API 2000 LC/MS/MS TurboIonSpray Ion Source Manual

Transcription

1 Table of Contents API 2000 LC/MS/MS TurboIonSpray Ion Source Manual Table of Contents API 2000 TurboIonSpray Ion Source Manual....1 AboutThisManual...1 Introduction...1 TurboIonSpraySourceComponents...3 Installation...6 OptimizingtheTurboIonSpray...7 ChangingThePeekTubingTransferLine...12 ChangingTheStainlessSteelSprayerTube...14 Changing The Grounded Union/Splitter Fitting...14 AppendixA-ConsumableParts...16 AppendixB-Troubleshooting...17 Introduction...17 i

2 Table of Contents ii

3 API 2000 TurboIonSpray Ion Source Manual About This Manual Conventions This manual contains the instructions required to operate the API 2000 TurboIonSpray ion source. Within this manual, the following conventions are used: WARNING! Indicates an operation that may cause personal injury if precautions are not followed. CAUTION! Indicates an operation that may cause damage to the instrument if precautions are not followed. Introduction NOTE: Emphasizes significant information in a procedure or description. TurboIonSpray is an Atmospheric Pressure Ion Source in which pre-formed ions in solution are emitted into the gas phase with or without the application of heat. In this way, quasi-molecular ions can be generated from very labile and high molecular weight compounds with no thermal degradation. TurboIonSpray is the technique by which a liquid sample is pumped by a liquid chromatograph (LC) pump or syringe drive through a sprayer probe, which is maintained at a high voltage, and is nebulized into the Ion Source creating a mist of highly charged droplets. Once inside the Ion Source, the droplets evaporate causing the ions to enter the gas phase by a low energy process called Ion Evaporation. The use of an orthogonal heated gas extends the rugged and versatile technique of TurboIonSpray to accept higher flow rates with improved sensitivity. TurboIonSpray will accept flows from 5 to 1000 µl/min of solvent compositions from 100% aqueous to 100% organic, such as acetonitrile, without splitting. This allows the use of 1 mm, 2 mm, and 4.6 mm analytical columns with or without splitting. A heater probe directs a jet of heated dry gas (up to a maximum of 500 C) at the mist produced by the sprayer. The gas is sprayed across the orifice at an angle of approximately 45 with respect to the Curtain Plate. The liquid spray emerging from the TurboIonSpray is directed at an angle of about 45 from the opposite direction (or 135 ). The TurboIonSpray effluent and the heated dry gas intersect at an angle of approximately 90 near the orifice. This interaction helps focus the TurboIonSpray stream and increases the rate of droplet evaporation resulting in an increased ion signal. 1

4 Features Specifications TurboIonSpray Setup TurboIonSpray is ideally suited for LC/MS/MS quantitative analyses. The sensitivity increases that are achieved with this technique are both flow rate and analyte dependent. In the conventional IonSpray source sensitivity decreases with increased flow rate, while the heated TurboIonSpray process increases ionization efficiency, especially at the higher flow rates; this results in improved sensitivity. Sensitivity is compound dependent and compounds of extremely high polarity and low surface activity usually show the greatest sensitivity increases. The TurboIonSpray technique is mild enough to be used with labile compounds such as peptides, proteins, and thermally labile pharmaceuticals. NOTE: The difference between conventional IonSpray and TurboIonSpray is that in the TurboIonSpray, the ionization of the sample is achieved with additional heater gas delivered to aid in the desolvation of the spray. Typically the TurboIonSpray is used at higher flow rates (greater than or equal to 40 µl/min). The following list outlines the features of the TurboIonSpray ion source: Able to function as conventional IonSpray source when the Heater Gas is turned off. Able to function with flow rates from 1 µl/min to 1000 µl/min. Able to vaporize 100% aqueous to 100% organic solvents. Temperature Range Probe temperature from 0 to 500 C Liquid Chromatograph Interfaces to any liquid chromatograph system 2

