Tuesday, July 31, 2012

Automatic Gas Chromatographs


Model 301-C
Automatic Gas Chromatographs
 
     
Guest Blog Post By Dr. John N. Driscoll

The HNU Model 301-C Gas Chromatographs (GCs) is a microprocessor-based instrument designed for the automatic chromatographic separation, identification, and quantitation of chemical, components in gas or liquid samples. Hundreds of different applications can be done with HNU's wide range of detectors to choose from, special columns and gas or liquid injection systems. A variety of data outputs are available for interfacing with customer's PC's, process control computers, etc. PID Analyzers will provide turnkey installations for our equipment. The Model301-C Automatic GCs provide the ability to generate laboratory quality results in a reliable field on-line instrument.

INTRODUCTION
The microprocessor-based Model 301-C is a completely automated, computer-based Gas Chromatographs (GC's) designed for unattended, continuous monitoring of a wide variety of chemicals in ambient, in-plant and on-line applications.  These instruments offer auto calibration and optional multiple user-programmable alarms.  The entire system is operated from a keyboard and display and can be easily networked using the optional HNU-NET software. 
The Model 301-C (see figure below) has three (3) standard modes of operation; Calibration, Analytical, and Standby. 
  
301-C Rackmount GC
          
The Model 301-C has three (3) standard modes of operation; Calibration, Analytical, and Standby. An additional (optional) mode of operation: Quickscan (rapidly scans points w/o column) to pinpoint high levels or problems at various sampling locations.
The use of the automated air sampling system or liquid sampling valve provides unattended continuous monitoring of ambient air by automatically injecting air (or other gas streams) onto the analytical columns. An automatic liquid valve with vaporizer is used with liquid samples. These GC's are available with a wider range of GC detectors than any other manufacturer.

We offer five detector options; photoionization (PID), far ultraviolet (FUV), flame ionization (FID), flame photometric (FPD) and thermal conductivity. This provides a wide variety of detectors for customers application. All of the regulators, valves, and gauges needed to control the pressure and flow of the gases are located for easy use and are clearly marked on the GC. Flow rates vary from 10-80 ml/min. depending on the application.

COLUMNS 
The precolumn and analytical column perform the separation of the chemical components in a sample. The precolumn separates undesirable components (interferences) from a sample so that only the compounds of interest pass through it onto the analytical column.  The precolumn is typically a short piece of the same type of material as the main (longer) analytical column but a polar pre column could be used to remove polar compounds that coelute with a species of interest. The analytical column performs the major separation of the components in a sample so that an independent measurement can be made of one or more specific compounds. The columns can be any type of packed (1/8", 1/16" and micropack) or capillary (0.32 mm I.D. or 0.53 mm I.D.) columns (Fig.1 below) suitable for the application.
OVEN
The oven contains the heating elements, fan, and columns. The 301-C column is compact and is useful for routine applications. The oven has factory set isothermal temperature control from 10°C above ambient to 150 °C offering fast analysis of high boiling point and semi-volatile compounds, as well as excellent reproducibility of retention times.
The automatic air sampling allows unattended, continuous monitoring of ambient air by automatically injecting air (or other gas stream) samples onto the analytical columns.
AUTOMATIC GAS/LIQUID SAMPLING SYSTEMS
The automatic air sampling system, consisting of the sample pump, lo-port valve, sample loop, and columns is configured for precolumn backflush operation.  Precolumn backflush is used to perform an analysis quickly by retaining and eliminating or "backflushing" the late eluting components in a sample that are of no interest.  Precolumn backflush helps avoid contamination of subsequent analyses with very late eluting peaks and helps keep analysis times to a minimum.
SAMPLE PUMP-GAS
The pump pulls the sample into the GC at a flow rate of approximately 250-300 mL/min. (without tubing).  Any length of tubing attached to the "sample in" port decreases the flow rate of the pump. The pump is capable of drawing sample through approximately 100 feet of 1/4" or 1/8" tubing from a remote location.
10 PORT VALVE 
The 10-port valve directs the flow of sample through the system.  This valve has ten piping connections, or polls, and five (5) internal passages. The 10-port valve, switches between two operating positions -precolumn backflush and inject. The precolumn backflush position is the 'resting' position, and is used to prevent unwanted compounds from entering the analytical column. The inject position is used to introduce sample into the sample loop for injection onto the column. The 10-port valve's air actuated solenoid diverts air pressure to one side or the other of a piston, causing the valve to switch back and forth between it two positions.  The gas supply for the 10-port valve instrument grade compressed air. 
SAMPLE LOOP
The sample loop receives the sample from the 10-Port valve, holds it while it equilibrates to atmospheric pressure, and provides a constant sample volume for injection onto the column (s). A buffer loop provides expansion space for the sample while it is equilibrated to atmospheric pressure. The sample loop is attached to two ports of the 10-port valve and can be varied in size as needed for a given column or application.
LIQUID SYSTEMS
The Model 301-C consists of a fluidics system that includes a pump and a liquid metering valve with a vaporization system to vaporize the liquid sample. The hardware in the fluidics system is configured differently in each mode of operation and varies with each application.

