Patent Description:
Inductively Coupled Plasma Optical Emission Spectrometry is an elemental analysis technique that derives its analytical data from the emission spectra of elements excited within a high temperature plasma. The purpose of the optical system in an ICP-OES is to separate element specific wavelengths of light that are emitted from the excited sample, and to focus the resolved light onto a detector as efficiently as possible.

The ICP-OES is comprised of five sections: a sample introduction region, a torch, a focussing optics region, a wavelength separation device (polychromator) and a detector. Light exiting the polychromator is focused onto the detector.

Traditionally, optical detection was carried out using a photomultiplier tube. Solid state charge transfer devices (CTDs) have replaced these in recent times. Charge transfer devices can be grouped broadly into two categories: Charge Injection Devices (CIDs) and Charge Coupled Devices (CCDs). Each is comprised of a doped silicon wafer forming a 2D pixel array. CIDs offer significant benefits in the field of atomic spectroscopy, when compared with other CTDs.

The detector in an OES is desirably capable of detecting light across a wide range of wavelengths in the visible and ultraviolet parts of the spectrum. For example, the iCap <NUM> Plus Series ICP-OES supplied by Thermo Fisher Scientific, Inc offers virtually continuous wavelength coverage across a range of <NUM> to <NUM>.

CID detectors require cooling to temperatures below zero Celsius - typically, -<NUM> to -<NUM> degrees Celsius - to achieve best performance. Cooling reduces the amount of dark CID detectors require cooling to temperatures below zero Celsius - typically, -<NUM> to -<NUM> degrees Celsius - to achieve best performance. Cooling reduces the amount of dark current/noise. Moreover, CID detectors are purged with an inert gas such as argon or nitrogen that contains little or no water (around or less than <NUM> parts per million (<NUM> ppm) of water). Such dry inert gas purging optimises transmission of light having a wavelength below around <NUM>.

If the CID detector is not purged, over time relatively moist air can leak into the detector chamber. Additionally or alternatively, the detector chamber may be purged with gas containing residual water, such as may happen if, for example, poorer quality purge gases such as welding argon are employed.

In either case, upon cooling the detector to its preferred sub-zero operating temperature, any water present will freeze on the detector surface. Reduced detector performance then occurs in the region of the frozen water and, in worst cases, the frozen water can irreparably damage the detector such that it must be replaced. Furthermore, the frozen water may reduce light transmission through the detector.

The present invention seeks to address this problem.

<CIT> relates to photoelectric sensors having an apparatus which removes dust from a lens thereof.

<CIT> relates to a photodetector comprising: a light guide path guiding the light from the spectroscope to a photodetector element; a cooling mechanism for cooling the photodetector element, a case shielding the temperature control mechanism from the air; and a purge mechanism for purging the gas inside the case through a ventilation hole by providing the ventilation hole in the case. Sensors for measuring the humidity are provided. After purging from the case inside until the humidity in the case comes down to a preset value or lower, cooling by the cooling mechanism is initiated.

The purge gas is preferably a dry purge gas, i.e. a gas that is substantially free of water vapour. Examples include argon or dry nitrogen. By monitoring a time since purge gas was supplied to the detector of the OES following shut down (for example, by determining an amount of time from power down of the spectrometer, which typically causes stopping of a supply of purge gas to the detector, or by measuring a length of time following shut off of the purge gas to the detector through, for example, operation of a valve), an informed decision can be taken over the amount of time necessary to purge the detector again when a start-up procedure commences. In other words, the purging time on restart is not fixed but instead is a variable time that depends upon the length of time since shut down of purging. In addition, moisture content within the detector may be measured using a humidity sensor following stopping of a supply of purge gas to the detector. By measuring the moisture content, an informed decision can be taken over whether the detector may be cooled safely and/or the amount of time necessary to purge the detector before cooling of the detector. Therefore, the purging time on restart of application of purge gas to the detector is a variable time that may depend upon the moisture content within the detector.

In general quantitative terms, the longer the detector has not been purged, the longer it is necessary to purge it when restarting. Software (for example) in the protection device, preferably that is run or executed on the processor, can determine the time needed to purge the detector in order to have confidence that the atmosphere surrounding the detector is sufficiently water vapour free that cooling of the detector will avoid damage to it and/or avoid a reduction of its performance. On the other hand, measurement of the time since cessation of the supply of purge gas to the detector and/or measurement of moisture content in the detector following cessation of the supply of purge gas to the detector allows purging to take place upon restart for a period no longer than necessary. This in turn results in a shorter time before the OES is ready to use again and a consequential reduction in the volume of purge gas needed.

Further advantages of the invention will be apparent from the appended dependent claims.

The invention may be put into practice in a number of ways and some specific embodiments will now be described by way of example only and with reference to the accompanying drawings in which:.

<FIG> shows a schematic diagram of an ICP-OES <NUM> according to an embodiment of the present invention. The ICP-OES <NUM> comprises a power supply <NUM>, a sample introduction region <NUM>, an ICP torch <NUM> powered by a radio-frequency generator <NUM>, focussing optics region <NUM>, a wavelength selection device <NUM>, a detector <NUM>, a purge gas supply <NUM>, a cooling device <NUM>, and a protection device <NUM> comprising a timer <NUM> and a processor <NUM>.

