RU6906U1 - ATOMIC-ABSORPTION MERCURY ANALYZER - Google Patents

Description

Atomic absorption mercury analyzer.

The inventive utility model relates to analytical chemistry, in particular, to spectral atomic absorption analysis with a differential scheme for measuring mercury concentration.

Known atomic absorption mercury analyzer with Zeeman correction of non-selective absorption HGG-3 (Scintrex, Canada, see Appendix 2), containing a resonant radiation source of mercury placed in a pulsed magnetic field, a lens, a light flux divider, an analytical cell, measuring and reference photodetectors channels. When the magnetic field is off, the intensity of the analytical radiation is measured, which depends on the amount of non-selective absorption and absorption due to mercury atoms. When the field is on, the intensity of the reference radiation is measured, which depends on the value of non-selective absorption. Further processing of the signal occurs according to the usual algorithm for differential atomic absorption methods.

The disadvantages of the analogue include a high detection limit of mercury in air (160 ng / m3), a narrow dynamic measurement range (2 orders of magnitude), and weight and power consumption for portable analyzers.

The closest in technical essence to the proposed utility model is an atomic absorption mercury analyzer with Zeeman correction of non-selective absorption 1 (see Appendix 1), consisting of a resonant radiation source of mercury placed in a constant magnetic field, an inclined plate, a polarizing modulator, consisting of an optoacoustic modulator (REM) and a linear polarizer, analytical cell, photo detector and signal processing unit. In the case of ascending along magnetic lines of force, only the a-components of the Zeeman triplet are recorded, with one c-component playing the role of the analytical line and the other as the comparison line. Two signals are displayed in the registration block at the first and second harmonics of the modulation frequency, the first of which is proportional to the concentration of mercury atoms in the analytical cell, and the second is proportional to the total intensity of the a-components. Further signal processing takes place in the microprocessor according to the usual algorithm for differential atomic absorption analysis, adjusted for non-resonant radiation.

The disadvantages of the prototype include a high detection limit when determining the mercury content in atmospheric air (5 ng / m at a device time constant of 5 seconds), design complexity (using a PMT-142, which requires a voltage in the range of 1500 - 2400 V, the need to form reference voltage at a double modulation frequency for the operation of a synchronous detector), a significant variation in the dynamic range of the measured mercury contents from device to device. Really,

Μl GOl N21 / 00

the intensity of radiation It passing through the optical system and incident on the photo detector is described by the formula from

1, b1k () (T. + T,) + (p, -p;) (T, + T,) cos5 + 2p ,, - T2) sm6 (1)

where 1o / 2 is the intensity of one c-component, K is the transmittance associated with the geometric factor and the magnitude of non-selective absorption, PX and pu are the transmittance of the medium along the x and y axes directed under

angle of 45 ° to the long axis PEM, Tj jI (A,) exp (-JQj (i) n (r) dr) dX-coefficient

selective transmission associated with the absorption of mercury atoms of the CTi component; 5 — phase incursion for a light wave. In the real case, the phase incursion is the sum of the phase incursion A introduced by PEM and the static phase incursion 5 ° due to the residual voltage in the optical parts. Thus, substituting 5 6o + AsinQt in formula (1), after simple mathematical transformations, we obtain for the constant component of the signal So, for the first Si and second S2 harmonics, the following relations:

So Y ((T, + T,) ((p, Cp /) + Jo (A) cos6o (p, -p /)) + 2p, pDT, -T,) Jo (A) sin5

S, J, (A) ((T, -T,) 2p, p cos6o- (T, + T,) (p, -p /) sin6o) (3)

S, J, (A) ((T, + T,) (p, -p /) cos6o - (T, -T,) 2p, p sinSo) (4)

In the ideal case, 5 ° O and then relations (3) and (4) have the form

S, J, (A) (T, -T,) p, p, (5)

(A) (T, + T,) (p, -p /) (6)

and then the following formula holds for the analytical signal S, ln ((b-S (t)) / (b + S (t))) (7)

where S (t) Si / 82, b 2Ji (A) pxpy / J2 (A) (px - Py) is the normalization constant. However, in real conditions it is 6 ° 0. Then, the appearance of mercury atoms (this corresponds to an increase in the difference TI - T2), as can be seen from formulas (3) and (4), leads to a change in the magnitude of both the first and second harmonics. In this case, signal processing according to formula (7) can lead to a significant distortion of the calibration curve at high mercury concentrations, depending on the value of 5 °. The experiment showed that for different devices the dynamic range of the measured concentrations varies 100 times due to variations in the maximum detectable concentration.

The objective of the utility model is to create an atomic absorption analyzer with a low detection limit, a wide dynamic range, stability of analytical characteristics and a simpler design.

The task is achieved by the fact that in an atomic absorption mercury analyzer consisting of a source of resonant radiation of mercury placed in a constant magnetic field, an optical-acoustic modulator (PEM), a linear polarizer, an analytical cell, a photodetector, a current / voltage converter, a recording system, and a microprocessor, As a photodetector, a photodiode is used, characterized by a large quantum yield (the quantum yield of the photodiode is 70–95%, and the PMT-142 used in mercury spectrometers is 5–7%). To amplify the photodiode of the photodiode, a current / voltage converter with a conversion coefficient of 10 is used. The spectral sensitivity of SiC semiconductor photodiodes has a maximum in the region of the resonance mercury line I, 255 nm, and rapidly decreases for wavelengths in both directions from the position of the mercury line. Consequently, a pair of photodiode current / voltage converter is equivalent to a PMT in terms of photocurrent gain and spectral sensitivity bandwidth and significantly exceeds it in quantum efficiency. Thus, in the case when the detection limit is determined by shot noise, the use of a photodiode to register resonant radiation instead of a PMT will reduce the detection limit by 3-5 times.

