JP2003329639A - Nox gas detecting method and device therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make an NOX gas detecting method and a device therefor stably detectable NOX gas for a long period of time. <P>SOLUTION: A detection pole, a counter pole, and a reference pole are provided for an alkali metal ionic conductor, and nitrite salt of an alkali metal is added to the counter pole only. With the potential of the detection pole intermittently made to be operation potential on the order of -150 mV by means of pulses of a duty ratio of about 1/5 and 10 sec or less in width, the detection pole and the counter pole are connected to each other. During the other periods, the detection pole is disconnected from the counter pole or from the reference pole. NOX gas is detected from a current flowing through the counter pole/ detection pole during a latter half of a pulse. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to detection of NOx using an alkali metal ion conductor, and more particularly to improvement of long-term stability of a NOx sensor used.

[0002]

2. Description of the Related Art The inventors have developed a NOx sensor using an alkali metal ion conductor such as NASICON. Such a NOx sensor is a ppb level NOx
It is necessary to build a network for monitoring air pollution or to reduce traffic pollution. As is well known, since NO can be converted into NO2 by a catalytic converter, the sensor needs to be able to detect at least NO2 and needs not to be interfered with by CO2 or H2O.

The inventors have provided a detection electrode, a reference electrode, and a counter electrode on an alkali metal ion conductor, and added an alkali metal nitrite or the like only to the counter electrode to make the potential of the detection electrode negative with respect to the reference electrode. Proposed NOx sensor (Japanese Patent Laid-Open No. 11-27)
1266). This sensor receives almost no interference of H2O and CO2 and can detect NO2 of ppb order, and the electrode reaction is as follows. Na ⁺ + NO2 + e ⁻ → NaNO2 (detection electrode) (1) NaNO2 → Na ⁺ + NO2 + e ⁻ (counter electrode) (2) This sensor is heated to about 150 ℃ and NO2 is applied to the detection electrode.
When is touched, Na ions are replenished from NASICON and NaNO2 is produced. In response to this, NaNO2 is decomposed at the counter electrode, and alkali metal nitrite at the counter electrode is consumed. As a result, the sensor output decreased in a short period of time and did not reach the target life of at least one month.

In order to compensate for the consumption of alkali metal nitrite, which is the active material of the counter electrode, the inventors applied a reverse bias voltage with the detection electrode as the + counter electrode, and the metal nitrite accumulated in the detection electrode was applied. It was proposed to decompose and transport Na ions from the detection electrode to the counter electrode (Japanese Patent Laid-Open No. 2001-194337).
issue). The idea here is to apply a reverse bias voltage to the sensor periodically to initialize the sensor so that the states of the detection electrode and the counter electrode do not change from the initial state. The inventor then arrived at the present invention by considering extending the sensor life while allowing the metal nitrite to be consumed on the counter electrode.

[0005]

SUMMARY OF THE INVENTION The basic object of the present invention is to provide a new detection method and device for extending the life of the NOx sensor (claims 1-8).

[0006]

The NOx gas detection method according to the present invention uses an alkali metal ion conductor provided with a detection electrode, a counter electrode and a reference electrode, and a counter electrode containing a nitrite or a nitrate of an alkali metal for detection. In a method of applying a negative potential to a reference electrode with respect to a reference electrode to detect NOx gas from a current between a counter electrode and a detection electrode, the negative potential is pulse-wise applied to the detection electrode at a duty ratio of 1/3 or less. Then, while detecting the NOx gas from the current between the counter electrode and the detection electrode when applying the pulse,
When the pulse is not applied, the detection electrode is made to have the same potential as the reference electrode, or the electrical connection with the reference electrode is cut off (claim 1).

Preferably, except when the pulse is applied,
The electrical connection between the detection electrode and the reference electrode is cut off (claim 2). Particularly preferably, the detection electrode is electrically cut off from both the reference electrode and the counter electrode except when the pulse is applied. Here,
Electrically connecting or electrically disconnecting means electrically connecting or disconnecting in an accessory circuit of the sensor, and has nothing to do with being connected by an ionic conductor in the sensor.

Preferably, the NOx gas is detected from the current of the latter half of the pulse, particularly after the end of the peak of the sensor current (sensor output) at the beginning of the pulse (claim 3). Particularly preferably, the sensor is aged under the conditions described in claim 1, the sensor is moved to a stable region after the initial reduction in sensitivity is completed, and NOx gas is detected (claim 4).