5 TurboIonSpray Nebulizer and Heater Gases (Gas 1 and 2) Zero Grade Air regulated to 100 psi TurboIonSpray Source Components Ion Source Housing API 2000 TurboIonSpray Ion Source Manual The ion source housing forms a seal with the instrument interface assembly to prevent any solvent or sample from escaping into the laboratory environment. Any liquid or vapour entering the source is safely transferred to a waste receptacle and fume hood with the aid of the source exhaust pump. The source housing incorporates two motion mechanisms (that adjust the position of the sprayer, described below), two windows to allow the user to view the position and condition of the sprayer, a Peek tubing transfer line, a grounded union/splitter fitting, two latches used for locking the source in place and integrated voltage/gas connectors. The connections for the voltage, heater and gases are made automatically once the source is locked to the interface.. TurboIonSpray Ion Source Sprayer Probe The TurboIonSpray source contains a fixed angle adjustable sprayer probe. The sprayer probe is adjustable in two directions: towards the orifice from a scale of 0 to 12 (adjusted by the horizontal adjusting ring mounted on the TurboIonSpray probe), and up and down towards the orifice from 0 to 12 by the Lateral Adjustment control on the top of the Ion Source. 3

6 Sprayer Probe Sprayer Probe and Adjustments The TurboIonSpray probe gas, or Nebulizer gas (Gas 1) and voltage (IS) are set at the applications computer using the software. The connections for the voltage and gas are made automatically once the source is locked to the interface. Transfer Line and Grounded Union/Splitter A short piece of Peek tubing (30 cm, ID) connects the sprayer probe inlet to a grounded fitting that can be used either as a union or splitter. Since the liquid being sprayed from the source is in contact with high voltages, the transfer line must be connected to the grounded fitting. Peek Tubing Grounded Union/Splitter PEEK Tubing and Grounded Union/Splitter 4

7 WARNING! The use of the Peek tubing transfer line connected to the grounded fitting is mandatory since it prevents any exposed peripherals connected to the source from floating at high voltage. Heater Probe WARNING! Peek tubing (or fused silica) with Peek fittings must be used as the transfer line. Metal tubing or fittings must not be used. The use of metal may result in the exposure of high voltage to the user. Refer to the procedure for changing the grounded fitting from a union connection to a splitter described later in this document. The heater probe temperature is set in the applications computer software by modifying the TEM parameter. The correlation between the parameter value of TEM and the actual heater temperature is direct (for example TEM 350 is equal to 350 C at the heater). The heater gas is also set in the applications computer software. The connections for the heater and gas are made automatically once the source is locked to the interface. The heater probe temperature is maintained by the Temperature Control Board (TCB) mounted behind the IonSource panel (below the IonSource housing). The TCB adjusts the flow of power to the heater element as a function of the difference between the actual heater temperature and the temperature setting in the applications computer. The probe temperature is monitored by a thermocouple connected directly to the heater element which maintains the temperature within ±5 degrees of the applications computer setting. The operating range for the probe is from approximately 100 to 500 C. NOTE: The temperature is controlled by monitoring the output of a thermocouple connected to the heater surrounding the metal tube. At the temperature control board the thermocouple output is compared with the temperature setting, the difference determines the power flow to the heater. The operating temperature should be adjusted relative to LC mobile phase composition and flow rate. In general, a combination of heat and gas flow that just allows the liquid spray to reach dryness before reaching the curtain plate will provide optimum performance. Heater gas flows of approximately 6 L/min can be considered optimum for conditions where heat is required. Ionization efficiency is improved with the input of heat for all liquid flow rates. However, at lower flow rates (<20 µl/min) the gains are small. Heat inputs may be desirable under low flow conditions for the additional reason of enhancing in-source (orifice-skimmer) fragmentation. Temperatures as low as C have proven useful for enhancing this capability (phosphopeptide mapping for instance) when operating at low flow rates. 5

8 Exhaust Pump The TurboIonSpray Source requires that the Source Exhaust system is properly connected and functioning (refer to the figure Exhaust Supply Connection Points under Source Exhaust Pump in the Optimizing the TurboIonSpray section). A filtered nitrogen or air gas supply (free from pump oil) is delivered to the Source Exhaust pump at 60 psi pressure at a flow of at least 4 L per minute. The Source Exhaust pump is used to vent solvent vapors which develop in the Ion Source plenum. It is recommended that these vapors be passed through a trap vented to a fume hood or outside port. WARNING! Failure to provide proper ventilation of the ion source can result in hazardous vapors being released into the laboratory environment. Installation The TurboIonSpray source connects to the Vacuum Interface housing. Two latches mounted on the source housing secure the TurboIonSpray against the vacuum interface. To install the TurboIonSpray on the API instrument: 1. Ensure that the two source latches are in the unlocked (up) position. 2. Align the two guide pins on the source with the vacuum interface receptacles. Guide Pins Guide Pin Alignment 3. Slide the source along the guide pins towards the interface as far as it can go without using excessive force. 4. Turn the two source latches to the locked (down) position. 5. The TurboIonSpray Nebulizer and Heater Gas (Gas 1 and 2), the sprayer high voltage and heater connectors are automatically engaged once the source is locked in position. 6

