Routine Maintenance for Atomic Absorption Spectrophotometers

Routine Maintenance for Atomic

Absorption Spectrophotometers

Author

Margaret A. Cunliffe

Application Note

Atomic Absorption

Introduction

Instruments in good operating condition are a necessity in any analytical laboratory.

This level of integrity can be achieved by a regular maintenance schedule with

minimal work. The four main areas of such a program for atomic absorption

spectrophotometers include:

• General instrument maintenance

• Gas supply maintenance

• Flame component maintenance

• Furnace component maintenance

The benefits of routine maintenance include:

• Increased instrument lifetime

• Reduced downtime

• Overall improvement in instrument performance; giving the operator greater

confidence in the validity of his analytical results

Whatever source is used, the supply must be continuous and

have a delivery pressure of 420 kPa (60 psi). The air must be

clean, dry and oil free. Approximately 50% of all gas unit fail-

ures are caused by moisture or other impurities inthe air

supply.

Excessive noise in the readout has also been attributed to

contaminated air. An air filter assembly is therefore an essen-

tial component of the atomic absorption spectrophotometer,

and its inclusion in the air supply installation is mandatory.

Weekly, check the air filter for particle and moisture accumu-

lation. When necessary, dismantle the air filter assembly and

clean the filter element, bowl, and drain valve components.

Use the following procedure for dismantling and cleaning the

air filters supplied with the instrument.

1. Shut off the air supply and allow the system pressure to

bleed off.

2. Unscrew the filter bowl, complete with automatic drain

valve.

3. Unscrew the retaining ring and push the drain valve back

into the bowl.

4. Unscrew the baffle carefully, and remove the filter and

filter shield.

5. Clean the filter bowl, drain valve components, baffle, and

filter shield by washing in a solution of soap and water.

DO NOT USE ORGANIC SOLVENTS AS THEY WILL

DESTROY THE BOWL AND VALVE COMPONENTS. Rinse

thoroughly in fresh water.

6. Clean the filter element by washing in ethyl alcohol or

similar solvent.

7. Ensure that all components are properly dried before

reassembly.

Nitrous Oxide Supply

The nitrous oxide used for atomic absorption spectrophotom-

etry must be oil free. If a heated regulator is not used, loss of

regulation can occur due to the expansion cooling effect

encountered when nitrous oxide is drawn from a cylinder.

This can lead to erratic results and create a potential flash-

back situation with manual gas control units: An acceptable

heated regulator may be ordered from any Agilent sales

office. The consumption rate is dependent on the application,

but is usually 10–20 liters per minute.

General Instrument Maintenance

Dust and condensed vapors can accumulate on the instru-

ment case, and corrosive liquids can be spilled on the instru-

ment. To minimize damage, wipe off the instrument with a

damp, soft cloth using water or a mild detergent solution. DO

NOT USE ORGANIC SOLVENTS. The sample compartment

windows and the lamp windows can accumulate dust or fin-

gerprints. In such cases, clean the windows with a soft tissue

moistened with a methanol or ethanol and water solution. If

the windows are not clean, the operator will observe noisy

lamp signals and non-reproducible analytical results.

The remaining optical components are sealed, but they should

not be exposed to corrosive vapors or a dusty atmosphere. In

laboratories where high concentrations of dust or vapors are

unavoidable, schedule a yearly check by a service engineer to

maintain the efficiency of optical light transmission in the

instrument. There is no need for an operator to clean the

sealed optical components.

Gas Supply Maintenance

Three gases are suitable for flame M. Air and nitrous oxide

are used as combustion support gases (oxidants). Acetylene

is used as the fuel gas. Each gas is supplied to the instrument

through piped supply systems and rubber hoses. Copper or

copper alloy tubing may be used for the oxidant gases.

Acetylene should only be supplied through stainless steel or

black iron pipe. Check connections regularly between the sup-

ply and instrument for leaks, especially when tanks are

changed using a soap solution or commercial leak detector.

Check the rubber hoses connected to the instrument for fray-

ing and cracking. In addition, each time a tank is changed,

check the regulators and valves for proper operation.

