Headspace GC FID / RSD values are high

Discussions about GC and other "gas phase" separation techniques.

5 posts Page 1 of 1
Hello!

I am developing a method on headspace (Shimadzu AOC 6000 Plus) gas chromatography (Shimadzu Nexis 2030) with FID detector. I have to develope, optimize and validate a method that will analyse 16 different analytes (polar and non polar) in sludge.

RSD/% of Areas for Metoxy-2-propanol, Etoxy-2-propanol, Ethyl-benzene, m-,p-Xylene, o-Xylene are big (later eluting analytes with high boiling point).

For example:

I made a solution which consists of:

Analyte ______________V/V volume / volume (%)
Acetone_______________ 3%
Ethyl-acetate___________ 8%
Metoxy-2-propanol______ 10%
Etoxy-2-propanol________ 20%
Ethyl-benzene__________ 20%
Xylene________________ 39%

10 vials ( vial volume =20 mL) with 800 uL of the prepared solution are analyzed.

Conditions
Incubation temp: 80°C
Incubation time: 30 min
Syringe temp:150°C
Fill strokes count: 0
Injection flow rate: 3mL/min
Injection volume: 250 ul
Inlet temp: 200°C
Carrier gas: Nitrogen
Column: SH-I-624Sil MS 60 m x 0,25 mmID x 1,40 um df
Total flow: 154,5 mL/min
Purge flow: 3 mL/min
Column flow: 1,5 mL/min
Linear velocity: 27,6 cm/s
Split ratio: 1:100
Oven temperature program:
----------- 35°C 17,5 min
10°C/min 100°C 3,5 min
40°C/min 240°C 1 min
Total program time: 32,00 min
Detector temp: 240°C

Analyte ______________ RSD (%)
Acetone_______________ 5,80%
Ethyl-acetate___________ 10,32%
Metoxy-2-propanol______ 25,39%
Etoxy-2-propanol________ 23,68%
Ethyl-benzene__________ 23,19%
m-,p-Xylene____________ 23,24%
o-Xylene_______________ 23,65%

Why is RSD so high for later eluting analytes (with high boiling point)? I already excluded a possibilty of leaky vials (used new caps with new septums).

Any suggestions?
You may be exceeding the sample loading capacity of the column.

You have for example Ethyl Benzene, density 0.67ug/ul x 800ul = 696ug x 0.2(percent volume) = 139.2 ug in the sample.

Typical sample loading capacity of a 0.25mmID column is 50-100ng.

Split ratio 1:100 x 139.2 ug = 1.392ug = 1392ng of Ethyl Benzene introduced onto the column, about 10x more than the column can process efficiently.

If my math is correct.

A 0.53mmID column though has a capacity of 1000-2000ng, that would handle the sample as you currently process it.
The past is there to guide us into the future, not to dwell in.
Dear James,

Thank you for your answer.

Here lies my problem.
Method I have to develope has to quantitatively determine all analytes in sludge in the range of LOQ-100% (v/v).
That is way I have to use Area Normalization.

How can I overcome the problem of high RSD when the v/v percentage is high without changing the column?
studentjelena wrote:
Dear James,

Thank you for your answer.

Here lies my problem.
Method I have to develope has to quantitatively determine all analytes in sludge in the range of LOQ-100% (v/v).
That is way I have to use Area Normalization.

How can I overcome the problem of high RSD when the v/v percentage is high without changing the column?



You need to calibrate for the linear range of the instrument then either reduce sample size or dilute sample into the range of the instrument.

No detector has infinite linear range. If you calibrate with that standard at 20% Ethyl Benzene then analyze a sample that is 100% Ethyl Benzene you most likely will not have a quantitative final result of 100%.

You need a minimum of 3 concentration levels to form a linear curve fit, 5 concentrations is much better. I run some calibration curves that have up to 8 concentration levels.

How do you calculate the LOQ if you are using a single calibration point at 20%v/v? If you calculate it, then dilute a standard to that level does it quantify at the expected concentration or is the peak even visible at all?

Try an experiment taking the original mixture, then diluting it by 10, then by 10 again, and again until you can't see the peaks of the analytes. See if those responses give a straight line when plotted as concentration versus area. The portion that does give a straight line is the linear range of the instrument. Then plan your sample preparation to put the results within that range of concentrations.
The past is there to guide us into the future, not to dwell in.
Hi James.


Thank you for you suggestions.

I absolutly agree with you about calibration and linear range. So, in conclusion I could not quantitatively determine VOCs in sludge ranging from LOQ-100% with precisions and certainty because FID detector isn't probably sensitive to too high (~ 100%) concentration?

LOD and LOQ determination:

I would dilute until all of my peaks cannot be detected. Then I would take 4 low concentrations and made a graph: concentration (v/v%) vs. Area and calculate the slope and the standard deviation. LOQ = 10 × SD / slope and LOD = 3.3 × SD / slope.

My plan:
10 repetitions with a mix of all 16 analytes with equal v/v% in 800 uL. I am starting with 4.96% v/v of each analyte (40 uL of each analyte in a total volume of 800 uL). Acetonitrile as a solvent.
I could detect 0.0062% v/v per analyte in a mix of 16 analytes but my repeatabilty of areas are not good. Sometimes all of the peaks show up sometime they do not (another problem I am experiencing).

I don't think is sample overloading in case of 20% v/v ethyl benzene because I analyzed cyclohexane who is 20% and isooctane 25% v/v and the RSD value is fine.

Standard 1
Analyte_____________ V/V%_____________ RSD/%

Methanol_____________ 3______________ 3.05
Isopropanol___________5_____________ 11.75
Cyclohexane_________ 20_____________ 6.84
Butanol______________ 22 ____________ 34.79
Toluene______________ 50____________ 24.57

Standard 2

Ethanol________________ 8 ____________ 4.15
Methyl-ethyl-ketone____ 10____________ 5.35
Isooctane______________ 25 ___________ 5.25
Methyl-isobutyl-ketone___ 7 __________ 20.43
Butyl-acetate____________ 50 __________ 23.08

Thank you for answering
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