[Home] [Master Table of Contents]

[SPCCP Index] [Appendix D Index]

Appendix D - Training Program

Photoionization/Flame Ionization Detectors

Objectives/Samples of Behavior:

1. Explain the operating principle of the PID & FID

2. Describe the calibration and operation of the PID & FID

3. Discuss the limitations of the PID & FID

4. Describe the advantages and applications of the PID & FID

A.  Photoionization Detectors (PID)

1. Principles of operation

1. Ultraviolet (UV) lamps emit UV energy - photons - to the sample air

2. UV - edge of ionizing radiation

3. Molecules of contaminants in the air are ionized

4. Ions (charged particles) are collected at electrodes

5. Collection generates a current & subsequent meter reading "ppm"

6. UV lamps : 9.5 eV, 10.2 eV, 10.6 eV, 11.7 eV

7. UV energy must be equal to or greater than the E needed to remove the e in question

8. Energy required = ionization potential of the chemical

9. IPs found in NIOSH Pocket Guide

Using 10.6 eV lamp:

2. Calibration - before each use / each day:

1. Zero:

"Zero gas"

Clean air

2. Span:

"Span gas" - isobutylene

Flush bag, attach tubing from instrument

Set instrument to span gas conc

3. Operation

1. Turn on, warm up

2. Place probe in suspect area

3. Can set alarm to various levels

4. Can run continuously or log data - e.g. record peak conc every 15 sec

5. Units:

ppm of span gas used - optimum conditions

ppm - cal gas equivalents or just "PID units"

4. Limitations

1. Water vapor, IP 12.59 - scatters, absorbs UV

2. Non-ionizable gases & vapors - blocking

3. Hi conc - inaccurate readings, e.g. drumhead space

4. Does not distinguish between substances - screens

5. Groups of chemical vapors: total ionizations

6. Can't "see" some chemicals - IP

7. Relative response - cal gas 100%; different gas ~75%, 110%, etc (ppm - cal gas equivalents)

5. Advantages / applications:

1. Wide variety of substances - good for fast screening

2. Detects both organic & inorganic

3. Sensitive to low conc

4. Helps in protective equipment selection

5. General IH work, IRP sites, emergency response, confined spaces, soil gas monitoring, etc.

B.  Flame Ionization Detectors (FID)

1. Principles of operation:

1. Hydrogen gas is used to produce a flame in the detector

2. Flame is used to ionize organic molecules

- Will not pick up CO₂, water vapor, etc.

3. Ions collected at electrodes & produce current / meter reading as with PID

4. IP not an issue - essentially all organics detected (IP 15.4 or less)

2. Calibration - before each use / each day:

1. Zero:  zero gas or clean ambient air

2. Span:  span gas - methane

3. Operation:

1. Insert external hydrogen fuel tank

2. Turn on, warm up (self-test - diagnostics)

3. Start pump, light hydrogen gas flame

4. Place probe in suspect areas

5. Can also alarm, data log

6. "FID units" or "ppm - cal gas equivalent"

4. Limitations

1. Can only read organic vapors / gases

2. Cannot distinguish between organics - get total organics

3. Needs external fuel source

4. Flame may extinguish at high vapor conc (or O₂)

5. Relative response - e.g. most substances do not produce a reading of 100 ppm when their actual conc is 100 ppm

5. Advantages / applications

1. Wide variety - detects more organics than PID

2. IP not a problem

3. Sensitive to low conc

4. Often used with gas chromatograph (GC) to help ID chemicals

5. Applications as with PID

6. Tremendous sampling variety when PID & FID used together