Because potentially toxic gases are used or produced in the

flame, it is necessary to use a suitable exhaust system with a

minimum capacity of 6 m

/min (200 cfm). A simple smoke

test will indicate if it is functioning properly.

Compressed Air Supply

Air may be supplied to the instrument from cylinders, a house

air system, or small compressor. Cylinders are the most

expensive source of air, particularly where large amounts are

consumed and cylinders must be changed frequently. If com-

pressed air from an in-house supply is used, a filter/regulator

assembly must be installed in the input line to the instrument.

An acceptable “Air Service Unit” (Part No. 01 102093 00) may

be ordered from any Agilent sales office.

Acetylene Supply

Acetylene is the only combustible gas which is normally used

in MS. The gas must be supplied packed in acetone. Some

companies supply acetylene packed in proprietary solvents,

but unfortunately the disadvantages outweigh the advan-

tages. The major disadvantage is that the solvent may be car-

ried over into the instrument and corrode the internal tubing,

causing a potential explosion hazard. Ensure that the acetylene

is at least 99.6% pure “M Grade” and packed in acetone.

The delivery pressure must be regulated and never exceed

105 kPa (15 psi). Check the instrument operation manual for

the correct delivery pressure for the particular instrument

being used. In addition, check the acetylene cylinder pressure

daily, and maintain in excess of 700 kPa (100 psi) to prevent

acetone from entering the gas line and degrading analytical

results or causing damage to the instrument.

Flame Component Maintenance

The flame component section of the instrument can be divid-

ed into three areas; the nebulizer, spray chamber and burner.

Each requires routine maintenance to assure optimum

performance.

Nebulizer

The nebulizer area of the flame component consists of the

capillary tubing and the nebulizer body. Always ensure that

the plastic capillary tubing used for aspirating solutions is cor-

rectly fitted to the nebulizer capillary. Any leakage of air, tight

bends, or kinks will cause unsteady, non-reproducible

readings.

At times the plastic capillary tubing can become clogged and

it will be necessary to cut off the clogged section or fit a new

piece of capillary tubing (about 15 cm long). in any event,

make sure the plastic capillary tubing fits tightly on the nebu-

lizer capillary. The nebulizer capillary can also become

clogged. If this occurs, proceed as follows:

1. TURN THE FLAME OFF.

2. Remove the plastic capillary tubing from the nebulizer.

3. Remove the nebulizer from the bung.

4. Dismantle the nebulizer as described in the instrument

operation manual or the instruction manual supplied with

the nebulizer.

5. Place the nebulizer in an ultrasonic cleaner containing

0.5% liquid soap solution such as Triton X-100 for 5 to

10 minutes. If the ultrasonic bath fails to clear the block-

age, pass a burr-free nebulizer wire CAREFULLY through

the nebulizer and then repeat the ultrasonic cleaning

procedure.

6. Re-assemble the nebulizer in accordance with the

instructions.

7. Install the cleaned nebulizer.

Replace the plastic capillary tubing.

If blockages are allowed to build up and are not removed,

the analytical signal will steadily drop until no absorbance

is observed.

8. Check the nebulizer body, capillary, and venturi occasion-

ally for corrosion. Nebulizer problems can be minimized

by taking care to always aspirate 50–500 mL of distilled

water at the end of each working day.

Spray Chamber

As the sample leaves the nebulizer it strikes the glass bead

and breaks into an aerosol of fine droplets. The efficiency of

the glass bead can be degraded by surface cracks, pitting and

the accumulation of solid material. The reduction in bead effi-

ciency can cause lower absorbance readings and noisy sig-

nals. When removing the nebulizer for inspection, always

check the glass bead. Look for pitting, cracks, breakage,

ensure that the adjusting mechanism operates properly and

that the bead is correctly positioned over the nebulizer outlet

(venturi).

