Gas-vloeistofchromatografie (GLC)

• GLC is used for analyzing mixtures of volatile compounds with boiling points typically below 700 K, but can also be used for less volatile compounds after derivatization.
• GLC offers advantages such as separating complex mixtures, short analysis times, reliability, and requiring small sample amounts, making it sensitive.

Introduction to GLC

• In Gas Chromatography (GC), the mobile phase is an inert gas, ensuring no interaction with the components being separated.
• The migration speed of components depends on their chemical properties.
• Gas-Solid Chromatography (GSC) involves a solid stationary phase where adsorption occurs, separating components based on their adsorption equilibria, however has limited applications, mainly for low molecular weight gases.
• Gas-Liquid Chromatography (GLC) uses a liquid stationary phase adsorbed or chemically bonded to a solid, where compounds are separated based on their distribution between the gas and liquid phases and having broader applications.

GLC Theory

• The GLC separation principle is based on distribution, with the distribution coefficient K defined as the ratio of the concentration of the analyte in the stationary phase (Cs) to its concentration in the mobile phase (Cm): K = Cs/Cm.

Van Deemter Equation

• The Van Deemter equation helps describe peak broadening in chromatography.
• HETP = A + B/u + Cu, where HETP is the Height Equivalent to a Theoretical Plate, representing column efficiency
• A is eddy diffusion (related to the variety of path lengths through the column packing).
• B is longitudinal molecular diffusion (diffusion of the analyte along the column length).
• C is mass transfer (resistance to mass transfer between phases).
• The equation can be refined to HETP = A + (B/u) + Cmu, expanding 'C' into other factors.
• The longitudinal diffusion term (B) is significant in gases due to the high diffusion coefficient (Dm), which is 10,000 times greater in gases than liquids.
• A plot of HETP against linear velocity (u) typically shows a minimum, indicating an optimal flow rate for minimizing band broadening.

GLC Apparatus

• Key components of a GLC system include a carrier gas source, injection port, column, oven, and detector.

Carrier Gas Source

• The carrier gas is typically a chemically inert gas such as He, Ar, N2, or H2.
• The choice of carrier gas depends on the detection system, viscosity, purity, safety, and cost.
• Accurate pressure regulation and measurement are essential.
• Gas flow rates can be measured using a soap bubble flow meter, which measures the time a soap bubble takes to travel between two points in a burette

Sample Injection System

• The sample injection system introduces the sample (mixture of two or more components in liquid or gas form) into the GLC system.
• Injection can be automated using an autosampler.
• The injection port typically includes a rubber or silicone septum and a septum purge to remove contaminants.
• Good column efficiency requires appropriate sample size and injection speed.
• The injection block is heated to a temperature 50°C above the boiling point of the least volatile component (direct injection).
• Gaseous samples require special injection syringes. Liquid samples are introduced using microsyringes through the septum.

Types of Sample Injection

• Direct injection: The entire sample enters the column; suitable for packed columns and large-diameter capillary columns.
• Split injection: Only a fraction of the sample enters the column; used for capillary columns.
• Splitless injection.
• PTV injection (Programmable Temperature Vaporizing): Similar to SSL but with rapid heating/cooling. The temperature can be initially low to selectively remove volatile solvents.
• On-column injection: The sample is injected directly onto the column in liquid form, eliminating discrimination of volatile components.

Split/Splitless Injector

• Injection is commonly achieved using a split/splitless injector (SSL), including a septum, gas inlet, split valve, heated metal block, glass tube, and liner.

Columns

• Columns may be made of glass, stainless steel, aluminum, or copper, with typical lengths ranging from 1 to 100 meters; they can be U-shaped or spiral.
• Column internal diameters range from 0.15 mm onwards. Columns are classified into two types based on diameter: packed or capillary.
• Packed columns (diameter > 1 mm) were the first type used (from 1950). They are filled with a packing material consisting of a finely divided solid support. The stationary phase is a thin liquid film coated on the support.
• Capillary columns (diameter < 1 mm) came later (1960-1980) and are open tubular.
• Open Tubular (OT) columns have a thin liquid film of stationary phase coated on the inner wall of the capillary tube
• Capillary columns offer higher efficiency compared to packed columns.