Further, the NOx gas detecting apparatus of the present invention is provided with a detection electrode, a counter electrode and a reference electrode on an alkali metal ion conductor, and a sensor containing an alkali metal nitrite or nitrate is used as the counter electrode for detection. In a device for detecting a NOx gas from a current between a counter electrode and a detection electrode by applying a negative potential to a reference electrode to the reference electrode, the negative potential is ⅓ to the detection electrode.
Pulse driving means for applying a pulse with the following duty ratio and for making the detection electrode equipotential with respect to the reference electrode or interrupting electrical connection with the reference electrode except when the pulse is applied. And a current detecting means for detecting the NOx gas from the current between the counter electrode and the detection electrode when the pulse is applied (claim 5).

Preferably, the detection electrode is electrically cut off from the reference electrode except when the pulse is applied, and particularly preferably the detection electrode is electrically cut off from both the reference electrode and the counter electrode except when the pulse is applied. Preferably, the current detection means detects NOx gas from the current in the latter half of the pulse (claim 6). Preferably, the sensor is aged under the conditions described in claim 5 and is moved to a stable range after the initial reduction in sensitivity is completed (claim 7). Particularly preferably, the duty ratio is 1/4.
~ 1/20, pulse period 30 seconds ~ 1 minute, pulse width 3
Second to 10 seconds (claim 8).

[0011]

According to the NOx gas detecting method and apparatus of the present invention, the detection electrode and the reference electrode are always kept at the same potential or electrically cut off, and are pulsed at a duty ratio of 1/3 or less. , NOx is added to the detection electrode with reference to the reference electrode, and NOx is detected from the current between the counter electrode and the detection electrode during pulse application.
Detect gas. It was predicted that since a current flows in a pulsed manner between the counter electrode and the detection electrode, the consumption of the active material on the counter electrode should decrease in proportion to the duty ratio, and the sensor life should be extended accordingly. However, in reality, when a potential is applied in a pulsed manner between the detection electrode and the reference electrode (pulse drive),
The value of the current between the counter electrode and the detection electrode is different from when the potential is continuously applied to the detection electrode (continuous drive), and NOx sensitivity higher than that of continuous drive is obtained by pulse drive. When detecting NOx gas of, the lower limit of detection concentration can be lowered. Further, even if the electric potential was applied to the detection electrode in a pulsed manner, the sensitivity decreased for about 10 days after the start of use of the sensor, similarly to the case where the electric potential was continuously applied to the detection electrode. However, when a potential was applied to the detection electrode in a pulsed manner, a stable region of sensitivity appeared after the initial decrease in sensitivity. Since the stable period of sensitivity continues for 30 days or more, for example, the stable region can be used to extend the life of the NOx sensor (claims 1 and 5).

When the detection electrode is cut off from the reference electrode except when the pulse is applied, no current flows between the detection electrode and the counter electrode except when the pulse is applied (claim 2).

The response waveform when a pulse is applied to the detection pole is 2
There is a peak of the sensor current, which is divided into two parts and has a weak correlation with the NOx concentration in the first half. After passing this peak,
The sensor current has a linear relationship with the NOx concentration. Therefore, if the NOx gas is detected from the current in the latter half of the pulse, the NOx concentration can be detected with high accuracy (claims 3 and 6). The absolute value of the time derivative of the sensor current on the trailing side of the peak (the latter half of the peak) is smaller as the NOx concentration is higher. Therefore, it is possible to detect the NOx concentration from this,
It is not impossible to detect other than the sensor current in the latter half of the pulse.

Further, even if the sensor is used while applying a negative potential in a pulsed manner to the detection electrode, the initial sensor sensitivity is largely different from the case where the potential (bias voltage) is continuously applied to the detection electrode. There is no. However, if the sensing electrode is connected to the counter electrode in a pulsed manner, then a stable region of sensor sensitivity will occur. Therefore, if the sensor is aged for a period in which initial sensitivity deterioration occurs, and then NOx detection is started, N
The Ox concentration can be detected (claims 4 and 7).