The variation of the dynamic range from device to device is noticeably reduced due to the use of the constant component of the photocurrent SQ as normalization in the formation of the analytical signal Sa. In this case, Sa is calculated by the formula (7), where S (t) Si / So, b 4Ji (A) pxpy / (px + Py) is the normalization constant. Indeed, to realize the maximum sensitivity, parameter A is chosen so that Ji (A), the factor for the first harmonic signal in formula (3), is maximum. With a value of A «135, the factor Ji (A) is close to the maximum value and is equal to Ji (A)« 0.53, while Jo (A) «О, J2 (A) 0.48. Taking these relations into account, we rewrite formula (2) for the signal So in the form

(T, + T,) (p, 4p;) (8)

Note that in the utility model, the transmission difference along the x and y axis (px - pu) arises due to spurious polarization on the optical parts and its value is quite small, while in the prototype this difference is specially created using an inclined plate (to form the second harmonics) and it is equal to (P Ru) 0.1- The absence of an inclined plate again reduces the variation of the dynamic range, because since (px - Py) and 6o are caused by spurious effects, then (p - Py) sin6o «О and then the signal Si is practically independent of the parasitic polarization parameters:

S, b.) (T, -T,) p, p cos6o (9)

Experimental verification showed that the dynamic range of different devices varies by no more than 20%.

FIG. 1. Block - scheme of a utility model. Figure 2. Block - registration system diagram.

Fig.Z. Dependence of the detection limit of mercury in air (at the S / N 3 level) on the value of Z for the photodiode, where I is the total intensity of the a-components.

A block diagram of a utility model is shown in FIG. one.

The atomic absorption analyzer contains a resonant radiation source 1, an exciting high-frequency generator 2, a permanent magnet 3, two lenses 4, an optical acoustic modulator (REM) 5, a crystal oscillator 6, a polarizer 7, an analytical cell 8, a photodiode 9, a current / voltage transducer 10, registration system 11, microprocessor 12, digital indicator / control panel 13.

The block diagram of the registration system is presented in figure 2.

The registration system consists of a narrowband amplifier 14, synchronous

detector 15, DC detector 16.

Consider the action of a utility model using an example of a lamp with the Hg204 isotope as a radiation source. Under the influence of a magnetic field 3, the emission resonance mercury line X 254 nm is split into an unbiased TC component and two shifted CT components. When observing the radiation of the light source 1 along the magnetic lines of force, a + - and st components are observed with circular polarization clockwise and counterclockwise, respectively. The magnitude of the magnetic field is selected in such a way that the a-component is shifted to the region of maximum absorption of mercury atoms and thus fulfill the role of an analytical line, and the art component leaves the absorption line and plays the role of a comparison line. To temporarily separate the intensities of the a + - and a.-components, we use a PEM 5 optoacoustic modulator and a linear polarizer 7. In the absence of mercury atoms in the analytical cell 8, the intensities of a + - and a.-components are equal. With the appearance of absorbing atoms, the intensity of the a + component decreases, since its spectral position coincides with the maximum of the mercury absorption circuit in air, and the intensity of the a component practically remains the same, since it is on the edge of the absorption circuit. As a result, a Si signal appears at the modulation frequency associated with the concentration of mercury in the analytical cell. To ensure selectivity, a signal So proportional to the photocurrent constant is used as a normalization signal. The signals Si and So are amplified in the recording system 11, and the signal Si is extracted using a narrow-band amplifier 14 and a synchronous detector 15 tuned to the modulation frequency, and the signal So is extracted using a detector 15. The final signal processing occurs in the microprocessor 12 according to formula (7) and the result is displayed on digital indicator 13. In the case of using a lamp with another isotope or with a natural mixture, the utility model works in the same way, only the measurement sensitivity will change.

Thus, in the utility model, the detection limit is reduced and, accordingly, the dynamic range is expanded 5 times due to the use of a photodiode as a photodetector and amounts to 1 ng / m (see Fig. 3),

the dynamic range of measured concentrations was stabilized by using a new registration system and the design, adjustment and alignment of the device were simplified by eliminating the inclined plate in the optical path, a narrow-band amplifier and a second-harmonic synchronous detector in the registration system and PMT together with a power supply.

List of references.

1.A.A. Gapeev, S.E. Sholupov, M.N. Slyadnev. Zeeman modulation polarization spectrometry as a variant of atomic absorption analysis: possibilities and limitations. Journal of Analytical Chemistry, 1996, vol. 51, No. 8, p. 85864

2.S.E.Sholupov, A.A. Ganeyev. Zeeman atomic alzofiop spectrometry using high frequency modulated light polarization. Spectrochim. Acta, 1995, v.SOB, N10, p. 12271236.

Claims (2)

1. Atomic absorption mercury analyzer containing an optically coupled resonant radiation source placed between the pole pieces of a permanent magnet, a lens, an optoacoustic modulator, optically coupled polarizer, an analytical cell, a second lens and a photodetector, as well as a recording system and a microprocessor, characterized in that the photodetector is made in the form of a photodiode, the introduced current-voltage converter is connected at the input to the photodiode, and at the output, to the recording system, and the optoacoustic modulator optically connected to a polarizer. 2. The device according to claim 1, characterized in that the registration system is made in the form of series-connected narrow-band amplifier and synchronous detector, in parallel with which a DC detector of the photodiode is connected.