Types of Open Tubular columns

• Support Coated Open Tubular (SCOT) columns have a thin film of support material coated on the inner wall of the capillary before coating with stationary phase.
• Wall Coated Open Tubular (WCOT) columns have the stationary phase directly coated as a thin film on the capillary wall.
• Fused Silica Open Tubular (FSOT) columns use specially purified silica with an outer polyimide layer for strength and flexibility.

Packed columns (Column Material)

• Packed Columns, these have a diameter of 2 to 4 mm and a length of 2 to 3 m (spiraled).
• They are filled with finely divided support material with a particle size of 0.1 to 0.6 mm, balancing efficiency and pressure drop.
• The support material is typically treated diatomaceous earth (kieselguhr).
• The preparation process can lead to unwanted silanol groups that cause peak broadening and tailing. Commercial Products like CHROMOSORB are used

Column Thermostatting

• Column temperature is a critical variable that needs controlling.
• This is done using a thermostatted oven.
• The optimal temperature is a function of the boiling points of the components and the degree of separation needed.
• For components with widely varying boiling points, temperature programming (continuous or step-wise) is more appropriate.
• Higher resolution is achieved at lower temperatures.

Stationary Phase

• The stationary phase is an organic liquid, that is either adsorbed or chemically bonded onto a solid support or the column wall, either with packed columns or open tubular (SCOT / WCOT, FSOT) columns.
• Desirable characteristics: high viscosity over a wide temperature range, low volatility, thermal stability (high boiling points), chemical inertness, and appropriate polarity.
• The "like dissolves like" principle applies when selecting the stationary phase.
• An ideal stationary phase should be a TOC (Total Organic Carbon).

Detector

• The detector measures changes in the composition of the gas exiting the column by converting a physical property change into an electrical signal (e.g., electric current).
• Characteristics of an ideal detector: sensitive, stable, linear response, wide temperature range, short response time, high reliability, and ease of use.
• Types of detectors: Thermal Conductivity Detector (TCD), Flame Ionization Detector (FID), Electron Capture Detector (ECD) and Mass Spectrometer (MS).

Silanization

• Silanization is used to reduce peak broadening and tailing.
• It involves reacting silanol groups (-Si-OH) with a silanizing agent like chlorosilanes such as: -SI-OH + CI-SI-CI → -SI-O-SI-CI+HCI, followed by washing with methanol: -Si-O-SI-CI + CH3OH -> SI-O-SI-OCH3 + HCl.

Thermal Conductivity Detector (TCD)

• The Thermal Conductivity Detector (TCD), measures the difference in thermal conductivity between the pure carrier gas and the gas mixture of carrier gas and separated components.
• The TCD is also known as katharometer.
• The TCD consists of a metal block with two passages containing identical filaments of platinum or tungsten, forming part of a Wheatstone bridge. One passage serves as a reference, while the other measures the analyte.
• The TCD uses the Wheatstone bridge arrangement.

TCD Operation

• If the carrier gas flows through both channels, the bridge is balanced, i.e UA = UB.
• If a component is present in the carrier gas, the temperature of the filament changes and the electrical resistance is altered.
• With a component in the carrier gas, the bridge becomes unbalanced (UA ≠ UB) resulting in a signal (proportional to concentration).

TCD advantages and disadvantages

• TCD advantages: simplicity, fairly linear response, universality (detects organic and inorganic compounds), and non-destructive analysis.
• TCD disadvantages: limited sensitivity (10⁻⁸ g/mL), making it suitable for packed or capillary columns.

TCD - Wheatstone bridge

• With reference to the "Spanningswet" (Voltage Law):
• VDA = VCD which implies I1R1 = I2R2
• VAB=VBC which implies IRR=IMRM since
• Therefore: implying R1 = R2 and RR=RM.

Flame Ionization Detector (FID)

• In the FID, ionization occurs through a hydrogen-air flame.
• In a hydrogen-air flame there are relatively few ions resulting in a low conductance and electrical current.
• The introduction of an organic component leads to ionization, increasing conductance and electrical current.
• The components of an FID: collector (anode) and a cathode.

FID advantages and disadvantages

• FID Advantages: simple and robust.
• FID Advantages: Linear range (107).
• FID Advantages: Highly sensitive (especially for hydrocarbons with C-H bonds).
• FID Advantages: Low noise (insensitive to CO2, NOx, H2O, SO2, etc.).
• FID Advantages: High sensitivity (10-13 g/mL) suits packed and capillary columns.
• FID Disadvantages: Destructive. Requires H2 and air.