The sample introduction region <NUM> comprises a nebulizer <NUM>, a spray chamber <NUM> and a peristaltic pump <NUM> powered by the power supply <NUM>. A liquid sample is extracted from a sample source <NUM> and supplied to the nebulizer <NUM> by the peristaltic pump <NUM>. The nebulizer <NUM> contains an inert gas, typically argon, provided by the inert gas supply <NUM>] to which it is connected. The nebulizer <NUM> is employed to nebulize the liquid sample into a stream of droplets (sample aerosol) and may, for example, be of the pneumatic, ultrasonic or grid type. Whilst <FIG> shows the inert gas supply <NUM> and the purge gas supply <NUM> to be in two separate locations, the skilled person would understand that the inert gas supply <NUM> may be located within the same gas box as the purge gas supply <NUM>. The skilled person would also understand that a single source, such as a tank or canister containing argon, may be used as the inert gas supply <NUM> and the purge gas supply <NUM>.

The output of the nebulizer <NUM> is coupled to the spray chamber <NUM> such that the sample aerosol entrained with the inert gas is provided to an input of the spray chamber <NUM>.

The spray chamber <NUM> removes large droplets from the sample aerosol and provides the sample aerosol entrained in inert gas to an injector tube <NUM> of the ICP torch <NUM> downstream of the spray chamber <NUM>. The large droplets collect and coalesce in the spray chamber leave the sample introduction region via a drain 22a fluidly connected to the spray chamber <NUM>. The spray chamber <NUM> is coupled to the ICP torch <NUM> such that the output of the spray chamber <NUM> is introduced to a first end 30a of the ICP torch <NUM>.

Alternatively, if the sample is a solid, then this is introduced to the ICP torch <NUM> following laser ablation or spark/arc ablation. In the former method, a pulsed UV laser is focused on the sample and creates a plume of ablated material which can be swept into the ICP torch by use of gas. In the latter an electrical arc/spark is used to ablate material from a sample which is swept in to the ICP torch using a gas.

The ICP torch <NUM> is employed to create and sustain a plasma <NUM> formed from an inert gas, such as argon. The plasma <NUM> is an electrically conducting gaseous mixture containing sufficient cations and electrons to maintain electrical conductance.

The ICP torch <NUM> comprises three concentric tubes usually made of quartz - a central tube <NUM>, an outer tube <NUM> and an intermediate tube <NUM> disposed therebetween. A second end 30b of the torch <NUM>, opposite to the first end 30a, is placed inside an induction coil <NUM> supplied with a radio-frequency electric current generated by the radio frequency generator <NUM>. The induction coil <NUM> generates an alternating magnetic field at the second end 30b of the torch <NUM>. The ICP torch <NUM> is connected to the inert gas supply <NUM> such that the inert gas flows through the central, outer and intermediate tubes <NUM>, <NUM>, <NUM> of the ICP torch <NUM>. The flow of inert gas in the outer tube <NUM> serves two purposes, firstly it is employed to form the plasma <NUM>, and secondarily it is employed to cool the ICP torch <NUM>. The flow of inert gas in the intermediate tube <NUM> is employed to shift the plasma position within the ICP torch <NUM>. The flow of inert gas in the central tube <NUM> is employed to transport the sample aerosol to the plasma <NUM> proximal to the second end 30b of the torch <NUM>.

To generate the plasma <NUM> within the ICP torch <NUM>, a Tesla coil (not shown) is employed to generate a spark which introduces free electrons. The free electrons are accelerated in alternating directions by the alternating magnetic field created by the induction coil <NUM>. The accelerated electrons collide with atoms of the inert gas and the collisions may cause excitation and ionization of the atoms. The process continues until the rate of release of new electrons in collisions is balanced by the rate of recombination of electrons with ions. This produces the plasma <NUM>, which consists mostly of atoms with a rather small fraction of free electrons and ions.

As mentioned above, the sample aerosol entrained in inert gas enters the central tube <NUM> at the first end 30a of the torch <NUM>. The sample aerosol is transported to the plasma proximal to the second end of the tube 30b. The plasma comprises a pre-heating zone proximal 32a to the first end of the torch 30a, a normal analytical zone 32b proximal to the second end of the torch 30b and an initial radiation zone 30c disposed therebetween. Once the sample aerosol is within the pre-heating zone 32a, the sample aerosol is desolvated, vapourized and dissociated into atoms. The sample atoms output from the pre-heating zone 32a pass to the initial radiation zone 32c and subsequently to the normal analytical zone 32b. Within these zones, the sample atoms are excited and ionized. It is considered that most of the excitation and ionization occurs as a result of collisions of sample atoms with energetic electrons. Subsequently, the excited and ionized sample atoms exit the plasma <NUM> and consequently cool. On cooling, the excited and ionized sample atoms electronically relax by emitting photons having wavelengths characteristic of energy levels in the atoms.

The second end 30b of the ICP torch <NUM> is aligned with the optics <NUM> of the focussing optics region <NUM> which are in turn aligned with a wavelength selection device <NUM>. Photons emitted from the normal analytical zone are collected and focussed onto an entrance aperture <NUM> of the wavelength selection device <NUM> by the optics <NUM> of the focussing optics of region <NUM>. The entrance aperture <NUM> of the wavelength selection device <NUM> may be radially or axially aligned with the plasma <NUM>. The wavelength selection device <NUM> typically employs a diffraction grating to separate the photons into different wavelengths and may be, for example, a monochromator or polychromator.