9 6. Connect the liquid sample tubing to the grounded fitting which is connected to the inlet end of the sprayer probe. Ensure that all fittings are properly seated in order to minimize dead volumes. NOTE: In order to reduce the band broadening of sample with solvent, thus maximizing sensitivity, the use of ID Peek, or fused silica tubing between the TurboIonSpray source and the injector is recommended. To remove the TurboIonSpray from the API instrument: WARNING! Ion source may be hot for several minutes after it is removed from the instrument. 1. Stop all scans and place the instrument in Standby or Overnight Quit status. 2. Turn off the liquid flow entering the ion source. 3. Disconnect the liquid sample tubing from the grounded fitting. 4. Turn the two source latches to the unlocked (up) position. The sprayer high voltage and heater connectors are automatically disengaged once the source is unlocked. 5. Slide the source along the guide pins away from the interface to remove it. Optimizing the TurboIonSpray The following section outlines the practical considerations which must be made when optimizing the TurboIonSpray performance. It is intended to provide the qualitative information necessary to aid the operator in quantifying the separate operating parameters. TurboIonSpray performance depends on the following factors: Sprayer position TurboIonSpray (IS) voltage Nebulizer gas (NEB) Curtain gas (CUR) Heater Gas flow and Temperature (Gas 2 and TEM) Declustering Potential (DP) and Focusing Potential (FP) voltage (compound dependent) Solvent Composition Optimum performance on standard compounds is relatively easy to achieve and little adjustment or tuning is required once the Source is optimized. With experience, the operator will develop a personal optimization method which works best. In general, once the sprayer is set up and optimum factors have been determined, little or no readjustment of the values is required on a day-to-day basis. For tuning purposes a compound with a known molecular ion should be introduced either by continuous infusion or flow injection. Infusion is preferred because it provides a continuous flow of sample. The tuning compound should have characteristics similar to the sample to be analyzed during normal operation and should be introduced at the same liquid flow rate. 7

10 CAUTION! When optimizing the Sprayer position make certain not to spray directly down the orifice. Spraying down the orifice may contaminate the Vacuum Interface and Vacuum Chamber ion optics and could impact instrument performance. CAUTION! To avoid possible damage to the instrument, always view the sprayer tip through the side port when adjusting its position. CAUTION! If unattended user operation is intended through the use of sample control, Ensure that LC shut-off is in use to prevent flooding of the plenum chamber. TurboIonSpray Probe Position The position of the TurboIonSpray Probe relative to the Orifice and to the Heater Probe is an important factor in optimizing the TurboIonSpray performance. The probe should point between 5 and 10 mm off axis with respect to the center of the orifice. The distance of the Heater Probe from the orifice plane is fixed, but the TurboIonSpray can be adjusted using the scale on the side of the Sample inlet arm. Changing from low solvent flow rates (40µL/min) to high solvent flow rates (1mL/min) requires that the TurboIonSpray be repositioned further away from the orifice to prevent solvent penetration through the orifice into the mass spectrometer. OPTIMAL TURBOIONSPRAY TARGET CURTAIN PLATE 40 µl/min ORIFICE 200 µl/min 1mL/min TurboIonSpray Positioning Across the Orifice Also as the aqueous composition of the carrier solvent increases at high flow rates (1 ml/ min), the more visible the spray becomes and the further away from the orifice it should be directed. See the figure above where the areas indicated in the diagram for the different flow rates are the optimum target areas for the TurboIonSpray liquid spray. The circle immediately around the orifice (for example the part of the orifice plate which is visible when viewing the front of the interface) should remain clear of solvent or solvent drops at all times. 8

