while the invention has been shown and described with reference to a number of embodiments thereof , it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims . fig1 shows , by way of example , a rectangular sample support plate 100 with 63 ( columns a - i ; rows 1 - 7 ) circular sample sites 102 in plan view . the shape of the sample support and / or the number and arrangement of the sample sites on the sample support are only to be understood as an example , of course . there can be a large variety of deviations from the embodiment explicitly shown here without deviating from the principle of the invention . the circular sample sites 102 are intended to hold the prepared samples . the sample sites 102 can be surrounded by a milled - in groove , for example , to largely prevent the matrix solution from flowing away ; they can additionally , or alternatively , be hydrophilic , in contrast to the areas between sample sites , which have a hydrophobic coating . these areas serve to provide a spatial demarcation from adjacent sample sites , among other things . normally the prepared samples should not touch these areas . if this does happen despite all the care taken in the sample preparation , the risk of cross - contamination thus created can be detected by the method explained as part of this disclosure without excessive procedural effort , in particular without having to resort to any further devices in addition to the maldi ion source and a mass analyzer . for the purpose of detecting a contamination , the desorption laser , which transfers the prepared analyte molecules into the gaseous phase and ionizes them by accurately bombarding the sample sites coated with samples , is directed once or several times at the areas between the individual sample sites 102 , and is activated so that the emitted laser light pulse hits a predetermined point on the area between sample sites , see black rectangles 104 in fig1 . this is carried out in certain phases of a deposition procedure , or alternatively of an analytical measurement procedure . if these points of the areas between sample sites are contaminated with substance material which should never have got there , then the laser energy produces ions which can be mass - selectively acquired and detected with a mass analyzer , for example a time - of - flight mass spectrometer . this obviates the need to use additional optical imaging apparatus , as is known from the prior art , thus significantly reducing the apparatus and procedural effort required for the contamination test . so if ions are detected at a point on the sample support where none should be present , i . e ., on an area between sample sites , this is an indication of contamination and can lead to appropriate corrective measures . it is possible , for example , to provide the sample sites adjacent to a contaminated point with a digital or electronic label , which could be called “ suspicion of contamination ”, for example , in order to draw a user &# 39 ; s attention to the fact that the spectra obtained from these sample sites are possibly falsified by cross - contamination and require special attention during the evaluation . it is also possible to alert a user immediately to a positive indication of contamination via an acoustic , optical or other type of alarm signal while the procedure is still being carried out , so that the points identified can be inspected more closely . in fig1 , several sampling patterns are depicted by the rectangles 104 , which can be used all together , in turn or as alternatives . a very time - saving type is shown in row 7 of the sample site matrix , where every area between two adjacent sample sites 102 in the same row is sampled just once with the desorption laser ; in this example centrally on an imaginary line connecting the centers of the sample sites 102 . this type of sampling is especially suitable for procedures where prepared sample sites ( a - i1 ; a - i2 ; etc .) are examined row by row for analyte molecules . in this case the positioning device which moves the light beam guide of the desorption laser and the sample support 100 relative to each other can , in an intermediate step , position the desorption laser so as to sample between two sample sites before it is directed at the next sample site to be analyzed . it is essentially unimportant here whether the positioning is carried out via an xy shift of the sample support , a change in the axis along which the laser light is incident , or both . all conceivable relative positioning devices shall be covered by the invention described here . furthermore , it is understood that the investigation can take place row by row ( a - i1 ; a - i2 ; etc .) and / or column by column ( a1 - 7 ; b1 - 7 ; etc .). in addition to the sample sites immediately adjacent in a row ( 1 - 7 ) and / or a column ( a - i ), the areas between sample sites can also be sampled at the point where four sample sites are diagonally closest . this is depicted in fig1 , by way of example , with the aid of the sample site groups g6 , g7 , h6 and h7 and also h5 , h6 , i5 and i6 , where a sampling point is indicated at the intersection of the imaginary diagonal lines connecting these four sample sites . with this embodiment , it is also possible to check the slightly lower risk of contamination across diagonal separations . in addition , fig1 shows , with the aid of sample sites b2 , d3 and f4 as an example , the density and / or frequency with which the surroundings of a sample site 102 can be sampled in order to detect any contamination . in the case of sample site b2 , there are fourteen sampling points , which are arranged more or less in a circle around the sample site ; for d3 there are seven , and for f4 five . these numbers must be seen only as illustrating the method , but not limiting it . in principle , the higher the repetition rate of the desorption laser , i . e . the higher the frequency with which the laser shots can be fired , the more sampling points can be