The detector <NUM> is aligned with the wavelength selection device <NUM> such that photons exiting the wavelength selection device <NUM> impinge on the detector <NUM>. The detector <NUM> may be a charge injection device (CID), which is a well-known device and so will not be described in further detail. In an alternative embodiment, the detector may be a charge coupled device (CCD). The output from the detector <NUM> is measured and processed by a set of electronics <NUM>. The set of electronics <NUM> may measure the number and wavelengths of the photons to identify and determine a concentration of the elements within the sample. Typically, the detector <NUM> and the set of electronics <NUM> are powered by the power supply <NUM>. The output from the set of electronics is provided to a computer <NUM>, which monitors, collects and displays the data of the ICP-OES on a display screen (not shown).

Prior to sample introduction into the ICP-OES <NUM>, the detector <NUM> must be purged for a start-up time period and subsequently cooled. As discussed above, purging and subsequent cooling of the detector reduces the amount of dark current/noise and so prepares the ICP-OES <NUM> for introduction and measurement of a sample. The purging and subsequent cooling of the detector is controlled by a protection device <NUM> comprising a timer <NUM> and a processor <NUM>. In some embodiments, the processor <NUM> may be a processor of the computer <NUM>. In the embodiment shown, however, the processor <NUM> is separate from the computer <NUM>.

For purging of the detector <NUM>, purge gas is applied to the detector <NUM> via a purge gas supply line <NUM> connected to the purge gas supply <NUM>. Purge gas is an inert gas such as argon or nitrogen that contains little or no water (around or less than <NUM> parts per million (ppm) of water). This may be powered by the power supply <NUM> which is used to power the other components of the ICP-OES <NUM>. The timer <NUM> is employed to measure a parameter indicative of a shut down time period. The shut down time period is the time period for which purge gas is not applied to the detector. The timer <NUM> may be, for example, a battery operated timer or a capacitor based timer and more specific details of the operation of the timer will be discussed with reference to later Figures. The processor <NUM> is connected to the timer <NUM> and employed to determine a start-up time period based upon the parameter measured by the timer <NUM>. Purge gas is applied to the detector <NUM> during the start-up period and this may be triggered by the processor <NUM>, or alternatively this may be triggered automatically upon switching on the ICP-OES instrument <NUM>. More specific details of the operation of the processor <NUM> will be discussed with reference to <FIG>, <FIG> and <FIG>. Preferably, the determined start-up period should be as short as possible, thereby allowing the user to make maximum use of the instrument and consuming less purge gas, whilst being as long as necessary for the detector to be properly purged. Therefore, in a preferred embodiment, the determined start-up period may be the minimum duration of purging for which the detector is properly purged.

Once purged, the cooling device <NUM>, for example a Peltier cooling device, is employed to cool the detector <NUM> to temperatures below zero degrees Celsius. The processor <NUM> may be configured to trigger cooling of the detector <NUM> after elapse of the start-up time period. Once the detector <NUM> has been purged and cooled, a sample may be introduced to the ICP-OES <NUM> via the sample introduction region <NUM>.

In a preferred embodiment of the present invention, the timer <NUM> of <FIG> comprises a timing circuit <NUM> as depicted in <FIG>. The timing circuit <NUM> comprises a capacitor <NUM>, a resistor <NUM> and a voltmeter <NUM>. Preferably, the impendence of the resistor may be <NUM> Ohm and the capacitance of the capacitor may be <NUM>µF such that the time constant for the timing circuit may be <NUM> minutes. Preferably, the value for Vcc may be <NUM> V. The capacitor <NUM> and the resistor <NUM> may be connected in series with the power supply <NUM> thereby forming an RC circuit connected to ground such that the value for Vcc is the voltage across the power supply <NUM>. The capacitor based circuit has advantages over a real-time clock that requires a battery since a battery has a limited lifetime, which is generally much less than the lifetime of the instrument itself. The instrument would therefore need maintenance as the battery would need replacement. In contrast, the circuitry based timer of the present invention, which can be made from low-cost components, is designed to function as intended for the lifetime of the instrument. Furthermore, the degree of time accuracy required of the circuitry based timer is not high so that it is well suited to the described use.

On turning on the ICP-OES <NUM>, the capacitor <NUM> is charged according to equation (<NUM>): V=Vcc (<NUM> - <IMG>) wherein V is the voltage across the capacitor <NUM> at any time t, Vcc is the voltage across the power supply <NUM> and T is the time constant for the timing circuit <NUM>. In a completely discharged state, the voltage across the capacitor <NUM> is zero and the voltage across the resistor <NUM> is equal to the voltage of the power supply <NUM>. On turning on the ICP-OES <NUM>, current flows from the power supply <NUM> to the capacitor <NUM>. The capacitor <NUM> begins to charge thereby increasing a voltage across the capacitor <NUM> exponentially with time, as depicted in <FIG>. Once the capacitor <NUM> is fully charged, current stops flowing to the capacitor <NUM>, the voltage across the capacitor <NUM> is approximately equal to the voltage across the power supply <NUM> (Vcc) and the voltage across the resistor <NUM> is approximately zero.

On turning off the ICP-OES <NUM>, the capacitor <NUM> is discharged according to equation (<NUM>): V=Vcc (<IMG>). During discharge, the voltage across the capacitor <NUM> drops exponentially with time as current flows away from the capacitor <NUM> and towards the power supply <NUM>, as depicted in <FIG>. Once the capacitor <NUM> is fully discharged, the current stops flowing, the voltage drop across the resistor <NUM> is approximately equal to Vcc and the voltage across the capacitor <NUM> is zero.

In the circuit of <FIG>, the voltmeter <NUM> configured to measure the voltage across the capacitor <NUM> is a digital voltmeter comprising an operational amplifier 114a connected to an analogue digital converter 114b (ADC). Of course other voltmeters may equally be employed to measure the voltage across the capacitor <NUM>.