While the nebulizer and glass bead are removed from the

instrument for inspection, the spray chamber and liquid trap

should be removed, dismantled, and cleaned. Discard the liq-

uid in the liquid trap and wash both the spray chamber and

liquid trap thoroughly with laboratory detergent and warm

water. Rinse completely with distilled water and dry all com-

ponents. Refill the liquid trap and reassemble the spray cham-

ber, checking for any distortion of O-rings or blockages in the

gas inlets. Reconnect the drain hose. If a bottle or jug is used

to collect the waste solutions, check that the hose is not

below the level of the waste. If the hose is below that level,

absorbance readings will steadily decrease with occasional

abrupt increases as intermittent drainage of the spray cham-

ber occurs. Therefore, it is necessary to daily check the level

of the waste and to dispose of it frequently. This is imperative

when using organic solvents because of the potential hazards

introduced by flammable liquids. Only wide necked, plastic

containers can safely be used to collect the waste solutions.

Burner

The final area of concern in the flame component is the

burner. During aspiration of certain solutions, carbon and/or

salt deposits can build up on the burner causing changes in

the fuel/oxidant ratio and flame profile, potential clipping of

the optical beam, and degradation of the analytical signal. To

minimize the accumulation of salts, a dilute solution of acid

(HNO

) may be aspirated between samples. However, if salts

continue to build up, turn off the flame and use the brass

cleaning strip supplied with the instrument. Insert the strip in

the burner slot and move it back and forth through the slot.

This should dislodge any particles which will then be carried

away once the flame is lit and water aspirated.

DO NOT USE SHARP OBJECTS such as razors to clean the

burner as they can nick the slot and form areas where salt

and carbon can accumulate at an accelerated rate.

If this type of cleaning is inadequate, remove the burner,

invert, and soak it in warm soapy water. A scrub brush will

facilitate cleaning. Soaking may also be done in dilute acid

(0.5% HNO

). Ultrasonic cleaners containing dilute non-ionic

detergent only are another alternative for cleaning. After

cleaning, thoroughly rinse the burner with distilled water and

dry before installing in the instrument. NEVER DISASSEMBLE

THE BURNER FOR CLEANING. IMPROPERLY RE-ASSEMBLED

BURNERS WILL LEAK COMBUSTIBLE GAS MIXTURES,

POTENTIALLY CAUSING EXPLOSIONS.

Each day after all analyses are completed, 50–100 mL of dis-

tilled water should be aspirated to clean the nebulizer, spray

chamber, and burner. This is even more important after aspi-

rating solutions containing high concentrations of Cu, Ag, and

Hg, since these elements can form explosive acetylides. The

entire burner/nebulizer assembly should be disassembled and

thoroughly cleaned after analyzing these types of solutions.

The burner should be removed weekly, scrubbed with a

laboratory detergent, and rinsed with distilled water.

Furnace Component Maintenance

The graphite furnace accessory maintenance can be divided

into three major areas; the gas and water supplies, the work-

head, and the autosampler. Each plays an important role in

obtaining valid analytical results. The following general

maintenance program refers to the GTA-95.

Gas and Water Supplies

Normally the gases used in FAAS are inert gases such as N

and Ar. Either one may be used, but must be clean, dry, and of

high purity. The regulated pressure should be 100–340 kPa

(15–50 psi). At times the incorporation of air may be useful to

fully ash a sample. However, air should not be used at ash tem-

peratures higher than 500 °C because of the accelerated rate of

graphite component deterioration at elevated temperatures.

The water supply, used to cool the furnace, may be supplied

either from a laboratory tap or a cooling-recirculating pump. If

a recirculating pump is used the water must be kept below

40 °C. The water used must be clean and free of corrosive

contamination. The flow should be 1.5–2 liters/minute.

Maximum permissible pressure is 200 kPa (30 psi).

Workhead

The workhead is a closed assembly with quartz windows on

either end. Before starting an analysis, check the windows for

dust or fingerprints. If needed, clean both sides of the quartz

windows with a soft tissue moistened with an alcohol/water

solution. Never use coarse cloths or abrasive cleaning agents.

While the windows are removed, inspect the gas inlets on the

window mountings. If the graphite components have deterio-

rated extensively, graphite particulates may have dropped into

the gas inlets, blocking the proper flow of gas. This will cause

further graphite deterioration at an accelerated rate and lead

to poor analytical performance. To clean, carefully blow out

the particulates with a supply of air. Inspect the inside of the

window mountings and clean off any sample residue which

may have deposited over time.

In the center of the workhead are the graphite components.

