DETECTORS Helium Ionization Detector - HID Overview
only helium carrier and make-up gas, and is sensitive to the low ppm range. ... With insufficient make-up flow, the chlorinated peaks will be ...
DETECTORS
Helium Ionization Detector - HID
Overview
The Helium Ionization Detector is a universal detector, responding to all molecules except neon. It requires
only helium carrier and make-up gas, and is sensitive to the low ppm range. The HID is particularly useful for
volatile inorganics to which the FID and other selective detectors will not respond, like NOx, CO, CO2, O2,
N2, H2S and H2. It is a robust detector that, unlike the TCD, has no filaments to burn out. The SRI HID
consists of a detector body, a collector electrode, an arc electrode assembly, and a thermostatted heater block
which can be heated to 375oC. In SRI GCs, the HID is mounted on the right-hand side of the Column Oven.
HID detector removed from GC and heater block
Collector
electrode Pointed
HID
electrode
Arc
Sample-laden
window
carrier gas inlet
Makeup
gas inlet
Arc
Detector body
HID detector between TCD and FID detectors cross
on an SRI GC Flat
electrode
Close-up of the same HID detector Support brace
DETECTORS
HID - Helium Ionization Detector
Theory of Operation
The SRI HID detector uses two electrodes which support a low current arc through the helium make-up
gas flow. The helium molecules between the electrodes are elevated from ground state to form a helium plasma
cloud. As the helium molecules collapse back to ground state, they give off a photon. The sample molecules
are ionized when they collide with these photons. All compounds having an ionization potential lower than
17.7eV are ionized upon contact with photons from the helium cloud. The ionized component molecules are
then attracted to a collector electrode, amplified, and output to the PeakSimple data system.
Collector Ground
electrode electrode
Photons in the
make-up gas
stream
Helium plasma cloud
Sample-laden
carrier flow Makeup gas
flow
Ionization reaction: Purple arc
photons colliding
with sample
molecules
Arc electrode
NOTE: If the arc electrode is covered with TeflonTM (translucent) insulation, it should leave 1mm of its tip
exposed. If the flat electrode is covered with ceramic (white) insulation, then the tip should be flush with the
edge of the insulation sleeve. There should be a 1-2mm gap between the arc electrodes, and this gap should
be centered in the arc cross.
DETECTORS
Helium Ionization Detector - HID
Expected Performance
HID noise run
Columns: 1m Mol. Sieve, 2m Hayesep-D
Carrier: Helium @ 10mL/min
HID gain = HIGH
HID current = 70
HID temp = 200oC
Temperature Program:
Initial Hold Ramp Final
80oC 15.00 0.00 80oC
HID noise averages less than 100µV from peak to peak
Test Analysis of 1cc 1000ppm C1-C6
Method: valve injection
Column: 1m (3’) Silica Gel
Carrier: Helium @ 10mL/min
HID gain = HIGH
HID temp = 150oC
HID make-up = 29psi @
40mL/min
Temperature program:
Initial Hold Ramp Final
50oC 1.00 10.00 220oC
220oC 10.00 0.00 220oC
Results:
Component Retention Area
O2/N2 0.766 3350.0970
Methane 1.066 1163.1965
Ethane 3.550 2161.0940
Propane 8.083 3001.6200
Butane 12.850 3958.3250
Pentane 16.950 4849.9755
Hexane 20.800 5023.0105
total 23507.3185
DETECTORS
HID - Helium Ionization Detector
General Operating Procedure
1. Set the HID amplifier gain switch to HIGH for most applications from the ppm level to 1%. Use the
MEDIUM gain setting for slightly more concentrated samples.
2. Set the helium make-up gas flow to 40mL/min, and the helium carrier gas flow to 10mL/min. The make-up
gas flow is critical to the HID’s performance. With insufficient make-up flow, the chlorinated peaks will be
inverted on the chromatogram; see the chromatograms compared on the HID Make-up Gas Flow page. Clean,
high purity helium is best; moisture, air, and other contaminants can cause problems.
3. Set the HID temperature to 200oC. This temperature will help prevent moisture accumulation in the
detector’s arc assembly.
4. Zero the data system signal, then switch ON the HID current; the switch is located on the GC’s front control
panel under “DETECTOR PARAMETERS.” Set the HID current at 100 using the trimpot setpoint on the top
edge of the front control panel.