The output of the operational amplifier 114a is connected to its inverting input so that the voltage output from the operational amplifier 114a is the same as the voltage input to the voltage amplifier, which is the voltage across the capacitor <NUM>. In other words, the operational amplifier 114a forms a voltage follower (also referred to as a unity-gain amplifier, a buffer amplifier, or an isolation amplifier). The output voltage of the operational amplifier is 114a the same as the voltage across the capacitor <NUM> and is converted to a digital signal by the ADC 114b. The digital signal is stored and processed by the processor <NUM> as discussed in further detail in <FIG>.

The operation of the timer <NUM> and the processor <NUM> of the protection device <NUM> of <FIG> is illustrated in <FIG>. In step <NUM>, the power supply <NUM> for the ICP-OES <NUM> is switched on, for example, by a user. In this embodiment, the power supply <NUM> for the purge gas supply <NUM> and the timing circuit <NUM> is the same as that used to power the other components of the ICP-OES <NUM>. Therefore, turning on the ICP-OES <NUM> triggers turning on of the purge gas supply <NUM> and charging of the capacitor <NUM>. Of course, as discussed in relation to <FIG>, the power supply for the purge gas supply <NUM> and the power supply for the timing circuit <NUM> may be different from each other and/or from that of the other ICP-OES components. Furthermore, turning on of the timing circuit <NUM> may be triggered in response to sensing of the application of purge gas to the detector <NUM>.

Once the ICP-OES <NUM> has been switched on connection of the purge gas supply <NUM> to the detector <NUM> is detected in step <NUM> by using a sensor (not shown). The sensor may be a pressure switch configured to measure the pressure of the purge gas within the supply line <NUM>. If the purge gas supply <NUM> is not connected to the detector <NUM>, then the processor <NUM> receives a signal from the pressure switch indicative of a lack of connection. The processor <NUM> processes this signal and triggers a display screen (not shown) to display a notification to a user to check the connection between the purge gas supply <NUM> and the detector <NUM> (step <NUM>). The processor <NUM> may in addition or by way of alternative process this signal and trigger turning off of the timing circuit <NUM>. The processor <NUM> also turns off or inhibits turning on of the cooling device <NUM> thereby preventing cooling of the detector <NUM> (step <NUM>).

If the purge gas supply <NUM> is connected to the detector <NUM>, then the processor <NUM> receives a signal from the pressure switch confirming connection. The processor <NUM> processes this signal and may trigger a display screen (not shown) to display a notification to a user that the purge gas supply <NUM> and the detector <NUM> are connected. The processor <NUM> may in addition or by way of alternative process this signal and trigger turning on of the timing circuit <NUM>.

Once the ICP-OES <NUM> has been switched on, the capacitor <NUM> of the timing circuit <NUM> starts to charge (i.e. the voltage across the capacitor <NUM> starts to increase) (step <NUM>) and the application of purge gas to the detector <NUM> commences, as explained in relation to <FIG> and as shown in <FIG>. In the embodiment of <FIG>, the power supply <NUM> is configured to power the components of ICP-OES, the protection device <NUM> and the application of purge gas <NUM> to the detector. Therefore, turning on the ICP-OES <NUM> simultaneously triggers charging of the capacitor <NUM> and the application of purge gas to the detector <NUM>.

Alternatively, the other components of the ICP-OES <NUM> may not be powered by the same power supply <NUM> as that used to power the application of purge gas and used to power the protection device <NUM>. Therefore, turning on the ICP-OES <NUM> may not cause charging of the capacitor <NUM> of the timing circuit <NUM>. Instead, turning on the purge gas supply <NUM> may simultaneously trigger the charging of the capacitor <NUM> of the timing circuit <NUM>.

In a further alternative embodiment, the power supply for the protection device <NUM> could be different from that used to power the application of purge gas to the detector <NUM>. A sensor (not shown), which may be the same sensor as that used to check connection of the purge gas supply <NUM> to the detector <NUM>, may be employed to detect starting and cessation of the application of purge gas to the detector <NUM>. The sensor may generate corresponding control signals that are received by the processor <NUM> and trigger turning on/off of the timing circuit <NUM>. The sensor may trigger turning on of the timing circuit <NUM> on starting application of the purge gas to the detector <NUM> and turning off of the timing circuit <NUM> on cessation of the application of purge gas to the detector <NUM>. As discussed above, the sensor may be a pressure switch configured to measure the pressure of purge gas within the purge gas supply line <NUM>. In one example, the purge gas supply line <NUM> may be valve operated and the opening and closing of the valve (not shown) may be sensed by the sensor.

Next, one option is to charge the capacitor <NUM> to its maximum voltage and measure the time period for the capacitor <NUM> to reach the maximum voltage (t<NUM>) (step <NUM>). The time period may be measured by the processor <NUM> using its internal clock as reference. The maximum voltage is approximated to be the voltage of the power supply <NUM> (Vcc), which may already be known. Both the maximum voltage (Vcc) and the corresponding time period (t<NUM>) are stored in a memory of the processor <NUM> (step <NUM>). Subsequently, in step <NUM>, the processor <NUM> determines the time constant (T) for the timing circuit <NUM> using equation (<NUM>) and the stored maximum voltage (Vcc) and time for the capacitor <NUM> to reach the maximum voltage (t<NUM>).