11 The best position is usually a few millimeters off axis to the bottom of the curtain plate aperture. Multiply charged proteins and peptides introduced at a few microliters per minute usually require the sprayer to be as close as possible to the Curtain Plate. TurboIonSpray Voltage (IS) In positive mode, singly charged compounds usually require a high probe voltage of between 4000 to 5500 V. In negative mode, compounds usually require a lower voltage, between 3000 to V. NOTE: If the TurboIonSpray voltage is set too high a blue glow can be seen at the tip of the TurboIonSpray indicating a corona discharge. This will result in decreased sensitivity and stability of the ion signal. Nebulizer Gas (Gas 1) Optimize for signal stability and sensitivity. Typically a value of 10 to 45 psi is used as applied by the applications computer. Curtain Gas Flow (CUR) The Curtain Gas ensures a stable clean environment for the sample ions entering the mass spectrometer. The gas curtain prevents air or solvent from entering the analyzer region of the instrument while permitting the sample ions to be drawn into the vacuum chamber by the electrical fields generated between the Vacuum Interface and the TurboIonSpray needle. The presence of the solvent vapor or moisture in the analyzer region of the mass spectrometer contaminates the Q0 Rod Set causing a reduction in resolution, stability, sensitivity, and an increase in chemical background noise. As a general rule, the Curtain Gas flow should be set as high as possible without reducing the signal significantly (for example start at a lower value and increase the flow until the signal starts to decrease). In order to prevent instrument contamination the Curtain Gas flow should be optimized at the highest possible setting, but never below 11 psi, that does not result in a significant reduction in signal intensity. It is suggested that the TurboIonSpray be operated with curtain gas settings adjusted to the highest flow rate possible (never below 11) without signal loss. Heater Gas (Gas 2) The Heater Gas (Gas 2) aids in the evaporation of solvent which aids in increasing the ionization of the sample. The higher the liquid flow or the higher the aqueous composition of the solvent, the higher the Heater Gas temperature and gas flow required. However, too high a temperature can cause premature vaporization of the solvent, and result in a high chemical background noise, while too high a Heater Gas flow can produce a noisy, or unstable signal. 9

12 The table below provides recommended operating conditions for the TurboIonSpray at three different flow rates. For each flow rate, the Curtain Gas flow rate (from setting 11 to 45 at the applications computer) should be as high as possible. The solvent composition used for optimization was 1/1 Water/Acetonitrile. These conditions represent a starting point from which to optimize TurboIonSpray. By an iterative process, the various settings can be optimized using Flow Injection Analysis to obtain maximum signal-to-noise for the compound of interest. TurboIonSpray Suggested Operating Settings Table Flow Rate µl/min TurboProbe Temp. ( C) 40 µl/min. 300 (200 to 500) 200 µl/min. 400 (300 to 500) 1 ml/min. 475 (450 to 500) Heater Gas (GAS2) Flow (L/min) PSI 6 65 (4 to 8) (40-90) 7 75 (4 to 8) (40-90) 8 90 (4 to 8) (40-90) V = Vertical position H = Horizontal position Nebulizer Setting (GAS 1) 20 (10-30) 30 (15-35) 50 (20-55) Position V H Turbo Temperature (TEM) The quantity and type of sample affect the optimal TurboIonSpray temperature. At higher flow rates the optimal temperature increases. A more significant factor is the composition of the solvent. As the organic content of the solvent increases the optimal probe temperature should decrease. With solvents consisting of 100 percent methanol or acetonitrile the probe performance may optimize as low as 300 C. Aqueous solvents consisting of 100 per cent water at flows approximately 1mL/min require a minimum probe temperature of 425 C. Normal optimization is usually performed in increments of 25 C. The TurboIonSpray is normally used with sample flow rates of 40 µl/min to 1000 µl/ min. The heat is used to increase the rate of evaporation and this improves ionization efficiency resulting in increased sensitivity. See the table above for recommended temperatures. CAUTION! Do not operate the TurboIonSpray with probe temperatures greater than 500 o C. Declustering Potential (DP) and Focusing Potential (FP) Voltages Optimal Declustering Potential and Focusing Potential operating conditions with the TurboIonSpray source should be set high enough to reduce the chemical noise but low enough to avoid fragmentation. Start with the Focusing Potential (FP) at 300V and the Declustering Potential (DP) at 20V. 10