SAMPLE PUMP-LIQUIDA liquid metering pump with a ceramic head is used to bring the sample to the liquid valve.

LIQUID VALVE
The purpose of the liquid  metering valve is to meter or proportion a constant quantity (0.25 to 10 m1) of a liquid or liquefied gas into a carrier gas stream and, following this, to vaporize the metered liquid rapidly and completely. The liquid sampling valve  directs the flow of sample through the system. The valve switches between two positions which are dependent on the mode of operation. The valve is located at the bottom of the oven and is maintained above the oven temperature to vaporize the sample. ).  The sample loop (size may be from 0.2 to 10 mL) as needed for a specific column or application.

PRECOLUMN BACKFLUSHThe precolumn backflush mode is used only with the automatic air sampling system; it is not used with manual syringe injections. During precolumn backflush operation, the pump pulls an air sample into the sample loop on the 10-port valve.  The valve switches from its resting position to the "inject" position so that the precolumn is in series with the analytical column.  The contents of the sample loop are injected onto the precolunm where some separation of the components occurs.  The compounds of interest pass through the precolumn to the analytical column where the major component separation occurs. After a user-defined period of time, the 10-port valve switches back to its resting "precolumn backflush" position, and the compounds that are of no interest and do not elute from the precolumn are "backflushed' off of the precolumn and vented.  Backflushing prevents the compounds that are of no interest from entering the analytical column.

FIRMWARE/ELECTRONICS
The firmware controlled Electronics System initiates all sample point (optional multipoint) functions and valve switching. In addition, it regulates the chromatographic and temperature control functions; provides data storage for the 8 hour shift reports; and performs diagnostic tests on the oven temperature, power supply voltages, carrier gas (N2) pressure, calibration peak height, autozero, CPU board function, RAM function, and the printer.

DETECTORS 
The detectors quantitatively measure the concentration of the components in the sample.  Six detectors are available with the Model 301-C  PID, FUV, FID, FPD, TCD .  The instrument's dual-detector capability allows any two of the three interchangeable detectors to be run separately or in series.
Teflon tubing is supplied with the GC. 
PHOTOIONIZATION DETECTOR 
The PID provides a response to a wide range of organic and some inorganic compounds at part per billion (ppb) levels. The HNU PID consists of an ultraviolet lamp and an ion chamber.  The detector measures the concentration of gases present in a sample  using the method of pbotoionization.Photoionization occurs when a molecule absorbs a photon (light energy) of sufficient energy, creating a positive ion and an electron as shown below:
R  + hn  = R+  + e-
The sample drawn into the ion chamber is exposed to photons generated by the ultraviolet lamp. Molecules in the sample with ionization potentials less than or equal to the energy level of the lamp are ionized. The ionization potential is that energy in electron volts (eV) needed to free an electron from a molecule. A positively biased accelerator electrode repels these ions, causing them to travel to the collecting electrode, where an analog signal proportional to the concentration of the sample is generated.  The signal is amplified to provide an analog output for graphic recording or electronic integration. Ultraviolet lamps are available in four energies; 8.3, 9.5, 10.2, 10.6 and 11.7 electron volts (eV).  