Alternatively, once the capacitor <NUM> has started charging, the time period (t<NUM>) for the capacitor to reach a pre-determined voltage (V<NUM>) may be measured (step <NUM>). Both the pre-determined voltage (V<NUM>) and the corresponding time period (t<NUM>) are stored in the memory of the processor <NUM>. Subsequently, in step <NUM>, the pre-determined voltage (V<NUM>) and the corresponding time period (t<NUM>) are processed to determine the time constant (T) of the timing circuit <NUM> using equation (<NUM>). It is noted that the time constant (T) could be calculated using the known values for impedance of the resistor <NUM> and capacitance of the capacitor <NUM>. However, measuring the time periods t<NUM> and t<NUM> to determine the time constant (T) of the timing circuit <NUM> is more accurate, since the value of capacitance may vary over their lifetime and over components by more than <NUM>%.

Typically, the voltage across the capacitor <NUM> is measured regularly, for example, at intervals of <NUM> to <NUM> seconds, preferably at intervals of <NUM> to <NUM> seconds, such as every <NUM> seconds. The last stored value of the voltage before shut down is measured as V<NUM>, as indicated in <FIG> and step <NUM> of <FIG>.

Once the power supply <NUM> has been switched off, and so the ICP-OES <NUM> has been shut down, (step <NUM>), the application of purge gas to the detector <NUM> stops and the voltage across the capacitor <NUM> decreases as the capacitor <NUM> discharges (step <NUM>). This is discussed in relation to <FIG> and is depicted in <FIG>.

Subsequently, the power supply <NUM> is switched on again and so the application of purge gas to the detector <NUM> restarts (step <NUM>). The voltage across the capacitor <NUM> on switching on the power supply <NUM>, the restart voltage (V<NUM>), is measured (step <NUM>), as depicted in <FIG>. The restart voltage (V<NUM>) is stored and processed by the processor <NUM>.

In step <NUM>, the processor compares the restart voltage (V<NUM>) with the last stored value, V<NUM>, which may be equal to VCC if the capacitor was fully charged before shut down.

If the capacitor was not fully charged before shut down, then the processor <NUM> determines the time period for which the purge gas supply <NUM> has been switched off using equation (<NUM>) with V<NUM>, V<NUM> and the time constant (T) previously determined in step <NUM>. In more detail, the processor inputs the restart voltage (V<NUM>) and the time constant (T) into equation (<NUM>) to determine the time for the voltage across the capacitor <NUM> to drop from the maximum voltage (Vcc) to the restart voltage (V<NUM>). The processor then inputs V<NUM> into equation (<NUM>) to determine the time for the voltage across the capacitor to drop from Vcc to V<NUM>. The processor then subtracts these values from each other to determine the shut down time period (toff) (the time for which the ICP-OES <NUM>, and so the purge gas supply <NUM>, is switched off).

Typically, the capacitor has become fully charged prior to shut down. Consequently, the restart voltage (V<NUM>) may be input into equation (<NUM>) and the time period for which the ICP-OES <NUM> is switched off is equal to the time for the voltage across the capacitor to drop from maximum voltage (Vcc) to the restart voltage (V<NUM>).

Note that the impedance of all other loads is much smaller than the value of the resistor which leads to the approximation that the Vcc net is connected to ground during power-down.

In steps <NUM>, <NUM> and <NUM>, the processor compares the shut down time period (toff) for which the ICP-OES <NUM> was switched off to first and second threshold time values (ta) and (tb). The first threshold time period (ta) is less than the second threshold time period (tb). The first threshold time period (ta) may be, for example, <NUM> minutes. The second threshold time period may be, for example, <NUM> minutes. The first and second threshold time periods (ta, tb) may be experimentally determined. The first and second threshold time periods (ta, tb) may also be stored in a memory of the processor <NUM>.

If the shut down time period (toff) is less than the first threshold time value (ta), then no further application of purge gas to the detector <NUM> prior to cooling is required (step <NUM>). Subsequently, the processor <NUM> generates a trigger signal that triggers cooling of the detector (step <NUM>), for example, by triggering switching on of the cooling device <NUM>. The first threshold time value (ta) is the maximum time period for which the application of purge gas may be turned off before cooling of the detector <NUM> safely. If the application of purge gas is turned off for a time period greater than the first threshold time value (ta), then cooling of the detector may result in damage or reduced performance.

If the shut down time period (toff) is greater than the first threshold time value (ta), then the processor <NUM> triggers turning off or inhibits turning on of the cooling device <NUM> (steps <NUM>, <NUM>).

The processor then calculates the start-up time period (tstart), which is the time period for which purge gas is applied to the detector <NUM> (steps <NUM>, <NUM>).

The start-up time period (tstart) may be selected by the processor <NUM> from a number of pre-determined time periods which are pre-determined experimentally using a safety factor. The start-up time period (tstart) is selected from the pre-determined time periods based upon the shut down time period (toff). The selection may also be based upon the performance quality desired by the user. For example, for a certain shut-down time period (toff), there may be two possible pre-determined time periods for the start-up time period (tstart). If the user desires a high performance measurement, then the longer of those pre-determined time periods may be selected as the start-up time period (tstart).