13 NOTE: The fragmentation energy of a compound is a function of its structure and molecular weight. Generally lower molecular weight compounds require less energy - lower Declustering Potential and Focusing Potential voltages to induce fragmentation. For some labile compounds, DP values of between 5 to 10 may be required with the TurboIonSpray. In general terms, the higher the Declustering Potential and Focusing Potential voltages, the greater the energy imparted to the ions entering the analyzing region of the mass spectrometer. The energy helps to decluster the ions and to reduce the chemical noise in the spectrum resulting in an increase in sensitivity. Increasing the voltages beyond optimal conditions can induce fragmentation before the ions enter the mass filters resulting in a decrease in sensitivity. In some instances this fragmentation can prove a valuable tool providing additional structural information. Solvent Composition Commonly used solvents and modifiers are acetonitrile, methanol, propanol, water, acetic acid, formic acid, ammonium formate and ammonium acetate. The modifiers such as TEA, sodium phosphate, TFA and dodecyl sodium sulfate are not commonly used because they complicate the spectrum with their ion mixtures and cluster combinations. They may also suppress the strength of the target compound ion signal. The standard concentration of ammonium formate or ammonium acetate is from 2 to 10 millimole per liter for positive ions and 2 to 50 millimole per liter for negative ions. The concentration of the organic acids is 0.01% to 0.5% by volume. Source Exhaust Pump The Source Exhaust system is required for TurboIonSpray operation. The exhaust pump draws the solvent vapors from the enclosed source chamber and delivers them to a trap at the rear of the instrument chassis where they can be collected. The Source Exhaust system is interlocked to the system electronics, such that if the source exhaust pump is not operating to specification the instrument electronics are disabled. The exhaust system lowers the pressure in the source slightly below atmospheric. If the pressure in the source rises beyond a preset trip point, the instrument High Voltage Power Supply is disabled. 11

14 . e Curtain Gas Supply Exhaust Supply Valve Waste Out Gas1/Gas2 Exhaust Supply Connection Points Exhaust Waste Out Operating Tips 1. When running the TurboIonSpray, with or without the Heater Probe on, run the Curtain Gas at as high a flow rate as possible. The Curtain Gas should optimize about 30% higher when TurboIonSpray is in use. This will help to: a) prevent penetration of the Curtain Gas which can produce a noisy signal, b) prevent contamination of the orifice, and c) increase the overall signal to noise ratio 2. The liquid spray from the TurboIonSpray should be directed away from the orifice in order to: a) prevent contamination of the orifice, b) prevent piercing of the Curtain Gas which can create an unstable signal, and c) prevent electrical shorting due to the presence of the liquid. 3. The higher the liquid flow or the higher the aqueous composition of the solvent, the higher the temperature and Heater Gas (Gas 2) flow required. However, too high a temperature can cause thermal degradation of the compound, and result in a high chemical background noise, while too high a heater gas flow can produce a noisy, or unstable signal. Changing The Peek Tubing Transfer Line A Peek tubing transfer line is used to carry the sample and solvent from the Sample Inlet to the metal sprayer tube where it is nebulized and ionized. The standard TurboIonSpray set-up uses a 30 cm, (65 µm) ID Peek tubing transfer line. 12

15 WARNING! The use of the Peek tubing transfer line connected to the grounded fitting is mandatory since it prevents any exposed peripherals connected to the source from floating at high voltage. To exchange the Peek tubing transfer lines in the TurboIonSpray in the event of blockage of the existing line: 1. Remove the TurboIonSpray and set on its side. 2. Unscrew the Peek tubing fittings from the probe inlet and grounded union/splitter fitting and discard the old tubing (the fittings may be re-used if they have not been over-tightened and damaged). Peek Tubing 3. Obtain a new piece of Peek tubing (it must be at least 30 cm long) and place Peek connection fittings on each end. WARNING! Peek tubing (or fused silica) with Peek fittings must be used as the transfer line. Metal tubing or fittings must not be used. The use of metal may expose the user to high voltage. 4. Connect the Peek tubing to the probe inlet and grounded union/splitter fitting. 13

