150 split ratio. Identifying the n-alkanes is easy. They're obviously visible. But consistent ID of inter-alkane peaks becomes difficult, particularly after system maintenance. I've tried calculating retention indeces. I say plural because I've done Kovats, Linear, Linear interpolation, cubic spline, bessel spline, etc. When I calculate them for the same peaks on a run of the same sample after inlet maintenance (column clip included), the indeces for most peaks is fine, (+/- 0-2 points). But for the heavier inter-alkanes, the indeces shift upwards of 8 points. Is this a "normal" phenomena? Does this mean there is something "wrong" with my system? Do I need to change a setting in my method? Being able to consistently id these smaller peaks from one sample set to the next is critical to my experiments and any assistance would be GREATLY appreciated!

When you clip the column are you adjusting the head pressure lower to account for the shorter length so you have the same linear velocity through the column?

[AICMM]

I am curious, and it really is none of my business, but why are you doing isothermal between C6 and C20??

Best regards,

AICMM

[sarakuhl]

I am curious, and it really is none of my business, but why are you doing isothermal between C6 and C20??

Best regards,

AICMM

To clarify, my temperature ramp is linear. I don't know why I said my method was isothermal. Sorry.

[sarakuhl]

I'm running an isothermal method looking at crude oil from C6-C20 on a ZB1-inferno (30x0.25x0.25), 150 split ratio. Identifying the n-alkanes is easy. They're obviously visible. But consistent ID of inter-alkane peaks becomes difficult, particularly after system maintenance. I've tried calculating retention indeces. I say plural because I've done Kovats, Linear, Linear interpolation, cubic spline, bessel spline, etc. When I calculate them for the same peaks on a run of the same sample after inlet maintenance (column clip included), the indeces for most peaks is fine, (+/- 0-2 points). But for the heavier inter-alkanes, the indeces shift upwards of 8 points. Is this a "normal" phenomena? Does this mean there is something "wrong" with my system? Do I need to change a setting in my method? Being able to consistently id these smaller peaks from one sample set to the next is critical to my experiments and any assistance would be GREATLY appreciated!

When you clip the column are you adjusting the head pressure lower to account for the shorter length so you have the same linear velocity through the column?

No... ?

[James_Ball]

I'm running an isothermal method looking at crude oil from C6-C20 on a ZB1-inferno (30x0.25x0.25), 150 split ratio. Identifying the n-alkanes is easy. They're obviously visible. But consistent ID of inter-alkane peaks becomes difficult, particularly after system maintenance. I've tried calculating retention indeces. I say plural because I've done Kovats, Linear, Linear interpolation, cubic spline, bessel spline, etc. When I calculate them for the same peaks on a run of the same sample after inlet maintenance (column clip included), the indeces for most peaks is fine, (+/- 0-2 points). But for the heavier inter-alkanes, the indeces shift upwards of 8 points. Is this a "normal" phenomena? Does this mean there is something "wrong" with my system? Do I need to change a setting in my method? Being able to consistently id these smaller peaks from one sample set to the next is critical to my experiments and any assistance would be GREATLY appreciated!

When you clip the column are you adjusting the head pressure lower to account for the shorter length so you have the same linear velocity through the column?

No... ?

Most systems with electronic flow control have to calculate the flow using the column internal diameter and length to know what head pressure will deliver a certain flow rate (ml/min or cm/sec whichever it is set to use). The shorter the column the less head pressure required to achieve the desired flow. If you trim column but do not tell the instrument the new length, or manually adjust the head pressure if not using electronic flow control, then the controller will apply too much head pressure for the flow required. Also I don't believe it is a linear relationship as temperature increases, so you may have 1.2ml/minute initial flow when you expect 1.0ml/minute and maybe 1.3 or 1.4ml/minute at maximum temperature when you expect 1.0ml/min. If that is happening it could be throwing off the later eluting analytes more than the early analytes.

[Henrylozano]

I'm running an isothermal method looking at crude oil from C6-C20 on a ZB1-inferno (30x0.25x0.25), 150 split ratio. Identifying the n-alkanes is easy. They're obviously visible. But consistent ID of inter-alkane peaks becomes difficult, particularly after system maintenance. I've tried calculating retention indeces. I say plural because I've done Kovats, Linear, Linear interpolation, cubic spline, bessel spline, etc. When I calculate them for the same peaks on a run of the same sample after inlet maintenance (column clip included), the indeces for most peaks is fine, (+/- 0-2 points). But for the heavier inter-alkanes, the indeces shift upwards of 8 points. Is this a "normal" phenomena? Does this mean there is something "wrong" with my system? Do I need to change a setting in my method? Being able to consistently id these smaller peaks from one sample set to the next is critical to my experiments and any assistance would be GREATLY appreciated!

What's your GC detector?