Alternatively, the start-up time period (tstart) may be calculated. The calculation of the start-up time period (tstart) is based upon the value of the voltage on start-up (V<NUM>) using equation (<NUM>). The processor <NUM> determines the time for the capacitor <NUM> to charge up to a threshold voltage value (VT) from the restart voltage (V<NUM>). The threshold voltage value (VT) is set based upon the voltage across the capacitor once the capacitor has been charged for a time period sufficiently long enough for a proper purge to occur. A proper purge is a purge for a certain period of time so that the atmosphere surrounding the detector is sufficiently water vapour free and subsequent cooling of the detector will not cause damage to it and/or reduce f its performance. By way of example, the threshold voltage value may be the voltage which the capacitor has after a charging time of <NUM> minutes. The processor <NUM> determines this by calculating the time (tT) for the capacitor to charge from zero volts to the threshold voltage value (VT) using equation <NUM>. The processor <NUM> then determine the time (t<NUM>) for the capacitor to charge from zero volts to the restart voltage (V<NUM>). The processor then subtracts t<NUM> from tT thereby determining the start-up time period (tstart).

If the shut down time period (toff) is greater than the first threshold time value (ta) but less than the second threshold time value (tb), then once the start-up time period (tstart) has been determined (step <NUM>), the processor <NUM> triggers the display screen (not shown) to display a notification to a user (step <NUM>). The notification indicates that the detector <NUM> should be purged for the start-up time period (tstart) for optimal performance. The notification also indicates that the user can choose to take reduced performance measurements before the detector <NUM> is ready (i.e. without further purging of the detector <NUM> and without cooling of the detector <NUM>).

Subsequently, the processor <NUM> generates a signal to restart or maintain application of purge gas to the detector <NUM> for the start-up time period (tstart) (step <NUM>). For example, the processor <NUM> may control switching on and off of the purge gas supply <NUM> or may control valve operation of the purge gas supply line <NUM>. Once the detector <NUM> has been purged for the start-up time period (tstart), the processor <NUM> triggers cooling of the detector (step <NUM>). For example, the processor <NUM> may trigger switching on of the cooling device <NUM> once the start-up time period (tstart) has elapsed.

If the shut down time period (toff) is greater than the second threshold time value (tb) (step <NUM>), then once the start-up time period (tstart) has been determined (step <NUM>), the processor <NUM> triggers the display screen (not shown) to display a notification to a user (step <NUM>). This notification indicates that measurements should not be taken before purging of the detector <NUM> for the start-up time period (tstart). Therefore, in this case, a user does not have the option of taking reduced performance measurements before the detector <NUM> is ready. Subsequently, the processor <NUM> generates a signal to restart or maintain application of purge gas to the detector <NUM> for the start-up time period (tstart) (step <NUM>). For example, the processor <NUM> may control switching on and off of the purge gas supply <NUM> or may control valve operate of the purge gas supply line <NUM>. Once the detector <NUM> has been purged for the start-up time period (tstart), the processor <NUM> triggers cooling of the detector (step <NUM>). For example, the processor <NUM> may trigger switching on of the cooling device <NUM> once the start-up time period (tstart) has elapsed.

<FIG> is a schematic diagram of an ICP-OES <NUM>' in accordance with the disclosure. The arrangement of <FIG> is essentially the same to that of <FIG> and so will not be described in detail to avoid repetition. The difference between the arrangement of <FIG> and of <FIG> is that, in <FIG>, the protection device <NUM>' comprises a humidity sensor <NUM> instead of a timer. The humidity sensor <NUM> is located within a region of the detector <NUM> where cooling takes place. However, the humidity sensor <NUM> could instead be located within the purge gas supply line <NUM>. The humidity sensor <NUM> is connected to the processor <NUM> and is employed to measure a parameter indicative of moisture content in the detector <NUM> following cessation of the application of purge gas to the detector <NUM>. The humidity sensor <NUM> employs an electronic hygrometer that measures the capacitance of its environment, which relates directly to relative humidity. Although the absolute capacitance may vary from sensor to sensor, individual sensors should be very repeatable. This type of hygrometer comprises capacitor having a dielectric material placed between two electrically conductive plates. The dielectric material is configured to absorb water vapour from its environment. The electrical capacitance across the capacitor increases as the dielectric material absorbs water vapour. The humidity sensor <NUM> may be calibrated by performing a two point calibration. The calibration data may be stored within the sensor. For example, the humidity sensor may be a camera and the calibration data may be stored within the flash of the camera. The humidity sensor <NUM> may alternatively employ, for example, dry and wet bulb hygrometers or dew point hygrometers. The processor <NUM> is employed to determine the start-up time period based upon the parameter indicative of moisture content. The humidity sensor <NUM> and the processor <NUM> are described in more detail in <FIG>.

In step <NUM>, the power supply for the ICP-OES <NUM>' is turned on, for example, manually by a user. In this embodiment, the power supply powering the application of purge gas to the detector <NUM> is the same as that powering the other components of the ICP-OES <NUM>'.

Therefore, turning on the ICP-OES <NUM>' triggers the application of purge gas to the detector <NUM>. Of course, the power supply for the purge gas supply may be different from that of the other ICP-OES components.

Once the ICP-OES <NUM>' has been switched on, a sensor (not shown) detects connection of the purge gas supply <NUM> to the detector <NUM> in step <NUM>. The sensor may be a pressure switch configured to measure the pressure of the purge gas within the supply line <NUM>. If the purge gas supply <NUM> is not connected to the detector <NUM>, then the processor <NUM> receives a signal from the pressure switch indicative of lack of connection, processes this signal and triggers the display screen (not shown) to display a notification to a user to check the purge gas connection (step <NUM>). The processor <NUM> also turns off or inhibits turning on of the cooling device <NUM> thereby preventing cooling of the detector <NUM> (step <NUM>).

