WO1993012442A1

WO1993012442A1 - Method for measuring liquid scintillation samples deposited on multi-well sample plates

Info

Publication number: WO1993012442A1
Application number: PCT/FI1991/000380
Authority: WO
Inventors: Tapio YRJÖNEN; Timo Oikari
Original assignee: Wallac Oy
Priority date: 1991-12-10
Filing date: 1991-12-10
Publication date: 1993-06-24

Abstract

The invention shows a method for measuring liquid scintillation samples deposited in the sample wells of multi-well sample plates. The method corrects the measured values of the samples being affected by scintillation photons from other samples of the plate. The correction is done by determining the affection of the other samples of the multi-well sample plate.

Description

METHOD FOR MEASURING LIQUID SCINTILLATION SAMPLES DEPOSITED ON MULTI-WELL SAMPLE PLATES

BACKGROUND OF THE INVENTION

Liquid scintillation counters are commonly used to measure the count rate or activity of samples containing low energy beta particles or corresponding particles emitting

radionuclides such as tritium and carbon-14.

The range of the low energy beta particles in the sample is generally few tens of micrometers at the most. As a

consequence, the sample to be measured has to be placed in direct contact with a scintillation medium, which comprises a solvent or solvents and a solute or solutes present in a few percent by weight of the solutions. In interaction processes the most of the kinetic energy of the beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons, whose amount is proportional to the energy of the beta particle. These scintillation photons are detected usually by two, in coincidence operating, photomultiplier tubes producing electric pulses. The height of the pulses are proportional to the amount of emitted scintillation photons and thus proportional to the energy of the interacted beta particle.

When measuring sample activities with liquid scintillation counters, the basic problem is the counting efficiency due to the quenching of the sample.

It is known in the liquid scintillation counting that the reduction of the counting efficiency due to the quenching of the sample can be corrected by a means of a quench curve which describes the relationship between the counting efficiency and the amount of the quench of the sample.

Normally liquid scintillation counters are provided with one detector and they are designed to measure samples in 7 ml or 20 ml glass or plastic vials.

A novel liquid scintillation counter, which counts samples directly from multi-well sample plates is published under International Patent Publication Number WO 89/12838, which apparatus counts liquid scintillation or corresponding samples directly from sample plates which comprises several separate sample wells or vials. The apparatus has one or several detectors in order to count one or several samples at a time. The sample plate is placed in the counting position or before counting position manually or

automatically on a rigid plate holder made of photon attenuating material and having holes for the wells of the sample plate. The walls of the holes are reflecting or scattering in order to guide the photons from the liquid scintillation sample to the detectors, built of two

photomultiplier tubes operating in coincidence and situated on the opposite sides of the holes of the plate holder. The wells of the sample plate can be closed by an adhesive transparent tape. The apparatus can be used also for counting gamma radiation emitting samples if the holes of the sample plate are surrounded by gamma radiation

sensitive detectors.

Another novel scintillation counting system for in-situ measurement of radioactive samples in a multiple-well plate is presented under European Patent Publication Number

0425767A1. This apparatus is provided with multiple

photomultiplier tubes positioned adjacent to the sample wells containing the scintillator for simultaneously

measuring the radioactivity of multiple samples with only a single photomultiplier tube sensing the scintillations from each well and converting the sensed scintillations into corresponding electrical pulses. The electrical pulses from each photomultiplier tube are processed to discriminate between pulses attributable to sample events within the wells and pulses attributable to non-sample events such as photomultiplier tube noise. The discrimination is effected by determining whether a selected number of electrical pulses occurs with a prescribed time interval, the

occurrence of the selected number of pulses within the prescribed time interval signifying a sample event. Only the electrical pulses attributable to sample events are supplied to a pulse analyzer.

The multi-well sample plates have typically eight row of wells, which diameter is 7 - 8 mm arranged in twelve columns with 9 millimeters distance between the center points of the wells. The typical volumes of sample wells of such 96-well sample plates are 200 - 400 microliters depending on the height of the plate. When the wells of the multi-well sample plate are separate, it can placed before counting on a rigid sample plate holder made of photon attenuating material and having with thru-holes for the wells of the sample plate as shown in patent application published under an international publication number WO 89/12838. As a consequence, an optically isolated

compartment is formed around each sample well of the sample plate. Unfortunately most of the commercially available multi-well sample plates are transparent and the wells are joined together with ribs or resp. in order to stiffen said sample plate. As a consequence of this is that it is impossible to isolate the wells optically. As a further consequence of this is that some amount of the