targeted per time period in order to check for contamination . repetition rates of 2 to 10 kilohertz or more prove to be helpful here . it is also particularly favorable if the maldi ion source is coupled to a fast mass analyzer , such as a time - of - flight mass spectrometer . but it is also possible to use other types of mass analyzer , depending on the setting of the repetition rate . fig3 a and 3b show mass - spectrometric signals acquired with an ion detector . one signal has no ion signature ( fig3 a ), in other words a zero signal , and the other has a specific ion signature ( fig3 b ). the ion signatures in fig3 b originate from α - cyano - 4 - hydroxycinnamic acid , in the upper part of the diagram , and sinapic acid , in the lower part of the diagram . they contain not only ionized matrix molecule ions but also matrix molecule cluster ions , labeled in each case , as they typically occur with maldi ionization of these matrices . for the sample support in fig1 , the areas between sample sites are uncoated and often consist of bare metal . if a desorption laser samples such a metal surface , the mass - spectrometric signal can look like the one shown in fig3 a , i . e ., without any detectable ion signature , or generally without a detectable ion signal above the noise . if , on the other hand , matrix solution from a sample site 102 gets onto an area between sample sites in the course of a preparation , or if it is applied there unintentionally , sampling with the desorption laser at this point on the area between sample sites will produce a mass - spectrometric signal with a characteristic ion signature , as shown in fig3 b ; this signal depends on the matrix used for preparing the samples . it is understood that the two matrix ion spectra from fig3 b are only examples of an ion signature indicating a contamination , and that there are further examples , and especially alternative matrix substances , which are not all stated or pictured here for reasons of clarity and brevity . regarding a further embodiment , fig2 shows a similar image to fig1 . the difference lies in the signal which is defined for an area that is not contaminated during the preparation . in the case of the sample support from fig2 , the areas between sample sites are coated with a substance 206 , which , when sampled with a desorption laser , exhibits a characteristic signal shape , for example a characteristic ion signature , in the detector signal interrogated in temporal relation to the sampling . this substance can be a matrix substance , for example , as can be seen in fig3 b . in the case of a sample support with pre - coated areas between sample sites , a contamination could be a difference between the detected ion signature and the expected characteristic ion signature . if , in an example , the areas between sample sites on a sample support are coated with cyano - 4 - hydroxycinnamic acid ( hatched area 206 in fig2 ), and the samples on sample sites 102 are prepared with sinapic acid , an ion signature as in the upper part of fig3 b would indicate a clean area without contamination , whereas the presence of sinapic acid on the sampled areas between sample sites would , with high probability , result in a superposition of the two panels of fig3 b . this would be different to the pure cyano - 4 - hydroxycinnamic acid spectrum , and would be interpreted as contamination . the invention should naturally also include the case where the matrix incorrectly applied to the area between sample sites covers the base coat layer of the other matrix so completely that only the sinapic acid signature from fig3 b is detected by sampling with the desorption laser in the explained example . in any case , the result is a difference to the pure cyano - 4 - hydroxycinnamic acid signature as a reference signal and this difference is taken as an indicator of contamination . in a very simple version of the method , when the clean state of the area between sample sites is defined by a zero signal such as the one shown in fig3 a , it may be sufficient to acquire the signal of the ion detector in a mass - independent way , i . e . not resolved according to m / z masses . the integral over all the channels shown in fig3 a would result in an intensity value close to zero , for example , and would indicate a clean area within the tolerance range of the noise . in contrast , an intensity integral over the spectra shown in fig3 b would result in a value significantly higher than the noise , which would suggest contamination . in some embodiments it can also be worthwhile carrying out a mass - selective measurement with the mass spectrometer , but one which is limited to a narrow mass range . in the upper part of the diagram shown in fig3 b , there is a high intensity peak at 212 . 045 u ( atomic mass unit = dalton ), for example . the time - of - flight mass spectrometer used in this example could therefore be limited to the transmission of ions in the range from , say , 211 . 5 u to 212 . 5 u . alternatively , the time - of - flight mass spectrometer can transmit ions in a larger mass region , although the ion detector is only interrogated in a narrow mass range . the intensity of an ion signal acquired in such an isolated way will nevertheless be sufficient in most cases to exceed the omnipresent noise in order to indicate contamination . the invention has been described above with reference to different , special example embodiments . it is understood , however , that various aspects or details of the invention can be modified without deviating from the scope of the invention . in particular , measures disclosed in connection with different embodiments can be combined in any way if this appears feasible to a person skilled in the art . in addition , the above description serves only as an illustration of the invention and not as a limitation of the scope of protection , which is exclusively defined by the enclosed claims , taking into account any equivalents which may possibly exist .