At frequent, regular intervals, remove the graphite tube atom-

izer and inspect the inside of the graphite shield. Ensure that

the bore and the injector hole area are free of loose carbon or

sample residue. Check the electrodes on either end of the

graphite shield for proper tapering. If the tapering is worn or

burnt, the electrodes will not make the correct contact with

the graphite tubing, causing fluctuations in applied power

resulting in irreproducibility. The electrodes also have a series

of gas inlets which must be free of loose carbon or sample

residue.

Above the graphite shield is the titanium chimney. Injected

sample or sample residue from the ash/atomize cycles may

deposit in this area. A cotton swab soaked with alcohol can

be used to clean both the inside and outside of the chimney.

Alternatively, the titanium chimney may be soaked in dilute

acid to remove deposits.

Autosampler

The components of the autosampler requiring routine mainte-

nance are the rinse bottle, syringe, and capillary tubing, the

proper care of which will minimize contamination and improve

reproducibility of analytical results.

Regularly remove the rinse bottle for cleaning. This involves

soaking the bottle in 20% HNO

followed by rinsing with

distilled-deionized water. Refill the bottle with a solution of

0.01–0.05% HNO

in distilled-deionized water. The solution

may also include 0.005% v/v Triton X-100 R. The Triton helps

maintain the sample capillary in clean condition and assists in

obtaining good precision.

At times, graphite particulates may accumulate on the capil-

lary tip and should be carefully removed with a tissue. If these

particulates are not removed, the dispensing characteristics

of the capillary may change. Contamination of the capillary

may become a problem when using some matrix modifiers. In

such cases, direct the capillary to a vial containing 20% HNO

draw up 70 µL, and stop the autosampler while the capillary is

in the vial. After a period of a few minutes, the autosampler

RESET should be utilized to rinse out the acid solution. This

will clean the internal and external areas of the capillary.

Similarly, organic residues can be removed by directing the

capillary to a vial of acetone and repeating the above proce-

dure. The PTFE capillary should be treated carefully during

cleaning and operation. If bends or kinks appear, it can take

time to reshape, and while doing so the repeatability of injec-

tion may be degraded. If the capillary tip is damaged, the dam-

aged portion should be cut off at a 90° angle with a sharp

scalpel or razor blade.

The final area of the autosampler maintenance schedule is

the syringe. Daily, check for bubbles in both the capillary and

syringe. Any bubbles in the system can cause dispensing

errors and lead to erroneous results. Follow the instructions

in the operating manual to free the system of bubbles. If the

bubbles continue to cling to the syringe, it may need cleaning.

The syringe can be washed with a mild detergent solution and

thoroughly rinsed with deionized water. Ensure that contami-

nation is not introduced through the syringe. Be particularly

careful not to bend the plunger while washing the syringe.

Conclusion

Attached is a routine maintenance schedule for atomic

absorption spectrophotometers (Figure 1). By adhering to this

program, the overall integrity of the atomic absorption spec-

trophotometer can be maintained and the laboratory analyst

will reap the benefits of increased instrument lifetime, reduced

downtime, and gain greater confidence in the analytical results.

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Agilent shall not be liable for errors contained herein or

for incidental or consequential damages in connection

with the furnishing, performance, or use of this material.

Information, descriptions, and specifications in this

publication are subject to change without notice.

Printed in the USA

November 1, 2010

AA039

Maintenance Schedule (Flame AA)

Daily Completed

1. Check Gas

2. Check Exhaust system with smoke test

3. Empty the drain receptacle

4. Clean lamp and sample compartment windows

5. Rinse spray chamber with 50-100 mL of distilled water

Weekly

1. Disassemble spray chamber

(a) Check glassbead

(b) Check nebulizer components

(d) Scrub the burner

(e) Change the liquid in the liquid trap

(f) Check the O-rings

2. Check air filter assembly

3. Wipe off instrument

4. At Time of Gas Tank Change

5. Check for leaks

6. Check for operation of the regulators

7. Check for operation of the shut off valves

8. Check the gas supply hoses

Yearly

1. Schedule an Agilent service engineer to perform Preventive Maintenance

Figure 1. Routine maintenance schedule for atomic absorption spectrophotometers.

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