If the purge gas supply <NUM> is connected to the detector, then the processor <NUM> receives a signal from the sensor confirming connection. Subsequently, the processor <NUM> processes this signal and triggers the humidity sensor <NUM> to measure a humidity value (hm) indicative of moisture content within the detector <NUM> (step <NUM>). The measured humidity value (hm) is stored in the memory of the processor <NUM>.

Subsequently, in steps <NUM>, <NUM> and <NUM>, the processor compares the measured humidity value (hm) to a first threshold humidity value (h<NUM>) and a second threshold humidity value (h<NUM>). The first threshold humidity value (h<NUM>) is less than the second threshold humidity value (h<NUM>). The first and second threshold humidity values (h<NUM>, h<NUM>) may be experimentally determined. The first and second threshold humidity values (h<NUM>, h<NUM>) may also be stored in a memory of the processor <NUM>.

Typically, the first threshold humidity value (h<NUM>) is a maximum level of humidity of a detector which may be cooled safely. If the measured humidity value (hm) is less than the first threshold humidity value (h<NUM>), then no further purging of the detector <NUM> is required before cooling (step <NUM>). Subsequently, the processor <NUM> triggers cooling of the detector <NUM>, for example, by triggering turning on of the cooling device <NUM> (step <NUM>).

If the measured humidity value (hm) is greater than the first threshold humidity value (h<NUM>), then the processor <NUM> triggers turning off or inhibits turning on of the cooling device <NUM> (steps <NUM>, <NUM>). The processor then calculates the start-up time period (tstart), which is the time period for which purge gas is applied to the detector <NUM>.

The start-up time period (tstart) may be selected by the processor <NUM> from a number of pre-determined time periods which are pre-determined experimentally using a safety factor. The start-up time period (tstart) is selected from the pre-determined time periods based upon the humidity value (hm). The pre-determined time periods may be stored in a memory of the processor <NUM>. The selection may also be based upon the performance quality desired by the user. For example, for a certain humidity value (hm), there may be two possible pre-determined time periods. If the user desires a high performance measurement, then the longer of those pre-determined time periods may be selected as the start-up time period (tstart).

If the measured humidity value (hm) is less than the second threshold humidity value (h<NUM>) but greater than the first threshold humidity value (h<NUM>) (step <NUM>), then the processor <NUM> triggers the display screen (not shown) to display a notification to a user (step <NUM>). The notification indicates that for optimal performance the detector <NUM> should be purged for the start-up time period (tstart). The notification also indicates that the user can choose to take reduced performance measurements before the detector <NUM> is ready (i.e. without further purging of the detector <NUM> and without cooling of the detector <NUM>). Subsequently, the processor <NUM> triggers purging of the detector <NUM> for the start-up time period (tstart) for example <NUM> minutes (step <NUM>). For example, the processor <NUM> may control turning on/off of the purge gas supply <NUM> or valve operation of the purge gas supply line <NUM>. Subsequently, the processor <NUM> triggers cooling of the detector <NUM>, for example, by triggering turning on of the cooling device <NUM> (step <NUM>).

If the measured humidity value (hm) is greater than the second threshold humidity value (h<NUM>) (step <NUM>), then the processor <NUM> triggers the display screen (not shown) to display a notification to a user (step <NUM>). This notification indicates that the detector <NUM> should be purged for the start-up time period (tstart) before any cooling occurs and/or measurements are taken. Therefore, in this case, a user does not have the option of taking reduced performance measurements before the detector <NUM> is ready. The start-up time period (tstart) for hm>h<NUM> is greater than the start-up time period for hm<h<NUM>.

Subsequently, the processor <NUM> triggers purging of the detector <NUM> for the start-up time period (tstart). The processor <NUM> does not trigger cooling of the detector <NUM> in this instance. Instead, following purging of the detector, the humidity of the purge gas is measured again. If, when re-measured, the measured humidity value (hm) is less than the first threshold humidity value (h<NUM>), then the processor <NUM> triggers cooling of the detector <NUM> (step <NUM>). If, when re-measured, the humidity of the purge gas (hm) is greater than the first threshold humidity value (h<NUM>) but less than the second threshold humidity value (h<NUM>), then the detector <NUM> may be cooled once purged for the start-up time period (tstart), as discussed above. Therefore, the protection device <NUM> ensures that the detector <NUM> is not cooled until a safe level of humidity has been reached.

As depicted in steps <NUM>, <NUM> and <NUM> during cooling of the detector <NUM>, the humidity of the purge gas is measured, for example, every ten minutes. The measured humidity values (hm) are stored in the memory of the processor <NUM>. The processor <NUM> compares the measured humidity values (hm) to the first and second threshold humidity values (h<NUM>, h<NUM>) (steps <NUM>, <NUM>, <NUM>). If a measured humidity value (hm) is less than the first threshold humidity value (h<NUM>), then cooling of the detector <NUM> may be continued. If a measured humidity value (hm) is greater than the first threshold humidity value (h<NUM>), then the processor <NUM> causes cooling of the detector <NUM> to stop (steps <NUM>, <NUM>). By way of example, the processor <NUM> may trigger turning off of the cooling device <NUM>, in response to the measured humidity value (hm) being greater than the first threshold humidity value (h<NUM>). The processor may then trigger purging of the detector <NUM> for the start-up time period (tstart). The duration of the start-up time period (tstart) depends upon whether the humidity of the purge gas is less than or greater than the second threshold humidity value (h<NUM>). Therefore, the protection device <NUM> dynamically controls purging and cooling of the detector <NUM> to ensure that damage to the detector is avoided.