16 Changing The Stainless Steel Sprayer Tube The standard TurboIonSpray set-up uses a (100 µm) ID metal sprayer tube. The procedure outlined below should be used to exchange the metal tube in the TurboIonSpray in the event of blockage or degradation of the sprayer tip. 1. Remove the TurboIonSpray and set on its side. 2. Disconnect the Peek transfer line from the probe inlet. 3. Unscrew and remove the black inlet probe fitting. A Peek union, metal spring and the metal sprayer tube can now be removed from the probe. 4. Unscrew the fitting holding the metal tube to the Peek union. Remove the tube from the fitting and discard. Do not discard the fittings. Metal Sprayer tube removed 5. Place a new sprayer tube and ferrule in the fitting and screw into the Peek union. Ensure that the sprayer tube is placed as far as it can go into the Peek union to guarantee a leak-proof seal (two wrenches should also be used to tighten the fittings). 6. Place the metal spring and metal tube/peek union back into the probe. 7. Place the black inlet probe fitting over the Peek union and tighten. This fitting is used to adjust the protrusion of the metal sprayer tube tip at the end of the probe. The metal tube should protrude approximately 0.75 mm, although a protrusion between 0.5 to 1.25 mm is also acceptable. Changing The Grounded Union/Splitter Fitting A Peek tubing transfer line connects the sprayer probe inlet to a grounded fitting that can be used either as a union or splitter. Since the liquid being sprayed from the source is in contact with high voltages, the transfer line must be connected to the grounded fitting. 14

17 WARNING! The use of the Peek tubing transfer line connected to the grounded fitting is mandatory since it prevents any exposed peripherals connected to the source from floating at high voltage. To change between the union and splitter insert fittings: 1. Remove the TurboIonSpray and set on its side. 2. Disconnect the Peek transfer line from the ground fitting. 3. Disconnect any other fittings or tubing that may be connected to the grounded fitting. 4. The insert within the grounded fitting should now move freely and can be removed by tilting the source and allowing it to fall out. 5. Place the new insert into the fitting and align it such that all the openings match. 6. Connect the Peek transfer line to the grounded fitting and any other fittings or tubing that may be used. WARNING! Peek tubing (or fused silica) with Peek fittings must be used as the transfer line. Metal tubing or fittings must not be used. The use of metal may result in the exposure of high voltage to the user. 15

18 Appendix A - Consumable Parts Appendix A - Consumable Parts The following is a list of parts included in the Kit of consumable parts (part number ) supplied with the TurboIonSpray assembly. Item Part No. Description (Quantity) Probe Spring (1) ID Peek Tubing (60 cm) Probe Inlet Nut (1) Viton O-Ring, 7/64 ID (2) Viton O-Ring, 4 mm ID (2) Metal Electrode Tube (2) Peek LC Fitting (2) Peek Union (1) Splitter Tee Insert, 0.25 mm bore (1) Peek Ferrule (4) Viton O-Ring, 2 mm ID (2) 16

19 Appendix B - Troubleshooting Appendix B - Troubleshooting Introduction The troubleshooting procedures given here are suitable for use by operators who have access to an ohm meter and are familiar with its use. These procedures do not require removal of the instrument covers and do not expose the operator to hazardous voltages. The heater control circuits are protected by a resetable over current device. This device may be tripped by excessive heater current or by failure to approach the desired operating temperature within about 5 minutes. When tripped the device will remove power from the heater. To reset the protective device turn power off the system for about one minute or remove the ionizer from the system for about one minute. Diagnosis of the nature of the fault is done by means of ohm meter readings taken across the pins of the ionizer 24 pin connector on the ionizer which is shown below. Ionizer 24 Pin Connector 17

20 Appendix B - Troubleshooting FAILURE MODES HEATED IONIZER Table # CAUSE SYMPTOMS DIAGNOSIS CURE 1 Short circuit in heater. No heat. Heater resistance measured across pins 7 and 19 is less than 15 ohms. 2 Open circuit in heater. No heat. Heater resistance measured across pins 7 and 19 is greater than 25 ohms. 3 Heater grounded. No heat. Heater resistance from pin 9 to ionizer frame is less than 100K ohms. 4 RTD short circuit. No heat. RTD resistance measured across pins 9 and 10 is less than 100 ohms. 5 RTD open circuit. No heat. RTD resistance measured across pins 9 and 10 is greater than 300 ohms. 6 RTD grounded. No heat. RTD resistance from pin 10 to ionizer frame is less than 100K ohms. 7 Poor thermal contact between the RTD and the heater. When starting the heater with the temperature set to greater than 300 C, the heat comes on for about 5 minutes and then goes off. The heater works normally when the temperature is set to 150 C Call service to replace heater. Call service to replace heater. Call service to replace heater. Call service to replace RTD. Call service to replace RTD. Call service to replace RTD. Call service to correct clips attaching the RTD to the heater. 18