The pulse cycle is, for example, N every 30 seconds to 1 minute.
From the viewpoint of detecting the Ox concentration, 30 seconds to 1 minute is preferable. Next, during the pulse, the peak of the sensor current ends, so that the NOx concentration can be accurately measured, the pulse width is preferably 3 seconds to 10 seconds, and the duty ratio is 1 / to increase the life of the sensor. 4 to 1/20 is preferable. This condition is to extend the life of the sensor at a practical detection interval as much as possible and to extract the sensor current after the end of the initial peak at the time of pulse-on (claim 8).

[0016]

EXAMPLE A NOx gas detecting method and an NOx gas detecting apparatus of an example will be described with reference to FIGS. For simplicity, NOx gas will be referred to as NOx, and NO will be converted to NO2 by an appropriate converter and detected.
The detection of will be described.

In the NOx gas detector 2 of FIG. 1, 4
Is an NOx sensor, 6 is an insulating substrate such as alumina, 8 is a heater using a Pt film or ruthenium oxide film, and 10 is a heater power supply. NOx by heater 8
The sensor 4 is, for example, 150 ° C. (generally 100 to 200
(° C).

Reference numeral 12 denotes a sodium ion conductor such as NASICON, which may be a lithium ion conductor or an alkali metal ion conductor. 14 is a detection pole,
Here, 16 is a noble metal film such as gold, and 16 is a counter electrode, which is a film in which an alkali metal nitrite or an alkali metal nitrate such as NaNO2 or NaNO3 is added to a noble metal such as gold. The type of the alkali metal in the nitrate or the nitrite is the same as the type of the alkali metal ion used in the ionic conductor 12 since the sensor 4 is driven by current, and the alkali metal nitrate or the alkali metal nitrite is, for example, The saturated aqueous solution is added dropwise to the counter electrode 16. Reference numeral 18 is a reference electrode, which is also made of a noble metal film such as gold. Reference numeral 20 denotes a sealing portion using glass or the like, which is for shielding the reference electrode 18 from the atmosphere (atmosphere) to be detected, but the sealing portion 20 may not be provided. The structure itself of the NOx sensor 4 is described in Japanese Patent Application Laid-Open No. 2001-194337 and Japanese Patent Application Laid-Open No. 11-194337.
It is known from US Pat. In addition, a third component such as an alkaline earth metal nitrite may be added to the counter electrode from the viewpoint of improving moisture resistance.

Reference numeral 22 is a reference power source, and with respect to the detection electrode 14,
This is for applying a bias voltage of about −100 to −200 mV at a potential based on the reference electrode 18 at a predetermined duty ratio in a pulsed manner. Reference numeral 24 is a relay, which is an example of a switch, and may be a semiconductor switch or the like as long as it has low resistance and voltage loss can be ignored. When the relay 24 is opened, the detection electrode 14 is electrically cut off from the counter electrode 16 and the reference electrode 18. When the relay 24 is closed, the detection electrode and the counter electrode are electrically connected, and the detection electrode is opposite to the counter electrode. It is placed at a given potential. Reference numeral 26 denotes an operational amplifier which, when the relay 24 is closed, sets the potential of the detection electrode 14 to −100 with respect to the reference electrode 18.
Keep about -200 mV negative. An ammeter 28 is connected to the counter electrode 16 and closes the relay 24 to detect the detection electrode 1
When a bias voltage is applied to 4 in a pulsed manner, a current value at a predetermined timing in the latter half of the pulse is output. Instead of providing the relay 24 between the operational amplifier 26 and the reference power source 22, a relay 25 may be provided between the ammeter 28 and the counter electrode 16, or both the relays 24 and 25 may be provided. Further, a function generator or the like may be provided instead of the reference power source 22 without providing the relay 24. With this arrangement, the output pulse of the function generator keeps the detection pole at a negative potential with respect to the reference pole, and keeps the detection pole at the same potential as the reference pole during the other period.