<FIG> is a schematic diagram of an ICP-OES <NUM>" in accordance with the present invention. The arrangement of <FIG> is essentially the same to that of <FIG> and <FIG> except that the protection device <NUM>" comprises both the timer <NUM> of <FIG> and the humidity sensor <NUM> of <FIG> and <FIG>. The timer <NUM> and the humidity sensor <NUM> are connected to the processor <NUM>. The timer <NUM> is employed to measure a parameter indicative of the shut down time period following cessation of an application of purge gas to the detector <NUM>. The shut down time period, as discussed above, is the time period for which purge gas is not applied to the detector <NUM>. Specific details of the timer <NUM> operation have been discussed in relation to <FIG> and <FIG>. The processor <NUM> is employed to determine the start-up time period based upon the shut-down time period. During the start-up time period, purge gas is applied to the detector <NUM> based upon the parameter. The processor <NUM> may be configured to trigger application of purge gas to the detector <NUM> or maintain application of purge gas to the detector <NUM> during the start-up time period, preferably from the beginning of the start-up period. The humidity sensor <NUM> is employed to measure a parameter indicative of moisture content in the detector <NUM> following cessation of the application of purge gas to the detector <NUM> and/or following purging of the detector <NUM> for the start-up time period.

It can be seen that the timer <NUM> can be used alone as shown in <FIG> as a means to determine the shut-down period, or it can be used as a back-up to a humidity sensor (or pressure or other sensor) as shown in <FIG>, which can be useful if the humidity sensor (or pressure sensor etc.) should fail. Alternatively, the humidity sensor (or pressure or other sensor) may be used as a back-up to the timer <NUM> as a means to determine the shut-down time period.

Alternatively, the timer <NUM> and humidity sensor <NUM> may be used simultaneously in the embodiment of <FIG>. For instance, the processor120 may process signals/measurements received from both the timer <NUM> and the humidity sensor <NUM>. If either measurement indicate that the detector <NUM> should be purged before cooling, then the processor <NUM> will trigger purging of the detector <NUM>. The processor <NUM> will also prevent cooling of the detector <NUM> until both the measurement/signal received from the timer <NUM> and the measurement/signal received from the humidity sensor <NUM> indicate that cooling of the detector <NUM> may be performed safely.

For example, the timer <NUM> may be used to determine the start-up time period (tstart) and the humidity sensor <NUM> may be used to measure the humidity value (hm) after purging the detector for the start-up time period (tstart). The measured humidity value (hm) may then be compared to the first threshold humidity value (h<NUM>). If the measured humidity value (hm) is less than the first threshold humidity value (h<NUM>), then the processor <NUM> may trigger cooling of the detector <NUM>. If the measured humidity value (hm) is greater than the first threshold humidity value (h<NUM>), then the processor <NUM> may prevent cooling of the detector <NUM> and instead trigger further purging of the detector <NUM>.

Alternatively, the start-up time period (tstart) determined using the humidity sensor <NUM> and the timer <NUM> each may be compared and averaged. In this case, the humidity sensor <NUM> is employed to measure a parameter indicative of moisture content in the detector <NUM> following cessation of the application of purge gas to the detector <NUM>. Once an averaged start-up time period has been determined, the methodology in <FIG> or <FIG> may be followed.

The embodiments of the present invention relate to an ICP-OES. However, it will be understood that the present invention is applicable to various types of an OES spectrometer implementing a detector that is cooled to optimize performance. For example, the invention may be applied to other types of optical emission spectrometer (OES) such as a Spark Ablation OES, a Microwave Induced Plasma (MIP) OES, or the like.

The skilled person would readily appreciate that the power supply used to power the components of the ICP-OES, to power the application of purge gas to the detector and to power the timer and processor may be the same or alternatively, each of these elements may have a separate power supply. For example, the timer may be battery operated.

The skilled person would also understand that, whilst the processor and timer have been described as separate features, they could, of course, be arranged to form a single unit. Of course, whilst the sensor employed in the ICP-OES of <FIG> to <NUM> is a humidity sensor, other sensors may be alternatively or additionally employed to indicate a characteristic of the atmosphere in the detector and therefrom the shut down time period. For example, a pressure sensor may be employed in the detector <NUM> or the purge gas supply line <NUM> and pressure of the purge gas may be measured to indicate the shut-down period. In one such embodiment, the amount that the pressure is below a threshold pressure (or below the last measured pressure before shut down) can indicate the shutdown time period (toff). Subsequently, after the purge gas is supplied again to the detector, when the pressure has reached a certain threshold, and optionally maintained at that level for a certain time, it can be determined that the detector is properly purged so that the detector may be cooled. Alternatively, the application of purge gas to the detector <NUM> may be controlled by a solenoid valve and operation of the solenoid valve may be monitored to indicate the shut down time period.

Claim 1:
An Optical Emission Spectrometer (OES) (<NUM>) comprising:
a detector (<NUM>) to which purge gas can be supplied; and
a protection device (<NUM>, <NUM>");
wherein the protection device (<NUM>) comprises:
a timer (<NUM>) configured to measure a parameter indicative of a shut down time period following cessation of an application of purge gas to the detector (<NUM>); and
a processor (<NUM>) connected to the timer (<NUM>), wherein the processor (<NUM>) is configured to determine a start-up time period during which purge gas is supplied to the detector (<NUM>) prior to cooling of the detector (<NUM>), the start-up time period being based upon the parameter measured by the timer (<NUM>).