scintillation photons produced by the absorption of the beta particle in certain sample well may travel to other sample wells and thus producing an undesired increase in observed count rates in those wells. This phenomenom is called as optical crosstalk . It is known that the use of opaque multi-well sample plates can reduce optical

crosstalk as mentioned in TopCount Topics PAN0005 6/91, published by Packard Instrument Company, Meriden USA 1991. Unfortunately in many applications transparent multi-well sample plates are preferred and the most of commercially available multi-well sample plates are transparent. In somewhat different field of penetrating (gamma)

radiation detection is presented in US patent no. 4 348 588 (Yrjönen et al.). That patent shows a system for

measurement of radioactivity of a number of radioactive samples by a radiation detector which will be affected by radiation from a number of other sources having

predetermined locations in addition to the sample being measured consist of first measuring the values of activity measured by the detector when a standard radiation source of known activity is placed in the detector and in each of the predetermined locations to determine the effect of the other sources on the measurements of the detector, storing the values thus determined and thereafter compensating the values of radioactivity of each sample measured by the detector with stored values to obtain the actual values of the radioactivity of the sample. However no optical cross- talk is involved in that patent.

SUMMARY OF THE INVENTION

The present invention shows a novel method and an apparatus for measuring liquid scintillation or corresponding samples deposited in the sample wells of any multi-well sample plates. The apparatus has one or several detectors, built of two photomultiplier tubes operating in coincidence and situated on the opposite sides of the multi-well sample plate, in order to count one or several samples at a time. The method corrects measured values of the samples

deposited on multi-well sample plate by a detector which will be affected by scintillation photons from other

samples of the plate. Said correction is done by

pre-determining affection of the other samples of the multi-well sample plate.

The above and other features and advantages of this

invention will become better understood by reference to the detailed description that follows, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cut out of a side view of a transparent

multi-well sample plate in a counting position.

FIG. 2 shows the values of the samples measured by the

liquid scintillation counter without cross talk correction .

FIG. 3 shows the values of FIG. 2 with cross talk

correction according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the FIG.1 there is a cut-out of a multi-well sample plate 10 comprising sample wells 11, 12 and 13. The walls 14 and bottoms 15 of said sample plate are light permeable. Said sample wells are closed by a transparent lid 21 or transparent adhesive tape 21 or transparent heat sealed foil or corresponding means. Said sample wells 11, 12 and 13 contain samples 16, 17 and 18. In the situation shown in FIG. 1 the sample well 12 containing the sample 17 is in the counting position: it is viewed by

photomultiplier tubes 19a and 19b thru aperture holes 20a and 20b. In addition to the scintillation photons produced by the sample 17 said photomultiplier tubes 19a and 19b observe scintillation photons produced by other samples (e.g.16 and 18) in the other sample wells (e.g.11 and 13), because the walls 14 between said sample wells are

transparent. For compensating this optical crosstalk the inventors have found out a new method, which mathematiaal argumentation is the following: For simplyfying mathematical notations the sample wells of the 96-well multi-well sample plate are numbered in the following way:

When a sample in the sample well i is the counting

position, the corresponding detector measures it by

counting efficiency E_i. In addition said detector measures other samples in other sample wells j. These counting efficiencies compared to E_i are defined as

crosstalk-coefficients C_i,j (i=1,...,96, j=1,...,96). The shorter is the distance between said detector and a sample, the larger is the crosstalk-coefficient.

Thus a detector, which measures a sample in a well i, detects count rate

I_i = B_i + E_i*C_i,j*A_j, (j = 1,...,96), (Eq. 1) where A_j = activity of a sample in a sample well j,

B_i = background count rate of the sample well i.

This equation can be written a matrix form

I = B + E*C*A, (Eq. 2)

By solving A gives A = C^-1*E^-1*(I-B), (Eq. 3) which can be written as

A_i = C_i,j ^-1*E_j ^-1*(I_j - B_j), (Eq. 4)

(i = 1,...,96, j = 1,...,96). Eq. 4 can be simplified by writting

which is efficiency and background corrected countrate, and finally

A_i = C_i,j ^-1*I_{c,j '} ( Eq . 5) (i = 1,...,96, j = 1,...,96).

Equations 3, 4 and 5 shows activities of the samples A_i can be calculated from detected count rates I_i, if the elements of matrixes B, E^-1 and C^-1 are known in priori. A, I, B and I_c are matrixes, which have 96 rows and 1 column.

E is a matrix has 96 rows and 96 columns, but only the diagonal elements deviates from zero.

C is a symmetric matrix with 96 rows and 96 columns. Its diagonal elements are one and other element are smaller than one.