Reference numeral 30 denotes a pulse generator which generates a pulse at a predetermined cycle of about 30 seconds to 1 minute, and a waveform shaping section 32 shapes this into a square wave pulse having a predetermined width of about 3 seconds to 10 seconds.
The relay drive 34 drives the relay 24 to turn it on / off. Further, the relay drive 34 has a predetermined timing in the latter half of the pulse (immediately before the end of the pulse or in the latter half of the pulse width 1 /
3), a sampling signal is sent to the ammeter 28, and the ammeter 28 outputs the current value at this point. The concentration converter 36 converts this current value into the NOx concentration. Since the current value in the embodiment is determined by the sum of the offset current at the NOx concentration of 0 and the current proportional to the NOx concentration, the concentration conversion unit 36 calculates the offset current value for each sensor 4 from the measured current value. A coefficient for multiplying a value excluding the offset current and converting the NOx concentration is stored.

In the embodiment, the relay 24 is intermittently turned on at a predetermined duty ratio so that the electric potential is applied between the detection electrode 14 and the reference electrode 18 and the detection electrode 14 and the counter electrode 16 are connected only during that period. I am trying to connect. However, a bias voltage may be constantly applied between the detection electrode 14 and the reference electrode 18, the electric path between the detection electrode 14 and the counter electrode 16 may be intermittently connected, and the others may be cut off.

FIG. 2 shows the bias potential applied to the detection electrode and the sensor output. The potential of the detection electrode is shown with reference to the reference electrode, for example, about -100 mV to -200 mV. When the sensor is driven in a cycle T, the ON time for turning on the relay 24 is T1 and the off time is T2. The ratio (T1 / T) is preferably 1/3 or less, more preferably 1/4 to 1/20. The on-time T1 is preferably 3 seconds or more and 10 seconds or less, more preferably 5 seconds or more and 10 seconds or less. The cycle T determines the NOx detection cycle, and is preferably 30 seconds or more and 1 minute or less, and is, for example, 30 seconds or 1 minute.

In the embodiment, as shown in FIG. 2A, the electric potential E is applied to the detection electrode by a square wave pulse, but the waveform of the electric potential applied to the detection electrode is not limited to the square wave. For example, in Figure 2 (2),
The initial peak of the pulse is terminated during the first wide pulse, and a second narrow pulse is added thereto at an interval of, for example, about 1 to 5 seconds, and the sensor is synchronized with the second pulse. Sample the current.

FIG. 2C shows the sensor output. In the embodiment, the current flowing from the detection electrode to the counter electrode is the sensor output, and the current value in the first half of the pulse has a very weak correlation with the NOx concentration.
Since the linearity between the current value and the NOx concentration is obtained in the latter half of the pulse, the current value in the latter half of the pulse is sampled. Further, in the embodiment, the purpose is not to forcibly return the Na ions that have moved from the counter electrode to the detection electrode to the counter electrode, but to send the consumption of Na ions at the counter electrode, so that the detection electrode is not applied while the pulse is not applied. The electrical connection between the counter electrode and the counter electrode is cut off, and the current is forcibly fixed to zero. The term latter half of the pulse is used in Fig. 2 (2).
When the waveform of the pulse is not a simple square wave as in, the first pulse and the second pulse are regarded as a part of one pulse as a whole, and at the timing of applying a negative potential to the detection electrode, Also, it means the latter half of the pulse as a whole.

In FIGS. 3 and 4, the detection electrode is pulsed by −15.
The sensor output when a bias voltage of 0 mV is applied is shown. The results of adding NaNO2 to the counter electrode will be described below. In the waveforms of FIGS. 3 and 4, the period T is 38 seconds, the time T1 for applying the pulse voltage is 8 seconds, and the sensor temperature is 150 seconds.
C. and the potential of the detection electrode with respect to the reference electrode is -150 mV. Further, the NO2 concentration is about 40 ppb, which is the data within 10 days after starting the use of the sensor under this condition. When a bias voltage of −150 mV is applied to the detection electrode in a pulsed manner, a peak of the sensor output is generated and then the peak is rapidly attenuated, and a region close to a steady value can be seen. Then, the range in which the peak almost ends and the sensor current approaches the steady value is the region where the NOx concentration can be measured. When the sensor temperature is changed from 150 ° C. in FIG. 3 to 200 ° C. in FIG. 4, the peak width is sharply reduced, and NOx can be detected with a shorter pulse. However, when the sensor temperature is increased, the consumption amount of the active material of the counter electrode is increased for the same pulse width.

