MSL935004A Maintain instruments and equipment

Gas Chromatograph

Contents: Quiz on last lesson

GC and its components -Review of basic Cleaning and maintenance of main components Trouble shootings Calculations involved in GC

Basic review of GC-8A Shimadzu

What are Applications of GC?

• separation volatile and thermally stable organic compounds

• quantitative & qualitative

• Operates at high temperature usually 5-10 oC higher than highest bp of mix

Important to know that:

Carrier Gas (Mobile phase): • Essential in moving analyte through column • High Ultra Purity (less then 10ppm impurities)

• Restricted choice

• Choice of N2 , Ar, He, Air, H2,

• Storage • Inert tubing

Comparison of gases

Gas Application Comments

He General carrier or make-up gas Excellent but expensive Can not use with ECD unless with make up gas

N2 General make-up gas Carrier gas (packed column)

Cheap Not good for capillary as carrier gas (gives long run times)

H2 Carrier gas (capillary columns) Combustion gas (FID, NPD & FPD)

Highly explosive (oven trip sensor & careful venting) Best carrier gas (capillary)

Air Combustion gas (FID, NPD & FPD) Cheap and readily available

Injector: • Via micro-syringe • Sample not large for optimum column efficiency • Slow Injection of large sample causes band

broadening (loss of resolution) • Temperature should be about 50°C higher than the

boiling point of the least volatile component of the sample

Packed -Inert solid supported material (glass or stainless steel coil of 1-5m length and 5mm diameter) filled with stationary phase or coated with liquid stationary phase. Carbowax- a brand name for polyethylene glycol

Column (Stationary phase):

Capillary-

stationary phase is coated on inert solid tube Common polymers use as stationary phases are polysilioxanes

• Polar or Non-polar stationary phase

• Temp must be controlled

Detector: Detector Selectivity Detectability

Flame ionization (FID)

Most organic compounds 100 pg

Thermal conductivity (TCD)

Universal 1 ng

Electron capture (ECD)

Halides, nitrates, nitriles, peroxides,

anhydrides, organometallics

50 fg

Nitrogen-phosphorus Nitrogen, phosphorus

10 pg

Flame photometric (FPD)

Sulphur, phosphorus, tin, boron, arsenic,

germanium, selenium, chromium

100 pg

Photo-ionization (PID)

Aliphatics, aromatics, ketones, esters,

aldehydes, amines, heterocyclics,

organosulphurs, some organometallics

2 pg

Maintenance of GC

Preventive maintenance of your instruments will results in peak performance. Frequency of maintenance schedule will depends on how often you are using the instrument, if a GC is running multiple samples in an 8.0 hrs day, then you would be required to perform:

Daily: check of leakage, performance of standards, run calibration curve and gas pressure.

Weekly: check gas pressure cylinders, replace if necessary.

Change septum, injector liner, perform manual leak test.

Monthly: Check gas leak from primary gas supply to GC.

Otherwise depending on how many samples are tested/day, how clean the samples are? Condition of syringe (auto sampler), environmental factors (temperature) will need to be considered to allow you in adopting a suitable schedule.

Septa • High temp. septa are recommended to prevent injector

and column contamination. • Septum should be changed 30-50 manual injection and

75-100 for auto sampler injection. • If septa leak, air will get inside column and it will rapidly

damage the column. • When changing septa, column temp. should be reduced

to 35oC • Sign of leakage or bleeding septa (ghost peak or poor

reproducibility.

Injection Technique:

• Sample injection should be smooth and rapid, with quick removal of the syringe after injection, in order to avoid peak broadening.

• Injection volume is major issue, auto sampler is the solution. In manual injection try to rinsed and flushing the syringe 6 times.

Column • Check for a sample problem by injecting a reference standard. • If you get a good chromatogram, the problem most likely is sample related. If the chromatogram is not satisfactory, the problem probably is column or instrument related. • Check for a column problem by replacing the column with a duplicate column, one known to provide good results under proper conditions. If results are good, the problem is related to the original column. If the symptom persists, the problem is related to the instrument. • Incorrect installation of column is one source of leakage • It is important to turn oven off and wait for 10-15 minute to cool,

then turn carrier gas off (to prevent oxidation of column) before changing or installing a column.

A dirty detector creates noisy baselines.

• If the TCD is displaying problems such as a wandering baseline, increased noise level, or changes in response on a checkout chromatogram, it is probably contaminated with deposits from such things as column bleed or dirty samples.

• The TCD is cleaned by a process known as bakeout. Bakeout should be performed only after you have confirmed that the carrier gas and the flow system

components are leak and contaminant free.

• must turn off the TCD and cap the detector column fitting to prevent irreparable damage to the filament caused by oxygen entering the detector.

1. Turn the detector off.

2. Remove the column from the detector and cap the detector column fitting.

3. Set the reference gas flow rate between 20 and 30 mL/min. Set the detector temperature to 400oC.

4. Allow thermal cleaning to continue for several hours. Then cool the system to normal operating temperatures.

Trouble shootings

Please see GC resource on Moodle

What does tM represent?

The time between sample injection and an analyte peak reaching a detector at the end

of the column is termed the retention time, tR

Basic Definitions & Calculations

Term

tm

tr

t’r

Capacity factor, k’

Separation factor,

Definition/equation

-Retention time of unrestrained peak

-Retention time a component

-Adjusted retention time t’r = tr - tm

𝑘′ = 𝑡′

𝑟

𝑡𝑚

=𝑡𝑟−𝑡𝑚

𝑡𝑚

( ideal 1-5)

α= k ‘2 / k ‘1

Term

Resolution, R

Definition/equation

Measure of separation between two peaks:

𝑅𝑠 =2(𝑡𝑟2 − 𝑡𝑟1)

𝑊1 + 𝑊2

𝑊1 = 𝑏𝑎𝑠𝑒𝑤𝑖𝑑𝑡ℎ 𝑝𝑒𝑎𝑘 1

W2= 𝑏𝑎𝑠𝑒𝑤𝑖𝑑𝑡ℎ 𝑝𝑒𝑎𝑘 2

Term

Peak Asymmetry factor, AF

Number of theoretical plate, N

Height Equivalent to a Theoretical Plate,

Definition/equation

𝐴𝐹 =𝐵

𝐴𝑎𝑡 𝑊𝑎. 1

Where A = leading peak halfwidth

B= tailing peak halfwidth

N = 5.45 (tr/W1/2)2

where w1/2 is the peak width at half-height

HETP=L/N

where L is the length of the column

Sensitivity:

Slope of calibration plot (external stand)

LOD= 3 𝑥 𝑁𝑜𝑖𝑠𝑒 𝑝∓𝑝

𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦

LOQ = 10 𝑥 𝑁𝑜𝑖𝑠𝑒 𝑝∓𝑝

𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦

System Suitability Test

The following criteria should be used to determine system suitability

Injection of standard solution in 6 replicate

Relative standard deviation in peak area and retention time should be evaluated

RSD should be Peak Area (2%) and retention time (1%)