The method in practice is the following:

Further definations:

Table I. A simplified notations of the crosstalk-coefficients of a sample, which is in the sample well i :

1

In above table it is assumed that the crosstalk caused by the sample which is in the sample well i is limited in the area shown in the table. In practise this corresponds the situation, when ³H-samples are measured in a transparent multi-well sample plate. The main purpose of above table is to show that due to the symmetry of the counting conditions there is in this case only six different

crosstalk-coefficients. For example according to the table

C5=C_i-26,i, C_i-22,i, C_i+22,i, C_i+26,i.

I. Crosstalk standardization The purpose of this crosstalk standardization is to define and store for further use crosstalk-coefficients C_i _{, j}, counting efficiencies Ei and background count rates B_i.

For this purpose a crosstalk-standardization plate is prepared:

It can be seen that an active sample is deposited in the sample well G11 (or 83) and background samples for the defination of the crosstalk coefficients is deposited in the sample wells E09 (57), E10 (58), E11 (59), F10 (70), F11 (71) and for the defination of background in the sample well A01 (1).

The standardization procedure is as follows: 1) At first, background sample which is in the sample well A01 is measured by detector 1. This background count rate is stored and it is used for subtracting background count rates in the following steps.

2) Second the active sample, which is in the sample well G11 (83) is measured by each detectors. The counting efficiencies Ei of each detector for further use are calculated from formula

E_i = (I_i - BG)/A, (Eq. 6) where I_i = measured count rate of the standard (G11),

BG = background (A01) count rate,

A = activity of the standard (G11) or highest measured count rate. 3) Third the background samples which are in the sample wells E09 (57), E10 (58), E11 (59), F10 (70), F11 (71) are measured by detector 1. Crosstalk-coefficients are

calculated from measured count rates by substituting detector 1 counting efficiency E₁, background count rate BG and activity A:

C1 = (I_F11-BG)/E₁*A,

C2 = (I_F10-BG)/E₁*A, C3 = (I_{E 11}-BG)/E₁*A, (Eq. 7)

C4 = (I_E10-BG)/E₁*A,

C1 = (I_E09-BG)/E*A,

These coefficients are substituted into correct position of the C-matrix. After this the inversion C-1 of the C-matrix is computed and C^-1-matrix is stored for further use.

E-matrix need not to be inverted, because its non-diagonal elements are zero, which means that E_i,i ^-1 = 1/E_i,i.

The applicants have found out that crosstalk-coefficients can be calculated theoretically: crosstalk count rate, which is observed in a sample well i can be approximated using equation

I_i = A_j*(D_i,j/ (4*3. 14*r_{i ,j} ²) *e^-u _{i ,j}*r_{i ,j}, ( Eq. 8 )

(j=1, ... ,96, j<>i), where is D_i,j/4*3.14*r_i,j ² is the relative space angle in which sample i is seen from sample j. The distance between sample i and sample j is ri, j. Total linear absorption coefficient u_{i ,j}can be experimentally defined and its magnitude depends for example on the optical properties of the sample plate.

It can be seen from Eq. 8 that

E_i*C_{i, j} = (D_i,j/(4*3.14*r_i,j ²)*e^-u _i,j*r_i,j, (Eq. 9) (j=1, ... ,96, j<>i), is the counting efficiency by which detector, which

measures sample i, detects other samples j. The

E_i*C_i,j-values can be substituted to E*C-matrix in Eq. 2. Then (E*C)^-1 is computed and stored for further use.

II. Correcting measured count rates of the samples to be analyzed.

Every sample of the sample plate is measured. Activities or corrected count rates of the samples are calculated from Eq. 5 or Eq. 4.

The method according to the present invention is not confined the above description alone, but it may show even considerable variation within the scope of the patent claims.

Claims

1. Method for correcting measuring values in liquid scintillation counter having at least one detection means, said liquid scintillation counter being adapted to measure samples deposited on sample plates, said detection means being affected by the scintillation photons from sample to be analyzed and also being affected by the scintillation photons from other samples on the same sample plate, characterized by

- determining degree of affection from said other samples, - measuring samples to be analyzed to obtain their first measuring values,

- correcting said first measuring values by said determined degree of affection to obtain their corrected measuring values.

2. Method according to claim 1, characterized by that said degree of affection is a contribution expressed as the number of scintillation pulses.

3. Method according to claim 1, characterized by

determining theoretically said degree of affection,

whereafter correcting the measuring values by said

determined degree of affection.

4. Method according to claim 1, characterized by measuring degree of affection of at least one isotope sample,

whereafter correcting the measuring values by said

determined degree of affection.