In FIG. 5, the sensor temperature is set to 150 ° C.
7 shows a response waveform on the 7th day after the start of use of the sensor when a bias voltage of −150 mV was applied to the detection electrode with respect to the reference electrode for 8 seconds at a cycle of 2 seconds. This response waveform is measured by extending a normal pulse width of 8 seconds to one cycle of 38 seconds. The difference between the current values of NO2 238 ppb and NO2 2 ppb is the net sensor output. At the peak immediately after the pulse application, these differences are small, and the difference in the current values increases as the peak ends and approaches the steady value. To do. Therefore, the sensor output to be sampled is preferably in the latter half of the pulse, particularly after the peak of the sensor current is almost finished.

FIG. 6 shows the relationship between the NO2 concentration and the sensor output under the condition that the sensor temperature is 150 ° C. and the sensing electrode is −150 mV with respect to the reference electrode for 8 seconds in 38 second cycle. This is the data on the 5th day from the start of use of the sensor, and there is an offset current of about 10 nA, and the sensor output has a linear relationship with the NO2 concentration. Therefore, if the offset current value and the NO2 concentration per sensor current are stored for each sensor, the NOx concentration can be obtained even if the sensor is replaced.

FIG. 7 shows the relationship between the sensor output and the potential of the detection electrode when the sensor temperature is 150 ° C. and the bias voltage is constantly applied between the detection electrode and the reference electrode. The larger the absolute value of the bias voltage, the larger the sensor output, and the faster the consumption of the active material of the counter electrode, the faster.
In addition, since the sensor output becomes smaller when the bias voltage is 0 side than -100 mV, the potential of the detection electrode is-
100 to -200 mV is preferable.

FIG. 8 shows the time-dependent data of the sensor for 41 days. The vertical axis represents the relative NO2 sensitivity based on the first day of use of the sensor that has not been aged and the first day as a reference. Here, NO2 of 200 ppb is the detection target. Further, the sensor temperature was 150 ° C., and in the comparative example, the potential of the detection electrode was constantly kept at −150 mV, and in the embodiment, a potential of −150 mV was applied to the detection electrode in a pulsed manner for 8 seconds in a 38 second cycle. The counter electrode was connected to the detection electrode during the pulse, and the current value 5 seconds after the pulse was applied was used as the sensor output.

The sensitivity on the first day was 0.4 nA / ppb in the example,
In the comparative example, it is 0.1 nA / ppb. It can be seen that when a bias potential is applied to the detection electrode in a pulsed manner, the value of the sensor current differs between pulse application and continuous application, and the mechanism that determines the output current of the sensor differs between pulse application and continuous application.

For about 10 days from the start of use, the sensitivity is similarly lowered in both Examples and Comparative Examples. However, in the embodiment, the sensor sensitivity reaches the stable range after about 10 days from the start of use, and thereafter, the sensor sensitivity is stable for at least 30 days, and NOx with high reliability can be detected. On the other hand, in the comparative example, the stable range of the sensor output does not exist,
The sensor sensitivity decreases monotonically. Therefore, if the sensor is aged under pulse driving conditions for about 10 days and then the measurement of the NOx concentration is started, the NOx concentration can be stably measured.

[Brief description of drawings]

FIG. 1 is a block diagram of a NOx gas detection device according to an embodiment.

FIG. 2 is a waveform diagram of a detection pole potential and a sensor output in the embodiment, (1) shows a waveform of the detection pole potential E in the embodiment, and (2) shows a waveform of the detection pole potential in the modified example. (3) shows the waveform of the sensor output in the embodiment.

FIG. 3: Sensor temperature 150 ° C., detection pole potential −150
The figure shows the waveform of the sensor output when the drive cycle is 38 seconds and the pulse application time is 8 seconds in mV.

FIG. 4 shows a sensor temperature of 200 ° C. and a detection pole potential of −150.
The figure shows the waveform of the sensor output when the drive cycle is 38 seconds and the pulse application time is 8 seconds in mV.

FIG. 5: Sensor temperature 150 ° C., detection pole potential −150
The figure shows the waveform of the sensor output in 2 ppb of NO2 and 238 ppb of NO2 when the driving cycle is 38 seconds in mV.

FIG. 6: Sensor temperature 150 ° C., detection pole potential −150
mV, drive cycle 38 seconds, pulse application time 8 seconds, showing the relationship between sensor output and NO2 concentration

FIG. 7 is a diagram showing the relationship between the detection electrode potential and the sensor output when the detection electrode potential is continuously applied at a sensor temperature of 150 ° C.

