diff --git a/.Rbuildignore b/.Rbuildignore
index abbad15..d887687 100644
--- a/.Rbuildignore
+++ b/.Rbuildignore
@@ -9,3 +9,5 @@
 ^CRAN-RELEASE$
 ^cran-comments.md$
 ^\.github$
+^.*\.Rproj$
+^\.Rproj\.user$
diff --git a/.gitignore b/.gitignore
index 1396e56..acc9fb3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@
 *.o
 *.rds
 CRAN-RELEASE
+.Rproj.user
diff --git a/MALDIquant.Rproj b/MALDIquant.Rproj
new file mode 100644
index 0000000..21a4da0
--- /dev/null
+++ b/MALDIquant.Rproj
@@ -0,0 +1,17 @@
+Version: 1.0
+
+RestoreWorkspace: Default
+SaveWorkspace: Default
+AlwaysSaveHistory: Default
+
+EnableCodeIndexing: Yes
+UseSpacesForTab: Yes
+NumSpacesForTab: 2
+Encoding: UTF-8
+
+RnwWeave: Sweave
+LaTeX: pdfLaTeX
+
+BuildType: Package
+PackageUseDevtools: Yes
+PackageInstallArgs: --no-multiarch --with-keep.source
diff --git a/R/as.matrix-functions.R b/R/as.matrix-functions.R
index e9472de..60c7428 100644
--- a/R/as.matrix-functions.R
+++ b/R/as.matrix-functions.R
@@ -25,6 +25,7 @@
   m
 }
 
+
 ## .as.binary.matrix
 ##  internal function to convert a matrix with NA to a binary one
 ##
@@ -41,3 +42,26 @@
   mode(m) <- "integer"
   m
 }
+
+## .as.occurrence.list
+##  internal function to create a list of peaks occurrence
+##
+## params:
+##  l: list of AbstractMassObject objects
+##
+## returns:
+##  a list
+.as.occurrence.list <- function(l) {
+  
+  .stopIfNotIsMassObjectList(l)
+  
+  mass <- .unlist(lapply(l, function(x)x@mass))
+  uniqueMass <- sort.int(unique(mass))
+  n <- lengths(l)
+  r <- rep.int(seq_along(l), n)
+  
+  i <- findInterval(mass, uniqueMass)
+  
+  return(list(r = as.integer(r), i = as.integer(i), masses = uniqueMass))
+  
+}
diff --git a/R/filterPeaks-functions.R b/R/filterPeaks-functions.R
index 9595939..33ad79e 100644
--- a/R/filterPeaks-functions.R
+++ b/R/filterPeaks-functions.R
@@ -51,44 +51,45 @@ filterPeaks <- function(l, minFrequency, minNumber, labels,
   minNumber <- rep_len(minNumber, nl)
   mergeWhitelists <- mergeWhitelists[1]
 
-  ## binary peak matrix (mask)
-  m <- .as.binary.matrix(.as.matrix.MassObjectList(l))
+  ## use peaks occurrence list
+  o <- .as.occurrence.list(l)
 
-  ## whitelist
-  w <- matrix(0L, nrow=nrow(m), ncol=ncol(m))
-
-  ## group indices by labels
+ # group indices by labels
   idx <- lapply(ll, function(x)which(labels == x))
 
   ## collect whitelists
+  
+  w <- matrix(FALSE, nrow = nl, ncol = length(o$masses))
+  
   for (i in seq_along(idx)) {
-    wl <- .whitelist(m, idx[[i]],
-                     minFrequency=minFrequency[i], minNumber=minNumber[i])
+    
+    wl <- .whitelistoccur(o, idx[[i]], minFrequency=minFrequency[i], minNumber=minNumber[i])
+    
     if (sum(wl)) {
       if (mergeWhitelists) {
         ## R uses columnwise recycling
         w <- t(t(w) | wl)
       } else {
         ## R uses columnwise recycling
-        w[idx[[i]], ] <- t(t(w[idx[[i]], , drop=FALSE]) | wl)
+        w[i, ] <- t(t(w[i, , drop=FALSE]) | wl)
       }
     } else {
       warning("Empty peak whitelist for level ", sQuote(ll[i]), ".")
     }
+    
   }
-
-  ## apply whitelist
-  w <- w & m
-
+  
   ## turn matrix back into MassPeaks objects
-  for (i in seq_along(l)) {
-    j <- which(as.logical(m[i, ]))
-    include <- which(w[i, j])
-    l[[i]]@mass <- l[[i]]@mass[include]
-    l[[i]]@intensity <- l[[i]]@intensity[include]
-    l[[i]]@snr <- l[[i]]@snr[include]
+  
+  for (i in seq_along(idx)) {
+    for (j in idx[[i]]) {
+      wmask <- w[i, o$i[o$r == j]]
+      l[[j]]@mass <- l[[j]]@mass[wmask]
+      l[[j]]@intensity <- l[[j]]@intensity[wmask]
+      l[[j]]@snr <- l[[j]]@snr[wmask]
+    }
   }
-
+  
   l
 }
 
@@ -133,3 +134,48 @@ filterPeaks <- function(l, minFrequency, minNumber, labels,
 
   colSums(m[rows, , drop=FALSE]) >= minPeakNumber
 }
+
+
+.whitelistoccur <- function(l, rows, minFrequency, minNumber) {
+
+  ## test arguments
+  if (is.na(minFrequency) && is.na(minNumber)) {
+    stop(sQuote(minFrequency), " or ", sQuote(minNumber),
+         " has to be a meaningful number!")
+  }
+
+  if (!is.na(minFrequency) && minFrequency < 0L) {
+    minFrequency <- 0L
+    warning(sQuote("minFrequency"),
+            " < 0 does not make sense! Using 0 instead.")
+  }
+
+  if (!is.na(minNumber) && minNumber < 0L) {
+    minNumber <- 0L
+    warning(sQuote("minNumber"), " < 0 does not make sense! Using 0 instead.")
+  }
+
+  if (!is.na(minFrequency) && !is.na(minNumber)) {
+    warning(sQuote("minFrequency"), " and ", sQuote("minNumber"),
+            " arguments are given. Choosing the higher one.")
+  }
+
+  ## calculate minimal number of peaks
+
+  keep.rows <- (l$r %in% rows)
+  l$r <- l$r[keep.rows]
+  l$i <- l$i[keep.rows]
+  l$i <- split(l$i, factor(l$i))
+
+  minPeakNumber <- max(minFrequency * length(unique(l$r)), minNumber, na.rm=TRUE)
+
+  above.min <- function(x) { length(x) >= minPeakNumber }
+  wl.vals <- unlist(lapply(l$i, above.min))
+
+  wl <- array(FALSE, length(l$masses))
+  wl[as.numeric(names(l$i))] <- wl.vals
+
+  return(c(wl))
+
+}
+