Detector selectivity (and sensitivity) varies with each lamp.  The PID becomes more selective as the lamp energy decreases since it is capable of ionizing fewer compounds.  The detector becomes less selective as the lamp energy increases. It is capable of ionizing a larger number of compounds.  The best lamp for an application is typically one with an energy level just above the ionization potential of the compounds of interest. The 10.2 eV lamp provides the. maximum sensitivities for those compounds it detects. The PID has a linear range of better than seven decades (>107), is nondestructive, and can be used in series with other detector.
The Far Ultraviolet Detector (FUV) provides a nearly universal response (except for the noble gases) to organic and inorganic compounds at low part per million (ppm) levels.  The internal volume of the FUV is only 40 mL making it an ideal choice for use with capillary columns. The FUVAD consists of a 10.2 eV ultraviolet lamp, an absorption cell, and a photodiode. The photodiode produces a constant background current within the detector.  The ultraviolet lamp generates light energy (photons) in the far ultraviolet (120-150 nm) range. As molecules in a sample pass through the absorption cell of the detector, they absorb the photons emitted from the UV lamp, creating negative ions which cause a decrease in the background current measured by the photodiode. The pbotodiode senses and measures this decrease in background current and generates an analog signal proportional to the concentration of the sample.  The signal is amplified and linearized to provide in analog output for graphic recording or electronic integration. The FUV has a linear range of better than four decades (>104), is non-destructive, and can be used in series with other detectors.
FLAME PHOTOMETRIC DETECTOR
The sample is burned in a hydrogen rich flame which excites sulfur or phosphorus to a low lying electronic level. This is followed by a resultant relaxation to the ground state with a corresponding emission of a blue (S) or green (P) photon. This type of emission is termed chemiluminescence. The emission is at 394 nm for Sulfur and 525 nm for phosphorus. The S:C selectivity ratio is > 10,000:1. The HNU detector uses rare earth filters instead of interference filters for S & P to improve detection limits and eliminate some of the deficiencies of interference filters Detection limits in the 5 pg  and 20 pg range for P and S respectively. The HNU Flame Photometric Detector (FPD) incorporates arare earth glass filter for improved performance. Our FPD offers a two to three-fold increase in sensitivity for sulfur (S) and phosphorous (P) compared to detectors with interference filters. Since band pass is no longer dependent on the viewing angle, results are more reproducible-regardless of flame geometry.  Sensitivity is significantly increased due to improved interference rejection and a larger effective aperture. Only HNU's FPD offers rare earth glass filters for more reproducibleresults and increased sensitivity.
FLAME IONIZATION DETECTOR
The process of ionization which occurs in organic compounds when the carbon- carbon bond is broken via a thermal process in the flame that results in the formation of carbon ions. These ions are collected in the flame by applying a positive potential to the  FID jet and the ions are pushed to the collection electrode where the current is measured. The response (current) is proportional to the concentration and is measured with an electrometer/amplifier. An FID consists of a combustion/ion chamber, a flame, a voltage source for the accelerating electrode (usually applied to the jet) and an amplifier capable of measuring down to one to five picoamperes full scale.
The FID is a mass sensitive detector, the output of which is directly proportional to the ratio of the compound’s carbon mass to the total compound mass. Thus, the sample is destroyed in the flame. 
THERMAL CONDUCTIVITY DETECTOR Measures difference between the thermal transfer characteristics of the gas and a reference gas, generally helium but hydrogen or nitrogen can be used depending on the application. The sample and reference filaments are two legs of a Wheatstone Bridge. A constant current is applied with a resultant in a rise in filament temperature. As the sample passes through the detector, the resistance changes as the reference gas is replaced by the sample which has a lower thermal conductivity. This difference in resistance is proportional to the concentration. The response is universal since the detector responds to any compound that conducts heat. The minimum detection limit is in the 100-200 ppm. The maximum concentration is 100%. 

OUTPUTS
There are two (digital signals) RS232C ports on these GCs. One is for the serial printer, the other can be used to send the signal to a PB for control/data collection with HNU Net software (optional). The other output  provides 0-1V and 0-10V analog outputs. The alarm outputs are optically isolated to minimize noise. A third (optional ) output is 4-20 mA (consult factory) for PLCs, Process Control Computers, or recorders.
OPERATION
The Model 301-B can be operated through the internal fimware or the external HNU Net software. 
INTERNAL FIRMWARE/SOFTWARE
The internal firmware allows the Model 301-C  to communicate with the keyboard and any external device. It contains the operating system and programming for the GC. If there is a power outage, the system will automatically restart and calibrate the GC. Consult HNU systems if you have any questions. HNU has two software packages for remote control of the  301-C (optional) GCs. The ViewGraph software allows the remote viewing and capture of a chromatogram from a PC with Windows. HNU Net  is used to control the 301-C remotely and to collect data. This program runs from an external PC.   
AUTO ZERO/CALIBRATION
These GCs will autozero at the beginning of each run, this eliminates zero drift for these GCs. The instruments automatically calibrate every twenty four hours.

SUMMARY
These GC's are remarkably versatile instruments designed for the chromatographic separation, identification, and quantitation of chemical components in gas or liquid samples.
The Model 301-C GCs with their wide range of detectors, gas or liquid sample injection systems, and the ability to accommodate packed or capillary columns provides a wide range of environmental, plant, and on-line applications. The exceptional stability, minimum maintenance, ease of use and flexibility of these GCs make them the ideal choice for the monitoring needs of both large and small companies.

Contact Dr. Driscoll to inquire about your application or to request a quotation today.


2 comments:

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