FIG. 8: Sensor temperature 150 ° C., detection pole potential −150
mV, drive cycle 38 seconds, pulse application time 8 seconds, NO
2 Diagram showing sensor output in 200 ppb

[Explanation of symbols] 2 NOx gas detector 4 NOx sensor 6 insulating substrate 8 heater 10 Heater power supply 12 Na ion conductor 14 detection poles 16 opposite poles 18 reference pole 20 Sealing part 22 Reference power supply 24,25 relay 26 operational amplifier 28 ammeter 30 pulse generator 32 Wave shaping section 34 relay drive 36 Density converter

   ─────────────────────────────────────────────────── ─── Continued front page    (71) Applicant 000112439             Figaro Giken Co., Ltd.             1-5-3 Senba Nishi, Minoh City, Osaka Prefecture (71) Applicant 396020800             Japan Science and Technology Agency             4-8 Hommachi, Kawaguchi City, Saitama Prefecture (72) Inventor Norio Miura             1-11-18-1402 Hirao, Chuo-ku, Fukuoka City, Fukuoka Prefecture (72) Inventor Yamazoe ▲ Noboru ▼             4-32 Matsugaoka, Kasuga City, Fukuoka Prefecture (72) Inventor Manabu Tani             5-4-11 Higashi-Kojiya, Ota-ku, Tokyo             Inside Blue Co., Ltd. (72) Inventor Hozumi Futa             5-4-11 Higashi-Kojiya, Ota-ku, Tokyo             Inside Blue Co., Ltd. (72) Inventor Kazuyasu Kaneyasu             1-5-3 Senba-nishi, Minoh-shi Figaro             Ken Co., Ltd.

Claims (8)

[Claims] 1. An alkali metal ionic conductor is provided with a detection electrode, a counter electrode and a reference electrode, and a sensor in which an alkali metal nitrite or nitrate is contained in the counter electrode is used, and the detection electrode is a negative electrode with respect to the reference electrode. In a method for detecting a NOx gas from a current between a counter electrode and a detection electrode by applying a potential, the negative potential is pulse-wise applied to the detection electrode at a duty ratio of 1/3 or less, and the counter electrode / detection at the time of pulse application The NOx gas is detected from the current between the electrodes, and the detection electrode is made to have the same potential as the reference electrode or the electrical connection with the reference electrode is cut off except when the pulse is applied. , NOx gas detection method. 2. The NOx gas detection method according to claim 1, wherein the electrical connection between the detection electrode and the reference electrode is interrupted except when the pulse is applied. 3. The NOx according to claim 1 or 2, wherein NOx gas is detected from the current in the latter half of the pulse.
x Gas detection method. 4. The NOx gas is detected by aging the sensor under the conditions described in claim 1, moving the sensor to a stable region after the initial reduction in sensitivity is completed, and detecting NOx gas. The NOx gas detection method according to claim 3. 5. An alkali metal ionic conductor is provided with a detection electrode, a counter electrode and a reference electrode, and a sensor containing alkali metal nitrite or nitrate is used as the counter electrode, and the detection electrode is negative with respect to the reference electrode. In a device for detecting NOx gas from the current between the counter electrode and the detection electrode by applying a negative potential to the detection electrode in a pulsed manner at a duty ratio of 1/3 or less, and except when the pulse is applied. Is a pulse drive means for making the detection electrode equipotential with respect to the reference electrode or for interrupting the electrical connection with the reference electrode, and the NOx gas is generated from the current between the counter electrode and the detection electrode when the pulse is applied. An NOx gas detection device, characterized in that a current detection means for detection is provided. 6. The NOx gas detection device according to claim 5, wherein the current detection means detects NOx gas from the current in the latter half of the pulse. 7. The sensor is a sensor that is aged under the conditions described in claim 5 and is moved to a stable region after the initial reduction in sensitivity is completed. Item 6. A NOx gas detection device. 8. The duty ratio is 1/4 to 1/20,
The NOx gas detection device according to claim 6 or 7, wherein the pulse period is 30 seconds to 1 minute and the pulse width is 3 seconds to 10 seconds.