5. When the HID is OFF and the signal zeroed, and the HID is then turned ON, the milliVolt offset at HIGH
gain setting should be 200-800mV. A higher offset means more sensitivity, but less dynamic range. If the offset
is less than 200, the arc and ground electrodes are probably too close.
6. Observe the arc window; if you can see the purple arc between the ground and arc electrodes, proceed to
step 7. If the arc goes sideways to the detector body instead of down to the ground electrode, then the gap
between the electrodes is too large. If you cannot see the arc,
A. Use a multimeter to check the voltage between the arc and ground electrodes. With the HID
current at 100, the voltage reading should be greater than 200VDC (our readings average around
240VDC).
B. Look through the arc window at the arc and ground electrodes. If they appear to be touching,
disconnect the red electrode lead wire then check the continuity between the electrodes using a multimeter;
the reading should be open or infinite.
C. If the continuity between the electrodes is not open, re-gap the electrodes.
7. Let the milliVolt reading stabilize, then begin the analytical run.
DETECTORS
Helium Ionization Detector - HID
HID Make-up Gas Flow
The following chromatograms were produced by an SRI HID equipped GC. Excepting the make-up gas
flows, all run conditions are identical. The first chromatogram resulted from a make-up gas flow of 20mL/min.
Drastically different in appearance from the first, the second chromatogram was produced with a make-up gas
flow of 10mL/min. In the absence of sufficient make-up gas flow, the chlorinated peaks are negative. Not
every HID has the same optimum make-up flow; experiment with different flow rates until you find the best
range for your detector.
Sample: 0.5mL 1000ppm C1-C6 Temperature program:
Column: 30m MXT1-5 Initial Hold Ramp Final
Carrier: Helium @ 10mL/min 50oC 1.00 10.00 140oC
Results:
Component Retention Area Make-up: 20mL/min
Solvent 0.305 4791.9566
Benzene 0.896 14.5888
TCE 1.145 17.9614
Toluene 1.790 19.6294
PCE 2.305 21.3786
Ethyl Benzene 2.998 23.5176
Bromoform 3.221 22.0414
Ortho Xylene 3.470 26.3280
Total: 4937.4018
Results:
Component Retention Area Make-up: 10mL/min
Solvent 0.381 1771.5762
Benzene 0.876 622.0096
TCE 1.266 527.2432
Toluene 1.771 571.1129
Ethyl Benzene 2.993 379.2581
Ortho Xylene 3.468 312.9010
Total: 4184.1010
DETECTORS
HID - Helium Ionization Detector
Cleaning the HID
If your HID baseline seems noisy, try cleaning the electrodes following the steps below. Over time, the HID
electrodes can develop a coating of soot, which can cause the arc
to flicker or change position, resulting in sudden baseline jumps.
Green wire
1. Unclip the amplifier lead and slide it off the collector electrode.
Unclip and remove the leads from the pointed and flat electrodes
(note that the green wire is
connected to the pointed
electrode, and the red wire is
connected to the flat
Red wire electrode).
2. Remove the the arc and
ground electrodes by loosening
the 1/8” fittings that hold the
electrodes in the arc cross.
3. Remove the collector electrode by loosening the 1/4” fitting that secures
it in the detector body.
4. Use a piece of 100-400 grain sandpaper to clean the surface of the
collector electrode and the point of the ground electrode. Sand the tip of
the arc electrode so that it is flush against the ceramic insulation, and to
remove any residue. While handling the electrodes, try to minimize hand
contact by holding them with a clean paper towel.
5. Remove any sanding residue from the electrodes using a paper towel
optionally moistened with methanol or another quick-evaporating solvent. Use the arc window
6. Replace the electrodes and check for to check arc and
Screw ground electrode
clamp proper alignment. The collector electrode
positioning.
stop should extend about 4mm into the detector
body. An existing screw clamp stop on the
collector electrode should allow replacement
4mm Ground electrode
without readjustment. Should
adjustment be required, loosen
the screw clamp to position the 1-2mm gap
electrode, then tighten it to hold
the position. To position the arc Arc electrode
and ground electrodes, remove the arc cross from the detector body by
loosening the 1/4” fitting connecting the two parts of the detector (this
fitting also secures the support brace). The ground and arc electrodes should have a gap of
about 1-2mm (0.040-0.080”) between them, with the gap centered in the arc cross. Hold
the arc cross up to the light and verify the electrodes’ positions by looking through the arc
window. Once the electrodes are positioned, tighten them securely with a wrench.
